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Sample records for intergranular cracking mechanism

  1. Mechanisms of stress corrosion cracking and intergranular attack in Alloy 600 in high temperature caustic and pure water

    SciTech Connect

    Bandy, R.; van Rooyen, D.

    1984-01-01

    In recent years, several studies have been conducted on the intergranular stress corrosion cracking (SCC) and intergranular attack (IGA) of Alloy 600. A combination of SCC and IGA has been observed in Alloy 600 tubing on the hot leg of some operating steam generators in pressurized water reactor (PWR) nuclear power plants, and sodium hydroxide along with several other chemical species have been implicated in the tube degradations. SCC has been observed above and within the tube sheet, whereas IGA is generally localized within the tube sheet. Alloy 600 is also susceptible to SCC in pure and primary water. Various factors that influence SCC and IGA include metallurgical conditions of the alloy, concentrations of alkaline species, impurity content of the environment, temperature and stress. The mechanisms of these intergranular failures, however, are not well understood. Some of the possible mechanisms of the SCC and IGA in high temperature water and caustic are described in this paper.

  2. UNDERSTANDING THE MECHANISMS CONTROLLING ENVIRONMENTALLY-ASSISTED INTERGRANULAR CRACKING OF NICKEL-BASE ALLOYS

    SciTech Connect

    Gary S. Was

    2004-02-13

    Creep and IG cracking of nickel-base alloys depend principally on two factors--the deformation behavior and the effect of the environment. We have shown that both contribute to the observed degradation in primary water. The understanding of cracking does not lie wholly within the environmental effects arena, nor can it be explained only by intrinsic mechanical behavior. Rather, both processes contribute to the observed behavior in primary water. In this project, we had three objectives: (1) to verify that grain boundaries control deformation in Ni-16Cr-9Fe at 360 C, (2) to identify the environmental effect on IGSCC, and (3) to combine CSLBs and GBCs to maximize IGSCC resistance in Ni-Cr-Fe in 360 C primary water. Experiments performed in hydrogen gas at 360 C confirm an increase in the primary creep rate in Ni-16Cr-9Fe at 360 C due to hydrogen. The creep strain transients caused by hydrogen are proposed to be due to the collapse of dislocation pile-ups, as confirmed by observations in HVEM. The observations only partially support the hydrogen-enhanced plasticity model, but also suggest a potential role of vacancies in the accelerate creep behavior in primary water. In high temperature oxidation experiments designed to examine the potential for selective internal oxidation in the IGSCC process, cracking is greatest in the more oxidizing environments compared to the low oxygen potential environments where nickel metal is stable. In Ni-Cr-Fe alloys, chromium oxides form preferentially along the grain boundaries, even at low oxygen potential, supporting a potential role in grain boundary embrittlement due to preferential oxidation. Experiments designed to determine the role of grain boundary deformation on intergranular cracking have established, for the first time, a cause-and-effect relationship between grain boundary deformation and IGSCC. That is, grain boundary deformation in Ni-16Cr-9Fe in 360 C primary water leads to IGSCC of the deformed boundaries. As well

  3. Mechanisms of intergranular fracture

    SciTech Connect

    Farkas, D.

    1999-08-01

    The authors present a study of the atomistic mechanisms of crack propagation along grain boundaries in metals and alloys. The failure behavior showing cleavage crack growth and/or crack-tip dislocation emission is demonstrated using atomistic simulations for an embedded-atom model. The simulations follow the quasi-equilibrium growth of a crack as the stress intensity applied increases. Dislocations emitted from crack tips normally blunt the crack and inhibit cleavage, inducing ductile behavior. When the emitted dislocations stay near the crack tip (sessile dislocations), they do blunt the crack but brittle cleavage can occur after the emission of a sufficient number of dislocations. The fracture process occurs as a combination of dislocation emission/micro-cleavage portions that are controlled by the local atomistic structure of the grain boundary. The grain boundary is shown to be a region where dislocation emission is easier, a mechanism that competes with the lower cohesive strength of the boundary region.

  4. Intergranular Strain Evolution near Fatigue Crack Tips in Polycrystalline Metals

    SciTech Connect

    Zheng, Lili; Gao, Yanfei; Lee, Sooyeol; Barabash, Rozaliya; Lee, Jinhaeng; Liaw, Peter K

    2011-01-01

    The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.

  5. Intergranular strain evolution near fatigue crack tips in polycrystalline metals

    NASA Astrophysics Data System (ADS)

    Zheng, L. L.; Gao, Y. F.; Lee, S. Y.; Barabash, R. I.; Lee, J. H.; Liaw, P. K.

    2011-11-01

    The deformation field near a steady fatigue crack includes a plastic zone in front of the crack tip and a plastic wake behind it, and the magnitude, distribution, and history of the residual strain along the crack path depend on the stress multiaxiality, material properties, and history of stress intensity factor and crack growth rate. An in situ, full-field, non-destructive measurement of lattice strain (which relies on the intergranular interactions of the inhomogeneous deformation fields in neighboring grains) by neutron diffraction techniques has been performed for the fatigue test of a Ni-based superalloy compact tension specimen. These microscopic grain level measurements provided unprecedented information on the fatigue growth mechanisms. A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is adopted to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Predictions from perfect grain-boundary simulations exhibit the same lattice strain distributions as in neutron diffraction measurements, except for discrepancies near the crack tip within about one-tenth of the plastic zone size. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find a significantly improved agreement between predicted and measured lattice strains inside the fatigue process zone. Consequently, the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.

  6. Computer Simulation of Intergranular Stress Corrosion Cracking via Hydrogen Embrittlement

    SciTech Connect

    Smith, R.W.

    2000-04-01

    Computer simulation has been applied to the investigation of intergranular stress corrosion cracking in Ni-based alloys based on a hydrogen embrittlement mechanism. The simulation employs computational modules that address (a) transport and reactions of aqueous species giving rise to hydrogen generation at the liquid-metal interface, (b) solid state transport of hydrogen via intergranular and transgranular diffusion pathways, and (c) fracture due to the embrittlement of metallic bonds by hydrogen. A key focus of the computational model development has been the role of materials microstructure (precipitate particles and grain boundaries) on hydrogen transport and embrittlement. Simulation results reveal that intergranular fracture is enhanced as grain boundaries are weakened and that microstructures with grains elongated perpendicular to the stress axis are more susceptible to cracking. The presence of intergranular precipitates may be expected to either enhance or impede cracking depending on the relative distribution of hydrogen between the grain boundaries and the precipitate-matrix interfaces. Calculations of hydrogen outgassing and in gassing demonstrate a strong effect of charging method on the fracture behavior.

  7. Further Study of near Solidus Intergranular Cracking in Inconel 718

    NASA Technical Reports Server (NTRS)

    Thompson, R. G.

    1981-01-01

    A series of tests, performed to determine the strain necessary to initiate intergranular cracking in Inconel 718 as a function of temperature, contained enough scatter near the melting temperature that questions remained as to the best curve of curves to fit to the data. Fracture surface analysis showed that the scatter was due to incipient melting in the grain boundary region. The melting contributed to low fracture strain but had only a small on the incipient cracking strain. Gleeble tests, which could be interrupted by water quenching, were used to study the incipient intergranular melting of Inconel 718. This modified weld simulation test provided a sufficiently rapid quench to preserve the intergranular microstructure created during incipient melting. This structure was studied both microscopically and with energy dispensive X-ray analysis. The implications of incipient melting and low-strain incipient cracking on the development of microfissuring envelopes are discussed.

  8. Intergranular stress corrosion cracking and selective internal oxidation of nickel-chromium-iron alloys in hydrogenated steam

    NASA Astrophysics Data System (ADS)

    Capell, Brent M.

    2005-07-01

    Selective internal oxidation (SIO) is a mechanism of grain boundary embrittlement through the formation of intergranular oxides of Cr2O3. SIO is proposed as a mechanism to explain intergranular stress corrosion cracking (IGSCC) of Ni-base alloys in pressurized water reactor environments. The purpose of this work is to investigate SIO through a series of experiments using controlled-purity alloys in a controlled, low-pressure steam environment in which the oxygen potential is varied. Five alloys; Ni-9Fe, Ni-5Cr, LCr (Ni-5Cr-9Fe), CD85 (Ni-16Cr-9Fe) and HCr (Ni-30Cr-9Fe), were used in corrosion coupon exposure tests and constant extension rate tensile (CERT) tests at 550°C and 400°C in an environment consisting of a controlled mixture of hydrogen, water vapor and argon. The hydrogen-to-water vapor partial pressure ratio (PPR) was varied between 0.001 and 0.9 to control the oxygen partial pressure. The Ni-9Fe, Ni-5Cr and LCr alloys formed a uniform Ni(OH)2 film at PPR values less than 0.09 while CD85 and HCr formed Cr2O 3 oxide films over the entire PPR range. Corrosion coupon results also show the formation of highly localized oxide particles at grain boundaries. Focused ion beam analysis revealed that intergranular oxides were observed at significant depths (>150 nm) down grain boundaries and the oxide morphology depended on the alloy composition and PPR value. Diffusion of oxygen along the grain boundary accounted for the growth of intergranular oxides. CERT test results showed that intergranular cracking was caused by creep-induced microvoid coalescence only at 550°C and did not depend on PPR. At 400°C, the cracking behavior depended on the PPR and resulted in a mixture of creep-induced microvoid coalescence and brittle intergranular failure. The cracked boundary fraction was higher at a PPR value where a Ni(OH)2 surface film formed. Alloy composition influenced cracking and the cracked boundary fraction decreased as the alloy chromium content increased. The

  9. Ultrasonic inspection reliability for intergranular stress corrosion cracks

    SciTech Connect

    Heasler, P G; Taylor, T T; Spanner, J C; Doctor, S R; Deffenbaugh, J D

    1990-07-01

    A pipe inspection round robin entitled Mini-Round Robin'' was conducted at Pacific Northwest Laboratory from May 1985 through October 1985. The research was sponsored by the US Nuclear Regulatory Commission, Office of Nuclear Regulatory Research under a program entitled Evaluation and Improvement of NDE Reliability for Inservice Inspection of Light Water Reactors.'' The Mini-Round Robin (MRR) measured the intergranular stress corrosion (GSC) crack detection and sizing capabilities of inservice inspection (ISI) inspectors that had passed the requirements of IEB 83-02 and the Electric Power Research Institute (EPRI) sizing training course. The MRR data base was compared with an earlier Pipe Inspection Round Robin (PIRR) that had measured the performance of inservice inspection prior to 1982. Comparison of the MRR and PIRR data bases indicates no significant change in the inspection capability for detecting IGSCC. Also, when comparing detection of long and short cracks, no difference in detection capability was measured. An improvement in the ability to differentiate between shallow and deeper IGSCC was found when the MRR sizing capability was compared with an earlier sizing round robin conducted by the EPRI. In addition to the pipe inspection round robin, a human factors study was conducted in conjunction with the Mini-Round Robin. The most important result of the human factors study is that the Relative Operating Characteristics (ROC) curves provide a better methodology for describing inspector performance than only probability of detection (POD) or single-point crack/no crack data. 6 refs., 55 figs., 18 tabs.

  10. STUDY OF GRAIN BOUNDARY CHARACTER ALONG INTERGRANULAR STRESS CORROSION CRACK PATHS IN AUSTENITIC ALLOYS

    SciTech Connect

    Guertsman, Valery Y.; Bruemmer, Stephen M.

    2001-05-25

    Samples of austenitic stainless alloys were examined by means of scanning and transmission electron microscopy. Misorientations were measured by electron backscattered diffraction. Grain boundary distributions were analyzed with special emphasis on the grain boundary character along intergranular stress-corrosion cracks and at crack arrest points. It was established that only coherent twin S3 boundaries could be considered as "special" ones with regard to crack resistance. However, it is possible that twin interactions with random grain boundaries may inhibit crack propagation. The results suggest that other factors besides geometrical ones play an important role in the intergranular stress-corrosion cracking of commercial alloys.

  11. Proceedings: 1991 EPRI workshop on secondary-side intergranular corrosion mechanisms

    SciTech Connect

    Partridge, M.J.; Zemitis, W.S. )

    1992-08-01

    A workshop on Secondary-Side Intergranular Corrosion Mechanisms'' was organized by EPRI as an effort to give those working in this area an opportunity to share their results, ideas, and plans. Topics covered included: (1) caustic induced intergranular attack/stress corrosion cracking (IGA/IGSCC), (2) plant experience, (3) boric acid as an IGA/IGSCC remedial measure, (4) lead induced IGA/IGSCC, and (5) acid induced IGA/IGSCC.

  12. A three-dimensional multiscale model of intergranular hydrogen-assisted cracking

    NASA Astrophysics Data System (ADS)

    Rimoli, J. J.; Ortiz, M.

    2010-07-01

    We present a three-dimensional model of intergranular hydrogen-embrittlement (HE) that accounts for: (i) the degradation of grain-boundary strength that arises from hydrogen coverage; (ii) grain-boundary diffusion of hydrogen; and (iii) a continuum model of plastic deformation that explicitly resolves the three-dimensional polycrystalline structure of the material. The polycrystalline structure of the specimen along the crack propagation path is resolved explicitly by the computational mesh. The texture of the polycrystal is assumed to be random and the grains are elastically anisotropic and deform plastically by crystallographic slip. We use the impurity-dependent cohesive model in order to account for the embrittling of grain boundaries due to hydrogen coverage. We have carried out three-dimensional finite-element calculations of crack-growth initiation and propagation in AISI 4340 steel double-cantilever specimens in contact with an aggressive environment and compared the predicted initiation times and crack-growth curves with the experimental data. The calculated crack-growth curves exhibit a number of qualitative features that are in keeping with observation, including: an incubation time followed by a well-defined crack-growth initiation transition for sufficiently large loading; the existence of a threshold intensity factor K Iscc below which there is no crack propagation; a subsequent steeply rising part of the curve known as stage I; a plateau, or stage II, characterized by a load-insensitive crack-growth rate; and a limiting stress-intensity factor K Ic , or toughness, at which pure mechanical failure occurs. The calculated dependence of the crack-growth initiation time on applied stress-intensity factor exhibits power-law behavior and the corresponding characteristic exponents are in the ball-park of experimental observation. The stage-II calculated crack-growth rates are in good overall agreement with experimental measurements.

  13. Laser Peening of Alloy 600 to Improve Intergranular Stress Corrosion Cracking Resistance in Power Plants

    SciTech Connect

    Chen, H; Rankin, J; Hackel, L; Frederick, G; Hickling, J; Findlan, S

    2004-04-20

    Laser peening is an emerging modern process that impresses a compressive stress into the surface of metals or alloys. This treatment can reduce the rate of intergranular stress corrosion cracking and fatigue cracking in structural metals or Alloy 600 needed for nuclear power plants.

  14. An Electrochemical Framework to Explain Intergranular Stress Corrosion Cracking in an Al-5.4%Cu-0.5%Mg-0.5%Ag Alloy

    NASA Technical Reports Server (NTRS)

    Little, D. A.; Connolly, B. J.; Scully, J. R.

    2001-01-01

    A modified version of the Cu-depletion electrochemical framework was used to explain the metallurgical factor creating intergranular stress corrosion cracking susceptibility in an aged Al-Cu-Mg-Ag alloy, C416. This framework was also used to explain the increased resistance to intergranular stress corrosion cracking in the overaged temper. Susceptibility in the under aged and T8 condition is consistent with the grain boundary Cu-depletion mechanism. Improvements in resistance of the T8+ thermal exposure of 5000 h at 225 F (T8+) compared to the T8 condition can be explained by depletion of Cu from solid solution.

  15. The role of Hydrogen and Creep in Intergranular Stress Corrosion Cracking of Alloy 600 and Alloy 690 in PWR Primary Water Environments ? a Review

    SciTech Connect

    Rebak, R B; Hua, F H

    2004-07-12

    Intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) of Alloy 600 in PWR steam generator environment has been extensively studied for over 30 years without rendering a clear understanding of the essential mechanisms. The lack of understanding of the IGSCC mechanism is due to a complex interaction of numerous variables such as microstructure, thermomechanical processing, strain rate, water chemistry and electrochemical potential. Hydrogen plays an important role in all these variables. The complexity, however, significantly hinders a clearer and more fundamental understanding of the mechanism of hydrogen in enhancing intergranular cracking via whatever mechanism. In this work, an attempt is made to review the role of hydrogen based on the current understanding of grain boundary structure and chemistry and intergranular fracture of nickel alloys, effect of hydrogen on electrochemical behavior of Alloy 600 and Alloy 690 (e.g. the passive film stability, polarization behavior and open-circuit potential) and effect of hydrogen on PWSCC behavior of Alloy 600 and Alloy 690. Mechanistic studies on the PWSCC are briefly reviewed. It is concluded that further studies on the role of hydrogen on intergranular cracking in both inert and primary side environments are needed. These studies should focus on the correlation of the results obtained at different laboratories by different methods on materials with different metallurgical and chemical parameters.

  16. Hydrogen Induced Intergranular Cracking of Nickel-Base Alloys.

    DTIC Science & Technology

    1980-07-01

    propagating crack according to Rice [38]. 75 FIGURE 32. Evan’s diagram [39]. 80 FIGURE 33. Diagram showing schematically the effect of promoter...increase in permeability at -470mV SCE but no effect at -580mV SCE? One could consider this with the help of the Evans’ diagram shown in Figure 32...Cathode) ’corr A I Mixed 7corrosion (Anode) Aoj Polarization 109 current density FIGURE 32. Evans’ diagram 1391. grain boundaries can not dissolve

  17. Mechanism of Intergranular Penetration of Ga in an Aluminum Alloy

    NASA Astrophysics Data System (ADS)

    Ding, Boxiong; Hoagland, Richard

    1998-03-01

    The intergranular penetration rates of gallium in 7050-T74 aluminum alloy were examined at temperatures from 25C to 180C under stress free condition. The results provide an estimate of activation energy of the penetration process. The penetration of Ga is observed to occur along the grain boundary and also spread over the surface, but much more slowly. Experiments were also performed at 23C involving solid Ga. These results together with mechanism controlling the intergranular penetration of Ga in Al will be discussed. This work was supported by DARPA.

  18. Influence of local stress and strain on intergranular cracking of 316L stainless steel in supercritical water

    NASA Astrophysics Data System (ADS)

    West, Elaine Ann

    The objective of this study was to determine how the deformation propensities of individual grains of 316L stainless steel influence intergranular cracking behavior in supercritical water (SCW). The grain-to-grain variations in deformation propensities were estimated from the Schmid and Taylor factors of grains. Resulting stress inhomogeneities and strain incompatibilities which were evaluated to determine the conditions that promoted intergranular cracking in SCW. Proton irradiation of 316L caused hardening and radiation induced depletion of chromium at grain boundaries and was found to increase intergranular cracking severity. The SCW environment increased the crack density on the gage surfaces of the specimen by a factor of 18 compared to a 400°C argon environment. Intergranular cracks preferentially occurred along grain boundaries oriented perpendicular to the tensile axis and adjacent to grains with low Schmid factors. The Schmid-Modified Grain Boundary Stress (SMGBS) model was developed to analyze local grain boundary stresses. The model was validated by showing that the Schmid factor dependence of cracking in SCW could be predicted from the trace inclination distribution, and confirmed that cracking was driven by the normal stresses acting on grain boundaries. The similar dependencies of slip discontinuity and intergranular cracking on trace inclination, Schmid factor, Taylor factor, and grain boundary character suggest that slip discontinuity contributes to intergranular cracking. Grains with low Taylor factors decreased slip discontinuity propensity at grain boundaries with trace inclinations >50° because they provided multiple favorably oriented slip systems on which deformation could occur. Grain boundary engineering reduced the intergranular cracking propensity of 316L stainless steel in SCW by virtue of the fact that special grain boundaries were more resistant to intergranular cracking in SCW than random high angle grain boundaries. The findings of

  19. Hydrogen Induced Intergranular Cracking of Nickel-Base Alloys.

    DTIC Science & Technology

    1982-02-01

    locations as being an important step in the embrittle- ment mechanism. It was first suggested by Bastien and Azou (1) that mobile dislocations carry...W-7405-ENG-48 and the Office of Naval Research under contract number N00014-78-C-0002/NR 036-127. References 1. P. Bastien and P. Azou , C.R. Acad. Sci

  20. Investigation of intergranular stress corrosion cracking in the fuel pool at Three Mile Island Unit 1

    SciTech Connect

    Czajkowski, C.J.

    1985-01-01

    An intergranular stress corrosion cracking failure of 304 stainless steel pipe in 2000 ppM B as H/sub 3/BO/sub 3/ + H/sub 2/O at 100/sup 0/C has been investigated. Constant extension rate testing has produced an intergranular type failure in material in air. Chemical analysis was performed on both the base metal and weld material, in addition to fractography, EPR testing and optical microscopy in discerning the mode of failure. Various effects of Cl/sup -/, O/sub 2/, and MnS are discussed. The results have indicated that the cause of failure was the severe sensitization coupled with probable contamination by S and possibly by Cl ions.

  1. Intergranular stress-corrosion cracking of austenitic stainless steels in PWR boric-acid storage systems

    SciTech Connect

    Macdonald, D.D.; Cragnolino, G.A.; Olemacher, J.; Chen, T.Y.; Dhawale, S.

    1982-08-01

    A review is presented of the available literature on the intergranular stress corrosion cracking (IGSCC) of austenitic stainless steels at temperatures below 100/sup 0/C, as well as the results of an experimental investigation of the IGSCC of Types 304, 304L, and 316L stainless steels conducted in boric acid environments of the type employed in pressurized nuclear reactors (PWRs) for nuclear shim control. The susceptibility of furnace sensitized Type 304SS to IGSCC was studied using slow strain rate tests as a function of pH, temperature, potential, and concentration of suspected contaminants: chloride, thiosulfate, and tetrathionate. Possible alternate alloys, such as Types 304L and 316L stainless steels, were also tested under those specific conditions that render Type 304SS susceptible to cracking. Corrosion potentials that can be attained in air-saturated boric acid solutions in the presence of the above mentioned species were measured in order to evaluate the propensity towards intergranular cracking under conditions simulating those that prevail in service.

  2. Mitigation of Intergranular Stress Corrosion Cracking in Al-Mg by Electrochemical Potential Control

    NASA Astrophysics Data System (ADS)

    McMahon, M. E.; Scully, J. R.; Burns, J. T.

    2017-08-01

    Intergranular stress corrosion cracking in the Al-Mg alloy AA5456-H116 is suppressed via cathodic polarization in 0.6 M NaCl, saturated (5.45 M) NaCl, 2 M MgCl2, and saturated (5 M) MgCl2. Three zones of intergranular stress corrosion cracking (IG-SCC) susceptibility correlate with pitting potentials of unsensitized AA5456-H116 and pure β phase (Al3Mg2) in each solution. These critical potentials reasonably describe the influence of α Al matrix and β phase dissolution rates on IG-SCC severity. Complete inhibition occurred at applied potentials of -1.0 V and -1.1 V versus saturated calomel electrode ( V SCE) in 0.6 M NaCl. Whereas only partial mitigation of IG-SCC was achieved at -0.9 V SCE in 0.6 M NaCl and at -0.9, -1.0, and -1.1 V SCE in the more aggressive environments. Correlation of pitting potentials in bulk environments with IG-SCC behavior suggests an effect of bulk environment [Cl-] and pH on the stabilized crack tip chemistry.

  3. Cohesive zone model for intergranular slow crack growth in ceramics: influence of the process and the microstructure

    NASA Astrophysics Data System (ADS)

    Romero de la Osa, M.; Estevez, R.; Olagnon, C.; Chevalier, J.; Tallaron, C.

    2011-10-01

    Ceramic polycrystals are prone to slow crack growth (SCG) which is stress and environmentally assisted, similarly to observations reported for silica glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and the relative humidity. In addition, evidence is available on the influence of the microstructure on the SCG rate with an increase in the crack velocity with decreasing the grain size. Crack propagation takes place beyond a load threshold, which is grain size dependent. We present a cohesive zone model for the intergranular failure process. The methodology accounts for an intrinsic opening that governs the length of the cohesive zone and allows the investigation of grain size effects. A rate and temperature-dependent cohesive model is proposed (Romero de la Osa M, Estevez R et al 2009 J. Mech. Adv. Mater. Struct. 16 623-31) to mimic the reaction-rupture mechanism. The formulation is inspired by Michalske and Freiman's picture (Michalske and Freiman 1983 J. Am. Ceram. Soc. 66 284-8) together with a recent study by Zhu et al (2005 J. Mech. Phys. Solids 53 1597-623) of the reaction-rupture mechanism. The present investigation extends a previous work (Romero de la Osa et al 2009 Int. J. Fracture 158 157-67) in which the problem is formulated. Here, we explore the influence of the microstructure in terms of grain size, their elastic properties and residual thermal stresses originating from the cooling from the sintering temperature down to ambient conditions. Their influence on SCG for static loadings is reported and the predictions compared with experimental trends. We show that the initial stress state is responsible for the grain size dependence reported experimentally for SCG. Furthermore, the account for the initial stresses enables the prediction of a load threshold below which no crack growth is observed: a crack arrest takes place when the crack path meets a region in compression.

  4. Slow Strain Rate Tensile Testing to Assess the Ability of Superalloys to Resist Environment-Assisted Intergranular Cracking

    NASA Technical Reports Server (NTRS)

    Gabb, Timothy P.; Telesman, Jack; Banik, Anthony; McDevitt, Erin

    2014-01-01

    Intergranular fatigue crack initiation and growth due to environmental degradation, especially at notched features, can often limit the fatigue life of disk superalloys at high temperatures. For clear comparisons, the effects of alloy composition on cracking in air needs to be understood and compared separately from variables associated with notches and cracks such as effective stress concentration, plastic flow, stress relaxation, and stress redistribution. The objective of this study was to attempt using simple tensile tests of specimens with uniform gage sections to compare the effects of varied alloy composition on environment-assisted cracking of several powder metal and cast and wrought superalloys including ME3, LSHR, Udimet 720, ATI 718Plus alloy, Haynes 282, and Inconel 740. Slow and fast strain-rate tensile tests were found to be a useful tool to compare propensities for intergranular surface crack initiation and growth. The effects of composition and heat treatment on tensile fracture strain and associated failure modes were compared. Environment interactions were determined to often limit ductility, by promoting intergranular surface cracking. The response of various superalloys and heat treatments to slow strain rate tensile testing varied substantially, showing that composition and microstructure can significantly influence environmental resistance to cracking.

  5. Effects of water chemistry on intergranular cracking of irradiated austenitic stainless steels

    SciTech Connect

    Chung, H.M.; Ruther, W.E.; Sanecki, J.E.; Hins, A.; Kassner, T.F.

    1995-12-31

    To determine the effects of water chemistry on the susceptibility to irradiation-assisted stress corrosion cracking (IASCC) in austenitic stainless steels, constant-extension-rate tests were conducted in simulated BWR environments on several heats of high- and commercial-purity (HP and CP) Type 304 SS specimens from BWR components irradiated to fluences up to 2.4 {times} 10{sup 21} n cm{sup {minus}2} (E > 1 MeV). Effects of dissolved oxygen (DO) and electrochemical potential (ECP) in 289 C water were investigated. Dependence of susceptibility to intergranular stress corrosion cracking (IGSCC) on DO was somewhat different for the two materials. Susceptibility of the HP heats, less influenced by DO and ECP, was higher than that of CP material for all DO and fluence levels. Percent IGSCC in the CP material was negligible for DO < 0.01 ppm or ECP <{minus}140 mV SHE. Results of analysis by Auger electron spectroscopy indicated that the HP neutron absorber tubes were characterized by relatively lower concentrations of Cr, Ni, and Li and relatively higher concentrations of F and N on grain boundaries than those of the CP materials. It is suggested that a synergism between irradiation-induced grain-boundary Cr depletion and fabrication-related fluorine contamination plays an important role in the stress corrosion cracking behavior of the HP neutron absorber tubes.

  6. Creep and intergranular cracking of Ni-Cr-Fe-C in 360[degree]C argon

    SciTech Connect

    Angeliu, T.M. ); Was, G.S. )

    1994-06-01

    The influence of carbon and chromium on the creep and intergranular (IG) cracking behavior of controlled-purity Ni-xCr-9Fe-yC alloys in 360 C argon was investigated using constant extension rate tension (CERT) and constant load tension (CLT) testing. The CERT test results at 360 C show that the degree of IG cracking increases with decreasing bulk chromium or carbon content. The CLT test results at 360 C and 430 C reveal that, as the amounts of chromium and carbon in solution decrease, the steady-state creep rate increases. The occurrence of severe IG cracking correlates with a high steady-state creep rate, suggesting that creep plays a role in the IG cracking behavior in argon at 360 C. The failure mode of IG cracking and the deformation mode of creep are coupled through the formation of grain boundary voids that interlink to form grain boundary cavities, resulting in eventual failure by IG cavitation and ductile overload of the remaining ligaments. Grain boundary sliding may be enhancing grain boundary cavitation by redistributing the stress from inclined to more perpendicular boundaries and concentrating stress at discontinuities for the boundaries oriented 45 deg with respect to the tensile axis. Additions of carbon or chromium, which reduce the creep rate over all stress levels, also reduce the amount of IG fracture in CERT experiments. A damage accumulation model was formulated and applied to CERT tests to determine whether creep damage during a CERT test controls failure. Results show that, while creep plays a significant role in CERT experiments, failure is likely controlled by ductile overload caused by reduction in area resulting from grain boundary void formation and interlinkage.

  7. Status report: Intergranular stress corrosion cracking of BWR core shrouds and other internal components

    SciTech Connect

    1996-03-01

    On July 25, 1994, the US Nuclear Regulatory Commission (NRC) issued Generic Letter (GL) 94-03 to obtain information needed to assess compliance with regulatory requirements regarding the structural integrity of core shrouds in domestic boiling water reactors (BWRs). This report begins with a brief description of the safety significance of intergranular stress corrosion cracking (IGSCC) as it relates to the design and function of BWR core shrouds and other internal components. It then presents a brief history of shroud cracking events both in the US and abroad, followed by an indepth summary of the industry actions to address the issue of IGSCC in BWR core shrouds and other internal components. This report summarizes the staff`s basis for issuing GL 94-03, as well as the staff`s assessment of plant-specific responses to GL 94-03. The staff is continually evaluating the licensee inspection programs and the results from examinations of BWR core shrouds and other internal components. This report is representative of submittals to and evaluations by the staff as of September 30, 1995. An update of this report will be issued at a later date.

  8. Localized deformation as a key precursor to initiation of intergranular stress corrosion cracking of austenitic stainless steels employed in nuclear power plants

    NASA Astrophysics Data System (ADS)

    Karlsen, Wade; Diego, Gonzalo; Devrient, Bastian

    2010-11-01

    Cold-work has been associated with the occurrence of intergranular cracking of stainless steels employed in light water reactors. This study examined the deformation behavior of AISI 304, AISI 347 and a higher stacking fault energy model alloy subjected to bulk cold-work and (for 347) surface deformation. Deformation microstructures of the materials were examined and correlated with their particular mechanical response under different conditions of temperature, strain rate and degree of prior cold-work. Select slow-strain rate tensile tests in autoclaves enabled the role of local strain heterogeneity in crack initiation in pressurized water reactor environments to be considered. The high stacking fault energy material exhibited uniform strain hardening, even at sub-zero temperatures, while the commercial stainless steels showed significant heterogeneity in their strain response. Surface treatments introduced local cold-work, which had a clear effect on the surface roughness and hardness, and on near-surface residual stress profiles. Autoclave tests led to transgranular surface cracking for a circumferentially ground surface, and intergranular crack initiation for a polished surface.

  9. Intergranular tellurium cracking of nickel-based alloys in molten Li, Be, Th, U/F salt mixture

    NASA Astrophysics Data System (ADS)

    Ignatiev, Victor; Surenkov, Alexander; Gnidoy, Ivan; Kulakov, Alexander; Uglov, Vadim; Vasiliev, Alexander; Presniakov, Mikhail

    2013-09-01

    , and intergranular corrosion does not take place. In the fuel salt with [U(IV)]/[U(III)] = 4-20 the potentials of uranium alloy formation with the main components of the tested alloys are not reached, that's why alloys and intermetallic compounds are not formed on the surface of the investigated chromium-nickel alloys. Under such conditions any intergranular tellurium corrosion of the selected alloys does not occur. In the fuel salt with [U(IV)/]/[U(III)] = 100 the potentials of uranium alloy formation with the main components of the tested alloys are not also reached. Under such redox conditions any traces intergranular tellurium IGC on the HN80MTY and H80M-VI alloys specimens are not found. Certain signs of incipient IGC in the form of tellurium presence on the grain boundaries in the HN80MTB and EM-721 alloys surface layer and formation of not too deep cracks on HN80MTB alloy surface were revealed at [U(IV)/]/[U(III)] = 100. With this uranium ratio in the presence of corrosion products on the surface of all of the alloys films, containing tellurium, metals of the construction alloys and carbon, are formed. In the melt with [U(IV)]/[U(III)] = 500 in all of the alloys tested the tellurium IGC took place. The HN80MTY alloy shows the maximum resistance to tellurium IGC. The intensity of tellurium IGC of the alloy (the K parameter) is by 3-5 times lower as compared to other alloys. The EM-721 alloy has the minimal resistance to tellurium IGC (K = 9200 pc m/cm, the depth of cracks is up to 434 μm). The studies have shown, that the intensity of the nickel alloys IGC is controlled by the [U(IV)]/[U(III)] ratio, and its dependence on this parameter is of threshold character. Providing the uranium ratio value's monitoring and regulation, it is possible to control the tellurium corrosion and in such a way to eliminate IGC completely or to minimize its value. The alloys strength characteristics and their structure were changed insignificantly after testing within the [U

  10. OBSERVATIONS AND IMPLICATIONS OF INTERGRANULAR STRESS CORROSION CRACK GROWTH OF ALLOY 152 WELD METALS IN SIMULATED PWR PRIMARY WATER

    SciTech Connect

    Toloczko, Mychailo B.; Olszta, Matthew J.; Overman, Nicole R.; Bruemmer, Stephen M.

    2013-08-15

    Significant intergranular (IG) crack growth during stress corrosion cracking (SCC) tests has been documented during tests in simulated PWR primary water on two alloy 152 specimens cut from a weldment produced by ANL. The cracking morphology was observed to change from transgranular (TG) to mixed mode (up to ~60% IG) during gentle cycling and cycle + hold loading conditions. Measured crack growth rates under these conditions often suggested a moderate degree of environmental enhancement consistent with faster growth on grain boundaries. However, overall SCC propagation rates at constant stress intensity (K) or constant load were very low in all cases. Initial SCC rates up to 6x10-9 mm/s were occasionally measured, but constant K/load growth rates dropped below ~1x10-9 mm/s with time even when significant IG engagement existed. Direct comparisons were made among loading conditions, measured crack growth response and cracking morphology during each test to assess IGSCC susceptibility of the alloy 152 specimens. These results were analyzed with respect to our previous SCC crack growth rate measurements on alloy 152/52 welds.

  11. Reduction of Intergranular Cracking Susceptibility by Precipitation Control in 2.25Cr Heat-Resistant Steels

    NASA Astrophysics Data System (ADS)

    Sung, Hyun Je; Heo, Nam Hoe; Kim, Sung-Joon

    2017-03-01

    This research is performed to decrease reheat cracking susceptibility in the T/P23 heat-resistant steels (2.25Cr1.5WVNbTi), in other words, to reduce phosphorus and sulfur segregation concentration at the prior austenite grain boundary/carbide interfaces (GCIs) and the carbide-free prior austenite grain boundaries (carbide-free PAGBs) causing intergranular cracking. The increase of bulk vanadium content reduces the amount of M23C6 carbides consuming carbon atoms which can repulse phosphorus and sulfur from interfaces, but promotes the precipitation reaction of V-rich MX carbo-nitrides. Such a precipitation reaction results in the lower segregation concentration of phosphorus or no sulfur at the GCIs and the carbide-free PAGBs. This is because the carbon atoms remaining after precipitation reaction segregates to the interfaces and repels phosphorus and sulfur. Also, tungsten segregation can increase the cohesive grain boundary strength as one of the grain boundary strengtheners. Consequently, the lower segregation concentration of the impurities and the segregation of tungsten atoms lower the intergranular cracking susceptibility.

  12. Reduction of Intergranular Cracking Susceptibility by Precipitation Control in 2.25Cr Heat-Resistant Steels

    NASA Astrophysics Data System (ADS)

    Sung, Hyun Je; Heo, Nam Hoe; Kim, Sung-Joon

    2017-01-01

    This research is performed to decrease reheat cracking susceptibility in the T/P23 heat-resistant steels (2.25Cr1.5WVNbTi), in other words, to reduce phosphorus and sulfur segregation concentration at the prior austenite grain boundary/carbide interfaces (GCIs) and the carbide-free prior austenite grain boundaries (carbide-free PAGBs) causing intergranular cracking. The increase of bulk vanadium content reduces the amount of M23C6 carbides consuming carbon atoms which can repulse phosphorus and sulfur from interfaces, but promotes the precipitation reaction of V-rich MX carbo-nitrides. Such a precipitation reaction results in the lower segregation concentration of phosphorus or no sulfur at the GCIs and the carbide-free PAGBs. This is because the carbon atoms remaining after precipitation reaction segregates to the interfaces and repels phosphorus and sulfur. Also, tungsten segregation can increase the cohesive grain boundary strength as one of the grain boundary strengtheners. Consequently, the lower segregation concentration of the impurities and the segregation of tungsten atoms lower the intergranular cracking susceptibility.

  13. AN ULTRASONIC PHASED ARRAY EVALUATION OF INTERGRANULAR STRESS CORROSION CRACK (IGSCC) DETECTION IN AUSTENITIC STAINLESS STEEL PIPING WELDS

    SciTech Connect

    Diaz, Aaron A.; Anderson, Michael T.; Cinson, Anthony D.; Crawford, Susan L.; Cumblidge, Stephen E.

    2010-07-22

    Research is being conducted for the U.S. Nuclear Regulatory Commission at the Pacific Northwest National Laboratory to assess the effectiveness and reliability of advanced nondestructive examination (NDE) methods for the inspection of light water reactor (LWR) components and challenging material/component configurations. This study assessed the effectiveness of far-side inspections on wrought stainless steel piping with austenitic welds, as found in thin-walled, boiling water reactor (BWR) component configurations, for the detection and characterization of intergranular stress corrosion cracks (IGSCC).

  14. Origins of Negative Strain Rate Dependence of Stress Corrosion Cracking Initiation in Alloy 690, and Intergranular Crack Formation in Thermally Treated Alloy 690

    NASA Astrophysics Data System (ADS)

    Kim, Young Suk; Kim, Sung Soo

    2016-09-01

    We show that enhanced stress corrosion cracking (SCC) initiation in cold-rolled Alloy 690 with decreasing strain rate is related to the rate of short-range ordering (SRO) but not to the time-dependent corrosion process. Evidence for SRO is provided by aging tests on cold-rolled Alloy 690 at 623 K and 693 K (350 °C and 420 °C), respectively, which demonstrate its enhanced lattice contraction and hardness increase with aging temperature and time, respectively. Secondary intergranular cracks formed only in thermally treated and cold-rolled Alloy 690 during SCC tests, which are not SCC cracks, are caused by its lattice contraction by SRO before SCC tests but not by the orientation effect.

  15. Inhibitory effect of boric acid on intergranular attack and stress corrosion cracking of Alloy 600 in high temperature water

    SciTech Connect

    Kawamura, H.; Hirano, H.; Koike, M.; Suda, M.

    1995-09-01

    The inhibitory effect of boric acid on the Intergranular Attack and Stress Corrosion Cracking (IGA/SCC) propagation behavior of steam generator (SG) tubing was studied under accelerated test conditions. Based on the analysis results of stress intensity factors at IGA/SCC crack tips, the notched C-ring tests were carried out to evaluate the effect of stress intensity and boric acid on the IGA/SCC crack propagation. The A.C. impedance measurement and Auger electron spectroscopy (AES) were also conducted to clarify the inhibitory effect of boric acid. Notched C-ring test results indicated that IGA/SCC crack velocity of alloy 600 increased gradually with increasing stress intensity factor in the range 4 to about 26 MPa{center_dot}m{sup 1/2}, which might be loaded on the IGA/SCC crack tips of actual SG tubes under PWR secondary conditions. Adding boric acid slightly retarded the crack velocity in both all volatile treatment (AVT) water and caustic solutions. IGA/SCC crack velocities were lower in nearly neutral solutions than in alkali or acidic solutions. Furthermore, A.C. impedance studies showed that the polarization resistances of oxide films formed in boric acid solutions were higher than those of films formed in acidic and alkali solutions. AES analysis revealed that boron content in the oxide films formed in acidic solution containing boric acid was lowest. Good agreement was obtained between the IGA/SCC inhibitory effect of boric acid and the formation of the stable oxide films containing boron.

  16. Effect of mechanical treatment on intergranular corrosion of 6064 alloy bars

    NASA Astrophysics Data System (ADS)

    Sláma, P.; Nacházel, J.

    2017-02-01

    Aluminium Al-Mg-Si-type alloys (6xxx-series) exhibit good mechanical properties, formability, weldability and good corrosion resistance in various environments. They often find use in automotive industry and other applications. Some alloys, however, particularly those with higher copper levels, show increased susceptibility to intergranular corrosion. Intergranular corrosion (IGC) is typically related to the formation of microgalvanic cells between cathodic, more noble phases and depleted (precipitate-free) zones along grain boundaries. It is encountered mainly in AlMgSi alloys containing Cu, where it is thought to be related to the formation Q-phase precipitates (Al4Mg8Si7Cu2) along grain boundaries. The present paper describes the effects of mechanical working (extrusion, drawing and straightening) and artificial aging on intergranular corrosion in rods of the 6064 alloy. The resistance to intergranular corrosion was mapped using corrosion tests according to EN ISO 11846, method B. Corrosion tests showed dependence of corrosion type on mechanical processing of the material. Intergranular, pitting and transgranular corrosion was observed. Artificial ageing influenced mainly the depth of the corrosion.

  17. Use of Slow Strain Rate Tensile Testing to Assess the Ability of Several Superalloys to Resist Environmentally-Assisted Intergranular Cracking

    NASA Technical Reports Server (NTRS)

    Gabb, Timothy P.; Telesman, Jack; Banik, Anthony; McDevitt, Erin

    2014-01-01

    Intergranular fatigue crack initiation and growth due to environmental degradation, especially at notched features, can often limit the fatigue life of disk superalloys at high temperatures. For clear comparisons, the effects of alloy composition on cracking in air needs to be understood and compared separately from variables associated with notches and cracks such as effective stress concentration, plastic flow, stress relaxation, and stress redistribution. The objective of this study was to attempt using simple tensile tests of specimens with uniform gage sections to compare the effects of varied alloy composition on environment-assisted cracking of several powder metal and cast and wrought superalloys including ME3, LSHR, Udimet 720(TradeMark) ATI 718Plus(Registered TradeMark) alloy, Haynes 282(Trademark), and Inconel 740(TradeMark) Slow and fast strain-rate tensile tests were found to be a useful tool to compare propensities for intergranular surface crack initiation and growth. The effects of composition and heat treatment on tensile fracture strain and associated failure modes were compared. Environment interactions were determined to often limit ductility, by promoting intergranular surface cracking. The response of various superalloys and heat treatments to slow strain rate tensile testing varied substantially, showing that composition and microstructure can significantly influence environmental resistance to cracking.

  18. Intergranular stress corrosion cracking of alloy 600 and x-750 in high-temperature deaerated water/steam

    NASA Astrophysics Data System (ADS)

    Shen, Yulin; Shewmon, Paul G.

    1991-08-01

    Intergranular stress corrosion cracking (IGSCC) has been studied in two similar nickel-base alloys, alloys 600 and X-750, in deaerated steam at elevated temperatures (380 °C). In both cases, IGSCC occurs through the nucleation, growth, and linkup of grain boundary voids rather than by grain boundary dissolution. The rate of IGSCC is much faster in both alloys when they are heat-treated so that carbon is kept in solution instead of precipitating as grain boundary carbides. This higher carbon content allows the formation of a higher methane pressure in grain boundary voids. Slip impingement on the grain boundary plays a central role in nucleating a set of voids with a spacing of about 0.2 μm. A second set of more closely spaced bubbles develops between these through the action of stress-assisted diffusion of material from the first set of bubbles.

  19. Microstructural characterization on intergranular stress corrosion cracking of Alloy 600 in PWR primary water environment

    NASA Astrophysics Data System (ADS)

    Lim, Yun Soo; Kim, Hong Pyo; Hwang, Seong Sik

    2013-09-01

    Stress corrosion cracks in Alloy 600 compact tension specimens tested at 325 °C in a simulated primary water environment of a pressurized water reactor were analyzed using microscopic equipment. Oxygen diffused into the grain boundaries just ahead of the crack tips from the external primary water. As a result of oxygen penetration, Cr oxides were precipitated on the crack tips and the attacked grain boundaries. The oxide layer in the crack interior was revealed to consist of double (inner and outer) layers. Cr oxides were found in the inner layer, with NiO and (Ni,Cr) spinels in the outer layer. Cr depletion (or Ni enrichment) zones were created in the attacked grain boundary, the crack tip, and the interface between the crack and matrix, which means that the formation of Cr oxides was due to the Cr diffusion from the surrounding matrix. The oxygen penetration and resultant metallurgical changes around the crack tip are believed to be significant factors affecting the PWSCC initiation and growth behaviors of Alloy 600. For interpretation of color in Fig. 4, the reader is referred to the web version of this article.

  20. Corrosion cracking

    SciTech Connect

    Goel, V.S.

    1985-01-01

    This book presents the papers given at a conference on alloy corrosion cracking. Topics considered at the conference included the effect of niobium addition on intergranular stress corrosion cracking, corrosion-fatigue cracking in fossil-fueled-boilers, fracture toughness, fracture modes, hydrogen-induced thresholds, electrochemical and hydrogen permeation studies, the effect of seawater on fatigue crack propagation of wells for offshore structures, the corrosion fatigue of carbon steels in seawater, and stress corrosion cracking and the mechanical strength of alloy 600.

  1. Intergranular Cracking as a Major Cause of Long-Term Capacity Fading of Layered Cathodes

    DOE PAGES

    Liu, Hao; Wolf, Mark; Karki, Khim; ...

    2017-05-26

    Capacity fading has limited commercial layered Li-ion battery electrodes to <70% of their theoretical capacity. Higher capacities can be achieved initially by charging to higher voltages, however, these gains are quickly eroded by a faster fade in capacity. In-creasing lifetimes and reversible capacity is contingent on identifying the origin of this capacity fade to inform electrode design and synthesis. We must understand how the battery reactions change following capacity loss after long-term cycling. Using operando X-ray diffraction, we followed the reaction of a LiNi0.8Co0.15Al0.05O2 (NCA) electrode after months of charge-discharge cycles. Furthermore, the heterogeneous reaction kinetics observed during extended cyclesmore » quantitatively explain the capacity loss, which is ultimately attributed to inter-granular fracturing that degrades the connectivity of sub-surface grains within the polycrystalline NCA aggregate.« less

  2. Thermo-Mechanical Fatigue Crack Growth of RR1000.

    PubMed

    Pretty, Christopher John; Whitaker, Mark Thomas; Williams, Steve John

    2017-01-04

    Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.

  3. Thermo-Mechanical Fatigue Crack Growth of RR1000

    PubMed Central

    Pretty, Christopher John; Whitaker, Mark Thomas; Williams, Steve John

    2017-01-01

    Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles. PMID:28772394

  4. Mechanics of Interface Cracks

    DTIC Science & Technology

    1990-09-27

    tip fields along with a correspondence of these fields to the well characterized small strain (HRR) fields in homogeneous media . In particular, it...crack dimension. Our results showed that for cases involving two elastic-plastic media that the fields, in both materials, are parts of a single...of an geneous media (e.g., Hutchinson, 1983). In one sense the work infinite crack embedded in an infinite bimaterial body (see Fig. complimented

  5. Predicting susceptibility of alloy 600 to intergranular stress corrosion cracking using a modified electrochemical potentiokinetic reactivation test

    SciTech Connect

    Ahn, M.K.; Kwon, H.S.; Lee, J.H.

    1995-06-01

    Modified double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were applied to evaluate the degree of sensitization (DOS) for alloy 600 aged at 700 C. Results of the modified DL-EPR test were compared to intergranular stress corrosion cracking (IGSCC) susceptibilities determined in deaerated 0.01 M sodium tetrathionate under deformation at a constant strain rate of 0.93 {times} 10{sup {minus}6}/s. By analyzing the effects of solution concentration, temperature, and scan rate on the electrochemical response in the EPR tests and the morphologies, the optimal modified DL-EPR test condition for alloy 600 was obtained in 0.01 M sulfuric acid + 10 ppm potassium thiocyanate at 25 C and at a scan rate of 0.5 mV/s. The standard DL-EPR test, performed under conditions of 0.5 M H{sub 2}SO{sub 4} + 0.01 M KCNS at 30 C and a scan rate of 1.67 mV/s, provided very poor discriminating power for various DOS of alloy 600 because general and pitting corrosion occurred, in addition to intergranular corrosion. The modified test, however, was highly discriminating because of its highly selective corrosion attack at grain boundaries. IGSCC occurred in samples of alloy 600 aged for < 20 h, and susceptibility to IGSCC{sub s} increased with decreasing aging times up to 1 h, showing maximum IGSCC{sub s} in the sample aged for 1 h. IGSCC{sub s} for the alloy was found to be associated closely with the chromium-depleted profile across grain boundaries. The deeper and narrower chromium-depleted zone produced greater IGSCC{sub s}. It was demonstrated that DOS measured by the modified DL-EPR test was correlated more closely with IGSCC{sub s} than was DOS measured by the standard EPR test. This correlation resulted from the fact that the modified EPR test selectively attacked the more deeply chromium-depleted regions in comparison to the standard EPR test.

  6. Assessment of susceptibility of Type 304 stainless steel to intergranular stress corrosion cracking in simulated Savannah River Reactor environments

    SciTech Connect

    Ondrejcin, R.S.; Caskey, C.R. Jr.

    1989-12-01

    Intergranular stress corrosion cracking (IGSCC) of Type 304 stainless steel rate tests (CERT) of specimens machined was evaluated by constant extension from Savannah River Plant (SRP) decontaminated process water piping. Results from 12 preliminary CERT tests verified that IGSCC occurred over a wide range of simulated SRP envirorments. 73 specimens were tested in two statistical experimental designs of the central composite class. In one design, testing was done in environments containing hydrogen peroxide; in the other design, hydrogen peroxide was omitted but oxygen was added to the environment. Prediction equations relating IGSCC to temperature and environmental variables were formulated. Temperature was the most important independent variable. IGSCC was severe at 100 to 120C and a threshold temperature between 40C and 55C was identified below which IGSCC did not occur. In environments containing hydrogen peroxide, as in SRP operation, a reduction in chloride concentration from 30 to 2 ppB also significantly reduced IGSCC. Reduction in sulfate concentration from 50 to 7 ppB was effective in reducing IGSCC provided the chloride concentration was 30 ppB or less and temperature was 95C or higher. Presence of hydrogen peroxide in the environment increased IGSCC except when chloride concentration was 11 ppB or less. Actual concentrations of hydrogen peroxide, oxygen and carbon dioxide did not affect IGSCC. Large positive ECP values (+450 to +750 mV Standard Hydrogen Electrode (SHE)) in simulated SRP environments containing hydrogen peroxide and were good agreement with ECP measurements made in SRP reactors, indicating that the simulated environments are representative of SRP reactor environments. Overall CERT results suggest that the most effective method to reduce IGSCC is to reduce chloride and sulfate concentrations.

  7. Effect of boron on intergranular hot cracking in Ni-Cr-Fe superalloys containing niobium

    NASA Technical Reports Server (NTRS)

    Thompson, R. G.

    1990-01-01

    Solidification mechanisms had a dominant influence on microfissuring behavior of the test group. Carbon modified the Laves formation significantly and showed that one approach to alloy design would be balancing carbide formers against Laves formers. Boron's strong effect on microfissuring can be traced to its potency as a Laves former. Boron's segregation to grain boundaries plays at best a secondary role in microfissuring.

  8. Statistical crack mechanics

    SciTech Connect

    Dienes, J.K.

    1983-01-01

    An alternative to the use of plasticity theory to characterize the inelastic behavior of solids is to represent the flaws by statistical methods. We have taken such an approach to study fragmentation because it offers a number of advantages. Foremost among these is that, by considering the effects of flaws, it becomes possible to address the underlying physics directly. For example, we have been able to explain why rocks exhibit large strain-rate effects (a consequence of the finite growth rate of cracks), why a spherical explosive imbedded in oil shale produces a cavity with a nearly square section (opening of bedding cracks) and why propellants may detonate following low-speed impact (a consequence of frictional hot spots).

  9. Effects of neutron irradiation on hydrogen-induced intergranular fracture in a low activation 9%Cr-2%W steel

    NASA Astrophysics Data System (ADS)

    Kimura, A.; Kayano, H.; Narui, M.

    1991-03-01

    Hydrogen charging changed the fracture mode in tensile tests at room temperature from ductile shear rupture to intergranular cracking, resulting in a considerable reduction of the ductility of a low activation 9%Cr-2%W martensitic steel. The critical hydrogen charging current density required to cause hydrogen-induced intergranular cracking was reduced by neutron irradiation, suggesting that neutron irradiation enhanced hydrogen-induced intergranular cracking. This hydrogen-induced intergranular cracking was not caused by irreversible damage due to hydrogen charging, since it disappeared after aging at room temperature. The recovery rate of the fracture mode from intergranular cracking to ductile rupture during aging at room temperature was reduced by irradiation. A mechanism of irradiation-induced enhancement of hydrogen embrittlement in a low activation 9%Cr-2%W martensitic steel is proposed.

  10. Contribution of solution pH and buffer capacity to suppress intergranular stress corrosion cracking of sensitized type 304 stainless steel at 95 C

    SciTech Connect

    Zhang, S.; Shibata, T.; Haruna, T. . Dept. of Materials Science and Processing)

    1999-05-01

    Controlling pH of high-temperature water to [approximately]pH 7 at 300 C by adding lithium hydroxide (LiOH) into the coolant system of a pressurized water reactor (PWR) successfully has been mitigating the corrosion of PWR component materials. The effects of solution pH and buffer capacity on intergranular stress corrosion cracking (IGSCC) of sensitized type 304 stainless steel ([SS] UNS S30400) was examined at 95 C by slow strain rate technique (SSRT) with an in-situ cracking observation system. It was found that an increase in solution pH or buffer capacity increased crack initiation time and decreased mean crack initiation frequency, but exerted almost no effect on crack propagation. This inhibition effect on IGSCC initiation was explained as resulting from a retarding effect of solution pH and buffer capacity on the decrease in pH at crack nuclei caused by the hydrolysis of metal ions dissolved when the passive film was ruptured by strain in SSRT.

  11. Mechanisms of stress relief cracking in titanium stabilised austenitic stainless steel

    NASA Astrophysics Data System (ADS)

    Chabaud-Reytier, M.; Allais, L.; Caes, C.; Dubuisson, P.; Pineau, A.

    2003-11-01

    The heat affected zone (HAZ) of AISI 321 welds may exhibit a serious form of cracking during service at high temperature. This form of damage, called 'stress relief cracking', is known to be due to work hardening but also to aging due to Ti(C,N) precipitation on dislocations which modifies the mechanical behaviour of the HAZ. The present study aims to analyse the latter embrittlement mechanism in one specific heat of 321 stainless steel. To this end, different HAZs are simulated using an annealing heat-treatment, followed by various cold rolling and aging conditions. Then, we study the effects of work hardening and aging on Ti(C,N) precipitation, on the mechanical (hardness, tensile and creep) behaviour of the simulated HAZs and on their sensitivity to intergranular crack propagation through stress relaxation tests performed on pre-cracked CT type specimens tested at 600 °C. It is shown that work hardening is the main parameter of the involved mechanism but that aging does not promote crack initiation although it leads to titanium carbide precipitation. Therefore, the role of Ti(C,N) precipitation on stress relief cracking mechanisms is discussed. An attempt is made to show that solute drag effects are mainly responsible for this form of intergranular damage, rather than Ti(C,N) precipitation.

  12. Critical analysis of alloy 600 stress corrosion cracking mechanisms in primary water

    SciTech Connect

    Rios, R. |; Noel, D.; Bouvier, O. de; Magnin, T.

    1995-04-01

    In order to study the mechanisms involved in the stress-corrosion cracking (SCC) of Alloy 600 in primary water, the influence of the relevance of physicochemical and metallurgical parameters was assessed: hydrogen and oxygen overpressures, microstructure, and local chemical composition. The obtained results show that, even if the dissolution/oxidation seems to be the first and necessary step responsible for crack initiation and if hydrogen effects can also be involved in cracking, neither a dissolution/oxidation model nor a hydrogen model appears sufficient to account for cracking. Moreover, fractographic examinations performed on specimens` fracture surfaces lead to the fact that attention should be paid to a cleavage like microcracking mechanism involving interactions between corrosion and plasticity at the vicinity of grain boundaries. A corrosion-enhanced plasticity model is proposed to describe the intergranular and transgranular cracking in Alloy 600.

  13. Mechanisms of Forming Intergranular Microcracks and Microscopic Surface Discontinuities in Welds

    DTIC Science & Technology

    1992-06-01

    Cracking 22 2.4 Stress Corrosion Cracking 29 2.5 Laser Microscope 31 CHAPTER 3 3.0 Description of Specimens 32 CHAPTER 4 4.0 Observation of...Expanded Comments 130 B.1 Laser Microscopy 130 B.2 Hot Cracking 132 B.3 Cold Cracking 136 B.4 Stress Corrosion Cracking 149 B.5 Miscellaneous 155 B.6... cracking may occur shortly after welding or as delayed cracking , after a significant length of time. Stress corrosion cracking

  14. Fracture mechanics parameters for small fatigue cracks

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.

    1992-01-01

    This paper presents a review of some common small-crack test specimens, the underlying causes of the small-crack effect, and the fracture-mechanics parameters that have been used to correlate or predict their growth behavior. This review concentrates on continuum mechanics concepts and on the nonlinear behavior of small cracks. The paper reviews some stress-intensity factor solutions for small-crack test specimens and develops some simple elastic-plastic J integral and cyclic J integral expressions that include the influence of crack-closure. These parameters were applied to small-crack growth data on two aluminum alloys, and a fatigue life prediction methodology is demonstrated. For these materials, the crack-closure transient from the plastic wake was found to be the major factor in causing the small-crack effect.

  15. The role of grain boundary chemistry and structure in the environmentally-assisted intergranular cracking of nickel-base alloys

    SciTech Connect

    Was, G.S.

    1992-07-01

    Stress corrosion cracking tests in constant extension rate tensile (CERT) and constant load tensile (CLT) tests were conducted on Ni-xCr- 9Fe-yC in Ar, water, and a LiOH-boric acid solution. Cr and C improve the resistance of Ni-base alloys to IG cracking in both Ar and water at 360C. Since creep plays a role in IG cracking, one possible explanation for the role of the environment involves its effect on the creep. Experiments were conducted on the role of C in the deformation behavior and failure mode of Ni-16Cr-9Fe. Constant load experiments were conducted on Ni-16Cr-9Fe to determine if the CLT test is more aggressive than CERT. The electron backscattering technique in a SEM is being developed in order to extend the IG cracking studies to grain sizes typical of commercial alloys, 20-30 microns.

  16. Low-cycle fatigue-cracking mechanisms in fcc crystalline materials

    NASA Astrophysics Data System (ADS)

    Zhang, P.; Qu, S.; Duan, Q. Q.; Wu, S. D.; Li, S. X.; Wang, Z. G.; Zhang, Z. F.

    2011-01-01

    The low-cycle fatigue (LCF) cracking behavior in various face-centered-cubic (fcc) crystalline materials, including Cu single crystals, bicrystals and polycrystals, Cu-Al and Cu-Zn alloys, ultrafine-grained (UFG) Al-Cu and Cu-Zn alloys, was systematically investigated and reviewed. In Cu single crystals, fatigue cracking always nucleates along slip bands and deformation bands. The large-angle grain boundary (GB) becomes the preferential site in bicrystals and polycrystals. In addition, fatigue cracking can also nucleate along slip bands and twin boundaries (TBs) in polycrystalline materials. However, shear bands and coarse deformation bands are observed to the preferential sites for fatigue cracking in UFG materials with a large number of GBs. Based on numerous observations on fatigue-cracking behavior, the fatigue-cracking mechanisms along slip bands, GBs, TBs, shear bands and deformation bands were systematically compared and classified into two types, i.e. shear crack and impingement crack. Finally, these fatigue-cracking behaviors are discussed in depth for a better understanding of their physical nature and the transition from intergranular to transgranular cracking in various fcc crystalline materials. These comprehensive results for fatigue damage mechanisms should significantly aid in obtaining the optimum design to further strengthen and toughen metallic materials in practice.

  17. High temperature fatigue crack propagation in a nickel base superalloy and investigation of the intergranular fracture process

    SciTech Connect

    Kirkwood, B.L.

    1982-01-01

    The high temperature fatigue behavior of a nickel base superalloy was studied to determine the effect of grain boundary cavitation on the crack propagation rate. It was found that the introduction of cavities into a specimen prior to the fatigue test increases the crack propagation rate significantly over specimens which did not have cavities introduced into them. Companion fatigue specimens were cycled under similar conditions until they attained different levels of cyclic stress intensity in order to observe the changes that occur in the cavity spacing within the plastic zone as the stress intensity increases. The cavity spacing was observed with shadowed two stage TEM replicas taken from the plastic zone near the crack tip. It was found that the cavities nucleate continuously throughout the test, with the cavity spacing becoming progressively smaller as the cyclic stress intensity increases. It also was found that the cavity spacing decreases as one goes through the plastic zone toward the crack tip. A computer analysis of the diffusional growth rate of a void in the plastic zone was done to determine a theoretical value of the minimum cavity spacing which would give the observed crack propagation.

  18. An investigation on quench cracking behavior of superalloy Udimet 720LI using a fracture mechanics approach

    SciTech Connect

    Mao, J.; Keefer, V.L.; Chang, K.M.; Furrer, D.

    2000-04-01

    Quench cracking can be a serious problem in the heat treatment of high strength superalloys. A new fracture mechanics approach, quench cracking toughness (K{sub Q}), was introduced to evaluate the on-cooling quench cracking resistance of superalloy Udimet 720LI. A fully automatic computer controlled data acquisition and processing system was set up to track the on-cooling quenching process and to simulate the quench cracking. The influences of grain size, cooling rate, solution temperature, and alloy processing routes on quench cracking resistance were investigated. Research results indicate that quench cracking revealed a typical brittle and intergranular failure at high temperatures, which causes a lower quench cracking toughness in comparison to fracture toughness at room temperature. Fine grain structures show the higher quench cracking resistance and lower failure temperatures than intermediate grain structures at the same cooling rates. Moreover, higher cooling rate results in lower cracking toughness under the same grain size structures. In comparison of processing routes, powder metallurgy (PM) alloys show higher cracking resistance than cast and wrought (CW) alloys for fine grain structures at the same cooling rates. However, for immediate grain structure, there is no obvious difference of K{sub Q} between the two processing route in this study.

  19. Crack branching in carbon steel. Fracture mechanisms

    NASA Astrophysics Data System (ADS)

    Syromyatnikova, A. S.; Alekseev, A. A.; Levin, A. I.; Lyglaev, A. V.

    2010-04-01

    The fracture surfaces of pressure vessels made of carbon steel that form during crack branching propagation are examined by fractography. Crack branching is found to occur at a crack velocity higher than a certain critical value V > V c . In this case, the material volume that is involved in fracture and depends on the elastoplastic properties of the material and the sample width has no time to dissipate the energy released upon crack motion via the damage mechanisms intrinsic in the material under given deformation conditions (in our case, via cracking according to intragranular cleavage).

  20. Kinetic evaluation of intergranular fracture in austenitic stainless steels

    SciTech Connect

    Simonen, E.P.; Bruemmer, S.M.

    1995-12-31

    A second, higher-dose threshold exists for irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels in non-oxidizing environments. The data supporting this concept have stimulated interest in the mechanical aspects of intergranular (IG) fracture. Cracking in a non-oxidizing environment suggests that mechanically-induced IG fracture may play an important role in the IASCC mechanism under these conditions. Radiation alters deformation processes in austenitic alloys and may influence the fracture mode during either in-situ or post-irradiation straining. Radiation effects that must be considered include radiation strengthening, radiation creep and radiation-induced flow localization. The present evaluation relates these radiation-induced phenomena to IG fracture relevant to IASCC. The evaluation indicates that radiation strengthening retards matrix deformation and allows intergranular fracture to occur at higher stresses and lower temperatures than expected for unirradiated stainless steel.

  1. MECHANICS OF CRACK BRIDGING UNDER DYNAMIC LOADS

    SciTech Connect

    N. SRIDHAR; ET AL

    2001-02-01

    A bridging law for fiber reinforced composites under dynamic crack propagation conditions has been derived. Inertial effects in the mechanism of fiber pullout during dynamic propagation of a bridged crack are critically examined for the first time. By reposing simple shear lag models of pullout as problems of dynamic wave propagation, the effect of the frictional coupling between the fibers and the matrix is accounted for in a fairly straightforward way. The solutions yield the time-dependent relationship between the crack opening displacement and the bridging traction. Engineering criteria and the role of material and geometrical parameters for significant inertial effects are identified.

  2. Crack tip mechanics in periodically layered composites

    NASA Astrophysics Data System (ADS)

    Jha, Mahendra

    failure mechanisms of microvoid nucleation, growth and coalescence is employed within the framework of small deformation plasticity theory. Evolution of plastic zone and damage in the ductile layer is monitored with increasing load. High plastic strain localization is found to occur along the interface. Fracture initiation in the ductile phase and crack renucleation in the brittle layer ahead of the crack are predicted for the system under consideration.

