Crack tip deformation and fatigue crack growth
NASA Technical Reports Server (NTRS)
Liu, H.-W.
1981-01-01
Recent research on fatigue crack growth is summarized. Topics discussed include the use of the differential stress intensity factor to characterize crack tip deformation, the use of the unzipping model to study the growth of microcracks and the fatigue crack growth in a ferritic-martensitic steel, and the development of a model of fatige crack growth threshold. It is shown that in the case of small yielding, the differential stress intensity factor provides an adequate description of cyclic plastic deformation at the crack tip and correlates well with the crack growth rate. The unzipping model based on crack tip shear decohesion process is found to be in good agreement with the measured crack growth and striation spacing measurements. The proposed model of crack growth threshold gives correct predictions of the crack growth behavior in the near-threshold region.
NASA Technical Reports Server (NTRS)
Namkung, Min; Clendenin, C. Gerald; Wincheski, Buzz; Fulton, James P.; Todhunter, Ronald G.; Simpson, John W.
1994-01-01
Fatigue-testing system includes automated subsystem continuously tracking location of fatigue-crack tip in metal or other highly electrically conductive specimen. Fatigue-crack-tip-locating subsystem also searches specimen to find initial fatigue crack and its tip and to trace out hidden fatigue cracks and other flaws inside specimen. Subsystem operates under overall control of personal computer, which also controls load frame applying prescribed cyclic stresses to specimen. Electromagnetic flaw detector based on eddy-current principle scanned over surface of specimen. Flaw detector described in "Electromagnetic Flaw Detector Is Easier To Use" (LAR-15046). System provides automated control and monitoring of fatigue experiments, saving time for researchers and enabling experiments to run unattended 24 hours a day. All information on crack-tip trajectories and rates of growth of cracks recorded automatically, so researchers have access to more information.
Semi-empirical crack tip analysis
NASA Technical Reports Server (NTRS)
Chudnovsky, A.; Ben Ouezdon, M.
1988-01-01
Experimentally observed crack opening displacements are employed as the solution of the multiple crack interaction problem. Then the near and far fields are reconstructed analytically by means of the double layer potential technqiue. Evaluation of the effective stress intensity factor resulting from the interaction of the main crack and its surrounding crazes in addition to the remotely applied load is presented as an illustrative example. It is shown that crazing (as well as microcracking) may constitute an alternative mechanism to Dugdale-Berenblatt models responsible for the cancellation of the singularity at the crack tip.
Crack Tip Dislocation Nucleation in FCC Solids
NASA Astrophysics Data System (ADS)
Knap, J.; Sieradzki, K.
1999-02-01
We present results of molecular dynamic simulations aimed at examining crack tip dislocation emission in fcc solids. The results are analyzed in terms of recent continuum formulations of this problem. In mode II, Au, Pd, and Pt displayed a new unanticipated mechanism of crack tip dislocation emission involving the creation of a pair of Shockley partials on a slip plane one plane below the crack plane. In mode I, for all the materials examined, Rice's continuum formulation [J. Mech. Phys. Solids 40, 239 (1992)] underestimated the stress intensity for dislocation emission by almost a factor of 2. Surface stress corrections to the emission criterion brought the agreement between continuum predictions and simulations to within 20%.
NASA Technical Reports Server (NTRS)
Hudak, S. J., Jr.; Davidson, D. L.; Chan, K. S.
1983-01-01
Crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading when linear damage accumulation procedures are employed. Crack closure is believed to control the crack growth retardation, although the specific closure mechanism is debatable. Information on the relative contributions to crack closure from: (1) plasticity left in the wake of the advancing crack and (2) crack tip residual stresses is provided. The delay period and corresponding crack growth rate transients following overloads are systematically measured as a function of load ratio (R) and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth as measured by crack tip opening loads and delta K sub eff. The latter measurements are obtained using a scanning electron microscope equipped with a cyclic loading stage; measurements are quantified using a relatively new stereoimaging technique. Combining experimental results with analytical predictions suggests that both plastic wake and residual stress mechanism are operative, the latter becoming predominate as R increases.
Atomistic observation of a crack tip approaching coherent twin boundaries
Liu, L.; Wang, J.; Gong, S. K.; Mao, S. X.
2014-01-01
Coherent twin boundaries (CTBs) in nano-twinned materials could improve crack resistance. However, the role of the CTBs during crack penetration has never been explored at atomic scale. Our in situ observation on nano-twinned Ag under a high resolution transmission electron microscope (HRTEM) reveals the dynamic processes of a crack penetration across the CTBs, which involve alternated crack tip blunting, crack deflection, twinning/detwinning and slip transmission across the CTBs. The alternated blunting processes are related to the emission of different types of dislocations at the crack tip and vary with the distance of the crack tip from the CTBs. PMID:24637906
Experimental investigation of creep crack tip deformation using moire interferometry
NASA Astrophysics Data System (ADS)
Kang, B. S.-J.; Zhuang, Y.-N.
High temperature moire interferometry was applied to obtain full-field creep crack tip displacements of a three-point bend Al 2024-T4 specimen under constant temperature of 200 C up to 720 hr. C* was evaluated by the moire data obtained at discrete time intervals. Test results indicated that under steady-state creep condition, the creep crack tip v-displacement rate agreed with the asymptotic solution based on C*-integral, however, no creeping behavior was observed for the crack tip u-displacement field after t = 276 hr. This discrepancy may be due to the initial large creep crack tip blunting and cavitation damage which alter the creep crack tip singular field such that the C*-integral is no longer applicable to characterize steady-state creep crack tip field. It is suggested that the size and shape of material grain boundaries may play an important role on the creeping behavior of the material.
The noncontinuum crack tip deformation behavior of surface microcracks
NASA Astrophysics Data System (ADS)
Morris, W. L.
1980-07-01
The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.
Crack tip plasticity in single crystal UO2: Atomistic simulations
Yongfeng Zhang; Paul C. Millett; Michael Tonks; Bulent Biner; Xiang-Yang Liu; David A. Andersson
2012-11-01
The fracture behavior of single crystal uranium dioxide is studied using molecular dynamics simulations at room temperature. Initially, an elliptical notch is created on either {111} or {110} planes, and tensile loading is applied normal to the crack planes. For cracks on both planes, shielding of crack tips by plastic deformation is observed, and crack extension occurs for crack on {111} planes only. Two plastic processes, dislocation emission and phase transformation are identified at crack tips. The dislocations have a Burgers vector of ?110?/2, and glide on {100} planes. Two metastable phases, the so-called Rutile and Scrutinyite phases, are identified during the phase transformation, and their relative stability is confirmed by separate density- functional-theory calculations. Examination of stress concentration near crack tips reveals that dislocation emission is not an effective shielding mechanism. The formation of new phases may effectively shield the crack provided all phase interfaces formed near the crack tips are coherent, as in the case of cracks residing on {110} planes.
Bruemmer, Stephen M.; Thomas, Larry E.
2010-04-05
The fundamental basis for mechanistic understanding and modeling of SCC remains in question for many systems. Specific mechanisms controlling SCC can vary with changes in alloy characteristics, applied/residual stress or environmental conditions. The local crack electrochemistry, crack-tip mechanics and material metallurgy are the main factors controlling crack growth. These localized properties are difficult or impossible to measure in active cracks. Nevertheless, it is essential to quantitatively interrogate these crack-tip conditions if mechanistic understanding is to be obtained. A major recent advance has been the ability to investigate SCC cracks and crack tips using high-resolution ATEM techniques. ATEM enables the characterization of SCC cracks including trapped tip solution chemistries, corrosion product/film compositions and structures, and elemental composition gradients and defect microstructures along the crack walls and at the crack tip. A wide variety of methods for imaging and analyses at resolutions down to the atomic level can be used to examine the crack and corrosion film characteristics. Surface films and reaction layers have been examined by cross-sectional TEM techniques, but little work had been conducted on environmentally induced internal cracks until that of Lewis and co-workers [1-3] and the current authors [4-17]. This capability combined with modern ATEM techniques has enabled exciting new insights into corrosion processes occurring at buried interfaces and is being used to identify mechanisms controlling IGSCC in boiling water reactor (BWR) and pressurized water reactor (PWR) components. The objective of this paper is to summarize certain results focused on IGSCC of Fe- base and Ni-base stainless alloys in high-temperature water environments. Representative crack-tip examples will be shown to illustrate specific aspects that are characteristic of SCC in the material/environment combinations. Differences and similarities in crack-tip
Measurement and analysis of critical crack tip processes during fatigue crack growth
NASA Technical Reports Server (NTRS)
Davidson, D. L.; Hudak, S. J.; Dexter, R. J.
1985-01-01
The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied.
Stress corrosion crack tip microstructure in nickel-based alloys
Shei, S.A.; Yang, W.J.
1994-04-01
Stress corrosion cracking behavior of several nickel-base alloys in high temperature caustic environments has been evaluated. The crack tip and fracture surfaces were examined using Auger/ESCA and Analytical Electron Microscopy (AEM) to determine the near crack tip microstructure and microchemistry. Results showed formation of chromium-rich oxides at or near the crack tip and nickel-rich de-alloying layers away from the crack tip. The stress corrosion resistance of different nickel-base alloys in caustic may be explained by the preferential oxidation and dissolution of different alloying elements at the crack tip. Alloy 600 (UNS N06600) shows good general corrosion and intergranular attack resistance in caustic because of its high nickel content. Thermally treated Alloy 690 (UNS N06690) and Alloy 600 provide good stress corrosion cracking resistance because of high chromium contents along grain boundaries. Alloy 625 (UNS N06625) does not show as good stress corrosion cracking resistance as Alloy 690 or Alloy 600 because of its high molybdenum content.
The analysis of crack tip fields in ferroelastic materials
NASA Astrophysics Data System (ADS)
Carka, Dorinamaria; Landis, Chad M.
2011-09-01
The stress and strain fields near a stationary crack in a ferroelastic material are analyzed. The constitutive response of the material is taken to be characteristic of a polycrystalline sample assembled from randomly oriented tetragonal single crystal grains. The constitutive law accounts for the strain saturation, asymmetry in tension versus compression, Bauschinger effects, reverse switching, and strain reorientation that can occur in these materials due to the non-proportional loading that can arise near a crack tip. Detailed finite element calculations are carried out to determine the stress and strain fields in the vicinity of the crack tip, and to compute values for the J-integral on various integration paths around the tip. The results of the calculations are discussed in relation to results for growing cracks and for stationary cracks in standard elastic-plastic materials.
Intergranular Strain Evolution near Fatigue Crack Tips in Polycrystalline Metals
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.
An analysis of ductile rupture modes at a crack tip
NASA Astrophysics Data System (ADS)
Needleman, A.; Tvergaard, V.