  3. Subcritical crack growth and mechanical weathering: a new consideration of how moisture influences rock erosion rates.

    NASA Astrophysics Data System (ADS)

    Eppes, Martha-Cary; Keanini, Russell; Hancock, Gregory S.

    2016-04-01

    The contributions of moisture to the mechanical aspects of rock weathering and regolith production are poorly quantified. In particular, geomorphologists have largely overlooked the role of subcritical crack growth processes in physical weathering and the fact that moisture strongly influences the rates of those processes. This influence is irrespective of the function that moisture plays in stress loading mechanisms like freezing or hydration. Here we present a simple numerical model that explores the efficacy of subcritical crack growth in granite rock subaerially exposed under a range of moisture conditions. Because most weathering-related stress loading for rocks found at, or near, Earth's surface (hereafter surface rocks) is cyclic, we modeled crack growth using a novel combination of Paris' Law and Charles' Law. This combination allowed us to apply existing empirically-derived data for the stress corrosion index of Charles' Law to fatigue cracking. For stress, we focused on the relatively straightforward case of intergranular stresses that arise during solar-induced thermal cycling by conductive heat transfer, making the assumption that such stresses represent a universal minimum weathering stress experienced by all surface rocks. Because all other tensile weathering-related stresses would be additive in the context of crack growth, however, our model can be adapted to include other stress loading mechanisms. We validated our calculations using recently published thermal-stress-induced cracking rates. Our results demonstrate that 1) weathering-induced stresses as modeled herein, and as published by others, are sufficient to propagate fractures subcritically over long timescales with or without the presence of water 2) fracture propagation rates increase exponentially with respect to moisture, specifically relative humidity 3) fracture propagation rates driven by thermal cycling are strongly dependent on the magnitude of diurnal temperature ranges and the

  4. Crack initiation mechanisms in IASCC of stainless steel alloys

    SciTech Connect

    Cookson, J.M.; Was, G.S.; Andresen, P.L.

    1995-12-31

    An abnormally high oxygen concentration was recently discovered in a high purity stainless steel alloy widely used in IASCC studies. This led to an investigation into the role of oxygen on the initiation of intergranular cracking in irradiated samples in high temperature water. The concentration of oxygen in the alloys correlated with the number of cracks initiated in the proton irradiated region of samples strained in water containing 0.5 {micro}S/cm H{sub 2}SO{sub 4} at 288 C. This suggests that the presence of oxygen, in the form of spinel oxide particles, can lead to a substantial increase in the likelihood of crack initiation. This effect is only observed in irradiated samples strained in water, not in either unirradiated (non-sensitized) samples strained in water or irradiated samples strained in argon This paper examines the possible role of oxides in promoting crack initiation and the implications for IASCC.

  5. Theoretical studies on the mechanical behavior of granular materials under very low intergranular stresses

    NASA Technical Reports Server (NTRS)

    French, Kenneth W., Jr.

    1986-01-01

    The salient aspects of the theoretical modeling of a conventional triaxial test (CTC) of a cohesionless granular medium with stress and strain rate loading are described. Included are a controllable gravitational body force and provision for low confining pressure and/or very low intergranular stress. The modeling includes rational, analytic, and numerical phases, all in various stages of development. The numerical evolutions of theoretical models will be used in final design stages and in the analysis of the experimental data. In this the experimental design stage, it is of special interest to include in the candidate considerations every anomaly found in preliminary terrestrial experimentation. Most of the anomalies will be eliminated by design or enhanced for measurement as the project progresses. The main aspect of design being not the physical apparatus but the type and trajectories of loading elected. The major considerations that have been treated are: appearance and growth of local surface aberrations, stress-power coefficients, strain types, optical strain, radial bead migration, and measures of rotation for the proper stress flux.

  6. Analysis of Internal Crack Healing Mechanism under Rolling Deformation

    PubMed Central

    Gao, Haitao; Ai, Zhengrong; Yu, Hailiang; Wu, Hongyan; Liu, Xianghua

    2014-01-01

    A new experimental method, called the ‘hole filling method’, is proposed to simulate the healing of internal cracks in rolled workpieces. Based on the experimental results, the evolution in the microstructure, in terms of diffusion, nucleation and recrystallisation were used to analyze the crack healing mechanism. We also validated the phenomenon of segmented healing. Internal crack healing involves plastic deformation, heat transfer and an increase in the free energy introduced by the cracks. It is proposed that internal cracks heal better under high plastic deformation followed by slow cooling after rolling. Crack healing is controlled by diffusion of atoms from the matrix to the crack surface, and also by the nucleation and growth of ferrite grain on the crack surface. The diffusion mechanism is used to explain the source of material needed for crack healing. The recrystallisation mechanism is used to explain grain nucleation and growth, accompanied by atomic migration to the crack surface. PMID:25003518

  7. Cracking mechanism of PuO/sub 2/ fuel hot pressed in graphite dies

    SciTech Connect

    Taylor, D.H.

    1981-01-01

    Internal cracking in PuO/sub 2/ fuel is caused by gas pressure during hot pressing. Surface cracks are caused by tensile stresses arising from the phase change of ..cap alpha..-PuO/sub 2/ to ..gamma..-PuO/sub 2/ on reoxidation. To control cracking, process variables were chosen to minimize fuel reduction and to give large intergranular porosity. 19 figures.

  8. Propagation of stress corrosion cracks in alpha-brasses

    SciTech Connect

    Beggs, Dennis Vinton

    1981-01-01

    Transgranular and intergranular stress corrosion cracks were investigated in alpha-brasses in a tarnishing ammoniacal solution. Surface observation indicated that the transgranular cracks propagated discontinuously by the sudden appearance of a fine crack extending several microns ahead of the previous crack tip, often associated with the detection of a discrete acoustic emission (AE). By periodically increasing the deflection, crack front markings were produced on the resulting fracture surfaces, showing that the discontinuous propagation of the crack trace was representative of the subsurface cracking. The intergranular crack trace appeared to propagate continuously at a relatively blunt crack tip and was not associated with discrete AE. Under load pulsing tests with a time between pulses, ..delta..t greater than or equal to 3 s, the transgranular fracture surfaces always exhibited crack front markings which corresponded with the applied pulses. The spacing between crack front markings, ..delta..x, decreased linearly with ..delta..t. With ..delta..t less than or equal to 1.5 s, the crack front markings were in a one-to-one correspondence with applied pulses only at relatively long crack lengths. In this case, ..delta..x = ..delta..x* which approached a limiting value of 1 ..mu..m. No crack front markings were observed on intergranular fracture surfaces produced during these tests. It is concluded that transgranular cracking occurs by discontinuous mechanical fracture of an embrittled region around the crack tip, while intergranular cracking results from a different mechanism with cracking occurring via the film-rupture mechanism.

  9. Modeling the Interactions Between Multiple Crack Closure Mechanisms at Threshold

    NASA Technical Reports Server (NTRS)

    Newman, John A.; Riddell, William T.; Piascik, Robert S.

    2003-01-01

    A fatigue crack closure model is developed that includes interactions between the three closure mechanisms most likely to occur at threshold; plasticity, roughness, and oxide. This model, herein referred to as the CROP model (for Closure, Roughness, Oxide, and Plasticity), also includes the effects of out-of plane cracking and multi-axial loading. These features make the CROP closure model uniquely suited for, but not limited to, threshold applications. Rough cracks are idealized here as two-dimensional sawtooths, whose geometry induces mixed-mode crack- tip stresses. Continuum mechanics and crack-tip dislocation concepts are combined to relate crack face displacements to crack-tip loads. Geometric criteria are used to determine closure loads from crack-face displacements. Finite element results, used to verify model predictions, provide critical information about the locations where crack closure occurs.

  10. Mechanism of irradiation assisted stress corrosion crack initiation in thermally sensitized 304 stainless steel

    NASA Astrophysics Data System (ADS)

    Onchi, T.; Dohi, K.; Soneda, N.; Navas, Marta; Castaño, M. L.

    2005-04-01

    Thermally sensitized 304 stainless steels, irradiated up to 1.2 × 1021 n/cm2 (E > 1 MeV), were slow-strain-rate-tensile tested in 290 °C water containing 0.2 ppm dissolved oxygen (DO), followed by scanning and transmission electron microscopic examinations, to study mechanism of irradiation-assisted-stress-corrosion-crack (IASCC) initiation. Intergranular (IG) cracking behaviors changed at a border fluence (around 1 × 1020 n/cm2), above which deformation twinning were predominant and deformation localization occurred earlier with increasing fluence. The crack initiation sites tended to link to the deformation bands, indicating that the crack initiation may be brought about by the deformation bands interacted with grain boundaries. Thus the border fluence is equivalent to the IASCC threshold fluence for the sensitized material, although the terminology of IASCC is originally given to the non-sensitized materials without microstructural definition. The IASCC threshold fluence was found to change with irradiation conditions. Changes in IASCC susceptibility and IASCC threshold fluence with fluence and DO were further discussed.

  11. Mechanics of the crack path formation

    NASA Technical Reports Server (NTRS)

    Rubinstein, Asher A.

    1989-01-01

    A detailed analysis of experimentally obtained curvilinear crack path trajectories formed in a heterogeneous stress field is presented. Experimental crack path trajectories were used as data for numerical simulations, recreating the actual stress field governing the development of the crack path. Thus, the current theories of crack curving and kinking could be examined by comparing them with the actual stress field parameters as they develop along the experimentally observed crack path. The experimental curvilinear crack path trajectories were formed in the tensile specimens with a hole positioned in the vicinity of a potential crack path. The numerical simulation, based on the solution of equivalent boundary value problems with the possible perturbations of the crack path, is presented here.

  12. The role of grain boundary chemistry and structure in the environmentally-assisted intergranular cracking of nickel-base alloys. Progress report, August 1, 1991--July 31, 1992

    SciTech Connect

    Was, G.S.

    1992-07-01

    Stress corrosion cracking tests in constant extension rate tensile (CERT) and constant load tensile (CLT) tests were conducted on Ni-xCr- 9Fe-yC in Ar, water, and a LiOH-boric acid solution. Cr and C improve the resistance of Ni-base alloys to IG cracking in both Ar and water at 360C. Since creep plays a role in IG cracking, one possible explanation for the role of the environment involves its effect on the creep. Experiments were conducted on the role of C in the deformation behavior and failure mode of Ni-16Cr-9Fe. Constant load experiments were conducted on Ni-16Cr-9Fe to determine if the CLT test is more aggressive than CERT. The electron backscattering technique in a SEM is being developed in order to extend the IG cracking studies to grain sizes typical of commercial alloys, 20-30 microns.

  13. Deformation mechanics of deep surface flaw cracks

    NASA Technical Reports Server (NTRS)

    Francis, P. H.; Nagy, A.; Beissner, R. E.

    1972-01-01

    A combined analytical and experimental program was conducted to determine the deformation characteristics of deep surface cracks in Mode I loading. An approximate plane finite element analysis was performed to make a parameter study on the influence of crack depth, crack geometry, and stress level on plastic zones, crack opening displacement, and back surface dimpling in Fe-3Si steel and 2219-T87 aluminum. Surface replication and profiling techniques were used to examine back surface dimple configurations in 2219-T87 aluminum. Interferometry and holography were used to evaluate the potential of various optical techniques to detect small surface dimples on large surface areas.

  14. Relating Ab Initio Mechanical Behavior of Intergranular Glassy Films in Γ-Si3N4 to Continuum Scales

    NASA Astrophysics Data System (ADS)

    Ouyang, L.; Chen, J.; Ching, W.; Misra, A.

    2006-05-01

    Nanometer thin intergranular glassy films (IGFs) form in polycrystalline ceramics during sintering at high temperatures. The structure and properties of these IGFs are significantly changed by doping with rare earth elements. We have performed highly accurate large-scale ab initio calculations of the mechanical properties of both undoped and Yittria doped (Y-IGF) model by theoretical uniaxial tensile experiments. Uniaxial strain was applied by incrementally stretching the super cell in one direction, while the other two dimensions were kept constant. At each strain, all atoms in the model were fully relaxed using Vienna Ab initio Simulation Package VASP. The relaxed model at a given strain serves as the starting position for the next increment of strain. This process is carried on until the total energy (TE) and stress data show that the "sample" is fully fractured. Interesting differences are seen between the stress-strain response of undoped and Y-doped models. For the undoped model, the stress-strain behavior indicates that the initial atomic structure of the IGF is such that there is negligible coupling between the x- and the y-z directions. However, once the behavior becomes non- linear the lateral stresses increase, indicating that the atomic structure evolves with loading [1]. To relate the ab initio calculations to the continuum scales we analyze the atomic-scale deformation field under this uniaxial loading [1]. The applied strain in the x-direction is mostly accommodated by the IGF part of the model and the crystalline part experiences almost negligible strain. As the overall strain on the sample is incrementally increased, the local strain field evolves such that locations proximal to the softer spots attract higher strains. As the load progresses, the strain concentration spots coalesce and eventually form persistent strain localization zone across the IGF. The deformation pattern obtained through ab initio calculations indicates that it is possible to

  15. Intergranular stress corrosion cracking: A rationalization of apparent differences among stress corrosion cracking tendencies for sensitized regions in the process water piping and in the tanks of SRS reactors

    SciTech Connect

    Louthan, M.R.

    1990-09-28

    The frequency of stress corrosion cracking in the near weld regions of the SRS reactor tank walls is apparently lower than the cracking frequency near the pipe-to-pipe welds in the primary cooling water system. The difference in cracking tendency can be attributed to differences in the welding processes, fabrication schedules, near weld residual stresses, exposure conditions and other system variables. This memorandum discusses the technical issues that may account the differences in cracking tendencies based on a review of the fabrication and operating histories of the reactor systems and the accepted understanding of factors that control stress corrosion cracking in austenitic stainless steels.

  16. Linking elastic, mechanical and transport properties in anisotropically cracked rocks

    NASA Astrophysics Data System (ADS)

    Schubnel, A.; Benson, P.; Nasseri, F.; Gueguen, Y.; Meredith, P.; Young, R.

    2007-12-01

    Damage and crack porosity can result in a decrease of the mechanical strength of the rock, the development of elastic and mechanical anisotropy and the enhancement of transport properties. Using Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, it is possible to calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation using the solid grains and fluid elastic properties. Using the same tool, we show that the anisotropy, the shear wave splitting and the dispersion of elastic waves can be derived for anisotropic crack fabrics. Mechanically, the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (KIC) of rocks. We show that KIC can show large amounts of anisotropy as well, the degree and orientation of which being largely constrained once again by the microcrack fabric. NIC can predict relatively well KIC at high crack density, by simply using dimensionless crack densities inverted from velocities. A decrease of 50% for crack densities larger than 1, 80% for crack densities larger than 5 is predicted, in close agreement with our observed experimental variation of KIC. At the microscale, this can be interpreted by the fact that the main fracture is strongly interacting with the pre-existing microcrack fabric. Finally, and above the percolation threshold, macroscopic fluid flow also depends on the porosity, crack density and aspect ratio. Using the permeability model of Guéguen and Dienes (1989) and the crack density and aspect ratio recovered from the elastic wave velocity inversion, we successfully predict the evolution of permeability with pressure for direct comparison with the laboratory measurements. These combined experimental and modelling results illustrate the importance of understanding the details of how rock microstructures change in response to an external stimulus in predicting the

  17. Molecular-dynamics Simulation-based Cohesive Zone Representation of Intergranular Fracture Processes in Aluminum

    NASA Technical Reports Server (NTRS)

    Yamakov, Vesselin I.; Saether, Erik; Phillips, Dawn R.; Glaessgen, Edward H.

    2006-01-01

    A traction-displacement relationship that may be embedded into a cohesive zone model for microscale problems of intergranular fracture is extracted from atomistic molecular-dynamics simulations. A molecular-dynamics model for crack propagation under steady-state conditions is developed to analyze intergranular fracture along a flat 99 [1 1 0] symmetric tilt grain boundary in aluminum. Under hydrostatic tensile load, the simulation reveals asymmetric crack propagation in the two opposite directions along the grain boundary. In one direction, the crack propagates in a brittle manner by cleavage with very little or no dislocation emission, and in the other direction, the propagation is ductile through the mechanism of deformation twinning. This behavior is consistent with the Rice criterion for cleavage vs. dislocation blunting transition at the crack tip. The preference for twinning to dislocation slip is in agreement with the predictions of the Tadmor and Hai criterion. A comparison with finite element calculations shows that while the stress field around the brittle crack tip follows the expected elastic solution for the given boundary conditions of the model, the stress field around the twinning crack tip has a strong plastic contribution. Through the definition of a Cohesive-Zone-Volume-Element an atomistic analog to a continuum cohesive zone model element - the results from the molecular-dynamics simulation are recast to obtain an average continuum traction-displacement relationship to represent cohesive zone interaction along a characteristic length of the grain boundary interface for the cases of ductile and brittle decohesion. Keywords: Crack-tip plasticity; Cohesive zone model; Grain boundary decohesion; Intergranular fracture; Molecular-dynamics simulation

  18. Mechanism of corrosion fatigue cracking of automotive coil spring steel

    NASA Astrophysics Data System (ADS)

    Nam, Tae-Heum; Kwon, Min-Seok; Kim, Jung-Gu

    2015-11-01

    The AISI 300M ultra-high strength steel was applied for the automotive suspension coil spring. Recently, some premature failures were reported, which caused by synergistic effect of cyclic mechanical stress and corrosion, namely corrosion fatigue cracking. In this study, the accurate mechanism of corrosion fatigue cracking for coil spring steel was studied for the proper prevention method against the catastrophic failure. Fatigue life was evaluated in 5 wt% NaCl solution under the anodic dissolution and hydrogen embrittlement conditions, which is simulated by applying constant potentials. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis indicated that the corrosion fatigue cracking was initiated at the MnS inclusion of the pit initiation site. The calculation of hydrogen production corresponding to each corrosion fatigue test condition revealed the two operating mechanisms of the cracking process. The corrosion fatigue cracking failure of coil spring steel was mainly caused by the anodic dissolution combined with hydrogen embrittlement.

  19. Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: effect of hydration.

    PubMed

    Kruzic, J J; Nalla, R K; Kinney, J H; Ritchie, R O

    2003-12-01

    Few studies have focused on a description of the fracture toughness properties of dentin in terms of resistance-curve (R-curve) behavior, i.e., fracture resistance increasing with crack extension, particularly in light of the relevant toughening mechanisms involved. Accordingly, in the present study, fracture mechanics based experiments were conducted on elephant dentin in order to determine such R-curves, to identify the salient toughening mechanisms and to discern how hydration may affect their potency. Crack bridging by uncracked ligaments, observed directly by microscopy and X-ray tomography, was identified as a major toughening mechanism, with further experimental evidence provided by compliance-based experiments. In addition, with hydration, dentin was observed to display significant crack blunting leading to a higher overall fracture resistance than in the dehydrated material. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of dentin and in predicting its failure in vivo.

  20. Thermal-mechanical fatigue crack growth in Inconel X-750

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1985-01-01

    Thermal-mechanical fatigue crack growth (TMFCG) was studied in a 'gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.

  1. Thermal-mechanical fatigue crack growth in Inconel X-750

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1984-01-01

    Thermal-mechanical fatigue crack growth (TMFCG) was studied in a gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.

  2. Thermal-mechanical fatigue crack growth in aircraft engine materials

    NASA Astrophysics Data System (ADS)

    Dai, Yi

    1993-08-01

    This thesis summarizes the major technical achievements obtained as a part of a collaborative research and development project between Ecole Polytechnique and Pratt & Whitney Canada. These achievements include: (1) a thermal-mechanical fatigue (TMF) testing rig which is capable of studying the fatigue behaviors of gas turbine materials under simultaneous changes of temperatures and strains or stress; (2) an advanced alternative current potential drop (ACPD) measurement system which is capable of performing on-line monitoring of fatigue crack initiation and growth in specimen testing under isothermal and TMF conditions; (3) fatigue crack initiation and short crack growth data for the titanium specimens designed with notch features associated with bolt holes of compressor discs; (4) thermal-mechanical fatigue crack growth data for two titanium alloys being used in PWC engine components, which explained the material fatigue behavior encountered in full-scale component testing; (5) a complete fractographic analysis for the tested specimens which enhanced the understanding of the fatigue crack growth mechanisms and helped to establish an analytical crack growth model; and (6) application of the ACPD fatigue crack monitoring technique to single tooth firtree specimen (STFT) LCF testing of PWA 1480 single crystal alloy. Finally, a comprehensive discussion concerning the results pertaining to this research project is presented.

  3. Fracture Mechanics Analyses for Interface Crack Problems - A Review

    NASA Technical Reports Server (NTRS)

    Krueger, Ronald; Shivakumar, Kunigal; Raju, Ivatury S.

    2013-01-01

    Recent developments in fracture mechanics analyses of the interfacial crack problem are reviewed. The intent of the review is to renew the awareness of the oscillatory singularity at the crack tip of a bimaterial interface and the problems that occur when calculating mode mixity using numerical methods such as the finite element method in conjunction with the virtual crack closure technique. Established approaches to overcome the nonconvergence issue of the individual mode strain energy release rates are reviewed. In the recent literature many attempts to overcome the nonconvergence issue have been developed. Among the many approaches found only a few methods hold the promise of providing practical solutions. These are the resin interlayer method, the method that chooses the crack tip element size greater than the oscillation zone, the crack tip element method that is based on plate theory and the crack surface displacement extrapolation method. Each of the methods is validated on a very limited set of simple interface crack problems. However, their utility for a wide range of interfacial crack problems is yet to be established.

  4. Mechanisms of time-dependent crack growth at elevated temperature

    SciTech Connect

    Saxena, A.; Stock, S.R.

    1990-04-15

    Objective of this 3-y study was to conduct creep and creep-fatigue crack growth experiments and to characterize the crack tip damage mechanisms in a model material (Cu-1wt%Sb), which is known to cavitate at grain boundaries under creep deformation. Results were: In presence of large scale cavitation damage and crack branching, time rate of creep crack growth da/dt does not correlate with C[sub t] or C[sup *]. When cavitation damage is constrained, da/dt is characterized by C[sub t]. Area fraction of grain boundary cavitated is the single damage parameter for the extent of cavitation damage ahead of crack tips. C[sub t] is used for the creep-fatigue crack growth behavior. In materials prone to rapid cavity nucleation, creep cracks grow faster initially and then reach a steady state whose growth rate is determined by C[sub t]. Percent creep life exhausted correlates with average cavity diameter and fraction of grain boundary area occupied by cavities. Synchrotron x-ray tomographic microscopy was used to image individual cavities in Cu-1wt% Sb. A methodology was developed for predicting the remaining life of elevated temperature power plant components; (C[sub t])[sub avg] was used to correlate creep-fatigue crack growth in Cr-Mo and Cr-Mo-V steel and weldments.

  5. Part 2. Metallurgical factors governing the H-assisted intergranular cracking of peak-aged Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C)

    SciTech Connect

    Gaudett, M.A.; Scully, J.R.

    2000-01-01

    A previous study (Part 1) showed that the solution-treated and aged (STA) (i.e., peak-aged) condition of Beta-C Ti, ({sigma}{sub 0.2 pct y} = 865 MPa), as measured by reductions in the fracture initiation stress with predissolved H content and the introduction of an intergranular (IG) fracture mode. It was also shown that yield-strength elevation and the subsequent enhancement in the local hydrostatic stresses within the notch root are not the controlling factors in the H-assisted IG fracture initiation of the STA condition. Previous work (Part 1) implicates a microstructural feature or condition associated with the 500 C aging treating. In this study, it is shown that localized internal hydride precipitation at the grain boundaries or alpha beta interfaces was not detected by a variety of experimental methods over the range of internal H contents for which IG fracture initiation was observed. It was also shown that grain-boundary alpha colonies or films are not responsible for the IG fracture initiation in the STA condition. A measured increase in hydrogen embrittlement (HG) susceptibility as a function of aging time at 500 C is consistent with the segregation or depletion of a critical species at the grain boundary. However, grain-boundary segregation/depletion could not be detected with Auger electron spectroscopy (AES) of specimens fracturing in a vacuum. Compression tests used to characterize and compare the alloys' slip behavior showed that plastic deformation is concentrated at or near the grain boundaries in the STA condition. Therefore, a possible intergranular fracture initiation mechanism that includes the effects of hydrogen and localized deformation is discussed.

  6. Mechanisms of fatigue damage and crack growth in advanced materials

    NASA Astrophysics Data System (ADS)

    Ritchie, Robert O.

    2001-03-01

    In terms of in-service failures, cyclic fatigue is the most prevalent form of fracture. Despite the wealth of information on fatigue failures in traditional structural materials such as (ductile) metals and alloys, far less is understood about the susceptibility of the newer advanced materials, such as (brittle) intermetallics, ceramics and their composites. In this presentation, the mechanics and mechanisms of fatigue damage and crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile metallic materials, and corresponding behavior in the more brittle advanced materials. This is achieved by considering the process of subcritical crack growth as a mutual competition between intrinsic mechanisms of microstructural damage ahead of the crack tip, which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip, which impede it. This approach is shown to be important for the understanding of the structural fatigue properties of advanced materials, such as monolithic and composite ceramics, and a range of intermetallics (e.g., TiAl, MoSi2, Nb3Al), as the mechanisms of fatigue in these brittle materials are conceptually distinct from that associated with the well known metal fatigue. Examples of the application and life-prediction methodologies for such materials in fatigue-critical situations will be given from the aerospace and bioengineering industries.

  7. Crack Tip Plasticity in Dynamic Fracture Mechanics.

    DTIC Science & Technology

    1978-04-01

    0.1. Ant.r.d) ~IIIi. . • ~~~~• ~~~~~~~~~~~~~~~~ ~~~ . - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - 1 CRACK TIP PLASTICITY IN DYNANI C FRACTU ...force——from the material’s fracture property——the resistance. The material property represents the energy dissipated ~n flow into the crack tip and...the flow stress varied - arbitrarily along the length of the strip yield zone. The flow stress val- ues were assigned in accord with a known strain

  8. Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth.

    PubMed

    Mughrabi, Haël

    2015-03-28

    In this survey, the origin of fatigue crack initiation and damage evolution in different metallic materials is discussed with emphasis on the responsible microstructural mechanisms. After a historical introduction, the stages of cyclic deformation which precede the onset of fatigue damage are reviewed. Different types of cyclic slip irreversibilities in the bulk that eventually lead to the initiation of fatigue cracks are discussed. Examples of trans- and intercrystalline fatigue damage evolution in the low cycle, high cycle and ultrahigh cycle fatigue regimes in mono- and polycrystalline face-centred cubic and body-centred cubic metals and alloys and in different engineering materials are presented, and some microstructural models of fatigue crack initiation and early crack growth are discussed. The basic difficulties in defining the transition from the initiation to the growth of fatigue cracks are emphasized. In ultrahigh cycle fatigue at very low loading amplitudes, the initiation of fatigue cracks generally occupies a major fraction of fatigue life and is hence life controlling.

  9. Predicting weld solidification cracking using damage mechanics -- LDRD summary report

    SciTech Connect

    Dike, J.J.; Brooks, J.A.; Bammann, D.J.; Li, M.; Krafcik, J.S.; Yang, N.Y.C.

    1997-04-01

    This report summarizes the efforts to develop and validate a finite element based model to predict weld solidification cracking behavior. Such a model must capture the solidification behavior, the thermal behavior in the weld pool region, the material mechanical response, and some failure criteria to determine when solidification cracking will occur. For such a program to be successful, each aspect of the model had to be accurately modeled and verified since the output of one portion of the model served as the input to other portions of the model. A solidification model which includes dendrite tip and eutectic undercooling was developed and used in both the thermal and mechanical finite element analysis. High magnification video techniques were developed to measure strains for validation of the mechanical predictions using a strain rate and temperature dependent constitutive model. This model was coupled with a ductile void growth damage model and correlated with experimental observations to determine capabilities of predicting cracking response. A two phase (solid + liquid) material model was also developed that can be used to more accurately capture the mechanics of weld solidification cracking. In general, reasonable agreement was obtained between simulation and experiment for location of crack initiation and extent of cracking for 6061-T6 aluminum. 35 refs.

  10. Dissolution Condensation Mechanism of Stress Corrosion Cracking in Liquid Metals: Driving Force and Crack Kinetics

    NASA Astrophysics Data System (ADS)

    Glickman, Evgeny E.

    2011-02-01

    Stress corrosion cracking (SCC) in aqueous solution is driven by exothermic reactions of metal oxidation. This stimulus, as well as classical mechanisms of SCC, does not apply to SCC in liquid metals (LMs). In the framework of the dissolution-condensation mechanism (DCM), we analyzed the driving force and crack kinetics for this nonelectrochemical mode of SCC that is loosely called "liquid metal embrittlement" (LME). According to DCM, a stress-induced increase in chemical potential at the crack tip acts as the driving force for out-of-the-tip diffusion mass transfer that is fast because diffusion in LMs is very fast and surface energy at the solid-liquid interface is small. In this article, we review two versions of DCM mechanism, discuss the major physics behind them, and develop DCM further. The refined mechanism is applied then to the experimental data on crack velocity V vs stress intensity factor, the activation energy of LME, and alloying effects. It is concluded that DCM provides a good conceptual framework for analysis of a unified kinetic mechanism of LME and may also contribute to SCC in aqueous solutions.

  11. Research on anti crack mechanism of bionic coupling brake disc

    NASA Astrophysics Data System (ADS)

    Shi, Lifeng; Yang, Xiao; Zheng, Lingnan; Wu, Can; Ni, Jing

    2017-09-01

    According to the biological function of fatigue resistance possessed by biology, this study designed a Bionic Coupling Brake Disc (BCBD) which can inhibit crack propagation as the result of improving fatigue property. Thermal stress field of brake disc was calculated under emergency working condition, and circumferential and radial stress field which lead to fatigue failure of brake disc were investigated simultaneously. Results showed that the maximum temperature of surface reached 890°C and the maximum residual tensile stress was 207 Mpa when the initial velocity of vehicle was 200 km/h. Based on the theory of elastic plastic fracture mechanics, the crack opening displacement and the crack front J integrals of the BCBD and traditional brake disc (TBD) with pre-cracking were calculated, and the strength of crack front was compared. Results revealed the growth behavior of fatigue crack located on surface of brake disc, and proved the anti fatigue resistance of BCBD was better, and the strength of crack resistance of BCBD was much stronger than that of TBD. This simulation research provided significant references for optimization and manufacturing of BCBD.

  12. Crack Turning Mechanics of Composite Wing Skin Panels

    NASA Technical Reports Server (NTRS)

    Yuan, F. G.; Reeder, James R. (Technical Monitor)

    2001-01-01

    The safety of future composite wing skin integral stiffener panels requires a full understanding of failure mechanisms of these damage tolerance critical structures under both in-plane and bending loads. Of primary interest is to derive mathematical models using fracture mechanics in anisotropic cracked plate structures, to assess the crack turning mechanisms, and thereby to enhance the residual strength in the integral stiffener composite structures. The use of fracture mechanics to assess the failure behavior in a cracked structure requires the identification of critical fracture parameters which govern the severity of stress and deformation field ahead of the flaw, and which can be evaluated using information obtained from the flaw tip. In the three-year grant, the crack-tip fields under plane deformation, crack-tip fields for anisotropic plates and anisotropic shells have been obtained. In addition, methods for determining the stress intensity factors, energy release rate, and the T-stresses have been proposed and verified. The research accomplishments can be summarized as follows: (1) Under plane deformation in anisotropic solids, the asymptotic crack-tip fields have been obtained using Stroh formalism; (2) The T-stress and the coefficient of the second term for sigma(sub y), g(sub 32), have been obtained using path-independent integral, the J-integral and Betti's reciprocal theorem together with auxiliary fields; (3) With experimental data performed by NASA, analyses indicated that the mode-I critical stress intensity factor K(sub Q) provides a satisfactory characterization of fracture initiation for a given laminate thickness, provided the failure is fiber-dominated and crack extends in a self-similar manner; (4) The high constraint specimens, especially for CT specimens, due to large T-stress and large magnitude of negative g(sub 32) term may be expected to inhibit the crack extension in the same plane and promote crack turning; (5) Crack turning out of

  13. Creep, Fatigue and Environmental Interactions and Their Effect on Crack Growth in Superalloys

    NASA Technical Reports Server (NTRS)

    Telesman, J.; Gabb, T. P.; Ghosn, L. J.; Smith, T.

    2017-01-01

    Complex interactions of creep/fatigue/environment control dwell fatigue crack growth (DFCG) in superalloys. Crack tip stress relaxation during dwells significantly changes the crack driving force and influence DFCG. Linear Elastic Fracture Mechanics, Kmax, parameter unsuitable for correlating DFCG behavior due to extensive visco-plastic deformation. Magnitude of remaining crack tip axial stresses controls DFCG resistance due to the brittle-intergranular nature of the crack growth process. Proposed a new empirical parameter, Ksrf, which incorporates visco-plastic evolution of the magnitude of remaining crack tip stresses. Previous work performed at 704C, extend the work to 760C.

  14. Characterization of Cracking and Crack Growth Properties of the C5A Aircraft Tie-Box Forging

    NASA Technical Reports Server (NTRS)

    Piascik, Robert S.; Smith, Stephen W.; Newman, John A.; Willard, Scott A.

    2003-01-01

    Detailed destructive examinations were conducted to characterize the integrity and material properties of two aluminum alloy (7075-T6) horizontal stabilizer tie box forgings removed.from US. Air Force C5A and C5B transport aircraft. The C5B tie box forging was,found to contain no evidence of cracking. Thirteen cracks were found in the CSA,forging. All but one of the cracks observed in the C5A component were located along the top cap region (one crack was located in the bottom cap region). The cracks in the C5A component initiated at fastener holes and propagated along a highly tunneled intergranular crack path. The tunneled crack growth configuration is a likelv result of surface compressive stress produced during peening of the .forging suijace. The tie box forging ,fatigue crack growth, fracture and stress corrosion cracking (SCC) properties were characterized. Reported herein are the results of laboratory air ,fatigue crack growth tests and 95% relative humidity SCC tests conducted using specimens machined from the C5A ,forging. SCC test results revealed that the C5A ,forging material was susceptible to intergranular environmental assisted cracking: the C5A forging material exhibited a SCC crack-tip stress-intensity factor threshold of less than 6 MPadn. Fracture toughness tests revealed that the C5A forging material exhibited a fracture toughness that was 25% less than the C5B forging. The C5A forging exhibited rapid laboratory air fatigue crack growth rates having a threshold crack-tip stress-intensity factor range of less than 0.8 MPa sup m. Detailed fractographic examinations revealed that the ,fatigue crack intergranular growth crack path was similar to the cracking observed in the C5A tie box forging. Because both fatigue crack propagation and SCC exhibit similar intergranular crack path behavior, the damage mechanism resulting in multi-site cracking of tie box forgings cannot be determined unless local cyclic stresses can be quantified.

  15. Intergranular fracture of gamma titanium aluminides under hot working conditions

    SciTech Connect

    Seetharaman, V.; Semiatin, S.L.

    1998-07-01

    A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results.

  16. Modeling Selective Intergranular Oxidation of Binary Alloys

    SciTech Connect

    Xu, Zhijie; Li, Dongsheng; Schreiber, Daniel K.; Rosso, Kevin M.; Bruemmer, Stephen M.

    2015-01-07

    Intergranular attack of alloys under hydrothermal conditions is a complex problem that depends on metal and oxygen transport kinetics via solid-state and channel-like pathways to an advancing oxidation front. Experiments reveal very different rates of intergranular attack and minor element depletion distances ahead of the oxidation front for nickel-based binary alloys depending on the minor element. For example, a significant Cr depletion up to 9 µm ahead of grain boundary crack tips were documented for Ni-5Cr binary alloy, in contrast to relatively moderate Al depletion for Ni-5Al (~100s of nm). We present a mathematical kinetics model that adapts Wagner’s model for thick film growth to intergranular attack of binary alloys. The transport coefficients of elements O, Ni, Cr, and Al in bulk alloys and along grain boundaries were estimated from the literature. For planar surface oxidation, a critical concentration of the minor element can be determined from the model where the oxide of minor element becomes dominant over the major element. This generic model for simple grain boundary oxidation can predict oxidation penetration velocities and minor element depletion distances ahead of the advancing front that are comparable to experimental data. The significant distance of depletion of Cr in Ni-5Cr in contrast to the localized Al depletion in Ni-5Al can be explained by the model due to the combination of the relatively faster diffusion of Cr along the grain boundary and slower diffusion in bulk grains, relative to Al.

  17. Elasto-plastic fracture mechanics of crack growth in soil

    NASA Astrophysics Data System (ADS)

    Hallett, P. D.; Newson, T. A.

    2003-04-01

    A predominant variable in soil structure formation and degradation is crack propagation. Empirical models exist to predict fracture but these do not describe the underlying physical processes. Theoretical fracture mechanics models have been applied to soil, but most are not applicable when soil is in a wet, plastic state. Since the onset of crack formation in soil tends to occur in this condition, physically sound elasto-plastic fracture mechanics approaches are long overdue. We address this weakness by applying a new elasto-plastic fracture mechanics approach to describe crack formation in plastic soil. Samples are fractured using a deep-notch (modified 4-point) bend test, with data on load transmission, sample bending, crack growth, and crack mouth opening collected to assess the crack opening angle (COA), the crack tip opening angle (CTOA) and the plastic energy dissipation rate (Dpl). These are all material properties that can be used directly to predict and describe crack propagation. CTOA will be used to discuss the results here, although a full description of the other parameters will be provided in the conference presentation. It provides a powerful parameter for describing soil cracking since CTOA is induced by soil shrinkage (an easily measured parameter) and can be used to describe elasto-plastic fracture in finite element modelling packages. The test variables we have studied to date are clay platelet orientation, soil texture, clay mineralogy, and pore water salinity. All samples were formed by consolidating a soil slurry with a 120 kPa vertical stress. Tests on pure kaolinite showed that platelet orientation did not affect CTOA which was 0.23 ± 0.02 for both conditions. Soil texture did have a marked influence, however, with silica sand:kaolinite mixes of 20:80 and 40:60 reducing CTOA to 0.14 ± 0.02 and 0.12 ± 0.01 respectively. These lower values of CTOA indicate that less strain is required to induce fracture when the amount of clay is lowered

  18. Understanding cracking failures of coatings: A fracture mechanics approach

    NASA Astrophysics Data System (ADS)

    Kim, Sung-Ryong

    A fracture mechanics analysis of coating (paint) cracking was developed. A strain energy release rate (G(sub c)) expression due to the formation of a new crack in a coating was derived for bending and tension loadings in terms of the moduli, thicknesses, Poisson's ratios, load, residual strain, etc. Four-point bending and instrumented impact tests were used to determine the in-situ fracture toughness of coatings as functions of increasing baking (drying) time. The system used was a thin coating layer on a thick substrate layer. The substrates included steel, aluminum, polycarbonate, acrylonitrile-butadiene-styrene (ABS), and Noryl. The coatings included newly developed automotive paints. The four-point bending configuration promoted nice transversed multiple coating cracks on both steel and polymeric substrates. The crosslinked type automotive coatings on steel substrates showed big cracks without microcracks. When theoretical predictions for energy release rate were compared to experimental data for coating/steel substrate samples with multiple cracking, the agreement was good. Crosslinked type coatings on polymeric substrates showed more cracks than theory predicted and the G(sub c)'s were high. Solvent evaporation type coatings on polymeric substrates showed clean multiple cracking and the G(sub c)'s were higher than those obtained by tension analysis of tension experiments with the same substrates. All the polymeric samples showed surface embrittlement after long baking times using four-point bending tests. The most apparent surface embrittlement was observed in the acrylonitrile-butadiene-styrene (ABS) substrate system. The impact properties of coatings as a function of baking time were also investigated. These experiments were performed using an instrumented impact tester. There was a rapid decrease in G(sub c) at short baking times and convergence to a constant value at long baking times. The surface embrittlement conditions and an embrittlement toughness

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    MedlinePlus

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    MedlinePlus

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  1. Inclusion size effect on the fatigue crack propagation mechanism and fracture mechanics of a superalloy

    NASA Astrophysics Data System (ADS)

    Denda, Takeshi; Bretz, Perter L.; Tien, John K.

    1992-02-01

    Low cycle fatigue life of nickel-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependencies between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grained IN718, which is a representative turbine disc material for jet engines. There is a faceted-striated crack transition on the fracture surfaces. This faceted-striated transition also appears in the da/dN vs crack length curves. It is observed that the faceted crack propagation time can be more than 50 pct of total lifetime in the low cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given.

  2. Role of Intergranular Films in Toughened Ceramics

    SciTech Connect

    Becher, P.F.; Hsueh, C.H.; More, K.L.; Painter, G.S.; Sun, E.Y.

    1999-05-10

    Self-reinforced silicon nitride ceramics rely the generation of elongated grains that act as reinforcing elements to gain increases in fracture toughness. However, the size and number of the reinforcing grains must be controlled, along with the matrix grain size, to optimize the fracture toughness and strength. Furthermore, the toughening processes of crack bridging are dependent upon retention of these reinforcing grains during crack extension by an interfacial debonding process. Both the debonding process and the resultant toughening effects are found to be influenced by the composition of the sintering aids which typical are incorporated into the amorphous intergranular films found in these ceramics. Specifically, it is shown that the interface between the intergranular glass and the reinforcing grains is strengthened in the presence of an epitaxial SiAlON layer. In addition, the interface strength increases with the Al and 0 content of the SiAlON layer. Micromechanics modeling indicates that stresses associated with thermal expansion mismatch are a secondary factor in interfacial debonding in these specific systems. On the other hand, first principles atomic cluster calculations reveal that the debonding behavior is consistent with the formation of strong Si-0 and Al-O bonds across the glass-crystalline interface.

  3. Low-pH SCC: Mechanical effects on crack propagation

    SciTech Connect

    Beavers, J.A.; Hagerdorn, E.L.

    1996-09-06

    A better definition of the role of mechanical factors on low-pH stress corrosion crack propagation is needed to aid in the prediction of crack growth rates on operating pipelines and to develop strategies to mitigate this form of cracking. The overall objective of the project was to determine the roles and synergistic effects of pressure, pressure fluctuations, and hydrotesting on low-pH stress corrosion crack growth. All testing was performed in a low-pH electrolyte (NS4 solution) under cyclic load conditions on pre-cracked specimens of one X-65 line pipe steel. The cyclic load conditions in the testing were related to field conditions using the J-integral parameter. This project consisted of the following three tasks, Task 1 - Development of Test Protocol, Task 2 - Mechanical Effects, and Task 3 - Effects of Hydrotesting. The purposes of Task 1 were to prepare the test specimens and experimental apparatus and to establish a standard test protocol for conducting the cyclic load tests and analyzing the test data. The specimen preparation procedures and environmental conditions were similar to those used in a previous project for TransCanada PipeLines (TCPL). The most significant difference between the tests performed in this project and the previous research was in the mode of loading. The previous work was performed under constant extension rate loading while this project was performed under cyclic load conditions. It is difficult to relate test conditions under constant extension rate loading with field conditions. However, the cyclic load conditions in the laboratory test can be directly related to field test conditions using the J-integral parameter. Modifications also were necessary in the data analysis procedure to account for the change in loading mode.

  4. Multiple-shock initiation via statistical crack mechanics

    SciTech Connect

    Dienes, J.K.; Kershner, J.D.

    1998-12-31

    Statistical Crack Mechanics (SCRAM) is a theoretical approach to the behavior of brittle materials that accounts for the behavior of an ensemble of microcracks, including their opening, shear, growth, and coalescence. Mechanical parameters are based on measured strain-softening behavior. In applications to explosive and propellant sensitivity it is assumed that closed cracks act as hot spots, and that the heating due to interfacial friction initiates reactions which are modeled as one-dimensional heat flow with an Arrhenius source term, and computed in a subscale grid. Post-ignition behavior of hot spots is treated with the burn model of Ward, Son and Brewster. Numerical calculations using SCRAM-HYDROX are compared with the multiple-shock experiments of Mulford et al. in which the particle velocity in PBX 9501 is measured with embedded wires, and reactions are initiated and quenched.

  5. Crack propagation in Hastelloy X

    SciTech Connect

    Weerasooriya, T.; Strizak, J.P.

    1980-05-01

    The fatigue and creep crack growth rates of Hastelloy X were examined both in air and impure helium. Creep crack growth rate is higher in air and impure helium at 650/sup 0/C. Initial creep crack growth from the original sharp fatigue crack is by an intergranular mode of fracture. As the cracking accelerates at higher stress intensities, growth is by a mixed mode of both intergranular and transgranular fracture. Fatigue crack growth rate increases with increasing temperature and decreasing frequency for the range of stress intensities reported in the literature and is lower in impure helium than in air.

  6. Fracture Mechanics of Crack Growth During Sonic-IR Inspection

    NASA Astrophysics Data System (ADS)

    Chen, J. C.; Riddell, W. T.; Lick, Kyle; Wong, Chang-Hwa

    2007-03-01

    In past studies, we showed that cracks synthesized under carefully controlled conditions will propagate when subjected to sonic IR testing. The extent or severity of the propagation observed depended on several parameters including the stress intensity factor (which corresponds to crack growth rate) under which the crack was synthesized, the tightness of the crack closure, and the initial crack length. Furthermore, we showed that crack propagation during sonic IR testing occurs for 2024 aluminum, titanium and 304 stainless steel specimens. In this study, we extend the range of experimental conditions for synthesizing cracks to further elucidate their effect on the crack propagation, and we focus more specifically on the stress intensity factor. The stress intensity factor not only determines the rate of crack growth, but it has two profound effects on crack characteristics: the establishment of plastic zones around the crack tip and the variation of the topography of the mating crack surfaces. These two factors strongly affect crack propagation.

  7. Mechanisms and Modelling of Environment-Dependent Fatigue Crack Growth in a Nickel Based Superalloy

    DTIC Science & Technology

    1991-12-12

    controlling mechanisms of this environment-dependent crack growth stage in Alloy 718 in order to develop the ability to predict the crack growth performance...stage crack-tip oxidation mechanism. According to this mechanism, the oxygen partial pressure controls the preferential formation of the oxide layers at...network. The reduction in grain boundary ductility due to oxidation is balanced by considering the effective strain at the crack tip resulting from

  8. Elastic plastic fracture mechanics methodology for surface cracks

    NASA Technical Reports Server (NTRS)

    Ernst, Hugo A.; Lambert, D. M.

    1994-01-01

    The Elastic Plastic Fracture Mechanics Methodology has evolved significantly in the last several years. Nevertheless, some of these concepts need to be extended further before the whole methodology can be safely applied to structural parts. Specifically, there is a need to include the effect of constraint in the characterization of material resistance to crack growth and also to extend these methods to the case of 3D defects. As a consequence, this project was started as a 36 month research program with the general objective of developing an elastic plastic fracture mechanics methodology to assess the structural reliability of pressure vessels and other parts of interest to NASA which may contain flaws. The project is divided into three tasks that deal with (1) constraint and thickness effects, (2) three-dimensional cracks, and (3) the Leak-Before-Burst (LBB) criterion. This report period (March 1994 to August 1994) is a continuation of attempts to characterize three dimensional aspects of fracture present in 'two dimensional' or planar configuration specimens (Chapter Two), especially, the determination of, and use of, crack face separation data. Also, included, are a variety of fracture resistance testing results (J(m)R-curve format) and a discussion regarding two materials of NASA interest (6061-T651 Aluminum alloy and 1N718-STA1 nickel-base super alloy) involving a bases for like constraint in terms of ligament dimensions, and their comparison to the resulting J(m)R-curves (Chapter Two).

  9. Clarification of stress corrosion cracking mechanism on nickel base alloys in steam generators for their long lifetime assurance

    SciTech Connect

    Nagano, Hiroo; Kajimura, Haruhiko

    1995-12-31

    Thermally treated (TT) Alloys 600 (16%Cr-8%Fe-bal.Ni) and 690 (30%Cr-10%Fe-bal.Ni) have been successfully used in the steam generators of operating pressurized water reactors (PWRs). This paper deals with intergranular stress corrosion cracking (IGSCC) mechanisms in Ni-base alloys in various corrosive environments such as deaerated water, air-saturated chloride medium, and caustic solutions at high temperatures with focus on Cr content and Cr carbide precipitation at grain boundaries in the alloys. Nickel base alloys of high purity, or with different Cr, C, and B contents with different heat treatments were put to various corrosion tests. SCC resistance of Alloy 600 is affected differently by water chemistry of environments, while Alloy 690 is almost immune to the environments investigated: (1) Cr depletion at grain boundaries is clearly detrimental to IGSCC resistance of Alloy 600 in air-saturated water containing Cl{sup {minus}} ions at 300 C. (2) High purity Alloy 600 has weaker SCC resistance in deaerated water at 360 C than commercially available Alloy 600. Cr depletion along grain boundaries is detrimental to the IGSCC resistance, however its detrimental effect disappears when Cr carbides precipitate at grain boundaries in semi-continuous or continuous way. The NiCr{sub 2}O{sub 4} film formed on the metal surfaces enhances the IGSCC resistance. Similar relationship between Cr depletion and Cr carbide precipitation is also observed in Alloy 600 in deaerated caustic solutions at high temperatures. (3) Concerning intergranular attack (IGA), which occurs in oxidizing caustic solutions at high temperature, existence of semi-continuous or continuous Cr carbides improves the IGA resistance regardless of Cr depletion. A dual layer corrosion protective film composed of an upper layer of NiO and lower layer of Cr{sub 2}O{sub 3} formed on metal surfaces, of which formation is accelerated by selective Cr carbide dissolution, may be responsible for the IGA resistance.

  10. Intergranular stress distributions in polycrystalline aggregates of irradiated stainless steel

    NASA Astrophysics Data System (ADS)

    Hure, J.; El Shawish, S.; Cizelj, L.; Tanguy, B.

    2016-08-01

    In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with recently proposed physically-based crystal plasticity constitutive equations validated for neutron-irradiated austenitic stainless steel. Intergranular normal stress probability density functions are found with respect to plastic strain and irradiation level, for uniaxial loading conditions. In addition, plastic slip activity jumps at grain boundaries are also presented. Intergranular normal stress distributions describe, from a statistical point of view, the potential increase of intergranular stress with respect to the macroscopic stress due to grain-grain interactions. The distributions are shown to be well described by a master curve once rescaled by the macroscopic stress, in the range of irradiation level and strain considered in this study. The upper tail of this master curve is shown to be insensitive to free surface effect, which is relevant for IGSCC predictions, and also relatively insensitive to small perturbations in crystallographic texture, but sensitive to grain shapes.

  11. Multiscale Modeling of Intergranular Fracture in Aluminum: Constitutive Relation For Interface Debonding

    NASA Technical Reports Server (NTRS)

    Yamakov, V.; Saether, E.; Glaessgen, E. H.

    2008-01-01

    Intergranular fracture is a dominant mode of failure in ultrafine grained materials. In the present study, the atomistic mechanisms of grain-boundary debonding during intergranular fracture in aluminum are modeled using a coupled molecular dynamics finite element simulation. Using a statistical mechanics approach, a cohesive-zone law in the form of a traction-displacement constitutive relationship, characterizing the load transfer across the plane of a growing edge crack, is extracted from atomistic simulations and then recast in a form suitable for inclusion within a continuum finite element model. The cohesive-zone law derived by the presented technique is free of finite size effects and is statistically representative for describing the interfacial debonding of a grain boundary (GB) interface examined at atomic length scales. By incorporating the cohesive-zone law in cohesive-zone finite elements, the debonding of a GB interface can be simulated in a coupled continuum-atomistic model, in which a crack starts in the continuum environment, smoothly penetrates the continuum-atomistic interface, and continues its propagation in the atomistic environment. This study is a step towards relating atomistically derived decohesion laws to macroscopic predictions of fracture and constructing multiscale models for nanocrystalline and ultrafine grained materials.

  12. Mechanical behavior and failure mechanism of pre-cracked specimen under uniaxial compression

    NASA Astrophysics Data System (ADS)

    Liu, Ting; Lin, Baiquan; Yang, Wei

    2017-08-01

    As a desirable permeability enhancement method, hydraulic slotting has been widely used for enhanced coal bed methane (ECBM) recovery in China. Aiming at the problem that the action mechanism of the slot on the mechanical properties of the slotted coal is still unclear, this paper investigates the effects of flaw inclination on the strength, deformation and cracking process of the pre-cracked specimens. The result shows that the stress-strain curves can be divided into three categories based on the stress behaviors, dropping step by step or dropping sharply, after the peak. With an increase of the flaw inclination, the strength and elastic modulus of the pre-cracked specimen increases gradually, which is verified by the numerical simulation and theoretical results. Analysis of the cracking processes indicates that the initiation position of the first crack in specimens with various flaw inclinations is different, which is caused by the various distributions of tensile and compressive stress concentration zones. The distribution of the stress field controls the cracking process which will in turn affect the stress field distribution. With the propagation of the cracks, the tensile stress concentration zones expand and the concentration degree lowers gradually, while the compressive stress concentration zones show the opposite variation trend. Based on the above results, an optimized slot arrangement method has been proposed for the field application of hydraulic slotting.

  13. Behavior of cracked cylinders under combined thermal and mechanical loading

    SciTech Connect

    Ignaccolo, S.

    1996-12-01

    Nuclear pressure vessels and pipings can be submitted in their life to severe mechanical and thermal loadings. Engineering methods easy to apply, but sufficiently accurate, are needed to assess the flaws. In the field of non-linear fracture mechanics a lot of work has been achieved for structures submitted to mechanical loadings. But for thermal loadings, and particularly for thermal gradients, only few contributions are available. The authors propose, here, to present the main results of a complete set of finite element computations, conducted in France by CEA, EDF and FRAMATOME, on cracked cylinders submitted to combined mechanical and thermal loads. The interaction between these two types of loads is analyzed in the cases of austenitic and ferritic structures. Moreover, these results are compared to the predictions obtained by simplified engineering methods (R6 procedure, J{sub SA16}, and J{sub EDF} approaches). Their domain of validity is also discussed.

  14. The effect of material heterogeneity and random loading on the mechanics of fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Srivatsan, T. S.; Sambandham, M.; Bharucha-Reid, A. T.

    1985-01-01

    This paper reviews experimental work on the influence of variable amplitude or random loads on the mechanics and micromechanisms of fatigue crack growth. Implications are discussed in terms of the crack driving force, local plasticity, crack closure, crack blunting, and microstructure. Due to heterogeneity in the material's microstructure, the crack growth rate varies with crack tip position. Using the weakest link theory, an expression for crack growth rate is obtained as the expectation of a random variable. This expression is used to predict the crack growth rates for aluminum alloys, a titanium alloy, and a nickel steel in the mid-range region. It is observed, using the present theory, that the crack growth rate obeys the power law for small stress intensity factor range, and that the power is a function of a material constant.

  15. Characterization of Cracking Mechanisms of Carbon Anodes Used in Aluminum Industry by Optical Microscopy and Tomography

    NASA Astrophysics Data System (ADS)

    Amrani, Salah; Kocaefe, Duygu; Kocaefe, Yasar; Bhattacharyay, Dipankar; Bouazara, Mohamed; Morais, Brigitte

    2016-10-01

    The objective of this work is to understand the different mechanisms of crack formation in dense anodes used in the aluminum industry. The first approach used is based on the qualitative characterization of the surface cracks and the depth of these cracks. The second approach, which constitutes a quantitative characterization, is carried out by determining the distribution of the crack width along its length as well as the percentage of the surface containing cracks. A qualitative analysis of crack formation was also carried out using 3D tomography. It was observed that mixing and forming conditions have a significant effect on crack formation in green anodes. The devolatilization of pitch during baking causes the formation and propagation of cracks in baked anodes in which large particles control the direction of crack propagation.

  16. Circumferential cracking in steam generator tubes repaired by mechanical sleeving

    SciTech Connect

    Stubbe, J.; Pierson, E.; Laire, C.; Nedden, L. zur; Somville, P.; Royen, P. Van

    1995-12-31

    After one service cycle, leaks were detected in Doel 4 steam generator (SG) tubes repaired by mechanical sleeving (hydraulically + roll expanded). Two tubes were pulled and examined, one of them showing a big leak and the second being, pulled randomly. They both revealed through wall circumferential primary water stress corrosion cracking (PWSCC) at the upper hydraulic transition so that it was concluded that the problem was generic. A thorough assessment of the root causes of failure was undertaken, including stress and strain direct measurement by X-ray diffraction and photoelasticity, local stresses and temperature evaluation by calculation and stress corrosion cracking tests. Stress corrosion tests were carried out in 10 % NAOH environment, on mock-ups manufactured from reserve tubing of the plant simulating not only the upper joint but also the complete assembly (two joints). An estimate of the expected life was performed by comparison with reference mock-ups representative of the roll transitions (including the kiss roll). The findings are that the hydraulic expansion may generate high residual stresses, in spite of the very low residual deformations. Concerning, the temperature however, there are some indications that it could be substantially lower at the level of the cracking than at the tube to tubesheet roll transitions, which makes the quantified evaluation somewhat inaccurate. It is concluded that repair by mechanical sleeving is influenced by many parameters, including details of the installation procedure. Lifetime may be very limited when applied to PWSCC sensitive tubes and must be evaluated by appropriate testing. In particular, corrosion mock-ups should represent the entire sleeve, with both joints.

  17. New probabilistic fracture mechanics approach with neural network-based crack modeling: Its application to multiple cracks problem

    SciTech Connect

    Yoshimura, Shinobu; Lee, J.S.; Yagawa, Genki; Sugioka, Kiyoshi; Kawai, Tadahiko

    1995-11-01

    Studies on efficient utilization and life extension of operating nuclear power plants (NPPs) have become increasingly important since ages of the first-generation NPPs are approaching their design lives. In order to predict a remaining life of each plant, it is necessary to select those critical components that strongly influence the plant life, and to evaluate their remaining lives by considering aging effects of materials and other factors. This paper proposes a new method to incorporate sophisticated crack models, such as interaction and coalescence of multiple surface cracks, into probabilistic fracture mechanism (PFM) computer programs using neural networks. First, hundreds of finite element (FE) calculations of a plate containing multiple surface cracks are performed by parametrically changing crack parameters such as sizes and locations. A fully automated 3D FE analysis system is effectively utilized here. Second, the back-propagation neural network is trained using the FE solutions, i.e. crack parameters vs. their corresponding stress intensity factors (SIFs). After a sufficient number of training iterations, the network attains an ability to promptly output SIFs for arbitrary combinations of crack parameters. The well trained network is then incorporated into the parallel PFM program which runs on one of massively parallel computers composed of 512 processing units. To demonstrate its fundamental performances, the present computer program is applied to evaluate failure probabilities of aged reactor pressure vessels considering interaction and coalescence of two dissimilar semi-elliptical surface cracks.

  18. Elastic-Plastic Fracture Mechanics Analysis of Small Cracks

    DTIC Science & Technology

    1982-09-01

    for small cracks. The data usually involves a plot of the cyclic streys ( Aa ) required to propagate a crack versus the crack size. For large cracks...the data follows the line predicted by LEFM (Aal a - constant) while for short cracks Aa is less than predicted by LEFX. By normalizing the crack length...Prediction, ASTh STP 687, J. Bi. Chang, Ed., American Society of Tzsting and Materi- als, 1979, 16-42. 12. Newman. 3. C., Jr. and RaJu, I. S., "An Empirical

  19. Theoretical study of the elasticity, mechanical behavior, electronic structure, interatomic bonding, and dielectric function of an intergranular glassy film model in prismatic β-Si3N4

    NASA Astrophysics Data System (ADS)

    Ching, W. Y.; Rulis, Paul; Ouyang, Lizhi; Aryal, Sitaram; Misra, Anil.

    2010-06-01

    Microstructures such as intergranular glassy films (IGFs) are ubiquitous in many structural ceramics. They control many of the important physical properties of polycrystalline ceramics and can be influenced during processing to modify the performance of devices that contain them. In recent years, there has been intense research, both experimentally and computationally, on the structure and properties of IGFs. Unlike grain boundaries or dislocations with well-defined crystalline planes, the atomic scale structure of IGFs, their fundamental electronic interactions, and their bonding characteristics are far more complicated and not well known. In this paper, we present the results of theoretical simulations using ab initio methods on an IGF model in β-Si3N4 with prismatic crystalline planes. The 907-atom model has a dimension of 14.533Å×15.225Å×47.420Å . The IGF layer is perpendicular to the z axis, 16.4Å wide, and contains 72 Si, 32 N, and 124 O atoms. Based on this model, the mechanical and elastic properties, the electronic structure, the interatomic bonding, the localization of defective states, the distribution of electrostatic potential, and the optical dielectric function are evaluated and compared with crystalline β-Si3N4 . We have also performed a theoretical tensile experiment on this model by incrementally extending the structure in the direction perpendicular to the IGF plane until the model fully separated. It is shown that fracture occurs at a strain of 9.42% with a maximum stress of 13.9 GPa. The fractured segments show plastic behavior and the formation of surfacial films on the β-Si3N4 . These results are very different from those of a previously studied basal plane model [J. Chen , Phys. Rev. Lett. 95, 256103 (2005)10.1103/PhysRevLett.95.256103] and add insights to the structure and behavior of IGFs in polycrystalline ceramics. The implications of these results and the need for further investigations are discussed.

  20. Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy

    PubMed Central

    Withers, P. J.

    2015-01-01

    To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored. PMID:25624521

  1. Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy.

    PubMed

    Withers, P J

    2015-03-06

    To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored.

  2. NASGRO(registered trademark): Fracture Mechanics and Fatigue Crack Growth Analysis Software

    NASA Technical Reports Server (NTRS)

    Forman, Royce; Shivakumar, V.; Mettu, Sambi; Beek, Joachim; Williams, Leonard; Yeh, Feng; McClung, Craig; Cardinal, Joe

    2004-01-01

    This viewgraph presentation describes NASGRO, which is a fracture mechanics and fatigue crack growth analysis software package that is used to reduce risk of fracture in Space Shuttles. The contents include: 1) Consequences of Fracture; 2) NASA Fracture Control Requirements; 3) NASGRO Reduces Risk; 4) NASGRO Use Inside NASA; 5) NASGRO Components: Crack Growth Module; 6) NASGRO Components:Material Property Module; 7) Typical NASGRO analysis: Crack growth or component life calculation; and 8) NASGRO Sample Application: Orbiter feedline flowliner crack analysis.

  3. Fracture processes and mechanisms of crack growth resistance in human enamel

    NASA Astrophysics Data System (ADS)

    Bajaj, Devendra; Park, Saejin; Quinn, George D.; Arola, Dwayne

    2010-07-01

    Human enamel has a complex micro-structure that varies with distance from the tooth’s outer surface. But contributions from the microstructure to the fracture toughness and the mechanisms of crack growth resistance have not been explored in detail. In this investigation the apparent fracture toughness of human enamel and the mechanisms of crack growth resistance were evaluated using the indentation fracture approach and an incremental crack growth technique. Indentation cracks were introduced on polished surfaces of enamel at selected distances from the occlusal surface. In addition, an incremental crack growth approach using compact tension specimens was used to quantify the crack growth resistance as a Junction of distance from the occlusal surface. There were significant differences in the apparent toughness estimated using the two approaches, which was attributed to the active crack length and corresponding scale of the toughening mechanisms.

  4. Elastic plastic fracture mechanics methodology for surface cracks

    NASA Technical Reports Server (NTRS)

    Ernst, Hugo A.; Boatwright, D. W.; Curtin, W. J.; Lambert, D. M.

    1993-01-01

    The Elastic Plastic Fracture Mechanics (EPFM) Methodology has evolved significantly in the last several years. Nevertheless, some of these concepts need to be extended further before the whole methodology can be safely applied to structural parts. Specifically, there is a need to include the effect of constraint in the characterization of material resistance to crack growth and also to extend these methods to the case of 3D defects. As a consequence, this project was started as a 36 month research program with the general objective of developing an EPFM methodology to assess the structural reliability of pressure vessels and other parts of interest to NASA containing defects. This report covers a computer modelling algorithm used to simulate the growth of a semi-elliptical surface crack; the presentation of a finite element investigation that compared the theoretical (HRR) stress field to that produced by elastic and elastic-plastic models; and experimental efforts to characterize three dimensional aspects of fracture present in 'two dimensional', or planar configuration specimens.

  5. Indirect measurement of the viscosity of the intergranular glass phase in yttria-sintered silicon nitride

    NASA Technical Reports Server (NTRS)

    Dittmar, Mark B.; Drummond, Charles H., III

    1991-01-01

    Dense, sintered Si3N4 possesses a residual intergranular glass phase which softens at high temperatures, resulting in degradation of the ceramic's mechanical properties at high temperatures. An important parameter in the determination of the high temperature mechanical properties of sintered Si3N4 is the temperature-viscosity relationship of the intergranular glass. A method for indirectly measuring the intergranular glass viscosity at a given temperature using physical modelling of a two phase glass crystal microstructure and beam bending viscometry measurements of Si3N4 is described. Intergranular glass viscosities obtained by this method are presented for a yttria sintered Si3N4.

  6. Local Crack Branching as a Mechanism for Instability in Dynamic Fracture

    NASA Astrophysics Data System (ADS)

    Sharon, Eran; Gross, Steven P.; Fineberg, Jay

    1995-06-01

    The motion of a crack in dynamic fracture has been shown to be governed by a dynamical instability causing oscillations in its velocity and structure on the fracture surface. We present experimental evidence indicating that the mechanism for instability is attempted local crack branching. At the instability onset, a crack will locally change its topology and sprout small, microscopic side branches. The trajectories of these local branches are independent of the crack velocity and exhibit scaling behavior. A connection between microscopic and macroscopic crack branching is established.

  7. Phenomena and mechanisms of crack propagation in glass-ceramics.

    PubMed

    Apel, E; Deubener, J; Bernard, A; Höland, M; Müller, R; Kappert, H; Rheinberger, V; Höland, W

    2008-10-01

    Lithium disilicate, leucite and apatite glass-ceramics have become state-of-the-art framework materials in the fabrication of all-ceramic dental restorative materials. The goal of this study was to examine the crack propagation behaviour of these three known glass-ceramic materials after they have been subjected to Vickers indentation and to characterize their crack opening profiles (delta(meas) vs. (a-r)). For this purpose, various methods of optical examination were employed. Optical microscopy investigations were performed to examine the crack phenomena at a macroscopic level, while high-resolution techniques, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), were employed to investigate the crack phenomena at a microscopic level. The crack patterns of the three glass-ceramics vary from fairly straightforward to more complex, depending on the amount of residual glass matrix present in the material. The high-strength lithium disilicate crystals feature a high degree of crosslinking, thereby preventing crack propagation. In this material, the crack propagates only through the residual glass phase, which constitutes 30%-40% by volume. Having a high glass content of more than 65% by volume, the leucite and apatite glass-ceramics show far more complex crack patterns. Cracks in the leucite glass-ceramic propagate through both the glass and crystal phase. The apatite glass-ceramic shows a similar crack behaviour as an inorganic-organic composite material containing nanoscale fillers, which are pulled out in the surroundings of the crack tip. The observed crack behaviour and the calculated K(tip) values of the three types of glass-ceramics were compared to the K(IC) values determined according to the SEVNB method.

  8. Dynamic fracture mechanics analysis for an edge delamination crack

    NASA Technical Reports Server (NTRS)

    Rizzi, Stephen A.; Doyle, James F.

    1994-01-01

    A global/local analysis is applied to the problem of a panel with an edge delamination crack subject to an impulse loading to ascertain the dynamic J integral. The approach uses the spectral element method to obtain the global dynamic response and local resultants to obtain the J integral. The variation of J integral along the crack front is shown. The crack behavior is mixed mode (Mode 2 and Mode 3), but is dominated by the Mode 2 behavior.

  9. Updated Fatigue-Crack-Growth And Fracture-Mechanics Software

    NASA Technical Reports Server (NTRS)

    Forman, Royce G.; Shivakumar, Venkataraman; Newman, James C., Jr.

    1995-01-01

    NASA/FLAGRO 2.0 developed as analytical aid in predicting growth and stability of preexisting flaws and cracks in structural components of aerospace systems. Used for fracture-control analysis of space hardware. Organized into three modules to maximize efficiency in operation. Useful in: (1) crack-instability/crack-growth analysis, (2) processing raw crack-growth data from laboratory tests, and (3) boundary-element analysis to determine stresses and stress-intensity factors. Written in FORTRAN 77 and ANSI C.

  10. Accommodating and cracking mechanisms in low-cycle fatigue

    NASA Technical Reports Server (NTRS)

    Pineau, A.

    1978-01-01

    The three main stages of fatigue life (accommodation, crack initiation and crack growth) are briefly reviewed. The cyclic behavior of annealed or predeformed face-centered cubic metals is described. Moreover, two types of alloys (Al-4-Cu and WASPALOY) are examined regarding the influence of the interactions between the precipitates and the dislocations on the cyclic behavior. Data on the percent of life to crack initiation (for a microcrack smaller than about 100 microns) are also given. Finally, experimental and theoretical results on crack growth rates in lowcycle fatigue are described.

  11. Comparative tensile and torsion tests as a method of determination of the crack growth mechanism in corrosion cracking

    SciTech Connect

    Marichev, V.A.

    1987-11-01

    The author present and review tensile and torsional testing techniques for determining the contribution of electrochemical corrosion and hydrogen embrittlement to the mechanics of crack propagation in aluminium and molybdenum alloys and in a nickel-chromium steel. They suggest and analyze testing parameters and considerations whose implementations are shown to increase the accuracy of tensile and torsion determinations of fracture and embrittlement behavior.

  12. Thermal-Mechanical Response of Cracked Satin Weave CFRP Composites at Cryogenic Temperatures

    NASA Astrophysics Data System (ADS)

    Watanabe, S.; Shindo, Y.; Narita, F.; Takeda, T.

    2008-03-01

    This paper examines the thermal-mechanical response of satin weave carbon fiber reinforced polymer (CFRP) laminates with internal and/or edge cracks subjected to uniaxial tension load at cryogenic temperatures. Cracks are considered to occur in the transverse fiber bundles and extend through the entire thickness of the fiber bundles. Two-dimentional generalized plane strain finite element models are developed to study the effects of residual thermal stresses and cracks on the mechanical behavior of CFRP woven laminates. A detailed examination of the Young's modulus and stress distributions near the crack tip is carried out which provides insight into material behavior at cryogenic temperatures.

  13. THERMAL-MECHANICAL RESPONSE OF CRACKED SATIN WEAVE CFRP COMPOSITES AT CRYOGENIC TEMPERATURES

    SciTech Connect

    Watanabe, S.; Shindo, Y.; Narita, F.; Takeda, T.

    2008-03-03

    This paper examines the thermal-mechanical response of satin weave carbon fiber reinforced polymer (CFRP) laminates with internal and/or edge cracks subjected to uniaxial tension load at cryogenic temperatures. Cracks are considered to occur in the transverse fiber bundles and extend through the entire thickness of the fiber bundles. Two-dimentional generalized plane strain finite element models are developed to study the effects of residual thermal stresses and cracks on the mechanical behavior of CFRP woven laminates. A detailed examination of the Young's modulus and stress distributions near the crack tip is carried out which provides insight into material behavior at cryogenic temperatures.

  14. Cracking mechanisms in thermally cycled Ti-6Al-4V reinforced with SiC fibers

    SciTech Connect

    Thomin, S.H.; Dunand, D.C.; Noel, P.A.

    1995-04-01

    A titanium alloy (Ti-6Al-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200 C and 700 C in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix cracks emanating from the specimen surface. These cracks also give oxygen access to fibers, further resulting in fiber degradation. The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating within cracks. Matrix stresses are determined using an analytical model, considering stress relaxation by matrix creep and the temperature dependence of materials properties. Matrix fatigue from these cyclically varying stresses (mechanism (1)) cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to be another necessary damage mechanism. Furthermore, an approximate solution for the stress intensity resulting from crack wedging by oxide formation (mechanism (3)) is given, which may be an operating mechanism as well for long cracks.

  15. Augmented finite-element method for arbitrary cracking and crack interaction in solids under thermo-mechanical loadings

    PubMed Central

    Jung, J.; Do, B. C.

    2016-01-01

    In this paper, a thermal–mechanical augmented finite-element method (TM-AFEM) has been proposed, implemented and validated for steady-state and transient, coupled thermal–mechanical analyses of complex materials with explicit consideration of arbitrary evolving cracks. The method permits the derivation of explicit, fully condensed thermal–mechanical equilibrium equations which are of mathematical exactness in the piece-wise linear sense. The method has been implemented with a 4-node quadrilateral two-dimensional (2D) element and a 4-node tetrahedron three-dimensional (3D) element. It has been demonstrated, through several numerical examples that the new TM-AFEM can provide significantly improved numerical accuracy and efficiency when dealing with crack propagation problems in 2D and 3D solids under coupled thermal–mechanical loading conditions. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’. PMID:27242303

  16. Computational modeling of the mechanism of hydrogen embrittlement (HE) and stress corrosion cracking (SCC) in metals

    NASA Astrophysics Data System (ADS)

    Cendales, E. D.; Orjuela, F. A.; Chamarraví, O.

    2016-02-01

    In this article theoretical models and some existing data sets were examined in order to model the two main causes (hydrogen embrittlement and corrosion-cracking under stress) of the called environmentally assisted cracking phenomenon (EAC). Additionally, a computer simulation of flat metal plate subject to mechanical stress and cracking due both to hydrogen embrittlement and corrosion was developed. The computational simulation was oriented to evaluate the effect on the stress-strain behavior, elongation percent and the crack growth rate of AISI SAE 1040 steel due to three corrosive enviroments (H2 @ 0.06MPa; HCl, pH=1.0; HCl, pH=2.5). From the computer simulation we conclude that cracking due to internal corrosion of the material near to the crack tip limits affects more the residual strength of the flat plate than hydrogen embrittlement and generates a failure condition almost imminent of the mechanical structural element.

  17. Fracture mechanics analyses of partial crack closure in shell structures

    NASA Astrophysics Data System (ADS)

    Zhao, Jun

    2007-12-01

    This thesis presents the theoretical and finite element analyses of crack-face closure behavior in shells and its effect on the stress intensity factor under a bending load condition. Various shell geometries, such as spherical shell, cylindrical shell containing an axial crack, cylindrical shell containing a circumferential crack and shell with double curvatures, are all studied. In addition, the influence of material orthotropy on the crack closure effect in shells is also considered. The theoretical formulation is developed based on the shallow shell theory of Delale and Erdogan, incorporating the effect of crack-face closure at the compressive edges. The line-contact assumption, simulating the crack-face closure at the compressive edges, is employed so that the contact force at the closure edges is introduced, which can be translated to the mid-plane of the shell, accompanied by an additional distributed bending moment. The unknown contact force is computed by solving a mixed-boundary value problem iteratively, that is, along the crack length, either the normal displacement of the crack face at the compressive edges is equal to zero or the contact pressure is equal to zero. It is found that due to the curvature effects crack closure may not always occur on the entire length of the crack, depending on the direction of the bending load and the geometry of the shell. The crack-face closure influences significantly the magnitude of the stress intensity factors; it increases the membrane component but decreases the bending component. The maximum stress intensity factor is reduced by the crack-face closure. The significant influence of geometry and material orthotropy on rack closure behavior in shells is also predicted based on the analytical solutions. Three-dimensional FEA is performed to validate the theoretical solutions. It demonstrates that the crack face closure occurs actually over an area, not on a line, but the theoretical solutions of the stress intensity

  18. The study on ``load relief`` mechanism of multiple cracks in thick-wall cylinder

    SciTech Connect

    Zhang, Y.H.; Huang, Z.Z.; Tan, Y.; Chen, L.Y.; Pan, B.Z.

    1995-11-01

    In this paper, the stress field on a given cross section in a thick-wall cylinder with single or multiple cracks is analyzed by means of 3-D photoelastic. Based on the study of the effect of crack on stress field, the concept of ``Additional Bending Moment`` is presented and the expression for non-dimensional ABM, M, is derived. The ``load relief`` mechanism of multiple cracks in a thick-wall cylinder is studied.

  19. Controlling stress corrosion cracking in mechanism components of ground support equipment

    NASA Technical Reports Server (NTRS)

    Majid, W. A.

    1988-01-01

    The selection of materials for mechanism components used in ground support equipment so that failures resulting from stress corrosion cracking will be prevented is described. A general criteria to be used in designing for resistance to stress corrosion cracking is also provided. Stress corrosion can be defined as combined action of sustained tensile stress and corrosion to cause premature failure of materials. Various aluminum, steels, nickel, titanium and copper alloys, and tempers and corrosive environment are evaluated for stress corrosion cracking.

  20. Three-dimensional EBSD characterization of thermo-mechanical fatigue crack morphology in compacted graphite iron

    SciTech Connect

    Pirgazi, Hadi; Ghodrat, Sepideh; Kestens, Leo A.I.

    2014-04-01

    In cylinder heads made of compacted graphitic iron (CGI), heating and cooling cycles can lead to localized cracking due to thermo-mechanical fatigue (TMF). To meticulously characterize the complex crack path morphology of CGI under TMF condition, in relation to microstructural features and to find out how and by which mechanisms the cracks predominantly develop, three-dimensional electron back scattering diffraction (EBSD) was employed. Based on the precise quantitative microstructural analysis, it is found that graphite particles not only play a crucial role in the crack initiation, but also are of primary significance for crack propagation, i.e. crack growth is enhanced by the presence of graphite particles. Furthermore, the density of graphite particles on the fracture plane is more than double as high as in any other arbitrary plane of the structure. The obtained results did not indicate a particular crystallographic preference of fracture plane, i.e. the crystal plane parallel to the fracture plane was nearly of random orientation. - Highlights: • Crystallographic features of a thermo-mechanical fatigue (TMF) crack were studied. • Wide-field 3D EBSD is used to characterize the TMF crack morphology. • Data processing was applied on a large length scale of the order of millimeters. • Graphite density in the fracture plane is much higher than any other random plane. • It is revealed that crack growth is enhanced by the presence of graphite particles.

  1. Mechanisms of coke formation and fouling in thermal cracking

    SciTech Connect

    Lott, R.K.; Rangwala, H.A.; Hsi, C.

    1995-12-31

    When heavy oil is cracked to produce distillate, coking of the reacting liquid is, in general, preceded by formation of a new, highly viscous liquid phase, rich in coke precursors. Results from pilot-scale experiments using feedstocks from Gudao (China) reported here show that inert-gas stripping of light distillates from the reacting liquid strongly inhibits coking and possibly the partition of precursors into the new phase. Heavy oil, rich in asphaltene, is often reported to have a high coking propensity. This paper provides experimental evidence to show that the asphaltene concentration is not the most critical factor in the coking propensity of heavy oil. Autoclave tests show that the liquid product could contain more than 40% of asphaltene, and yield only 60% of the coke produced by similar tests in which the liquid product contains less than 20% asphaltene. The solubility of asphaltene in the reaction liquid is the most crucial factor affecting coke yield. It controls the coking mechanisms and the fouling tendency of the resulting coke.

  2. Mechanical properties and crack growth behavior of polycrystalline copper using molecular dynamics simulation

    NASA Astrophysics Data System (ADS)

    Qiu, Ren-Zheng; Li, Chi-Chen; Fang, Te-Hua

    2017-08-01

    This study investigated the mechanical properties and crack propagation behavior of polycrystalline copper using a molecular dynamics simulation. The effects of temperature, grain size, and crack length were evaluated in terms of atomic trajectories, slip vectors, common neighbor analysis, the material’s stress-strain diagram and Young’s modulus. The simulation results show that the grain boundary of the material is more easily damaged at high temperatures and that grain boundaries will combine at the crack tip. From the stress-strain diagram, it was observed that the maximum stress increased as the temperature decreased. In contrast, the maximum stress was reduced by increasing the temperature. With regard to the effect of the grain size, when the grain size was too small, the structure of the sample deformed due to the effect of atomic interactions, which caused the grain boundary structure to be disordered in general. However, when the grain size was larger, dislocations appeared and began to move from the tip of the crack, which led to a new dislocation phenomenon. With regards to the effect of the crack length, the tip of the crack did not affect the sample’s material when the crack length was less than 5 nm. However, when the crack length was above 7.5 nm, the grain boundary was damaged, and twinning structures and dislocations appeared on both sides of the crack tip. This is because the tip of the crack was blunt at first before sharpening due to the dislocation effect.

  3. Cryogenic mechanical response of multilayer satin weave CFRP composites with cracks

    NASA Astrophysics Data System (ADS)

    Watanabe, S.; Shindo, Y.; Takeda, T.; Narita, F.

    2008-07-01

    We deal with the thermomechanical response of multilayer satin weave carbon-fiber-reinforced polymer (CFRP) laminates with internal and/or edge cracks and temperature-dependent material properties subjected to tensile loading at cryogenic temperatures. The composite material is assumed to be under the generalized plane strain. Cracks are located in the transverse fiber bundles and extend to the interfaces between two fiber bundles. A finite-element model is employed to study the influence of residual thermal stresses on the mechanical behavior of multilayer CFRP woven laminates with cracks. Numerical calculations are carried out, and Young's modulus and stress distributions near the crack tip are shown graphically.

  4. Uncommon deformation mechanisms during fatigue-crack propagation in nanocrystalline alloys.

    PubMed

    Cheng, Sheng; Lee, Soo Yeol; Li, Li; Lei, Changhui; Almer, Jon; Wang, Xun-Li; Ungar, Tamas; Wang, Yinmin; Liaw, Peter K

    2013-03-29

    The irreversible damage at cracks during the fatigue of crystalline solids is well known. Here we report on in situ high-energy x-ray evidence of reversible fatigue behavior in a nanocrystalline NiFe alloy both in the plastic zone and around the crack tip. In the plastic zone, the deformation is fully recoverable as the crack propagates, and the plastic deformation invokes reversible interactions of dislocation and twinning in the nanograins. But around the crack tip lies a regime with reversible grain lattice reorientation promoted by a change of local stress state. These observations suggest unprecedented fatigue deformation mechanisms in nanostructured systems that are not addressed theoretically.

  5. Scaling of Crack Surfaces and Implications for Fracture Mechanics

    NASA Astrophysics Data System (ADS)

    Morel, Stéphane; Schmittbuhl, Jean; Bouchaud, Elisabeth; Valentin, Gérard

    2000-08-01

    The scaling laws describing the roughness development of crack surfaces are incorporated into the Griffith criterion. We show that, in the case of a Family-Vicsek scaling, the energy balance leads to a purely elastic brittle behavior. On the contrary, it appears that an anomalous scaling reflects an R-curve behavior associated with a size effect of the critical resistance to crack growth in agreement with the fracture process of heterogeneous brittle materials exhibiting a microcracking damage.

  6. Scaling of crack surfaces and implications for fracture mechanics

    PubMed

    Morel; Schmittbuhl; Bouchaud; Valentin

    2000-08-21

    The scaling laws describing the roughness development of crack surfaces are incorporated into the Griffith criterion. We show that, in the case of a Family-Vicsek scaling, the energy balance leads to a purely elastic brittle behavior. On the contrary, it appears that an anomalous scaling reflects an R-curve behavior associated with a size effect of the critical resistance to crack growth in agreement with the fracture process of heterogeneous brittle materials exhibiting a microcracking damage.

  7. Controlled Mechanical Cracking of Metal Films Deposited on Polydimethylsiloxane (PDMS)

    PubMed Central

    Polywka, Andreas; Stegers, Luca; Krauledat, Oliver; Riedl, Thomas; Jakob, Timo; Görrn, Patrick

    2016-01-01

    Stretchable large area electronics conform to arbitrarily-shaped 3D surfaces and enables comfortable contact to the human skin and other biological tissue. There are approaches allowing for large area thin films to be stretched by tens of percent without cracking. The approach presented here does not prevent cracking, rather it aims to precisely control the crack positions and their orientation. For this purpose, the polydimethylsiloxane (PDMS) is hardened by exposure to ultraviolet radiation (172 nm) through an exposure mask. Only well-defined patterns are kept untreated. With these soft islands cracks at the hardened surface can be controlled in terms of starting position, direction and end position. This approach is first investigated at the hardened PDMS surface itself. It is then applied to conductive silver films deposited from the liquid phase. It is found that statistical (uncontrolled) cracking of the silver films can be avoided at strain below 35%. This enables metal interconnects to be integrated into stretchable networks. The combination of controlled cracks with wrinkling enables interconnects that are stretchable in arbitrary and changing directions. The deposition and patterning does not involve vacuum processing, photolithography, or solvents.

  8. Crack propagation and the material removal mechanism of glass-ceramics by the scratch test.

    PubMed

    Qiu, Zhongjun; Liu, Congcong; Wang, Haorong; Yang, Xue; Fang, Fengzhou; Tang, Junjie

    2016-12-01

    To eliminate the negative effects of surface flaws and subsurface damage of glass-ceramics on clinical effectiveness, crack propagation and the material removal mechanism of glass-ceramics were studied by single and double scratch experiments conducted using an ultra-precision machine. A self-manufactured pyramid shaped single-grit tool with a small tip radius was used as the scratch tool. The surface and subsurface crack propagations and interactions, surface morphology and material removal mechanism were investigated. The experimental results showed that the propagation of lateral cracks to the surface and the interaction between the lateral cracks and radial cracks are the two main types of material peeling, and the increase of the scratch depth increases the propagation angle of the radial cracks and the interaction between the cracks. In the case of a double scratch, the propagation of lateral cracks and radial cracks between paired scratches results in material peeling. The interaction between adjacent scratches depends on the scratch depth and separation distance. There is a critical separation distance where the normalized material removal volume reaches its peak. These findings can help reduce surface flaws and subsurface damage induced by the grinding process and improve the clinical effectiveness of glass-ceramics used as biological substitute and repair materials.

  9. Effects of Water Intrusion on Mechanical Properties of and Crack Propagation in Coal

    NASA Astrophysics Data System (ADS)

    Yao, Qiangling; Chen, Tian; Ju, Minghe; Liang, Shun; Liu, Yapeng; Li, Xuehua

    2016-12-01

    Studying the mechanical properties of and crack propagation in coal after water intrusion is necessary to tackle a number of geological engineering problems such as those associated with underground water storage in collieries and support for underground roadways in coal mines. To study the mechanical properties and crack development, 12 coal samples with moisture contents of 0, 2.37, 3.78 and 5.29 % were prepared for acoustic emission tests under uniaxial compression. Over about 6 days, the coal samples absorbed moisture from a humidifier in three different phases. In this period, uniaxial tests show that the peak stress, elastic modulus, strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased. It was further found that, by analysing the relationship between the stiffness and stress and the accumulated acoustic emission counts, all the phases of crack development can be evaluated. This is useful for studying the effect of water intrusion on crack propagation and for calculating the mechanical properties of the coal such as the elastic modulus. This investigation also quantifies the percentage of the stress thresholds for crack closure, crack initiation, and crack damage that constitutes the peak stress. These stress thresholds do not change with moisture content. Our results are of great significance for water storage in coal mines, for determination of pillar dimensions in coal mines, and for expanding the knowledge base of the mechanical properties of coal and the characteristics of crack propagation.

  10. Mechanisms and Kinetics of Environmentally Assisted Cracking: Current Status, Issues, and Suggestions for Further Work

    NASA Astrophysics Data System (ADS)

    Lynch, S. P.

    2013-03-01

    Mechanisms and kinetics of metal-induced embrittlement, hydrogen-embrittlement, and stress-corrosion cracking are discussed, and long-standing controversies are addressed by reviewing critical observations. Recommendations are also made regarding further work (including repetition of previous work using more advanced measurement and characterisation techniques) that should be carried out in order to resolve some of the contentious issues. The evidence to date suggests that adsorption-based mechanisms, involving weakening of substrate interatomic bonds so that dislocation emission or decohesion is facilitated, accounts for embrittlement in many systems. Embrittling adsorbed species include some metal atoms, hydrogen, and complex ions produced by de-alloying. Other viable mechanisms of embrittlement include those based on (1) dissolution of anodic grain-boundary regions, and (2) decohesion at grain boundaries owing to segregated hydrogen and impurities. The hydrogen-enhanced localised-plasticity mechanism, based on solute hydrogen facilitating dislocation activity in the plastic zone ahead of cracks, makes a contribution in some cases, but is relatively unimportant compared with these other mechanisms for most fracture modes. The film-induced cleavage mechanism, proposed especially for stress-corrosion cracking in systems involving de-alloying at crack tips, is questionable on numerous grounds, and is probably not viable. Rate-controlling processes for environmentally assisted cracking are not well established, except for solid-metal induced embrittlement where surface self-diffusion of embrittling atoms to crack tips controls cracking kinetics. In some systems, adsorption kinetics are probably rate-controlling for liquid-metal embrittlement, hydrogen-environment embrittlement, and stress-corrosion cracking. In other cases, rate-controlling processes could include the rate of anodic or cathodic reactions at and behind crack tips (responsible for producing embrittling

  11. Microstructural indicators of transition mechanisms in time-dependent fatigue crack growth in nickel base superalloys

    NASA Astrophysics Data System (ADS)

    Heeter, Ann E.

    Gas turbine engines are an important part of power generation in modern society, especially in the field of aerospace. Aerospace engines are design to last approximately 30 years and the engine components must be designed to survive for the life of the engine or to be replaced at regular intervals to ensure consumer safety. Fatigue crack growth analysis is a vital component of design for an aerospace component. Crack growth modeling and design methods date back to an origin around 1950 with a high rate of accuracy. The new generation of aerospace engines is designed to be efficient as possible and require higher operating temperatures than ever seen before in previous generations. These higher temperatures place more stringent requirements on the material crack growth performance under creep and time dependent conditions. Typically the types of components which are subject to these requirements are rotating disk components which are made from advanced materials such as nickel base superalloys. Traditionally crack growth models have looked at high temperature crack growth purely as a function of temperature and assumed that all crack growth was either controlled by a cycle dependent or time dependent mechanism. This new analysis is trying to evaluate the transition between cycle-dependent and time-dependent mechanism and the microstructural markers that characterize this transitional behavior. The physical indications include both the fracture surface morphology as well as the shape of the crack front. The research will evaluate whether crack tunneling occurs and whether it consistently predicts a transition from cycle-dependent crack growth to time-dependent crack growth. The study is part of a larger research program trying to include the effects of geometry, mission profile and environmental effects, in addition to temperature effects, as a part of the overall crack growth system. The outcome will provide evidence for various transition types and correlate those

  12. An extension of fracture mechanics/technology to larger and smaller cracks/defects.

    PubMed

    Abé, Hiroyuki

    2009-01-01

    Fracture mechanics/technology is a key science and technology for the design and integrity assessment of the engineering structures. However, the conventional fracture mechanics has mostly targeted a limited size of cracks/defects, say of from several hundred microns to several tens of centimeters. The author and his group has tried to extend that limited size and establish a new version of fracture technology for very large cracks used in geothermal energy extraction and for very small cracks/defects or damage often appearing in the combination of mechanical and electronic components of engineering structures. Those new versions are reviewed in this paper.

  13. Cracked finite elements proposed for NASTRAN. [based on application of finite element method to fracture mechanics

    NASA Technical Reports Server (NTRS)

    Aberson, J. A.; Anderson, J. M.

    1973-01-01

    The recent introduction of special crack-tip singularity elements, usually referred to as cracked elements, has brought the power and flexibility of the finite-element method to bear much more effectively on fracture mechanics problems. This paper recalls the development of two cracked elements and presents the results of some applications proving their accuracy and economy. Judging from the available literature on numerical methods in fracture mechanics, it seems clear that the elements described have been used more extensively than any others in practical fracture mechanics applications.

  14. An extension of fracture mechanics/technology to larger and smaller cracks/defects

    PubMed Central

    Abé, Hiroyuki

    2009-01-01

    Fracture mechanics/technology is a key science and technology for the design and integrity assessment of the engineering structures. However, the conventional fracture mechanics has mostly targeted a limited size of cracks/defects, say of from several hundred microns to several tens of centimeters. The author and his group has tried to extend that limited size and establish a new version of fracture technology for very large cracks used in geothermal energy extraction and for very small cracks/defects or damage often appearing in the combination of mechanical and electronic components of engineering structures. Those new versions are reviewed in this paper. PMID:19907123

  15. Generation mechanism of nonlinear ultrasonic Lamb waves in thin plates with randomly distributed micro-cracks.

    PubMed

    Zhao, Youxuan; Li, Feilong; Cao, Peng; Liu, Yaolu; Zhang, Jianyu; Fu, Shaoyun; Zhang, Jun; Hu, Ning

    2017-08-01

    Since the identification of micro-cracks in engineering materials is very valuable in understanding the initial and slight changes in mechanical properties of materials under complex working environments, numerical simulations on the propagation of the low frequency S0 Lamb wave in thin plates with randomly distributed micro-cracks were performed to study the behavior of nonlinear Lamb waves. The results showed that while the influence of the randomly distributed micro-cracks on the phase velocity of the low frequency S0 fundamental waves could be neglected, significant ultrasonic nonlinear effects caused by the randomly distributed micro-cracks was discovered, which mainly presented as a second harmonic generation. By using a Monte Carlo simulation method, we found that the acoustic nonlinear parameter increased linearly with the micro-crack density and the size of micro-crack zone, and it was also related to the excitation frequency and friction coefficient of the micro-crack surfaces. In addition, it was found that the nonlinear effect of waves reflected by the micro-cracks was more noticeable than that of the transmitted waves. This study theoretically reveals that the low frequency S0 mode of Lamb waves can be used as the fundamental waves to quantitatively identify micro-cracks in thin plates. Copyright © 2017 Elsevier B.V. All rights reserved.

  16. Fracture mechanics applied to elevated temperature crack growth

    NASA Technical Reports Server (NTRS)

    Jordan, E. H.; Meyers, G. J.

    1989-01-01

    Twenty-six isothermal crack growth tests were performed on Hastelloy-X tubular specimens at a variety of temperatures and strain ranges. Conditions were selected to include nominally elastic and nominally plastic conditions. A number of parameters including the stress intensity factor, strain intensity factor, J-integral, Crack Opening Displacement, and Tompkins model were examined for their ability to correlate the data. Test conditions were selected such that growth rates at a single value of the parameter were obtained at radially different crack lengths, thus exploring the geometry independence of the correlating parameter. None of the parameters were fully satisfactory. However, COD calculated from J-integral appeared to be the most successful.

  17. Environmental cracking of Alloy 600 in BWR environments

    SciTech Connect

    Ljungberg, L.G.

    1991-03-01

    Alloy 600 may be sensitized to intergranular stress corrosion cracking (IGSCC) in both BWR normal water chemistry (NWC) and in hydrogen water chemistry (HWC). Carbide precipitation causes chromium depleted grain boundaries, which are susceptible to IGSCC in NWC but not in HWC. The mechanism for chromium depletion is complex due to the existence of two chromium carbides. Segregated phosphorous enhances IGSCC of Alloy 600 with chromium depleted grain boundaries in NWC. Under all other conditions phosphorous seems harmless. Grain boundary segregated sulfur is suspect of enhancing IGSCC in both NWC and HWC, but no verifying tests have been performed. Boron and nitrogen have a strong tendency to segregate to grain boundaries in Alloy 600, but seem to be harmless. Only few crack propagation data relevant to BWR conditions exist for Alloy 600. The paper describes mechanisms for sensitization, stress corrosion cracking, and crack propagation rates. 94 refs., 21 figs., 1 tab.

  18. Energy absorption mechanisms during crack propagation in metal matrix composites

    NASA Technical Reports Server (NTRS)

    Murphy, D. P.; Adams, D. F.

    1979-01-01

    The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models.

  19. Environment enhanced fatigue crack propagation in metals: Inputs to fracture mechanics life prediction models

    NASA Technical Reports Server (NTRS)

    Gangloff, Richard P.; Kim, Sang-Shik

    1993-01-01

    This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.

  20. Measurement and Modeling of Hydrogen Environment-Assisted Cracking in a Ni-Cu-Al-Ti Superalloy

    NASA Astrophysics Data System (ADS)

    Burns, James T.; Harris, Zachary D.; Dolph, Justin D.; Gangloff, Richard P.

    2016-03-01

    This research improves H decohesion mechanism-based modeling of intergranular stress corrosion cracking in a Ni-Cu superalloy, Monel K-500. New cracking data plus improved model parameters lead to accurate predictions of the cathodic potential dependencies of K TH and H diffusion-limited d a/d t II for Monel K-500 under slow-rising K in 0.6 M NaCl solution. Experiments and modeling demonstrate that IGSCC is eliminated for applied potentials more positive than a critical level between -900 and -840 mVSCE, but slow-subcritical cracking persists by a microvoid-based mechanism.

  1. Fracture mechanics of matrix cracking and delamination in glass/epoxy laminates

    NASA Technical Reports Server (NTRS)

    Caslini, M.; Zanotti, C.; Obrien, T. K.

    1986-01-01

    This study focused on characterizing matrix cracking and delamination behavior in multidirectional laminates. Static tension and tension-tension fatigue tests were conducted on two different layups. Damage onset, accumulation, and residual properties were measured. Matrix cracking was shown to have a considerable influence on residual stiffness of glass epoxy laminates, and could be predicted reasonably well for cracks in 90 deg piles using a simple shear lag analysis. A fracture mechanics analysis for the strain energy release rate associated with 90 deg ply-matrix crack formation was developed and was shown to correlate the onset of 90 deg ply cracks in different laminates. The linear degradation of laminate modulus with delamination area, previously observed for graphite epoxy laminates, was predicted for glass epoxy laminates using a simple rule of mixtures analysis. The strain energy release rate associated with edge delamination formation under static and cyclic loading was difficult to analyze because of the presence of several contemporary damage phenomena.

  2. About the mechanism of stress corrosion cracking of Alloy 600 in high temperature water

    SciTech Connect

    Rebak, R.B.; Szklarska-Smialowska, Z.

    1995-12-31

    Alloy 600 is a material commonly used to construct the tubing in the steam generators (SG) of pressurized light water reactors (PWR) and of CANDU heavy water reactors. It is well established which variables and to which extent they influence the crack growth rate (CGR) in Alloy 600 exposed to high temperature (deaerated) water (HTW), especially in very aggressive conditions. There is evidence that the same variables that influence CGR also control the crack induction time. However, there are only a few data on crack induction time and no detailed explanation of the events that lead to the nucleation of a crack on an apparent smooth tube surface. In this paper, a critical review of the mechanisms of stress corrosion cracking (SCC) is given and, an interpretation of the events occurring during the long ({approx} 15 y) induction times observed in plant is postulated.

  3. A model for the effective thermal and mechanical properties of cracked UO/sub 2/ pellets

    SciTech Connect

    Williford, R.E.; Lanning, D.D.; Mohr, C.L.

    1982-02-01

    An alternate thermal-mechanical behavior model for cracked UO/sub 2/ pelletized fuel is presented. It is recognized that fuel cracking and relocation cause some of the initial pellet-cladding gap (the ''free area'') to be moved into the fuel in the form of cracks. The introduction of this free area into the fuel causes the fuel effective thermal conductivity and effective elastic moduli to be simultaneously reduced to values significantly less than laboratory data for solid pellets. Hooke's Law and a crack compliance model are used to deduce the effective fuel conductivity and moduli from simultaneous in-reactor measurements of rod power, fuel center temperature, and cladding elongation. The fuel-cladding ''gap'' is considered as another ''crack,'' and is also described by the crack compliance model, which predicts that there is always some finite amount of fuelcladding contact. The primary thermal mechanical feedback mechanism is found to be due to crack closure effects on fuel effective thermal conductivity, rather than gap closure effects on gap conductance.

  4. Underclad cracking of pressure vessel steels for light-water reactors

    SciTech Connect

    Lopez, H.F.

    1987-06-01

    Although fracture mechanics analyses have shown that underclad cracks have no detrimental effect on the integrity of thick walled pressure vessels (40 year service), in order to avoid unexpected failures the US Nuclear Regulatory Commission has issued Regulatory Guide 1.43 which sets limits on the extent of fissures permitted and describes acceptable means of controlling the weld cladding processes. Cavitation and intergranular fissuring in SA508-2 and 22NiMoCr37 steels can occur in the presence or absence of intergranular particles. The observations of intergranular fissuring and cavitation in those HAZ free from overlapping effects are attributed to grain boundary segregation. Other probable void nucleation sites are the grain boundary-lath interface intersections which facilitate the formation of grain boundary discontinuities.

  5. Magnetic Field Twisting by Intergranular Downdrafts

    NASA Astrophysics Data System (ADS)

    Taroyan, Youra; Williams, Thomas

    2016-10-01

    The interaction of an intergranular downdraft with an embedded vertical magnetic field is examined. It is demonstrated that the downdraft may couple to small magnetic twists leading to an instability. The descending plasma exponentially amplifies the magnetic twists when it decelerates with depth due to increasing density. Most efficient amplification is found in the vicinity of the level, where the kinetic energy density of the downdraft reaches equipartition with the magnetic energy density. Continual extraction of energy from the decelerating plasma and growth in the total azimuthal energy occurs as a consequence of the wave-flow coupling along the downdraft. The presented mechanism may drive vortices and torsional motions that have been detected between granules and in simulations of magnetoconvection.

  6. Molecular dynamics based study and characterization of deformation mechanisms near a crack in a crystalline material

    NASA Astrophysics Data System (ADS)

    Zhang, Jiaxi; Ghosh, Somnath

    2013-08-01

    Modeling crack propagation in crystalline materials is a challenging enterprise due to complexities induced by the interaction of the crack with various deformation mechanisms such as dislocation, micro twin, stacking faults etc.. As a first step toward the development of physics-based models of deformation in the presence of a crack, this paper proposes a comprehensive approach based on molecular dynamics simulations of a crystalline material with an embedded crack. The MD-based framework invokes a sequence of four tasks to accomplish the overall goal, viz. (i) MD simulation, (ii) characterization of atomic-level crack and deformation mechanisms, (iii) quantification of atomic-level deformation mechanisms and crack, and (iv) response analysis. Effective characterization methods like CNA, DXA and deformation gradient analysis followed by quantification are able to delineate the crack length/opening, dislocation structure and microtwins at a high resolution. Interactions of the crack with the dislocation networks and microtwins under mode I loading conditions are investigated for different lattice orientations. Crystal orientation has significant effect on the mechanisms activation and evolution. An important study is made through partitioning of the total energy into recoverable elastic energy, defect energy and inelastic dissipation, and correlating them with deformation characteristics such as dislocation density and twin volume fraction. Finally, a simple mechanistic model of deformation is developed, which associates dislocation density evolution with the stress-strain response in a crystalline material in the presence of a crack. Results show good quantitative agreement of material softening and hardening behavior with direct MD simulation results. The model can be further used to estimate the range of strain-rates that may be applied for physically meaningful MD simulations.

  7. Crack growth of 10M Ni-Mn-Ga material in cyclic mechanical loading

    NASA Astrophysics Data System (ADS)

    Aaltio, I.; Ge, Y.; Pulkkinen, H.; Sjöberg, A.; Söderberg, O.; Liu, X. W.; Hannula, S.-P.

    The 10M martensitic Ni-Mn-Ga single crystal materials are usually applied in the magneto-mechanical actuators. Therefore, it is important to know the possible effect of the long-term cyclic shape changes on their structure and behavior. This can be evaluated with the mechanical fatigue testing. In the present study, the single crystal 10M Ni-Mn-Ga samples of different compositions were applied to strain-controlled uniaxial mechanical cycling in the multivariant state at ambient temperature. The experiments revealed distinctive changes of the twin variant structure, especially in the mobile twin area, density of twin boundaries, and in the tendency for fatigue crack growth. Characterization of the crack surface showed that the cracks in the microscale grow in a step-wise manner on specific crystallographic planes, i.e, twin boundary planes, but that the macroscopic crack does not occur only along crystallographic directions.

  8. Assessment of rock mechanical behavior considering stress dependent stiffness of the cracked domain within crack tensor-based approach

    NASA Astrophysics Data System (ADS)

    Takemura, T.; Panaghi, K.; Golshani, A.; Takahashi, M.; Sato, M.

    2015-12-01

    The analyses dedicated to media with prevalent discontinuities such as rocks has mostly been limited to inevitable simplifications to make engineering judgments on the material behavior feasible. Such assumptions, though favorable in numerical simulations, usually lead to overestimations in aseismic design of earthen structures. One of the forbidding tasks in modeling rock behavior is taking the stress dependency of stiffness into consideration which implies more complicated formulations. Although the theoretical relationship for such computations has already been proposed by scholars, there still remains some gaps in the real-world application of the aforementioned. The crack tensor-based formulation in describing stress-strain behavior of cracked rock is a case in point in which the fourth-rank crack tensor effect is usually ignored due to adopting equal normal and shear stiffnesses for the medium. Once the stiffnesses were distinguished in different values, the accompanying condition imposed by the formulation requires computation of fourth rank tensor which has not been obtained in a practical manner so far. In the present study, we aim to acquire the values via experimental measurements and implement the results to further improve the accuracy of the formulation used in characterizing mechanical behavior of rock samples.

  9. Effectiveness of Fiber Reinforcement on the Mechanical Properties and Shrinkage Cracking of Recycled Fine Aggregate Concrete

    PubMed Central

    Nam, Jeongsoo; Kim, Gyuyong; Yoo, Jaechul; Choe, Gyeongcheol; Kim, Hongseop; Choi, Hyeonggil; Kim, Youngduck

    2016-01-01

    This paper presents an experimental study conducted to investigate the effect of fiber reinforcement on the mechanical properties and shrinkage cracking of recycled fine aggregate concrete (RFAC) with two types of fiber—polyvinyl alcohol (PVA) and nylon. A small fiber volume fraction, such as 0.05% or 0.1%, in RFAC with polyvinyl alcohol or nylon fibers was used for optimum efficiency in minimum quantity. Additionally, to make a comparative evaluation of the mechanical properties and shrinkage cracking, we examined natural fine aggregate concrete as well. The test results revealed that the addition of fibers and fine aggregates plays an important role in improving the mechanical performance of the investigated concrete specimens as well as controlling their cracking behavior. The mechanical properties such as compressive strength, splitting tensile strength, and flexural strength of fiber-reinforced RFAC were slightly better than those of non-fiber-reinforced RFAC. The shrinkage cracking behavior was examined using plat-ring-type and slab-type tests. The fiber-reinforced RFAC showed a greater reduction in the surface cracks than non-fiber-reinforced concrete. The addition of fibers at a small volume fraction in RFAC is more effective for drying shrinkage cracks than for improving mechanical performance. PMID:28773256

  10. Effectiveness of Fiber Reinforcement on the Mechanical Properties and Shrinkage Cracking of Recycled Fine Aggregate Concrete.

    PubMed

    Nam, Jeongsoo; Kim, Gyuyong; Yoo, Jaechul; Choe, Gyeongcheol; Kim, Hongseop; Choi, Hyeonggil; Kim, Youngduck

    2016-02-26

    This paper presents an experimental study conducted to investigate the effect of fiber reinforcement on the mechanical properties and shrinkage cracking of recycled fine aggregate concrete (RFAC) with two types of fiber-polyvinyl alcohol (PVA) and nylon. A small fiber volume fraction, such as 0.05% or 0.1%, in RFAC with polyvinyl alcohol or nylon fibers was used for optimum efficiency in minimum quantity. Additionally, to make a comparative evaluation of the mechanical properties and shrinkage cracking, we examined natural fine aggregate concrete as well. The test results revealed that the addition of fibers and fine aggregates plays an important role in improving the mechanical performance of the investigated concrete specimens as well as controlling their cracking behavior. The mechanical properties such as compressive strength, splitting tensile strength, and flexural strength of fiber-reinforced RFAC were slightly better than those of non-fiber-reinforced RFAC. The shrinkage cracking behavior was examined using plat-ring-type and slab-type tests. The fiber-reinforced RFAC showed a greater reduction in the surface cracks than non-fiber-reinforced concrete. The addition of fibers at a small volume fraction in RFAC is more effective for drying shrinkage cracks than for improving mechanical performance.

  11. Ultra-sensitive Pressure sensor based on guided straight mechanical cracks

    PubMed Central

    Choi, Yong Whan; Kang, Daeshik; Pikhitsa, Peter V.; Lee, Taemin; Kim, Sang Moon; Lee, Gunhee; Tahk, Dongha; Choi, Mansoo

    2017-01-01

    Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed. PMID:28059136

  12. Thermal-mechanical modeling and experimental validation of weld solidification cracking in 6061-T6 aluminum

    SciTech Connect

    Dike, J.J.; Brooks, J.A.; Bammann, D.J.; Li, M.

    1997-12-31

    Finite element simulation using an internal state variable constitutive model coupled with a void growth and damage model are used to study weld solidification cracking of 6061-T6 aluminum. Calculated results are compared with data from an experimental program determining the locations of failure as a function of weld process parameters and specimen geometry. Two types of weld solidification cracking specimen were studied. One specimen, in which cracking did not occur, was used to evaluate finite element simulations of the thermal response and calculations of average strain across the weld. The other specimen type was used to determine the location of crack initiation as a function of weld process parameters. This information was used to evaluate the finite element simulations of weld solidification cracking. A solidification model which includes dendrite tip and eutectic undercooling was used in both thermal and mechanical finite element analyses. A strain rate and temperature history dependent constitutive model is coupled with a ductile void growth damage model in the mechanical analyses. Stresses near the weld pool are examined to explain results obtained in the finite element analyses and correlated with experimental observations. Good agreement is obtained between simulation and experiment for locations of crack initiation and extent of cracking. Some effects of uncertainties in material parameters are discussed.

  13. Ultra-sensitive Pressure sensor based on guided straight mechanical cracks

    NASA Astrophysics Data System (ADS)

    Choi, Yong Whan; Kang, Daeshik; Pikhitsa, Peter V.; Lee, Taemin; Kim, Sang Moon; Lee, Gunhee; Tahk, Dongha; Choi, Mansoo

    2017-01-01

    Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

  14. Crack Turning and Arrest Mechanisms for Integral Structure

    NASA Technical Reports Server (NTRS)

    Pettit, Richard; Ingraffea, Anthony

    1999-01-01

    In the course of several years of research efforts to predict crack turning and flapping in aircraft fuselage structures and other problems related to crack turning, the 2nd order maximum tangential stress theory has been identified as the theory most capable of predicting the observed test results. This theory requires knowledge of a material specific characteristic length, and also a computation of the stress intensity factors and the T-stress, or second order term in the asymptotic stress field in the vicinity of the crack tip. A characteristic length, r(sub c), is proposed for ductile materials pertaining to the onset of plastic instability, as opposed to the void spacing theories espoused by previous investigators. For the plane stress case, an approximate estimate of r(sub c), is obtained from the asymptotic field for strain hardening materials given by Hutchinson, Rice and Rosengren (HRR). A previous study using of high order finite element methods to calculate T-stresses by contour integrals resulted in extremely high accuracy values obtained for selected test specimen geometries, and a theoretical error estimation parameter was defined. In the present study, it is shown that a large portion of the error in finite element computations of both K and T are systematic, and can be corrected after the initial solution if the finite element implementation utilizes a similar crack tip discretization scheme for all problems. This scheme is applied for two-dimensional problems to a both a p-version finite element code, showing that sufficiently accurate values of both K(sub I) and T can be obtained with fairly low order elements if correction is used. T-stress correction coefficients are also developed for the singular crack tip rosette utilized in the adaptive mesh finite element code FRANC2D, and shown to reduce the error in the computed T-stress significantly. Stress intensity factor correction was not attempted for FRANC2D because it employs a highly accurate

  15. The cracking mechanism of silicon particles in an A357 aluminum alloy

    NASA Astrophysics Data System (ADS)

    Yeh, Jien-Wei; Liu, Wen-Pin

    1996-11-01

    The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurlandrss and that of Low et al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.

  16. Mechanical analysis of asphalt stabilized permeable base to inhibit reflective cracking

    NASA Astrophysics Data System (ADS)

    Luo, Min

    2017-09-01

    Asphalt stabilized drainage base has good drainage performance, can effectively rule out the water in pavement structure, reduce the occurrence of water damage, and maintain good pavement performance. Based on the mechanical analysis of the affect of asphalt stabilized permeable base on the inhibition of reflection crack, using the finite element software to simulate the stress characteristics of the asphalt and to do the mechanical analysis of asphalt pavement cracks at the grass-roots level to the pavement after stretching process, by comparing the pavement crack tip stress intensity factor of original pavement structure and set up the ATPB layer, respectively, to study the effect of asphalt stabilized permeable base on inhibition of reflection cracking.

  17. Safety assessment of Cracked K-joint Structure Based on Fracture Mechanics

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Pengyu, Yan; Jianwei, Du; Fuhai, Cai

    2017-05-01

    The K-joint is the main bearing structure of lattice jib crane. During frequent operation of the crane, surface cracks often occur at its weld toe, and then continue to expand until failure. The safety of the weak structure K-joint of the crane jib can be evaluated by BS7910 failure assessment standard in order to improve its utilization. The finite element model of K-joint structure with cracks is established, and its mechanical properties is analyzed by ABAQUS software, the results show that the crack depth has a great influence on the bearing capacity of the structure compared with the crack length. It is assumed that the K-joint with the semi-elliptical surface crack under the action of the tension propagate stably under the condition that the c/a (ratio of short axis to long axis of ellipse) is about 0.3. The safety assessment of K-joint with different lengths crack is presented according to the 2A failure assessment diagram of BS7910, and the critical crack of K-joint under different loads can be obtained.

  18. Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

    NASA Astrophysics Data System (ADS)

    Suresh, S.; Vasudévan, A. K.; Bretz, P. E.

    1984-02-01

    The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with Δ K 0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data for vacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.

  19. Grain boundary defects initiation at the outer surface of dissimilar welds: Corrosion mechanism studies

    SciTech Connect

    Bouvier, O. De; Yrieix, B.

    1995-12-31

    Dissimilar welds located on the primary coolant system of the French PWR plants exhibit grain boundary defects in the true austenitic zones of the first buttering layer. If grain boundaries reach the interface, they can extend to the martensitic band. Those defects are filled with compact oxides. In addition, the ferritic base metal presents some pits along the interface. Nowadays, three mechanisms are proposed to explain the initiation of those defects: stress corrosion cracking, intergranular corrosion and high temperature intergranular oxidation. This paper is dealing with the study of the mechanisms involved in the corrosion phenomenon. Intergranular corrosion tests performed on different materials show that only the first buttering layer, even with some {delta} ferrite, is sensitized. The results of stress corrosion cracking tests in water solutions show that intergranular cracking is possible on a bulk material representative of the first buttering layer. It is unlikely on actual dissimilar welds where the ferritic base metal protects the first austenitic layer by galvanic coupling. Therefore, the stress corrosion cracking assumption cannot explain the initiation of the defects in aqueous environment. The results of the investigations and of the corrosion studies led to the conclusion that the atmosphere could be the only possible aggressive environment. This conclusion is based on natural atmospheric exposure and accelerated corrosion tests carried out with SO{sub 2} additions in controlled atmosphere. They both induce a severe intergranular corrosion on true sensitized austenitic materials.

  20. Grain egression: A new mechanism of fatigue-crack initiation in Ti-6Al-4V

    NASA Astrophysics Data System (ADS)

    Gilbert, Jeremy L.; Piehler, Henry R.

    1989-09-01

    A new mechanism of fatigue-crack initiation (FCI), grain egression, was observed in the course of investigating corrosion-fatigue crack initiation in Ti-6A1-4V hip prostheses fabricated using three different processes. Extensive scanning electron microscopy (SEM) was used to document this new mechanism as well as the other FCI mechanisms operating. Grain egression entails the fracture and egression of primary α grains from the surface of the sample, resulting in a sharp pit that subsequently acts as the site of crack initiation. The different sizes and morphologies of the grain-egression sites observed are very similar to the sizes and morphologies of the pri-mary α grains resulting from the three different fabrication processes, providing further evidence for grain egression as an operative FCI mechanism.

  1. Establishing the Foundations of the Mechanics of Fracture of Materials Compressed Along Cracks (Review)

    NASA Astrophysics Data System (ADS)

    Guz, A. N.

    2014-01-01

    The basic results of establishing the foundations of the mechanics of fracture of homogeneous materials compressed along cracks and inhomogeneous (composite) materials compressed along interface cracks are analyzed. These results were obtained using elastic, plastic, and viscoelastic material models. This review consists of three parts. The first part discusses the basic concept that the start (onset) of fracture is the mechanism of local instability near the cracks located in a single plane or parallel planes. The fracture criterion and the basic problems arising in this division of fracture mechanics are also formulated. Two basic approaches to establishing the foundations of the mechanics of fracture of materials compressed along cracks are outlined. One approach, so-called beam approximation, is based on various applied theories of stability of thin-walled systems (including the Bernoulli, Kirchhoff-Love, Timoshenko-type hypotheses, etc.). This approach is essentially approximate and introduces an irreducible error into the calculated stresses. The other approach is based on the basic equations and methods of the three-dimensional linearized theory of stability of deformable bodies for finite and small subcritical strains. This approach does not introduce major errors typical for the former approach and allows obtaining results with accuracy acceptable for mechanics. The second part offers a brief analysis of the basic results obtained with the first approach and a more detailed analysis of the basic results obtained with the second approach, including the consideration of the exact solutions for interacting cracks in a single plane and in parallel planes and results for some structural materials. The third part reports new results for interacting cracks in very closely spaced (or coinciding, as an asymptotic case) planes. These results may be considered a transition from the second approach (three-dimensional linearized theory of elastic stability) to the

  2. Thermo-mechanical simulations of early-age concrete cracking with durability predictions

    NASA Astrophysics Data System (ADS)

    Havlásek, Petr; Šmilauer, Vít; Hájková, Karolina; Baquerizo, Luis

    2017-09-01

    Concrete performance is strongly affected by mix design, thermal boundary conditions, its evolving mechanical properties, and internal/external restraints with consequences to possible cracking with impaired durability. Thermo-mechanical simulations are able to capture those relevant phenomena and boundary conditions for predicting temperature, strains, stresses or cracking in reinforced concrete structures. In this paper, we propose a weakly coupled thermo-mechanical model for early age concrete with an affinity-based hydration model for thermal part, taking into account concrete mix design, cement type and thermal boundary conditions. The mechanical part uses B3/B4 model for concrete creep and shrinkage with isotropic damage model for cracking, able to predict a crack width. All models have been implemented in an open-source OOFEM software package. Validations of thermo-mechanical simulations will be presented on several massive concrete structures, showing excellent temperature predictions. Likewise, strain validation demonstrates good predictions on a restrained reinforced concrete wall and concrete beam. Durability predictions stem from induction time of reinforcement corrosion, caused by carbonation and/or chloride ingress influenced by crack width. Reinforcement corrosion in concrete struts of a bridge will serve for validation.

  3. Applicability of the fracture toughness master curve to irradiated highly embrittled steel and intergranular fracture

    SciTech Connect

    Nanstad, Randy K; Sokolov, Mikhail A; McCabe, Donald E

    2008-01-01

    The Heavy-Section Steel Irradiation (HSSI) Program at Oak Ridge National Laboratory has evaluated a submerged-arc (SA) weld irradiated to a high level of embrittlement and a temper embrittled base metal that exhibits significant intergranular fracture (IGF) relative to representation by the Master Curve. The temper embrittled steel revealed that the intergranular mechanism significantly extended the transition temperature range up to 150 C above To. For the irradiated highly embrittled SA weld study, a total of 21 1T compact specimens were tested at five different temperatures and showed the Master Curve to be nonconservative relative to the results, although that observation is uncertain due to evidence of intergranular fracture.

  4. Applied Stress Affecting the Environmentally Assisted Cracking

    NASA Astrophysics Data System (ADS)

    Vasudevan, A. K.

    2013-03-01

    Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.

  5. Mechanics of finite cracks in dissimilar anisotropic elastic media considering interfacial elasticity

    DOE PAGES

    Juan, Pierre -Alexandre; Dingreville, Remi

    2016-10-31

    Interfacial crack fields and singularities in bimaterial interfaces (i.e., grain boundaries or dissimilar materials interfaces) are considered through a general formulation for two-dimensional (2-D) anisotropic elasticity while accounting for the interfacial structure by means of an interfacial elasticity paradigm. The interfacial elasticity formulation introduces boundary conditions that are effectively equivalent to those for a weakly bounded interface. This formalism considers the 2-D crack-tip elastic fields using complex variable techniques. While the consideration of the interfacial elasticity does not affect the order of the singularity, it modifies the oscillatory effects associated with problems involving interface cracks. Constructive or destructive “interferences” aremore » directly affected by the interface structure and its elastic response. Furthermore, this general formulation provides an insight on the physical significance and the obvious coupling between the interface structure and the associated mechanical fields in the vicinity of the crack tip.« less

  6. Mechanics of finite cracks in dissimilar anisotropic elastic media considering interfacial elasticity

    NASA Astrophysics Data System (ADS)

    Juan, Pierre-Alexandre; Dingreville, Rémi

    2017-02-01

    Interfacial crack fields and singularities in bimaterial interfaces (i.e., grain boundaries or dissimilar materials interfaces) are considered through a general formulation for two-dimensional (2-D) anisotropic elasticity while accounting for the interfacial structure by means of an interfacial elasticity paradigm. The interfacial elasticity formulation introduces boundary conditions that are effectively equivalent to those for a weakly bounded interface. This formalism considers the 2-D crack-tip elastic fields using complex variable techniques. While the consideration of the interfacial elasticity does not affect the order of the singularity, it modifies the oscillatory effects associated with problems involving interface cracks. Constructive or destructive "interferences" are directly affected by the interface structure and its elastic response. This general formulation provides an insight on the physical significance and the obvious coupling between the interface structure and the associated mechanical fields in the vicinity of the crack tip.

  7. Mechanics of finite cracks in dissimilar anisotropic elastic media considering interfacial elasticity

    SciTech Connect

    Juan, Pierre -Alexandre; Dingreville, Remi

    2016-10-31

    Interfacial crack fields and singularities in bimaterial interfaces (i.e., grain boundaries or dissimilar materials interfaces) are considered through a general formulation for two-dimensional (2-D) anisotropic elasticity while accounting for the interfacial structure by means of an interfacial elasticity paradigm. The interfacial elasticity formulation introduces boundary conditions that are effectively equivalent to those for a weakly bounded interface. This formalism considers the 2-D crack-tip elastic fields using complex variable techniques. While the consideration of the interfacial elasticity does not affect the order of the singularity, it modifies the oscillatory effects associated with problems involving interface cracks. Constructive or destructive “interferences” are directly affected by the interface structure and its elastic response. Furthermore, this general formulation provides an insight on the physical significance and the obvious coupling between the interface structure and the associated mechanical fields in the vicinity of the crack tip.

  8. Analytical and Experimental Study of Near-Threshold Interactions Between Crack Closure Mechanisms

    NASA Technical Reports Server (NTRS)

    Newman, John A.; Riddell, William T.; Piascik, Robert S.

    2003-01-01

    The results of an analytical closure model that considers contributions and interactions between plasticity-, roughness-, and oxide-induced crack closure mechanisms are presented and compared with experimental data. The analytical model is shown to provide a good description of the combined influences of crack roughness, oxide debris, and plasticity in the near-threshold regime. Furthermore, analytical results indicate that closure mechanisms interact in a non-linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.

  9. Crack healing in cross-ply composites observed by dynamic mechanical analysis

    NASA Astrophysics Data System (ADS)

    Nielsen, Christian; Nemat-Nasser, Sia

    2015-03-01

    Cross-ply composites with healable polymer matrices are characterized using dynamic mechanical analysis (DMA). The [90,0]s samples are prepared by embedding layers of unidirectional glass or carbon fibers in 2MEP4FS, a polymer with thermally reversible covalent cross-links, which has been shown to be capable of healing internal cracks and fully recovering fracture toughness when the crack surfaces are kept in contact. After fabrication, cracks in the composites' transverse plies are observed due to residual thermal stresses introduced during processing. Single cantilever bending DMA measurements show the samples exhibit periods of increasing storage moduli with increasing temperature. These results are accurately modeled as a one-dimensional composite, which captures the underlying physics of the phenomenon. The effect of cracks on the stiffness is accounted for by a shear-lag model. The predicted crack density of the glass fiber composite is shown to fall within a range observed from microscopy images. Crack healing occurs as a function of temperature, with chemistry and mechanics-based rationales given for the onset and conclusion of healing. The model captures the essential physics of the phenomenon and yields results in accord with experimental observations.

  10. Fracture mechanics applied to nonisothermal fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Jordan, E. H.; Meyers, G. J.

    1986-01-01

    Twelve nonisothermal fatigue crack growth tests were performed on Hastelloy-X tubular specimens in which strain and temperature varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 982 C. A number of parameters, including the stress intensity factor, strain intensity factor, and J-integral, were examined for their ability to correlate the data. There was no decisive difference between the success of the three parameters. Each parameter correlated data from different strain ranges to within no worse than a factor of 2.1 on da/dn. The effect of strain temperature cycle shape was investigated and found to be moderate, while a strain hold of 1 min had very little effect. An attempt was made to predict nonisothermal test results from isothermal data. These predictions were better than those made by using peak test temperature isothermal data but still not within scatter.

  11. A computerized test system for thermal-mechanical fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1986-01-01

    A computerized testing system to measure fatigue crack growth under thermal-mechanical fatigue conditions is described. Built around a servohydraulic machine, the system is capable of a push-pull test under stress-controlled or strain-controlled conditions in the temperature range of 25 to 1050 C. Temperature and mechanical strain are independently controlled by the closed-loop system to simulate the complex inservice strain-temperature relationship. A d-c electrical potential method is used to measure crack growth rates. The correction procedure of the potential signal to take into account powerline and RF-induced noises and thermal changes is described. It is shown that the potential drop technique can be used for physical mechanism studies and for modelling crack tip processes.

  12. A computerized test system for thermal-mechanical fatigue crack growth

    NASA Technical Reports Server (NTRS)

    Marchand, N.; Pelloux, R. M.

    1986-01-01

    A computerized testing system to measure fatigue crack growth under thermal-mechanical fatigue conditions is described. Built around a servohydraulic machine, the system is capable of a push-pull test under stress-controlled or strain-controlled conditions in the temperature range of 25 to 1050 C. Temperature and mechanical strain are independently controlled by the closed-loop system to simulate the complex inservice strain-temperature relationship. A d-c electrical potential method is used to measure crack growth rates. The correction procedure of the potential signal to take into account powerline and RF-induced noises and thermal changes is described. It is shown that the potential drop technique can be used for physical mechanism studies and for modelling crack tip processes.

  13. Effects of hydrogen on mechanical properties in irradiated austenitic stainless steels

    NASA Astrophysics Data System (ADS)

    Morisawa, J.; Kodama, M.; Nishimura, S.; Asano, K.; Nakata, K.; Shima, S.

    1994-09-01

    To investigate the hydrogen effect on mechanical properties of solution annealed Type 304 stainless steel, tensile tests of neutron irradiated materials were conducted after a hydrogen charging and discharging process (hydrogen treatment). Elongation was less with increasing neutron fluence after hydrogen treatment than that of as-irradiated specimens. Intergranular cracking occurred by the hydrogen treatment in heavier irradiated specimens, in which the Cr depleted zone along grain boundary was observed. Embrittlement and intergranular cracking after the hydrogen treatment were estimated to be attributed to the Cr depleted zone at the grain boundary due to neutron irradiation.

  14. Fatigue crack growth bridging mechanisms in titanium metal-matrix composites

    NASA Astrophysics Data System (ADS)

    Tamin, Mohd Nasir

    1997-09-01

    The bridging fatigue crack growth damage mechanisms in a unidirectional SiC/Ti MMC include matrix cracking, fiber/matrix interface debonding and sliding along bridging fibers and fracture of these fibers. The basic components of these mechanisms are examined in this program. The evolution characteristics of residual stresses indicated that extensive stress relaxation occurred in the Ti-alloy matrix phase of the composite following post-fabrication cool down to 600sp° C. Parametric study on the SiC fiber coating materials showed that the effective residual stress component has an inverse relationship with the thickness of the composite reaction zone. The debonding shear strength of the composite is determined based on localized shear stress distribution along the fiber/matrix interface at the onset of debonding. The resulting shear strength is found to decrease from 221.2 MPa at ambient temperature to 138.6 MPa at 650sp° C. An interphase debonding model, which combines fracture mechanics equations with finite element results on interphase shear stress and bridging fiber traction range, is proposed to establish a distribution of debonding lengths along a fiber-bridged matrix crack length. The longest debonding lengths in a SiC/Ti MMC was predicted along the first intact fiber at the crack mouth and the lengths decrease for fibers located closer to the crack tip. In addition, the debonding crack length increases with increasing temperature. The driving force for the interface debond crack, however, has an inverse relationship with the test temperature. The concurrent damage events of fiber stress evolution and continuous fiber strength degradation were postulated into a fiber fracture criterion to describe the fracture process of a bridging fiber. Although the strength properties of SiC SCS-6 fibers are found to be unaffected by test temperature of 650sp° C and below, temperature influenced the fracture process of these fibers through the density of cracks in the

  15. Development of Probabilistic Fracture Mechanics Analysis Code for Pipes with Stress Corrosion Cracks

    NASA Astrophysics Data System (ADS)

    Machida, Hideo; Arakawa, Manabu; Yamashita, Norimichi; Yoshimura, Shinobu

    Risk-Informed integrity management methodologies have been developed for Japanese nuclear power plants. One of the issues of concern is the reliability assessment of piping with flaws due to stress corrosion cracking (SCC). Therefore, the probabilistic fracture mechanics analysis code has been developed, which can perform the reliability assessment for austenitic stainless steel piping with flaws due to SCC. This paper describes technical basis of this code. This method is based on Monte-Carlo technique considering many sample cases in a piping section, where the initiation and growth of cracks are calculated and piping failures, including leaks and rapture, are evaluated. A notable feature is that multiple cracks can be treated, consequently, assessment of coalescence of cracks and intricate break evaluation of piping section have been included. Moreover, the in-service inspection (ISI) and integrity evaluation by Fitness-for-Service (FFS) code are integrated into the analysis, and the contribution to failure probability decrease can be assessed. Key parameters are determined on a probability basis with the designated probability type throughout the procedure. Size, location and time of crack initiation, coefficients of crack growth due to SCC and factors for piping failure are included in those parameters. With this method the reliability level of the piping through the operation periods can be estimated and the contribution of various parameters including ISI can be quantitatively evaluated.

  16. Experimental study on crack coalescence mechanisms of pre-existing flaws under blast loading

    NASA Astrophysics Data System (ADS)

    Yue, Zhong-wen; Yang, Ren-shu; Ma, Xin-min; Guo, Dong-ming

    2008-11-01

    The PMMA model transmission-type experiment of dynamic caustics was carried out to simulate the fracture blasting process of material containing pre-existing flaws using the dynamic caustic-test system. The mechanism of the fracture coalescence among four prefabricated flaws with echelon geometry distribution was studied under blast loading. The experiment results show that two wing cracks respectively coalescing with the flaw F2 and flaw F3 appear at both tips of the flaw F1 closest to the blasthole. Whereas the flaw F4 doesn't produce wing cracks, the flaw F2 and flaw F3 also respectively generate two wing cracks which don't link up the flaw F4. Crack propagation is greatly affected by preexisting flaws. During the whole fracture process, the wing crack velocity oscillates with the increase of crack propagating time. The value of dynamic stress intensity factor reaches the maximum in a moment and then gradually decreases. The changes of dynamic stress intensity factor also oscillate in the whole time. Furthermore, the value of dynamic stress intensity factor KdII is smaller than that of dynamic stress intensity factor KdI. The results of the present research can provide the theoretical basis for the study on blasting of rock containing flaws.