A N ELASTIC-VISCOPLASTIC model of a ductile, porous solid is used to study the influence of the nucleation and growth of micro-voids in the material near the tip of a crack. Conditions of small scale yielding are assumed, and the numerical analyses of the stress and strain fields are based on finite strain theory, so that crack tip blunting is fully accounted for. An array of large inclusions or inclusion colonies, with a relatively low strength, results in large voids near the crack tip at a rather early stage, whereas small second phase particles in the matrix material between the inclusions require large strains before cavities nucleate. Various distributions of the large inclusions, and various critical strains for nucleation of the small scale voids between the inclusions, are considered. Localization of plastic flow plays an important role in determining the failure path between the crack tip and the nearest larger void, and the path is strongly sensitive to the distribution of the large inclusions. Values of the J-integral and the crack opening displacement at fracture initiation are estimated, together with values of the tearing modulus during crack growth, and these values are related to experimental results.
Crack tip field and fatigue crack growth in general yielding and low cycle fatigue
NASA Technical Reports Server (NTRS)
Minzhong, Z.; Liu, H. W.
1984-01-01
Fatigue life consists of crack nucleation and crack propagation periods. Fatigue crack nucleation period is shorter relative to the propagation period at higher stresses. Crack nucleation period of low cycle fatigue might even be shortened by material and fabrication defects and by environmental attack. In these cases, fatigue life is largely crack propagation period. The characteristic crack tip field was studied by the finite element method, and the crack tip field is related to the far field parameters: the deformation work density, and the product of applied stress and applied strain. The cyclic carck growth rates in specimens in general yielding as measured by Solomon are analyzed in terms of J-integral. A generalized crack behavior in terms of delta is developed. The relations between J and the far field parameters and the relation for the general cyclic crack growth behavior are used to analyze fatigue lives of specimens under general-yielding cyclic-load. Fatigue life is related to the applied stress and strain ranges, the deformation work density, crack nucleus size, fracture toughness, fatigue crack growth threshold, Young's modulus, and the cyclic yield stress and strain. The fatigue lives of two aluminum alloys correlate well with the deformation work density as depicted by the derived theory. The general relation is reduced to Coffin-Manson low cycle fatigue law in the high strain region.
In situ examination of moving crack tips in ordered intermetallics.
Heuer, J.; Lam, N. Q.; Okamoto, P. R.; Stubbins, J. F.
1999-01-25
Recent studies have shown that high stress concentrations at moving crack tips in the intermetallic compound NiTi can induce a crystalline-to-amorphous (C-A) transformation of the crack tip region. This stress-induced C-A transformation has a temperature dependence and crystallization behavior similar to those of ion irradiation-induced C-A transformation of NiTi. The present study examines if these similarities between stress- and irradiation-induced amorphization hold true for two other intermetallic compounds, CuTi and Ni{sub 3}Ti. In situ straining was performed in an intermediate-voltage transmission electron microscope. The presence or absence of an amorphous phase was determined by dark field imaging and selected area diffraction of crack tip regions. Crack tips in both CuTi and Ni{sub 3}Ti were found to remain crystalline upon fracture. The observed absence of stress-induced amorphization in Ni{sub 3}Ti is consistent with its known absence during irradiation, but the absence in CuTi differs from its known irradiation-induced amorphization behavior. Reasons for the similarity and difference are discussed.
3D crack tip fields for FCC single crystals
Cuitino, A.M.; Ortiz, M.
1995-12-31
Cracks in single crystals are of concern in a number of structural and non-structural applications, ranging form single-crystal turbine blades and rotors to metal interconnect lines in microcircuits. In this paper we present 3D numerical simulations of the crack-tip fields of a Cu single crystal, including stress, strain and slip activity patterns. The orientation of the crack tip is along the crystallographic orientation (101), while the crack plane is (010). A material model based on dislocation mechanics is used in these simulations. This model correctly predicts the observed behavior of Cu, including the basic hardening characteristics of single crystals, orientation dependence and stage I-II-III structure of the stress-strain curves, the observed levels of latent hardening and their variation with orientation and deformation in the primary system and slip activities and dislocation densities. We use the FEM within the context of finite deformation plasticity. In the figure below, we show the finite element mesh composed by 12-noded tetrahedrons with 6-noded triangular faces. The model simulates half of a beam, which is subjected to a concentrated load at 1/8 of total length from the support. Detailed results of the stress, deformation and slip activity are presented at different radii from crack tip and at different depths from the surface. In general, the results show a strong difference in the slip activity pattern form the interior to the exterior, while smaller differences are encountered in the stress and strain fields.
Genetic identification of crack-tip parameters using thermoelastic isopachics
NASA Astrophysics Data System (ADS)
Dulieu-Barton, J. M.; Worden, K.
2003-02-01
A curve-fitting routine based on a genetic algorithm is used to generate noise-free cardioid curves from thermoelastic data obtained from the neighbourhood of a crack-tip. The technique is benchmarked using simulated thermoelastic data. The generated cardioid curves are used to determine the stress intensity factors (SIFs) from slots and real cracks loaded in mode 1 and mixed-mode. The derived SIFs show excellent agreement with theory and confirm the validity of the approach. The curve-fitting approach allows more information to be obtained from the thermoelastic data than previous analysis routines and will provide a basis for further development of thermoelastic stress analysis for application in crack-tip stress studies.
CRACK TIP OPENING DISPLACEMENT AND ANGLE FOR A GROWING CRACK IN CARBON STEEL
LAM, POH-SANG
2005-01-18
The crack tip opening displacements and angles (CTOD/CTOA) are calculated with finite element method based on the test data of a set of constraint-dependent J-R curves for A285 carbon steel. The values of the CTOD/CTOA are initially high at initiation, but rapidly decrease to a nearly constant value. When the common practice is adopted by using only the constant part of CTOD/CTOA as the fracture criterion, the crack growth behavior is shown to be severely underestimated. However, with a bilinear form of CTOD/CTOA fracture criterion which approximates the initial non-constant portion, the experimental load vs. crack extension curves can be closely predicted. Furthermore, it is demonstrated that the CTOD/CTOA is crack tip constraint dependent. The values of CTOD/CTOA for specimens with various ratios of crack length to specimen width (a/W) are reflected by the J-R curves and their slopes.
Near tip stress and strain fields for short elastic cracks
NASA Technical Reports Server (NTRS)
Soediono, A. H.; Kardomateas, G. A.; Carlson, R. L.
1994-01-01
Recent experimental fatigue crack growth studies have concluded an apparent anomalous behavior of short cracks. To investigate the reasons for this unexpected behavior, the present paper focuses on identifying the crack length circumstances under which the requirements for a single parameter (K(sub I) or delta K(sub I) if cyclic loading is considered) characterization are violated. Furthermore, an additional quantity, the T stress, as introduced by Rice, and the related biaxiality ratio, B, are calculated for several crack lengths and two configurations, the single-edge-cracked and the centrally-cracked specimen. It is postulated that a two-parameter characterization by K and T (or B) is needed for the adequate description of the stress and strain field around a short crack. To further verify the validity of this postulate, the influence of the third term of the Williams series on the stress, strain and displacement fields around the crack tip and in particular on the B parameter is also examined. It is found that the biaxiality ratio would be more negative if the third term effects are included in both geometries. The study is conducted using the finite element method with linearly elastic material and isoparametric elements and axial (mode I) loading. Moreover, it is clearly shown that it is not proper to postulate the crack size limits for 'short crack' behavior as a normalized ratio with the specimen width, a/w; it should instead be stated as an absolute, or normalized with respect to a small characteristic dimension such as the grain size. Finally, implications regarding the prediction of cyclic (fatigue) growth of short cracks are discussed.
Finite strain crack tip fields in soft incompressible elastic solids.
Krishnan, Venkat R; Hui, Chung Yuen; Long, Rong
2008-12-16
A finite element model (FEM) is used to study the behavior of the large deformation field near the tip of a crack in a soft incompressible plane stress fracture specimen loaded in mode I. Results are obtained for the case of a neo-Hookean solid (ideal rubber) and a hyperelastic solid with exponentially hardening behavior. In contrast to the predictions of linear elastic fracture mechanics (LEFM), the near tip stress fields are dominated by the opening stress which shows a 1/R singularity for the neo-Hookean material and a -1/(R ln R) singularity for the exponential hardening solid. We found very similar qualitative behavior in the near tip stress fields despite the very large difference in strain hardening behavior of the two material models. Our result shows that the near tip opening stress is controlled by the far field energy release rate for large applied loads. PMID:19053624
Advanced turbine blade tip seal system
NASA Technical Reports Server (NTRS)
Zelahy, J. W.
1981-01-01
An advanced blade/shroud system designed to maintain close clearance between blade tips and turbine shrouds and at the same time, be resistant to environmental effects including high temperature oxidation, hot corrosion, and thermal cycling is described. Increased efficiency and increased blade life are attained by using the advanced blade tip seal system. Features of the system include improved clearance control when blade tips preferentially wear the shrouds and a superior single crystal superalloy tip. The tip design, joint location, characterization of the single crystal tip alloy, the abrasive tip treatment, and the component and engine test are among the factors addressed. Results of wear testing, quality control plans, and the total manufacturing cycle required to fully process the blades are also discussed.
Crack tip blunting and cleavage under dynamic conditions
NASA Astrophysics Data System (ADS)
Rajan, V. P.; Curtin, W. A.
2016-05-01
In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures (Lin et al., 1987). Here, crack tip blunting and cleavage under dynamic conditions are explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: (1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and (2) what happens in the dynamic "overloaded" situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed as a "dynamic cleavage test", in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, ductile crack tip behavior ultimately prevails since a single emission event can blunt and arrest the crack, preventing further cleavage. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears unrelated to dynamic effects at the crack tip.
Effect of material damage on the stress-strain state near a crack tip in creep
NASA Astrophysics Data System (ADS)
Astaf'ev, V. I.; Grigorova, T. V.; Pastukhov, V. A.
1992-02-01
The asymptotic stress and damage fields near the tip of a growing crack are determined for a creep-damaged material described by Rabotnov-Hayhurst-Leckie constitutive equations. It is found that the singular stress field, characteristic of the crack theory, is absent near the crack tip, which is consistent with the results of finite element solutions for tearing cracks. A crack growth law is obtained which provides a qualitative description of the crack growth process in stainless steels under constant loading.
FURTHER EXAMINATION OF CRACK TIP MICROSTRUCTURES IN F82H ON THE LOWER SHELF
Gelles, David S.; Odette, George R.; Spatig, P.
2003-09-03
Dislocation microstructures have been further examined near the crack tip of a compact tension specimen of unirradiated F82H loaded to 25.6 MPa m square root at –196 degrees C after fatigue precracking. A specimen was prepared by sectioning, dimple grinding and ion milling to produce electron transparency just ahead of the crack tip. Further ion milling has allowed improved examination of the microstructure immediately ahead of the crack tip. It is found that subgrain structure is relatively unaffected near the crack tip whereas 3 micron from the crack tip, dislocation loop structure was identified.