  17. Simulation of crack propagation in fiber-reinforced concrete by fracture mechanics

    SciTech Connect

    Zhang Jun; Li, Victor C

    2004-02-01

    Mode I crack propagation in fiber-reinforced concrete (FRC) is simulated by a fracture mechanics approach. A superposition method is applied to calculate the crack tip stress intensity factor. The model relies on the fracture toughness of hardened cement paste (K{sub IC}) and the crack bridging law, so-called stress-crack width ({sigma}-{delta}) relationship of the material, as the fundamental material parameters for model input. As two examples, experimental data from steel FRC beams under three-point bending load are analyzed with the present fracture mechanics model. A good agreement has been found between model predictions and experimental results in terms of flexural stress-crack mouth opening displacement (CMOD) diagrams. These analyses and comparisons confirm that the structural performance of concrete and FRC elements, such as beams in bending, can be predicted by the simple fracture mechanics model as long as the related material properties, K{sub IC} and ({sigma}-{delta}) relationship, are known.

  18. Transient thermal-mechanical behavior of cracked glass-cloth-reinforced epoxy laminates at low temperatures

    SciTech Connect

    Shindo, Y.; Ueda, S.

    1997-06-01

    We consider the transient thermal-mechanical response of cracked G-10CR glass-cloth-reinforced epoxy laminates with temperature-dependent properties. The glass-cloth-reinforced epoxy laminates are suddenly cooled on the surfaces. A generalized plane strain finite element model is used to study the influence of warp angle and crack formation on the thermal shock behavior of two-layer woven laminates at low temperatures. Numerical calculations are carried out, and the transient temperature distribution and the thermal-mechanical stresses are shown graphically.

  19. Deformation and cracking of irradiated austenitic stainless steels

    SciTech Connect

    Carter, R.D.; Atzmon, M.; Was, G.S.

    1995-12-31

    Samples of proton-irradiated 304L stainless steel were deformed by constant extension rate tensile tests at strain rates of 3 {times} 10{sup {minus}7} s{sup {minus}1} and 3 {times} 10{sup {minus}8} s{sup {minus}1} to strains of up to 10% at 288--350 C in argon. Minor cracking was observed in and around spinel inclusions in the material, however no intergranular cracking of the type observed in water environments was found. Thus intergranular cracking cannot occur by a radiation hardening mechanism alone. The microstructures that resulted from irradiation and deformation were characterized using electron microscopy. Surface slip band formation is observed on one or two {l_brace}111{r_brace} slip systems in each grain. The slip bands correspond to dislocation channels in the material as identified by transmission electron microscopy. The channels form by activation of grain-boundary dislocation sources, with the emitted dislocations sweeping through the grain interior to the opposing rain boundaries. During this process, the dislocations remove the radiation-produced defects. Slip band and dislocation channel densities increase with increasing strain in the samples. These results are used to interpret stress corrosion cracking behavior in this material.

  20. Impact of hydraulic suction history on crack growth mechanics in soil

    NASA Astrophysics Data System (ADS)

    Yoshida, S.; Hallett, P. D.

    2008-05-01

    The mechanics of crack formation and the influence of soil stress history were described using the crack tip opening angle (CTOA) measured with fractography. Two soils were studied: a model soil consisting of 40% Ca-bentonite and 60% fine silica sand and a remolded paddy soil with similar clay content and mineralogy. Fracture testing used deep-notch bend specimens formed by molding soils at the liquid limit into rectangular bars, equilibrating to soil water suction ranging from 5 kPa to 50 kPa (with some 50 kPa specimens wetted to 5 kPa), and inserting a crack 0.4× specimen thickness. Bend tests at a constant displacement rate of 1 mm min-1 provided data on applied force and load point displacement. The growth and geometry of the cracks were quantified from a series of images to determine the CTOA. Modulus of rupture, evaluated from the peak force, increased as water suction increased. However, rewetting did not alter the peak stress from the 50 kPa value, indicating that shrinkage-induced consolidation was more important than the soil water suction at the onset of testing. CTOA measured during stable crack growth decreased with drying. CTOA decreased even further when specimens equilibrated initially to 50 kPa were rewetted to 5 kPa. These results suggested that CTOA was primarily governed by the stiffness, although rewetting probably altered the capillary stresses in advance of the crack tip. Our future work will combine CTOA with a model that couples hydrological and mechanical processes to take into account the dependency of CTOA on the soil water regime so that crack propagation in soil can be predicted.

  1. Intergranular attack of alloy 600: Simulation and remedial action tests: Final report

    SciTech Connect

    Daret, J.; Feron, D.

    1989-02-01

    The intergranular attack (IGA) that affects alloy 600 tubes in the tube sheet crevices of PWR steam generators is hard to simulate in laboratory studies. For this study, a special apparatus was designed with a range of representative materials, mechanical conditions and geometry. The design also took account of sludge piles, thermal fluxes and water chemistry. During a first series of seven model boiler tests, chemical parameters and test procedures were adjusted to finally obtain a field prototypical degradation of tubing over a significant length within the tube sheet crevice for the case of caustic pollution. IGA was not produced for the river water in-leakage case. A second series of model boiler tests also showed the possibility of producing a representative IGA by initially filling the tube sheet crevices with concentrated caustic solutions. A third series of five model boiler tests aimed at studying in the effectiveness of remedial actions on either virgin of IGA affected tubing. Tube sheet crevice flushing operations using the natural circulation procedure showed a poor efficiency for moving concentrated contaminants, but they succeeded in forcing chemicals additives (acetic acid or boric acid) within the non-occluded portions of crevices. This off-line treatment resulted in a reduction in the progression rate of the pre-existing IGA by a factor of 2 to 3. Simulation of this treatment on virgin material showed that this result was obtained because acetate or borate shifted the cation-to-anion equivalent ratio to well under one. However this off-line treatment could not prevent the occurrence of intergranular stress corrosion cracking (IGSCC) near the top of crevices, since caustic continued to hideout under full power operation. The best remedial action consisted of a combination of off-line and on-line boric acid treatment. IGSCC was prevented both on virgin and IGA affected tubes. 3 figs., 3 tabs.

  2. Small punch test evaluation of intergranular embrittlement of an alloy steel

    SciTech Connect

    Baik, J.M.; Buck, O.; Kameda, J.

    1983-12-01

    The ductile-brittle transition temperature in steel is commonly determined using Charpy V-notch impact specimens as specified by ASTM E23-81. In some specific cases, however, the use of this standardized test specimen may be impractical, if not impossible. For instance, it is well known that ferritic steels show a substantial degradation of the mechanical properties after long time exposure to an irradiation environment. Because of the increase in strength and the reduction in ductility due to neutron irradiation, the Charpy V-notch transition temperature is raised causing concern from a safety point of view. To study these radiation effects, a test specimen much smaller than the standard Charpy V-notch specimen would be extremely desirable for two reasons. First, to study neutron damage small specimens take less space within a reactor. Secondly, the damage achieved in simulation experiments, such as proton or electron accelerators, is limited to small penetration depths. Several efforts on the development of such a small test specimen, similar to that used to determine the ductility of sheet metal, as recommended by ASTM E643-78, have been described in the literature. The paper reports on correlations between small punch (SP) and Charpy V-notch (CVN) test results obtained on temper-embrittled NiCr steel. The ductile-brittle transition temperature (DBTT) with intergranular embrittlement being induced by grain boundary segregation of specific impurities was determined. The relation between test results discussed in terms of the micromechanisms of intergranular cracking. It is suggested that in radiation embrittlement investigations similar correlations may be obtained.

  3. Effect of Microstructure on Low Temperature Cracking Behavior of EN82H Welds

    SciTech Connect

    W. J. Mills; C. M. Brown; M. G. Burke

    2001-04-30

    As-fabricated EN82H welds are susceptible to low temperature embrittlements in 54 degree C hydrogenated water. Values of J[sub]IC in water are typically 90% to 98% lower than those in air due to a fracture mechanism transition from microvoid coalescence to hydrogen-included intergranular fracture. Environmental J[sub]IC testing demonstrated that a high temperature (1093 degree C) anneal and furnace-cool alleviates the material's susceptibility to hydrogen-induced intergranular cracking. To identify metallurgical and compositional features that are responsible for the material's environment-sensitive behavior, detailed characterization of the microstructure and grain boundary chemistry for the as-fabricated and as-annealed materials was performed. Results from light optical microscopy, analytical electron microscopy, electron probe microanalysis, Auger electron spectroscopy and mechanical property characterization are used to provide insight into the observed low temperature embrittlement phenomenon. The key microstructural feature responsible for low temperature cracking in as-fabricated welds appears to be fine niobium and titanium-rich carbonitrides that cover most grain boundaries. These precipitates are effective hydrogen traps that promote hydrogen-induced intergranular cracking. Dissolution the fine carbonitrides during the 1093 degree C anneal reduces grain boundary trapping sites, which accounts for the improved fracture resistance displayed by the annealed weld. The role of strength level in promoting low temperature embrittlement is evaluated by cold-rolling the annealed weld to increase its yield strength from 280 to 640 MPa. The annealed and cold-rolled weld exhibits high toughness in 54 degree C water and shows no evidence of hydrogen-induced intergranular cracking, thereby demonstrating that strength is not a primary cause of low temperature embrittlement.

  4. A New Approximate Fracture Mechanics Analysis Methodology for Composites with a Crack or Hole

    NASA Technical Reports Server (NTRS)

    Tsai, H. C.; Arocho, A.

    1990-01-01

    A new approximate theory which links the inherent flaw concept with the theory of crack tip stress singularities at a bi-material interface was developed. Three assumptions were made: (1) the existence of inherent flaw (i.e., damage zone) at the tip of the crack, (2) a fracture of the filamentary composites initiates at a crack lying in the matrix material at the interface of the matrix/filament, and (3) the laminate fails whenever the principal load-carrying laminae fails. This third assumption implies that for a laminate consisting of 0 degree plies, cracks into matrix perpendicular to the 0 degree filaments are the triggering mechanism for the final failure. Based on this theory, a parameter bar K sub Q which is similar to the stress intensity factor for isotropic materials but with a different dimension was defined. Utilizing existing test data, it was found that bar K sub Q can be treated as a material constant. Based on this finding a fracture mechanics analysis methodology was developed. The analytical results are correlated well with test results. This new approximate theory can apply to both brittle and metal matrix composite laminates with crack or hole.

  5. Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system

    NASA Astrophysics Data System (ADS)

    Kang, Daeshik; Pikhitsa, Peter V.; Choi, Yong Whan; Lee, Chanseok; Shin, Sung Soo; Piao, Linfeng; Park, Byeonghak; Suh, Kahp-Yang; Kim, Tae-Il; Choi, Mansoo

    2014-12-01

    Recently developed flexible mechanosensors based on inorganic silicon, organic semiconductors, carbon nanotubes, graphene platelets, pressure-sensitive rubber and self-powered devices are highly sensitive and can be applied to human skin. However, the development of a multifunctional sensor satisfying the requirements of ultrahigh mechanosensitivity, flexibility and durability remains a challenge. In nature, spiders sense extremely small variations in mechanical stress using crack-shaped slit organs near their leg joints. Here we demonstrate that sensors based on nanoscale crack junctions and inspired by the geometry of a spider's slit organ can attain ultrahigh sensitivity and serve multiple purposes. The sensors are sensitive to strain (with a gauge factor of over 2,000 in the 0-2 per cent strain range) and vibration (with the ability to detect amplitudes of approximately 10 nanometres). The device is reversible, reproducible, durable and mechanically flexible, and can thus be easily mounted on human skin as an electronic multipixel array. The ultrahigh mechanosensitivity is attributed to the disconnection-reconnection process undergone by the zip-like nanoscale crack junctions under strain or vibration. The proposed theoretical model is consistent with experimental data that we report here. We also demonstrate that sensors based on nanoscale crack junctions are applicable to highly selective speech pattern recognition and the detection of physiological signals. The nanoscale crack junction-based sensory system could be useful in diverse applications requiring ultrahigh displacement sensitivity.

  6. Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system.

    PubMed

    Kang, Daeshik; Pikhitsa, Peter V; Choi, Yong Whan; Lee, Chanseok; Shin, Sung Soo; Piao, Linfeng; Park, Byeonghak; Suh, Kahp-Yang; Kim, Tae-il; Choi, Mansoo

    2014-12-11

    Recently developed flexible mechanosensors based on inorganic silicon, organic semiconductors, carbon nanotubes, graphene platelets, pressure-sensitive rubber and self-powered devices are highly sensitive and can be applied to human skin. However, the development of a multifunctional sensor satisfying the requirements of ultrahigh mechanosensitivity, flexibility and durability remains a challenge. In nature, spiders sense extremely small variations in mechanical stress using crack-shaped slit organs near their leg joints. Here we demonstrate that sensors based on nanoscale crack junctions and inspired by the geometry of a spider's slit organ can attain ultrahigh sensitivity and serve multiple purposes. The sensors are sensitive to strain (with a gauge factor of over 2,000 in the 0-2 per cent strain range) and vibration (with the ability to detect amplitudes of approximately 10 nanometres). The device is reversible, reproducible, durable and mechanically flexible, and can thus be easily mounted on human skin as an electronic multipixel array. The ultrahigh mechanosensitivity is attributed to the disconnection-reconnection process undergone by the zip-like nanoscale crack junctions under strain or vibration. The proposed theoretical model is consistent with experimental data that we report here. We also demonstrate that sensors based on nanoscale crack junctions are applicable to highly selective speech pattern recognition and the detection of physiological signals. The nanoscale crack junction-based sensory system could be useful in diverse applications requiring ultrahigh displacement sensitivity.

  7. Corrosion fatigue of small cracks: Mechanics and chemistry: Final technical progress report, July 1, 1984-October 31, 1987

    SciTech Connect

    Wei, R.P.

    1988-03-07

    The principal contributions of this study have been briefly summarized. Firstly, electrochemical reaction control of corrosion fatigue crack growth in ferrous alloys was demonstrated. The controlling reactions are those involving equilibration of the crack-tip bare surfaces with its neighboring filmed or oxidized surfaces. Secondly, the electrolyte within the crack serves as a high resistance path that separates the crack tip region from the bulk (external) environment. Because of this high resistance, the crack tip region is effectively shielded and the reactions are affected little by the externally imposed conditions, with the crack-tip potential tending to remain very near the free corrosion potential. Thirdly, with increases in test temperature, it becomes increasingly difficult to displace the crack-tip potential away from the free corrosion potential. In other words, because of the increased reaction rate at higher temperatures, larger currents must be imposed to maintain the surface at a prescribed potential. The increased currents, however, result in greater potential drop along the crack, and thereby reduces the change in potential at the crack tip under a fixed applied potential. The details of electrochemical conditions and of the mechanisms of reactions, however, are much more complex, and would require further study. Fourthly, the results provide further support for hydrogen embrittlement as the mechanism for crack growth enhancement for the austenitic stainless and the carbon strengthened steels. Lastly, the role of grain boundaries in influencing short-crack growth response has be elucidated.

  8. Effect of prior cold work on intergranular and transgranular corrosion in type 304 stainless steels: Quantitative discrimination by image analysis

    SciTech Connect

    Garcia, C.; Martin, F.; De Tiedra, P.; Heredero, J.A.; Aparicio, M.L.

    2000-03-01

    Stainless steel (AISI 304 [UNS S30400]) was evaluated as a function of prior cold work and several thermal sensitization treatments. Traditional experimental techniques such as ASTM A262, practice A; modified Strauss test; electrochemical potentiokinetic reactivation (EPR); and electrochemical potentiokinetic reactivation double-loop (EPRDL) were used. Microstructural studies also were conducted. Analysis of the results showed the presence of transgranular attack (TGA) and intergranular attack (IGA) and a transition phenomenon between them. The IGA and TBA contributions to this transition phenomenon were analyzed and quantified by a new procedure based on quantitative metallography performed by image analysis. Using this new methodology, it was possible to determine the most dangerous degrees of deformation for the development of intergranular corrosion (IGC), transgranular corrosion (TGC), intergranular stress corrosion cracking (IGSCC), and transgranular stress corrosion cracking (TGSCC) for different sensitization conditions.

  9. A fracture mechanics approach for estimating fatigue crack initiation in carbon and low-alloy steels in LWR coolant environments

    SciTech Connect

    Park, H. B.; Chopra, O. K.

    2000-04-10

    A fracture mechanics approach for elastic-plastic materials has been used to evaluate the effects of light water reactor (LWR) coolant environments on the fatigue lives of carbon and low-alloy steels. The fatigue life of such steel, defined as the number of cycles required to form an engineering-size crack, i.e., 3-mm deep, is considered to be composed of the growth of (a) microstructurally small cracks and (b) mechanically small cracks. The growth of the latter was characterized in terms of {Delta}J and crack growth rate (da/dN) data in air and LWR environments; in water, the growth rates from long crack tests had to be decreased to match the rates from fatigue S-N data. The growth of microstructurally small cracks was expressed by a modified Hobson relationship in air and by a slip dissolution/oxidation model in water. The crack length for transition from a microstructurally small crack to a mechanically small crack was based on studies on small crack growth. The estimated fatigue S-N curves show good agreement with the experimental data for these steels in air and water environments. At low strain amplitudes, the predicted lives in water can be significantly lower than the experimental values.

  10. Boric acid application guidelines for intergranular corrosion inhibition

    SciTech Connect

    Piskor, S.R. . Nuclear Services Div.)

    1990-12-01

    A significant fraction of the operating Pressurized Water Reactor steam generators have used or are using boric acid as an inhibitor to control stress corrosion cracking, intergranular attack, or denting. Boric acid is applied on line, or by means of crevice flushing, low power soaks, or a combination of these methods. When boric acid is used, it is important to have knowledge about its chemical and physical properties, its effect on corrosion, and its correct application. The data on these subjects may be found in a diversity of sources, which are often not readily available or convenient to use. In addition, new information has recently become available. This report has been prepared and revised to be comprehensive treatise on boric acid relevant to its application in nuclear steam generators. Relevant boric acid information from 1987--89 has been added to provide the latest available data from laboratory testing and power plant application. 5 figs.

  11. Boric acid application guidelines for intergranular corrosion inhibition: Topical report

    SciTech Connect

    Hermer, R.E.

    1987-12-01

    A significant fraction of the operating Pressurized Water Reactor steam generators have used or are using boric acid as an inhibitor to control stress corrosion cracking, intergranular attack, or denting. Boric acid is applied via crevice flushing, low power soaks, on-line, or using a combination of these methods. When boric acid is used it is important to have knowledge about its chemical and physical properties, its effect on corrosion, and how it should be correctly applied. The data on these subjects may be found in a diversity of sources, which are often not readily available or convenient to use. This document has been prepared to be a comprehensive treatise on boric acid relevant to its application in nuclear steam generators. 49 refs., 31 figs., 16 tabs.

  12. NASCRAC - A computer code for fracture mechanics analysis of crack growth

    NASA Technical Reports Server (NTRS)

    Harris, D. O.; Eason, E. D.; Thomas, J. M.; Bianca, C. J.; Salter, L. D.

    1987-01-01

    NASCRAC - a computer code for fracture mechanics analysis of crack growth - is described in this paper. The need for such a code is increasing as requirements grow for high reliability and low weight in aerospace components. The code is comprehensive and versatile, as well as user friendly. The major purpose of the code is calculation of fatigue, corrosion fatigue, or stress corrosion crack growth, and a variety of crack growth relations can be selected by the user. Additionally, crack retardation models are included. A very wide variety of stress intensity factor solutions are contained in the code, and extensive use is made of influence functions. This allows complex stress gradients in three-dimensional crack problems to be treated easily and economically. In cases where previous stress intensity factor solutions are not adequate, new influence functions can be calculated by the code. Additional features include incorporation of J-integral solutions from the literature and a capability for estimating elastic-plastic stress redistribution from the results of a corresponding elastic analysis. An example problem is presented which shows typical outputs from the code.

  13. An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

    NASA Astrophysics Data System (ADS)

    Yan, Wei; Cai, J. B.; Chen, W. Q.

    2011-01-01

    A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.

  14. Subcritical crack-growth behavior of borosilicate glass under cyclic loads: Evidence of a mechanical fatigue effect

    SciTech Connect

    Dill, S.J.; Dauskardt, R.H.; Bennison, S.J.

    1997-03-01

    Amorphous glasses are generally considered immune to mechanical fatigue effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue effect occurs at near threshold subcritical crack-growth rates (da/dt < 3 {times} 10{sup {minus}8} m/s) as the crack extension per cycle approaches the dimensions of the borosilicate glass network. While subcritical crack growth under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack growth, lower growth rates actually exceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack-growth rates. Likely mechanisms for the mechanical fatigue effect are presented.

  15. Mapping the cyclic plastic zone to elucidate the mechanisms of crack tip deformation in bulk metallic glasses

    NASA Astrophysics Data System (ADS)

    Scudino, S.; Shahid, R. N.; Escher, B.; Stoica, M.; Li, B. S.; Kruzic, J. J.

    2017-02-01

    Developing damage-tolerant bulk metallic glasses (BMGs) requires knowledge of the physical mechanisms governing crack propagation. While fractography suggests that fatigue crack propagation occurs in an incremental manner, conclusive evidence of alternating crack tip blunting and resharpening is lacking. By mapping the strain fields in both the monotonic and cyclic plastic zones, it is shown that the characteristic compressive stresses required to resharpen the crack tip are developed in a BMG upon unloading. This result confirms the mechanism of fatigue crack propagation in BMGs. Broader implications of these findings are that the effect of shear banding is rather diffuse and plastic deformation ahead of a stress concentration, such as a crack tip, appears to extend well beyond the extent of visible shear bands on the sample surface.

  16. The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

    NASA Astrophysics Data System (ADS)

    London, B.; Nelson, D. V.; Shyne, J. C.

    1988-10-01

    Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored in K-decreasing tests performed under R = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔKth decreased. Threshold values varied from 2.8 MPa m1/2 (200 °C temper) to 9.5 MPa m1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C.

  17. Molecular dynamics simulations of intergranular fracture in UO2 with nine empirical interatomic potentials

    SciTech Connect

    Yongfeng Zhang; Paul C Millett; Michael R Tonks; Xian-Ming Bai; S Bulent Biner

    2014-09-01

    The intergranular fracture behavior of UO2 was studied using molecular dynamics simulations with a bicrystal model. The anisotropic fracture behavior due to the different grain boundary characters was investigated with the View the MathML source symmetrical tilt S5 and the View the MathML source symmetrical tilt S3 ({1 1 1} twin) grain boundaries. Nine interatomic potentials, seven rigid-ion plus two core–shell ones, were utilized to elucidate possible potential dependence. Initiating from a notch, crack propagation along grain boundaries was observed for most potentials. The S3 boundary was found to be more prone to fracture than the S5 one, indicated by a lower energy release rate associated with the former. However, some potential dependence was identified on the existence of transient plastic deformation at crack tips, and the results were discussed regarding the relevant material properties including the excess energies of metastable phases and the critical energy release rate for intergranular fracture. In general, local plasticity at crack tips was observed in fracture simulations with potentials that predict low excess energies for metastable phases and high critical energy release rates for intergranular fracture.

  18. Damage Tolerance Assessment Handbook. Volume 1. Introduction Fracture Mechanics Fatigue Crack Propagation

    DTIC Science & Technology

    1993-10-01

    Volume ,: Introduction NJ 08405 Fracture Mechanics Fatigue Crack Propagation Research and Special Programs Administration John A. Volpe National...Load-displacement plot [Adapted from John M. Barson/Stanley T. Rolfe, Fracture and Fatigue Control in Structures. Applications of Fracture Mechanics...Methods ASTM STP 527, American Society for Testing and Materials, Philadelphia, PA, 1973. 2-19. Ratwani, M.M. and Wilhem , DP. Develonment and EvaluAtion of

  19. Investigations on crack generation mechanism and crack reduction by buffer layer insertion in thermal-plasma-jet crystallization of amorphous silicon films on glass substrate

    NASA Astrophysics Data System (ADS)

    Tanaka, Keisuke; Hayashi, Shohei; Morisaki, Seiji; Higashi, Seiichiro

    2015-01-01

    The crack generation mechanism and the effect of crack reduction by buffer SiO2 layer insertion in thermal-plasma-jet (TPJ) crystallization of an amorphous silicon film on a glass substrate have been investigated. The crack generation was clearly observed 13.7 s after TPJ irradiation using a high-speed camera, which indicates that cracks are generated not during heating, but during cooling. From the measurement and simulation of substrate deformations, it was clarified that the substrate deformed convexly during heating and it consequently deformed concavely after cooling owing to the substrate surface densification. This result indicated that the tensile stress generated by the concave deformation is the origin of cracks. The deposition of the buffer SiO2 layer generated compressive stress, which minimizes accumulation of tensile stress after TPJ annealing. The number of cracks in unit length significantly decreased owing to the decrease in tensile stress with the increase in the thickness of the buffer SiO2 layer.

  20. ON THE ORIGIN OF INTERGRANULAR JETS

    SciTech Connect

    Yurchyshyn, V. B.; Goode, P. R.; Abramenko, V. I.; Steiner, O.

    2011-08-01

    We observe that intergranular jets, originating in the intergranular space surrounding individual granules, tend to be associated with granular fragmentation, in particular, with the formation and evolution of a bright granular lane (BGL) within individual granules. The BGLs have recently been identified as vortex tubes by Steiner et al. We further discover the development of a well-defined bright grain located between the BGL and the dark intergranular lane to which it is connected. Signatures of a BGL may reach the lower chromosphere and can be detected in off-band H{alpha} images. Simulations also indicate that vortex tubes are frequently associated with small-scale magnetic fields. We speculate that the intergranular jets detected in the New Solar Telescope (NST) data may result from the interaction between the turbulent small-scale fields associated with the vortex tube and the larger-scale fields existing in the intergranular lanes. The intergranular jets are much smaller and weaker than all previously known jet-like events. At the same time, they appear much more numerous than the larger events, leading us to the speculation that the total energy release and mass transport by these tiny events may not be negligible in the energy and mass-flux balance near the temperature minimum atop the photosphere. The study is based on the photospheric TiO broadband (1.0 nm) filter data acquired with the 1.6 m NST operating at the Big Bear Solar Observatory. The data set also includes NST off-band H{alpha} images collected through a Zeiss Lyot filter with a passband of 0.025 nm.

  1. On the Origin of Intergranular Jets

    NASA Astrophysics Data System (ADS)

    Yurchyshyn, V. B.; Goode, P. R.; Abramenko, V. I.; Steiner, O.

    2011-08-01

    We observe that intergranular jets, originating in the intergranular space surrounding individual granules, tend to be associated with granular fragmentation, in particular, with the formation and evolution of a bright granular lane (BGL) within individual granules. The BGLs have recently been identified as vortex tubes by Steiner et al. We further discover the development of a well-defined bright grain located between the BGL and the dark intergranular lane to which it is connected. Signatures of a BGL may reach the lower chromosphere and can be detected in off-band Hα images. Simulations also indicate that vortex tubes are frequently associated with small-scale magnetic fields. We speculate that the intergranular jets detected in the New Solar Telescope (NST) data may result from the interaction between the turbulent small-scale fields associated with the vortex tube and the larger-scale fields existing in the intergranular lanes. The intergranular jets are much smaller and weaker than all previously known jet-like events. At the same time, they appear much more numerous than the larger events, leading us to the speculation that the total energy release and mass transport by these tiny events may not be negligible in the energy and mass-flux balance near the temperature minimum atop the photosphere. The study is based on the photospheric TiO broadband (1.0 nm) filter data acquired with the 1.6 m NST operating at the Big Bear Solar Observatory. The data set also includes NST off-band Hα images collected through a Zeiss Lyot filter with a passband of 0.025 nm.

  2. Adhesion mechanisms of bituminous crack sealant to aggregate and laboratory test development

    NASA Astrophysics Data System (ADS)

    Hajialiakbari Fini, Elham

    Crack sealing is a common pavement maintenance treatment because it extends pavement service life. However, crack sealant often fails prematurely due to a loss of adhesion. Since current test methods are mostly empirical and only provide a qualitative measure of bond strength, they cannot predict sealant adhesive failure accurately. Hence, there is an urgent need for test methods based on bituminous sealant rheology that can better predict sealant field performance. This study introduces three laboratory tests aimed to assess the bond property of hot-poured crack sealant to pavement crack walls. The three tests are designed to serve the respective needs of producers, engineers, and researchers. The first test implements the principle of surface energy to measure the thermodynamic work of adhesion, which is the energy spent in separating the two materials at the interface. The work of adhesion is reported as a measure of material compatibility at an interface. The second test is a direct adhesion test, a mechanical test which is designed to closely resemble both the installation process and the crack expansion due to thermal loading. This test uses the Direct Tension Test (DTT) device. The principle of the test is to apply a tensile force to detach the sealant from its aggregate counterpart. The maximum load, Pmax, and the energy to separation, E, are calculated and reported to indicate interface bonding. The third test implements the principles of fracture mechanics in a pressurized circular blister test. The apparatus is specifically designed to conduct the test for bituminous crack sealant, asphalt binder, or other bitumen-based materials. In this test, a fluid is injected at a constant rate at the interface between the substrate (aggregate or a standard material) and the adhesive (crack sealant) to create a blister. The fluid pressure and blister height are measured as functions of time; the data is used to calculate Interfacial Fracture Energy (IFE), which is a

  3. Is Frost Cracking By Segregation Ice Growth One of the Mechanisms That Erode Bedrock River Margins?

    NASA Astrophysics Data System (ADS)

    Alden, L. L.; Sklar, L. S.

    2014-12-01

    Rivers cut vertically and laterally into bedrock. However, control on the width of bedrock rivers is an unsolved problem. In alpine settings, frost cracking is one of the mechanisms that break down bedrock. Segregation ice drives growth of ice lenses within rock masses. When the temperature of the rock is within the "frost cracking window" of -3 to -8 °C, ice lenses can attract liquid water. Expanding ice lenses can exert sufficient pressure to fracture the rock. We hypothesize that alpine rivers may promote segregation ice growth at the river margin by supplying water, but also may inhibit frost cracking by supplying heat. We find support for this hypothesis in data collected along the Tuolumne and Mokelumne rivers in the Sierra Nevada, California. A 1D heat flow model predicts that frost cracking should occur above 2325 masl in this area. To test for a river effect, we measured fracture density along the Tuolumne River at ~2600 masl, finding that density at the river margin is significantly greater than on adjacent hillslopes in the Cathedral Peak granodiorite. We then deployed data loggers on the Mokelumne River (at 2486 masl) over the winter of 2013/2014 to record water, surface and subsurface rock temperatures at varying depths and distances from the river. Temperatures within the frost cracking window were only recorded at a distance of ~5 m from the river, suggesting an insulating effect from the river and snow cover. Rock temperatures 1 m deep equilibrated at ~ 2 °C, significantly colder than predicted by the 1D model. Ongoing work includes terrestrial LIDAR scans to detect erosion of the river bank at the Mokelumne site, and development of a 2D heat flow model to predict subsurface rock temperatures for varying surface boundary conditions and channel morphology. We expect that further analysis will reveal systematic relationships between the surface boundary conditions and rock temperature at depth, enabling predictive modeling of frost cracking

  4. Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water

    NASA Astrophysics Data System (ADS)

    Moss, Tyler; Was, Gary S.

    2017-04-01

    The objective of this study is to determine whether stress corrosion crack initiation of Alloys 600 and 690 occurs by the same mechanism in subcritical and supercritical water. Tensile bars of Alloys 690 and 600 were strained in constant extension rate tensile experiments in hydrogenated subcritical and supercritical water from 593 K to 723 K (320 °C to 450 °C), and the crack initiation behavior was characterized by high-resolution electron microscopy. Intergranular cracking was observed across the entire temperature range, and the morphology, structure, composition, and temperature dependence of initiated cracks in Alloy 690 were consistent between hydrogenated subcritical and supercritical water. Crack initiation of Alloy 600 followed an Arrhenius relationship and did not exhibit a discontinuity or change in slope after crossing the critical temperature. The measured activation energy was 121 ± 13 kJ/mol. Stress corrosion crack initiation in Alloy 690 was fit with a single activation energy of 92 ± 12 kJ/mol across the entire temperature range. Cracks were observed to propagate along grain boundaries adjacent to chromium-depleted metal, with Cr2O3 observed ahead of crack tips. All measures of the SCC behavior indicate that the mechanism for stress corrosion crack initiation of Alloy 600 and Alloy 690 is consistent between hydrogenated subcritical and supercritical water.

  5. Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water

    NASA Astrophysics Data System (ADS)

    Moss, Tyler; Was, Gary S.

    2017-01-01

    The objective of this study is to determine whether stress corrosion crack initiation of Alloys 600 and 690 occurs by the same mechanism in subcritical and supercritical water. Tensile bars of Alloys 690 and 600 were strained in constant extension rate tensile experiments in hydrogenated subcritical and supercritical water from 593 K to 723 K (320 °C to 450 °C), and the crack initiation behavior was characterized by high-resolution electron microscopy. Intergranular cracking was observed across the entire temperature range, and the morphology, structure, composition, and temperature dependence of initiated cracks in Alloy 690 were consistent between hydrogenated subcritical and supercritical water. Crack initiation of Alloy 600 followed an Arrhenius relationship and did not exhibit a discontinuity or change in slope after crossing the critical temperature. The measured activation energy was 121 ± 13 kJ/mol. Stress corrosion crack initiation in Alloy 690 was fit with a single activation energy of 92 ± 12 kJ/mol across the entire temperature range. Cracks were observed to propagate along grain boundaries adjacent to chromium-depleted metal, with Cr2O3 observed ahead of crack tips. All measures of the SCC behavior indicate that the mechanism for stress corrosion crack initiation of Alloy 600 and Alloy 690 is consistent between hydrogenated subcritical and supercritical water.

  6. Detection of cracking and damage mechanisms in brittle granites by moment tensor analysis of acoustic emission signals

    NASA Astrophysics Data System (ADS)

    Xu, Shi-da; Li, Yuan-hui; Liu, Jian-po

    2017-05-01

    An acoustic emission (AE) testing of rock cracking was performed under uniaxial loading conditions by precut varisized circular holes in selected brittle granites. Based on AE-source location technique and AE-theory for moment tensor analysis, rules of the temporal-spatial evolution of micro-cracks in different failure mechanisms were explored and types of micro-cracks were analyzed as well. The results revealed that the micro-cracks are uniquely easy to generate in the positions where stress are concentrated. Tensile fractures are easy to form on the roof and floor of a circular hole, while shear fractures are easy to be found on both sides. The locations of initial cracks generated around the holes in the loading process are the direction or vertical direction of maximum principle stress. Macroscopic crack orientation agrees with the direction of maximum principle stress approximately. As the size of circular opening increases and the relative size of pillar decreases, shear cracks are dominant with the percentage more than 45%, tension cracks are fewer, accounted for less than 40% of the total events, and mixed-mode cracks represent a minimum proportion, despite the decrease of percentage of shear cracks. The findings of this work can serve for supporting design of tunnel or roadway to avoid collapse.

  7. Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

    NASA Astrophysics Data System (ADS)

    Hall, M. M.

    2011-02-01

    Fitness-for-service evaluations of engineered components that are subject to environment assisted cracking (EAC) often require analyses of potentially large crack extensions through regions of variable stress intensity. However, there are few EAC data and models that directly address the effects of variable stress intensity factor on EAC crack growth. The model developed here is used to evaluate stress corrosion cracking (SCC) data that were obtained on a high-strength beta-titanium alloy under conditions of variable crack mouth opening displacement (CMOD) rate. SCC of this Ti alloy in ambient temperature, near-neutral NaCl aqueous solution is thought to be due to hydrogen environment assisted cracking (HEAC). As the model equations developed here do not admit to a closed form solution for crack velocity as a function of applied stress intensity factor, K, a semiquantitative graphical solution is used to rationalize the crack growth data. The analyses support a previous suggestion that the observed crack growth rate behavior can be attributed to the effect of crack tip strain rate on rates of mechanical disruption and repair of an otherwise protective crack tip oxide film. Model elements introduced here to HEAC modeling include (1) an expression relating corrosion-active surface area to crack tip strain rate and repassivation rate, (2) an expression relating the critical grain boundary hydrogen to the applied stress intensity factor, and (3) an expression relating CTSR to both applied and crack advance strain rate components. Intergranular crack advance is modeled assuming diffusive segregation of corrosion-generated hydrogen to grain boundary trap sites causing embrittlement of the fracture process zone (FPZ). The model equations developed here provide a quantitative basis for understanding the physical significance of K-variation effects and, with additional development, will provide an engineering tool for analysis of crack growth in a variable K field.

  8. Drying and cracking mechanisms in a starch slurry.

    PubMed

    Goehring, Lucas

    2009-09-01

    Starch-water slurries are commonly used to study fracture dynamics. Drying starch cakes benefit from being simple, economical, and reproducible systems, and have been used to model desiccation fracture in soils, thin-film fracture in paint, and columnar joints in lava. In this paper, the physical properties of starch-water mixtures are studied, and used to interpret and develop a multiphase transport model of drying. Starch cakes are observed to have a nonlinear elastic modulus, and a desiccation strain that is comparable to that generated by their maximum achievable capillary pressure. It is shown that a large material porosity is divided between pore spaces between starch grains, and pores within starch grains. This division of pore space leads to two distinct drying regimes, controlled by liquid and vapor transport of water, respectively. The relatively unique ability for drying starch to generate columnar fracture patterns is shown to be linked to the unusually strong separation of these two transport mechanisms.

  9. Drying and cracking mechanisms in a starch slurry

    NASA Astrophysics Data System (ADS)

    Goehring, Lucas

    2009-09-01

    Starch-water slurries are commonly used to study fracture dynamics. Drying starch cakes benefit from being simple, economical, and reproducible systems, and have been used to model desiccation fracture in soils, thin-film fracture in paint, and columnar joints in lava. In this paper, the physical properties of starch-water mixtures are studied, and used to interpret and develop a multiphase transport model of drying. Starch cakes are observed to have a nonlinear elastic modulus, and a desiccation strain that is comparable to that generated by their maximum achievable capillary pressure. It is shown that a large material porosity is divided between pore spaces between starch grains, and pores within starch grains. This division of pore space leads to two distinct drying regimes, controlled by liquid and vapor transport of water, respectively. The relatively unique ability for drying starch to generate columnar fracture patterns is shown to be linked to the unusually strong separation of these two transport mechanisms.

  10. Grain-by-grain study of the mechanisms of crack propagation during iodine stress corrosion cracking of Zircaloy-4

    SciTech Connect

    Haddad, R.E.; Dorado, A.O.

    1994-12-31

    This paper describes the tests conducted to determine the conditions leading to cracking of a specified grain of metal, during the iodine stress corrosion cracking (SCC) of zirconium alloys, focusing on the crystallographic orientation of crack paths, the critical stress conditions, and the significance of the fractographic features encountered. In order to perform crystalline orientation of fracture surfaces, a specially heat-treated Zircaloy-4 having very large grains, grown up to the wall thickness, was used. Careful orientation work has proved that intracrystalline pseudo-cleavage occurs only along basal planes. the effects of anisotropy, plasticity, triaxiality, and residual stresses originated in thermal contraction have to be considered to account for the influence of the stress state. A grain-by-grain calculation led to the conclusion that transgranular cracking always takes place on those bearing the maximum resolved tensile stress perpendicular to basal planes. Propagation along twin boundaries has been identified among the different fracture modes encountered.

  11. Investigation of Hot Cracking Behavior in Transverse Mechanically Arc Oscillated Autogenous AA2014 T6 TIG Welds

    NASA Astrophysics Data System (ADS)

    Biradar, N. S.; Raman, R.

    2012-09-01

    Hot cracking studies on autogenous AA2014 T6 TIG welds were carried out. Significant cracking was observed during linear and circular welding test (CWT) on 4-mm-thick plates. Weld metal grain structure and amount of liquid distribution during the terminal stages of solidification were the key cause for hot cracking in aluminum welds. Square-wave AC TIG welding with transverse mechanical arc oscillation (TMAO) was employed to study the cracking behavior during linear and CWT. TMAO welds with amplitude = 0.9 mm and frequency = 0.5 Hz showed significant reduction in cracking tendency. The increase in cracking resistance in the arc-oscillated weld was attributed to grain refinement and improved weld bead morphology, which improved the weld metal ductility and uniformity, respectively, of residual tensile stresses that developed during welding. The obtained results were comparable to those of reported favorable results of electromagnetic arc oscillation.

  12. Crack Growth Mechanisms under Anti-Plane Shear in Composite Laminates

    NASA Astrophysics Data System (ADS)

    Horner, Allison Lynne

    The research conducted for this dissertation focuses on determining the mechanisms associated with crack growth in polymer matrix composite laminates subjected to anti-plane shear (mode III) loading. For mode III split-beam test methods were proposed, and initial evaluations were conducted. A single test method was selected for further evaluation. Using this test method, it was determined that the apparent mode III delamination toughness, GIIIc , depended on geometry, which indicated a true material property was not being measured. Transverse sectioning and optical microscopy revealed an array of transverse matrix cracks, or echelon cracks, oriented at approximately 45° and intersecting the plane of the delamination. Subsequent investigations found the echelon array formed prior to the onset of planar delamination advance and that growth of the planar delamination is always coupled to echelon array formation in these specimens. The evolution of the fracture surfaces formed by the echelon array and planar delamination were studied, and it was found that the development was similar to crack growth in homogenous materials subjected to mode III or mixed mode I-III loading, although the composite laminate architecture constrained the fracture surface development differently than homogenous materials. It was also found that, for split-beam specimens such as those used herein, applying an anti-plane shear load results in twisting of the specimen's uncracked region which gives rise to a mixed-mode I-III load condition. This twisting has been related to the apparent mode III toughness as well as the orientation of the transverse matrix cracks. A finite element model was then developed to study the mechanisms of initial echelon array formation. From this, it is shown that an echelon array will develop, but will become self-limiting prior to the onset of planar delamination growth.

  13. Fan-head shear rupture mechanism as a source of off-fault tensile cracking

    NASA Astrophysics Data System (ADS)

    Tarasov, Boris

    2016-04-01

    This presentation discusses the role of a recently identified fan-head shear rupture mechanism [1] in the creation of off-fault tensile cracks observed in earthquake laboratory experiments conducted on brittle photoelastic specimens [2,3]. According to the fan-mechanism the shear rupture propagation is associated with consecutive creation of small slabs in the fracture tip which, due to rotation caused by shear displacement of the fracture interfaces, form a fan-structure representing the fracture head. The fan-head combines such unique features as: extremely low shear resistance (below the frictional strength) and self-sustaining tensile stress intensification along one side of the interface. The variation of tensile stress within the fan-head zone is like this: it increases with distance from the fracture tip up to a maximum value and then decreases. For the initial formation of the fan-head high local stresses corresponding to the fracture strength should be applied in a small area, however after completions of the fan-head it can propagate dynamically through the material at low shear stresses (even below the frictional strength). The fan-mechanism allows explaining all unique features associated with the off-fault cracking process observed in photoelastic experiments [2,3]. In these experiments spontaneous shear ruptures were nucleated in a bonded, precut, inclined and pre-stressed interface by producing a local pressure pulse in a small area. Isochromatic fringe patterns around a shear rupture propagating along bonded interface indicate the following features of the off-fault tensile crack development: tensile cracks nucleate and grow periodically along one side of the interface at a roughly constant angle (about 80 degrees) relative to the shear rupture interface; the tensile crack nucleation takes place some distance behind the rupture tip; with distance from the point of nucleation tensile cracks grow up to a certain length within the rupture head zone

  14. Environmental degradation and life time prediction of generator retaining rings subject to stress corrosion cracking

    SciTech Connect

    Speidel, M.O.; Magdowski, R.

    1995-12-31

    Retaining rings are among the most highly stressed components of generator rotors. There is a history of service failures of such retaining rings in both fossil fired and in nuclear power systems. The predominant failure mode is intergranular stress corrosion cracking. For a complete life time prediction it is necessary to know the effect of materials, stresses, and environments on nucleation, re-initiation, and growth of stress corrosion cracks. The present paper contributes information on the above topics. This includes time to failure measurements of stressed cylindrical tensile specimens in water and re-initiation measurements on components containing stress corrosion cracks which had been stopped by a more benign environment. Also included are fracture mechanics stress corrosion crack growth rate measurements in two different commercial retaining ring materials as a function of stress intensity. Conclusions are drawn with respect to materials choice, operating conditions, and inspection intervals.

  15. Representing Matrix Cracks Through Decomposition of the Deformation Gradient Tensor in Continuum Damage Mechanics Methods

    NASA Technical Reports Server (NTRS)

    Leone, Frank A., Jr.

    2015-01-01

    A method is presented to represent the large-deformation kinematics of intraply matrix cracks and delaminations in continuum damage mechanics (CDM) constitutive material models. The method involves the additive decomposition of the deformation gradient tensor into 'crack' and 'bulk material' components. The response of the intact bulk material is represented by a reduced deformation gradient tensor, and the opening of an embedded cohesive interface is represented by a normalized cohesive displacement-jump vector. The rotation of the embedded interface is tracked as the material deforms and as the crack opens. The distribution of the total local deformation between the bulk material and the cohesive interface components is determined by minimizing the difference between the cohesive stress and the bulk material stress projected onto the cohesive interface. The improvements to the accuracy of CDM models that incorporate the presented method over existing approaches are demonstrated for a single element subjected to simple shear deformation and for a finite element model of a unidirectional open-hole tension specimen. The material model is implemented as a VUMAT user subroutine for the Abaqus/Explicit finite element software. The presented deformation gradient decomposition method reduces the artificial load transfer across matrix cracks subjected to large shearing deformations, and avoids the spurious secondary failure modes that often occur in analyses based on conventional progressive damage models.

  16. Creep-Environment Interactions in Dwell-Fatigue Crack Growth of Nickel Based Superalloys

    NASA Astrophysics Data System (ADS)

    Maciejewski, Kimberly; Dahal, Jinesh; Sun, Yaofeng; Ghonem, Hamouda

    2014-05-01

    A multi-scale, mechanistic model is developed to describe and predict the dwell-fatigue crack growth rate in the P/M disk superalloy, ME3, as a function of creep-environment interactions. In this model, the time-dependent cracking mechanisms involve grain boundary sliding and dynamic embrittlement, which are identified by the grain boundary activation energy, as well as, the slip/grain boundary interactions in both air and vacuum. Modeling of the damage events is achieved by adapting a cohesive zone (CZ) approach which considers the deformation behavior of the grain boundary element at the crack tip. The deformation response of this element is controlled by the surrounding continuum in both far field (internal state variable model) and near field (crystal plasticity model) regions and the intrinsic grain boundary viscosity which defines the mobility of the element by scaling up the motion of dislocations into a mesoscopic scale. This intergranular cracking process is characterized by the rate at which the grain boundary sliding reaches a critical displacement. A damage criterion is introduced by considering the grain boundary mobility limit in the tangential direction leading to strain incompatibility and failure. Results of simulated intergranular crack growth rate using the CZ model are generated for temperatures ranging from 923 K to 1073 K (650 °C to 800 °C), in both air and vacuum. These results are compared with those experimentally obtained and analysis of the model sensitivity to loading conditions, particularly temperature and oxygen partial pressure, are presented.

  17. Fracture-Based Mesh Size Requirements for Matrix Cracks in Continuum Damage Mechanics Models

    NASA Technical Reports Server (NTRS)

    Leone, Frank A.; Davila, Carlos G.; Mabson, Gerald E.; Ramnath, Madhavadas; Hyder, Imran

    2017-01-01

    This paper evaluates the ability of progressive damage analysis (PDA) finite element (FE) models to predict transverse matrix cracks in unidirectional composites. The results of the analyses are compared to closed-form linear elastic fracture mechanics (LEFM) solutions. Matrix cracks in fiber-reinforced composite materials subjected to mode I and mode II loading are studied using continuum damage mechanics and zero-thickness cohesive zone modeling approaches. The FE models used in this study are built parametrically so as to investigate several model input variables and the limits associated with matching the upper-bound LEFM solutions. Specifically, the sensitivity of the PDA FE model results to changes in strength and element size are investigated.

  18. Model Predictions of Chemically Controlled Slow Crack Growth with Application to Mechanical Effects in Geothermal Environments

    SciTech Connect

    Viani, B E

    2001-04-11

    Representative, simplified geothermal rock-fluid systems are investigated with a modeling approach to estimate how rock water interactions affect coupled properties related to mechanical stability and permeability improvement through fracturing. First, geochemical modeling is used to determine the evolution of fluid chemistry at temperatures up to 300 C when fluids are in contact with representative rocks of continental origin. Then, a kinetic crack growth model for quartz is used to predict growth rate for subcritical cracks in acidic and basic environments. The predicted growth rate is highly sensitive to temperature and pH in the ranges tested. At present, the model is limited to situations in which quartz controls the mechanical process of interest, such as well bore stability in silica cemented rocks and the opening of quartz filled veins to enhance permeability.

  19. The Role of Microtexture on Fatigue Lifetime Variability and Crack Initiation Mechanisms (Preprint)

    DTIC Science & Technology

    2011-10-01

    October 2011 4 . TITLE AND SUBTITLE THE ROLE OF MICROTEXTURE OF FATIGUE LIFETIME VARIABILITY AND CRACK INITIATION MECHANISMS (PREPRINT) 5a...CONTRACT NUMBER In-house 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 62102F 6 . AUTHOR(S) Christopher J. Szczepanski (Universal Technology...Preprint journal article to be submitted to Titanium 2011 World Conference. This document contains color. 14. ABSTRACT Commercial titanium alloys

  20. Analysis of cracked core spray injection line piping from the Quad Cities Units 1 and 2 boiling water reactors

    SciTech Connect

    Diercks, D.R.

    1983-12-01

    Elbow assemblies and adjacent piping from the loops A and B core spray injection lines of Quad Cities Units 1 and 2 Boiling Water Reactors have been examined in order to determine the nature and causes of coolant leakages and flaw indications detected during hydrostatic tests and subsequent ultrasonic inspections. The elbow assemblies were found to contain multiple intergranular cracks in the weld heat-affected zones. The cracking was predominantly axial in orientation in the forged elbow and wedge components, whereas mixed axial and circumferential cracking was seen in the wrought piping pieces. In at least two instances, axial cracks completely penetrated the circumferential weld joining adjacent components. Based upon the observations made in the present study, the failures were attributed to intergranular stress corrosion cracking caused by the weld-induced sensitized microstructure and residual stresses present; dissolved oxygen in the reactor coolant apparently served as the corrosive species. The predominantly axial orientation of the cracks present in the forged components is believed to be related to the banded microstructure present in these components. The metallographic studies reported are supplemented by x-radiography, chemical analysis and mechanical test results, determinations of the degree of sensitization present, and measurements of weld metal delta ferrite content.

  1. Subcritical crack propagation as a mechanism of crevasse formation and iceberg calving

    NASA Astrophysics Data System (ADS)

    Weiss, Jérôme

    Recent investigations of crevassing on alpine glaciers and ice shelves have been based on linear elastic fracture mechanics (LEFM). However, LEFM is unable to explain some aspects of crevasse formation such as the initiation of crevasse propagation from crystal-scale (mm) microcracks, the slow propagation of large fractures in ice shelves, and the acceleration of crevasse opening before breaking of the ice terminus. Here another mechanism to account for these observations is proposed: subcritical crevassing. Subcritical crack growth, documented in many materials though not yet explored in ice, is characterized by a crack velocity that scales as a power of the tensile stress intensity factor, but is much less than that associated with critical crack propagation. This mechanism allows crevasse propagation from mm-scale microcracks at velocities much lower than body wave speeds, and explains crevasse-opening accelerations in a natural way. Subcritical crevassing is theoretically explored for several simplified situations but is limited by a lack of available data on crevasse evolution.

  2. Mechanics and crack formation in the extracellular matrix with articular cartilage as a model system

    NASA Astrophysics Data System (ADS)

    Kearns, Sarah; Silverberg, Jesse; Bonassar, Lawrence; Cohen, Itai; Das, Moumita

    We investigate the mechanical structure-function relations in the extracellular matrix (ECM) with focus on crack formation and failure. As a model system, our study focuses on the ECM in articular cartilage (AC), the tissue that covers the ends of bones, and distributes load in joints including in the knees, shoulders, and hips. The strength, toughness, and crack resistance of native articular cartilage is unparalleled in materials made by humankind. This mechanical response is mainly due to its ECM. The ECM in AC has two major mechanobiological components: a network of the biopolymer collagen and a flexible aggrecan gel. We model this system as a biopolymer network embedded in a swelling gel, and investigate the conditions for the formation and propagation of cracks using a combination of rigidity percolation theory and energy minimization approaches. Our results may provide useful insights into the design principles of the ECM as well as of biomimetic hydrogels that are mechanically robust and can, at the same time, easily adapt to cues in their surroundings. This work was partially supported by a Cottrell College Science Award.

  3. Theoretical study of the elasticity, mechanical behavior, electronic structure, interatomic bonding, and dielectric function of an intergranular glassy film model in prismatic {beta}-Si{sub 3}N{sub 4}

    SciTech Connect

    Ching, W. Y.; Rulis, Paul; Aryal, Sitaram; Ouyang, Lizhi; Misra, Anil

    2010-06-01

    Microstructures such as intergranular glassy films (IGFs) are ubiquitous in many structural ceramics. They control many of the important physical properties of polycrystalline ceramics and can be influenced during processing to modify the performance of devices that contain them. In recent years, there has been intense research, both experimentally and computationally, on the structure and properties of IGFs. Unlike grain boundaries or dislocations with well-defined crystalline planes, the atomic scale structure of IGFs, their fundamental electronic interactions, and their bonding characteristics are far more complicated and not well known. In this paper, we present the results of theoretical simulations using ab initio methods on an IGF model in {beta}-Si{sub 3}N{sub 4} with prismatic crystalline planes. The 907-atom model has a dimension of 14.533 A x 15.225 A x 47.420 A . The IGF layer is perpendicular to the z axis, 16.4 A wide, and contains 72 Si, 32 N, and 124 O atoms. Based on this model, the mechanical and elastic properties, the electronic structure, the interatomic bonding, the localization of defective states, the distribution of electrostatic potential, and the optical dielectric function are evaluated and compared with crystalline {beta}-Si{sub 3}N{sub 4}. We have also performed a theoretical tensile experiment on this model by incrementally extending the structure in the direction perpendicular to the IGF plane until the model fully separated. It is shown that fracture occurs at a strain of 9.42% with a maximum stress of 13.9 GPa. The fractured segments show plastic behavior and the formation of surfacial films on the {beta}-Si{sub 3}N{sub 4}. These results are very different from those of a previously studied basal plane model [J. Chen et al., Phys. Rev. Lett. 95, 256103 (2005)] and add insights to the structure and behavior of IGFs in polycrystalline ceramics. The implications of these results and the need for further investigations are discussed.

  4. A Fracture-Mechanical Model of Crack Growth and Interaction: Application to Pre-eruptive Seismicity

    NASA Astrophysics Data System (ADS)

    Matthews, C.; Sammonds, P.; Kilburn, C.

    2007-12-01

    A greater understanding of the physical processes occurring within a volcano is a key aspect in the success of eruption forecasting. By considering the role of fracture growth, interaction and coalescence in the formation of dykes and conduits as well as the source mechanism for observed seismicity we can create a more general, more applicable model for precursory seismicity. The frequency of volcano-tectonic earthquakes, created by fracturing of volcanic rock, often shows a short-term increase prior to eruption. Using fracture mechanics, the model presented here aims to determine the conditions necessary for the acceleration in fracture events which produces the observed pre-eruptive seismicity. By focusing on the cause of seismic events rather than simply the acceleration patterns observed, the model also highlights the distinction between an accelerating seismic sequence ending with an eruption and a short-term increase which returns to background levels with no activity occurring, an event also observed in the field and an important capability if false alarms are to be avoided. This 1-D model explores the effects of a surrounding stress field and the distribution of multi-scale cracks on the interaction and coalescence of these cracks to form an open pathway for magma ascent. Similarly to seismic observations in the field, and acoustic emissions data from the laboratory, exponential and hyperbolic accelerations in fracturing events are recorded. Crack distribution and inter-crack distance appears to be a significant controlling factor on the evolution of the fracture network, dominating over the effects of a remote stress field. The generality of the model and its basis on fundamental fracture mechanics results makes it applicable to studies of fracture networks in numerous situations. For example looking at the differences between high temperature fracture processes and purely brittle failure the model can be similarly applied to fracture dynamics in the

  5. Finite element analysis of surface cracks in the Wilkins Ice Shelf using fracture mechanics

    NASA Astrophysics Data System (ADS)

    Plate, Carolin; Müller, Ralf; Gross, Dietmar; Humbert, Angelika; Braun, Matthias

    2010-05-01

    Ice shelves, located between the warming atmosphere and the ocean, are sensitive elements of the climate system. The Wilkins Ice Shelf is situated in the south-western part of the Antarctic Peninsula, a well known hot spot of global warming. Recent break-up events exemplified the potential of disintegration of the ice shelf. A multi interdisciplinary project consisting of remote sensing, modeling of the ice dynamics and fracture mechanics intends to improve the understanding of the impacts of temperature increase on ice shelf stability. As a part of this project the aim of this presentation is to demonstrate the fracture mechanical approach using finite elements and configurational forces. For fracture mechanical purposes the material behavior of ice is treated as a brittle solid, and linear fracture mechanics is used. Crucial to all methods in linear fracture mechanics is the evaluation of the stress intensity factor K which is a measure for the load concentration at the crack tip and which depends on the geometry of the body and on the applied loading. The computed value of K can be compared to the critical stress intensity factor Kc, a material property obtained from experimental examinations, to judge whether a crack will propagate. One very effective procedure to obtain the stress intensity factor takes advantage of configurational forces, which can be easily obtained in the finite element analysis. An initial investigation is based on a 2-dimensional analysis of a single crack with a mode-I load type using a static plane strain model in the finite element analysis software COMSOL and additional routines to compute and evaluate the configurational forces. Analytical solutions of simple geometry and load cases are called on in comparison. The application to the Wilkins Ice Shelf follows by using material parameters, geometries and loading situations, which are obtained from literature values, remote sensing data analysis and modeling of the ice dynamics

  6. Matrix Fatigue Cracking Mechanisms of Alpha(2) TMC for Hypersonic Applications

    NASA Technical Reports Server (NTRS)

    Gabb, Timothy P.; Gayda, John

    1994-01-01

    The objective of this work was to understand matrix cracking mechanisms in a unidirectional alpha(sub 2) TMC in possible hypersonic applications. A (0)(sub 8) SCS-6/Ti-24Al-11Nb (at. percent) TMC was first subjected to a variety of simple isothermal and nonisothermal fatigue cycles to evaluate the damage mechanisms in simple conditions. A modified ascent mission cycle test was then performed to evaluate the combined effects of loading modes. This cycle mixes mechanical cycling at 150 and 483 C, sustained loads, and a slow thermal cycle to 815 C. At low cyclic stresses and strains more common in hypersonic applications, environment-assisted surface cracking limited fatigue resistance. This damage mechanism was most acute for out-of-phase nonisothermal cycles having extended cycle periods and the ascent mission cycle. A simple linear fraction damage model was employed to help understand this damage mechanism. Time-dependent environmental damage was found to strongly influence out-of-phase and mission life, with mechanical cycling damage due to the combination of external loading and CTE mismatch stresses playing a smaller role. The mechanical cycling and sustained loads in the mission cycle also had a smaller role.

  7. Some considerations about the possible mechanisms of lead assisted stress corrosion cracking of steam generator tubing

    SciTech Connect

    Helie, M.; Lambert, I.; Santarini, G.

    1995-12-31

    A number of experimental studies have been performed since the examination of tubes pulled from nuclear power plants revealed the presence of lead associated with secondary side mixed Inter/Trans-Granular Stress Corrosion Cracking (IG/TG SCC). Various testing methods were employed on Alloys 600, 690, and 800, and the available studies involve Constant Elongation Rate Tests (CERTs), constant deformation tests in caustic, neutral, or acidic solutions, as well as electrochemical tests and surface layer analyses. Complementary tests are presented on the influence of the solution temperature, and of the elongation rate in CERTS. An analysis of the various results obtained is interpreted in the framework of a mechanism involving both anodic dissolution and fracture events. This analysis indicates that such a mechanism can account for the influence of lead by considering a lowering of the global activation energy of the repassivation processes without changes in the apparent activation energy of the crack growth rate. Comparison to in-service cracking also indicates that the influence of lead seems more predominant in neutral pH solutions than in caustic or acidic ones.

  8. The mechanics of moisture-expansion cracking in fired-clay ceramics

    NASA Astrophysics Data System (ADS)

    Hamilton, Andrea; Hall, Christopher

    2013-03-01

    Samian ware (or terra sigillata) is a type of fired-clay ceramic produced at a number of sites in France in the period 50 BC to 200 AD and widely traded in Western Europe. It has a characteristic high-gloss surface, formed by application of a non-calcareous clay slip to the green body before firing. New SEM observations show that the slip layer is frequently crazed, although the cracks are not usually visible to the unaided eye. We discuss the mechanics of the crazing, and show that the cracking is driven by rehydroxylation (RHX) moisture expansion. Observations and analysis aid in understanding the RHX dating of archaeological pottery by showing that craze networks permit efficient transport of moisture through the slip layer.

  9. Reaction Control System Thruster Cracking Consultation: NASA Engineering and Safety Center (NESC) Materials Super Problem Resolution Team (SPRT) Findings

    NASA Technical Reports Server (NTRS)

    MacKay, Rebecca A.; Smith, Stephen W.; Shah, Sandeep R.; Piascik, Robert S.

    2005-01-01

    The shuttle orbiter s reaction control system (RCS) primary thruster serial number 120 was found to contain cracks in the counter bores and relief radius after a chamber repair and rejuvenation was performed in April 2004. Relief radius cracking had been observed in the 1970s and 1980s in seven thrusters prior to flight; however, counter bore cracking had never been seen previously in RCS thrusters. Members of the Materials Super Problem Resolution Team (SPRT) of the NASA Engineering and Safety Center (NESC) conducted a detailed review of the relevant literature and of the documentation from the previous RCS thruster failure analyses. It was concluded that the previous failure analyses lacked sufficient documentation to support the conclusions that stress corrosion cracking or hot-salt cracking was the root cause of the thruster cracking and lacked reliable inspection controls to prevent cracked thrusters from entering the fleet. The NESC team identified and performed new materials characterization and mechanical tests. It was determined that the thruster intergranular cracking was due to hydrogen embrittlement and that the cracking was produced during manufacturing as a result of processing the thrusters with fluoride-containing acids. Testing and characterization demonstrated that appreciable environmental crack propagation does not occur after manufacturing.

  10. Aqueous sodium chloride induced intergranular corrosion of Al-Li-Cu alloys

    NASA Technical Reports Server (NTRS)

    Pizzo, P. P.; Daeschner, D. L.

    1986-01-01

    Two methods have been explored to assess the susceptibility of Al-Li-Cu alloys to intergranular corrosion in aqueous sodium chloride solution. They are: (1) constant extension rate testing with and without alternate-immersion preexposure and (2) metallographic examination after exposure to a NaCl-H2O2 corrosive solution per Mil-H-6088F. Intergranular corrosion was found to occur in both powder and ingot metallurgy alloys of similar composition, using both methods. Underaging rendered the alloys most susceptible. The results correlate to stress-corrosion data generated in conventional time-to-failure and crack growth-rate tests. Alternate-immersion preexposure may be a reliable means to assess stress corrosion susceptibility of Al-Li-Cu alloys.

  11. Role of Localized Deformation in Irradiation-Assisted Stress Corrosion Cracking Initiation

    NASA Astrophysics Data System (ADS)

    West, Elaine A.; McMurtrey, Michael D.; Jiao, Zhijie; Was, Gary S.

    2012-01-01

    Intergranular cracking of irradiated austenitic alloys depended on localized grain boundary stress and deformation in both high-temperature aqueous and argon environments. Tensile specimens were irradiated with protons to doses of 1 to 7 dpa and then strained in high-temperature argon, simulated boiling water reactor normal water chemistry, and supercritical water environments. Quantitative measurements confirmed that the initiation of intergranular cracks was promoted by (1) the formation of coarse dislocation channels, (2) discontinuous slip across grain boundaries, (3) a high inclination of the grain boundary to the tensile axis, and (4) low-deformation propensity of grains as characterized by their Schmid and Taylor factors. The first two correlations, as well as the formation of intergranular cracks at the precise locations of dislocation channel-grain boundary intersections are evidence that localized deformation drives crack initiation. The latter two correlations are evidence that intergranular cracking is promoted at grain boundaries experiencing elevated levels of normal stress.

  12. Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

    NASA Astrophysics Data System (ADS)

    Lammi, Christopher J.; Lados, Diana A.

    2012-01-01

    Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and K max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter Δ K- K max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.

  13. Some stochastic aspects of intergranular creep cavitation

    SciTech Connect

    Fariborz, S.J.; Farris, J.P.; Harlow, D.G.; Delph, T.J.

    1987-10-01

    We present some results obtained from a simplified stochastic model of intergranular creep cavitation. The probabilistic features of the model arise from the inclusion of random cavity placement on the grain boundary and time-discrete stochastic cavity nucleation. Among the predictions of the model are Weibull-distributed creep rupture failure times and a Weibull distribution of cavity radii. Both of these predictions have qualitative experimental support. 18 refs., 7 figs.

  14. Mechanisms of time-dependent crack growth at elevated temperature. Final project report, July 1, 1986--August 31, 1989

    SciTech Connect

    Saxena, A.; Stock, S.R.