Advanced optical blade tip clearance measurement system
NASA Technical Reports Server (NTRS)
Ford, M. J.; Honeycutt, R. E.; Nordlund, R. E.; Robinson, W. W.
1978-01-01
An advanced electro-optical system was developed to measure single blade tip clearances and average blade tip clearances between a rotor and its gas path seal in an operating gas turbine engine. This system is applicable to fan, compressor, and turbine blade tip clearance measurement requirements, and the system probe is particularly suitable for operation in the extreme turbine environment. A study of optical properties of blade tips was conducted to establish measurement system application limitations. A series of laboratory tests was conducted to determine the measurement system's operational performance characteristics and to demonstrate system capability under simulated operating gas turbine environmental conditions. Operational and environmental performance test data are presented.
Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy.
Withers, P J
2015-03-01
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
Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy
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
An investigation of crack-tip stress field criteria of predicting cleavage-crack initiation
Keeney-Walker, J.; Bass, B.R.; Landes, J.D. )
1991-09-01
Cleavage-crack initiation in large-scale wide-plate (WP) specimens could not be accurately predicted from small, compact (CT) specimens by using a linear-elastic fracture-mechanics, K{sub Ic}, methodology. In the wide-plate tests conducted by the Heavy-Section Steel Technology Program at Oak Ridge National Laboratory, crack initiation has consistently occurred at stress-intensity (K{sub I}) values ranging from two to four times those predicted by the CT specimens. Studies were initiated to develop crack-tip stress field criteria incorporating effects of geometry, size, and constraint that will lead to improved predictions of cleavage initiation in WP specimens from CT specimens. The work centers around nonlinear two-and three-dimensional finite-element analyses of the crack-tip stress fields in these geometries. Analyses were conducted on CT and WP specimens for which cleavage initiation fracture had been measured in laboratory tests. The local crack-tip field generated for these specimens were then used in the evaluation of fracture correlation parameters to augment the K{sub I} parameter for predicting cleavage initiation. Parameters of hydrostatic constraint and of maximum principal stress, measured volumetrically, are included in these evaluations. The results suggest that the cleavage initiation process can be correlated with the local crack-tip fields via a maximum principal stress criterion based on achieving a critical area within a critical stress contour. This criterion has been successfully applied to correlate cleavage initiation in 2T-CT and WP specimen geometries. 23 refs., 16 figs., 5 tabs.
Crack tip shielding observed with high-resolution transmission electron microscopy
Adhika, Damar Rastri; Tanaka, Masaki; Daio, Takeshi; Higashida, Kenji
2015-01-01
The dislocation shielding field at a crack tip was experimentally proven at the atomic scale by measuring the local strain in front of the crack tip using high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA). Single crystalline (110) silicon wafers were employed. Cracks were introduced using a Vickers indenter at room temperature. The crack tip region was observed using HRTEM followed by strain measurements using GPA. The measured strain field at the crack tip was compressive owing to dislocation shielding, which is in good agreement with the strain field calculated from elastic theory. PMID:26115957
Microstructure characterization and thermal behavior around crack tip under electropulsing
NASA Astrophysics Data System (ADS)
Wei, Shaopeng; Wang, Gang; Deng, Dewei; Rong, Yiming
2015-10-01
Electropulsing treatment is a practical method to arrest crack propagation. The microstructure characterization and research on the forming mechanism are difficult due to the small affected area (0.01-1 mm2), high-temperature gradient (102 K/mm) and change rate (104-107 K/s). In this paper, the 1045 steel plate with a preexisting crack subjected to high-voltage pulses was investigated. The surface morphologies and microstructure around the crack tip were observed using optical microscopy and scanning electron microscopy. Experimental results showed that the material around the tip melted, splashed and blunted under electropulsing treatment. The microstructure around the molten hole was divided into four distinct regions. An electro-thermal coupled model considering material ejection, cavity formation, current oscillation and temperature-dependent material properties was proposed to investigate the dynamic formation process of molten hole and gradient microstructure. The uneven temperature distribution, high cooling rate and insufficient carbon diffusion led to the formation of gradient microstructure.
Tipping point analysis of cracking in reinforced concrete
NASA Astrophysics Data System (ADS)
Perry, M.; Livina, V.; Niewczas, P.
2016-01-01
In this work, we demonstrate that tipping point analysis of strain data can provide reactive and predictive indicators of cracking and structural transitions in a reinforced concrete system. The method is able to detect trend-driven transitions in a short time series of approximately 2000 datapoints, providing a clear indication of when a concrete beam under gradual bending progresses from a linear to a nonlinear strain response. The method is also able to provide an early warning signal of the appearance of bifurcations, such as cracks, with a forewarning of 200-500 datapoints. The method, which was originally developed for applications in geophysics, shows promising results in the area of structural health monitoring, in particular, for real-time observations of civil constructions.
NASA Astrophysics Data System (ADS)
Tsamasphyros, G. J.; Kanderakis, G. N.; Marioli-Riga, Z. P.
2003-05-01
Composite patch repair of metallic structures has become a rapidly grown technology in the aerospace field due to the demand for significant increases in the useful life of both military and civilian aircraft. This has led to significant advances overall in the repair technology of cracked metallic structures. Adhesively bonded composite reinforcements offer remarkable advantages such as mechanical efficiency, repair time, cost reduction, high structural integrity, repair inspectability, damage tolerance to further causes of future strains, anticorrosion and antifretting properties. However, because of the different nature and properties of the materials that form a repair (metals, composites, adhesives), side-effects may occur: debonding due to high stress concentration in the vicinity of the crack, thermal residual stresses because of different thermal expansion coefficients of the adherents, etc. In this paper a three-dimensional finite elements analysis of the area around a patch repaired crack of a typical aircraft fuselage is performed, taking into account both the properties and the geometry of the involved materials. Examined in this case are 2024-T3 aluminum alloy as base material, FM-73 as the adhesive system and F4/5521 boron/epoxy prepreg as the patch material. Through the thickness stresses near the crack tip and along the patch edges with and without temperature effects are calculated and debonding near the crack tip is examined. Finally, the calculated results are compared with existing theories.
NASA Astrophysics Data System (ADS)
Xiao, J.; Qiu, S. Y.; Chen, Y.; Fu, Z. H.; Lin, Z. X.; Xu, Q.
2015-01-01
Alloy 690(TT) is widely used for steam generator tubes in pressurized water reactor (PWR), where it is susceptible to corrosion fatigue. In this study, the corrosion fatigue behavior of Alloy 690(TT) in simulated PWR environments was investigated. The microstructure of the plastic zone near the crack tip was investigated and labyrinth structures were observed. The relationship between the crack tip plastic zone and fatigue crack growth rates and the environment factor Fen was illuminated.
The structure of the near-tip field during transient elastodynamic crack growth
NASA Astrophysics Data System (ADS)
Freund, L. B.; Rosakis, A. J.
T HE PROCESS of dynamic crack growth in a nominally elastic malerial under conditions of plane strain or plane stress is considered. Of particular concern is the influence of the transient nature of the process on the stress field in the immediate vicinity of the crack tip during nonsteady growth. Asymptotically, the crack tip stress field is square root singular at the crack tip, with the angular variation of the singular field depending weakly on the instantaneous crack tip speed and with the instantaneous stress intensity factor being a scalar multiplier of the singular field. However, for a material particle at a small distance from the moving crack, the local stress field depends not only on instantaneous values of crack speed and stress intensity factor, but also on the past history of these lime-dependent quantities. A representation of the crack tip field is obtained in the form of an expansion about the crack up in powers of radial coordinate, with the coefficients depending on the time rates of change of crack tip speed and stress intensity factor. This representation is used to interpret some experimental observations, with the conclusion that the higher-order expansion provides an accurate description of crack tip fields under fairly severe transient conditions. In addition, some estimates are made of the practical limits of using a stress intensity factor field alone to characterize the local fields.
Microscopic observation of the tips of fast running cracks in PMMA
NASA Astrophysics Data System (ADS)
Zimmermann, C.; Schönert, K.
ALTHOUGH various theoretical models exist, little experimental data is available on the material behaviour in the ultimate vicinity of the tip of fast running cracks. Using a microscope coupled image converter camera, the tips of cracks running in PMMA (polymethylmethacrylate) at speeds between 250 m s -1 and 680 m s -1 were photographed. Due to the high aperture of the optical set-up, shadow optical effects could be greatly reduced. Thus it was possible to observe the contour of the crack flanks up to the crack tip, revealing the existence of fibrils in between the flanks. Seemingly the appearance of these fibrils is connected with the onset of crack branching. Having the crack pass a microlattice, which had been vapor deposited onto the specimen surface, the displacements around the crack tip could be determined. The recorded plastic zone is of triangular shape. Experimental results are compared with theoretical predictions.
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1982-01-01
An elastic-plastic (incremental and small strain) finite element analysis was used with a crack growth criterion to study crack initiation, stable crack growth, and instability under monotonic loading to failure of metallic materials. The crack growth criterion was a critical crack-tip-opening displacement (CTOD) at a specified distance from the crack tip, or equivalently, a critical crack-tip-opening angle (CTOA). Whenever the CTOD (or CTOA) equaled or exceeded a critical value, the crack was assumed to grow. Single values of critical CTOD were used in the analysis to model crack initiation, stable crack growth, and instability for 7075-T651 and 2024-T351 aluminum alloy compact specimens. Calculated and experimentally measured CTOD values at initiation agreed well for both aluminum alloys. These critical CTOD values were also used to predict failure loads on center-crack tension specimens and a specially-designed three-hole-crack tension specimen made of the two aluminum alloys and of 304 stainless steel. All specimens were 12.7 mm thick. Predicted failure loads for 7075-T651 aluminum alloy and 304 stainless steel specimens were generally within + or - 15 percent of experimental failure loads, whereas the predicted failure loads for 2024-T351 aluminum alloy specimens were generally within + or - 5 percent of the experimental loads.
NASA Technical Reports Server (NTRS)
Dawicke, D. S.; Sutton, M. A.
1993-01-01
The stable tearing behavior of thin sheets 2024-T3 aluminum alloy was studied for middle crack tension specimens having initial cracks that were: flat cracks (low fatigue stress) and 45 degrees through-thickness slant cracks (high fatigue stress). The critical crack-tip-opening angle (CTOA) values during stable tearing were measured by two independent methods, optical microscopy and digital image correlation. Results from the two methods agreed well. The CTOA measurements and observations of the fracture surfaces showed that the initial stable tearing behavior of low and high fatigue stress tests is significantly different. The cracks in the low fatigue stress tests underwent a transition from flat-to-slant crack growth, during which the CTOA values were high and significant crack tunneling occurred. After crack growth equal to about the thickness, CTOA reached a constant value of 6 deg and after crack growth equal to about twice the thickness, crack tunneling stabilized. The initial high CTOA values, in the low fatigue crack tests, coincided with large three-dimensional crack front shape changes due to a variation in the through-thickness crack tip constraint. The cracks in the high fatigue stress tests reach the same constant CTOA value after crack growth equal to about the thickness, but produced only a slightly higher CTOA value during initial crack growth. For crack growth on the 45 degree slant, the crack front and local field variables are still highly three-dimensional. However, the constant CTOA values and stable crack front shape may allow the process to be approximated with two-dimensional models.
Experimental characterization of crack tip deformation fields in Alloy 718 at high temperatures
Liu, J.; Lyons, J.; Sutton, M.; Reynolds, A.
1998-01-01
A series of fracture mechanics tests were conducted at temperatures of 650 C and 704 C in air, using Inconel 719. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 C during crack growth, and confirm that K{sub 1} is a viable continuum-based fracture parameter for creep crack growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.
Discrete dislocation plasticity and crack tip fields in single crystals
NASA Astrophysics Data System (ADS)
Van der Giessen, E.; Deshpande, V. S.; Cleveringa, H. H. M.; Needleman, A.
2001-09-01
Small-scale yielding around a stationary plane strain mode I crack is analyzed using discrete dislocation plasticity. The dislocations are all of edge character, and are modeled as line singularities in a linear elastic material. Superposition is used to represent the solution in terms of analytical fields for edge dislocations in a half-space and a numerical image solution that enforces the boundary conditions. The description of the dislocation dynamics includes the lattice resistance to dislocation motion, dislocation nucleation, interaction with obstacles and annihilation. A model planar crystal with three slip systems is considered. Two slip system orientations are analyzed that differ by a 90° rotation. The non-hardening, single crystal plasticity continuum slip solution of Rice (Mech. Mater. 6 (1987) 317) for this model crystal predicts that slip and kink bands emerge for both crystal geometries, while Drugan (J. Mech. Phys. Solids 49 (2001) 2155) has obtained kink band free solutions. For a reference set of parameter values, kink band free solutions are found in one orientation while the emergence of kink bands is seen in the other orientation. However, lowering the dislocation source density suppresses the formation of kink bands in this orientation as well. In all calculations, the opening stress in the immediate vicinity of the crack tip is much larger than predicted by continuum slip theory.
Sub-10-micrometer toughening and crack tip toughness of dental enamel.
Ang, Siang Fung; Schulz, Anja; Pacher Fernandes, Rodrigo; Schneider, Gerold A
2011-04-01
In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited R-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip's toughness (K(I0),K(III0)), the crack's closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine enamel was studied using atomic force microscopy (AFM). The mode I crack tip toughness K(I0) of cracks along enamel rod boundaries and across enamel rods exhibit a similar range of values: K(I0,Ir)=0.5-1.6MPa m(0.5) (based on Irwin's 'near-field' solution) and K(I0,cz)=0.8-1.5MPa m(0.5) (based on the cohesive zone solution of the Dugdale-Muskhelishvili (DM) crack model). The mode III crack tip toughness K(III0,Ir) was computed as 0.02-0.15MPa m(0.5). The crack-closure stress at the crack tip was computed as 163-770 MPa with a cohesive zone length and width 1.6-10.1μm and 24-44 nm utilizing the cohesive zone solution. Toughening elements were observed under AFM and SEM: crack bridging due to protein ligament and hydroxyapatite fibres (micro- and nanometer scale) as well as microcracks were identified. PMID:21316630
Environmental fatigue of an Al-Li-Cu alloy. Part 3: Modeling of crack tip hydrogen damage
NASA Technical Reports Server (NTRS)
Piascik, Robert S.; Gangloff, Richard P.
1992-01-01
Environmental fatigue crack propagation rates and microscopic damage modes in Al-Li-Cu alloy 2090 (Parts 1 and 2) are described by a crack tip process zone model based on hydrogen embrittlement. Da/dN sub ENV equates to discontinuous crack advance over a distance, delta a, determined by dislocation transport of dissolved hydrogen at plastic strains above a critical value; and to the number of load cycles, delta N, required to hydrogenate process zone trap sites that fracture according to a local hydrogen concentration-tensile stress criterion. Transgranular (100) cracking occurs for process zones smaller than the subgrain size, and due to lattice decohesion or hydride formation. Intersubgranular cracking dominates when the process zone encompasses one or more subgrains so that dislocation transport provides hydrogen to strong boundary trapping sites. Multi-sloped log da/dN-log delta K behavior is produced by process zone plastic strain-hydrogen-microstructure interactions, and is determined by the DK dependent rates and proportions of each parallel cracking mode. Absolute values of the exponents and the preexponential coefficients are not predictable; however, fractographic measurements theta sub i coupled with fatigue crack propagation data for alloy 2090 established that the process zone model correctly describes fatigue crack propagation kinetics. Crack surface films hinder hydrogen uptake and reduce da/dN and alter the proportions of each fatigue crack propagation mode.
Investigation on Crack Tip Transformation in NiTi Alloys: Effect of the Temperature
NASA Astrophysics Data System (ADS)
Sgambitterra, Emanuele; Maletta, Carmine; Furgiuele, Franco
2015-06-01
The effect of the temperature on crack tip transformation in Nickel-Titanium (NiTi) shape memory alloys was analyzed in this work by means of experimental and analytical approaches. In particular, single edge crack specimens were analyzed for two different values of the testing temperature in the pseudoelastic regime of the alloy, i.e., T = 298 K and T = 338 K. The thermal-dependent phase transition mechanisms occurring at the crack tip region were studied by analyzing data obtained from digital image correlation as well as by nanoindentation experiments performed near the crack tip. Finally, experimental results were compared with predictions of a recent analytical model. Results revealed that an increase in temperature causes a decrease of the phase transformation zone and that both the techniques are well suitable in capturing the thermal effect on the phase transformation mechanisms near the crack tip.
Fatigue-crack propagation in advanced aerospace materials: Aluminum-lithium alloys
Venkateswara Rao, K.T.; Ritchie, R.O.
1988-10-01
Characteristics of fatigue-crack propagation behavior are reviewed for recently developed commercial aluminum-lithium alloys, with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage-tolerant design. Specifically, crack-growth kinetics in Alcoa 2090-T8E41, Alcan 8090 and 8091, and Pechiney 2091 alloys, and in certain powder-metallurgy alloys, are examined as a function of microstructure, plate orientation, temperature, crack size, load ratio and loading sequence. In general, it is found that growth rates for long (> 10 mm) cracks are nearly 2--3 orders of magnitude slower than in traditional 2000 and 7000 series alloys at comparable stress-intensity levels. In additions, Al-Li alloys shown enhanced crack-growth retardations following the application of tensile overloads and retain superior fatigue properties even after prolonged exposure at overaging temperatures; however, they are less impressive in the presence of compression overloads and further show accelerated crack-growth behavior for microstructurally-small (2--1000 {mu}m) cracks (some three orders of magnitude faster than long cracks). These contrasting observations are attributed to a very prominent role of crack-tip shielding during fatigue-crack growth in Al-Li alloys, promoted largely by the tortuous and zig-zag nature of the crack-path morphologies. Such crack paths result in locally reduced crack-tip stress intensities, due to crack deflection and consequent crack wedging from fracture-surface asperities (roughness-induced crack closure); however, such mechanisms are far less potent in the presence of compressive loads, which act to crush the asperities, and for small cracks, where the limited crack wake severely restricts the shielding effect. 50 refs., 21 figs.
NASA Astrophysics Data System (ADS)
Nibur, Kevin A.; Somerday, Brian P.; Marchi, Chris San; Foulk, James W.; Dadfarnia, Mohsen; Sofronis, Petros
2013-01-01
Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. Thresholds for crack extension under rising displacement, K THi, for crack extension under constant displacement, K_{{THi}}^{*} , and for crack arrest under constant displacement K THa, were identified. These values were not found to be equivalent, i.e. K THi < K THa < K_{{THi}}^{*} . The hydrogen assisted fracture mechanism was determined to be strain controlled for all of the alloys in this study, and the micromechanics of strain controlled fracture are used to explain the observed disparities between the different threshold measurements. K THa and K THi differ because the strain singularity of a stationary crack is stronger than that of a propagating crack; K THa must be larger than K THi to achieve equivalent crack tip strain at the same distance from the crack tip. Hydrogen interacts with deformation mechanisms, enhancing strain localization and consequently altering both the nucleation and growth stages of strain controlled fracture mechanisms. The timing of load application and hydrogen exposure, i.e., sequential for constant displacement tests and concurrent for rising displacement tests, leads to differences in the strain history relative to the environmental exposure history and promotes the disparity between K_{{THi}}^{*} and K THi. K THi is the only conservative measurement of fracture threshold among the methods presented here.
Examination of dislocation structures near crack tip region of B2 NiAl alloys
Ng, B.C.; Simkin, B.; Crimp, M.A.
1997-12-31
The structure and distribution of dislocations in the region of crack tips and crack edges have been examined in single crystal stoichiometric B2 NiAl using electron channeling contrast imaging (ECCI). ECCI, which allows examination of near surface substructural crystalline defects in bulk specimens, has been carried out for undeformed as well as in-situ deformed 4-point bend specimens. Images of dislocations at crack tips and along the crack fracture surfaces have been studied. Observations have been correlated with variations in thermal treatment corresponding with brittle and toughened conditions. A higher dislocation density was observed over a larger region in front of crack tips for the toughened materials as compared to brittle material.
The Stress-Strain Condition Estimation of Detail in Crack Tip by Integral Strain Gauges
NASA Astrophysics Data System (ADS)
Syzrantsev, V.; Syzrantseva, K.
2016-04-01
The paper considers the task of stress-strain condition calculation of experimental sample in fatigue crack tip on weld boundary at its cyclic deforming. For this task decision authors use the information obtained by original means of cyclic strains measurement: Integral Strain Gauges. The results of carried experimental researches are compared with data of stress-strain condition estimation of detail in crack tip calculated by Finish Element Method.
Modeling of crack tip dislocation emission in B2 intermetallic alloys
NASA Astrophysics Data System (ADS)
Bartholomeusz, Michael F.; Wert, John A.
1992-04-01
A model has been previously proposed to describe the energy associated with emission of dissociated superlattice dislocations from crack tips in ordered intermetallic alloys. In the present paper, the model is applied to several B2 intermetallic alloys. The results of the analysis reveal a correlation between the range of slip system orientations for which emission of a dislocation from a crack tip is energetically favorable and the macroscopic fracture mode of the alloy.