    1990-04-15

    Objective of this 3-y study was to conduct creep and creep-fatigue crack growth experiments and to characterize the crack tip damage mechanisms in a model material (Cu-1wt%Sb), which is known to cavitate at grain boundaries under creep deformation. Results were: In presence of large scale cavitation damage and crack branching, time rate of creep crack growth da/dt does not correlate with C{sub t} or C{sup *}. When cavitation damage is constrained, da/dt is characterized by C{sub t}. Area fraction of grain boundary cavitated is the single damage parameter for the extent of cavitation damage ahead of crack tips. C{sub t} is used for the creep-fatigue crack growth behavior. In materials prone to rapid cavity nucleation, creep cracks grow faster initially and then reach a steady state whose growth rate is determined by C{sub t}. Percent creep life exhausted correlates with average cavity diameter and fraction of grain boundary area occupied by cavities. Synchrotron x-ray tomographic microscopy was used to image individual cavities in Cu-1wt% Sb. A methodology was developed for predicting the remaining life of elevated temperature power plant components; (C{sub t}){sub avg} was used to correlate creep-fatigue crack growth in Cr-Mo and Cr-Mo-V steel and weldments.

  15. Eddy current modeling by finite element method for evaluation of mechanical properties of the structure cracked in absolute probe

    NASA Astrophysics Data System (ADS)

    Harzallah, Salaheddine; Chabaat, Mohamed; Belgacem, Fethi Bin Muhammad

    2014-12-01

    In this paper, a nondestructive evaluation by sensor Eddy current is used as a tool to control cracks and micro-cracks in materials. A simulation by a numerical approach based on the finite element method is employed to detect cracks in materials and eventually to study their propagation using a crucial parameter such as a Stress Intensity Factor (SIF). This method has emerged as one of the most efficient techniques for prospecting cracks in materials, evaluating SIFs and analyzing crack's growth in the context of linear elastic fracture mechanics (LEFM). This technique uses extrapolation of displacements from results compared with those obtained by the integral interaction. On the other hand, crack's growth is analyzed as a model by combining the maximum circumferential stress criteria with the critical plane for predicting the direction of crack growth. Moreover, a constant crack growth increment is determined using the modified Paris's model. Furthermore, stress intensity factors needed for these models are calculated using the domain form of the J-integral interactions.

  16. Effects of strain rate and surface cracks on the mechanical behaviour of Balmoral Red granite

    NASA Astrophysics Data System (ADS)

    Mardoukhi, Ahmad; Mardoukhi, Yousof; Hokka, Mikko; Kuokkala, Veli-Tapani

    2017-01-01

    This work presents a systematic study on the effects of strain rate and surface cracks on the mechanical properties and behaviour of Balmoral Red granite. The tensile behaviour of the rock was studied at low and high strain rates using Brazilian disc samples. Heat shocks were used to produce samples with different amounts of surface cracks. The surface crack patterns were analysed using optical microscopy, and the complexity of the patterns was quantified by calculating the fractal dimensions of the patterns. The strength of the rock clearly drops as a function of increasing fractal dimensions in the studied strain rate range. However, the dynamic strength of the rock drops significantly faster than the quasi-static strength, and, because of this, also the strain rate sensitivity of the rock decreases with increasing fractal dimensions. This can be explained by the fracture behaviour and fragmentation during the dynamic loading, which is more strongly affected by the heat shock than the fragmentation at low strain rates. This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

  17. A study of the mechanism of primary water stress corrosion cracking of Alloy 600

    SciTech Connect

    Gourgues, A.F.; Andrieu, E.; Scott, P.M.

    1995-12-31

    Two aspects of the mechanism of stress corrosion cracking of Alloy 600 in pressurized water reactors (PWR) primary water have been studied in detail. Results are presented showing that grain boundaries of Alloy 600 are embrittled to a depth of several microns by exposure to primary water in an unstressed condition. It has been established that this embrittlement is not reversible by high temperature degassing and cannot be directly due to hydrogen. The results seem to support the hypothesis that oxygen atom penetration of grain boundaries is possible. However, no evidence of formation of grain boundary gas bubbles or oxides has been found. It is envisaged that this embrittlement process could sequentially act at the tip of a growing stress corrosion crack. The second phenomenon under study has been the plastic deformation behavior of Alloy 600 since it is known that cold work and stress have an important effect on stress corrosion cracking sensitivity. Results of plastic deformation during cyclic straining at various controlled strain rates are presented showing that Alloy 600 is not very sensitive to loading history and that cold work is of an essentially kinematic nature.

  18. Effects of strain rate and surface cracks on the mechanical behaviour of Balmoral Red granite.

    PubMed

    Mardoukhi, Ahmad; Mardoukhi, Yousof; Hokka, Mikko; Kuokkala, Veli-Tapani

    2017-01-28

    This work presents a systematic study on the effects of strain rate and surface cracks on the mechanical properties and behaviour of Balmoral Red granite. The tensile behaviour of the rock was studied at low and high strain rates using Brazilian disc samples. Heat shocks were used to produce samples with different amounts of surface cracks. The surface crack patterns were analysed using optical microscopy, and the complexity of the patterns was quantified by calculating the fractal dimensions of the patterns. The strength of the rock clearly drops as a function of increasing fractal dimensions in the studied strain rate range. However, the dynamic strength of the rock drops significantly faster than the quasi-static strength, and, because of this, also the strain rate sensitivity of the rock decreases with increasing fractal dimensions. This can be explained by the fracture behaviour and fragmentation during the dynamic loading, which is more strongly affected by the heat shock than the fragmentation at low strain rates.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

  19. Application of the damage mechanics to the description of multiple cracks development in shales

    NASA Astrophysics Data System (ADS)

    Izvekov, Oleg

    2014-05-01

    Oil and gas shales are one of the most perspective sources of hydrocarbons. Damage processes are in the focus of any technology of oil shales development because of their extremely low permeability. As a rule the aim of stimulation treatments is to make a system of multiple cracks. Real rock masses are almost heterogeneous. Strength of layered rocks like shales has anisotropic properties. Damage mechanics gives one of the natural ways of description of multiple cracks development. The phenomenological model of multiple cracks evolution in porous media based on general principles of thermodynamics [Kondaurov V.I., Izvekov O.Y., 2009] was generalized to the case of layered rocks. This model takes into account elastic domain existence, dependency of elastic domain on orientation of axis of anisotropy, reduction of elasticity modulus in active process, permeability and porosity change. The model involves latent energy of damage and elastic energy release due to damage evolution. In the report some coupled problems of damage and filtration are discussed. This work was supported by Russian President Grant for Young Scientists MK-7249.2013.5. Kondaurov V.I., Izvekov O.Y. A Model of Saturated Porous Media with an Elastic Brittle Skeleton // Proc. of the 4-th Biot Conference on Poromechanics, POROMECHANICS IV. - EStech Publications, Inc., PA,USA, 2009.

  20. Metallurgical investigation into ductility dip cracking in nickel based alloys

    NASA Astrophysics Data System (ADS)

    Noecker, Fredrick F., II

    A690 is a Ni-Cr-Fe alloy with excellent resistance to general corrosion, localized corrosion and stress corrosion cracking. However, the companion filler metal for A690, EN52, has been shown by several researchers to be susceptible to ductility dip cracking (DDC), which limits its widespread use in joining applications. The Gleeble hot ductility test was used to evaluate the DDC susceptibility of A600 and A690, along with their filler metals, EN82H and EN52, throughout the heating and cooling portions of a simulated weld reheat thermal cycle. Both macroscopic mechanical measures and microscopic measures of DDC were quantified and compared. Water quenching was conducted at select temperatures for subsequent microstructural characterization. Microstructural and microchemical characterization was carried out using scanning electron microscopy, transmission electron microscopy and analytical electron microscopy (AEM) techniques. The greatest resistance to DDC was observed in A600 and A690 during heating, where no DDC cracks formed even when the samples were fractured. Both A690 and EN52 were found to form an intermediate on-cooling dip in ductility and UTS, which corresponded to an increase in ductility dip crack length. The hot ductility and cracking resistance of EN82H remained high throughout the entire thermal cycle. DDC susceptibility in both EN52 and EN82H decreased when the thermal cycle was modified to promote coarsening/precipitation of intergranular carbides prior to straining. AEM analysis did not reveal any sulfur or phosphorous intergranular segregation in EN52 at 1600°F on-heating, on-cooling or after a 60 second hold. The ductility dip cracks were preferentially oriented at a 45° to the tensile axis and were of a wedge type appearance, both of which are characteristic of grain boundary sliding (GBS). Samples with microstructures that consisted of coarsened carbides and/or serrated grain boundaries, which are expected to decrease GBS, were found to be

  1. Three-dimensional morphology of pores and cracks in intact and mechanically deformed sandstones

    NASA Astrophysics Data System (ADS)

    Menendez, B.; David, C.; Wong, T.-F.; Martinez-Nistal, A.

    2003-04-01

    We have studied four different sandstones under confocal laser scanning microscopy (CLSM). In order to discriminate the void space from the grains, the samples were impregnated with a fluorescent dyed (Rhodamine B) resin and thin-sections with a thickness larger than usual were prepared and studied with CSLM. Two different kinds of samples have been studied: mechanically deformed samples of Darley Dale and Berea sandstones and intact samples of Rothbach and Bentheim sandstones. On each sample several three dimensional blocks have been investigated with size 228 by 152 microns and depths ranging from 35 to 100 microns. From each block a series of tens of parallel "virtual sections" has been recorded, separated by 1 or 2 microns in depth. First we show some examples on Darley Dale and Berea sandstone samples deformed in triaxial experiments. Rotating animations are built from series of 3D views of reconstructed crack networks taken step by step for different block orientations. When put together these 3D views nicely simulate a rotation of the 3D block. To create and run the animations we used the Confocal Assistant free software on a PC. Spectacular 3D animations representing crack networks in mechanically deformed samples are obtained this way in a very short time: some examples will be shown on the screen. Secondly we show on a poster some static 3D reconstructions of the pore and/or crack networks obtained using the Slicer Dicer software. For the intact samples we observe that pore (or grain) walls are smoother in Bentheim sandstone whereas in Rothbach sandstone the presence of a significant amount of coating clay minerals results in a visible surface roughness. Some differences in pore size and pore shape were also observed, with a more homogeneous distribution in Bentheim sandstone than in Rothbach sandstone. In both sandstones we observe the classical pore-to-throats junctions. Complex contact geometry between adjacent grains are sometimes observed. In the

  2. A BEM formulation applied in the mechanical material modelling of viscoelastic cracked structures

    NASA Astrophysics Data System (ADS)

    Oliveira, Hugo Luiz; Leonel, Edson Denner

    2016-12-01

    The present study aims at performing a mechanical analysis of 2D viscoelastic cracked structural materials using the Boundary Element Method (BEM). The mesh dimensionality reduction provided by the BEM and its accuracy in representing high gradient fields make this numerical method robust to solve fracture mechanics problems. Viscoelastic models address phenomena that provide changes on the mechanical material properties along time. Well-established viscoelastic models such as Maxwell, Kelvin-Voigt and Boltzmann are used in this study. The numerical viscoelastic scheme, which is based on algebraic BEM equations, utilizes the Euler method for time derivative evaluation. Therefore, the unknown variables at the structural boundary and its variations along time are determined through an ordinary linear system of equations. Moreover, time-dependent boundary conditions may be considered, which represent loading phases. The dual BEM formulation is adopted for modelling the mechanical structural behaviour of cracks bodies. Three examples are considered to illustrate the robustness of the adopted formulation. The results achieved by the BEM are in good agreement with reported data and numerical stability is observed.

  3. A BEM formulation applied in the mechanical material modelling of viscoelastic cracked structures

    NASA Astrophysics Data System (ADS)

    Oliveira, Hugo Luiz; Leonel, Edson Denner

    2017-03-01

    The present study aims at performing a mechanical analysis of 2D viscoelastic cracked structural materials using the Boundary Element Method (BEM). The mesh dimensionality reduction provided by the BEM and its accuracy in representing high gradient fields make this numerical method robust to solve fracture mechanics problems. Viscoelastic models address phenomena that provide changes on the mechanical material properties along time. Well-established viscoelastic models such as Maxwell, Kelvin-Voigt and Boltzmann are used in this study. The numerical viscoelastic scheme, which is based on algebraic BEM equations, utilizes the Euler method for time derivative evaluation. Therefore, the unknown variables at the structural boundary and its variations along time are determined through an ordinary linear system of equations. Moreover, time-dependent boundary conditions may be considered, which represent loading phases. The dual BEM formulation is adopted for modelling the mechanical structural behaviour of cracks bodies. Three examples are considered to illustrate the robustness of the adopted formulation. The results achieved by the BEM are in good agreement with reported data and numerical stability is observed.

  4. Problems of dynamic fracture mechanics without contact of the crack faces

    NASA Astrophysics Data System (ADS)

    Guz', A. N.; Zozulya, V. V.

    1994-10-01

    In this, the first part of the survey we have discussed only certain aspects of dynamic fracture mechanics. The surveyed material has been selected with a preference for the most highly developed parts of the theory, specifically those elements which have direct bearing on the second part of the survey. We have also included information on the dynamic fracture mechanics of initially stressed materials, in the development of which one of the authors has been a major contributor. Since many problems of dynamic fracture mechanics have been overlooked in the survey, we have added supplementary references to the literature. Various aspects of the strength and fracture of materials under dynamic loading are set forth in [11, 12, 40, 57, 60, 73, 80, 83]. Criteria of the initiation, motion, branching, and arrest of cracks are discussed in [7, 9, 60, 102, 111, 113, 124]. Among the most interesting elements of dynamic fracture mechanics are the problems of crack propagation. Certain analytical results pertinent to this topic have been obtained in [43-45, 47, 67-72, 78, 87, 92, 96, 97].

  5. Mechanical behaviour of metallic thin films on polymeric substrates and the effect of ion beam assistance on crack propagation

    SciTech Connect

    George, M. , E-Mail: matthieu.george@bnfl.com; Coupeau, C.; Colin, J.; Grilhe, J.

    2005-01-10

    The mechanisms of crack propagation in metallic films on polymeric substrates have been studied through in situ atomic force microscopy observations of thin films under tensile stresses and finite element stress calculations. Two series of films - ones deposited with ion beam assistance, the others without - have been investigated. The observations and stress calculations show that ion beam assistance can change drastically the propagation of cracks in coated materials: by improving the adhesion film/substrate, it slows down the delamination process, but in the same time enhances the cracks growth in the thickness of the material.

  6. The Thermo-Mechanical Problem of Internal and Edge Cracks in Multi-Layered Woven GFRP Laminates at Cryogenic Temperatures

    SciTech Connect

    Takeda, T.; Shindo, Y.; Narita, F.

    2004-06-28

    This paper presents the thermo-mechanical response of multi-layered G-11 woven glass/epoxy laminates with internal and/or edge cracks under tensile loading at cryogenic temperatures obtained from a two-dimensional finite element analysis. A condition of generalized plane strain is assumed to exist in the composite. Cracks are considered to occur in the transverse fiber bundles and extend through the entire thickness of the fiber bundles. The finite element model accounts for the temperature-dependent constituent properties. A detailed examination of the Young's modulus and stress distributions near the crack tip is carried out which provides insight into material behavior at cryogenic temperatures.

  7. The combined influence of chemical, metallurgical and mechanical factors on environment assisted cracking

    NASA Technical Reports Server (NTRS)

    Williams, D. P., III; Pao, P. S.; Wei, R. P.

    1979-01-01

    The principal aim of the paper is to re-emphasize and focus on both the multidisciplinary nature of the environment assisted cracking or embrittlement phenomenon. The multiplicity of factors involved in the embrittlement process is indicated, the mutual dependence of these factors and the influences of mechanical and environmental conditions are considered, and the interactions of various factors in determining the overall embrittlement response are discussed. The need for an interdisciplinary approach for resolving the major differences and for understanding embrittlement is outlined.

  8. Mechanical properties and examination of cracking in TMI-2 pressure vessel lower head material

    SciTech Connect

    Diercks, D.R.; Neimark, L.A.

    1993-09-01

    Mechanical tests have been conducted on material from 15 samples removed from the lower head of the Three Mile Island unit 2 nuclear reactor pressure vessel. Measured properties include tensile properties and hardness profiles at room temperature, tensile and creep properties at temperatures of 600 to 1200{degrees}C, and Charpy V-notch impact properties at {minus}20 to +300{degrees}C. These data, which were used in the subsequent analyses of the margin-to-failure of the lower head during the accident, are presented here. In addition, the results of metallographic and scanning electron microscope examinations of cladding cracking in three of the lower head samples are discussed.

  9. Code System for Fracture Mechanics Analysis of Circumferential Surface Cracks in Pipes.

    SciTech Connect

    BRUST, F.

    1999-07-28

    Version 00 The NRCPIPES software is designed to perform elastic and elastic-plastic fracture mechanics analysis for a circumferential surface cracked pipe, i.e., to establish the fracture-failure condition in terms of sustainable load (or stress) or displacement. The NRCPIPES software also includes several evaluation procedures and acceptance criteria for circumferential surface flaws based on the ASME Boiler and Pressure Vessel Code, Section XI criteria, the British R6 Revision 3 Option 1 criteria, and the original Net-Section-Collapse (limit-load) analysis.

  10. Damage Mechanisms and Controlled Crack Propagation in a Hot Pressed Silicon Nitride Ceramic. Ph.D. Thesis - Northwestern Univ., 1993

    NASA Technical Reports Server (NTRS)

    Calomino, Anthony Martin

    1994-01-01

    The subcritical growth of cracks from pre-existing flaws in ceramics can severely affect the structural reliability of a material. The ability to directly observe subcritical crack growth and rigorously analyze its influence on fracture behavior is important for an accurate assessment of material performance. A Mode I fracture specimen and loading method has been developed which permits the observation of stable, subcritical crack extension in monolithic and toughened ceramics. The test specimen and procedure has demonstrated its ability to generate and stably propagate sharp, through-thickness cracks in brittle high modulus materials. Crack growth for an aluminum oxide ceramic was observed to be continuously stable throughout testing. Conversely, the fracture behavior of a silicon nitride ceramic exhibited crack growth as a series of subcritical extensions which are interrupted by dynamic propagation. Dynamic initiation and arrest fracture resistance measurements for the silicon nitride averaged 67 and 48 J/sq m, respectively. The dynamic initiation event was observed to be sudden and explosive. Increments of subcritical crack growth contributed to a 40 percent increase in fracture resistance before dynamic initiation. Subcritical crack growth visibly marked the fracture surface with an increase in surface roughness. Increments of subcritical crack growth loosen ceramic material near the fracture surface and the fracture debris is easily removed by a replication technique. Fracture debris is viewed as evidence that both crack bridging and subsurface microcracking may be some of the mechanisms contributing to the increase in fracture resistance. A Statistical Fracture Mechanics model specifically developed to address subcritical crack growth and fracture reliability is used together with a damaged zone of material at the crack tip to model experimental results. A Monte Carlo simulation of the actual experiments was used to establish a set of modeling input

  11. Role of Grain Boundaries and Microstructure on the Environment Assisted Cracking of Pipeline Steels

    NASA Astrophysics Data System (ADS)

    Arafin, Muhammad

    2011-12-01

    In this research, two common types of environment assisted cracking (EAC) of pipeline steels, namely the intergranular stress corrosion cracking (IGSCC) and the hydrogen induced cracking (HIC), have been studied, and computer models have been developed to simulate the intergranular stress corrosion crack propagation behaviour in pipeline steel as well as to predict the intergranular fracture susceptibility, due to mechanical loading in non-corrosive environment, of polycrystalline materials. First, a new understanding of the IGSCC resistance of pipeline steel has been obtained by studying the grain boundary character and crystallographic orientation in both cracked and non-cracked pipeline steel samples using electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that the low-angle and certain types of special boundaries, known as the coincident site lattice (CSL) boundaries (S5, S11, and S13b types), are crack-resistant while the random high angle boundaries are prone to cracking. However, it has been also observed that the grain boundaries associated with {110} and {111} neighbour grain orientations having <110> and <111> rotation axis, respectively, are crack-resistant, while the cracked boundaries are mainly linked to the {100} orientation with <100> rotation axis. Subsequently, a novel integrated modeling approach, combining Voronoi Algorithm, Markov Chain theory, and Monte Carlo simulations, has been developed in order to predict the IGSCC behaviour of pipeline steels. The model takes both the physical microstructural features, such as the grain shape and grain size distribution, as well as the grain boundary characters and their orientations with respect to the external stress axis into account. The predicted crack propagation behaviour has been found to be in excellent agreement with the experimental crack-propagation and arrest data in API X65 pipeline steel. In addition, a texture based grain boundary character

  12. Environmentally assisted cracking in light water reactors.

    SciTech Connect

    Chopra, O. K.; Chung, H. M.; Clark, R. W.; Gruber, E. E.; Shack, W. J.; Soppet, W. K.; Strain, R. V.

    2007-11-06

    This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors (LWRs) from January to December 2002. Topics that have been investigated include: (a) environmental effects on fatigue crack initiation in carbon and low-alloy steels and austenitic stainless steels (SSs), (b) irradiation-assisted stress corrosion cracking (IASCC) of austenitic SSs in BWRs, (c) evaluation of causes and mechanisms of irradiation-assisted cracking of austenitic SS in PWRs, and (d) cracking in Ni-alloys and welds. A critical review of the ASME Code fatigue design margins and an assessment of the conservation in the current choice of design margins are presented. The existing fatigue {var_epsilon}-N data have been evaluated to define the effects of key material, loading, and environmental parameters on the fatigue lives of carbon and low-alloy steels and austenitic SSs. Experimental data are presented on the effects of surface roughness on fatigue crack initiation in these materials in air and LWR environments. Crack growth tests were performed in BWR environments on SSs irradiated to 0.9 and 2.0 x 10{sup 21} n x cm{sup -2}. The crack growth rates (CGRs) of the irradiated steels are a factor of {approx}5 higher than the disposition curve proposed in NUREG-0313 for thermally sensitized materials. The CGRs decreased by an order of magnitude in low-dissolved oxygen (DO) environments. Slow-strain-rate tensile (SSRT) tests were conducted in high-purity 289 C water on steels irradiated to {approx}3 dpa. The bulk S content correlated well with the susceptibility to intergranular SCC in 289 C water. The IASCC susceptibility of SSs that contain >0.003 wt. % S increased drastically. bend tests in inert environments at 23 C were conducted on broken pieces of SSRT specimens and on unirradiated specimens of the same materials after hydrogen charging. The results of the tests and a review of other data in the literature

  13. The effect of hot isostatic pressing on crack initiation, fatigue, and mechanical properties of two cast aluminum alloys

    NASA Astrophysics Data System (ADS)

    Rich, T. P.; Orbison, J. G.; Duncan, R. S.; Olivero, P. G.; Peterec, R. H.

    1999-06-01

    This article presents the results of an experimental materials testing program on the effect of hot isostatic pressing (HIP) on the crack initiation, fatigue, and mechanical properties of two cast aluminum alloys: AMS 4220 and 4225. These alloys are often used in castings for high temperature applications. Standard tensile and instrumented Charpy impact tests were performed at room and elevated temperatures. The resulting data quantify improvements in ultimate tensile strength, ductility, and Charpy impact toughness from the HIP process while indicating little change in yield strength for both alloys. In addition standard fracture mechanics fatigue tests along with a set of unique fatigue crack initiation tests were performed on the alloys. Hot isostatic pressing was shown to produce a significant increase in cycles to crack initiation for AMS 4225, while no change was evident in traditional da/dN fatigue crack growth. The data permits comparisons of the two alloys both with and without the HIP process.

  14. Mechanics of Matrix Cracking in Brittle-Matrix Fibre-Reinforced Composites

    NASA Astrophysics Data System (ADS)

    McCartney, L. N.

    1987-02-01

    Energy-balance calculations for a continuum model of cracking in a uniaxially fibre-reinforced composite having a brittle matrix are presented. It is assumed that the fibres are strong enough to remain intact when the matrix cracks across the entire cross section of the composite. By equating the energy availability for the cracking of continuum and discrete fibre models it is shown how the crack boundary condition relating fibre stress to crack opening must be selected. It is confirmed that the Griffith fracture criterion is valid for matrix cracking in composites. By considering the energy balance of long cracks it is shown that the limiting value of the stress intensity factor is independent of crack length and that it predicts a matrix-cracking strain that is consistent with the known result. An improved numerical method is described for solving a crack problem arising from the study of the cracking of brittle-matrix composites. Numerical results of high accuracy are obtained, which show how the cracking stress is related to the size of a pre-existing defect. Of special significance is the prediction of the correct threshold stress (i.e. matrix-cracking stress) below which matrix cracking is impossible no matter how large the pre-existing defect.

  15. Intergranular fracture in UO{sub 2}: derivation of traction-separation law from atomistic simulations

    SciTech Connect

    Zhang, Yongfeng; Millett, P.C.; Tonks, M.R.; Bai, Xian-Ming; Biner, S.B.

    2013-07-01

    In this study, the intergranular fracture behavior of UO{sub 2} was studied by molecular dynamics simulations using the Basak potential. In addition, the constitutive traction-separation law was derived from atomistic data using the cohesive-zone model. In the simulations a bicrystal model with the (100) symmetric tilt Σ5 grain boundaries was utilized. Uniaxial tension along the grain boundary normal was applied to simulate Mode-I fracture. The fracture was observed to propagate along the grain boundary by micro-pore nucleation and coalescence, giving an overall intergranular fracture behavior. Phase transformations from the Fluorite to the Rutile and Scrutinyite phases were identified at the propagating crack tips. These new phases are metastable and they transformed back to the Fluorite phase at the wake of crack tips as the local stress concentration was relieved by complete cracking. Such transient behavior observed at atomistic scale was found to substantially increase the energy release rate for fracture. Insertion of Xe gas into the initial notch showed minor effect on the overall fracture behavior. (authors)

  16. Intergranular fracture in UO2: derivation of traction-separation law from atomistic simulations

    SciTech Connect

    Yongfeng Zhang; Paul C Millett; Michael R Tonks; Xian-Ming Bai; S Bulent Biner

    2013-10-01

    In this study, the intergranular fracture behavior of UO2 was studied by molecular dynamics simulations using the Basak potential. In addition, the constitutive traction-separation law was derived from atomistic data using the cohesive-zone model. In the simulations a bicrystal model with the (100) symmetric tilt E5 grain boundaries was utilized. Uniaxial tension along the grain boundary normal was applied to simulate Mode-I fracture. The fracture was observed to propagate along the grain boundary by micro-pore nucleation and coalescence, giving an overall intergranular fracture behavior. Phase transformations from the Fluorite to the Rutile and Scrutinyite phases were identified at the propagating crack tips. These new phases are metastable and they transformed back to the Fluorite phase at the wake of crack tips as the local stress concentration was relieved by complete cracking. Such transient behavior observed at atomistic scale was found to substantially increase the energy release rate for fracture. Insertion of Xe gas into the initial notch showed minor effect on the overall fracture behavior.

  17. Characterization of microstructure, local deformation and microchemistry in Alloy 690 heat-affected zone and stress corrosion cracking in high temperature water

    NASA Astrophysics Data System (ADS)

    Lu, Zhanpeng; Chen, Junjie; Shoji, Tetsuo; Takeda, Yoichi; Yamazaki, Seiya

    2015-10-01

    With increasing the distance from the weld fusion line in an Alloy 690 heat-affected zone, micro-hardness decreases, kernel average misorientation decreases and the fraction of Σ3 boundaries increases. Chromium depletion at grain boundaries in the Alloy 690 heat-affected zone is less significant than that in an Alloy 600 heat-affected zone. Alloy 690 heat-affected zone exhibits much higher IGSCC resistance than Alloy 600 heat-affected zone in simulated pressurized water reactor primary water. Heavily cold worked Alloy 690 exhibits localized intergranular stress corrosion cracking. The effects of metallurgical and mechanical properties on stress corrosion cracking in Alloy 690 are discussed.

  18. Ultrasonic inspection of austenitic stainless steel welds with artificially produced stress corrosion cracks

    SciTech Connect

    Dugan, Sandra; Wagner, Sabine

    2014-02-18

    Austenitic stainless steel welds and nickel alloy welds, which are widely used in nuclear power plants, present major challenges for ultrasonic inspection due to the grain structure in the weld. Large grains in combination with the elastic anisotropy of the material lead to increased scattering and affect sound wave propagation in the weld. This results in a reduced signal-to-noise ratio, and complicates the interpretation of signals and the localization of defects. Mechanized ultrasonic inspection was applied to study austenitic stainless steel test blocks with different types of flaws, including inter-granular stress corrosion cracks (IGSCC). The results show that cracks located in the heat affected zone of the weld are easily detected when inspection from both sides of the weld is possible. In cases of limited accessibility, when ultrasonic inspection can be carried out only from one side of a weld, it may be difficult to distinguish between signals from scattering in the weld and signals from cracks.

  19. Ceramic fracture mode-intergranular vs transgranular fracture

    SciTech Connect

    Rice, R.W.

    1996-12-31

    Available data on intergranular fracture (IGF) vs transgranular fracture (TGF) of ceramics is summarized and significantly extended. At 22 C, where there is most data, TGF is normally dominant. IGF generally increases with decreasing grain size (G, mainly at G {le} 1-10 {mu}m), increasing grain boundary phase content and the occurrence of (1) slow crack growth, (2) mist, hackle, and crack branching, and (3) (mainly finer, substantial, grain boundary) porosity, and possibly with increasing elastic anisotropy. Possible effects of grain orientation, stress rate and character, as well as microstructural stresses from thermal expansion anisotropy (TEA) are discussed. At higher temperatures, there is a general shift to more IGF, especially with more grain boundary impurities, finer G, and probably higher elastic anisotropy. This shift often starts with IGF only at the fracture origin, and may not commence until temperatures of the order of 1500 C or more in some materials. While IGF is often attributed to weaker grain boundaries (implying lower strengths), it is also often associated with fine grain size, and thus the highest strengths at lower temperatures. IGF vs TGF reflects not just grain boundary strength (as often emphasized), but a balance of this versus the fracture toughness for grain fracture (usually via cleavage, which may also entail the multiplicity of cleavage planes). Several factors may interact to shift differing balances in different materials, e.g. some increase in IGF at larger G in TiB{sub 2} with high TEA, but more IGF increase in Al{sub 2}O{sub 3}, at larger G with less TEA, and no IGF increase in BeO and all TGF in B{sub 4}C having similar TEA to Al{sub 2}O{sub 3}.

  20. In situ electro-mechanical experiments and mechanics modeling of tensile cracking in indium tin oxide thin films on polyimide substrates

    NASA Astrophysics Data System (ADS)

    Peng, Cheng; Jia, Zheng; Bianculli, Dan; Li, Teng; Lou, Jun

    2011-05-01

    Indium tin oxide (ITO) thin films supported by polymer substrates have been widely used as transparent electrodes/interconnects in flexible electronics. Understanding the electro-mechanical behaviors of such material system is crucial for reliable operation of flexible devices under large deformation. In this paper, we performed in situ mechanical and electrical tests of ITO thin films with two different thicknesses (200 and 80 nm) deposited on polyimide substrates inside a scanning electron microscope. The crack initiation and propagation, crack density evolution and the corresponding electrical resistance variation were systematically investigated. It was found that cracks initiated at a higher tensile strain level and saturated with a higher density in thinner ITO films. Integrated with a coherently formulated mechanics model, the cohesive toughness and fracture strength of ITO thin films and the ITO/polyimide interfacial toughness were quantitatively determined. The experimentally observed thickness dependence of the saturated crack density in ITO thin films was also quantitatively verified by the model.

  1. R-curve behavior and stable crack growth at elevated temperature (1,500--1,650 C) in a Si[sub 3]N[sub 4]/SiC nanocomposite

    SciTech Connect

    Rouxel, T. . Lab. de Materiaux Ceramiques et Traitements de Surface); Wakai, Fumihiro; Sakaguchi, Shuji . Ceramic Science Dept.)

    1994-12-01

    The crack growth resistance behavior and the stable crack growth regime of a Si[sub 3]N[sub 4]/SiC composite have been examined at high temperature (1,500-1,650 C). Single edged notched beam specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R-curves have bee obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the T[sub g] of the intergranular glassy phase, the behavior changes from brittle to visco-plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack-tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.

  2. Flight monitor for jet engine disk cracks and the use of critical length criterion of fracture mechanics

    NASA Technical Reports Server (NTRS)

    Barranger, J. P.

    1973-01-01

    A disk crack detector is discussed which is intended to operate under flight conditions. It monitors the disk rim for surface cracks emanating from the blade root interface. An eddy current type sensor, with a remotely located capacitance/conductance bridge and signal analyzer, can reliably detect a simulated crack 3 mm long. The sensor was tested on a spinning turbine disk at 540 C. Tests indicate that the system is useful at disk rim velocities to 460 m/sec. By using fracture mechanics, it is shown for Inconel 718 th at a crack operating under a rim stress of 34 x ten to the 7th power N/sqm has a critical length of 18 mm.

  3. Micro-FTIR study of molecular orientation at crack tip in nylon 6/clay nanocomposite: insight into fracture mechanism.

    PubMed

    Xu, Wenfei; Lv, Ruihua; Na, Bing; Tian, Nana; Li, Zhujun; Fu, Qiang

    2009-07-23

    A study on the mechanism for the degraded toughness in nylon 6/clay nanocomposite is explored in this article. Such a nanocomposite exhibits lower specific essential work of fracture we and specific nonessential work of fracture betawp than its pure nylon 6 counterpart, as revealed by essential work of fracture (EWF) measurements. Furthermore, the molecular orientation in a small region (20x20 microm2) ahead of crack tip, obtained from micro-FTIR measurements for the first time, is found to be lower in the nanocomposite during crack initiation and propagation. The decreased molecular orientation, mostly resulted from severe microvoiding at crack tips, is responsible for the reduced specific essential work of fracture we. Meanwhile, the molecular orientation around crack tip also indicates that lower plastic deformation occurs in the plastic zone, which is well correlated with decreased specific nonessential work of fracture betawp in the nanocomposite.

  4. Crustal Rock Fracture Mechanics for Design and Control of Artificial Subsurface Cracks in Geothermal Energy Extraction Engineering ({Gamma}-Project)

    SciTech Connect

    Abe, Hiroyuki; Takahashi, Hideaki

    1983-12-15

    Recently a significant role of artificial and/or natural cracks in the geothermal reservoir has been demonstrated in the literatures (Abe, H., et al., 1983, Nielson, D.L. and Hullen, J.B., 1983), where the cracks behave as fluid paths and/or heat exchanging surfaces. Until now, however, there are several problems such as a design procedure of hydraulic fracturing, and a quantitative estimate of fluid and heat transfer for reservoir design. In order to develop a design methodology of geothermal reservoir cracks, a special distinguished research project, named as ''{Lambda}-Project'', started at Tohoku University (5 years project, 1983-1988). In this project a basic fracture mechanics model of geothermal reservoir cracks is being demonstrated and its validation is being discussed both theoretically and experimentally. This paper descibes an outline of ''{Lambda}-Project''.

  5. Near-tip dual-length scale mechanics of mode-I cracking in laminate brittle matrix composites

    NASA Technical Reports Server (NTRS)

    Ballarini, R.; Islam, S.; Charalambides, P. G.

    1992-01-01

    This paper presents the preliminary results of an on-going study of the near-tip mechanics of mode-I cracking in brittle matrix composite laminates. A finite element model is developed within the context of two competing characteristic lengths present in the composite: the microstructural length (the thickness of the layers) and a macro-length (crack-length, uncracked ligament size, etc.). For various values of the parameters which describe the ratio of these lengths and the constituent properties, the stresses ahead of a crack perpendicular to the laminates are compared with those predicted by assuming the composite is homogeneous orthotropic. The results can be used to determine the conditions for which homogenization can provide a sufficiently accurate description of the stresses in the vicinity of the crack-tip.

  6. Strain and texture evolution during mechanical loading of a crack tip in martensitic shape-memory NiTi.

    SciTech Connect

    Daymond, M. R.; Young, M. L.; Almer, J. D.; Dunand, D. C.; Queen's Univ.; Northwestern Univ.

    2007-06-01

    In situ synchrotron X-ray diffraction measurements are used to create two-dimensional maps of elastic strain and texture, averaged over a compact-tension specimen thickness, near a crack tip in a martensitic NiTi alloy. After fatigue crack propagation, the material ahead of the crack and in its wake exhibits a strong texture, which is eliminated by subsequent shape-memory heat treatment, indicating that this texture is due to detwinning, the main deformation mechanism of NiTi. Upon subsequent application of a static tensile stresses, the highly textured zone reappears and grows around the crack tip as the applied stress is increased. At the highest applied stress intensity of 35MPam1/2, large tensile strains are measured ahead of the crack tip and considerable elastic anisotropy is observed. This detwinning zone is similar to the plastic zone produced by dislocation slip present around cracks in other metals. The texture in this zone is not significantly altered after mechanical unloading, despite the development of substantial triaxial compressive residual strains in this zone.

  7. Analysis of crack propagation as an energy absorption mechanism in metal matrix composites

    NASA Technical Reports Server (NTRS)

    Adams, D. F.; Murphy, D. P.

    1981-01-01

    The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite.

  8. Bacterial division. Mechanical crack propagation drives millisecond daughter cell separation in Staphylococcus aureus.

    PubMed

    Zhou, Xiaoxue; Halladin, David K; Rojas, Enrique R; Koslover, Elena F; Lee, Timothy K; Huang, Kerwyn Casey; Theriot, Julie A

    2015-05-01

    When Staphylococcus aureus undergoes cytokinesis, it builds a septum, generating two hemispherical daughters whose cell walls are only connected via a narrow peripheral ring. We found that resolution of this ring occurred within milliseconds ("popping"), without detectable changes in cell volume. The likelihood of popping depended on cell-wall stress, and the separating cells split open asymmetrically, leaving the daughters connected by a hinge. An elastostatic model of the wall indicated high circumferential stress in the peripheral ring before popping. Last, we observed small perforations in the peripheral ring that are likely initial points of mechanical failure. Thus, the ultrafast daughter cell separation in S. aureus appears to be driven by accumulation of stress in the peripheral ring and exhibits hallmarks of mechanical crack propagation.

  9. Intergranular diffusion and embrittlement of a Ni-16Mo-7Cr alloy in Te vapor environment

    NASA Astrophysics Data System (ADS)

    Cheng, Hongwei; Li, Zhijun; Leng, Bin; Zhang, Wenzhu; Han, Fenfen; Jia, Yanyan; Zhou, Xingtai

    2015-12-01

    Nickel and some nickel-base alloys are extremely sensitive to intergranular embrittlement and tellurium (Te) enhanced cracking, which should be concerned during their serving in molten salt reactors. Here, a systematic study about the effects of its temperature on the reaction products at its surface, the intergranular diffusion of Te in its body and its embrittlement for a Ni-16Mo-7Cr alloy contacting Te is reported. For exposed to Te vapor at high temperature (823-1073 K), the reaction products formed on the surface of the alloy were Ni3Te2, CrTe, and MoTe2, and the most serious embrittlement was observed at 1073 K. The kinetic measurement in terms of Te penetration depth in the alloy samples gives an activation energy of 204 kJ/mol. Electron probe microanalysis confirmed the local enrichment of Te at grain boundaries. And clearly, the embrittlement was results from the intergranular diffusion and segregation of element Te.

  10. Assessment of Initial Test Conditions for Experiments to Assess Irradiation Assisted Stress Corrosion Cracking Mechanisms

    SciTech Connect

    Busby, Jeremy T; Gussev, Maxim N

    2011-04-01

    Irradiation-assisted stress corrosion cracking is a key materials degradation issue in today s nuclear power reactor fleet and affects critical structural components within the reactor core. The effects of increased exposure to irradiation, stress, and/or coolant can substantially increase susceptibility to stress-corrosion cracking of austenitic steels in high-temperature water environments. . Despite 30 years of experience, the underlying mechanisms of IASCC are unknown. Extended service conditions will increase the exposure to irradiation, stress, and corrosive environment for all core internal components. The objective of this effort within the Light Water Reactor Sustainability program is to evaluate the response and mechanisms of IASCC in austenitic stainless steels with single variable experiments. A series of high-value irradiated specimens has been acquired from the past international research programs, providing a valuable opportunity to examine the mechanisms of IASCC. This batch of irradiated specimens has been received and inventoried. In addition, visual examination and sample cleaning has been completed. Microhardness testing has been performed on these specimens. All samples show evidence of hardening, as expected, although the degree of hardening has saturated and no trend with dose is observed. Further, the change in hardening can be converted to changes in mechanical properties. The calculated yield stress is consistent with previous data from light water reactor conditions. In addition, some evidence of changes in deformation mode was identified via examination of the microhardness indents. This analysis may provide further insights into the deformation mode under larger scale tests. Finally, swelling analysis was performed using immersion density methods. Most alloys showed some evidence of swelling, consistent with the expected trends for this class of alloy. The Hf-doped alloy showed densification rather than swelling. This observation may be

  11. Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE#

    PubMed Central

    Oral, Ebru; Malhi, Arnaz S.; Muratoglu, Orhun K.

    2005-01-01

    Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is typically followed by melting to eliminate residual free radicals that cause oxidative embrittlement. Irradiation and subsequent melting reduce the strength and fatigue resistance of the polymer. We determined the radiation dose dependence and decoupled the effects of post-irradiation melting on the crystallinity, mechanical properties and fatigue crack propagation resistance of room temperature irradiated UHMWPE from those of irradiation alone. Stiffness and yield strength, were largely not affected by increasing radiation dose but were affected by changes in crystallinity, whereas plastic properties, ultimate tensile strength and elongation at break, were dominated at different radiation dose ranges by changes in radiation dose or crystallinity. Fatigue crack propagation resistance was shown to decrease with increase in radiation dose and with decrease in crystalline content. Morphology of fracture surfaces revealed loss of ductility with increase in radiation dose and more detrimental effects on ductility at lower radiation doses after post-irradiation melting. PMID:16105682

  12. Mechanism and estimation of fatigue crack initiation in austenitic stainless steels in LWR environments.

    SciTech Connect

    Chopra, O. K.; Energy Technology

    2002-08-01

    The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components. Figures I-9.1 through I-9.6 of Appendix I to Section III of the Code specify fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Existing fatigue strain-vs.-life ({var_epsilon}-N) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. This report provides an overview of fatigue crack initiation in austenitic stainless steels in LWR coolant environments. The existing fatigue {var_epsilon}-N data have been evaluated to establish the effects of key material, loading, and environmental parameters (such as steel type, strain range, strain rate, temperature, dissolved-oxygen level in water, and flow rate) on the fatigue lives of these steels. Statistical models are presented for estimating the fatigue {var_epsilon}-N curves for austenitic stainless steels as a function of the material, loading, and environmental parameters. Two methods for incorporating environmental effects into the ASME Code fatigue evaluations are presented. The influence of reactor environments on the mechanism of fatigue crack initiation in these steels is also discussed.

  13. Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks.

    PubMed

    Pineau, André

    2015-03-28

    The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.

  14. Fracture mechanics; Proceedings of the 22nd National Symposium, Atlanta, GA, June 26-28, 1990. Vols. 1 & 2

    NASA Technical Reports Server (NTRS)

    Ernst, Hugo A. (Editor); Saxena, Ashok (Editor); Mcdowell, David L. (Editor); Atluri, Satya N. (Editor); Newman, James C., Jr. (Editor); Raju, Ivatury S. (Editor); Epstein, Jonathan S. (Editor)

    1992-01-01

    Current research on fracture mechanics is reviewed, focusing on ductile fracture; high-temperature and time-dependent fracture; 3D problems; interface fracture; microstructural aspects of fatigue and fracture; and fracture predictions and applications. Particular attention is given to the determination and comparison of crack resistance curves from wide plates and fracture mechanics specimens; a relationship between R-curves in contained and uncontained yield; the creep crack growth behavior of titanium alloy Ti-6242; a crack growth response in three heat resistant materials at elevated temperature; a crack-surface-contact model for determining effective-stress-intensity factors; interfacial dislocations in anisotropic bimaterials; an effect of intergranular crack branching on fracture toughness evaluation; the fracture toughness behavior of exservice chromium-molybdenum steels; the application of fracture mechanics to assess the significance of proof loading; and a load ratio method for estimating crack extension.

  15. Fracture mechanics; Proceedings of the 22nd National Symposium, Atlanta, GA, June 26-28, 1990. Vols. 1 & 2

    NASA Technical Reports Server (NTRS)

    Ernst, Hugo A. (Editor); Saxena, Ashok (Editor); Mcdowell, David L. (Editor); Atluri, Satya N. (Editor); Newman, James C., Jr. (Editor); Raju, Ivatury S. (Editor); Epstein, Jonathan S. (Editor)

    1992-01-01

    Current research on fracture mechanics is reviewed, focusing on ductile fracture; high-temperature and time-dependent fracture; 3D problems; interface fracture; microstructural aspects of fatigue and fracture; and fracture predictions and applications. Particular attention is given to the determination and comparison of crack resistance curves from wide plates and fracture mechanics specimens; a relationship between R-curves in contained and uncontained yield; the creep crack growth behavior of titanium alloy Ti-6242; a crack growth response in three heat resistant materials at elevated temperature; a crack-surface-contact model for determining effective-stress-intensity factors; interfacial dislocations in anisotropic bimaterials; an effect of intergranular crack branching on fracture toughness evaluation; the fracture toughness behavior of exservice chromium-molybdenum steels; the application of fracture mechanics to assess the significance of proof loading; and a load ratio method for estimating crack extension.

  16. The radiation swelling effect on fracture properties and fracture mechanisms of irradiated austenitic steels. Part II. Fatigue crack growth rate

    NASA Astrophysics Data System (ADS)

    Margolin, B.; Minkin, A.; Smirnov, V.; Sorokin, A.; Shvetsova, V.; Potapova, V.

    2016-11-01

    The experimental data on the fatigue crack growth rate (FCGR) have been obtained for austenitic steel of 18Cr-10Ni-Ti grade (Russian analog of AISI 321 steel) irradiated up to neutron dose of 150 dpa with various radiation swelling. The performed study of the fracture mechanisms for cracked specimens under cyclic loading has explained why radiation swelling affects weakly FCGR unlike its effect on fracture toughness. Mechanical modeling of fatigue crack growth has been carried out and the dependencies for prediction of FCGR in irradiated austenitic steel with and with no swelling are proposed and verified with the obtained experimental results. As input data for these dependencies, FCGR for unirradiated steel and the tensile mechanical properties for unirradiated and irradiated steels are used.

  17. Lead induced stress corrosion cracking of Alloy 690 in high temperature water

    SciTech Connect

    Chung, K.K.; Lim, J.K.; Moriya, Shinichi; Watanabe, Yutaka; Shoji, Tetsuo

    1995-12-31

    Recent investigations of cracked steam generator tubes at nuclear power plants concluded that lead significantly contributed to cracking the Alloy 600 materials. In order to investigate the stress corrosion cracking (SCC) behavior of Alloy 690, slow strain rate tests (SSRT) and anodic polarization measurements were performed. The SSRTs were conducted in a lead-chloride solution (PbCl{sub 2}) and in a chloride but lead free solution (NaCl) at pH of 3 and 4.5 at 288 C. The anodic polarization measurements were carried out at 30 C using the same solutions as in SSRT. The SSRT results showed that Alloy 690 was susceptible to SCC in both solutions. In the lead chloride solution, cracking had slight dependence on lead concentration and pH. Cracking tend to increase with a higher lead concentration and a lower pH and was mainly intergranular and was to be a few tens to hundreds micrometers in length. In the chloride only solution, cracking was similar to the lead induced SCC. The results of anodic polarization measurement and electron probe micro analysis (EPMA) helped to understand lead induced SCC. Lead was a stronger active corrosive element but had a minor affect on cracking susceptibility of the alloy. While, chloride was quite different from lead effect to SCC. A possible mechanism of lead induced SCC of Alloy 690 was also discussed based on the test results.

  18. Gear fatigue crack prognosis using embedded model, gear dynamic model and fracture mechanics

    NASA Astrophysics Data System (ADS)

    Li, C. James; Lee, Hyungdae

    2005-07-01

    This paper presents a model-based method that predicts remaining useful life of a gear with a fatigue crack. The method consists of an embedded model to identify gear meshing stiffness from measured gear torsional vibration, an inverse method to estimate crack size from the estimated meshing stiffness; a gear dynamic model to simulate gear meshing dynamics and determine the dynamic load on the cracked tooth; and a fast crack propagation model to forecast the remaining useful life based on the estimated crack size and dynamic load. The fast crack propagation model was established to avoid repeated calculations of FEM and facilitate field deployment of the proposed method. Experimental studies were conducted to validate and demonstrate the feasibility of the proposed method for prognosis of a cracked gear.

  19. Fatigue Crack Growth Mechanisms for Nickel-based Superalloy Haynes 282 at 550-750 °C

    NASA Astrophysics Data System (ADS)

    Rozman, Kyle A.; Kruzic, Jamie J.; Sears, John S.; Hawk, Jeffrey A.

    2015-10-01

    The fatigue crack growth rates for nickel-based superalloy Haynes 282 were measured at 550, 650, and 750 °C using compact tension specimens with a load ratio of 0.1 and cyclic loading frequencies of 25 and 0.25 Hz. The crack path was observed to be primarily transgranular for all temperatures, and the observed effect of increasing temperature was to increase the fatigue crack growth rates. The activation energy associated with the increasing crack growth rates over these three temperatures was calculated less than 60 kJ/mol, which is significantly lower than typical creep or oxidation mechanisms; therefore, creep and oxidation cannot explain the increase in fatigue crack growth rates. Transmission electron microscopy was done on selected samples removed from the cyclic plastic zone, and a trend of decreasing dislocation density was observed with increasing temperature. Accordingly, the trend of increasing crack growth rates with increasing temperature was attributed to softening associated with thermally assisted cross slip and dislocation annihilation.

  20. Effects of temperature and environment on fatigue crack growth mechanisms in a 9% Cr 1% Mo steel

    SciTech Connect

    Cotterill, P.J.; Knott, J.F. )

    1992-10-01

    In this paper the environmental contribution of laboratory air to fatigue crack growth in a 9% Cr 1% Mo steel is assessed by a comparison of crack propagation rates in air and vacuum over a range of temperatures (25-625[degrees]C). In the Paris regime, growth rates in air are generally higher than those in vacuum, where there is little variation of da/dN with temperature. In contrast, the enhancing effect of the air atmosphere on crack growth rates is strongly temperature dependent. A variety of environment-assisted crack growth mechanisms are found to be operative at different temperature ranges, and evidence of these is provided by both an analysis of activation energies and a fractorgraphic investigation. The situation is different at lower stress intensities, where the threshold stress intensity range falls dramatically with increasing temperature in vacuum, and near-threshold growth rates at 525[degrees]C are higher in vacuum than in air. This effect is attributed to the occurrence of sever oxide-induced closure in air at elevated temperatures, where the crack is blocked with oxide at low stress intensities, reducing the crack driving force to a level below the intrinsic material threshold.

  1. Micromechanisms of intergranular brittle ftacture in intermetallic compounds

    NASA Astrophysics Data System (ADS)

    Vitek, V.

    1991-06-01

    Grain boundaries in intermetallic compounds such as Ni3A1 are inherently brittle. The reason is usually sought in grain boundary cohesion but in metals even brittle fracture is accompanied by some local plasticity and thus not only cohesion but also dislocation mobility in the boundary region need to be studied. We first discuss here the role of an irreversible shear deformation at the crack tip during microcrack propagation assuming that these two processes are concomitant. It is shown that a pre-existing crack cannot propagate in a brittle manner once the dislocation emission occurs. However, if a microcrack nucleates during loading it can propagate concurrently with the development of the irreversible shear deformation at the crack tip. The latter is then the major energy dissipating process. In the second part of this paper we present results of atomistic studies of grain boundaries in Ni3A1 and CU3Au which suggest that substantial structural differences exist between strongly and weakly ordered L12 alloys. We discuss then the consequence of these differences for intergranular brittleness in the framework of the above model for microcrack propagation. On this basis we propose an explanation for the intrinsic intergranular brittleness in some L12 alloys and relate it directly to the strength of ordering. Les joints de grains dans les composés intermétalliques de type Ni3AI sont de nature fragile. L'origine de cette fragilité est habituellement dans la cohésion des joints de grains. Dans les métaux, cependant, même la rupture fragile est accompagnée d'une certaine déformation plastique locale, de telle sorte que non seulement la cohésion mais aussi la mobilité des dislocations près des joints doit être étudiée. Nous discutons d'abord le rôle d'une déformation en cisaillement irréversible en tête de fissure pendant la propagation de cette fissure, en supposant que les deux processus sont concomitants. Nous montrons qu'une fissure préexistante ne

  2. Mechanism of Irradiation Assisted Cracking of Core Components in Light Water Reactors

    SciTech Connect

    Was, Gary S; Atzmon, Michael; Wang, Lumin

    2003-04-28

    The overall goal of the project is to determine the mechanism of irradiation assisted stress corrosion cracking (IASCC). IASCC has been linked to hardening, microstructural and microchemical changes during irradiation. Unfortunately, all of these changes occur simultaneously and at similar rates during irradiation, making attribution of IASCC to any one of these features nearly impossible to determine. The strategy set forth in this project is to develop means to separate microstructural from microchemical changes to evaluate each separately for their effect on IASCC. In the first part, post irradiation annealing (PIA) treatments are used to anneal the irradiated microstructure, leaving only radiation induced segregation (RIS) for evaluation for its contribution to IASCC. The second part of the strategy is to use low temperature irradiation to produce a radiation damage dislocation loop microstructure without radiation induced segregation in order to evaluate the effect of the dislocation microstructure alone.

  3. Shrinkage Cracking: A mechanism for self-sustaining carbon mineralization reactions in olivine rocks

    NASA Astrophysics Data System (ADS)

    Zhu, W.; Fusseis, F.; Lisabeth, H. P.; Xing, T.; Xiao, X.; De Andrade, V. J. D.; Karato, S. I.

    2015-12-01

    The hydration and carbonation of olivine results in an up to ~44% increase in solid molar volume, which may choke off of fluid supply and passivate reactive surfaces, thus preventing further carbonation reactions. The carbonation of olivine has ben studied extensively in the laboratory. To date, observations from these experimental studies indicate that carbonation reaction rates generally decrease with time and the extent of carbonation is limited in olivine rocks. Field studies, however, show that 100% hydration and carbonation occur naturally in ultramafic rocks. The disagreement between the laboratory results under controlled conditions and the field observations underlines the lack of understanding of the mechanisms responsible for the self-sustaining carbonation interaction in nature. We developed a state-of-the-art pressurized hydrothermal cell that is transparent to X-rays to characterize the real-time evolution of pore geometry during fluid-rock interaction using in-situ synchrotron-based X-ray microtomography. Through a time series of high-resolution 3-dimensional images, we document the microstructural evolution of a porous olivine aggregate reacting with a sodium bicarbonate solution at elevated pressure and temperature conditions. We observed porosity increases, near constant rate of crystal growth, and pervasive reaction-induced fractures. Based on the nanometer scale tomography data, we propose that shrinkage cracking is the mechanism responsible for producing new reactive surface and keep the carbonation reaction self-sustaining in our experiment. Shrinkage cracks are commonly observed in drying mud ponds, cooling lava flows and ice wedge fields. Stretching of a contracting surface bonded to a substrate of nearly constant dimensions leads to a stress buildup in the surface layer. When the stress exceeds the tensile strength, polygonal cracks develop in the surface layer. In our experiments, the stretching mismatch between the surface and interior of

  4. Fracture mechanics of propagating 3-D fatigue cracks with parametric dislocations

    NASA Astrophysics Data System (ADS)

    Takahashi, Akiyuki; Ghoniem, Nasr M.

    2013-07-01

    Propagation of 3-D fatigue cracks is analyzed using a discrete dislocation representation of the crack opening displacement. Three dimensional cracks are represented with Volterra dislocation loops in equilibrium with the applied external load. The stress intensity factor (SIF) is calculated using the Peach-Koehler (PK) force acting on the crack tip dislocation loop. Loading mode decomposition of the SIF is achieved by selection of Burgers vector components to correspond to each fracture mode in the PK force calculations. The interaction between 3-D cracks and free surfaces is taken into account through application of the superposition principle. A boundary integral solution of an elasticity problem in a finite domain is superposed onto the elastic field solution of the discrete dislocation method in an infinite medium. The numerical accuracy of the SIF is ascertained by comparison with known analytical solution of a 3-D crack problem in pure mode I, and for mixed-mode loading. Finally, fatigue crack growth simulations are performed with the Paris law, showing that 3-D cracks do not propagate in a self-similar shape, but they re-configure as a result of their interaction with external boundaries. A specific numerical example of fatigue crack growth is presented to demonstrate the utility of the developed method for studies of 3-D crack growth during fatigue.

  5. Toughened epoxy polymers: Fatigue crack propagation mechanisms. Ph.D. Thesis

    SciTech Connect

    Azimi, H.R.

    1994-01-01

    This study examines several mechanisms by which the fatigue crack propagation (FCP) resistance of shear-yielding thermoset polymers can be improved. Specifically, this research has four objectives as follows: first, to develop a mechanistic understanding of the FCP behavior of rubber-modified thermoset polymers; second, to understand the effect of strength and shape of the inorganic fillers on the FCP resistance and micromechanisms in filled epoxy polymers; third, to elucidate the nature of the interactions among the crack-tip shielding mechanisms in thermoset polymers subjected to cyclic loading and synergistically toughened with both rubber and inorganic particles (i.e., hybrid composites); fourth, to study the role of interfaces on the synergistic interactions in FCP behavior of hybrid composites. The model - matrix material consists of a diglycidyl ether of bisphenol A (DGEBA) based type epoxy cured with piperidine. Parallel to the first objective, the epoxy matrix was modified with rubber while changing volume fraction, type, and size of the rubber particles. To accomplish the second goal, the epoxy polymers were modified by a total 10 volume percent of either one of the following three types of inorganic modifiers: hollow glass spheres (HGS); solid glass spheres (SGS); and short glass fibers (SGF). The third goal was met by processing three different systems of hybrid epoxy composites modified by (1) CTBN rubber and HGS, (2) CTBN rubber and SGS, and (3) CTBN rubber and SGF. The total volume fraction of the two modifiers in each hybrid system was kept constant at 10 percent while systematically changing their ratio. To meet the fourth objective, the surface properties of the SGS particles in the hybrid system were altered using adhesion promoter. A mechanistic understanding of the FCP behavior of rubber-modified epoxies was achieved by relating fractographs to observed FCP behavior.

  6. Elevated temperature crack growth in aluminum alloys: Tensile deformation of 2618 and FVS0812 aluminum alloys

    NASA Technical Reports Server (NTRS)

    Leng, Yang; Gangloff, Richard P.

    1990-01-01

    Understanding the damage tolerance of aluminum alloys at elevated temperatures is essential for safe applications of advanced materials. The objective of this project is to investigate the time dependent subcritical cracking behavior of powder metallurgy FVS0812 and ingot metallurgy 2618 aluminum alloys at elevated temperatures. The fracture mechanics approach was applied. Sidegrooved compact tension specimens were tested at 175, 250, and 316 C under constant load. Subcritical crack growth occurred in each alloy at applied stress intensity levels (K) of between about 14 and 25 MPa/m, well below K (sub IC). Measured load, crack opening displacement and displacement rate, and crack length and growth rate (da/dt) were analyzed with several continuum fracture parameters including, the C-integral, C (sub t), and K. Elevated temperature growth rate data suggest that K is a controlling parameter during time dependent cracking. For FVS0812, da/dt is highest at 175 C when rates are expressed as a function of K. While crack growth rate is not controlled by C (sub t) at 175 C, da/dt appears to better correlate with C (sub t) at higher temperatures. Creep brittle cracking at intermediate temperatures, and perhaps related to strain aging, is augmented by time dependent transient creep plasticity at higher temperatures. The C (sub t) analysis is, however, complicated by the necessity to measure small differences in the elastic crack growth and creep contributions to the crack opening displacement rate. A microstructural study indicates that 2618 and FVS0812 are likely to be creep brittle materials, consistent with the results obtained from the fracture mechanics study. Time dependent crack growth of 2618 at 175 C is characterized by mixed transgranular and intergranular fracture. Delamination along the ribbon powder particle boundaries occurs in FVS0812 at all temperatures. The fracture mode of FVS0812 changes with temperature. At 175 C, it is characterized as dimpled rupture

  7. Fracture mechanics of bone--the effects of density, specimen thickness and crack velocity on longitudinal fracture.

    PubMed

    Behiri, J C; Bonfield, W

    1984-01-01

    The fracture mechanics parameters of critical stress intensity factor (Kc) and critical strain energy release rate (Gc) for longitudinal fracture of bovine tibia cortical bone were determined by the compact tension method. It was demonstrated that, for a given bone density, Kc and Gc depended on the loading rate, and resultant crack velocity, with a maximum in fracture toughness (Kc approximately 6.3 MNm-3/2, Gc approximately 2900 Jm-2) at a crack velocity approximately 10(-3) ms-1. For a given loading rate, or crack velocity, an increase in bone density, in the range from 1.92 to 2.02 Mgm-3, produced increases in Kc and Gc, but a variation in specimen thickness (from 0.5 to 2 mm) had no effect on the measured fracture mechanics parameters.

  8. Evaluation and Repair of Primary Water Stress Corrosion Cracking in Alloy 600/182 Control Rod Drive Mechanism Nozzles

    SciTech Connect

    Frye, Charles R.; Arey, Melvin L. Jr.; Robinson, Michael R.; Whitaker, David E.

    2002-07-01

    In February 2001, a routine visual inspection of the reactor vessel head of Oconee Nuclear Station Unit 3 identified boric acid crystals at nine of sixty-nine locations where control rod drive mechanism housings (CRDM nozzles) penetrate the head. The boric acid deposits resulted from primary coolant leaking from cracks in the nozzle attachment weld and from through-thickness cracks in the nozzle wall. A general overview of the inspection and repair process is presented and results of the metallurgical analysis are discussed in more detail. The analysis confirmed that primary water stress corrosion cracking (PWSCC) is the mechanism of failure of both the Alloy 182 weld filler material and the alloy 600 wrought base material. (authors)

  9. Influence of extrinsic crack deflection and delamination mechanisms on the cryogenic toughness of aluminum-lithium alloy 2090: Behavior in plate (T81) vs sheet (T83) material

    SciTech Connect

    Venkateswara Rao, K.T.; Ritchie, R.O.

    1989-02-01

    Cryogenic strength-toughness relationships are examined in 1.6-mm- thick sheet of commercial 2090-T8 aluminum-lithium alloy, and results compared with behavior in 12.7-mm-thick rolled plate. Unlike the significant increase in L-T fracture toughness exhibited by thick place sections at cryogenic temperatures, the thin sheet (of normally similar composition and microstructure) shows a marked decrease in toughness between 298 and 77 K. Such contrasting observations are attributed primarily to the low short-transverse toughness of the 2090-plate material, which results in enhanced through-thickness intergranular splitting during low-temperature fracture and hence to a prominent role of crack-divider delamination toughening. 23 refs., 6 figs., 1 tab.

  10. Stress corrosion crack initiation of alloy 600 in PWR primary water

    DOE PAGES

    Zhai, Ziqing; Toloczko, Mychailo B.; Olszta, Matthew J.; ...

    2017-04-27

    Stress corrosion crack (SCC) initiation of three mill-annealed alloy 600 heats in simulated pressurized water reactor primary water has been investigated using constant load tests equipped with in-situ direct current potential drop (DCPD) measurement capabilities. SCC initiation times were greatly reduced by a small amount of cold work. Shallow intergranular attack and/or cracks were found on most high-energy grain boundaries intersecting the surface with only a small fraction evolving into larger cracks and intergranular SCC growth. Crack depth profiles were measured and related to DCPD-detected initiation response. Lastly, we discuss processes controlling the SCC initiation in mill-annealed alloy 600.

  11. Applicability of Fracture Mechanics Methodology to Cracking and Fracture of Concrete.

    DTIC Science & Technology

    1986-02-01

    cracking and fracture. The publicized, annotated bibliography was the one by S. Mindess entitled "The Cracking and Fracture of Concrete: An Annotated...7 --- 109 157.0 Mindess , S.. J. S. Nadeau and J. M. Hay, Effects of Different Curing Conditions on Slow Crack Growth in Cement Paste, Cement and...Concrete Research. Vol. 4, 1974, pp. 953-965 158.0 Nadeau, J3. S.. S. Mindess and J3. MI. Hay, Slow Crack Growth in Cement Paste, Journal of the

  12. The Mode 1 mechanical crack tip stress field in hyperelastic and incompressible materials

    NASA Technical Reports Server (NTRS)

    Quigley, Claudia J.

    1995-01-01

    The finite deformation field of a plane strain Mode 1 crack in a hyperelastic and incompressible material was examined under the assumptions of small scale nonlinearity. Finite element analyses were performed for two different material laws, a Neo-Hookean material and a third order invariant of a Rivlin material. The numerical results for both materials were compared to the appropriate theoretical asymptotic solution. A local cavitation locus surrounding the crack tip was identified for the Neo-Hookean material. For the third order invariant Rivlin material, maximum values of the dominant stress component were found close to the surface of the crack, above and below the deformed crack tip.

  13. The Mode 1 mechanical crack tip stress field in hyperelastic and incompressible materials

    NASA Technical Reports Server (NTRS)

    Quigley, Claudia J.

    1995-01-01

    The finite deformation field of a plane strain Mode 1 crack in a hyperelastic and incompressible material was examined under the assumptions of small scale nonlinearity. Finite element analyses were performed for two different material laws, a Neo-Hookean material and a third order invariant of a Rivlin material. The numerical results for both materials were compared to the appropriate theoretical asymptotic solution. A local cavitation locus surrounding the crack tip was identified for the Neo-Hookean material. For the third order invariant Rivlin material, maximum values of the dominant stress component were found close to the surface of the crack, above and below the deformed crack tip.

  14. Stochastic mechanical degradation of multi-cracked fiber bundles with elastic and viscous interactions.

    PubMed

    Manca, Fabio; Giordano, Stefano; Palla, Pier Luca; Cleri, Fabrizio

    2015-05-01

    The mechanics of fiber bundles has been largely investigated in order to understand their complex failure modes. Under a mechanical load, the fibers fail progressively while the load is redistributed among the unbroken fibers. The classical fiber bundle model captures the most important features of this rupture process. On the other hand, the homogenization techniques are able to evaluate the stiffness degradation of bulk solids with a given population of cracks. However, these approaches are inadequate to determine the effective response of a degraded bundle where breaks are induced by non-mechanical actions. Here, we propose a method to analyze the behavior of a fiber bundle, undergoing a random distribution of breaks, by considering the intrinsic response of the fibers and the visco-elastic interactions among them. We obtain analytical solutions for simple configurations, while the most general cases are studied by Monte Carlo simulations. We find that the degradation of the effective bundle stiffness can be described by two scaling regimes: a first exponential regime for a low density of breaks, followed by a power-law regime at increasingly higher break density. For both regimes, we find analytical effective expressions described by specific scaling exponents.

  15. The effect of alloy composition on the mechanism of stress corrosion cracking of titanium alloys in aqueous environments

    NASA Technical Reports Server (NTRS)

    Boyd, J. D.; Williams, D. N.; Wood, R. A.; Jaffee, R. I.

    1972-01-01

    The effects of alloy composition on the aqueous stress corrosion of titanium alloys were studied with emphasis on determining the interrelations among composition, phase structure, and deformation and fracture properties of the alpha phase in alpha-beta alloys. Accomplishments summarized include the effects of alloy composition on susceptibility, and metallurgical mechanisms of stress-corrosion cracking.

  16. Complex nonlinear deformation of nanometer intergranular glassy films in beta-Si3N4.

    PubMed

    Chen, Jun; Ouyang, Lizhi; Rulis, Paul; Misra, Anil; Ching, W Y

    2005-12-16

    The mechanical properties of a model of Y-doped intergranular glassy film in silicon nitride ceramics are studied by large-scale ab initio modeling. By linking directly to its electronic structure, it is shown that this microstructure has a complex nonlinear deformation under stress and Y doping significantly enhances the mechanical properties. The calculation of the electrostatic potential across the film supports the space charge model in ceramic microstructures.

  17. Rate-dependent mode I interlaminar crack growth mechanisms in graphite/epoxy and graphite/PEEK

    NASA Technical Reports Server (NTRS)

    Gillespie, J. W., Jr.; Carlsson, L. A.; Smiley, A. J.

    1987-01-01

    In this paper the mode I fracture behavior of graphite/epoxy and graphite/PEEK composites is examined over four decades of crosshead rates (0.25-250 mm/min). Straight-sided double-cantilever-beam specimens consisting of unidirectional laminates were tested at room temperature. For graphite/epoxy the load-deflection response was linear to fracture, and stable slow crack growth initiating at the highest load level was observed for all rates tested. In contrast, mode I crack growth in the graphite/PEEK material was often unstable and showed stick-slip behavior. Subcritical crack growth occurring prior to the onset of fracture was observed at intermediate displacement rates. A mechanism for the fracture behavior of the graphite/PEEK material (based on viscoelastic, plastic, and microcrack coalescence in the process zone) is proposed and related to the observed rate-dependent phenomena.

  18. Finite element analysis of the plane strain crack-tip mechanical fields in pseudoelastic shape memory alloys

    NASA Astrophysics Data System (ADS)

    Baxevanis, T.; Chemisky, Y.; Lagoudas, D. C.

    2012-09-01

    The plane strain mechanical fields near a stationary crack tip in a pseudoelastic shape memory alloy (SMA) are analyzed via the finite element method. The small scale transformation assumption is employed for the calculations using displacement boundary conditions on a circular region that encloses the stress-induced phase transformation zone. The constitutive law used adopts the classical rate-independent small strain flow theory for the evolution equations of both the transformation and plastic strains. Results on the size and shape of the stress-induced transformation and plastic zone formed near the stationary crack are obtained and a fracture toughness criterion based on the J-integral is discussed in view of the observed path-dependence of J. Moreover, the obtained results are discussed in relation to results for stationary cracks in conventional elastic-plastic materials.

  19. Stress corrosion cracking susceptibilities of Fe3Al-based iron aluminides

    NASA Astrophysics Data System (ADS)

    Kim, Jung-Gu

    1995-04-01

    The susceptibilities to stress corrosion cracking (SCC) were investigated using the constant-potential U-bend SCC test. U-bend SCC evaluations were conducted on two iron aluminide compositions based on Fe3Al and containing 2 and 5 at.% Cr in acid-chloride (pH = 4200 ppm Cl), thiosulfate, and tetrathionate solutions at the freely-corroding conditions. Cracking failures occurred in the thiosulfate and tetrathionate solutions, but not in the acid-chloride solution. The iron aluminides were very susceptible to the sulfur-bearing environments in terms of SCC and aqueous corrosion characteristics. To investigate the effect of applied potential on the cracking behavior, U-bend tests were conducted in the acid-chloride solution at an anodic pitting potential and at cathodic hydrogen-evolution potentials. Cracking occurred within 200 h only at the highly negative cathodic potentials and only for the lower Cr composition. These results indicated that two iron aluminides investigated are susceptible to SCC in acid-chloride solution if the corrosion potential is sufficiently active to generate hydrogen, that the cracking mechanism was related to hydrogen embrittlement. The resistance to hydrogen embrittlement cracking increased with increasing Cr content, i.e., higher Cr levels were beneficial in minimizing this form of cracking. Increased resistance to cracking for the U-bend specimens is influenced by the chemical composition of the passive film. Metallographic examinations by scanning electron microscopy revealed that increasing Cr content decreased the proportion of transgranular cleavage cracking and increased the proportion of intergranular cracking.

  20. The analysis of fatigue crack growth mechanism and oxidation and fatigue life at elevated temperatures

    NASA Technical Reports Server (NTRS)

    Liu, H. W.

    1988-01-01

    Two quantitative models based on experimentally observed fatigue damage processes have been made: (1) a model of low cycle fatigue life based on fatigue crack growth under general-yielding cyclic loading; and (2) a model of accelerated fatigue crack growth at elevated temperatures based on grain boundary oxidation. These two quantitative models agree very well with the experimental observations.

  1. Structure, dynamic crack resistance and fracture mechanisms of quenched and tempered structural steels

    NASA Astrophysics Data System (ADS)

    Simonov, M. Yu.; Simonov, Yu. N.; Khanov, A. M.; Shaimanov, G. S.

    2013-03-01

    The structure of steels 09G2S, 25 and 40 after quenching and tempering at from 200 to 650°C is studied. An own method of the authors is used to estimate the impact toughness and the dynamic crack resistance of structural steels in various structural states. The micromechanisms of crack growth are studied in steels with various structures.

  2. Mechanical and metallurgical effects on low-pH stress corrosion cracking of natural gas pipelines

    SciTech Connect

    Harle, B.A.; Beavers, J.A.; Jaske, C.E.

    1995-12-01

    Stress corrosion cracking of natural gas pipelines in low-pH environments is a serious problem for the gas transmission industry. This paper describes results of an ongoing research program investigating crack growth of API X-65 and X-52 line pipe steels in a low-pH cracking environment using a J-integral technique. The overall objective of the work is to estimate crack growth rates on operating pipelines. In previous work, it was demonstrated that the technique could be utilized to reproduce the cracking observed in the field and that the J integral is a good parameter for characterizing crack growth behavior. Recent work has focused on the evaluation of the influence of loading parameters, such as displacement rate, and metallurgy, on crack growth. Testing has also been performed in which loading sequences involved: (a) a constant displacement rate, until cracking was detected, followed by maintaining a constant displacement; and, (b) slowly loading a specimen to fifty percent of its tensile strength in an inert, non-aqueous environment followed by loading in the low-pH environment.

  3. Oxidation products of Inconel alloys 600 and 690 in hydrogenated steam environments and their role in stress corrosion cracking

    NASA Astrophysics Data System (ADS)

    Ferguson, J. Bryce

    Inconel Alloys 600 and 690 are used extensively in components of Nuclear Pressurized Water Reactors (PWR) in the primary water loop which consists of H2 supersaturated steam. Alloy 600 has been found to crack intergranularly when exposed to primary water conditions. Alloy 690 was designed as a replacement and is generally regarded as immune to cracking. There is no consensus as to the mechanism which is responsible for cracking or the lack thereof in these alloys. In this work thermodynamic arguments for the stability of Ni and Cr compounds developed under pressurized water reactor environments ( PH2O and PH2 ) were experimentally tested. A mechanism is proposed to explain crack initiation and propagation alloy 600 along the grain boundaries where Cr2O3 has formed from the leaching of Cr from the matrix leaving behind a porous Ni-rich region. The mechanism is based on the thermodynamic potential for the transformation of a protective NiO surface layer into an amorphous non-protective Ni(OH)2 gel. This gel would also form along the grain boundaries and when hydrogenated steam reaches the porous Ni-rich regions. Crack initiation is then favored by tensile stressing of the grain boundary regions which can easily rupture the gelatinous film. The leaching of matrix Cr to form non-protective CrOOH gel at the crack tip followed by the exposure of fresh porous Ni to the environment also explains crack propagation in inconel alloy 600. The proposed crack initiation mechanism is not expected to occur in alloy 690 where a protective Cr2O 3 film covers the entire metal surface. However, crack propagation along the grain boundaries in alloy 600 and pre-cracked alloy 690 is expected to be active as hydroxide-forming reactions weaken the material at the grain boundaries.

  4. The growth of small corrosion fatigue cracks in alloy 2024

    NASA Technical Reports Server (NTRS)

    Piascik, Robert S.; Willard, Scott A.

    1993-01-01

    The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminum alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by crack initiation at constituent particle pits, intergranular microcracking for a less than 100 micrometers, and transgranular small crack growth for a micrometer. In aqueous 1 percent NaCl and at a constant anodic potential of -700 mV(sub SCE), small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of delta-K (less than 1 MPa square root of m) below long crack delta-K (sub th). When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Results suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks (a less than or equal to 100 micrometers). This is evidenced by similar small and long crack growth behavior at various levels of R. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for Al by 2024 exposed to salt water.

  5. Small-crack test methods

    NASA Astrophysics Data System (ADS)

    Larsen, James M.; Allison, John E.

    This book contains chapters on fracture mechanics parameters for small fatigue cracks, monitoring small-crack growth by the replication method, measurement of small cracks by photomicroscopy (experiments and analysis), and experimental mechanics of microcracks. Other topics discussed are the real-time measurement of small-crack-opening behavior using an interferometric strain/displacement gage; direct current electrical potential measurement of the growth of small cracks; an ultrasonic method for the measurement of the size and opening behavior of small fatigue cracks; and the simulation of short crack and other low closure loading conditions, utilizing constant K(max) Delta-K-decreasing fatigue crack growth procedures.

  6. Fracture Mechanics of Thin, Cracked Plates Under Tension, Bending and Out-of-Plane Shear Loading

    NASA Technical Reports Server (NTRS)

    Zehnder, Alan T.; Hui, C. Y.; Potdar, Yogesh; Zucchini, Alberto

    1999-01-01

    Cracks in the skin of aircraft fuselages or other shell structures can be subjected to very complex stress states, resulting in mixed-mode fracture conditions. For example, a crack running along a stringer in a pressurized fuselage will be subject to the usual in-plane tension stresses (Mode-I) along with out-of-plane tearing stresses (Mode-III like). Crack growth and initiation in this case is correlated not only with the tensile or Mode-I stress intensity factor, K(sub I), but depends on a combination of parameters and on the history of crack growth. The stresses at the tip of a crack in a plate or shell are typically described in terms of either the small deflection Kirchhoff plate theory. However, real applications involve large deflections. We show, using the von-Karman theory, that the crack tip stress field derived on the basis of the small deflection theory is still valid for large deflections. We then give examples demonstrating the exact calculation of energy release rates and stress intensity factors for cracked plates loaded to large deflections. The crack tip fields calculated using the plate theories are an approximation to the actual three dimensional fields. Using three dimensional finite element analyses we have explored the relationship between the three dimensional elasticity theory and two dimensional plate theory results. The results show that for out-of-plane shear loading the three dimensional and Kirchhoff theory results coincide at distance greater than h/2 from the crack tip, where h/2 is the plate thickness. Inside this region, the distribution of stresses through the thickness can be very different from the plate theory predictions. We have also explored how the energy release rate varies as a function of crack length to plate thickness using the different theories. This is important in the implementation of fracture prediction methods using finite element analysis. Our experiments show that under certain conditions, during fatigue crack

  7. On the toughening of brittle materials by grain bridging:promoting intergranular fracture through grain angle, strength, andtoughness

    SciTech Connect

    Foulk III, J.W.; Johnson, G.C.; Klein, P.A.; Ritchie, R.O.

    2007-11-15

    The structural reliability of many brittle materials such asstructural ceramics relies on the occurrence of intergranular, as opposedto transgranular, fracture in order to induce toughening by grainbridging. For a constant grain boundary strength and grain boundarytoughness, the current work examines the role of grain strength, graintoughness, and grain angle in promoting intergranular fracture in orderto maintain such toughening. Previous studies have illustrated that anintergranular path and the consequent grain bridging process can bepartitioned into five distinct regimes, namely: propagate, kink, arrest,stall and bridge. To determine the validity of the assumed intergranularpath, the classical penentration/deflection problem of a crack impingingon an interface is reexamined within a cohesive zone framework forintergranular and transgranular fracture. Results considering both modesof propagation, i.e., a transgranular and intergranular path, reveal thatcrack-tip shielding is a natural outcome of the cohesive zone approach tofracture. Cohesive zone growth in one mode shields the opposing mode fromthe stresses required for cohesive zone initiation. Although stablepropagation occurs when the required driving force is equivalent to thetoughness for either transgranular or intergranular fracture, the mode ofpropagation depends on the normalized grain strength, normalized graintoughness, and grain angle. For each grain angle, the intersection ofsingle path and multiple path solutions demarcates "strong" grains thatincrease the macroscopic toughness and "weak" grains that decrease it.The unstable transition to intergranular fracture reveals that anincreasinggrain toughness requires a growing region of the transgranularcohesive zone be at and near the peak cohesive strength. The inability ofthe body to provide the requisite stress field yields an overdriven andunstable configuration. The current results provide restrictions for theachievement of substantial toughening

  8. Gear Crack Propagation Investigation

    NASA Technical Reports Server (NTRS)

    1995-01-01

    Reduced weight is a major design goal in aircraft power transmissions. Some gear designs incorporate thin rims to help meet this goal. Thin rims, however, may lead to bending fatigue cracks. These cracks may propagate through a gear tooth or into the gear rim. A crack that propagates through a tooth would probably not be catastrophic, and ample warning of a failure could be possible. On the other hand, a crack that propagates through the rim would be catastrophic. Such cracks could lead to disengagement of a rotor or propeller from an engine, loss of an aircraft, and fatalities. To help create and validate tools for the gear designer, the NASA Lewis Research Center performed in-house analytical and experimental studies to investigate the effect of rim thickness on gear-tooth crack propagation. Our goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. In addition, we investigated the effect of rim thickness on crack propagation life. A finite-element-based computer program simulated gear-tooth crack propagation. The analysis used principles of linear elastic fracture mechanics, and quarter-point, triangular elements were used at the crack tip to represent the stress singularity. The program had an automated crack propagation option in which cracks were grown numerically via an automated remeshing scheme. Crack-tip stress-intensity factors were estimated to determine crack-propagation direction. Also, various fatigue crack growth models were used to estimate crack-propagation life. Experiments were performed in Lewis' Spur Gear Fatigue Rig to validate predicted crack propagation results. Gears with various backup ratios were tested to validate crack-path predictions. Also, test gears were installed with special crack-propagation gages in the tooth fillet region to measure bending-fatigue crack growth. From both predictions and tests, gears with backup ratios

  9. An anodic dissolution-based mechanism for the rapid cracking, ``pre-exposure`` phenomenon demonstrated by Al-Li-Cu alloys

    SciTech Connect

    Buchheit, R.G.; Wall, F.D.; Stoner, G.E.; Moran, J.P.

    1993-12-31

    Al-Li-Cu alloys have been observed to fail in less than 24 hours after removal from a one week immersion in aerated 3.5 w/o NaCl solution. Anodic dissolution-based mechanisms proposed previously for this phenomenon have been amended based on further experiment and characterization of the rapid cracking process. Amendments are based on studies of the relative electrochemical behavior of the microstructural elements in the subgrain boundary region, time to failure SCC testing in a simulated crack solution, evolution of crack potential and pH with time, fractographic examination of failed samples, and X-ray diffraction of films passivating crack walls. Results suggest that an active path exists along subgrain boundaries that is comprised of the highly reactive T{sub 1} (Al{sub 2}CuLi) precipitate phase and a solute depleted zone that does not readily passivate when exposed to the crack environment. The matrix phase does appear to passivate in the crack environment thereby confining the crack to the subgrain boundary region. This active path is enabled when cracks are isolated from a bulk environment, but is disabled otherwise. Potential and pH conditions required for cracking are discussed as is the formation of a hydrotalcite Li{sub 2}[Al{sub 2}(OH){sub 6}]{sub 2}{center_dot}CO{sub 3}{center_dot}3H{sub 2}O film that appears to be responsible for passivating crack walls.

  10. Mechanics of the injected pulsejet into gelatin gel and evaluation of the effect by puncture and crack generation and growth

    NASA Astrophysics Data System (ADS)

    Kato, T.; Arafune, T.; Washio, T.; Nakagawa, A.; Ogawa, Y.; Tominaga, T.; Sakuma, I.; Kobayashi, E.

    2014-08-01

    Recently, fluid jets have become widely used in medical devices and have been created and evaluated in clinical environments. Such devices are classified into two broad groups; those adopting continuous jets and those adopting discrete (or pulsed) jets. We developed a discrete jet device for brain cancer treatment, called a laser-induced liquid jet (LILJ) system. Although several studies have evaluated the availability and described the treatment mechanisms of fluid jet devices, the mechanisms of the fluid and injected material remain under-investigated. In this paper, we report the mechanism of frequent pulsejet injections into a viscoelastic biological material; namely, simulated gelatin brain tissue. The mechanism is evaluated by the injection depth, an easily measured parameter. To explain the injection mechanism, we propose that the pulsejet is pressured by forces introduced by resistance on the side surface of the hole and the reaction force proportionate to the injection depth. The pulsejet generated and propagated cracks in the gelatin, and the resistance eventually fractured the side surface of the hole. We evaluated the proposed model by measuring the behavior of pulsejets injected into gelatin by the LILJ. From the results, the following conclusions were obtained. First, the proposed model accurately describes the behavior of the injected pulsejet. Second, whether the hole or crack growth largely increases the final injection depth can be evaluated from differences in the decay constant. Finally, crack growth increases the final injection depth when the number of the injected pulsejets is greater than the inverse of the decay constant.

  11. Crack growth phenomena in micro-machined single crystal silicon and design implications for micro electro mechanical systems (MEMS)

    NASA Astrophysics Data System (ADS)

    Fitzgerald, Alissa Mirella

    The creation of micron-sized mechanisms using semiconductor processing technology is known collectively as MEMS, or Micro Electro Mechanical Systems. Many MEMS devices, such as accelerometers and switches, have mechanical structures fabricated from single crystal silicon, a brittle material. The reliability and longevity of these devices depends on minimizing the probability of fracture, and therefore requires a thorough understanding of crack growth phenomena in silicon. In this study, a special micro-machined fracture specimen, the compression-loaded double cantilever beam, was developed to study fracture phenomena in single crystal silicon on a size scale relevant to MEMS. The decreasing stress intensity geometry of this sample provided stable, controllable crack propagation in test sections as thin as 100 mum. Several common MEMS fabrication methods (plasma and chemical etch) were used to achieve a range of surface finishes. A 650 A thick titanium crack gage was used to directly measure crack extension as a function of time using the potential drop technique. High speed (100 MHz) data acquisition techniques were employed to capture fracture events on the sub-microsecond time scale. The stability of the sample design and the micron-scale resolution of the crack gage facilitated investigation into the existence of a stress corrosion effect in silicon. No evidence of sub-critical crack growth due to exposure to humid air was found in carefully controlled tests lasting up to 24 hours. Rapid crack propagation velocities (>1 km/s) during quasi-static loading were recorded using high speed data acquisition techniques. Unique evidence was found of reflected stress waves causing multiple, momentary arrests during rapid fracture events. These measurements, along with atomic force microscope scans of the fracture surfaces, offer new insight into the kinetics of the fracture process in silicon. Over 100 micro-machined samples were fractured in this research. Weibull

  12. The mechanism of stress-corrosion cracking in 7075 aluminum alloy

    NASA Technical Reports Server (NTRS)

    Jacobs, A. J.

    1970-01-01

    Various aspects of stress-corrosion cracking in 7075 aluminum alloy are discussed. A model is proposed in which the continuous anodic path along which the metal is preferentially attacked consists of two phases which alternate as anodes.

  13. Study of intergranular embrittlement in Fe-12Mn alloys

    SciTech Connect

    Lee, H.J.

    1982-06-01

    A high resolution scanning Auger microscopic study has been performed on the intergranular fracture surfaces of Fe-12Mn steels in the as-austenitized condition. Fracture mode below the ductile-brittle transition temperature was intergranular whenever the alloy was quenched from the austenite field. The intergranular fracture surface failed to reveal any consistent segregation of P, S, As, O, or N. The occasional appearance of S or O on the fracture surface was found to be due to a low density precipitation of MnS and MnO/sub 2/ along the prior austenite boundaries. An AES study with Ar/sup +/ ion-sputtering showed no evidence of manganese enrichment along the prior austenite boundaries, but a slight segregation of carbon which does not appear to be implicated in the tendency toward intergranular fracture. Addition of 0.002% B with a 1000/sup 0/C/1h/WQ treatment yielded a high Charpy impact energy at liquid nitrogen temperature, preventing the intergranular fracture. High resolution AES studies showed that 3 at. % B on the prior austenite grain boundaries is most effective in increasing the grain boundary cohesive strength in an Fe-12Mn alloy. Trace additions of Mg, Zr, or V had negligible effects on the intergranular embrittlement. A 450/sup 0/C temper of the boron-modified alloys was found to cause tempered martensite embrittlement, leading to intergranular fracture. The embrittling treatment of the Fe-12Mn alloys with and without boron additions raised the ductile-brittle transition by 150/sup 0/C. This tempered martensite embrittlement was found to be due to the Mn enrichment of the fracture surface to 32 at. % Mn in the boron-modified alloy and 38 at. % Mn in the unmodified alloy. The Mn-enriched region along the prior austenite grain boundaries upon further tempering is believed to cause nucleation of austenite and to change the chemistry of the intergranular fracture surfaces. 61 figures.

  14. Current understanding of stress-corrosion cracking

    SciTech Connect

    Parkins, R.N. )

    1992-12-01

    The mechanisms that cause stress corrosion cracking and the conditions in which they apply are reviewed. Attention is given to hydrogen-assisted cracking, film-induced cleavage, dissolution mechanisms, surface-mobility mechanism, cracking environments, deformation and cracking, and stochastic aspects of cracking. 70 refs.

  15. Real-time Observations of Rock Cracking and Weather Provide Insights into Thermal Stress-Related Processes of Mechanical Weathering.

    NASA Astrophysics Data System (ADS)

    Eppes, M. C.; Magi, B. I.; Keanini, R.

    2015-12-01

    The environmental conditions (weather and/or climate) that limit or drive mechanical weathering via thermal stress are poorly understood. Here we examine acoustic emission (AE) records of rock cracking in boulders sitting on the ground in humid-temperate (~1 year of data) and semi-arid (~3 years) locations. We compare on-site average ambient daily temperature for days in which cracking occurs to the average temperatures for those dates derived from local climate records. The temperatures characterizing days on which cracking occurs is similar for both stations (range = -10 C to +30 C); where 21% and 73% of cracking occurs on hot days (> 20C) in the humid and semi-arid climates respectively while 17% and 0.1% occurs on very cold days (-8C to -3C). When days during which cracking occurs are compared to climate averages, 81% (NC) and 51% (NM) of all cracking occurs on days with absolute temperature anomalies >1, regardless of the temperature. The proportion of cracking that occurs on anomalously hot or cold days rises to 92% and 77% when the data is normalized to account for uneven sampling of the days with extreme temperatures. In order to determine to what extent this trend holds true in a more complex setting, we examined an existing 100+ year record of rock falls from Yosemite Valley. Preliminary results, although more equivocal, are consistent with the boulder cracking AE data. We examine the AE datasets in the context of our previous numerical modeling of insolation-driven thermal stress in rock and hypothesize that there is an increased potential for fracture on days with extreme temperatures because 1) thermal-stress is dependent on temperature variance from far-field and/or average rock temperatures and 2) that days with climatologically extreme air temperatures result in maximums in such variance. An implication of our results is that environments with extreme weather variability may have higher thermal breakdown rates, including certain locations today and

  16. Pyrolytic carbon indentation crack morphology.

    PubMed

    Ely, J L; Stupka, J; Haubold, A D

    1996-06-01

    In studying fatigue and fracture behavior of brittle materials, Vickers diamond indentation cracks are often used. Many of the studies of indentation cracks use crack system models such as the radial-median crack or Palmqvist crack. These systems are also used to study small crack growth in brittle materials, and have been studied for pyrolytic carbon. However, the true morphology of these cracks in pyrolytic carbon coatings on graphite substrates have not been described. This study examined Vickers diamond and spherical ball indentation cracks in pyrolytic carbon coatings using several techniques, including serial metallographic cross sections, indentation fracture in bending, acoustic emission, and residual surface indentation scanning. The crack systems developed using these techniques were not typical of either radial median or Palmqvist systems. The morphology is unique to this material, possibly because of the coating thickness limitations. Given the difference in crack system, the application of standard indentation crack equations in studying fracture mechanics, especially for small cracks, must be questioned.

  17. A Problem of Fatigue Fracture Mechanics of a Two-Layer Material with Edge Cracks

    NASA Astrophysics Data System (ADS)

    Seyfullayev, A. I.; Rustamova, M. A.; Kerimova, Sh. A.

    2017-07-01

    A problem for an elastic half-plane consisting of two tightly coupled homogeneous isotropic elastic media and containing edge cracks is considered. Its lateral surface is subjected to a given external load. The growth of a fatigue crack in the two-layer half-plane is investigated. For some materials, the number of cycles to failure is calculated. The stress intensity factor is determined, and a numerical analysis is performed.

  18. Irradiation assisted stress corrosion cracking of controlled purity 304L stainless steels

    NASA Astrophysics Data System (ADS)

    Cookson, J. M.; Carter, R. D.; Damcott, D. L.; Atzmon, M.; Was, G. S.

    1993-06-01

    The effect of chromium, phosphorus, silicon and sulfur on the stress corrosion cracking of 304L stainless steel in CERT tests in high purity water or argon at 288°C following irradiation with 3.4 MeV protons at 400°C to 1 dpa, has been investigated using ultrahigh purity alloys (UHP) with controlled impurity additions. Grain boundary segregation of phosphorus or silicon due to proton irradiation was quantified using both Auger electron spectroscopy and scanning transmission electron microscopy, and the alloys with impurity element additions were observed to have greater grain boundary chromium depletion and nickel enrichment than the UHP alloy. The UHP alloy suffered severe cracking in CERT tests in water. Less cracking was found after CERT test of irradiated UHP+Por UHP+Si alloys, despite greater chromium depletion. This suggests a mitigating effect of phosphorus and silicon at grain boundaries. No cracking was found in argon tests, eliminating a purely mechanical embrittlement mechanism, but not eliminating a contribution from radiation hardening. Implanted hydrogen was not a factor in the intergranular cracking found.

  19. The effect of pre-stress cycles on fatigue crack growth - An analysis of crack growth mechanism. [in Al alloy plates

    NASA Technical Reports Server (NTRS)

    Kang, T. S.; Liu, H. W.

    1974-01-01

    Cyclic prestress increases subsequent fatigue crack growth rate in 2024-T351 aluminum alloy. This increase in growth rate, caused by the prestress, and the increased rate, caused by temper embrittlement as observed by Ritchie and Knott (1973), cannot be explained by the crack tip blunting model alone. Each fatigue crack increment consists of two components, a brittle and a ductile component. They are controlled by the ductility of the material and its cyclic yield strength, respectively.

  20. Subcritical crack growth in SiNx thin-film barriers studied by electro-mechanical two-point bending

    NASA Astrophysics Data System (ADS)

    Guan, Qingling; Laven, Jozua; Bouten, Piet C. P.; de With, Gijsbertus

    2013-06-01

    Mechanical failure resulting from subcritical crack growth in the SiNx inorganic barrier layer applied on a flexible multilayer structure was studied by an electro-mechanical two-point bending method. A 10 nm conducting tin-doped indium oxide layer was sputtered as an electrical probe to monitor the subcritical crack growth in the 150 nm dielectric SiNx layer carried by a polyethylene naphthalate substrate. In the electro-mechanical two-point bending test, dynamic and static loads were applied to investigate the crack propagation in the barrier layer. As consequence of using two loading modes, the characteristic failure strain and failure time could be determined. The failure probability distribution of strain and lifetime under each loading condition was described by Weibull statistics. In this study, results from the tests in dynamic and static loading modes were linked by a power law description to determine the critical failure over a range of conditions. The fatigue parameter n from the power law reduces greatly from 70 to 31 upon correcting for internal strain. The testing method and analysis tool as described in the paper can be used to understand the limit of thin-film barriers in terms of their mechanical properties.

  1. Gear crack propagation investigations

    NASA Technical Reports Server (NTRS)

    Lewicki, David G.; Ballarini, Roberto

    1996-01-01

    Analytical and experimental studies were performed to investigate the effect of gear rim thickness on crack propagation life. The FRANC (FRacture ANalysis Code) computer program was used to simulate crack propagation. The FRANC program used principles of linear elastic fracture mechanics, finite element modeling, and a unique re-meshing scheme to determine crack tip stress distributions, estimate stress intensity factors, and model crack propagation. Various fatigue crack growth models were used to estimate crack propagation life based on the calculated stress intensity factors. Experimental tests were performed in a gear fatigue rig to validate predicted crack propagation results. Test gears were installed with special crack propagation gages in the tooth fillet region to measure bending fatigue crack growth. Good correlation between predicted and measured crack growth was achieved when the fatigue crack closure concept was introduced into the analysis. As the gear rim thickness decreased, the compressive cyclic stress in the gear tooth fillet region increased. This retarded crack growth and increased the number of crack propagation cycles to failure.

  2. Analysis of internal crack propagation in silicon due to permeable pulse laser irradiation: study on processing mechanism of stealth dicing

    NASA Astrophysics Data System (ADS)

    Ohmura, Etsuji; Kawahito, Yuta; Fukumitsu, Kenshi; Okuma, Junji; Morita, Hideki

    2010-07-01

    Stealth dicing (SD) is an innovative dicing method developed by Hamamatsu Photonics K.K. In the SD method, a permeable nanosecond laser is focused inside a silicon wafer and scanned horizontally. A thermal shock wave propagates every pulse toward the side to which the laser is irradiated, then a high dislocation density layer is formed inside the wafer after the thermal shock wave propagation. In our previous study, it was concluded that an internal crack whose initiation is a dislocation is propagated when the thermal shock wave by the next pulse overlaps with this layer partially. In the experimental result, the trace that a crack is progressed gradually step by step was observed. In this study, the possibility of internal crack propagation by laser pulses was investigated. A two-dimensional thermal stress analysis based on the linear fracture mechanics was conducted using the stress distribution obtained by the axisymmetric thermal stress analysis. As a result, the validity of the hypothesis based on a heat transfer analysis result previously presented was supported. Also it was concluded that the internal crack is propagated by at least two pulses.

  3. Analysis of internal crack propagation in silicon due to permeable pulse laser irradiation: study on processing mechanism of stealth dicing

    NASA Astrophysics Data System (ADS)

    Ohmura, Etsuji; Kawahito, Yuta; Fukumitsu, Kenshi; Okuma, Junji; Morita, Hideki

    2011-02-01

    Stealth dicing (SD) is an innovative dicing method developed by Hamamatsu Photonics K.K. In the SD method, a permeable nanosecond laser is focused inside a silicon wafer and scanned horizontally. A thermal shock wave propagates every pulse toward the side to which the laser is irradiated, then a high dislocation density layer is formed inside the wafer after the thermal shock wave propagation. In our previous study, it was concluded that an internal crack whose initiation is a dislocation is propagated when the thermal shock wave by the next pulse overlaps with this layer partially. In the experimental result, the trace that a crack is progressed gradually step by step was observed. In this study, the possibility of internal crack propagation by laser pulses was investigated. A two-dimensional thermal stress analysis based on the linear fracture mechanics was conducted using the stress distribution obtained by the axisymmetric thermal stress analysis. As a result, the validity of the hypothesis based on a heat transfer analysis result previously presented was supported. Also it was concluded that the internal crack is propagated by at least two pulses.

  4. Discrimination between Fatigue Cracking and Mechanical Damage in Aircraft Fastener Holes by Eddy-Current Phase Rotation

    DTIC Science & Technology

    2016-08-01

    is a well-established NDI method that employs the induced magnetic fields and electrical currents produced by a small probe coil to detect defects...UNCLASSIFIED UNCLASSIFIED Discrimination between Fatigue Cracking and Mechanical Damage in Aircraft Fastener Holes by Eddy- Current Phase...using the nondestructive inspection technique of bolt-hole eddy- current (BHEC) testing. BHEC testing is based on detection of a disturbance in the

  5. Intergranular Strain Evolution During Biaxial Loading: A Multiscale FE-FFT Approach

    NASA Astrophysics Data System (ADS)

    Upadhyay, M. V.; Capek, J.; Van Petegem, S.; Lebensohn, R. A.; Van Swygenhoven, H.

    2017-05-01

    Predicting the macroscopic and microscopic mechanical response of metals and alloys subjected to complex loading conditions necessarily requires a synergistic combination of multiscale material models and characterization techniques. This article focuses on the use of a multiscale approach to study the difference between intergranular lattice strain evolution for various grain families measured during in situ neutron diffraction on dog bone and cruciform 316L samples. At the macroscale, finite element simulations capture the complex coupling between applied forces and gauge stresses in cruciform geometries. The predicted gauge stresses are used as macroscopic boundary conditions to drive a mesoscale full-field elasto-viscoplastic fast Fourier transform crystal plasticity model. The results highlight the role of grain neighborhood on the intergranular strain evolution under uniaxial and equibiaxial loading.

  6. Intergranular Strain Evolution During Biaxial Loading: A Multiscale FE-FFT Approach

    NASA Astrophysics Data System (ADS)

    Upadhyay, M. V.; Capek, J.; Van Petegem, S.; Lebensohn, R. A.; Van Swygenhoven, H.

    2017-03-01

    Predicting the macroscopic and microscopic mechanical response of metals and alloys subjected to complex loading conditions necessarily requires a synergistic combination of multiscale material models and characterization techniques. This article focuses on the use of a multiscale approach to study the difference between intergranular lattice strain evolution for various grain families measured during in situ neutron diffraction on dog bone and cruciform 316L samples. At the macroscale, finite element simulations capture the complex coupling between applied forces and gauge stresses in cruciform geometries. The predicted gauge stresses are used as macroscopic boundary conditions to drive a mesoscale full-field elasto-viscoplastic fast Fourier transform crystal plasticity model. The results highlight the role of grain neighborhood on the intergranular strain evolution under uniaxial and equibiaxial loading.

  7. Development of a novel technique to assess the vulnerability of micro-mechanical system components to environmentally assisted cracking.

    SciTech Connect

    Enos, David George; Goods, Steven Howard

    2006-11-01

    Microelectromechanical systems (MEMS) will play an important functional role in future DOE weapon and Homeland Security applications. If these emerging technologies are to be applied successfully, it is imperative that the long-term degradation of the materials of construction be understood. Unlike electrical devices, MEMS devices have a mechanical aspect to their function. Some components (e.g., springs) will be subjected to stresses beyond whatever residual stresses exist from fabrication. These stresses, combined with possible abnormal exposure environments (e.g., humidity, contamination), introduce a vulnerability to environmentally assisted cracking (EAC). EAC is manifested as the nucleation and propagation of a stable crack at mechanical loads/stresses far below what would be expected based solely upon the materials mechanical properties. If not addressed, EAC can lead to sudden, catastrophic failure. Considering the materials of construction and the very small feature size, EAC represents a high-risk environmentally induced degradation mode for MEMS devices. Currently, the lack of applicable characterization techniques is preventing the needed vulnerability assessment. The objective of this work is to address this deficiency by developing techniques to detect and quantify EAC in MEMS materials and structures. Such techniques will allow real-time detection of crack initiation and propagation. The information gained will establish the appropriate combinations of environment (defining packaging requirements), local stress levels, and metallurgical factors (composition, grain size and orientation) that must be achieved to prevent EAC.

  8. Stress Corrosion Cracking of Ni-Fe-Cr Alloys Relevant to Nuclear Power Plants

    NASA Astrophysics Data System (ADS)

    Persaud, Suraj

    Stress corrosion cracking (SCC) of Ni-Fe-Cr alloys and weld metals was investigated in simulated environments representative of high temperature water used in the primary and secondary circuits of nuclear power plants. The mechanism of primary water SCC (PWSCC) was studied in Alloys 600, 690, 800 and Alloy 82 dissimilar metal welds using the internal oxidation model as a guide. Initial experiments were carried out in a 480°C hydrogenated steam environment considered to simulate high temperature reducing primary water. Ni alloys underwent classical internal oxidation intragranularly resulting in the expulsion of the solvent metal, Ni, to the surface. Selective intergranular oxidation of Cr in Alloy 600 resulted in embrittlement, while other alloys were resistant owing to their increased Cr contents. Atom probe tomography was used to determine the short-circuit diffusion path used for Ni expulsion at a sub-nanometer scale, which was concluded to be oxide-metal interfaces. Further exposures of Alloys 600 and 800 were done in 315°C simulated primary water and intergranular oxidation tendency was comparable to 480°C hydrogenated steam. Secondary side work involved SCC experiments and electrochemical measurements, which were done at 315°C in acid sulfate solutions. Alloy 800 C-rings were found to undergo acid sulfate SCC (AcSCC) to a depth of up to 300 microm in 0.55 M sulfate solution at pH 4.3. A focused-ion beam was used to extract a crack tip from a C-ring and high resolution analytical electron microscopy revealed a duplex oxide structure and the presence of sulfur. Electrochemical measurements were taken on Ni alloys to complement crack tip analysis; sulfate was concluded to be the aggressive anion in mixed sulfate and chloride systems. Results from electrochemical measurements and crack tip analysis suggested a slip dissolution-type mechanism to explain AcSCC in Ni alloys.

  9. A phenomenological model for intergranular failure by r-type and wedge-type cavitation

    SciTech Connect

    Lee, S.B.; Miller, A.K.

    1995-07-01

    Equations to predict local intergranular failure (by r-type and wedge-type cavitation and the coupling between them) have been developed. The derivation has utilized physically based concepts such as thermal activation of the controlling processes, wedge cracking driven by grain boundary sliding, and cavity growth driven by diffusion. It has also been based upon phenomenological observations such as the variation in the steady-state creep rate with stress and temperature, incomplete healing of cavities under compression, and differences in life under slow-fast and fast-slow cycling. The model has been tested against data on the low-cycle fatigue life of 304 stainless steel under unequal ramp rates. The new equations simulate, for example, the differences in life produced by slow-fast, fast-slow, and equal ramp rate cycling in terms of their effects on internal cavitation. Together with the new equations` ability to treat monotonic creep rupture, these comparisons demonstrate that the intergranular failure equations are capable of simulating a number of phenomena of importance in life prediction for high-temperature structures.

  10. A study on electromigration-inducing intergranular fracture of fine silver alloy wires

    NASA Astrophysics Data System (ADS)

    Hsueh, Hao-Wen; Hung, Fei-Yi; Lui, Truan-Sheng

    2017-01-01

    In this study, Pd-coated Cu, Ag (purity = 4 N), and Ag alloy (Ag-8Au-3Pd) wires were employed to measure the tensile properties during current stressing using the so-called dynamic current tensile (DCT) test. Both the tensile strength and elongation of the wires decreased dramatically in the DCT test, particularly of the Ag-based wires, and the fracture morphology of the Cu-based and Ag-based wires was ductile fracture and intergranular fracture, respectively. Compared to the Cu-based wires, electromigration occurred more easily in the Ag-based wires, and it always generated voids and cracks at the grain boundaries; therefore, the fracture morphology of the Ag-based wires was intergranular fracture owing to the weakened grain boundary. Further, the results indicated that the Ag-based wires could not carry a higher current density than the Cu-based wires, primarily because their extremely low strength and elongation in current stressing might cause serious reliability problems.

  11. Detection of Intergranular Corrosion in Cold Plate Face Sheets

    NASA Technical Reports Server (NTRS)

    Winfree, William P.; Smith, Stephen W.; Piascik, Robert S.; Howell, Patricia A.

    2002-01-01

    Cold plates are critical for cooling electronic systems in the shuttle. As a result of the environmental conditions in which they operate, water can condense between them and a support shelf. In some cases, this water results in intergranular corrosion in the face sheet. If the intergranular corrosion sufficiently penetrates the face sheet, a coolant leak could occur and jeopardize cold plate operation. This paper examines techniques for detecting and characterizing the intergranular corrosion, to enable recertification of cold plates that have been in operation for 15 plus years. Intergranular corrosion was artificially induced in the face sheets of a series of cold plate specimens using an electrochemical process. Some of the cold plate specimens were separated for destructive characterization of the extent of corrosion produced by the electrochemical process and to insure the induced corrosion was intergranular. The rest of the specimens were characterized nondestructively using several techniques. X-ray tomography and ultrasonic techniques provided the best indication of corrosion in these specimens and will be the focus of this paper. An x-ray tomography technique was shown to be the most effective technique for characterizing depth of the intergranular corrosion. From these measurements, corrosion profile maps were developed that were consistent with subsequent destructive evaluations of the specimens. This enabled the assessment of NDE (ondestructive evaluation) standards to evaluate the viability of other NDE techniques. Due to system constraints, a different technique must be used to inspect an entire cold plate. An ultrasonic technique was shown to be very reliable for detection of corrosion in the unbacked regions of the face sheet. The ultrasonic technique was performed in an alcohol bath to avoid additional corrosion during the NDE evaluation. A pulse echo technique that focuses on the RMS value of the signal is shown to be very sensitive to the

  12. Investigation of Fatigue Crack Propagation in Spot-Welded Joints Based on Fracture Mechanics Approach

    NASA Astrophysics Data System (ADS)

    Hassanifard, S.; Bonab, M. A. Mohtadi; Jabbari, Gh.

    2013-01-01

    In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.

  13. Cyclic fatigue and fracture in pyrolytic carbon-coated graphite mechanical heart-valve prostheses: role of small cracks in life prediction.

    PubMed

    Dauskardt, R H; Ritchie, R O; Takemoto, J K; Brendzel, A M

    1994-07-01

    A fracture-mechanics based study has performed to characterize the fracture toughness and rates of cyclic fatigue-crack growth of incipient flaws in prosthetic heart-valve components made of pyrolytic carbon-coated graphite. Such data are required to predict the safe structural lifetime of mechanical heart-valve prostheses using damage-tolerant analysis. Unlike previous studies where fatigue-crack propagation data were obtained using through-thickness, long cracks (approximately 2-20 mm long), growing in conventional (e.g., compact-tension) samples, experiments were performed on physically small cracks (approximately 100-600 microns long), initiated on the surface of the pyrolytic-carbon coating to simulate reality. Small-crack toughness results were found to agree closely with those measured conventionally with long cracks. However, similar to well-known observations in metal fatigue, it was found that based on the usual computations of the applied (far-field) driving force in terms of the maximum stress intensity, Kmax, small fatigue cracks grew at rates that exceeded those of long cracks at the same applied stress intensity, and displayed a negative dependency on Kmax; moreover, they grew at applied stress intensities less than the fatigue threshold value, below which long cracks are presumed dormant. To resolve this apparent discrepancy, it is shown that long and small crack results can be normalized, provided growth rates are characterized in terms of the total (near-tip) stress intensity (incorporating, for example, the effect of residual stress); with this achieved, in principle, either form of data can be used for life prediction of implant devices. Inspection of the long and small crack results reveals extensive scatter inherent in both forms of growth-rate data for the pyrolytic-carbon material.

  14. Numerical, micro-mechanical prediction of crack growth resistance in a fibre-reinforced/brittle matrix composite

    NASA Technical Reports Server (NTRS)

    Jenkins, Michael G.; Ghosh, Asish; Salem, Jonathan A.

    1990-01-01

    Micromechanics fracture models are incorporated into three distinct fracture process zones which contribute to the crack growth resistance of fibrous composites. The frontal process zone includes microcracking, fiber debonding, and some fiber failure. The elastic process zone is related only to the linear elastic creation of new matrix and fiber fracture surfaces. The wake process zone includes fiber bridging, fiber pullout, and fiber breakage. The R-curve predictions of the model compare well with empirical results for a unidirectional, continuous fiber C/C composite. Separating the contributions of each process zone reveals the wake region to contain the dominant crack growth resistance mechanisms. Fractography showed the effects of the micromechanisms on the macroscopic fracture behavior.

  15. Numerical, micro-mechanical prediction of crack growth resistance in a fibre-reinforced/brittle matrix composite

    NASA Technical Reports Server (NTRS)

    Jenkins, Michael G.; Ghosh, Asish; Salem, Jonathan A.

    1990-01-01

    Micromechanics fracture models are incorporated into three distinct fracture process zones which contribute to the crack growth resistance of fibrous composites. The frontal process zone includes microcracking, fiber debonding, and some fiber failure. The elastic process zone is related only to the linear elastic creation of new matrix and fiber fracture surfaces. The wake process zone includes fiber bridging, fiber pullout, and fiber breakage. The R-curve predictions of the model compare well with empirical results for a unidirectional, continuous fiber C/C composite. Separating the contributions of each process zone reveals the wake region to contain the dominant crack growth resistance mechanisms. Fractography showed the effects of the micromechanisms on the macroscopic fracture behavior.

  16. Grain boundary oxidation and fatigue crack growth at elevated temperatures

    NASA Technical Reports Server (NTRS)

    Liu, H. W.; Oshida, Y.

    1986-01-01

    Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.

  17. Fatigue crack growth in an aluminum alloy-fractographic study

    NASA Astrophysics Data System (ADS)

    Salam, I.; Muhammad, W.; Ejaz, N.

    2016-08-01

    A two-fold approach was adopted to understand the fatigue crack growth process in an Aluminum alloy; fatigue crack growth test of samples and analysis of fractured surfaces. Fatigue crack growth tests were conducted on middle tension M(T) samples prepared from an Aluminum alloy cylinder. The tests were conducted under constant amplitude loading at R ratio 0.1. The stress applied was from 20,30 and 40 per cent of the yield stress of the material. The fatigue crack growth data was recorded. After fatigue testing, the samples were subjected to detailed scanning electron microscopic (SEM) analysis. The resulting fracture surfaces were subjected to qualitative and quantitative fractographic examinations. Quantitative fracture analysis included an estimation of crack growth rate (CGR) in different regions. The effect of the microstructural features on fatigue crack growth was examined. It was observed that in stage II (crack growth region), the failure mode changes from intergranular to transgranular as the stress level increases. In the region of intergranular failure the localized brittle failure was observed and fatigue striations are difficult to reveal. However, in the region of transgranular failure the crack path is independent of the microstructural features. In this region, localized ductile failure mode was observed and well defined fatigue striations were present in the wake of fatigue crack. The effect of interaction of growing fatigue crack with microstructural features was not substantial. The final fracture (stage III) was ductile in all the cases.

  18. Multi-scale modeling of microstructure dependent intergranular brittle fracture using a quantitative phase-field based method

    SciTech Connect

    Chakraborty, Pritam; Zhang, Yongfeng; Tonks, Michael R.

    2015-12-07

    In this study, the fracture behavior of brittle materials is strongly influenced by their underlying microstructure that needs explicit consideration for accurate prediction of fracture properties and the associated scatter. In this work, a hierarchical multi-scale approach is pursued to model microstructure sensitive brittle fracture. A quantitative phase-field based fracture model is utilized to capture the complex crack growth behavior in the microstructure and the related parameters are calibrated from lower length scale atomistic simulations instead of engineering scale experimental data. The workability of this approach is demonstrated by performing porosity dependent intergranular fracture simulations in UO2 and comparing the predictions with experiments.

  19. Quantity effect of radial cracks on the cracking propagation behavior and the crack morphology.

    PubMed

    Chen, Jingjing; Xu, Jun; Liu, Bohan; Yao, Xuefeng; Li, Yibing

    2014-01-01

    In this letter, the quantity effect of radial cracks on the cracking propagation behavior as well as the circular crack generation on the impacted glass plate within the sandwiched glass sheets are experimentally investigated via high-speed photography system. Results show that the radial crack velocity on the backing glass layer decreases with the crack number under the same impact conditions during large quantities of repeated experiments. Thus, the "energy conversion factor" is suggested to elucidate the physical relation between the cracking number and the crack propagation speed. Besides, the number of radial crack also takes the determinative effect in the crack morphology of the impacted glass plate. This study may shed lights on understanding the cracking and propagation mechanism in laminated glass structures and provide useful tool to explore the impact information on the cracking debris.

  20. Quantity Effect of Radial Cracks on the Cracking Propagation Behavior and the Crack Morphology

    PubMed Central

    Chen, Jingjing; Xu, Jun; Liu, Bohan; Yao, Xuefeng; Li, Yibing

    2014-01-01

    In this letter, the quantity effect of radial cracks on the cracking propagation behavior as well as the circular crack generation on the impacted glass plate within the sandwiched glass sheets are experimentally investigated via high-speed photography system. Results show that the radial crack velocity on the backing glass layer decreases with the crack number under the same impact conditions during large quantities of repeated experiments. Thus, the “energy conversion factor” is suggested to elucidate the physical relation between the cracking number and the crack propagation speed. Besides, the number of radial crack also takes the determinative effect in the crack morphology of the impacted glass plate. This study may shed lights on understanding the cracking and propagation mechanism in laminated glass structures and provide useful tool to explore the impact information on the cracking debris. PMID:25048684

  1. Crack barriers improve the mechanical and thermal properties of non-metallic sinter materials

    NASA Technical Reports Server (NTRS)

    Gruenthaler, K. H.; Heinrich, W.; Janes, S.; Nixdorf, J.

    1979-01-01

    Means of improving the tensile strength of ceramic composites by introducing ductile intermediate layers capable of absorbing the elastic energy at the rupture front are studied. Tests with an Al203 laminate with niobium inclusions showed that crack propagation could be successfully precluded by dissipation of the energy by deformation and/or delamination at the inclusion/matrix interface.

  2. Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

    NASA Astrophysics Data System (ADS)

    Sarkar, Aritra; Nagesha, A.; Parameswaran, P.; Sandhya, R.; Laha, K.

    2015-12-01

    Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873-923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.

  3. Two dimensional modeling of elastic wave propagation in solids containing cracks with rough surfaces and friction - Part II: Numerical implementation.

    PubMed

    Delrue, Steven; Aleshin, Vladislav; Truyaert, Kevin; Bou Matar, Olivier; Van Den Abeele, Koen

    2017-07-13

    Our study aims at the creation of a numerical toolbox that describes wave propagation in samples containing internal contacts (e.g. cracks, delaminations, debondings, imperfect intergranular joints) of known geometry with postulated contact interaction laws including friction. The code consists of two entities: the contact model and the solid mechanics module. Part I of the paper concerns an in-depth description of a constitutive model for realistic contacts or cracks that takes into account the roughness of the contact faces and the associated effects of friction and hysteresis. In the crack model, three different contact states can be recognized: contact loss, total sliding and partial slip. Normal (clapping) interactions between the crack faces are implemented using a quadratic stress-displacement relation, whereas tangential (friction) interactions were introduced using the Coulomb friction law for the total sliding case, and the Method of Memory Diagrams (MMD) in case of partial slip. In the present part of the paper, we integrate the developed crack model into finite element software in order to simulate elastic wave propagation in a solid material containing internal contacts or cracks. We therefore implemented the comprehensive crack model in MATLAB® and introduced it in the Structural Mechanics Module of COMSOL Multiphysics®. The potential of the approach for ultrasound based inspection of solids with cracks showing acoustic nonlinearity is demonstrated by means of an example of shear wave propagation in an aluminum sample containing a single crack with rough surfaces and friction. Copyright © 2017 Elsevier B.V. All rights reserved.

  4. Effect of dissolved oxygen content on stress corrosion cracking of a cold worked 316L stainless steel in simulated pressurized water reactor primary water environment

    NASA Astrophysics Data System (ADS)

    Zhang, Litao; Wang, Jianqiu

    2014-03-01

    Stress corrosion crack growth tests of a cold worked nuclear grade 316L stainless steel were conducted in simulated pressurized water reactor (PWR) primary water environment containing various dissolved oxygen (DO) contents but no dissolved hydrogen. The crack growth rate (CGR) increased with increasing DO content in the simulated PWR primary water. The fracture surface exhibited typical intergranular stress corrosion cracking (IGSCC) characteristics.

  5. Fracture Profile and Crack Propagation of Ultra-High Strength Hot-Stamped Boron Steel During Mechanical Trimming Process

    NASA Astrophysics Data System (ADS)

    Han, Xianhong; Yang, Kun; Chen, Sisi; Chen, Jun

    2015-10-01

    Mechanical trimming process for ultra-high strength boron steel after hot stamping was carried out in this study. Shear and tensile tests were designed to analyze the influences of stress state on the fracture mode; trimmed fracture surface and profile were observed and compared to other commonly used steels such as DP980 and Q235 etc.; the crack propagation during trimming process was studied through step-by-step tests. The observation and analysis reveal that the fracture mode of hot-stamped boron steel is highly related to the stress state, it belongs to cleavage fracture on low stress triaxiality but dimple fracture on high stress triaxiality. Such phenomenon is reflected in the trimming process, during which the stress state changes from shear-dominated state to tensile-dominated state. In addition, the burnish zone of trimmed boron steel is much smaller than other high strength steels, and the profile of cutting surface shows an `S'-like shape which is destructive to the trimming tool. Moreover, during the trimming process, most martensite laths near the cutting edge are stretched and rotated markedly to the direction of the shear band, and the main crack expands along those grain boundaries, which may penetrate through a few martensite laths and form small crack branches.

  6. An investigation of reheat cracking in the weld heat affected zone of type 347 stainless steel

    NASA Astrophysics Data System (ADS)

    Phung-On, Isaratat

    2007-12-01

    the shifting of the micro-indentations compared to their original locations. At the 80% stress level, the cracking mechanism was identified as the PFZ weakening, while at the 70% stress as the creep-like grain boundary sliding. A design of experiment (DOE) using a D-optimal design was successfully employed in this study to investigate the effects of microstructures on the reheat cracking susceptibility. The microstructures were modified by heat treatment prior to the reheat cracking test. The grain size and cooling rate were found to have moderate effects on cracking susceptibility. The amount (volume fraction) of MC carbide (NbC) had a significant effect on time to failure. The more NbC formed prior to test, the longer time to failure, and the more resistance to reheat cracking. On the other hand, the amount of GB carbide (M23C6) had an insignificant effect. The statistical interaction between MC carbide with other testing parameters also had strong effect. The PWHT temperature also had significant effect as can be predicted from the susceptibility C-curves. The heat treatment schedules, during cooling and during heating schedules, were also investigated. During cooling schedule was the same schedule done earlier in this study. On the other hand, during heating schedule allowed the sample cool to room temperature prior microstructure modification followed by the reheat cracking test. During heating schedule showed an improvement in resistance to reheat cracking. Microstructure of the crack samples showed the intergranular cracking path and wedge shapes along cracking boundaries. There was also the evidence of grain boundary sliding as a result of the creep-like grain boundary sliding cracking mechanism. SEM showed the intergranular cracking and grain separation with precipitates decorated on the fracture surfaces. The precipitates were identified as Nb-rich, MC carbide (NbC). The fracture surfaces showed micro-ductility and microvoid coalescence. The size of

  7. Comparison of interphase removal and oxidation embrittlement mechanisms of subcritical crack growth in SiC/SiC composites

    SciTech Connect

    Jones, R.H.; Henager, C.H. Jr.; Lewinsohn, C.A.; Windisch, C.F. Jr.

    1998-12-31

    Ceramic matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements are being made in the creep resistance of SiC fibers and hence in the high-temperature properties of SiC/SiC composites; however, more needs to be known about the stability of these materials in oxidizing environments before they become widely accepted. Experimental weight loss and crack growth data supports the conclusion that O{sub 2} enhanced crack growth of SiC/SiC occurs by more than one mechanism depending on the experimental conditions. An oxidation embrittlement mechanism (OEM) operates at temperatures below about 1,073--1,223 K and at O{sub 2} pressures of 2 x 10{sup 4} Pa and above while an interface removal mechanism (IRM) operates at temperatures exceeding 1,073 K and O{sub 2} pressure of 2 x 10{sup 3} Pa and below. The OEM, as proposed by Evans et al. (1), results from the reaction of O{sub 2} with SiC to form a glass layer on the fiber. The fracture stress of the fiber is reduced if this layer is thicker than a critical value (d > d{sub c}) and the temperature is below a critical value (T < T{sub g}) such that a sharp crack can be sustained in the layer. The IRM, as proposed by Henager et al. (8), results from the oxidation of the interfacial layer and the resulting relaxation of the bridging fibers. Interphase removal contributes to the stress relaxation of the fiber that occurs by creep. IRM occurs at T > T{sub g} or d < d{sub c}. This paper summarizes the evidence for these two mechanisms and attempts to define the conditions for their operation.

  8. Influence of temperature and carbon content on the cyclic deformation and fatigue behavior of {alpha}-iron. Part 2: Crack initiation and fatigue life

    SciTech Connect

    Sommer, C.; Mughrabi, H.; Lochner, D.

    1998-03-02

    The crack initiation mechanisms and the fatigue life of {alpha}-iron polycrystals have been characterized in plastic-strain-controlled tests with small to intermediate plastic strain ranges {Delta}{epsilon}{sub pl} in the temperature range from 220 to 400 K. The fatigue life of pure decarburized {alpha}-iron cannot be described generally by means of a Manson-Coffin law, except under conditions under which distinct cyclic hardening occurs. A Basquin law is found to be valid, when the effective stress component {sigma}{sup *} dominates (i.e. at low temperatures and small {delta}{epsilon}{sub pl}). At low temperatures and up to room temperature, intergranular crack initiation is observed. At higher temperatures, mostly transgranular cracks develop in decarburized {alpha}-iron in the valleys of the surface rumpling, while in {alpha}-iron with small carbon contents persistent slip bands form which then act as sites of transgranular crack initiation. Crack initiation rates and crack growth rates are evaluated in a semi-quantitative manner from crack depth distributions.

  9. Intergranular Properties of Zr-SUBSTITUTED Y123 Compounds

    NASA Astrophysics Data System (ADS)

    Sbârciog, C.; Redac, R. T.; Gr. Deac, I.; Pop, I.

    AC susceptibility measurements have been done to determine the intergranular properties for some 123-superconducting ceramics. From the imaginary part of the complex susceptibility, χ″(T), the temperature dependence of the critical current density was determined by using Bean's model. The intergranular current density was experimentally determined from AC susceptibility data by varying the field amplitude. All intragrain critical parameters are gradually degraded with increasing the concentration of the dopant substituting on the rare-earth side and with decreasing the sintering temperature during the sample preparation.

  10. Transverse crack initiation under combined thermal and mechanical loading of Fibre Metal Laminates and Glass Fibre Reinforced Polymers

    NASA Astrophysics Data System (ADS)

    van de Camp, W.; Dhallé, M. M. J.; Warnet, L.; Wessel, W. A. J.; Vos, G. S.; Akkerman, R.; ter Brake, H. J. M.

    2017-02-01

    The paper describes a temperature-dependent extension of the classical laminate theory (CLT) that may be used to predict the mechanical behaviour of Fibre Metal Laminates (FML) at cryogenic conditions, including crack initiation. FML are considered as a possible alternative class of structural materials for the transport and storage of liquified gasses such as LNG. Combining different constituents in a laminate opens up the possibility to enhance its functionality, e.g. offering lower specific weight and increased damage tolerance. To explore this possibility, a test programme is underway at the University of Twente to study transverse crack initiation in different material combinations under combined thermal and mechanical loading. Specifically, the samples are tested in a three-point bending experiment at temperatures ranging from 77 to 293 K. These tests will serve as a validation of the model presented in this paper which, by incorporating temperature-dependent mechanical properties and differential thermal expansion, will allow to select optimal material combinations and laminate layouts. By combining the temperature-dependent mechanical properties and the differential thermal contraction explicitly, the model allows for a more accurate estimate of the resulting thermal stresses which can then be compared to the strength of the constituent materials.

  11. Fracture Mechanics Analysis of an Annular Crack in a Three-concentric-cylinder Composite Model

    NASA Technical Reports Server (NTRS)

    Kuguoglu, Latife H.; Binienda, Wieslaw K.; Roberts, Gary D.

    2004-01-01

    A boundary-value problem governing a three-phase concentric-cylinder model was analytically modeled to analyze annular interfacial crack problems with Love s strain functions in order to find the stress intensity factors (SIFs) and strain energy release rates (SERRs) at the tips of an interface crack in a nonhomogeneous medium. The complex form of a singular integral equation (SIE) of the second kind was formulated using Bessel s functions in the Fourier domain, and the SIF and total SERR were calculated using Jacoby polynomials. For the validity of the SIF equations to be established, the SIE of the three-concentric-cylinder model was reduced to the SIE for a two-concentric-cylinder model, and the results were compared with the previous results of Erdogan. A preliminary set of parametric studies was carried out to show the effect of interphase properties on the SERR. The method presented here provides insight about the effect of interphase properties on the crack driving force.

  12. Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

    NASA Astrophysics Data System (ADS)

    Holroyd, N. J. Henry; Scamans, G. M.

    2011-12-01

    Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation energy, E a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al2CuMg where E a is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E a is independent of copper content for stress intensity factors below ~14 MNm-3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm-3/2, with E a , ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested

  13. Thermal-Mechanical Stress Analysis of Pressurized Water Reactor Pressure Vessel with/without a Preexisting Crack under Grid Load Following Conditions

    DOE PAGES

    Mohanty, Subhasish; Soppet, William K.; Majumdar, Saurin; ...

    2016-12-15

    In this paper, we present thermal-mechanical stress analysis of a pressurized water reactor pressure vessel and its hot-leg and cold-leg nozzles. Results are presented from thermal and thermal-mechanical stress analysis under reactor heat-up, cool-down, and grid load-following conditions. Analysis results are given with and without the presence of preexisting crack in the reactor nozzle (axial crack in hot leg nozzle). From the model results it is found that the stress-strain states are significantly higher in case of presence of crack than without crack. The stress-strain state under grid load following condition are more realistic compared to the stress-strain state estimatedmore » assuming simplified transients.« less

  14. Thermal–mechanical stress analysis of pressurized water reactor pressure vessel with/without a preexisting crack under grid load following conditions

    DOE PAGES

    Mohanty, Subhasish; Soppet, William K.; Majumdar, Saurin; ...

    2016-10-26

    In this paper, we present thermal-mechanical stress analysis of a pressurized water reactor pressure vessel and its hot-leg and cold-leg nozzles. Results are presented from thermal and thermal-mechanical stress analysis under reactor heat-up, cool-down, and grid load-following conditions. Analysis results are given with and without the presence of preexisting crack in the reactor nozzle (axial crack in hot leg nozzle). From the model results it is found that the stress-strain states are significantly higher in case of presence of crack than without crack. In conclusion, the stress-strain state under grid load following condition are more realistic compared to the stress-strainmore » state estimated assuming simplified transients.« less

  15. Thermal–mechanical stress analysis of pressurized water reactor pressure vessel with/without a preexisting crack under grid load following conditions

    SciTech Connect

    Mohanty, Subhasish; Soppet, William K.; Majumdar, Saurin; Natesan, Krishnamurti

    2016-10-26

    In this paper, we present thermal-mechanical stress analysis of a pressurized water reactor pressure vessel and its hot-leg and cold-leg nozzles. Results are presented from thermal and thermal-mechanical stress analysis under reactor heat-up, cool-down, and grid load-following conditions. Analysis results are given with and without the presence of preexisting crack in the reactor nozzle (axial crack in hot leg nozzle). From the model results it is found that the stress-strain states are significantly higher in case of presence of crack than without crack. In conclusion, the stress-strain state under grid load following condition are more realistic compared to the stress-strain state estimated assuming simplified transients.

  16. A finite element model on effects of impact load and cavitation on fatigue crack propagation in mechanical bileaflet aortic heart valve.