Chan, Kwai S; Chan, Candace K; Nicolella, Daniel P
2009-09-01
The risk of bone fracture increases with age because of a variety of factors that include, among others, decreasing bone quantity and quality. Despite recent advances, the roles of bone microstructure and trace mineralization in the fracture process are not well understood. In this study, we utilize a combination of in-situ fracture toughness testing, digital strain mapping, and X-ray photoelectron spectroscopy techniques to characterize the near-tip strain field, fracture toughness, and chemical elements on the fracture surface of bone specimens from donors of two ages (48-year-old and 78-year-old females). We show that age-related embrittlement of bone fracture is associated with higher near-tip strains by lamellar shear and crack deflection at lamellar interfaces in the young bone and their absence in the old bone. The different near-tip deformation behaviors may be associated with the presence of Si and Zn in the young bone but more Ca and P and the lack of Si and Zn in the old bone. PMID:19497396
Finite element solutions for crack-tip behavior in small-scale yielding
NASA Technical Reports Server (NTRS)
Tracey, D. M.
1976-01-01
The subject considered is the stress and deformation fields in a cracked elastic-plastic power law hardening material under plane strain tensile loading. An incremental plasticity finite element formulation is developed for accurate analysis of the complete field problem including the extensively deformed near tip region, the elastic-plastic region, and the remote elastic region. The formulation has general applicability and was used to solve the small scale yielding problem for a set of material hardening exponents. Distributions of stress, strain, and crack opening displacement at the crack tip and through the elastic-plastic zone are presented as a function of the elastic stress intensity factor and material properties.
Gołebiowski, B; Swiatnicki, W A; Gaspérini, M
2010-03-01
Microstructural changes occurring during fatigue tests of austenitic-ferritic duplex stainless steel (DSS) in air and in hydrogen-generating environment have been investigated. Hydrogen charging of steel samples during fatigue crack growth (FCG) tests was performed by cathodic polarization of specimens in 0.1M H(2)SO(4) aqueous solution. Microstructural investigations of specimens after FCG tests were carried out using transmission electron microscopy to reveal the density and arrangement of dislocations formed near crack tip. To determine the way of crack propagation in the microstructure, electron backscatter diffraction investigations were performed on fatigue-tested samples in both kinds of environment. To reveal hydrogen-induced phase transformations the atomic force microscopy was used. The above investigations allowed us to define the character of fatigue crack propagation and microstructural changes near the crack tip. It was found that crack propagation after fatigue tests in air is accompanied with plastic deformation; a high density of dislocations is observed at large distance from the crack. After fatigue tests performed during hydrogen charging the deformed zone containing high density of dislocations is narrow compared to that after fatigue tests in air. It means that hydrogenation leads to brittle character of fatigue crack propagation. In air, fatigue cracks propagate mostly transgranularly, whereas in hydrogen-generating environment the cracks have mixed transgranular/interfacial character. PMID:20500395
Three-dimensional observations of magnetic flux density around fatigue crack tips of bearing steels
NASA Astrophysics Data System (ADS)
Kida, Katsuyuki; Santos, Edson C.; Honda, Takashi; Tanabe, Hirotaka
2009-12-01
Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find these cracks. However, at present, it is not easy to distinguish the cracks that will grow fast and cause failure. We developed a three-dimensional scanning Hall probe microscope (3D-SHPM) and observed fatigue cracks at room temperature while they were growing. Four-point-bending fatigue tests were carried out using pre-cracked specimens (JIS-SUJ2, bearing steel). We observed the two-dimensional magnetic flux density distributions around the crack tips and found that there is a strong correlation between the changes in the magnetic flux densities and the crack growth. In order to understand this, we looked into all the three components of the magnetic flux densities, and found that they shape an arched bridge around a crack. We also found that the magnetic flux density moves in front of the crack tip along the crack growth direction.
Three-dimensional observations of magnetic flux density around fatigue crack tips of bearing steels
NASA Astrophysics Data System (ADS)
Kida, Katsuyuki; Santos, Edson C.; Honda, Takashi; Tanabe, Hirotaka
2010-03-01
Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find these cracks. However, at present, it is not easy to distinguish the cracks that will grow fast and cause failure. We developed a three-dimensional scanning Hall probe microscope (3D-SHPM) and observed fatigue cracks at room temperature while they were growing. Four-point-bending fatigue tests were carried out using pre-cracked specimens (JIS-SUJ2, bearing steel). We observed the two-dimensional magnetic flux density distributions around the crack tips and found that there is a strong correlation between the changes in the magnetic flux densities and the crack growth. In order to understand this, we looked into all the three components of the magnetic flux densities, and found that they shape an arched bridge around a crack. We also found that the magnetic flux density moves in front of the crack tip along the crack growth direction.
Effects of Tip Mass on Stability of Rotating Cantilever Pipe Conveying Fluid with Crack
NASA Astrophysics Data System (ADS)
Son, In Soo; Yoon, Han Ik; Lee, Sang Pil; Kim, Dong Jin
In this paper, the dynamic stability of a rotating cantilever pipe conveying fluid with a crack and tip mass is investigated by numerical method. That is, the effects of the rotating the rotating angular velocity, the mass ratio, the crack and tip mass on the critical flow velocity for flutter instability of system are studied. The equations of motion of rotating pipe are derived by using the extended Hamilton's principle. The crack section of pipe is represented by a local flexibility matrix connecting two undamaged pipe segments. The crack is assumed to be in the first mode of fracture and always opened during the vibrations. Finally, the stability maps of the cracked rotating pipe system as a rotating angular velocity and mass ratio β are presented.
NASA Astrophysics Data System (ADS)
Hoh, H. J.; Xiao, Z. M.; Luo, J.
2010-09-01
An analytical investigation on the plastic zone size of a crack near a coated circular inclusion under three different loading conditions of uniaxial tension, uniform tension and pure shear was carried out. Both the crack and coated circular inclusion are embedded in an infinite matrix, with the crack oriented along the radial direction of the inclusion. In the solution procedure, the crack is simulated as a continuous distribution of edge dislocations. With the Dugdale model of small-scale yielding [J. Mech. Phys. Solids 8 (1960) p. 100], two thin strips of yielded plastic zones are introduced at both crack tips. Using the solution for a coated circular inclusion interacting with a single dislocation as the Green's function, the physical problem is formulated into a set of singular integral equations. Using the method of Erdogan and Gupta [Q. J. Appl. Math. 29 (1972) p. 525] and iterative numerical procedures, the singular integral equations are solved numerically for the plastic zone sizes and crack tip opening displacement.
In situ SEM observation of microscale strain fields around a crack tip in polycrystalline molybdenum
NASA Astrophysics Data System (ADS)
Li, J. J.; Li, W. C.; Jin, Y. J.; Wang, L. F.; Zhao, C. W.; Xing, Y. M.; Lang, F. C.; Yan, L.; Yang, S. T.
2016-06-01
In situ scanning electron microscopy was employed to investigate the crack initiation and propagation in polycrystalline molybdenum under uniaxial tensile load at room temperature. The microscale grid pattern was fabricated using the sputtering deposition technology on the specimen surface covered with a fine square mesh copper grid. The microscale strain fields around the crack tip were measured by geometric phase analysis technique and compared with the theoretical solutions based on the linear elastic fracture mechanics theory. The results showed that as the displacement increases, the crack propagated mainly perpendicular to the tensile direction during the fracture process of molybdenum. The normal strain ɛ xx and shear strain ɛ xy are relatively small, and the normal strain ɛ yy holds a dominant position in the deformation fields and plays a key role in the whole fracture process of molybdenum. With the increase in displacement, the ɛ yy increases rapidly and the two lobes grow significantly but maintain the same shape and orientation. The experimental ɛ yy is in agreement with the theoretical solution. Along the x-axis in front of the crack tip, there is minor discrepancy between the experimental ɛ yy and theoretical ɛ yy within 25 μm from the crack tip, but the agreement between them is very good far from the crack tip (>25 μm).
Experimental analysis of crack tip fields in rubber materials under large deformation
NASA Astrophysics Data System (ADS)
Xiao, Xia; Song, Hai-Peng; Kang, Yi-Lan; Li, Xiao-Lei; Tan, Xiao-Hua; Tan, Hao-Yun
2012-04-01
A three-nested-deformation model is proposed to describe crack-tip fields in rubber-like materials with large deformation. The model is inspired by the distribution of the measured in-plane and out-of-plane deformation. The inplane displacement of crack-tip fields under both Mode I and mixed-mode (Mode I-II) fracture conditions is measured by using the digital Moiré method. The deformation characteristics and experimental sector division mode are investigated by comparing the measured displacement fields under different fracture modes. The out-of-plane displacement field near the crack tip is measured using the three-dimensional digital speckle correlation method.
NASA Astrophysics Data System (ADS)
Yamakov, V. I.; Warner, D. H.; Zamora, R. J.; Saether, E.; Curtin, W. A.; Glaessgen, E. H.
2014-04-01
This work investigates the dislocation nucleation processes that occur at the tip of a crack in aluminum under a broad range of crystallographic orientations and temperatures. A concurrent multiscale molecular dynamics - continuum simulation framework is employed. The results are then interpreted using a Peierls continuum model that uses finite temperature material properties derived from molecular dynamics simulation. Under ramped loading, partial dislocation nucleation at the crack tip is found to lead to both full dislocation emission and twinning, depending upon the orientation, temperature, and magnitude of the applied load in the simulation. The origins of the dependencies are made apparent by the Peierls continuum model. The continuum model suggests that in many instances dislocation nucleation from the crack tip can be considered to be a strain rate independent process, yet still temperature dependent through the temperature dependence of the stacking fault energies and elastic constants.
Phase Transformation Surfaces Analysis for SMA Around a Crack Tip with Curvature
NASA Astrophysics Data System (ADS)
Laydi, Mohamed Rachid; Lexcellent, Christian
2015-09-01
Under loading, for a shape memory alloy, the stress concentration around the crack tip, inside a plate, is a serious fracture problem. The tip shape constitutes an important data. In a recent paper (Lexcellent et al. in Int J Fract 169:1-13, 2011), the determination of phase transformation surfaces around a crack tip without curvature was investigated. For crack with curvature, the approximate stress field solution proposed by Creager and Paris is used (Int J Fract 3:247-252, 1967). The knowledge of phase transformation surfaces dimensions and shapes can help us to define a fracture criterion. The study is focussed on mode I because this opening mode is the most dangerous for the structure. Also, comparisons between experiments and computing can be made. Qualitative agreement was obtained.
Computer simulation of creep damage at crack tip in short fibre composites
NASA Astrophysics Data System (ADS)
Shuangyin, Zhang; Tsai, L. W.
1994-08-01
Creep damage at crack tip in short fibre composites has been simulated by using the finite element method (FEM). The well-known Schapery non-linear viscoelastic constitutive relationship was used to characterize time-dependent behaviour of the material. A modified recurrence equation was adopted to accelerate the iteration. Kachanov-Rabotnov's damage evolution law was employed. The growth of the damage zone with time around the crack tip was calculated and the results were shown with the so-called “digit photo”, which was produced by the printer.
NASA Astrophysics Data System (ADS)
Butt, Ali
Crack propagation in a solid rocket motor environment is difficult to measure directly. This experimental and analytical study evaluated the viability of real-time radiography for detecting bore regression and propellant crack propagation speed. The scope included the quantitative interpretation of crack tip velocity from simulated radiographic images of a burning, center-perforated grain and actual real-time radiographs taken on a rapid-prototyped model that dynamically produced the surface movements modeled in the simulation. The simplified motor simulation portrayed a bore crack that propagated radially at a speed that was 10 times the burning rate of the bore. Comparing the experimental image interpretation with the calibrated surface inputs, measurement accuracies were quantified. The average measurements of the bore radius were within 3% of the calibrated values with a maximum error of 7%. The crack tip speed could be characterized with image processing algorithms, but not with the dynamic calibration data. The laboratory data revealed that noise in the transmitted X-Ray intensity makes sensing the crack tip propagation using changes in the centerline transmitted intensity level impractical using the algorithms employed.
Liao, B.; Nan, Y.; Hu, Y.; Kang, D.T.
1998-02-01
The influence of hydrogen on the deformation ahead of the crack tip and the crack propagation were observed and studied in situ under transmission electron microscopy with dynamic tensile deformation for steel. The results show that hydrogen can promote local plastic deformation ahead of the crack tip and change the mode of crack propagation so that the crack will propagate in a zigzag path.
NASA Astrophysics Data System (ADS)
Hartmaier, Alexander; Gumbsch, Peter
2005-01-01
Metals with a body centered cubic crystal structure, like tungsten, exhibit a pronounced semibrittle regime at intermediate temperatures. In this regime their fracture toughness strongly depends on loading rate and temperature. Crack-tip plasticity has been studied with two-dimensional numerical simulations on different length scales. The method of discrete dislocation dynamics has been employed to test various assumptions made on the deformation mechanisms and the origin of the strong loading rate and temperature dependence of fracture toughness in this regime. A continuum elasticity-viscoplasticity model capable of describing larger plastic deformations yields complementary information with respect to the discrete dislocation method. Despite of their fundamental differences, both simulations consistently show that crack-tip plasticity can be described as a time-dependent microplastic deformation with well-defined activation energy and that the blunting of the crack tip plays an important role for the transition from semibrittle to ductile behavior. Based on general findings of the numerical simulations an Arrheniuslike relation between loading rate and temperature at points of constant fracture toughness is derived. This scaling relation shows the dominance of dislocation mobility as the rate limiting factor for fracture toughness and for the brittle-to-ductile transition itself. The results of our simulations are also consistent with experimental data gathered on tungsten single crystals. Thus, the proposed scaling relation can be used to predict fracture toughnesses in a wide range of temperatures and loading rates, based on only a small number of experiments.
Changes in magnetic flux density around fatigue crack tips of carbon tool steels
NASA Astrophysics Data System (ADS)
Honda, Takashi; Kida, Katsuyuki; Santos, Edson C.; Tanabe, Hirotaka
2010-03-01
Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find the cracks which eventually grow to cause failures. However, at present, it is not easy to distinguish, in the early stages of growth, the cracks which will grow fast and cause failure. We hypothesized that it may be possible to distinguish them by comparing changes in the magnetic flux density around the tips of those cracks that grew large enough to cause failure. In order to measure these changes in magnetic flux density, we developed a scanning Hall probe microscope and observed the fatigue cracks growing from artificial slits in carbon tool steels (JIS SKS93). We also compared the changes in magnetic flux density around crack tips which grew under different loads and found that there is a strong correlation between the magnetic flux density, crack growth and stress intensity factors. In order to understand this relation, we measured the changes in the magnetic flux density and residual tensile stress by using an X-ray system, and found that the magnetic flux density changes not only in the plastic deformation area but also in the area of elastic stress field with increased stress.
Changes in magnetic flux density around fatigue crack tips of carbon tool steels
NASA Astrophysics Data System (ADS)
Honda, Takashi; Kida, Katsuyuki; Santos, Edson C.; Tanabe, Hirotaka
2009-12-01
Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components it is important to find the cracks which eventually grow to cause failures. However, at present, it is not easy to distinguish, in the early stages of growth, the cracks which will grow fast and cause failure. We hypothesized that it may be possible to distinguish them by comparing changes in the magnetic flux density around the tips of those cracks that grew large enough to cause failure. In order to measure these changes in magnetic flux density, we developed a scanning Hall probe microscope and observed the fatigue cracks growing from artificial slits in carbon tool steels (JIS SKS93). We also compared the changes in magnetic flux density around crack tips which grew under different loads and found that there is a strong correlation between the magnetic flux density, crack growth and stress intensity factors. In order to understand this relation, we measured the changes in the magnetic flux density and residual tensile stress by using an X-ray system, and found that the magnetic flux density changes not only in the plastic deformation area but also in the area of elastic stress field with increased stress.
Crack-tip strain field mapping and the toughness of metallic glasses.
Hufnagel, Todd C; Vempati, Uday K; Almer, Jonathan D
2013-01-01
We have used high-energy x-ray scattering to map the strain fields around crack tips in fracture specimens of a bulk metallic glass under load at room temperature and below. From the measured strain fields we can calculate the components of the stress tensor as a function of position and determine the size and shape of the plastic process zone around the crack tip. Specimens tested at room temperature develop substantial plastic zones and achieve high stress intensities ((K(If) = 76 MPa m(1/2)) prior to fracture. Specimens tested at cryogenic temperatures fail at reduced but still substantial stress intensities (K(If) = 39 MPa m(1/2)) and show only limited evidence of crack-tip plasticity. We propose that the difference in behavior is associated with changes in the flow stress and elastic constants, which influence the number density of shear bands in the plastic zone and thus the strain required to initiate fracture on an individual band. A secondary effect is a change in the triaxial state of stress around the crack tip due to the temperature dependence of Poisson's ratio. It is likely that this ability to map elastic strains on the microscale will be useful in other contexts, although interpreting shifts in the position of the scattering peaks in amorphous materials in terms of elastic strains must be done with caution. PMID:24386172
Crack-Tip Strain Field Mapping and the Toughness of Metallic Glasses
Hufnagel, Todd C.; Vempati, Uday K.; Almer, Jonathan D.
2013-01-01
We have used high-energy x-ray scattering to map the strain fields around crack tips in fracture specimens of a bulk metallic glass under load at room temperature and below. From the measured strain fields we can calculate the components of the stress tensor as a function of position and determine the size and shape of the plastic process zone around the crack tip. Specimens tested at room temperature develop substantial plastic zones and achieve high stress intensities () prior to fracture. Specimens tested at cryogenic temperatures fail at reduced but still substantial stress intensities () and show only limited evidence of crack-tip plasticity. We propose that the difference in behavior is associated with changes in the flow stress and elastic constants, which influence the number density of shear bands in the plastic zone and thus the strain required to initiate fracture on an individual band. A secondary effect is a change in the triaxial state of stress around the crack tip due to the temperature dependence of Poisson's ratio. It is likely that this ability to map elastic strains on the microscale will be useful in other contexts, although interpreting shifts in the position of the scattering peaks in amorphous materials in terms of elastic strains must be done with caution. PMID:24386172
The effects of crack surface friction and roughness on crack tip stress fields
NASA Technical Reports Server (NTRS)
Ballarini, Roberto; Plesha, Michael E.
1987-01-01
A model is presented which can be used to incorporate the effects of friction and tortuosity along crack surfaces through a constitutive law applied to the interface between opposing crack surfaces. The problem of a crack with a saw-tooth surface in an infinite medium subjected to a far-field shear stress is solved and the ratios of Mode-I stress intensity to Mode-II stress intensity are calculated for various coefficients of friction and material properties. The results show that tortuosity and friction lead to an increase in fracture loads and alter the direction of crack propagation.
The effect of non-singular stresses on crack-tip constraint
NASA Astrophysics Data System (ADS)
Du, Z.-Z.; Hancock, J. W.
T HE EFFECT of the T-stress on the small-scale yielding field of a crack in plane strain conditions has been examined using modified boundary layer formulations. The numerically calculated stresses at the crack tip are represented by slip line fields for small-strain theory. Positive T-stresses cause plasticity to envelop the crack tip and exhibit a Prandtl field, corresponding to the limiting solution of the HRR field for a nonhardening material. Moderate compressive T-stresses reduce the direct stresses within the plastic zone by decreasing the hydrostatic stress by T. This causes a loss of J-dominance, and a stress distribution represented by an incomplete Prandtl field.
Crack-tip chemistry modeling of stage I stress corrosion cracking
Jones, R.H.; Simonen, E.P.
1991-10-01
Stage I stress corrosion cracking usually exhibits a very strong K dependence with Paris law exponents of up to 30. 2 Model calculations indicate that the crack velocity in this regime is controlled by transport through a salt film and that the K dependence results from crack opening controlled salt film dissolution. An ionic transport model that accounts for both electromigration through the resistive salt film and Fickian diffusion through the aqueous solution was used for these predictions. Predicted crack growth rates are in excellent agreement with measured values for Ni with P segregated to the grain boundaries and tested in IN H{sub 2}SO{sub 4} at +900 mV. This salt film dissolution may be applicable to stage I cracking of other materials.
Computation of the tip vortex flowfield for advanced aircraft propellers
NASA Technical Reports Server (NTRS)
Tsai, Tommy M.; Dejong, Frederick J.; Levy, Ralph
1988-01-01
The tip vortex flowfield plays a significant role in the performance of advanced aircraft propellers. The flowfield in the tip region is complex, three-dimensional and viscous with large secondary velocities. An analysis is presented using an approximate set of equations which contains the physics required by the tip vortex flowfield, but which does not require the resources of the full Navier-Stokes equations. A computer code was developed to predict the tip vortex flowfield of advanced aircraft propellers. A grid generation package was developed to allow specification of a variety of advanced aircraft propeller shapes. Calculations of the tip vortex generation on an SR3 type blade at high Reynolds numbers were made using this code and a parametric study was performed to show the effect of tip thickness on tip vortex intensity. In addition, calculations of the tip vortex generation on a NACA 0012 type blade were made, including the flowfield downstream of the blade trailing edge. Comparison of flowfield calculations with experimental data from an F4 blade was made. A user's manual was also prepared for the computer code (NASA CR-182178).
A Relationship Between Constraint and the Critical Crack Tip Opening Angle
NASA Technical Reports Server (NTRS)
Johnston, William M.; James, Mark A.