    PubMed

    Mohammadi, H; Klassen, R J; Wan, W-K

    2008-10-01

    Pyrolytic carbon mechanical heart valves (MHVs) are widely used to replace dysfunctional and failed heart valves. As the human heart beats around 40 million times per year, fatigue is the prime mechanism of mechanical failure. In this study, a finite element approach is implemented to develop a model for fatigue analysis of MHVs due to the impact force between the leaflet and the stent and cavitation in the aortic position. A two-step method to predict crack propagation in the leaflets of MHVs has been developed. Stress intensity factors (SIFs) are computed at a small initiated crack located on the leaflet edge (the worst case) using the boundary element method (BEM). Static analysis of the crack is performed to analyse the stress distribution around the front crack zone when the crack is opened; this is followed by a dynamic crack analysis to consider crack propagation using the finite element approach. Two factors are taken into account in the calculation of the SIFs: first, the effect of microjet formation due to cavitation in the vicinity of leaflets, resulting in water hammer pressure; second, the effect of the impact force between the leaflet and the stent of the MHVs, both in the closing phase. The critical initial crack length, the SIFs, the water hammer pressure, and the maximum jet velocity due to cavitation have been calculated. With an initial crack length of 35 microm, the fatigue life of the heart valve is greater than 60 years (i.e. about 2.2 x 10(9) cycles) and, with an initial crack length of 170 microm, the fatigue life of the heart valve would be around 2.5 years (i.e. about 9.1 x 10(7) cycles). For an initial crack length greater than 170 microm, there is catastrophic failure and fatigue cracking no longer occurs. A finite element model of fatigue analysis using Patran command language (PCL custom code) in MSC software can be used to evaluate the useful lifespan of MHVs. Similar methodologies can be extended to other medical devices under cyclic

  17. On the Effect of Local Grain-Boundary Chemistry on the Macroscopic Mechanical Properties of a High Purity Y2O3-Al2O3-Containing Silicon Nitride Ceramic: Role of Oxygen

    SciTech Connect

    Ziegler, A; McNaney, J M; Hoffman, M J; Ritchie, R O

    2004-11-23

    The effects of grain-boundary chemistry on the mechanical properties were investigated on high-purity silicon nitride ceramics, specifically involving the role of oxygen. Varying the grain-boundary oxygen content, by control of oxidizing heat treatments and sintering additives, was found to result in a transition in fracture mechanism from transgranular to intergranular fracture, with an associated increase in fracture toughness. This phenomenon is correlated to an oxygen-induced change in grain-boundary chemistry that appears to affect fracture by ''weakening'' the interface, facilitating debonding and crack advance along the boundaries, and consequently toughening by grain bridging. It is concluded that if the oxygen content in the thin grain-boundary films exceeds a lower limit, which is {approx}0.87 equiv% oxygen content, then the interfacial structure and bonding characteristics favor intergranular debonding during crack propagation; otherwise, transgranular fracture ensues.

  18. Tensile stress corrosion cracking of type 304 stainless steel irradiated to very high dose

    SciTech Connect

    Chung, H. M.; Ruther, W. E.; Strain, R. V.; Shack, W. J.

    2001-09-01

    Certain safety-related core internal structural components of light water reactors, usually fabricated from Type 304 or 316 austenitic stainless steels (SSs), accumulate very high levels of irradiation damage (20--100 displacement per atom or dpa) by the end of life. The data bases and mechanistic understanding of, the degradation of such highly irradiated components, however, are not well established. A key question is the nature of irradiation-assisted intergranular cracking at very high dose, i.e., is it purely mechanical failure or is it stress-commotion cracking? In this work, hot-cell tests and microstructural characterization were performed on Type 304 SS from the hexagonal fuel can of the decommissioned EBR-11 reactor after irradiation to {approximately}50 dpa at {approximately}370 C. Slow-strain-rate tensile tests were conducted at 289 C in air and in water at several levels of electrochemical potential (ECP), and microstructural characteristics were analyzed by scanning and transmission electron microcopies. The material deformed significantly by twinning and exhibited surprisingly high ductility in air, but was susceptible to severe intergranular stress corrosion cracking (IGSCC) at high ECP. Low levels of dissolved O and ECP were effective in suppressing the susceptibility of the heavily irradiated material to IGSCC, indicating that the stress corrosion process associated with irradiation-induced grain-boundary Cr depletion, rather than purely mechanical separation of grain boundaries, plays the dominant role. However, although IGSCC was suppressed, the material was susceptible to dislocation channeling at low ECP, and this susceptibility led to poor work-hardening capability and low ductility.

  19. Mechanical strength and subcritical crack growth under wet cyclic loading of glass-infiltrated dental ceramics.

    PubMed

    Salazar Marocho, Susana M; Studart, André R; Bottino, Marco A; Bona, Alvaro Della

    2010-05-01

    Evaluate the flexural strength (sigma) and subcritical crack growth (SCG) under cyclic loading of glass-infiltrated alumina-based (IA, In-Ceram Alumina) and zirconia-reinforced (IZ, In-Ceram Zirconia) ceramics, testing the hypothesis that wet environment influences the SCG of both ceramics when submitted to cyclic loading. Bar-shaped specimens of IA (n=45) and IZ (n=45) were fabricated and loaded in three-point bending (3P) in 37 degrees C artificial saliva (IA(3P) and IZ(3P)) and cyclic fatigued (F) in dry (D) and wet (W) conditions (IA(FD), IA(FW), IZ(FD), IZ(FW)). The initial sigma and the number of cycles to fracture were obtained from 3P and F tests, respectively. Data was examined using Weibull statistics. The SCG behavior was described in terms of crack velocity as a function of maximum stress intensity factor (K(Imax)). The Weibull moduli (m=8) were similar for both ceramics. The characteristic strength (sigma(0)) of IA and IZ was and 466MPa 550MPa, respectively. The wet environment significantly increased the SCG of IZ, whereas a less evident effect was observed for IA. In general, both ceramics were prone to SCG, with crack propagation occurring at K(I) as low as 43-48% of their critical K(I). The highest sigma of IZ should lead to longer lifetimes for similar loading conditions. Water combined with cyclic loading causes pronounced SCG in IZ and IA materials. The lifetime of dental restorations based on these ceramics is expected to increase by reducing their direct exposure to wet conditions and/or by using high content zirconia ceramics with higher strength. 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

  20. Investigation into the Impact of Hold Time, Thermal Mechanical Fatigue, Shotpeen, and Retardation on Fatigue Crack Growth in Inconel Dovetail Slots in Jet Engines

    NASA Astrophysics Data System (ADS)

    Joiner, Josiah W.

    2011-12-01

    Current jet engine industry studies are ongoing to develop a generic Inconel dovetail slot test case that will be used for calibrating a manufacturing-induced surface damage anomaly distribution curve for future probabilistic life assessments. The stress and temperature profile during the mission have been defined. This analysis will consist of a design of experiments on the Inconel dovetail slot test data. The test case includes thermal and mechanical stresses, as well as variations in hold time, stress and temperature regimes. Several DOEs will be created and run to help assess the impact of four crack growth mechanisms on the damage tolerance life for the different mission profiles: hold time, thermal mechanical fatigue, shotpeen, and retardation. For the sake of this study a parametric study is considered to be a DOE. Calculations will be completed for both surface and corner cracks. For surface cracks, a 2:1 aspect ratio semicircular initial flaw size of 15 x 30 mils will be used. For corner cracks, a 1:1 aspect ratio semicircular initial flaw size of 15 x 15 mils will be used. The calculations will be completed using a proprietary crack propagation code. The results of this study will reveal the mission profile at which each of the aforementioned effects begins to have a significant impact on the damage tolerance life. These studies are critical to ensuring the final test case adequately addresses each of these critical crack propagation drivers.

  1. Crack Growth Rate Modeling of a Titanium-Aluminide Alloy Under Thermal-Mechanical Cycling

    DTIC Science & Technology

    1991-12-01

    induced creep or environmental degradation [36]. This damage can occur on or below the surface of a material. One of the best examples of this is...Hastelloy-X, and B-1900 + hafnium . They were successful in predicting TMF crack growth rates to within a factor of five with all the parameters except...superalloys: MAR-M 509, B-1900 + hafnium (Hf), and MAR-M200 + Hf. They concludpd that at low growth rates, da/dN depends only on AK, . Out-of-phase TMF

  2. An Investigation of Mechanisms Effecting Environmental Stress Cracking in Titanium Alloy.

    DTIC Science & Technology

    1981-07-06

    of titanium alloy ( Ti6Al4V ) has been under an environmental stress cracking investigation. The acoustic emission (AE) techniques were used to monitor...in a closed loop metharol environment. The acoustic signals were received at a gain of 85 dB in a high bAndpass filter range of 100- 300 KHz and... temperature and annealing for a select time im- proved the fracture toughness in the alloy, and (2) in a plane strain load mode at a constant c rack

  3. Effect of boric acid on intergranular corrosion and on hideout return efficiency of sodium in the tube support plate crevices

    SciTech Connect

    Paine, J.P.N.; Shoemaker, C.E.; Campan, J.L.; Brunet, J.P.; Schindler, P.; Stutzmann, A.

    1995-12-31

    Sodium hydroxide is one of the main causes of intergranular attack/stress corrosion cracking (IGA/SCC) of alloy 600 steam generator (S.G.) tubes. Boric acid appears to be one of the possible remedies for intergranular corrosion process inhibition. In order to obtain data on boric acid injection efficiency, an experimental program was performed on previously corroded tubes. To prevent premature tube wall cracking, samples were sleeved on alloy 690 tubes. The objective of the tests was to evaluate, on a statistically valid number of samples, the effectiveness of boric acid and tube sleeving as possible remedies for IGA/SCC extension. Another independent experimental program was initiated to determine the hideout return efficiency in the tube support plate (TSP) and tubesheet (TS) crevices after a significant duration ({<=} 180 hours) of sodium hideout. The main objective of the first tests being a statistical evaluation of the efficiency of boric acid treatment, was not achieved. The tests did demonstrate that sleeving effectively reduces IGA/SCC growth. In an additional program, cracks were obtained on highly susceptible tubes when specimens were not sleeved. The companion tests performed in the same conditions but with an addition of boric acid did not show any IGA or cracks. These results seem to demonstrate the possible effect of boric acid in preventing the corrosion process. Results of the second tests did not demonstrate any difference in the amount of sodium piled up in the crevices before and after boric acid injection. They however showed an increase of the hideout return efficiency at the tube support plate level from 78 % without boric acid to 95 % when boric acid is present in the feed water.

  4. Irradiation-Assisted Stress Corrosion Cracking of Austenitic Stainless Steels in BWR Environments

    SciTech Connect

    Chen, Y.; Chopra, O. K.; Gruber, Eugene E.; Shack, William J.

    2010-06-01

    The internal components of light water reactors are exposed to high-energy neutron irradiation and high-temperature reactor coolant. The exposure to neutron irradiation increases the susceptibility of austenitic stainless steels (SSs) to stress corrosion cracking (SCC) because of the elevated corrosion potential of the reactor coolant and the introduction of new embrittlement mechanisms through radiation damage. Various nonsensitized SSs and nickel alloys have been found to be prone to intergranular cracking after extended neutron exposure. Such cracks have been seen in a number of internal components in boiling water reactors (BWRs). The elevated susceptibility to SCC in irradiated materials, commonly referred to as irradiation-assisted stress corrosion cracking (IASCC), is a complex phenomenon that involves simultaneous actions of irradiation, stress, and corrosion. In recent years, as nuclear power plants have aged and irradiation dose increased, IASCC has become an increasingly important issue. Post-irradiation crack growth rate and fracture toughness tests have been performed to provide data and technical support for the NRC to address various issues related to aging degradation of reactor-core internal structures and components. This report summarizes the results of the last group of tests on compact tension specimens from the Halden-II irradiation. The IASCC susceptibility of austenitic SSs and heat-affected-zone (HAZ) materials sectioned from submerged arc and shielded metal arc welds was evaluated by conducting crack growth rate and fracture toughness tests in a simulated BWR environment. The fracture and cracking behavior of HAZ materials, thermally sensitized SSs and grain-boundary engineered SSs was investigated at several doses (≤3 dpa). These latest results were combined with previous results from Halden-I and II irradiations to analyze the effects of neutron dose, water chemistry, alloy compositions, and welding and processing conditions on IASCC

  5. Fatigue Crack Growth Behavior of Nickel-base Superalloy Haynes 282 at 550-750 °C

    NASA Astrophysics Data System (ADS)

    Rozman, K. A.; Kruzic, J. J.; Hawk, J. A.

    2015-08-01

    The fatigue crack growth rates for nickel-based superalloy Haynes 282 were measured at temperatures of 550, 650, and 750 °C using compact tension specimens with a load ratio of 0.1 and cyclic loading frequencies of 25 Hz and 0.25 Hz. Increasing the temperature from 550 to 750 °C caused the fatigue crack growth rates to increase from ~20 to 60% depending upon the applied stress intensity level. The effect of reducing the applied loading frequency increased the fatigue crack growth rates from ~20 to 70%, also depending upon the applied stress intensity range. The crack path was observed to be transgranular for the temperatures and frequencies used during fatigue crack growth rate testing. At 750 °C, there were some indications of limited intergranular cracking excursions at both loading frequencies; however, the extent of intergranular crack growth was limited and the cause is not understood at this time.

  6. Elevated temperature crack growth

    NASA Technical Reports Server (NTRS)

    Kim, K. S.; Vanstone, R. H.

    1992-01-01

    The purpose of this program was to extend the work performed in the base program (CR 182247) into the regime of time-dependent crack growth under isothermal and thermal mechanical fatigue (TMF) loading, where creep deformation also influences the crack growth behavior. The investigation was performed in a two-year, six-task, combined experimental and analytical program. The path-independent integrals for application to time-dependent crack growth were critically reviewed. The crack growth was simulated using a finite element method. The path-independent integrals were computed from the results of finite-element analyses. The ability of these integrals to correlate experimental crack growth data were evaluated under various loading and temperature conditions. The results indicate that some of these integrals are viable parameters for crack growth prediction at elevated temperatures.

  7. Prediction of fatigue crack-growth patterns and lives in three-dimensional cracked bodies

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Raju, I. S.

    1986-01-01

    Fatigue crack growth patterns and lives for surface cracks, surface cracks at holes, and corner cracks at holes in three dimensional bodies were predicted using linear-elastic fracture mechanics concepts that were modified to account for crack-closure behavior. The predictions were made by using stress intensity factor equations for these crack configurations and the fatigue crack-growth (delta K against rate) relationship for the material of interest. The crack configurations were subjected to constant-amplitude fatigue loading under either remote tension or bending loads. The predicted crack growth patterns and crack growth lives for aluminum alloys agreed well with test data from the literature.

  8. Prediction of fatigue crack-growth patterns and lives in three-dimensional cracked bodies

    NASA Technical Reports Server (NTRS)

    Newman, J. C., Jr.; Raju, I. S.

    1984-01-01

    Fatigue crack growth patterns and lives for surface cracks, surface cracks at holes, and corner cracks at holes in three dimensional bodies were predicted using linear-elastic fracture mechanics concepts that were modified to account for crack-closure behavior. The predictions were made by using stress intensity factor equations for these crack configurations and the fatigue crack-growth (delta K against rate) relationship for the material of interest. The crack configurations were subjected to constant-amplitude fatigue loading under either remote tension or bending loads. The predicted crack growth patterns and crack growth lives for aluminum alloys agreed well with test data from the literature.

  9. Stress-induced crack path in Aji granite under tensile stress

    NASA Astrophysics Data System (ADS)

    Kudo, Yozo; Sano, Osam; Murashige, Naokuni; Mizuta, Yoshiaki; Nakagawa, Koji

    1992-12-01

    The double-torsion test using Aji granite was carried out to investigate the interaction between stress-induced crack path and mineral grains. Crack velocities were controlled at range 10-7 m/s to 10-1 m/s. After the stressed specimens were dyed, we checked the crack path by thin section analysis, using an optical microscope. The stress-induced crack path was divided into two types, transgranular and intergranular cracks, and each path was subdivided with respect to mineral grains. In spite of the extensive range of crack velocities, the ratios between the transgranular and intergranular crack lengths did not change. The crack paths were all jagged, and often showed detour around the grain boundary when faced with obstacles like hard grains or preexisting cracks. That is to say, quartz grain played an important role as an obstacle. Feldspar grain could change the crack path because of its cleavage plane. Biolite grain had a serious effect on the path even if its constitution ratio is very small. Fractal dimensions of the crack paths were calculated by three methods, as indicators of surface roughness. The fractal dimensions were shown in a slight trend with the change of crack velocity. This trend can be explained from the point of limited cracking rate in stress corrosion.

  10. Elevated temperature crack growth

    NASA Technical Reports Server (NTRS)

    Kim, K. S.; Vanstone, R. H.

    1989-01-01

    Alloy 718 crack growth experiments were conducted to assess the ability of the selected path-independent (P-I) integrals to describe the elevated temperature crack growth behavior. These tests were performed on single edge notch (SEN) specimens under displacement control with multiple extensometers to monitor the specimen and crack mouth opening displacement (CMOD). The displacements in these tests were sufficiently high to induce bulk cyclic inelastic deformation of the specimen. Under these conditions, the linear elastic fracture mechanics (LEFM) parameter K does not correlate the crack growth data. The experimentally measured displacement gradients at the end of specimen gage length were used as the boundary conditions in elastic-plastic finite element method (FEM) analyses. These analyses were performed with a node release approach using CYANIDE, a GEAE FEM code, which included a gap element which is capable of efficiently simulating crack closure. Excellent correlation was obtained between the experimentally measured and predicted variation of stress and CMOD with crack length and the stress-CMOD loops for Alloy 718 tests conducted at 538 C. This confirmed the accuracy of the FEM crack growth simulation approach. The experimentally measured crack growth rate data correlated well the selected P-I integrals. These investigations have produced significant progress in developing P-I integrals as non-linear fracture mechanics parameters. The results suggest that this methodology has the potential of accurately describing elevated temperature crack growth behavior under the combined influence of thermal cycling and bulk elastic-inelastic deformation states.

  11. Why is nacre strong? II. Remaining mechanical weakness for cracks propagating along the sheets

    NASA Astrophysics Data System (ADS)

    Okumura, K.

    2002-04-01

    In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a coarse-grained elastic energy for nacre, or stratified structure of hard and soft layers found in certain seashells . We then analyzed a crack running perpendicular to the layers and suggested one possible reason for the enhanced toughness of this substance. In the present paper, we consider a crack running parallel to the layers. We propose a new term added to the previous elastic energy, which is associated with the bending of layers. We show that there are two regimes for the parallel-fracture solution of this elastic energy; near the fracture tip the deformation field is governed by a parabolic differential equation while the field away from the tip follows the usual elliptic equation. Analytical results show that the fracture tip is lenticular, as suggested in a paper on a smectic liquid crystal (P.G. de Gennes, Europhys. Lett. 13, 709 (1990)). On the contrary, away from the tip, the stress and deformation distribution recover the usual singular behaviors (sqrt{x} and 1/sqrt{x}, respectively, where x is the distance from the tip). This indicates there is no enhancement in toughness in the case of parallel fracture.

  12. Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

    NASA Astrophysics Data System (ADS)

    Feng, Zhi-hui; Li, Jing-yuan; Wang, Yi-de

    2016-04-01

    The thermoplasticity of duplex stainless steel 2205 (DSS2205) is better than that of lean duplex steel 2101 (LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy (OM), electron backscatter diffraction (EBSD), Thermo-Calc software, and transmission electron microscopy (TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

  13. Magnetic field, temperature and mechanical crack performance of a GdBCO magnetic lens

    NASA Astrophysics Data System (ADS)

    Zhang, Z. Y.; Matsumoto, S.; Teranishi, R.; Kiyoshi, T.

    2012-11-01

    Magnetic field concentration by using the diamagnetism of a superconductor is a novel technique that has been experimentally demonstrated in magnetic lenses. A magnetic lens consists of a hollow superconductor cylinder with a tapered inner diameter within which the magnetic flux is concentrated by diamagnetism. Magnetic lenses are very promising for use in compact high magnetic field systems. However, magnetic lenses with large inner diameters are required to facilitate sample access during use. In this study, an optimized GdBaCuO magnetic lens with large inner and outer diameters was designed and its performance was investigated in liquid nitrogen, liquid helium, and a cryocooler-cooled cryostat. A lens with an inner diameter of 12 mm was constructed by stacking three specially machined GdBaCuO bulk pieces. This magnetic lens cracked due to the large flux jump that occurred when the concentrated field of the magnetic lens exceeded 10 T at 4.2 K. The cracked lens was subsequently impregnated with epoxy resin. A concentrated field of 12.42 T was realized when the background field was 8 T at 20 K and no flux jumps occurred. This result demonstrates that this GdBCO magnetic lens is promising for use in compact superconducting magnet systems.

  14. Intergranular fracture in irradiated Inconel X-750 containing very high concentrations of helium and hydrogen

    NASA Astrophysics Data System (ADS)

    Judge, Colin D.; Gauquelin, Nicolas; Walters, Lori; Wright, Mike; Cole, James I.; Madden, James; Botton, Gianluigi A.; Griffiths, Malcolm

    2015-02-01

    In recent years, it has been observed that Inconel X-750 spacers in CANDU reactors exhibits lower ductility with reduced load carrying capacity following irradiation in a reactor environment. The fracture behaviour of ex-service material was also found to be entirely intergranular at high doses. The thermalized flux spectrum in a CANDU reactor leads to transmutation of 58Ni to 59Ni. The 59Ni itself has unusually high thermal neutron reaction cross-sections of the type: (n, γ), (n, p), and (n, α). The latter two reactions, in particular, contribute to a significant enhancement of the atomic displacements in addition to creating high concentrations of hydrogen and helium within the material. Microstructural examinations by transmission electron microscopy (TEM) have confirmed the presence of helium bubbles in the matrix and aligned along grain boundaries and matrix-precipitate interfaces. Helium bubble size and density are found to be highly dependent on the irradiation temperature and material microstructure; the bubbles are larger within grain boundary precipitates. TEM specimens extracted from fracture surfaces and crack tips provide information that is consistent with crack propagation along grain boundaries due to the presence of He bubbles.

  15. Intergranular fracture in irradiated Inconel X-750 containing very high concentrations of helium and hydrogen

    SciTech Connect

    Judge, Colin D.; Gauquelin, Nicolas; Walters, Lori; Wright, Mike; Cole, James I.; Madden, James; Botton, Gianluigi A.; Griffiths, Malcolm

    2015-02-01

    In recent years, it has been determined that Inconel X-750 CANDU spacers have lost strength and material ductility following irradiation in reactor. The irradiated fracture behaviour of ex-service material was also found to be entirely intergranular. The heavily thermalized flux spectrum in a CANDU reactor results in transmutation of 58Ni to 59Ni. The 59Ni itself has unusually high thermal neutron reaction cross-sections of the type: (n, γ), (n, p), and (n,α). The latter two reactions, in particular, contribute to a significant enhancement of the atomic displacements in addition to creating high concentrations of hydrogen and helium within the material. Metallographic examinations by transmission electron microscopy (TEM) have confirmed the presence of helium bubbles in the matrix and aligned along grain boundaries and matrix-precipitate interfaces. He bubble size and density are found to be highly dependent on the irradiation temperature and material microstructure; the bubbles are larger within grain boundary precipitates. TEM specimens extracted from fracture surfaces and crack tips give direct evidence linking crack propagation with grain boundary He bubbles.

  16. Effect of intergranular surface attack on the fatigue and corrosion properties of AM-355 CRT material

    SciTech Connect

    Bhansali, K.J.; Liu, G.; Grendahl, S.M.; Champagne, V.K.; Pepi, M.S.

    1998-12-31

    A dynamic part, consisting of a stack of laminates, failed in the field after an unexpectedly short life. Failure analysis showed that a multitude of fatigue cracks originated from a combination of corrosion and fretting. Recent changes in the production process of the laminates resulted in the presence of an intergranular (IG) morphology on the surface. Due to the criticality of this part`s application, a better understanding of the influence of varying degrees of IG attack on fatigue properties was pursued. Coupon specimens were machined from actual components with different surface IG conditions and were subjected to fatigue testing. Results showed a direct relationship between the number of cycles to failure and the severity of surface IG attack. Potentiodynamic polarization measurements indicated that a sample with IG attack had a lower breakdown potential and an unstable passivation behavior as compared to that without an IG attack. The detrimental effects of surface IG attack on the crack initiation process, endurance limit and corrosion resistance is discussed in terms of the stress concentration and breakdown of the passivation layer.

  17. Microstructural Modeling of Intergranular Fracture in Tricrystals With Random Low- and High-Angle Grain Boundaries

    NASA Astrophysics Data System (ADS)

    Bond, David M.; Zikry, Mohammed A.

    2017-03-01

    Intergranular (IG) fracture behavior near triple junctions (TJs) in f.c.c. tricrystals with a variety of grain boundary (GB) misorientations has been investigated. Based on a dislocation-density GB interaction scheme, critical fracture conditions were coupled to evolving dislocation-density pileups and local stresses by using a dislocation-density-based crystalline plasticity formulation within a nonlinear finite-element framework to elucidate the effects of local GB structure, dislocation-GB interactions, and misorientations on IG crack propagation in f.c.c. crystalline materials. Tricrystals with low-angle GBs had higher fracture toughness than tricrystals with high-angle GBs. In TJs with a combination of random low- and high-angle GBs, the formation of dislocation-density pileups in the high-angle GB led to IG crack propagation along the high-angle GB rather than along the low-angle GB. These predictions, which are consistent with experimental observations, indicate that fracture behavior near TJs is controlled by highly local, evolving, and interrelated events, such as dislocation-density pileups and GB misorientations.

  18. Effect of heat treatment upon the fatigue-crack growth behavior of Alloy 718 weldments

    SciTech Connect

    Mills, W.J.; James, L.A.

    1981-05-01

    The microstructural features that influenced the room and elevated temperature fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination of fatigue fracture surfaces revealed that operative fatigue mechanisms were dependent on microstructure, temperatures and stress intensity factor. All specimens exhibited three basic fracture surface appearances at temperatures up to 538{degrees}C: crystallographic faceting at low stress intensity range ({Delta}K) levels, striation, formation at intermediate values, and dimples coupled with striations in the highest ({Delta}K) regime. At 649{degrees}C, the heat-treated welds exhibited extensive intergranular cracking. Laves and {delta} particles in the conventional heat-treated material nucleated microvoids ahead of the advancing crack front and caused on overall acceleration in crack growth rates at intermediate and high {Delta}K levels. The modified heat treatment removed many of these particles from the weld zone, thereby improving its fatigue resistance. The dramatically improved fatigue properties exhibited by the as-welded material was attributed to compressive residual stresses introduced by the welding process. 19 refs., 16 figs.

  19. Three-Dimensional Gear Crack Propagation Studies

    NASA Technical Reports Server (NTRS)

    Lewicki, David G.; Sane, Ashok D.; Drago, Raymond J.; Wawrzynek, Paul A.

    1998-01-01

    Three-dimensional crack growth simulation was performed on a split-tooth gear design using boundary element modeling and linear elastic fracture mechanics. Initial cracks in the fillet of the teeth produced stress intensity factors of greater magnitude (and thus, greater crack growth rates) than those in the root or groove areas of the teeth. Crack growth simulation was performed on a case study to evaluate crack propagation paths. Tooth fracture was predicted from the crack growth simulation for an initial crack in the tooth fillet region. Tooth loads on the uncracked mesh of the split-tooth design were up to five times greater than those on the cracked mesh if equal deflections of the cracked and uncracked teeth were considered. Predicted crack shapes as well as crack propagation life are presented based on calculated stress intensity factors, mixed-mode crack propagation trajectory theories, and fatigue crack growth theories.

  20. A study on the mechanism of stress corrosion cracking of duplex stainless steels in hot alkaline-sulfide solution

    NASA Astrophysics Data System (ADS)

    Chasse, Kevin Robert

    Duplex stainless steels (DSS) generally have superior strength and corrosion resistance as compared to most standard austenitic and ferritic stainless grades owing to a balanced microstructure of austenite and ferrite. As a result of having favorable properties, DSS have been selected for the construction of equipment in pulp and paper, chemical processing, nuclear, oil and gas as well as other industries. The use of DSS has been restricted in some cases because of stress corrosion cracking (SCC), which can initiate and grow in either the ferrite or austenite phase depending on the environment. Thorough understanding of SCC mechanisms of DSS in chloride- and hydrogen sulfide-containing solutions has been useful for material selection in many environments. However, understanding of SCC mechanisms of DSS in sulfide-containing caustic solutions is limited, which has restricted the capacity to optimize process and equipment design in pulp and paper environments. Process environments may contain different concentrations of hydroxide, sulfide, and chloride, altering corrosion and SCC susceptibility of each phase. Crack initiation and growth behavior will also change depending on the relative phase distribution and properties of austenite and ferrite. The role of microstructure and environment on the SCC of standard grade UNS S32205 and lean grade UNS S32101 in hot alkaline-sulfide solution were evaluated in this work using electrochemical, film characterization, mechanical testing, X-ray diffraction, and microscopy techniques. Microstructural aspects, which included residual stress state, phase distribution, phase ratio, and microhardness, were related to the propensity for SCC crack initiation in different simulated alkaline pulping liquors at 170 °C. Other grades of DSS and reference austenitic and superferritic grades of stainless steel were studied using exposure coupons for comparison to understand compositional effects and individual phase susceptibility

  1. Small Crack Growth and Fatigue Life Predictions for High-Strength Aluminium Alloys. Part 1; Experimental and Fracture Mechanics Analysis

    NASA Technical Reports Server (NTRS)

    Wu, X. R.; Newman, J. C.; Zhao, W.; Swain, M. H.; Ding, C. F.; Phillips, E. P.

    1998-01-01

    The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests A,ere conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and A weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plastisity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions, Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminum alloys was developed and demonstrated using the crack-closure model.

  2. Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries

    DOE PAGES

    Yan, Pengfei; Zheng, Jianming; Gu, Meng; ...

    2017-01-16

    LiNi1/3Mn1/3Co1/3O2 (NMC333) layered cathode is often fabricated as secondary particles of consisting of densely packed primary particles, which offers advantage of high energy density and alleviation of cathode side reactions/corrosions, but introduces other drawbacks, such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in the commercial NMC333 layered cathode by using advanced S/TEM. We found that the formation of the intragranular cracks is directly associated with high voltage cycling, which is an electrochemically driven and diffusion controlled process. The intragranular cracks were noticed to be characteristically initiated from grain interior, a consequencemore » of dislocation based crack incubation mechanism. This observation is in sharp contrast with the general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surface. As a result, our study indicates that maintain a structural stability is the key step toward high voltage operation of layered cathode materials.« less

  3. Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries

    PubMed Central

    Yan, Pengfei; Zheng, Jianming; Gu, Meng; Xiao, Jie; Zhang, Ji-Guang; Wang, Chong-Min

    2017-01-01

    LiNi1/3Mn1/3Co1/3O2-layered cathode is often fabricated in the form of secondary particles, consisting of densely packed primary particles. This offers advantages for high energy density and alleviation of cathode side reactions/corrosions, but introduces drawbacks such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in a commercial LiNi1/3Mn1/3Co1/3O2 cathode by using advanced scanning transmission electron microscopy. We find the formation of the intragranular cracks is directly associated with high-voltage cycling, an electrochemically driven and diffusion-controlled process. The intragranular cracks are noticed to be characteristically initiated from the grain interior, a consequence of a dislocation-based crack incubation mechanism. This observation is in sharp contrast with general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surfaces. Our study emphasizes that maintaining structural stability is the key step towards high-voltage operation of layered-cathode materials. PMID:28091602

  4. Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries

    NASA Astrophysics Data System (ADS)

    Yan, Pengfei; Zheng, Jianming; Gu, Meng; Xiao, Jie; Zhang, Ji-Guang; Wang, Chong-Min

    2017-01-01

    LiNi1/3Mn1/3Co1/3O2-layered cathode is often fabricated in the form of secondary particles, consisting of densely packed primary particles. This offers advantages for high energy density and alleviation of cathode side reactions/corrosions, but introduces drawbacks such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in a commercial LiNi1/3Mn1/3Co1/3O2 cathode by using advanced scanning transmission electron microscopy. We find the formation of the intragranular cracks is directly associated with high-voltage cycling, an electrochemically driven and diffusion-controlled process. The intragranular cracks are noticed to be characteristically initiated from the grain interior, a consequence of a dislocation-based crack incubation mechanism. This observation is in sharp contrast with general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surfaces. Our study emphasizes that maintaining structural stability is the key step towards high-voltage operation of layered-cathode materials.

  5. Crack layer theory

    NASA Technical Reports Server (NTRS)

    Chudnovsky, A.

    1984-01-01

    A damage parameter is introduced in addition to conventional parameters of continuum mechanics and consider a crack surrounded by an array of microdefects within the continuum mechanics framework. A system consisting of the main crack and surrounding damage is called crack layer (CL). Crack layer propagation is an irreversible process. The general framework of the thermodynamics of irreversible processes are employed to identify the driving forces (causes) and to derive the constitutive equation of CL propagation, that is, the relationship between the rates of the crack growth and damage dissemination from one side and the conjugated thermodynamic forces from another. The proposed law of CL propagation is in good agreement with the experimental data on fatigue CL propagation in various materials. The theory also elaborates material toughness characterization.

  6. Influence of circumferential notch and fatigue crack on the mechanical integrity of biodegradable magnesium-based alloy in simulated body fluid.

    PubMed

    Bobby Kannan, M; Singh Raman, R K; Witte, F; Blawert, C; Dietzel, W

    2011-02-01

    Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the mechanical integrity of an AZ91 magnesium alloy was studied using a constant extension rate tensile (CERT) method. The samples in two different geometries that is, circumferentially notched (CN), and circumferentially notched and fatigue cracked (CNFC), were tested in air and in simulated body fluid (SBF). The test results show that the mechanical integrity of the AZ91 magnesium alloy decreased substantially (∼50%) in both the CN and CNFC samples exposed to SBF. Fracture surface analysis revealed secondary cracks suggesting stress corrosion cracking susceptibility of the alloy in SBF.

  7. Grain boundary engineering for intergranular fracture and creep resistance

    SciTech Connect

    Palumbo, G.; Lehockey, E.M.; Lin, P.

    1996-12-31

    The effect of special grain boundary frequency on the bulk creep performance of 99.99% Ni at 84 MPa and 450{degrees}C (grain boundary sliding regime). Increasing the frequency of `special` grain boundaries (by thermomechanical processing) from 13% to 66% results in a 16-fold reduction in steady state creep rate and a 6-fold reduction in primary creep strain. Consistent with the previous intergranular fracture analysis, a moderate increase in special boundary frequency from 13% to 45% yields the greatest reduction in the creep strain parameters. Microstructural evaluation of the specimens following testing to 1.8% total strain showed that (1) cavitation had occurred exclusively at general grain boundaries (i.e., {Sigma}>29) and (2) no cavities were formed in the material containing 66% special grain boundaries. The results of this study provide considerable promise for a `grain boundary engineering` approach towards the mitigation of intergranular-creep and -fracture in practical engineering materials.

  8. Tailoring the intergranular phases in silicon nitride for improved toughness

    SciTech Connect

    Sun, E.Y.; Becher, P.F.; Plucknett, K.P.; Waters, S.B.; Hirao, K.; Brito, M.E.

    1996-12-31

    Intergranular glass phases can have a significant influence on fracture resistance (R-curve behavior) of Si nitride ceramics and appears to be related to debonding of the {beta}-Si{sub 3}N{sub 4}/oxynitride-glass interfaces. Applying the results from {beta}- Si{sub 3}N{sub 4}-whisker/oxynitride-glass model systems, self- reinforced Si nitrides with different sintering additive ratios were investigated. Si nitrides sintered with a lower Al{sub 2}O{sub 3}: Y{sub 2}O{sub 3} additive ratio exhibited higher stead-state fracture toughness together with a steeply rising R-curve. Analytical electron microscopy suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.

  9. The length of pre-existing fissures effects on the mechanical properties of cracked red sandstone and strength design in engineering.

    PubMed

    Wu, Jiangyu; Feng, Meimei; Yu, Bangyong; Han, Guansheng

    2017-08-30

    It is important to study the mechanical properties of cracked rock to understand the engineering behavior of cracked rock mass. Consequently, the influence of the length of pre-existing fissures on the strength, deformation, acoustic emission (AE) and failure characteristics of cracked rock specimen was analyzed, and the optimal selection of strength parameter in engineering design was discussed. The results show that the strength parameters (stress of dilatancy onset and uniaxial compressive strength) and deformation parameters (axial strain and circumferential strain at dilatancy onset and peak point) of cracked rock specimen decrease with the increase of the number of pre-existing fissures, and the relations which can use the negative exponential function to fit. Compared with the intact rock specimens, the different degrees of stress drop phenomena were produced in the process of cracked rock specimens when the stress exceeds the dilatancy onset. At this moment, the cracked rock specimens with the existence of stress drop are not instantaneous failure, but the circumferential strain, volumetric strain and AE signals increase burstingly. And the yield platform was presented in the cracked rock specimen with the length of pre-existing fissure more than 23mm, the yield failure was gradually conducted around the inner tip of pre-existing fissure, the development of original fissures and new cracks was evolved fully in rock. However, the time of dilatancy onset is always ahead of the the time of that point with the existence of stress drop. It indicates that the stress of dilatancy onset can be as the parameter of strength design in rock engineering, which can effectively prevent the large deformation of rock. Copyright © 2017 Elsevier B.V. All rights reserved.

  10. A comparison of the stress corrosion cracking susceptibility of commercially pure titanium grade 4 in Ringer's solution and in distilled water: a fracture mechanics approach.

    PubMed

    Roach, Michael D; Williamson, R Scott; Thomas, Joseph A; Griggs, Jason A; Zardiackas, Lyle D

    2014-01-01

    From the results of laboratory investigations reported in the literature, it has been suggested that stress corrosion cracking (SCC) mechanisms may contribute to early failures in titanium alloys that have elevated oxygen concentrations. However, the susceptibility of titanium alloys to SCC in physiological environments remains unclear. In this study, a fracture mechanics approach was used to examine the SCC susceptibility of CP titanium grade 4 in Ringer's solution and distilled de-ionized (DI) water, at 37°C. The study duration was 26 weeks, simulating the non-union declaration of a plated fracture. Four wedge loads were used corresponding to 86-95% of the alloy's ligament yield load. The longest cracks were measured to be 0.18 mm and 0.10 mm in Ringer's solution and DI water, respectively. SEM analysis revealed no evidence of extensive fluting and quasi-cleavage fracture features which, in literature reports, were attributed to SCC. We thus postulate that the Ringer's solution accelerated the wedge-loaded crack growth without producing the critical stresses needed to change the fracture mechanism. Regression analysis of the crack length results led to a significant best-fit relationship between crack growth velocity (independent variable) and test electrolyte, initial wedge load, and time of immersion of specimen in electrolyte (dependent variables).

  11. Measurement of intergranular attack in stainless steel using ultrasonic energy

    DOEpatents

    Mott, Gerry; Attaar, Mustan; Rishel, Rick D.

    1989-08-08

    Ultrasonic test methods are used to measure the depth of intergranular attack (IGA) in a stainless steel specimen. The ultrasonic test methods include a pitch-catch surface wave technique and a through-wall pulse-echo technique. When used in combination, these techniques can establish the extent of IGA on both the front and back surfaces of a stainless steel specimen from measurements made on only one surface.

  12. Intergranular degradation assessment via random grain boundary network analysis

    DOEpatents

    Kumar, Mukul; Schwartz, Adam J.; King, Wayne E.

    2002-01-01

    A method is disclosed for determining the resistance of polycrystalline materials to intergranular degradation or failure (IGDF), by analyzing the random grain boundary network connectivity (RGBNC) microstructure. Analysis of the disruption of the RGBNC microstructure may be assess the effectiveness of materials processing in increasing IGDF resistance. Comparison of the RGBNC microstructures of materials exposed to extreme operating conditions to unexposed materials may be used to diagnose and predict possible onset of material failure due to

  13. On the Relationship Between J-Integral and Crack Tip Opening Displacement in Elastic-Plastic Fracture Mechanics

    NASA Astrophysics Data System (ADS)

    Pereira, Marcos Venicius; Darwish, Fathi Aref; Campelo, Eduardo

    2013-08-01

    The relationship between J-integral ( J) and crack tip opening displacement (δ), considered fundamental for elastic-plastic fracture mechanics, can be established based on prior knowledge of the constraint factor m, which depends on the work hardening exponent and the material's yield strain. Both J and δ were simultaneously determined at fracture initiation and at different points along the resistance curves for a number of structural steels. The corresponding m values were calculated and then compared with the predictions made by different models. The results indicate that the experimentally determined m values are in fair agreement with the predictions made by ASTM over the whole range of flow parameters considered in this study. The Hutchinson-Rice-Rosengren singularity-based predictions result in overestimating m for steels considered to be of low strength and high strain hardening exponent. Predictions made by other models are predominantly higher in comparison with their experimental counterparts.

  14. Cycloid crack sequences on Europa: Relationship to stress history and constraints on growth mechanics based on cusp angles

    NASA Astrophysics Data System (ADS)

    Groenleer, Julie M.; Kattenhorn, Simon A.

    2008-01-01

    The original model developed to explain cycloidal cracks on Europa interprets cycloids as tensile fractures that grow in a curved path in response to the constantly rotating diurnal tidal stress field. Cusps form when a new cycloid crack segment propagates at an angle to the first in response to a rotation of the principal tidal stress orientation during a period of no crack growth. A recent revised model states that a cycloid cusp forms through the creation of a secondary fracture called a tailcrack at the tip of an existing cycloid segment during shearing motion induced by the rotating tidal stress field. As the tailcrack propagates away from the cusp, it becomes the next cycloid segment in the chain. The qualitative tailcrack model uniquely accounts for the normal and shear stresses that mechanically must resolve onto the tip of an existing cycloid segment at the instant of cusp formation. In this work, we provide a quantitative framework and test of the hitherto purely conceptual tailcrack model. We first present a relative age sequence inferred from geologic mapping of multiply cross-cutting cycloids in Europa's trailing hemisphere and place this into the context of the global stress history. The age sequence requires a cumulative minimum of 630° of shell reorientation due to nonsynchronous rotation to account for the observed range of orientations of cycloids of different ages. We determined the back-rotated longitudes of formation of two cycloid chain examples and used mathematical modeling of europan tidal stresses to show that the tailcrack model for cusp formation is not only viable, but places constraints on the overall development of a cycloid chain by controlling the timing of cusp development within Europa's orbit. For all cusps analyzed, the exact ratio of resolved shear to normal stress required to form the cusp angles by a process of tailcracking, as governed by the principles of linear elastic fracture mechanics, is produced at the tip of a

  15. A microscopic study of crack initiation mechanisms in 7075 aluminum alloy sheets.

    NASA Technical Reports Server (NTRS)

    Jones, D. L.; Liebowitz, H.

    1973-01-01

    A study of the opening mode of crack initiation in 7075-T6 aluminum alloy sheets has been conducted with the aid of a scanning electron microscope. Observations were made from several orientations, including the top view of the specimen which showed the notch profile and the edge view of the specimen which showed the entire notch front along the specimen thickness. It was found that the edge view exhibited the first signs of permanent deformation at about 55% of the breaking strength. These changes took the form of deformation bands which were aligned in the direction of the tensile axis and apparently defined limiting regions of homogeneous slip. It is felt that the appearance of microcracks at loads approaching the breaking strength was of fundamental importance in the formation of the final fracture surface. Many of these microcraks were initiated at intermetallic particles and other metallurgically weak regions on the notch surface. It was also possible to correlate the strain in the notch with the stress intensity factor for the various loads. Very large plastic strains were observed on the notch tip as compared to published values of elongation at fracture for unnotched specimens.

  16. Mechanisms of fatigue crack retardation following single tensile overloads in powder metallurgy aluminum alloys

    NASA Technical Reports Server (NTRS)

    Bray, G. H.; Reynolds, A. P.; Starke, E. A., Jr.

    1992-01-01

    In ingot metallurgy (IM) alloys, the number of delay cycles following a single tensile overload typically increases from a minimum at an intermediate baseline stress intensity range, Delta-K(B), with decreasing Delta-K(B) approaching threshold and increasing Delta-K(B) approaching unstable fracture to produce a characteristic 'U' shaped curve. Two models have been proposed to explain this behavior. One model is based on the interaction between roughness and plasticity-induced closure, while the other model only utilizes plasticity-induced closure. This article examines these models, using experimental results from constant amplitude and single overload fatigue tests performed on two powder metallurgy (PM) aluminum alloys, AL-905XL and AA 8009. The results indicate that the 'U'-shaped curve is primarily due to plasticity-induced closure, and that the plasticity-induced retardation effect is through-thickness in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plastic strain at the crack tip and overload plastic zone size are larger in the plane-stress surface regions than in the plane-strain interior regions. These results are not entirely consistent with either of the proposed models.

  17. The mechanics and physics of fracturing: application to thermal aspects of crack propagation and to fracking.

    PubMed

    Cherepanov, Genady P

    2015-03-28

    By way of introduction, the general invariant integral (GI) based on the energy conservation law is presented, with mention of cosmic, gravitational, mass, elastic, thermal and electromagnetic energy of matter application to demonstrate the approach, including Coulomb's Law generalized for moving electric charges, Newton's Law generalized for coupled gravitational/cosmic field, the new Archimedes' Law accounting for gravitational and surface energy, and others. Then using this approach the temperature track behind a moving crack is found, and the coupling of elastic and thermal energies is set up in fracturing. For porous materials saturated with a fluid or gas, the notion of binary continuum is used to introduce the corresponding GIs. As applied to the horizontal drilling and fracturing of boreholes, the field of pressure and flow rate as well as the fluid output from both a horizontal borehole and a fracture are derived in the fluid extraction regime. The theory of fracking in shale gas reservoirs is suggested for three basic regimes of the drill mud permeation, with calculating the shape and volume of the local region of the multiply fractured rock in terms of the pressures of rock, drill mud and shale gas. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

  18. A microscopic study of crack initiation mechanisms in 7075 aluminum alloy sheets.

    NASA Technical Reports Server (NTRS)

    Jones, D. L.; Liebowitz, H.

    1973-01-01

    A study of the opening mode of crack initiation in 7075-T6 aluminum alloy sheets has been conducted with the aid of a scanning electron microscope. Observations were made from several orientations, including the top view of the specimen which showed the notch profile and the edge view of the specimen which showed the entire notch front along the specimen thickness. It was found that the edge view exhibited the first signs of permanent deformation at about 55% of the breaking strength. These changes took the form of deformation bands which were aligned in the direction of the tensile axis and apparently defined limiting regions of homogeneous slip. It is felt that the appearance of microcracks at loads approaching the breaking strength was of fundamental importance in the formation of the final fracture surface. Many of these microcraks were initiated at intermetallic particles and other metallurgically weak regions on the notch surface. It was also possible to correlate the strain in the notch with the stress intensity factor for the various loads. Very large plastic strains were observed on the notch tip as compared to published values of elongation at fracture for unnotched specimens.

  19. Environment-assisted cracking of Fe-32% Mn-9% Al alloys in 3.5% sodium chloride solution

    SciTech Connect

    Chang, S.C.; Liu, J.Y.; Juang, H.K.

    1995-05-01

    The stress corrosion cracking (SCC) behavior of four austenitic Fe-32% Mn-9% Al alloys containing {approx} 1 wt% carbon was studied. All of the four alloys were susceptible to SCC in room temperature and 160 C NaCl solution. SCC was enhanced by applying an anodic potential. The crack path was transgranular, and cleavage-like regions were observed on the fracture surface. The corrosion-assisted microcleavage model proposed for the transgranular SCC behavior of face-centered cubic (fcc) materials very likely was the operating SCC mechanism of the austenitic Fe-Mn-Al alloys. Adding 1.23% Mo or 1.27% Si to the austenitic Fe-32% Mn-9% Al alloys did not change the SCC mechanism and crack paths. Adding 1.27% Si made the alloys more susceptible to SCC, but no such effect was observed with the addition of 1.23% Mo. All four alloys studied were susceptible to hydrogen embrittlement (HE) in 3.5% NaCl solution at large cathodic applied potentials, and the crack paths were intergranular. The addition of Mo, Cr, or Si in the alloy displaced the potential at which HE occurred to more cathodic values.

  20. Double-sided laser beam welded T-joints for aluminum-lithium alloy aircraft fuselage panels: Effects of filler elements on microstructure and mechanical properties

    NASA Astrophysics Data System (ADS)

    Han, Bing; Tao, Wang; Chen, Yanbin; Li, Hao

    2017-08-01

    In the current work, T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys for aircraft fuselage panels have been fabricated by double-sided fiber laser beam welding with different filler wires. A new type wire CW3 (Al-6.2Cu-5.4Si) was studied and compared with conventional wire AA4047 (Al-12Si) mainly on microstructure and mechanical properties. It was found that the main combined function of Al-6.2%Cu-5.4%Si in CW3 resulted in considerable improvements especially on intergranular strength, hot cracking susceptibility and hoop tensile properties. Typical non-dendritic equiaxed zone (EQZ) was observed along welds' fusion boundary. Hot cracks and fractures during the load were always located within the EQZ, however, this typical zone could be restrained by CW3, effectively. Furthermore, changing of the main intergranular precipitated phase within the EQZ from T phase by AA4047 to T2 phase by CW3 also resulted in developments on microscopic intergranular reinforcement and macroscopic hoop tensile properties. In addition, bridging caused by richer substructure dendrites within CW3 weld's columnar zone resulted in much lower hot cracking susceptibility of the whole weld than AA4047.

  1. Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

    NASA Astrophysics Data System (ADS)

    Lalpoor, M.; Eskin, D. G.; Katgerman, L.

    2009-12-01

    Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.

  2. Localized Deformation as a Primary Cause of Irradiation Assisted Stress Corrosion Cracking

    SciTech Connect

    Gary S. Was

    2009-03-31

    The objective of this project is to determine whether deformation mode is a primary factor in the mechanism of irradiation assisted intergranular stress corrosion cracking of austenitic alloys in light watert reactor core components. Deformation mode will be controlled by both the stacking fault energy of the alloy and the degree of irradiation. In order to establish that localized deformation is a major factor in IASCC, the stacking fault energies of the alloys selected for study must be measured. Second, it is completely unknown how dose and SFE trade-off in terms of promoting localized deformation. Finally, it must be established that it is the localized deformation, and not some other factor that drives IASCC.

  3. Hydrogen-environment-assisted cracking of an aluminum-zinc-magnesium(copper) alloy

    NASA Astrophysics Data System (ADS)

    Young, George Aloysius, Jr.

    There is strong evidence to indicate that hydrogen embrittlement plays a significant, if not controlling, role in the environmentally assisted cracking of 7XXX series aluminum alloys. In order to better understand hydrogen environment assisted cracking (HEAC), crack growth rate tests in the K-independent stage II crack growth regime were conducted on fracture mechanics specimens of an Al-6.09Zn-2.14Mg-2.19Cu alloy (AA 7050) and a low copper variant (Al-6.87Zn-2.65Mg-0.06Cu). Crack growth rate tests were performed in 90% relative humidity (RH) air between 25 and 90°C to assure hydrogen embrittlement control. The underaged, peak aged, and overaged tempers were investigated. Hydrogen uptake in humid air, hydrogen diffusion, and hydrogen trapping were investigated for each temper. Lastly, near crack tip hydrogen concentration depth profiles were analyzed via nuclear reaction analysis (NRA) and secondary ion mass spectroscopy (SIMS) using a liquid gallium, focused ion beam sputtering source (FIB/SIMS). The results of this study help explain and quantify empirically known trends concerning HEAC resistance and also establish new findings. In the copper bearing alloy, overaged tempers are more resistant but not immune to HEAC. Humid air is an aggressive environment for Al-Zn-Mg alloys because water vapor reacts with bare aluminum to produce high surface concentrations of hydrogen. This occurs in all tempers. Hydrogen diffuses from the near surface region to the high triaxial stress region ahead of the crack tip and collects at the high angle grain boundaries. The combination of tensile stress and high hydrogen concentration at the grain boundaries then causes intergranular fracture. Crack extension bares fresh metal and the process of hydrogen production, uptake, diffusion to the stressed grain boundary, and crack extension repeats. One reason increased degree of aging improves HEAC resistance in copper bearing 7XXX series alloys is that volume lattice and effective

  4. Microstructural dependence of aqueous-environment-assisted crack growth and hydrogen uptake in AA 7050

    NASA Astrophysics Data System (ADS)

    Young, Lisa Marie

    This goal of this research was to explain the effects of heat treatment, Cu content, and electrode potential (EApp) on short-transverse aqueous environment assisted cracking (EAC) in a precipitation hardened Al-Zn-Mg-(Cu) alloy. Substantial intergranular EAC susceptibility was observed in several underaged (UA) and peak aged (PA) tempers of AA 7050, where increasing E App produced a slow crack growth rate (da/dt) incubation and transition to fast da/dt. Above the transition potential, da/dt was dramatically increased by further increases in EApp. In contrast the overaged (OA) condition was highly EAC resistant, exhibiting transgranular da/dt ≤2 x 10-8 mm/sec or ≈ 10,000 times slower than PA. Crack growth rates in the low Cu alloy were several orders of magnitude higher than those exhibited by the high Cu material at similar EApp and were only slightly reduced on overaging. Thermal desorption spectroscopy (TDS) results showed enhanced hydrogen uptake in fast-cracking EAC regions compared to as-received hydrogen concentrations. Hydrogen analyses were complicated by the dependence of H-production and uptake on wake exposure time and a pH gradient in the occluded crack environment. Trends between applied anodic potential, crack wake H concentration (normalized by the wake exposure time), and aqueous da/dt were observed. Nuclear reaction analysis revealed unexpectedly high near-surface H concentrations ( ≈ 2000 wppm). The H-concentration profiles indicate that the observed da/dt can be rate-limited by bulk H-diffusion into the crack tip process zone, where EAC is promoted by a hydrogen embrittlement mechanism. The transition potential to fast da/dt increased in the anodic direction with increased isothermal aging time. Additionally, the presence of large Cu-containing second phase particles (S-phase) on high angle grain boundaries partially negated the beneficial effect of overaging on EAC in the Cu-containing alloy, an effect not observed in humid air cracking

  5. Role of oxygen diffusion at Ni/Cr2O3 interface in intergranular oxidation of Ni-Cr alloy

    NASA Astrophysics Data System (ADS)

    Medasani, Bharat; Sushko, Maria; Schreiber, Daniel; Rosso, Kevin; Bruemmer, Stephen

    Certain Ni-Cr alloys used in nuclear systems experience intergranular oxidation and stress corrosion cracking when exposed to high-temperature water leading to their degradation and unexpected failure. To develop a mechanistic understanding of grain boundary oxidation processes, we proposed a mesoscale metal alloy oxidation model that combines quantum Density Functional Theory (DFT) with mesoscopic Poisson-Nernst-Planck/classical DFT. This framework encompasses the chemical specificity of elementary diffusion processes and mesoscale reactive dynamics, and allows modeling oxidation processes on experimentally relevant length scales from first principles. As a proof of concept, a preliminary model was previously employed that limited oxygen diffusion pathways to those through the oxide phase and did not allow oxygen diffusion in the alloy or across oxide/alloy interfaces. In this work, we expand the model to include oxygen diffusion pathways along Ni/Cr2O3 interfaces and demonstrate the increasing importance of such pathways for intergranular oxidation of Ni-Cr alloys with high Cr content. This work is supported by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Simulations are performed using PNNL Institutional Computing facility.

  6. Investigating the mechanism of transgranular stress corrosion cracking in near-neutral pH environments on buried fuel transmission pipelines

    NASA Astrophysics Data System (ADS)

    Asher, Stefanie Lynn

    This research investigates the mechanism of transgranular stress corrosion cracking on fuel transmission pipelines. This research proposes that in near-neutral pH environments, hydrogen can be generated by the dissociation of carbonic acid and the reaction of metal ions with bicarbonate solutions, significantly increasing the available hydrogen for diffusion into the pipeline steel. This research has shown that TGSCC of pipeline steels is possible in simple groundwater solutions containing bicarbonate ions and carbon dioxide. Microstructural characterization coupled with hydrogen permeation indicates that the level of strain in the microstructure has the most influence on hydrogen diffusivity. Hydrogen accumulation occurs preferentially in at high energy discontinuous interfaces such as inclusion interfaces. It was determined that a stress concentration is required to facilitate sufficient hydrogen accumulation in the pipeline steel in order to initiate TGSCC. It was discovered that these stress concentrations develop from inclusions falling out of the pipeline surface. Slow strain rate tests found that TGSCC occurred in a wide range of compositions and temperatures as long as near-neutral conditions were maintained. Microcracks ahead of the crack tip provide evidence of hydrogen in these cracking processes. Morphology of these microcracks indicates that cracks propagate by the coalescence of microcracks with the main crack tip. Further research findings, scientific impact, and potential future work are also discussed.

  7. Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy

    PubMed Central

    Wang, S. D.; Xu, D. K.; Wang, B. J.; Sheng, L. Y.; Han, E. H.; Dong, C.

    2016-01-01

    Effect of solid solution treatment (T4) on stress corrosion cracking (SCC) behavior of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr (in wt.%) alloy has been investigated using slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution. The results demonstrated that the SCC susceptibility index (ISCC) of as-forged samples was 0.95 and its elongation-to-failure (εf) was only 1.1%. After T4 treatment, the SCC resistance was remarkably improved. The ISCC and εf values of T4 samples were 0.86 and 3.4%, respectively. Fractography and surface observation indicated that the stress corrosion cracking mode for as-forged samples was dominated by transgranular and partially intergranular morphology, whereas the cracking mode for T4 samples was transgranular. In both cases, the main cracking mechanism was associated with hydrogen embrittlement (HE). Through alleviating the corrosion attack of Mg matrix, the influence of HE on the SCC resistance of T4 samples can be greatly suppressed. PMID:27387817

  8. Effect of heat treatment upon the fatigue-crack growth behavior of Alloy 718 weldments. Part II. Microscopic behavior

    SciTech Connect

    Mills, W J; James, L A

    1981-05-01

    The microstructural features that influenced the room and elevated temperature fatigue-crack growth behavior of as-welded, conventional heat-treated, and modified heat-treated Alloy 718 GTA weldments were studied. Electron fractographic examination of fatigue fracture surfaces revealed that operative fatigue mechanisms were dependent on microstructure, temperature and stress intensity factor. All specimens exhibited three basic fracture surface appearances at temperatures up to 838{sup 0}C: crystallographic faceting at low stress intensity range ({Delta}K) levels, striation formation at intermediate values, and dimples coupled with striations in the highest {Delta}K regime. At 649{sup 0}C, the heat-treated welds exhibited extensive intergranular cracking. Laves and {delta}particles in the conventional heat-treated material nucleated microvoids ahead of the advancing crack front and caused an overall acceleration in crack growth rates at intermediate and high {Delta}K levels. The modified heat treatment removed many of these particles from the weld zone, thereby improving its fatigue resistance. The dramatically improved fatigue properties exhibited by the as-welded material was attributed to compressive residual stresses introduced by the welding process. 16 figures.

  9. Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy.

    PubMed

    Wang, S D; Xu, D K; Wang, B J; Sheng, L Y; Han, E H; Dong, C

    2016-07-08

    Effect of solid solution treatment (T4) on stress corrosion cracking (SCC) behavior of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr (in wt.%) alloy has been investigated using slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution. The results demonstrated that the SCC susceptibility index (ISCC) of as-forged samples was 0.95 and its elongation-to-failure (εf) was only 1.1%. After T4 treatment, the SCC resistance was remarkably improved. The ISCC and εf values of T4 samples were 0.86 and 3.4%, respectively. Fractography and surface observation indicated that the stress corrosion cracking mode for as-forged samples was dominated by transgranular and partially intergranular morphology, whereas the cracking mode for T4 samples was transgranular. In both cases, the main cracking mechanism was associated with hydrogen embrittlement (HE). Through alleviating the corrosion attack of Mg matrix, the influence of HE on the SCC resistance of T4 samples can be greatly suppressed.

  10. Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy

    NASA Astrophysics Data System (ADS)

    Wang, S. D.; Xu, D. K.; Wang, B. J.; Sheng, L. Y.; Han, E. H.; Dong, C.

    2016-07-01

    Effect of solid solution treatment (T4) on stress corrosion cracking (SCC) behavior of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr (in wt.%) alloy has been investigated using slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution. The results demonstrated that the SCC susceptibility index (ISCC) of as-forged samples was 0.95 and its elongation-to-failure (εf) was only 1.1%. After T4 treatment, the SCC resistance was remarkably improved. The ISCC and εf values of T4 samples were 0.86 and 3.4%, respectively. Fractography and surface observation indicated that the stress corrosion cracking mode for as-forged samples was dominated by transgranular and partially intergranular morphology, whereas the cracking mode for T4 samples was transgranular. In both cases, the main cracking mechanism was associated with hydrogen embrittlement (HE). Through alleviating the corrosion attack of Mg matrix, the influence of HE on the SCC resistance of T4 samples can be greatly suppressed.

  11. Fatigue crack growth behavior of pressure vessel steels and submerged arc weldments in a high-temperature pressurized water environment

    SciTech Connect

    Liaw, P.K.; Logsdon, W.A.; Begley, J.A. . Research and Development Center)

    1989-10-01

    The fatigue crack growth rate (FCGR) properties of SA508 Cl 2a and SA533 Gr A Cl 2 pressure vessel steels and the corresponding automatic submerged arc weldments were developed in a high-temperature pressurized water (HPW) environment at 288{degrees} C (550{degrees} F) and 7.2 MPa (1044 psi) at load ratios of 0.20 and 0.50. The properties were generally conservative compared to American Society of Mechanical Engineers Section XI water environment reference curve. The growth rate of fatigue cracks in the base materials, however, was faster in the HPW environment than in a 288{degrees} C (550{degrees} F) base line air environment. The growth rate of fatigue cracks in the two submerged arc weldments was also accelerated in the HPW environment but to a lesser degree than that demonstrated by the base materials. In the air environment, fatigue striations were observed, independent of material and load ratio, while in the HPW environment, some intergranular facets were present. The greater environmental effect on crack growth rates displayed by the base materials compared the weldments attributed to a different sulfide composition and morphology.

  12. Analysis of Subcritical Crack Growth in Dental Ceramics Using Fracture Mechanics and Fractography

    PubMed Central

    Taskonak, Burak; Griggs, Jason A.; Mecholsky, John J.; Yan, Jia-Hau

    2008-01-01

    .05) but did not have significantly different fracture toughness (P>0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram® Zirconia specimens (P≤0.05) but not for Vitadur Alpha specimens (P>0.05). Fatigue parameters, n and ln B, were 38.4 and −12.7 for Vitadur Alpha and were 13.1 and 10.4 for In-Ceram® Zirconia. Significance Moisture assisted subcritical crack growth had a more deleterious effect on In-Ceram® Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified fracture surface features that can potentially mislead investigators into misidentifying the critical flaw. PMID:17845817

  13. Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature

    NASA Astrophysics Data System (ADS)

    Gabrielli, F.; Pelloux, R. M.

    1982-06-01

    The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates (CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a given AK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail.

  14. Role of the adsorption factor in reduction of the long-term static crack resistance of high-strength steel in gaseous media

    SciTech Connect

    Romaniv, O.N.; Nikiforchin, G.N.; Tsirul'nik, A.T.

    1988-01-01

    The purpose of this work was to establish the role of the adsorption layer in subcritical crack growth in high-strength steel subject to long-term static loading in controlled-composition gaseous media. Investigations of long-term crack resistance were made in a sealed chamber on 45KhN2MFA steel with an edge crack loaded by four-point bending. The gaseous media were commercially pure hydrogen and helium. The most aggressive medium was deoxygenated and dried hydrogen and the least aggressive helium. The effect of helium moisture content on crack growth kinetics was evaluated. Subcritical crack growth mechanisms were determined, by fractography with a scanning electron microscope, to be typically intergranular. Hydrogen embrittlement occurred in three stages: establishment of adsorption equilibrium, adsorbed molecule dissociation into atomic hydrogen, and transport of hydrogen atoms into the prefailure zone. Local fracture occurred as a result of a reduction in critical stress in connection with hydrogen impregnation of the prefailure zone.

  15. Finite element analysis of type IV cracking in 2.25Cr-1Mo steel weldment based on micro-mechanistic approach

    NASA Astrophysics Data System (ADS)

    Goyal, Sunil; Laha, K.; Chandravathi, K. S.; Parameswaran, P.; Mathew, M. D.

    2011-08-01

    Creep studies were carried out on 2.25Cr-1Mo steel base metal and its fusion-welded weldments at 823 K over the stress range 100-240 MPa. The weldment possessed lower creep rupture strength than the base metal due to type IV failure at the outer edge of the heat-affected zone (HAZ). Premature failure of the weldment was associated with pronounced creep cavitation accompanied with localized creep deformation in the soft intercritical region of the HAZ that was sandwiched between relatively higher creep deformation-resistant microstructural regions. The cavitation was associated with coarse intergranular precipitates in the intercritical region of the HAZ. The type IV cracking in the intercritical region of the HAZ was found to initiate deep inside the weldment and propagate towards the specimen surface. Finite element analysis of stress and strain distributions across the weldment was carried out considering the micro-mechanical strength inhomogeneity across it to explain the observed features of type IV cracking. The estimated higher von-Mises and principal stresses deep inside the intercritical region of the HAZ of the weldment led to the localized creep deformation and preferential cavity nucleation and growth, resulting in type IV failure of the weldment. The role of intergranular precipitate particles in the intercritical region of the HAZ in facilitating creep cavity nucleation by the exhaustion of creep ductility of the material close to the precipitate was corroborated from finite element analysis of stress and strain distribution around the precipitates.

  16. Effect of boric acid on intergranular corrosion in tube support plate crevices. Final report

    SciTech Connect

    Brunet, J.P.; Campan, J.L.

    1993-10-01

    Intergranular attack on steam generator tubing is one important phenomenon involved in availability of Pressurized Water Reactors. Boric acid appears to be a possible candidate for inhibiting the corrosion process. The program performed in Cadarache was supposed to give statistical informations on the boric acid effect. It was based on a large number of samples initially attacked during a program performed by BABCOCK & WILCOX. These samples were sleeved onto Alloy 690 tubes, in order to prevent premature cracking. Unfortunately it was not possible to find chemical conditions able to produce significant additional corrosion; we postulated mainly due to a drastic reduction of the thermal flux resulting from the increase of the tube wall thickness under the tube support plates (TSP). The tests demonstrate that such sleeve could be a possible remedy of the corrosion when introduced under the TSP. The tests show indications of a possible beneficial effect of the boric acid, a large variability of the heats sensitivity to the IGA and a predominant effect of Na{sub 2}CO{sub 3} on IGA production.