2009-01-01
Of the various approaches used to model and predict fracture, the Crack Tip Opening Angle (CTOA) fracture criterion has been successfully used for a wide range of two-dimensional thin-sheet and thin plate applications. As thicker structure is considered, modeling the full three-dimensional fracture process will become essential. This paper investigates relationships between the local CTOA evaluated along a three-dimensional crack front and the corresponding local constraint. Previously reported tunneling crack front shapes were measured during fracture by pausing each test and fatigue cycling the specimens to mark the crack surface. Finite element analyses were run to model the tunneling shape during fracture, with the analysis loading conditions duplicating those tests. The results show an inverse relationship between the critical fracture value and constraint which is valid both before maximum load and after maximum load.
Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture toughness
Bass, B.R.; Bryson, J.W.; Theiss, T.J.; Rao, M.C.
1994-01-01
A program to develop and evaluate fracture methodologies for the assessment of crack-tip constraint effects on fracture toughness of reactor pressure vessel (RPV) steels has been initiated in the Heavy-Section Steel Technology (HSST) Program. Crack-tip constraint is an issue that significantly impacts fracture mechanics technologies employed in safety assessment procedures for commercially licensed nuclear RPVs. The focus of studies described herein is on the evaluation of two stressed-based methodologies for quantifying crack-tip constraint (i.e., J-Q theory and a micromechanical scaling model based on critical stressed volumes) through applications to experimental and fractographic data. Data were utilized from single-edge notch bend (SENB) specimens and HSST-developed cruciform beam specimens that were tested in HSST shallow-crack and biaxial testing programs. Results from applications indicate that both the J-Q methodology and the micromechanical scaling model can be used successfully to interpret experimental data from the shallow- and deep-crack SENB specimen tests. When applied to the uniaxially and biaxially loaded cruciform specimens, the two methodologies showed some promising features, but also raised several questions concerning the interpretation of constraint conditions in the specimen based on near-tip stress fields. Fractographic data taken from the fracture surfaces of the SENB and cruciform specimens are used to assess the relevance of stress-based fracture characterizations to conditions at cleavage initiation sites. Unresolved issues identified from these analyses require resolution as part of a validation process for biaxial loading applications. This report is designated as HSST Report No. 142.
In-situ SEM investigation of sub-microscale deformation fields around a crack-tip in silicon
NASA Astrophysics Data System (ADS)
Li, J. J.; Zhao, C. W.; Xing, Y. M.; Hou, X. H.; Fan, Z. C.; Jin, Y. J.; Wang, Y.
2012-12-01
A combination of in-situ scanning electron microscopy (SEM) and geometric phase analysis (GPA) was used to study the deformation fields around a crack-tip in single-crystal silicon under uniaxial tensile load. The sub-microscale silicon pillars grating was fabricated using holographic lithography followed by inductively coupled plasma etching. A series of SEM images of dynamic crack with the sub-microscale grating were obtained during tensile testing. The strain fields around the crack-tip were mapped by GPA. The strain fields were compared with the linear elastic fracture mechanics solutions. It was determined that the deformation is performed around the crack-tip area. The normal strain εxx and shear strain εxy are nearly zero, and the strain fields are dominated by the normal strain εyy component. With the increase of displacement load, the crack propagated mainly along the [010] crystal direction and the strains around the crack-tip increased gradually. It is noted that the theoretical prediction is lower than the experimental results from 0 to 2 μm ahead of the crack-tip. However, the agreement between experimental results and theoretical prediction is very good far from the crack-tip (>2 μm).
Effect of thermal activation energy on dislocation emission from an elliptically blunted crack tip
NASA Astrophysics Data System (ADS)
Zeng, Xin; Fang, Qi-Hong; Liu, You-Wen
2014-08-01
Thermal activation processes are of fundamental importance for the understanding and modeling the strength of structural materials. In this paper, the effect of thermal activation energy on dislocation emission from an elliptically blunted crack tip is researched. Critical stress intensity factors are calculated for an edge dislocation emission from an elliptically blunted crack under mode I and mode II loading conditions at high temperature. The results show that the impact of thermal activation processes is remarkable, the value of the critical stress intensity factor for dislocation emission decreases at high temperature, which means the applied loads for dislocation emission will decrease with increment of temperature.
NASA Astrophysics Data System (ADS)
Hagiwara, Naoto; Meyer, Michel
Crack tip opening displacement (CTOD, δ) tests were carried out for line pipe steels in buffer solutions, sand, and clay to evaluate initiation of hydrogen stress cracking (HSC) at surface defects in buried pipelines under cathodic protection. Four series of line pipe steels and two series of seam welds showed a similar tendency in cathodic current density (i) versus the critical CTOD (δc) curves, irrespective of types, pH and water content of the soils; δc showed a minimum (δHSC) when i>ith (ith=1mA/cm2) in all the testing conditions. δHSC increased with the increasing fracture toughness of the steel. Fluctuation of cathodic current density influenced δc when the maximum value of cathodic current density (imax) was larger than ith. HSC could be initiated at surface defects in pipelines only when imax>ith and δ≥δHSC.
Numerical Analysis of Crack Tip Plasticity and History Effects under Mixed Mode Conditions
NASA Astrophysics Data System (ADS)
Lopez-Crespo, Pablo; Pommier, Sylvie
The plastic behaviour in the crack tip region has a strong influence on the fatigue life of engineering components. In general, residual stresses developed as a consequence of the plasticity being constrained around the crack tip have a significant role on both the direction of crack propagation and the propagation rate. Finite element methods (FEM) are commonly employed in order to model plasticity. However, if millions of cycles need to be modelled to predict the fatigue behaviour of a component, the method becomes computationally too expensive. By employing a multiscale approach, very precise analyses computed by FEM can be brought to a global scale. The data generated using the FEM enables us to identify a global cyclic elastic-plastic model for the crack tip region. Once this model is identified, it can be employed directly, with no need of additional FEM computations, resulting in fast computations. This is done by partitioning local displacement fields computed by FEM into intensity factors (global data) and spatial fields. A Karhunen-Loeve algorithm developed for image processing was employed for this purpose. In addition, the partitioning is done such as to distinguish into elastic and plastic components. Each of them is further divided into opening mode and shear mode parts. The plastic flow direction was determined with the above approach on a centre cracked panel subjected to a wide range of mixed-mode loading conditions. It was found to agree well with the maximum tangential stress criterion developed by Erdogan and Sih, provided that the loading direction is corrected for residual stresses. In this approach, residual stresses are measured at the global scale through internal intensity factors.
Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture-toughness
Pennell, W.E.; Bass, B.R.; Bryson, J.W.; McAfee, W.J.; Theiss, T.J.; Rao, M.C.
1993-12-01
Uniaxial tests of single-edged notched bend (SENB) specimens with both deep- and shallow-flaws have shown elevated fracture-toughness for the shallow flaws. The elevation in fracture-toughness for shallow flaws has been shown to be the result of reduced constraint at the crack-tip. Biaxial loading has the potential to increase constraint at the crack-tip and thereby reduce some of the shallow-flaw, fracture-toughness elevation. Biaxial fracture-toughness tests have shown that the shallow-flaw, fracture-toughness elevation is reduced but not eliminated by biaxial loading. Dual-parameter, fracture-toughness correlations have been proposed to reflect the effect of crack-tip constraint on fracture-toughness. Test results from the uniaxial and biaxial tests were analyzed using the dual-parameter technology. Discrepancies between analysis results and cleavage initiation site data from fractographic examinations indicate that the analysis models are in need of further refinement. Addition of a precleavage, ductile-tearing element to the analysis model has the potential to resolve the noted discrepancies.
Biaxial loading and shallow-flaw effects on crack-tip constraint and fracture-toughness
Pennell, W.E.; Bass, B.R.; Bryson, J.W.; McAfee, W.J.; Theiss, T.J.; Rao, M.C.
1994-04-01
Uniaxial tests of single-edged notched bend (SENB) specimens with both deep- and shallow-flaws have shown elevated fracturetoughness for the shallow flaws. The elevation in fracture-toughness for shallow flaws has been shown to be the result of reduced constraint at the crack-tip. Biaxial loading has the potential to increase constraint at the crack-tip and thereby reduce some of the shallow-flaw, fracture-toughness elevation. Biaxial fracture-toughness tests have shown that the shallow-flaw, fracture-toughness elevation is reduced but not eliminated by biaxial loading. Dual-parameter, fracture-toughness correlations have been proposed to reflect the effect of crack-tip constraint on fracture-toughness. Test results from the uniaxial and biaxial tests were analyzed using the dual-parameter technology. Discrepancies between analysis results and cleavage initiation site data from fractographic examinations indicate that the analysis models are in need of further refinement. Addition of a precleavage, ductile-tearing element to the analysis model has the potential to resolve the noted discrepancies.
A linear least squares approach for evaluation of crack tip stress field parameters using DIC
NASA Astrophysics Data System (ADS)
Harilal, R.; Vyasarayani, C. P.; Ramji, M.
2015-12-01
In the present work, an experimental study is carried out to estimate the mixed-mode stress intensity factors (SIF) for different cracked specimen configurations using digital image correlation (DIC) technique. For the estimation of mixed-mode SIF's using DIC, a new algorithm is proposed for the extraction of crack tip location and coefficients in the multi-parameter displacement field equations. From those estimated coefficients, SIF could be extracted. The required displacement data surrounding the crack tip has been obtained using 2D-DIC technique. An open source 2D DIC software Ncorr is used for the displacement field extraction. The presented methodology has been used to extract mixed-mode SIF's for specimen configurations like single edge notch (SEN) specimen and centre slant crack (CSC) specimens made out of Al 2014-T6 alloy. The experimental results have been compared with the analytical values and they are found to be in good agreement, thereby confirming the accuracy of the algorithm being proposed.
ERRATUM: Genetic identification of crack-tip parameters using thermoelastic isopachics
NASA Astrophysics Data System (ADS)
Dulieu-Barton, J. M.; Worden, K.