  17. Structural Reliability of Ceramics at High Temperature: Mechanisms of Fracture and Fatigue Crack Growth

    SciTech Connect

    Reinhold H. Dauskardt

    2005-08-01

    Final report of our DOE funded research program. Aim of the research program was to provide a fundamental basis from which the mechanical reliability of layered structures may be understood, and to provide guidelines for the development of technologically relevant layered material structures with optimum resistance to fracture and subcritical debonding. Progress in the program to achieve these goals is described.

  18. Multi-scale modeling of microstructure dependent intergranular brittle fracture using a quantitative phase-field based method

    DOE PAGES

    Chakraborty, Pritam; Zhang, Yongfeng; Tonks, Michael R.

    2015-12-07

    In this study, the fracture behavior of brittle materials is strongly influenced by their underlying microstructure that needs explicit consideration for accurate prediction of fracture properties and the associated scatter. In this work, a hierarchical multi-scale approach is pursued to model microstructure sensitive brittle fracture. A quantitative phase-field based fracture model is utilized to capture the complex crack growth behavior in the microstructure and the related parameters are calibrated from lower length scale atomistic simulations instead of engineering scale experimental data. The workability of this approach is demonstrated by performing porosity dependent intergranular fracture simulations in UO2 and comparing themore » predictions with experiments.« less

  19. Determination of the equivalent intergranular void ratio - Application to the instability and the critical state of silty sand

    NASA Astrophysics Data System (ADS)

    Nguyen, Trung-Kien; Benahmed, Nadia; Hicher, Pierre-Yves

    2017-06-01

    This paper presents an experimental study of mechanical response of natural Camargue silty sand. The analysis of test results used the equivalent intergranular void ratio instead of the global void ratio. The calculation of equivalent intergranular void ratio requires the determination of parameter b which represents, physically, the fraction of active fines participating on the chain forces network, hence the strength of the soil. A new formula for determining the parameter b by using an approach based on the coordination number distribution and probability calculation is proposed. The validation of the developed relationship was done through back-analysis of published datasets in literature on the effect of fines content on silty sand behavior. It is shown that the equivalent intergranular void ratio calculated with the b value obtained by the new formula is able to provide strong correlation to not only the critical state of but also the onset of instability of various silty sands, in different terms as peak deviator stress, peak stress ratio or cyclic resistance. Therefore, it is suggested that the use of the equivalent void ratio concept and the new b calculating formula is highly desirable in predicting of the silty sand behavior.

  20. Fracture mechanics. [review of fatigue crack propagation and technology of constructing safe structures

    NASA Technical Reports Server (NTRS)

    Hardrath, H. F.

    1974-01-01

    Fracture mechanics is a rapidly emerging discipline for assessing the residual strength of structures containing flaws due to fatigue, corrosion or accidental damage and for anticipating the rate of which such flaws will propagate if not repaired. The discipline is also applicable in the design of structures with improved resistance to such flaws. The present state of the design art is reviewed using this technology to choose materials, to configure safe and efficient structures, to specify inspection procedures, to predict lives of flawed structures and to develop reliability of current and future airframes.

  1. Al-Li alloy AA2198's very high cycle fatigue crack initiation mechanism and its fatigue thermal effect

    NASA Astrophysics Data System (ADS)

    Xu, Luopeng; Cao, Xiaojian; Chen, Yu; Wang, Qingyuan

    2015-10-01

    AA2198 alloy is one of the third generation Al-Li alloys which have low density, high elastic modulus, high specific strength and specific stiffness. Compared With the previous two generation Al-Li alloys, the third generation alloys have much improved in alloys strength, corrosion resistance and weldable characteristic. For these advantages, the third generation Al-Li alloys are used as aircraft structures, such as C919 aviation airplane manufactured by China and Russia next generation aviation airplane--MS-21. As we know, the aircraft structures are usually subjected to more than 108 cycles fatigue life during 20-30 years of service, however, there is few reported paper about the third generation Al-Li alloys' very high cycle fatigue(VHCF) which is more than 108 cycles fatigue. The VHCF experiment of AA2198 have been carried out. The two different initiation mechanisms of fatigue fracture have been found in VHCF. The cracks can initiate from the interior of the testing material with lower stress amplitude and more than 108 cycles fatigue life, or from the surface or subsurface of material which is the dominant reason of fatigue failures. During the experiment, the infrared technology is used to monitor the VHCF thermal effect. With the increase of the stress, the temperature of sample is also rising up, increasing about 15 °C for every 10Mpa. The theoretical thermal analysis is also carried out.

  2. 3D Progressive Damage Modeling for Laminated Composite Based on Crack Band Theory and Continuum Damage Mechanics

    NASA Technical Reports Server (NTRS)

    Wang, John T.; Pineda, Evan J.; Ranatunga, Vipul; Smeltzer, Stanley S.

    2015-01-01

    A simple continuum damage mechanics (CDM) based 3D progressive damage analysis (PDA) tool for laminated composites was developed and implemented as a user defined material subroutine to link with a commercially available explicit finite element code. This PDA tool uses linear lamina properties from standard tests, predicts damage initiation with an easy-to-implement Hashin-Rotem failure criteria, and in the damage evolution phase, evaluates the degradation of material properties based on the crack band theory and traction-separation cohesive laws. It follows Matzenmiller et al.'s formulation to incorporate the degrading material properties into the damaged stiffness matrix. Since nonlinear shear and matrix stress-strain relations are not implemented, correction factors are used for slowing the reduction of the damaged shear stiffness terms to reflect the effect of these nonlinearities on the laminate strength predictions. This CDM based PDA tool is implemented as a user defined material (VUMAT) to link with the Abaqus/Explicit code. Strength predictions obtained, using this VUMAT, are correlated with test data for a set of notched specimens under tension and compression loads.

  3. Analysis of Mode I and Mode II Crack Growth Arrest Mechanism with Z-Fibre Pins in Composite Laminated Joint

    NASA Astrophysics Data System (ADS)

    Jeevan Kumar, N.; Ramesh Babu, P.

    2017-08-01

    This paper presents the numerical study of the mode I and mode II interlaminar crack growth arrest in hybrid laminated curved composite stiffened joint with Z-fibre reinforcement. A FE model of hybrid laminated skin-stiffener joint reinforced with Z-pins is developed to investigate the effect of Z- fibre pins on mode I and mode II crack growth where the delamination is embedded inbetween the skin and stiffener interface. A finite element model was developed using S4R element of a 4-node doubly curved thick shell elements to model the composite laminates and non linear interface elements to simulate the reinforcements. The numerical analyses revealed that Z-fibre pinning were effective in suppressing the delamination growth when propagated due to applied loads. Therefore, the Z-fibre technique effectively improves the crack growth resistance and hence arrests or delays crack growth extension.

  4. I Situ Electrochemical Scanning Tunneling Microscopy Study of Dealloying and Stress Corrosion Cracking of Copper - Alloys.

    NASA Astrophysics Data System (ADS)

    Chen, Jin-Syung Fred

    The mechanism of stress corrosion cracking (SCC) of Cu-30Au in 0.6 M NaCl was investigated by a series of experiments, in which samples were dealloyed (i.e., selective removal of copper atoms) by potentiostatic anodic polarization at zero applied stress (i) for varying lengths of time (10 seconds to 30 minutes) and then impact bent, and (ii) for 30 minutes followed by a period of time (5 seconds to 10 minutes) at the open circuit potential and then impact bent. The results indicate that dealloying at zero applied stress produces a surface porous layer that is capable, for a brief period of time (<= ~ 15 seconds), of inducing intergranular cleavage failure of the normally ductile FCC substrate. However, for time >15 seconds at open circuit potential, aging or coarsening reverses the ability of the surface layer to induce cleavage. In addition, samples were dealloyed and simultaneously stressed at various nominal values. At low values of applied stress, failure occurred by brittle intergranular cracking (IGSCC); and at high values of stress, failure occurred by brittle transgranular cracking (TGSCC). The results indicate that the mechanism of IGSCC is identical to that of TGSCC and can best be described by a modification of the "film-induced cleavage" model. The implication of the aging phenomenon to the film-induced cleavage model of stress corrosion cracking is also discussed. An electrochemical scanning tunneling microscope (ESTM) was built and used to study the in-situ dealloying process of thin-film Cu-Au alloys. Thin-films of Cu-75 at%Au alloy were prepared by thermal evaporation of the bulk alloy and deposition of the vapor onto heated mica. The surface structure of the thin film thus grown consists of terrace of well defined (111) planes separated by atomic height steps. The results from in-situ ESTM indicate that if applied potentials were lower than the critical potential (E_{rm c}), dissolution of Cu preferentially occurred at the low coordination sites

  5. The relationship between observed stress corrosion cracking fracture morphology and microstructure in Alloy 600

    SciTech Connect

    Symons, D.M.; Burke, M.G.; Foster, J.P.

    1997-12-31

    Microstructure is known to influence the stress corrosion cracking (SCC) behavior of Alloy 600 in both hydrogenated water and steam environments. This study evaluated the relative SCC response of a single heat of Alloy 600 as a function of microstructure in a hydrogenated doped-steam environment. The 400 C doped-steam environment was selected for the SCC tests to accelerate cracking. The material was evaluated in three conditions: (1) as-received (2) as-annealed, and (3) as-annealed + 26% deformation. Microstructural characterization was performed using analytical electron microscopy (AEM) techniques for the evaluation of carbide type and morphology, and general structure. Constant displacement (bolt-loaded) compact tension specimens were used to induce SCC. The as-annealed and as-annealed plus cold worked samples had two fracture morphologies: a rough intergranular SCC fracture morphology and a smooth intergranular fracture morphology. The SCC fracture in the as-received specimens was characterized by a classic intergranular morphology at low magnification, consistent with the microstructural evaluation of cross-sectional metallographic samples. More detailed examination revealed a pseudo-intergranular fracture morphology. This pseudo-intergranular morphology appears to be comprised of very fine cleavage-like microfacets. These observations may assist in understanding the difference in SCC fracture morphologies as reported in the open literature.

  6. Fracture mechanics analysis of cast duplex stainless steel elbows containing a surface crack

    SciTech Connect

    Delliou, P.A. le; Semete, P.; Ignaccolo, S.

    1998-12-31

    Some components of the primary loop of a PWR are made of cast duplex stainless steel. This kind of steel may age even at relatively low temperatures, (below 400 C, which is within the temperature range of PWR service conditions), leading to a significant decrease of its toughness. This is why a large research program was initiated on the fracture behavior of aged duplex stainless steel elbows in France. The main task of this program was to test three 2/3-scale models of aged PWR primary loop elbows. The first two tests (called SEM1 and SEM2) were conducted under in-plane closure bending at 320 C; the third (called SEM3) was conducted under constant internal pressure and in-plane closure bending at 60 C. The first two elbows contained a semi-elliptical notch machined into the outer surface of one flank, oriented either longitudinally (SEM1 test) or circumferentially (SEM2 test); the third elbow contained both notches described above, one on each flank. This paper presents the results of the experiments, the finite element calculations and the ductile fracture mechanics analyses that were performed.

  7. Biomolecule-assisted synthesis of single-crystalline selenium nanowires and nanoribbons via a novel flake-cracking mechanism

    NASA Astrophysics Data System (ADS)

    Zhang, Bin; Ye, Xingchen; Dai, Wei; Hou, Weiyi; Zuo, Fan; Xie, Yi

    2006-01-01

    Recently, the biomolecule-assisted synthesis method has been a new and promising focus in the preparation of various nanomaterials. But current works mainly focus on the synthesis of metal nanoparticles and nanowires using macro-biomolecules (e.g. virus, protein and DNA) as templates in the presence of a reducing agent. Beta-carotene, one of the most common bio-antioxidants, can be oxidized to form species with both hydrophilic and hydrophobic ends, which can provide an in situ soft template for the synthesis of nanomaterials. Herein, a simple beta-carotene-assisted method was developed for the first time to synthesize t-Se nanowires and nanoribbons with high crystallinity. We demonstrate that beta-carotene serves as not only the reducing agent, but also an in situ template in the preparation of Se one-dimensional nanostructures. It is found that the growth mechanism of Se nanomaterials is different from the familiar sphere-wire process. A novel flake-cracking mechanism is proposed. By this biomolecule-assisted route, Te one-dimensional nanostructures and Pd nanowires were also fabricated. The assisted-biomolecule in our method may be spread to carotenoids and other antioxidants, and thus broaden the application fields of biomolecules. Our preliminary investigations have shown that the facile, solution-phase biomolecule-assisted method can be potentially extended to the preparation of other low-dimensional nanostructures. The synthesized t-Se nanowires and nanoribbons may serve as templates to generate other tubular functional nanomaterials and find applications in the studies of structure-property relationships as well as in the fabrication of nanoscale optoelectronic devices.

  8. Mitigation of Crack Damage in Metallic Materials

    NASA Technical Reports Server (NTRS)

    Leser, Patrick E.; Newman, John A.; Smith, Stephen W.; Leser, William P.; Wincheski, Russell A.; Wallace, Terryl A.; Glaessgen, Edward H.; Piascik, Robert S.

    2014-01-01

    A system designed to mitigate or heal crack damage in metallic materials has been developed where the protected material or component is coated with a low-melting temperature film. After a crack is formed, the material is heated, melting the film which then infiltrates the crack opening through capillary action. Upon solidification, the healing material inhibits further crack damage in two ways. While the crack healing material is intact, it acts like an adhesive that bonds or bridges the crack faces together. After fatigue loading damages, the healing material in the crack mouth inhibits further crack growth by creating artificially-high crack closure levels. Mechanical test data show that this method sucessfully arrests or retards crack growth in laboratory specimens.

  9. Hydrogen embrittlement, grain boundary segregation, and stress corrosion cracking of alloy X-750 in low- and high-temperature water

    SciTech Connect

    Mills, W. J.; Lebo, M. R.; Kearns, J. J.

    1997-04-01

    The nature of intergranular stress corrosion cracking (SCC) of alloy X-750 was characterized in low- and high-temperature water by testing as-notched and precracked fracture mechanics specimens. Materials given the AH, BH, and HTH heat treatments were studied. While all heat treatments were susceptible to rapid low-temperature crack propagation (LTCP) below 150 C, conditions AH and BH were particularly susceptible. Low-temperature tests under various loading conditions (e.g., constant displacement, constant load, and increasing load) revealed that the maximum stress intensity factors (K{sub P{sub max}}) from conventional rising load tests provide conservative estimates of the critical loading conditions in highly susceptible heats, regardless of the load path history. For resistant heats, K{sub P{sub max}} provides a reasonable, but not necessarily conservative, estimate of the critical stress intensity factor for LTCP. Testing of as-notched specimens showed that LTCP will not initiate at a smooth surface or notch, but will readily occur if a cracklike defect is present. Comparison of the cracking response in water with that for hydrogen-precharged specimens tested in air demonstrated that LTCP is associated with hydrogen embrittlement of grain boundaries. The stress corrosion crack initiation and growth does occur in high-temperature water (>250 C), but crack growth rates are orders of magnitude lower than LTCP rates. The SCC resistance of HTH heats is far superior to that of AH heats as crack initiation times are two to three orders of magnitude greater and growth rates are one to two orders of magnitude lower.

  10. Evolution of Crack-Tip Transformation Zones in Superelastic Nitinol Subjected to in Situ Fatigue. a Fracture Mechanics And Synchrotron X-Ray Microdiffraction Analysis

    SciTech Connect

    Robertson, S.W.; Mehta, A.; Pelton, A.R.; Ritchie, R.O.; /UC, Berkeley /SLAC, SSRL

    2009-04-29

    The ultrahigh spatial resolution ({approx}1 {micro}m{sup 2}) of synchrotron X-ray microdiffraction is combined with fracture mechanics techniques to directly measure in situ three-dimensional strains, phases and crystallographic alignment ahead of a growing fatigue crack (100 cycles in situ) in superelastic Nitinol. The results provide some surprising insights into the growth of cracks in phase-transforming material at the microscale. Specifically, despite a macroscopic superelastic strain recovery of 6-8% associated with the phase transformation, individual austenite grains experience local strains of less than 1.5%. This observation indicates that it is the localized process of the accommodation of the transformation and subsequent loading of the martensite that provide the main source of the large recoverable strains. Furthermore, the plastic region ahead of the crack is composed of deformed martensite. This micromechanical transformation process is dependent upon the material texture, and directly influences the transformation zone size/shape as well as the crack path.

  11. Investigation of fatigue crack initiation from a non-metallic inclusion via high energy x-ray diffraction microscopy

    DOE PAGES

    Naragani, Diwakar; Sangid, Michael D.; Shade, Paul A.; ...

    2017-07-14

    Crack initiation at inclusions is a dominant, unavoidable and life-limiting failure mechanism of important structural materials. Fatigue progresses in a complex manner to find the ‘weakest link’ in the microstructure, leading to crack nucleation. In this study, fully 3-D characterization methods using high-energy synchrotron x-rays are combined with in-situ mechanical testing to study the crack initiation mechanism in a Ni-based superalloy specimen. The specimen was produced via powder metallurgy and seeded with a non-metallic inclusion. Two x-ray techniques were employed: absorption contrast computed micro-tomography (μ-CT) to determine the morphology of the inclusion and its location in the gauge section ofmore » the specimen; and far-field high-energy diffraction microscopy (FF-HEDM) to resolve the centroids, average orientations, and lattice strains of the individual grains comprising the microstructure surrounding the inclusion. Sequential μ-CT and FF-HEDM scans were carried out at both peak and zero applied stress following schedules of cyclic deformation. The µ-CT data showed the onset and location of crack initiation, and the FF-HEDM data provided temporal and spatial evolution of the intergranular strains. Strain partitioning and the associated stress heterogeneities that develop are shown to stabilize within a few loading cycles. Elasto-viscoplastic fast Fourier transform simulations were utilized to supplement interpretation of the experimental stress distributions and compared with the experimental stress distributions. In conclusion, appropriate conditions for crack nucleation in the form of stress gradients were demonstrated and created by virtue of the inclusion, specifically the residual stress state and local bonding state at the inclusion-matrix interface.« less

  12. Stress corrosion cracking of austenitic stainless steel core internal welds.

    SciTech Connect

    Chung, H. M.; Park, J.-H.; Ruther, W. E.; Sanecki, J. E.; Strain, R. V.; Zaluzec, N. J.

    1999-04-14

    Microstructural analyses by several advanced metallographic techniques were conducted on austenitic stainless steel mockup and core shroud welds that had cracked in boiling water reactors. Contrary to previous beliefs, heat-affected zones of the cracked Type 304L, as well as 304 SS core shroud welds and mockup shielded-metal-arc welds, were free of grain-boundary carbides, which shows that core shroud failure cannot be explained by classical intergranular stress corrosion cracking. Neither martensite nor delta-ferrite films were present on the grain boundaries. However, as a result of exposure to welding fumes, the heat-affected zones of the core shroud welds were significantly contaminated by oxygen and fluorine, which migrate to grain boundaries. Significant oxygen contamination seems to promote fluorine contamination and suppress thermal sensitization. Results of slow-strain-rate tensile tests also indicate that fluorine exacerbates the susceptibility of irradiated steels to intergranular stress corrosion cracking. These observations, combined with previous reports on the strong influence of weld flux, indicate that oxygen and fluorine contamination and fluorine-catalyzed stress corrosion play a major role in cracking of core shroud welds.

  13. A metallurgical evaluation of stress corrosion cracking in large diameter stainless steel piping

    SciTech Connect

    Wheeler, D.A.; Rawl, D.E. Jr.; Louthan, M.R. Jr.

    1990-01-01

    Ultrasonic testing (UT) of the stainless steel piping in the primary coolant water system of SRS reactors indicates the presence of short, partly-through-wall stress corrosion cracks in the heat-affected zone of approximately 7% of the circumferential pipe welds. These cracks are thought to develop by intergranular nucleation and mixed mode propagation. Metallographic evaluations have confirmed the UT indications of crack size and provided evidence that crack growth involved the accumulation of chloride inside the growing crack. It is postulated that the development of an oxygen depletion cell inside the crack results in the migration of chloride ions to the crack tip to balance the accumulation of positively charged metallic ions. The results of this metallurgicial evaluation, combined with structural assessments of system integrity, support the existence of leak-before-break conditions in the SRS reactor piping system. 13 refs., 9 figs.

  14. Shuttle Fuel Feedliner Cracking Investigation

    NASA Technical Reports Server (NTRS)

    Nesman, Tomas E.; Turner, Jim (Technical Monitor)

    2002-01-01

    This presentation provides an overview of material covered during 'Space Shuttle Fuel Feedliner Cracking Investigation MSFC Fluids Workshop' held November 19-21, 2002. Topics covered include: cracks on fuel feed lines of Orbiter space shuttles, fluid driven cracking analysis, liner structural modes, structural motion in a fluid, fluid borne drivers, three dimensional computational fluid dynamics models, fluid borne drivers from pumps, amplification mechanisms, flow parameter mapping, and flight engine flow map.

  15. Corrosion fatigue crack propagation in metals

    NASA Technical Reports Server (NTRS)

    Gangloff, Richard P.

    1990-01-01

    This review assesses fracture mechanics data and mechanistic models for corrosion fatigue crack propagation in structural alloys exposed to ambient temperature gases and electrolytes. Extensive stress intensity-crack growth rate data exist for ferrous, aluminum and nickel based alloys in a variety of environments. Interactive variables (viz., stress intensity range, mean stress, alloy composition and microstructure, loading frequency, temperature, gas pressure and electrode potential) strongly affect crack growth kinetics and complicate fatigue control. Mechanistic models to predict crack growth rates were formulated by coupling crack tip mechanics with occluded crack chemistry, and from both the hydrogen embrittlement and anodic dissolution/film rupture perspectives. Research is required to better define: (1) environmental effects near threshold and on crack closure; (2) damage tolerant life prediction codes and the validity of similitude; (3) the behavior of microcrack; (4) probes and improved models of crack tip damage; and (5) the cracking performance of advanced alloys and composites.

  16. Shear-tensile crack as a tool for reliable estimates of the non-double-couple mechanism: West Bohemia-Vogtland earthquake 1997 swarm

    NASA Astrophysics Data System (ADS)

    Šílený, Jan; Horálek, Josef

    2016-10-01

    Shear-tensile crack is a model for an earthquake mechanism that is more constrained than the moment tensor but that can still describe a non-shear focus. As such, the shear-tensile crack model is more robust than the moment tensor model and yields more reliable estimates for the earthquake mechanism. Such an advantage verifies the credibility of the non-double-couple component found for some events of the 1997 West Bohemia-Vogtland earthquake swarm. As expected, in several cases, a significantly resolved non-double-couple component was obtained where the moment tensor approach failed. Additionally, for non-shear sources, the shear-tensile crack model offers optimization of the Poisson number within the focus, concurrently with retrieval of the mechanism. However, results obtained for the joint inversion of the 1997 swarm indicate that resolution is low. A series of synthetic experiments indicated that limited observations during 1997 were not the cause. Rather, hypothetical experiments of both very good and extremely poor network configurations similarly yielded a low resolution for the Poisson number. Applying this method to data for recent swarms is irrelevant because the small non-double-couple components detected within the inversion are spurious and, thus, the events are pure double-couple phenomena.

  17. Mesoscale modeling of intergranular bubble percolation in nuclear fuels

    SciTech Connect

    Millett, Paul C.; Tonks, Michael; Biner, S. B.

    2012-04-15

    Phase-field simulations are used to examine the variability of intergranular fission gas bubble growth and percolation on uranium dioxide grain boundaries on a mesoscopic length scale. Three key parameters are systematically varied in this study: the contact angle (or dihedral angle) defining the bubble shape, the initial bubble density on the grain boundary plane, and the ratio of the gas diffusivity on the grain boundary versus the grain interiors. The simulation results agree well with previous experimental data obtained for bubble densities and average bubble areas during coalescence events. Interestingly, the rate of percolation is found to be highly variable, with a large dependency on the contact angle and the initial bubble density and little-to-no dependency on the grain boundary gas diffusivity.

  18. MESOSCALE MODELING OF INTERGRANULAR BUBBLE PERCOLATION IN NUCLEAR FUELS

    SciTech Connect

    Paul C. Millett; Michael Tonks; S. B. Biner

    2012-04-01

    Phase-field simulations are used to examine the variability of intergranular fission gas bubble growth and percolation on uranium dioxide grain boundaries on a mesoscopic length scale. Three key parameters are systematically varied in this study: the contact angle (or dihedral angle) defining the bubble shape, the initial bubble density on the grain boundary plane, and the ratio of the gas diffusivity on the grain boundary versus the grain interiors. The simulation results agree well with previous experimental data obtained for bubble densities and average bubble areas during coalescence events. Interestingly, the rate of percolation is found to be highly variable, with a large dependency on the contact angle and the initial bubble density, and little-to-no dependency on the grain boundary gas diffusivity.

  19. Amorphous intergranular phases control the properties of rodent tooth enamel

    NASA Astrophysics Data System (ADS)

    Gordon, Lyle M.; Cohen, Michael J.; MacRenaris, Keith W.; Pasteris, Jill D.; Seda, Takele; Joester, Derk

    2015-02-01

    Dental enamel, a hierarchical material composed primarily of hydroxylapatite nanowires, is susceptible to degradation by plaque biofilm-derived acids. The solubility of enamel strongly depends on the presence of Mg2+, F-, and CO32-. However, determining the distribution of these minor ions is challenging. We show—using atom probe tomography, x-ray absorption spectroscopy, and correlative techniques—that in unpigmented rodent enamel, Mg2+ is predominantly present at grain boundaries as an intergranular phase of Mg-substituted amorphous calcium phosphate (Mg-ACP). In the pigmented enamel, a mixture of ferrihydrite and amorphous iron-calcium phosphate replaces the more soluble Mg-ACP, rendering it both harder and more resistant to acid attack. These results demonstrate the presence of enduring amorphous phases with a dramatic influence on the physical and chemical properties of the mature mineralized tissue.

  20. Effects of carbon on intergranular fracture of iron

    SciTech Connect

    Shin, K.S.; Meshii, M.

    1984-01-01

    The effect of carbon on the grain boundary strength of iron which had been already reduced by sulfur segregation was investigated by a slow strain-rate tensile test at 77/sup 0/K. The amounts of sulfur and carbon segregation at grain boundaries were controlled by carbon contents and heat treatments of specimens and were determined by Auger electron spectroscopy after in-situ fracture under ultra-high vacuum. It was found that the primary effect of carbon is to displace sulfur from grain boundaries and thus to improve the grain boundary strength by reducing the embrittling effect of sulfur. The second effect of carbon is to increase the resistance to dislocation motion and thus to increase the apparent stress for intergranular fracture.

  1. Modeling crack propagation in polycrystalline microstructure using variational multiscale method

    DOE PAGES

    Sun, Shang; Sundararaghavan, Veera

    2016-01-01

    Crack propagation in a polycrystalline microstructure is analyzed using a novel multiscale model. The model includes an explicit microstructural representation at critical regions (stress concentrators such as notches and cracks) and a reduced order model that statistically captures the microstructure at regions far away from stress concentrations. Crack propagation is modeled in these critical regions using the variational multiscale method. In this approach, a discontinuous displacement field is added to elements that exceed the critical values of normal or tangential tractions during loading. Compared to traditional cohesive zone modeling approaches, the method does not require the use of any specialmore » interface elements in the microstructure and thus can model arbitrary crack paths. As a result, the capability of the method in predicting both intergranular and transgranular failure modes in an elastoplastic polycrystal is demonstrated under tensile and three-point bending loads.« less

  2. The study of crack resistance of TiAlN coatings under mechanical loading and thermal cycle testing

    SciTech Connect

    Akulinkin, Alexandr Shugurov, Artur Sergeev, Viktor; Panin, Alexey; Cheng, C.-H.

    2015-10-27

    The effect of preliminary ion bombardment of 321 stainless steel substrate on crack resistance of TiAlN coatings at uniaxial tension and thermal cycling is studied. The ion-beam treatment of the substrate is shown to substantially improve the adhesion strength of the coatings that prevents their delamination and spalling under uniaxial tension. The resistance to crack propagation and spalling by the thermal shock is higher in the TiAlN coating deposited onto the substrate subjected to Ti ion bombardment as compared to that in the TiAlN coating deposited onto the initial substrate.

  3. Hot-cracking mechanism in CO/sub 2/ laser beam welds of dissimilar metals involving PH martensitic stainless steels

    SciTech Connect

    Cieslak, M.J.

    1987-02-01

    Autogenous CO/sub 2/ laser beam welds were made between Alloy HP 9-4-20 and both 15-5 PH and PH 13-8 Mo stainless steel. Small scale circular-patch test specimens revealed that the combination involving the Nb-bearing alloy, 15-5 PH, was far more crack susceptible than the combination involving the Nb-free alloy, PH 13-8 Mo. Analytical electron microscopy was used to identify an NbC/austenite eutectic-like constituent as being responsible for the cracking phenomenon.

  4. The effect of microstructure on 650 C fatigue crack growth in P/M Astroloy

    NASA Technical Reports Server (NTRS)

    Gayda, J.; Miner, R. V.

    1983-01-01

    The effect of microstructure on fatigue crack propagation at 650 C has been studied in a P/M nickel-base superalloy, Astroloy. Crack propagation data were obtained in air and vacuum at 20 cpm with a modified compact tension specimen. The rate of crack growth, da/dn, was correlated with the stress intensity range. Key microstructural variables examined were grain size and the distribution and size of the strengthening gamma prime phase. A fine grain size less than 20 microns always promoted rapid, intergranular failure, while a large grain size promoted slower, transgranular failure which decreased as the size and volume fraction of aging gamma prime was manipulated so as to increase alloy strength. The rapid, intergranular mode of failure of the fine grain microstructures was suppressed in vacuum.

  5. Kinetics of water vapor reactions with intergranular fracture surfaces of Ni{sub 3}Al with and without boron

    SciTech Connect

    Lee, K.H.; Lukowski, J.T.; White, C.L.

    1996-11-15

    In recent years, there has appeared some convincing evidence that the brittleness normally observed in Ni{sub 3}Al polycrystals at ambient temperature and pressure is at least partially due to an environmental effect associated with trace levels of moisture present in many testing environments. In a previous study, the authors found that undoped Ni{sub 3}Al is susceptible to environmental embrittlement in either moist Ar or Ar + 5%H{sub 2} gas, indicating similar embrittling effect of both environments. In contrast to this, boron-doped Ni{sub 3}Al is not very susceptible to moisture-induced embrittlement whereas it is susceptible to gaseous hydrogen-induced intergranular embrittlement. In both H{sub 2}O and H{sub 2} environments, crack tip processes leading to embrittlement may involve gas phase transport to the crack tip, adsorption of the gaseous species (associative and dissociative), absorption and diffusion of atomic hydrogen. Once atomic hydrogen is produced at the surface, its absorption and diffusion should be the same regardless of whether it originates as environmental H{sub 2}O or as H{sub 2}.

  6. Characterizing the effect of creep on stress corrosion cracking of cold worked Alloy 690 in supercritical water environment

    NASA Astrophysics Data System (ADS)

    Zhang, Lefu; Chen, Kai; Du, Donghai; Gao, Wenhua; Andresen, Peter L.; Guo, Xianglong

    2017-08-01

    The effect of creep on stress corrosion cracking (SCC) was studied by measuring crack growth rates (CGRs) of 30% cold worked (CW) Alloy 690 in supercritical water (SCW) and inert gas environments at temperatures ranging from 450 °C to 550 °C. The SCC crack growth rate under SCW environments can be regarded as the cracking induced by the combined effect of corrosion and creep, while the CGR in inert gas environment can be taken as the portion of creep induced cracking. Results showed that the CW Alloy 690 sustained high susceptibility to intergranular (IG) cracking, and creep played a dominant role in the SCC crack growth behavior, contributing more than 80% of the total crack growth rate at each testing temperature. The temperature dependence of creep induced CGRs follows an Arrhenius dependency, with an apparent activation energy (QE) of about 225 kJ/mol.

  7. Cracking catalyst

    SciTech Connect

    Otterstedt, J. E. A.; Jaras, S. G.; Pudas, R.; Upson, L. L.

    1985-05-07

    A cracking catalyst having good resistance to metal poisoning has at least two particle fractions of different particle sizes, the cracking catalyzing zeolite material being concentrated to the coarser particle size fractions, and the finer particle size fractions being formed from material having relatively lower or no or insignificant cracking catalyzing activity. The particles of the finer particle size fractions have a matrix of kaolin and amorphous alumina--silica and may contain for example, an SO /SUB x/ eliminating additive such as Al/sub 2/O/sub 3/, CaO and/or MgO. The coarser particle size fractions having cracking catalyzing effect have a mean particle size of from 80 to 125 ..mu..m and the finer particle size fractions a mean particle size of from 30 to 75 ..mu..m. The coarser particle size fractions have a zeolite content of at least 20 weight % and may have a zeolite content of up to 100 weight %, the remainder consisting essentially of material which has relatively lower or no or insignificant cracking-catalyzing activity and which consists of kaolin and amorphous alumina-silica. The catalyst mass as a whole may have a zeolite content of up to 50 weight %.

  8. In vivo oxide-induced stress corrosion cracking of Ti-6Al-4V in a neck-stem modular taper: Emergent behavior in a new mechanism of in vivo corrosion.

    PubMed

    Gilbert, Jeremy L; Mali, Sachin; Urban, Robert M; Silverton, Craig D; Jacobs, Joshua J

    2012-02-01

    In vivo modular taper corrosion in orthopedic total joint replacements has been documented to occur for head-neck tapers, modular-body tapers, and neck-stem tapers. While the fretting corrosion mechanism by which this corrosion occurs has been described in the literature, this report shows new and as yet unreported mechanisms at play. A retrieved Ti-6Al-4V/Ti-6Al-4V neck-stem taper interface, implanted for 6 years is subjected to failure analysis to document taper corrosion processes that lead to oxide driven crack formation on the medial side of the taper. Metallurgical sectioning techniques and scanning electron microscopy analysis are used to document the taper corrosion processes. The results show large penetrating pitting attack of both sides of the taper interface where corrosion selectively attacks the beta phase of the microstructure and eventually consumes the alpha phase. The pitting attack evolves into plunging pits that ultimately develop into cracks where the crack propagation process is one of corrosion resulting in oxide formation and subsequent reorganization. This process drives open the crack and advances the front by a combination of oxide-driven crack opening stresses and corrosion attack at the tip. The oxide that forms has a complex evolving structure including a network of transport channels that provide access of fluid to the crack tip. This emergent behavior does not appear to require continued fretting corrosion to propagate the pitting and cracking. This new mechanism is similar to stress corrosion cracking where the crack tip stresses arise from the oxide formation in the crack and not externally applied tensile stresses. Copyright © 2011 Wiley Periodicals, Inc.

  9. Issues of intergranular embrittlement of VVER-type nuclear reactors pressure vessel materials

    NASA Astrophysics Data System (ADS)

    Zabusov, O.

    2016-04-01

    In light of worldwide tendency to extension of service life of operating nuclear power plants - VVER-type in the first place - recently a special attention is concentrated on phenomena taking place in reactor pressure vessel materials that are able to lead to increased level of mechanical characteristics degradation (resistibility to brittle fracture) during long term of operation. Formerly the hardening mechanism of degradation (increase in the yield strength under influence of irradiation) mainly had been taken into consideration to assess pressure vessel service life limitations, but when extending the service life up to 60 years and more the non-hardening mechanism (intergranular embrittlement of the steels) must be taken into account as well. In this connection NRC “Kurchatov Institute” has initiated a number of works on investigations of this mechanism contribution to the total embrittlement of reactor pressure vessel steels. The main results of these investigations are described in this article. Results of grain boundary phosphorus concentration measurements in specimens made of first generation of VVER-type pressure vessels materials as well as VVER-1000 surveillance specimens are presented. An assessment of non-hardening mechanism contribution to the total ductile-to- brittle transition temperature shift is given.

  10. The role of local strains from prior cold work on stress corrosion cracking

    NASA Astrophysics Data System (ADS)

    Ulaganathan, Jaganathan

    Several studies have recently reported that cold working exacerbates stress corrosion cracking (SCC) of materials in various environments, including those in which they were previously thought to be immune. While these studies usually consider cold work as a homogeneous effect, the presence of grain boundaries results in local strain concentrations that are inhomogeneously distributed within the microstructure. In order to understand the underlying mechanisms by which the local strains generated by cold work influences SCC, α-brass and Alloy 600 were used in this study. The microscopic changes in the local strains caused by cold work and by SCC were measured using electron backscatter diffraction (EBSD) and polychromatic X-ray microdiffraction (PXM). While the plastic strains were qualitatively expressed through the local misorientation calculated from the orientation data measured by both EBSD and PXM, the elastic strains were determined from the Laue patterns measured by PXM. The interaction between the local strains, and the crack initiation and propagation during SCC was studied by comparing the strain distribution from the same area measured before cold work, after cold work, and again after SCC. In this way, apart from obtaining insights on the interaction, the relative importance of pre-existing strain concentrations and those created by crack propagation can be identified. Additionally, statistical analysis of the EBSD data from uncracked and cracked grain boundaries in Alloy 600 showed the susceptibility of the boundaries to increase when they were surrounded by high local strain concentrations and when the grains sharing the boundary had similar deformation tendency, but to be independent of the grain boundary angle. Finally, one of the contributors for the changes in the strain distribution during SCC can be the corrosion process itself which was examined by intermittently measuring the changes in local strains caused by intergranular corrosion on an

  11. On post-weld heat treatment cracking in tig welded superalloy ATI 718Plus

    NASA Astrophysics Data System (ADS)

    Asala, G.; Ojo, O. A.

    The susceptibility of heat affected zone (HAZ) to cracking in Tungsten Inert Gas (TIG) welded Allvac 718Plus superalloy during post-weld heat treatment (PWHT) was studied. Contrary to the previously reported case of low heat input electron beam welded Allvac 718Plus, where HAZ cracking occurred during PWHT, the TIG welded alloy is crack-free after PWHT, notwithstanding the presence of similar micro-constituents that caused cracking in the low input weld. Accordingly, the formation of brittle HAZ intergranular micro-constituents may not be a sufficient factor to determine cracking propensity, the extent of heat input during welding may be another major factor that influences HAZ cracking during PWHT of the aerospace superalloy Allvac 718Plus.

  12. Subsurface Crack Initiation and Propagation Mechanism under the Super-Long Fatigue Regime for High Speed Tool Steel (JIS SKH51) by Fracture Surface Topographic Analysis

    NASA Astrophysics Data System (ADS)

    Shiozawa, Kazuaki; Morii, Yuuichi; Nishino, Seiichi

    In order to study the subsurface crack initiation and propagation mechanism of high strength steel under a very high cycle fatigue regime, computational simulation with fracture surface topographic analysis (FRASTA) was carried out for subsurface fatigue crack initiated specimens of high speed tool steel (JIS SKH51) obtained from the rotating bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye region on the fracture surface, which is a feature on the fracture surface in super long fatigue. This so-called GBF (granular-bright-facet) was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish-eye. It was clearly simulated by FRASTA that multiple microcracks were initiated and dispersed by the decohesion of a spherical carbide from the matrix around a nonmetallic inclusion, and converged into the GBF area during the fatigue process. After the formation of the GBF area, interior cracks grew radially and a fish-eye pattern formed on the fracture surface.

  13. Visualizing In Situ Microstructure Dependent Crack Tip Stress Distribution in IN-617 Using Nano-mechanical Raman Spectroscopy

    NASA Astrophysics Data System (ADS)

    Zhang, Yang; Mohanty, Debapriya P.; Tomar, Vikas

    2016-11-01

    Inconel 617 (IN-617) is a solid solution alloy, which is widely used in applications that require high-temperature component operation due to its high-temperature stability and strength as well as strong resistance to oxidation and carburization. The current work focuses on in situ measurements of stress distribution under 3-point bending at elevated temperature in IN-617. A nanomechanical Raman spectroscopy measurement platform was designed and built based on a combination of a customized open Raman spectroscopy (NMRS) system incorporating a motorized scanning and imaging system with a nanomechanical loading platform. Based on the scanning of the crack tip notch area using the NMRS notch tip, stress distribution under applied load with micron-scale resolution for analyzed microstructures is predicted. A finite element method-based formulation to predict crack tip stresses is presented and validated using the presented experimental data.

  14. First-principles study of the effects of halogen dopants on the properties of intergranular films in silicon nitride ceramics

    NASA Astrophysics Data System (ADS)

    Painter, Gayle S.; Becher, Paul F.; Kleebe, H.-J.; Pezzotti, G.

    2002-02-01

    The nanoscale intergranular films that form in the sintering of ceramics often occur as adherent glassy phases separating the crystalline grains in the ceramic. Consequently, the properties of these films are often equal in importance to those of the constituent grains in determining the ceramic's properties. The measured characteristics of the silica-rich phase separating the crystalline grains in Si3N4 and many other ceramics are so reproducible that SiO2 has become a model system for studies of intergranular films (IGF's). Recently, the influence of fluorine and chlorine dopants in SiO2-rich IGF's in silicon nitride was precisely documented by experiment. Along with the expected similarities between the halogens, some dramatically contrasting effects were found. But the atomic-scale mechanisms distinguishing the effects F and Cl on IGF behavior have not been well understood. First-principles density functional calculations reported here provide a quantum-level description of how these dopant-host interactions affect the properties of IGF's, with specific modeling of F and Cl in the silica-rich IGF in silicon nitride. Calculations were carried out for the energetics, structural changes, and forces on the atoms making up a model cluster fragment of an SiO2 intergranular film segment in silicon nitride with and without dopants. Results show that both anions participate in the breaking of bonds within the IGF, directly reducing the viscosity of the SiO2-rich film and promoting decohesion. Observed differences in the way fluorine and chlorine affect IGF behavior become understandable in terms of the relative stabilities of the halogens as they interact with Si atoms that have lost one if their oxygen bridges.

  15. Intergranular corrosion of an aluminum-magnesium-silicon-copper alloy

    SciTech Connect

    Burleigh, T.D.; Ludwiczak, E.; Petri, R.A.

    1995-01-01

    The intergranular (intercrystalline) corrosion (IGC) of a heat-treated aluminum-magnesium-silicon-copper alloy was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM revealed that tall chimneys of corrosion product formed on the surface above the pits during oxygenated salt water immersion. It was postulated that pitting corrosion occurred first and that the corrosion chimneys maintained the acidic, chloride pit environment that subsequently caused IGC (preferential dissolution of the region adjacent to the grain boundaries). TEM foils of the same alloy were immersed in a model pit solution (dilute hydrochloric acid) and showed IGC identical to the corrosion attack seen in the bulk samples. Potentiodynamic polarization in the dilute HCl solution verified that pure Al corroded many times faster than the bulk alloy. These results indicated IGC of this alloy occurred because the depleted region adjacent to the grain boundaries corroded rapidly in acidic solutions. The presence of pits with corrosion chimneys, or some type of occluded cells, must have maintained the acidic environment, which caused IGC.

  16. Micromechanics of intergranular creep failure under cyclic loading

    SciTech Connect

    Giessen, E. van der; Tvergaard, V.

    1996-07-01

    This paper is concerned with a micromechanical investigation of intergranular creep failure caused by grain boundary cavitation under strain-controlled cyclic loading conditions. Numerical unit cell analyses are carried out for a planar polycrystal model in which the grain material and the grain boundaries are modeled individually. The model incorporates power-law creep of the grains, viscous grain boundary sliding between grains as well as the nucleation and growth of grain boundary cavities until they coalesce and form microcracks. Study of a limiting case with a facet-size microcrack reveals a relatively simple phenomenology under either balanced loading, slow-fast loading or balanced loading with a hold period at constant tensile stress. Next, a (non-dimensionalized) parametric study is carried out which focuses on the effect of the diffusive cavity growth rate relative to the overall creep rate, and the effects of cavity nucleation and grain boundary sliding. The model takes account of the build up of residual stresses during cycling, and it turns out that this, in general, gives rise to a rather complex phenomenology, but some cases are identified which approach the simple microcrack behavior. The analyses provide some new understanding that helps to explain the sometimes peculiar behavior under balanced cyclic creep.

  17. Evaluation of stainless steels for their resistance to intergranular corrosion

    NASA Astrophysics Data System (ADS)

    Korostelev, A. B.; Abramov, V. Ya.; Belous, V. N.

    1996-10-01

    Austenitic stainless steels are being considered as structural materials for first wall/blanket systems in the International Thermonuclear Reactor (ITER). The uniform corrosion of stainless steels in water is well known and is not a critical issue limiting its application for the ITER design. The sensitivity of austenitic steels to intergranular corrosion (IGC) can be estimated rather accurately by means of calculation methods, considering structure and chemical composition of steel. There is a maximum permissible carbon content level, at which sensitization of stainless steel is eliminated: K = Cr eff - αC eff, where α-thermodynamic coefficient, Cr eff-effective chromium content (regarding molybdenum influence) and C eff-effective carbon content (taking into account nickel and stabilizing elements). Corrosion tests for 16Cr11Ni3MoTi, 316L and 316LN steel specimens, irradiated up to 2 × 10 22 n/cm 2 fluence have proved the effectiveness of this calculation technique for determination of austenitic steels tendency to IGC. This method is directly applicable in austenitic stainless steel production and enables one to exclude complicated experiments on determination of stainless steel susceptibility to IGC.

  18. Environmentally assisted cracking in light water reactors

    SciTech Connect

    Chopra, O.K.; Chung, H.M.; Gruber, E.E.

    1996-07-01

    This report summarizes work performed by Argonne National Laboratory on fatigue and environmentally assisted cracking (EAC) in light water reactors (LWRs) from April 1995 to December 1995. Topics that have been investigated include fatigue of carbon and low-alloy steel used in reactor piping and pressure vessels, EAC of Alloy 600 and 690, and irradiation-assisted stress corrosion cracking (IASCC) of Type 304 SS. Fatigue tests were conducted on ferritic steels in water that contained various concentrations of dissolved oxygen (DO) to determine whether a slow strain rate applied during different portions of a tensile-loading cycle are equally effective in decreasing fatigue life. Crack-growth-rate tests were conducted on compact-tension specimens from several heats of Alloys 600 and 690 in simulated LWR environments. Effects of fluoride-ion contamination on susceptibility to intergranular cracking of high- and commercial- purity Type 304 SS specimens from control-tensile tests at 288 degrees Centigrade. Microchemical changes in the specimens were studied by Auger electron spectroscopy and scanning electron microscopy to determine whether trace impurity elements may contribute to IASCC of these materials.

  19. Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

    NASA Astrophysics Data System (ADS)

    Chan, Kwai S.

    2014-07-01

    A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K th, for the onset of time-dependent crack growth. This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.

  20. Fatigue crack initiation in carbon and low-alloy steels in light water reactor environments : mechanism and prediction.

    SciTech Connect

    Chopra, O. K.; Shack, W. J.

    1998-01-27

    Section 111 of the ASME Boiler and Pressure Vessel Code specifies fatigue design curves for structural materials. The effects of reactor coolant environments are not explicitly addressed by the Code design curves. Recent test data illustrate potentially significant effects of light water reactor (LWR) coolant environments on the fatigue resistance of carbon and low-alloy steels. Under certain loading and environmental conditions, fatigue lives of test specimens may be shorter than those in air by a factor of {approx}70. The crack initiation and crack growth characteristics of carbon and low-alloy steels in LWR environments are presented. Decreases in fatigue life of these steels in high-dissolved-oxygen water are caused primarily by the effect of environment on growth of short cracks < 100 {micro}m in depth. The material and loading parameters that influence fatigue life in LWR environments are defined. Fatigue life is decreased significantly when five conditions are satisfied simultaneously, viz., applied strain range, service temperature, dissolved oxygen in water, and S content in steel are above a threshold level, and loading strain rate is below a threshold value. Statistical models have been developed for estimating the fatigue life of these steels in LWR environments. The significance of the effect of environment on the current Code design curve is evaluated.

  1. Intergranular fracture in some precipitation-hardened aluminum alloys at low temperatures

    SciTech Connect

    Kuramoto, S.; Itoh, G.; Kanno, M.

    1996-10-01

    Intergranular fracture at low temperatures from room temperature down to 4.2 K has been studied in some precipitation-hardened aluminum alloys. Microscopic appearance of intergranular facets is revealed to be greatly affected by the microstructure adjacent to the grain boundaries (GBs). When large precipitates on GBs and wide precipitation-free zones (PFZs) are present, coalescence of microvoids initiated at the GB precipitates causes the intergranular fracture with dimples. This fracture process is found to be unaffected by deformation temperature. On the other hand, in the presence of fine precipitates on GBs and narrow PFZs, matrix slip localization exerts significant influence on the fracture behavior. At low temperatures, large stress concentration at GBs leads to intergranular fracture, forming sharp ledges on the fracture surfaces, while at room temperature, the dynamic recovery process is thought to relax such stress concentration, resulting in a transgranular ductile rupture.

  2. Atomistic Structure, Strength, and Kinetic Properties of Intergranular Films in Ceramics

    SciTech Connect

    Garofalini, Stephen H

    2015-01-08

    Intergranular films (IGFs) present in polycrystalline oxide and nitride ceramics provide an excellent example of nanoconfined glasses that occupy only a small volume percentage of the bulk ceramic, but can significantly influence various mechanical, thermal, chemical, and optical properties. By employing molecular dynamics computer simulations, we have been able to predict structures and the locations of atoms at the crystal/IGF interface that were subsequently verified with the newest electron microscopies. Modification of the chemistry of the crystal surface in the simulations provided the necessary mechanism for adsorption of specific rare earth ions from the IGF in the liquid state to the crystal surface. Such results had eluded other computational approaches such as ab-initio calculations because of the need to include not only the modified chemistry of the crystal surfaces but also an accurate description of the adjoining glassy IGF. This segregation of certain ions from the IGF to the crystal caused changes in the local chemistry of the IGF that affected fracture behavior in the simulations. Additional work with the rare earth ions La and Lu in the silicon oxynitride IGFs showed the mechanisms for their different affects on crystal growth, even though both types of ions are seen adhering to a bounding crystal surface that would normally imply equivalent affects on grain growth.

  3. Measurement and Modeling of Hydrogen Environment-Assisted Cracking in Monel K-500

    NASA Astrophysics Data System (ADS)

    Gangloff, Richard P.; Ha, Hung M.; Burns, James T.; Scully, John R.

    2014-08-01

    Hydrogen environment-assisted cracking (HEAC) of Monel K-500 is quantified using slow-rising stress intensity loading with electrical potential monitoring of small crack propagation and elastoplastic J-integral analysis. For this loading, with concurrent crack tip plastic strain and H accumulation, aged Monel K-500 is susceptible to intergranular HEAC in NaCl solution when cathodically polarized at -800 mVSCE ( E A, vs saturated calomel) and lower. Intergranular cracking is eliminated by reduced cathodic polarization more positive than -750 mVSCE. Crack tip diffusible H concentration rises, from near 0 wppm at E A of -765 mVSCE, with increasing cathodic polarization. This behavior is quantified by thermal desorption spectroscopy and barnacle cell measurements of hydrogen solubility vs overpotential for planar electrodes, plus measured-local crevice potential, and pH scaled to the crack tip. Using crack tip H concentration, excellent agreement is demonstrated between measurements and decohesion-based model predictions of the E A dependencies of threshold stress intensity and Stage II growth rate. A critical level of cathodic polarization must be exceeded for HEAC to occur in aged Monel K-500. The damaging-cathodic potential regime likely shifts more negative for quasi-static loading or increasing metallurgical resistance to HEAC.

  4. Identification of crack path of inter- and transgranular fractures in sintered silicon nitride by in situ TEM.

    PubMed

    Ii, Seiichiro; Iwamoto, Chihiro; Matsunaga, Katsuyuki; Yamamoto, Takahisa; Ikuhara, Yuichi

    2004-01-01

    Inter- and/or transgranular crack paths in sintered silicon nitride (Si3N4) during fracture were investigated by in situ straining experiments in a transmission electron microscope at room temperature, using a high-precision micro-indenter. By this technique, cracks introduced in an in situ manner were observed to propagate in the grain interior and along grain boundaries. High-resolution electron microscopy (HREM) observation revealed that the crack propagation takes place at an interface between Si3N4 grains and an intergranular glassy film (IGF) in the case of intergranular fractures. According to the results by previous molecular dynamics simulations, a number of dangling bonds are present at the Si3N4/IGF interface, which should result in the observed fracture behavior at the interface. On the other hand, the crack path introduced during transgranular fracture of Si3N4 grains was found to be sharp and straight. The observed crack propagated towards [1120] inside the Si3N4 grain with the crack surface parallel to the (1100) plane. The HREM observations of crack walls revealed them to be atomically flat. The atomic termination of the crack walls was identified in combination with image simulations based on atomic models of the cleaved crack walls.

  5. Hydrogen embrittlement and stress corrosion cracking

    SciTech Connect

    Gibala, R.; Hehemann, R.F.

    1984-01-01

    Topics related to hydrogen embrittlement are discussed, taking into account an overview on hydrogen degradation phenomena, theories of hydrogen induced cracking of steels, the hydrogen embrittlement of steels, hydrogen trapping in iron and steels, some recent results on the direct observation of hydrogen trapping in metals and its consequences on embrittlement mechanisms, fracture mechanics and surface chemistry investigations of environment-assisted crack growth, the role of microstructure in hydrogen embrittlement, and hydrogen related second phase embrittlement of solids. Subjects in the area of stress corrosion cracking are also explored, giving attention to recent observations on the propagation of stress corrosion cracks and their relevance to proposed mechanisms of stress corrosion cracking, films and their importance in the nucleation of stress corrosion cracking in stainless steel, and fundamentals of corrosion fatigue behavior of metals and alloys. Stress corrosion cracking of ferritic and austenitic stainless steels is also considered along with embrittlement studies on metallic glasses.

  6. Three-dimensional measurements of fatigue crack closure

    NASA Technical Reports Server (NTRS)

    Grandt, A. F., Jr.

    1984-01-01

    Three dimensional fatigue crack opening profiles in transparent polymer test specimens were determined. The load required to separate crack faces was measured along the crack profile at various positions through the specimens thickness. Crack opening loads at the specimen surface (under plane stress conditions) were compared with measurements made under plane strain conditions the specimen interior. The fatigue crack opening load was correlated with fatigue crack retardation behavior caused by peak overloads, and the results discussed in terms of three dimensional aspects of the fatigue crack closure mechanism for fatigue crack retardation.

  7. Nondestructive Evaluation (NDE) Exploratory Development for Air Force Systems. Delivery Order 0001: Quick Reaction NDE and Characterization--Effects of Chemical Effects of Chemical Etching after Pre-Inspection Mechanical Cleaning on Fluorescent Penetrant Indications of Fatigue Cracks

    DTIC Science & Technology

    2011-08-01

    FPI crack indications degraded by mechanically cleaning cracked specimens made of Ti-6Al-4V and Inconel 718. The data showed that minimal etching...3.3 Etchant Solution (High Chromium Super Alloy – Class G) for Inconel 718 .......................... 3 3.4 Etch Rate Test Tabs...Media ...................... 16 Figure 9. SEM Images of Inconel Specimen 626-52 Cleaned with Dry Plastic Media ........................ 17 Figure 10

  8. Stress-corrosion cracking in BWR and PWR piping

    SciTech Connect

    Weeks, R.W.

    1983-07-01

    Intergranular stress-corrosion cracking of weld-sensitized wrought stainless steel piping has been an increasingly ubiquitous and expensive problem in boiling-water reactors over the last decade. In recent months, numerous cracks have been found, even in large-diameter lines. A number of potential remedies have been developed. These are directed at providing more resistant materials, reducing weld-induced stresses, or improving the water chemistry. The potential remedies are discussed, along with the capabilities of ultrasonic testing to find and size the cracks and related safety issues. The problem has been much less severe to date in pressurized-water reactors, reflecting the use of different materials and much lower coolant oxygen levels.

  9. Crack Formation in Cement-Based Composites

    NASA Astrophysics Data System (ADS)

    Sprince, A.; Pakrastinsh, L.; Vatin, N.

    2016-04-01

    The cracking properties in cement-based composites widely influences mechanical behavior of construction structures. The challenge of present investigation is to evaluate the crack propagation near the crack tip. During experiments the tension strength and crack mouth opening displacement of several types of concrete compositions was determined. For each composition the Compact Tension (CT) specimens were prepared with dimensions 150×150×12 mm. Specimens were subjected to a tensile load. Deformations and crack mouth opening displacement were measured with extensometers. Cracks initiation and propagation were analyzed using a digital image analysis technique. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The obtained experimental curve shows the stages of crack development.

  10. Ultrasound imaging of stress corrosion cracking

    NASA Astrophysics Data System (ADS)

    Hörchens, Lars; Wassink, Casper; Haines, Harvey

    2015-03-01

    The formation of cracks in a corrosive environment in combination with tensile stresses is known as stress corrosion cracking. This type of degradation mechanism can lead to sudden and rapid failure of a structure. In a colony of cracks, it is desired to determine the position and depth of individual cracks in order to assess the remaining strength of the structure. In the present paper, acoustical imaging using inverse wave field extrapolation is applied to a pipe coupon exhibiting stress corrosion cracking. It is shown that individual cracks in the colony can be identified and sized. Aside from the direct path into the pipe wall, reflections from the inner and outer surface of the sample are used to determine accurately the extent of the surface-breaking cracks within the material. The images obtained during a scan can be stacked together to provide a three-dimensional visualization of the colony of cracks.

  11. Thermal-Fatigue Crack-Growth Characteristics and Mechanical Strain Cycling Behavior of A-286 Discaloy, and 16-25-6 Austenitic Steels

    NASA Technical Reports Server (NTRS)

    Smith, Robert W.; Smith, Gordon T.

    1960-01-01

    Thermal-fatigue crack-growth characteristics of notched- and unnotched-disk specimens of A-286, Discaloy, hot-cold worked 16-25-6, and overaged 16-25-6 were experimentally studied. Separately controlled variables were total strain range (0.0043 to 0.0079 in./in.), maximum cycle temperature (1300 and 1100 F), and hold time at maximum temperature (O and 5 min). A limited number of mechanical, push-pull, constant-strain cycle tests at room temperature were made using notched and un-notched bars of the same materials. In these tests the number of cycles to failure as well as the variation of load change with accumulated cycles was measured, and the effects of mean stress were observed. Constant-strain-range mechanical-fatigue tests at room temperature revealed notched-bar fatigue life to be strongly influenced by mean stress. For a specific strain range, the longest fatigue life was always found to be associated with the least-tensile (or most compressive) mean stress. By defining thermal-fatigue life as the number of cycles required to produce a crack area of 6000 square mils, the relative thermal-fatigue resistances of the test materials were established. Notched-disk specimens of A-286 and Discaloy steels exhibited longer fatigue lives than either hot-cold worked or overaged 16-25-6. On the other hand, unnotched-disk specimens of Discaloy and hot-cold worked 16-25-6 had longer lives than A-286 and overaged 16-25-6. Separation of the crack-growth data into microstage and macrostage periods revealed that the macrostage period accounted for the greatest part of the difference among materials when tested in the notched configuration, while the microstage was largely responsible for the differences encountered in unnotched disks.

  12. Crack-free polydimethylsiloxane-bioactive glass-poly(ethylene glycol) hybrid monoliths with controlled biomineralization activity and mechanical property for bone tissue regeneration.

    PubMed

    Chen, Jing; Du, Yuzhang; Que, Wenxiu; Xing, Yonglei; Chen, Xiaofeng; Lei, Bo

    2015-12-01

    Crack-free organic-inorganic hybrid monoliths with controlled biomineralization activity and mechanical property have an important role for highly efficient bone tissue regeneration. Here, biomimetic and crack-free polydimethylsiloxane (PDMS)-modified bioactive glass (BG)-poly(ethylene glycol) (PEG) (PDMS-BG-PEG) hybrids monoliths were prepared by a facile sol-gel technique. Results indicate that under the assist of co-solvents, BG sol and PDMS and PEG could be hybridized at a molecular level, and effects of the PEG molecular weight on the structure, biomineralization activity, and mechanical property of the as-prepared hybrid monoliths were also investigated in detail. It is found that an addition of low molecular weight PEG can significantly prevent the formation of cracks and speed up the gelation of the hybrid monoliths, and the surface microstructure of the hybrid monoliths can be changed from the porous to the smooth as the PEG molecular weight increases. Additionally, the hybrid monoliths with low molecular weight PEG show the high formation of the biological apatite layer, while the hybrids with high molecular weight PEG exhibit negligible biomineralization ability in simulated body fluid (SBF). Furthermore, the PDMS-BG-PEG 600 hybrid monolith has significantly high compressive strength (32 ± 3 MPa) and modulus (153 ± 11 MPa), as well as good cell biocompatibility by supporting osteoblast (MC3T3-E1) attachment and proliferation. These results indicate that the as-prepared PDMS-BG-PEG hybrid monoliths may have promising applications for bone tissue regeneration.

  13. Crossover from nanoscopic intergranular hopping to conventional charge transport in pyrite thin films.

    PubMed

    Zhang, Xin; Manno, Michael; Baruth, Andrew; Johnson, Melissa; Aydil, Eray S; Leighton, Chris

    2013-03-26

    Pyrite FeS2 is receiving a resurgence of interest as a uniquely attractive thin film solar absorber based on abundant, low-cost, nontoxic elements. Here we address, via ex situ sulfidation synthesis, the long-standing problem of understanding conduction and doping in FeS2 films, an elusive prerequisite to successful solar cells. We find that an abrupt improvement in crystallinity at intermediate sulfidation temperatures is accompanied by unanticipated crossovers from intergranular hopping to conventional transport, and, remarkably, from hole-like to electron-like Hall coefficients. The hopping is found to occur between a small volume fraction of conductive nanoscopic sulfur-deficient grain cores (beneath our X-ray diffraction detection limits), embedded in nominally stoichiometric FeS2. In addition to placing constraints on the conditions under which useful properties can be obtained from FeS2 synthesized in diffusion-limited situations, these results also emphasize that FeS2 films are not universally p-type. Indeed, with no knowledge of the active transport mechanism we demonstrate that the Hall coefficient alone is insufficient to determine the sign of the carriers. These results elucidate the possible transport mechanisms in thin film FeS2 in addition to their influence on the deduced carrier type, an enabling advancement with respect to understanding and controlling doping in pyrite films.

  14. Modelling and measurement of crack closure and crack growth following overloads and underloads

    NASA Technical Reports Server (NTRS)

    Dexter, R. J.; Hudak, S. J.; Davidson, D. L.

    1989-01-01

    Ignoring crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading. Crack closure is believed to contribute to the crack growth retardation, although the specific closure mechanism is dabatable. The delay period and corresponding crack growth rate transients following overload and overload/underload cycles were systematically measured as a function of load ratio and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth, i.e. the effective stress intensity factor range. Experimental results are compared with the predictions of a Dugdale-type (1960) crack closure model, and improvements in the model are suggested.

  15. On the Mechanism of Surface Cracking in DC Cast 7XXX and 6XXX Extrusion Ingot Alloys

    NASA Astrophysics Data System (ADS)

    Benum, Steinar; Mortensen, Dag; Fjær, Hallvard; Øverlie, Hilde-Gunn; Reiso, Oddvin

    When applying the Hydro variant (Hycast Gas Cushion) of the Showa Denko gas slip technology for casting extrusion ingots of 7xxx alloys surface cracks occasionally occurred. Especially one alloy with 0.3 wt.% Cu caused problems. In order to identify the problem, the casting process for these alloys was simulated by a coupled stress, thermal and fluid flow model (ALSIM/ALSPEN). The simulations were designed as a factorial trial where casting speed, ramping of the speed, casting temperature, cone height of the starting block, cooling water efficiency and primary cooling were systematically varied. The hoop stress in the surface at the temperature when 97.5% of the material was solidified was used as a crack sensitivity indicator. Three stages were identified: (I) At the start a maximum hoop stress evolved, (II) then a minimum stress occurred before (III) the stress reached a stable level. For an AA6060 alloy the stress was found to be zero in the stable stage while the AA7108 alloy experienced tension stress also during the steady state regime. Based on the factorial analysis it was found that the stable stress increased most rapidly with increasing casting speed and decreased with an increased primary cooling and a reduced melt temperature.

  16. Unified nano-mechanics based probabilistic theory of quasibrittle and brittle structures: I. Strength, static crack growth, lifetime and scaling

    NASA Astrophysics Data System (ADS)

    Le, Jia-Liang; Bažant, Zdeněk P.; Bazant, Martin Z.

    2011-07-01

    Engineering structures must be designed for an extremely low failure probability such as 10 -6, which is beyond the means of direct verification by histogram testing. This is not a problem for brittle or ductile materials because the type of probability distribution of structural strength is fixed and known, making it possible to predict the tail probabilities from the mean and variance. It is a problem, though, for quasibrittle materials for which the type of strength distribution transitions from Gaussian to Weibullian as the structure size increases. These are heterogeneous materials with brittle constituents, characterized by material inhomogeneities that are not negligible compared to the structure size. Examples include concrete, fiber composites, coarse-grained or toughened ceramics, rocks, sea ice, rigid foams and bone, as well as many materials used in nano- and microscale devices. This study presents a unified theory of strength and lifetime for such materials, based on activation energy controlled random jumps of the nano-crack front, and on the nano-macro multiscale transition of tail probabilities. Part I of this study deals with the case of monotonic and sustained (or creep) loading, and Part II with fatigue (or cyclic) loading. On the scale of the representative volume element of material, the probability distribution of strength has a Gaussian core onto which a remote Weibull tail is grafted at failure probability of the order of 10 -3. With increasing structure size, the Weibull tail penetrates into the Gaussian core. The probability distribution of static (creep) lifetime is related to the strength distribution by the power law for the static crack growth rate, for which a physical justification is given. The present theory yields a simple relation between the exponent of this law and the Weibull moduli for strength and lifetime. The benefit is that the lifetime distribution can be predicted from short-time tests of the mean size effect on