2003-10-01
There are a number of unfortunate mistakes in the paper. This is entirely the fault of the authors who regretfully failed to agree on the coordinate system used for the curve-fit. In some detail, the errors are: (1) Page 177, column 2, line 7: Should read tanphi = K2/K1 not tan-1phi = K2/K1 as stated in the thermoelastic data section. This is purely a typographical error and has no effect on the results presented in the paper. (2) Page 178, column 1, line 2: Should read 8 MPa not 12.5 MPa for the 30° crack. This is purely a typographical error and has no effect on the results presented in the paper. (3) The legend 2phi in tables 2, 4, 6 and 8 should simply be phi. The effect of this on the various mode 1 examples is minimal because the value of phi is not used in the SIF determination. Relevant errata are: (4) Page 180, column 1, line 21: Instead of 'The mean estimated angle of rotation is 0.008 rad corresponding to 0.17°.' this should read 'The mean estimated angle of rotation is 0.017 rad corresponding to 0.97°.' (5) Page 181, column 1, line 10: Instead of 'The orientation value obtained was 0.05 rad corresponding to 9.17°.' this should read 'The orientation value obtained was 0.16 rad corresponding to 9.17°.' (6) Page 181, column 2, last line: Instead of 'Here the average of the three phi values is 1.9°.' this should read 'Here the average of the three phi values is 3.7°.' The most serious effect of the phi/2phi confusion is felt on the mixed-mode results in section 7 because the value of phi is used in the calculation of the SIFs. (7) Table 9 should read: corrected table Because of the changes to table 9 the text in section 7 from: ''Once again the most accurate value of phi is derived...'' to the end of that section should be disregarded. The following commentary is applicable: Here the most accurate value of phi is derived from the cardioid furthest from the crack-tip. The computed SIFs are as shown in table 9, where only the cardioid furthest from the
Pressure wall hole size and maximum tip-to-tip crack length following orbital debris penetration
NASA Technical Reports Server (NTRS)
Schonberg, William P.
1996-01-01
The threat of damage from high speed meteoroid and orbital debris particle impacts has become a significant design consideration in the development and construction of long duration earth-orbiting spacecraft. Historically, significant amounts of resources have been devoted to developing shielding for such structures as a means of reducing the penetration potential of high speed on-orbit impacts. These efforts have typically focused on simply whether or not the inner (or 'pressure') walls of candidate multi-wall structural systems would be perforated. Only recently the nature and extent of pressure wall penetration damage have begun to be explored. This report presents the results of a study whose objective was to characterize the hole formation and cracking phenomena associated with the penetration of the multi-wall systems being considered for the International Space Station Alpha (ISSA).
NASA Astrophysics Data System (ADS)
Sennour, M.; Laghoutaris, P.; Guerre, C.; Molins, R.
2009-09-01
Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr 2O 3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.
Time-resolved holography for the microscopic study of crack-tip motion in dynamic fracture
NASA Astrophysics Data System (ADS)
Steckenrider, John Scott
A novel laser system has been designed and developed for the time-resolved holographic analysis of dynamic crack propagation with a frame-to-frame resolution of from 28.3 to 161.6 ns. Average velocities of approximately 1300 m/s were measured in macroscopic (1.2 mm thick) glass specimens, and velocities of 1200 m/s were measured in microscopic (0.15 mm thick) glass specimens. Holographic analyses were conducted using both diffuse and direct laser illumination, demonstrating a broad range of applicability for this method. Normalized crack length versus time plots were generated for a series of specimens with pre-crack lengths of from 3.5 to 10.5 mm wing various pre-load levels and initiating explosive sizes. All specimens exhibited similar dynamic fracture behavior, particularly those outside of the region of shock of the initiating explosive (those of the longer pre-crack set). A best-fit to the theoretical prediction for mode I crack growth revealed a good match to the case for a specimen overloaded by 64 percent. However, no variation with applied load was evident, indicating that an immediate acceleration to a velocity of approximately 1200 m/s (near the empirically determined terminal velocity) was the more plausible fit. Since the terminal velocity was approximately the same for all specimens, this investigation also demonstrated that the terminal velocity is independent of crack-tip stresses, even for the most extreme explosive pulses. In addition, a different loading configuration was attempted in which the specimen was shielded from the explosive shock. The resulting crack growth behavior displayed a velocity well below the aforementioned terminal velocity, indicating an acceleration was present. However, owing to the lack of reproducibility in this method of loading, repeated experiments were not possible. Finally, applications of the described laser system for use in laser ultrasonics were investigated, and these results are included.
A study of near tip phenomena for cracks in a particulate composite
Rezvani, M.A.
1989-01-01
An experimental investigation using grids with a frequency of 125 lines/in. (5 lines/mm) was performed on inert propellant and pure binder at two different global head rates of 0.1 in./min (2.5 mm/min) and 1.0 in./min (25.4 mm/min). From the extracted data, displacements, strains, and dominant eigenvalue for displacement were calculated. An idealized model was used to explain the high strain zone ahead of inert propellant that caused severe blunting at the crack tip. Using the available algorithms and three dimensional photoelasticity, the dominant stress singularity order values were calculated in a four point single edged cracked bend specimen with both straight front and thumbnailed cracks. The free surface values are the same as for the inert propellant and in good agreement with analytical values. A boundary layer is observed in the singularity order which extends towards the mid-plane of the specimen. This region is about twenty percent of the distance from the free surface to mid-depth of the fractured body. The slow and fast head rates alter the global behavior of the specimen as well as the density of the displacement and strain contours. However, the near tip mechanisms are not altered.
Lee, S.Y.; Huang, E.-W.; Wu, W.; Liaw, P.K.; Paradowska, A.M.
2013-05-15
In-situ neutron diffraction was employed to directly measure the crystallographic-orientation-dependent (i.e. hkl) internal strains as a function of distance from the crack tip on the pre-cracked Hastelloy C-2000 compact-tension specimen. Both in-plane (IP) and through-thickness (TT) strain evolutions for various grain orientations were examined during tensile overloading and compressive underloading cycles. After overloading, underloading and their combination loadings were applied and unloaded, the significantly different (hkl) residual strain profiles were obtained in the vicinity of the crack tip. The load responses of the (200) grain orientation in both the IP and TT directions were more significant than those of any other orientations. It is suggested that the different orientation-dependent strain distributions around the crack tip are caused by the combined effects of elastic and plastic anisotropy of each (hkl) reflection upon loading and the subsequent development of residual stresses generated near the crack tip during unloading as a result of the plastic deformation. - Highlights: ► (hkl) strains are examined in situ using neutron diffraction. ► Distinct strain responses are developed around the crack tip under loading. ► The strain response of the (200) grain orientation is more significant. ► Possible mechanisms for the orientation-dependent strain responses are provided.
NASA Astrophysics Data System (ADS)
Begley, Matthew R.; Creton, Costantino; McMeeking, Robert M.
2015-11-01
A general asymptotic plane strain crack tip stress field is constructed for linear versions of neo-Hookean materials, which spans a wide variety of special cases including incompressible Mooney elastomers, the compressible Blatz-Ko elastomer, several cases of the Ogden constitutive law and a new result for a compressible linear neo-Hookean material. The nominal stress field has dominant terms that have a square root singularity with respect to the distance of material points from the crack tip in the undeformed reference configuration. At second order, there is a uniform tension parallel to the crack. The associated displacement field in plane strain at leading order has dependence proportional to the square root of the same coordinate. The relationship between the amplitude of the crack tip singularity (a stress intensity factor) and the plane strain energy release rate is outlined for the general linear material, with simplified relationships presented for notable special cases.
Sharpe, W.N.; Douglas, A.S.; Shapiro, J.M.
1988-03-15
Quantification of the dynamic fracture toughness of structural materials is essential to a wide range of problems - from nuclear accidents to ordnance applications. However, the difficulties associated with accurate measurements of cracks under dynamic loading are considerable. Thus there are no standardized procedures and few reliable results. This work describes a systematic study of the dynamic fracture toughness of SAE-01 tool steel, 4340 and HY100 steels and a tungsten, using the ISDG (Interferometric Strain/Displacement Gage) system which has very-high-frequency resolution. The major advantage of the method is that information is obtained very close to the crack tip, so that stress wave loading effects are accounted for. A detailed error analysis gives an uncertainty of -10% to +20% in the determination of fracture toughness, which compares with + or - 20% for published work.
Materials for advanced turbine engines. Volume 1: Advanced blade tip seal system
NASA Technical Reports Server (NTRS)
Zelahy, J. W.; Fairbanks, N. P.
1982-01-01
Project 3, the subject of this technical report, was structured toward the successful engine demonstration of an improved-efficiency, long-life, tip-seal system for turbine blades. The advanced tip-seal system was designed to maintain close operating clearances between turbine blade tips and turbine shrouds and, at the same time, be resistant to environmental effects including high-temperature oxidation, hot corrosion, and thermal cycling. The turbine blade tip comprised an environmentally resistant, activated-diffussion-bonded, monocrystal superalloy combined with a thin layer of aluminium oxide abrasive particles entrapped in an electroplated NiCr matrix. The project established the tip design and joint location, characterized the single-crystal tip alloy and abrasive tip treatment, and established the manufacturing and quality-control plans required to fully process the blades. A total of 171 blades were fully manufactured, and 100 were endurance and performance engine-tested.
Persaud, S Y; Carcea, A G; Huang, J; Korinek, A; Botton, G A; Newman, R C
2014-06-01
Alloy 800 (Fe-21Cr-33Ni) has been found susceptible to cracking in acid sulfate environments, but the mechanism is not well understood. Alloy 800 C-ring samples were exposed to an acid sulfate environment at 315°C and cracks were found with depths in excess of 300μm after 60h. Preparation of a TEM sample containing crack tips is challenging, but the ability to perform high-resolution microscopy at the crack tip would lend insight to the mechanism of acid sulfate stress corrosion cracking (AcSCC). The lift-out technique combined with a focused ion beam sample preparation was used to extract a crack tip along the cross-section of an acid sulfate crack in an Alloy 800 C-ring. TEM elemental analysis was done using EDS and EELS which identified a duplex oxide within the crack; an inner oxide consisting of a thin 3-4nm Cr-rich oxide and an outer oxide enriched in Fe and Cr. Preliminary conclusions and hypotheses resulted with respect to the mechanism of AcSCC in Alloy 800. PMID:24792448
NASA Technical Reports Server (NTRS)
Schonberg, William P.; Mohamed, Essam
1997-01-01
This report presents the results of a study whose objective was to develop first-principles-based models of hole size and maximum tip-to-tip crack length for a spacecraft module pressure wall that has been perforated in an orbital debris particle impact. The hole size and crack length models are developed by sequentially characterizing the phenomena comprising the orbital debris impact event, including the initial impact, the creation and motion of a debris cloud within the dual-wall system, the impact of the debris cloud on the pressure wall, the deformation of the pressure wall due to debris cloud impact loading prior to crack formation, pressure wall crack initiation, propagation, and arrest, and finally pressure wall deformation following crack initiation and growth. The model development has been accomplished through the application of elementary shock physics and thermodynamic theory, as well as the principles of mass, momentum, and energy conservation. The predictions of the model developed herein are compared against the predictions of empirically-based equations for hole diameters and maximum tip-to-tip crack length for three International Space Station wall configurations. The ISS wall systems considered are the baseline U.S. Lab Cylinder, the enhanced U.S. Lab Cylinder, and the U.S. Lab Endcone. The empirical predictor equations were derived from experimentally obtained hole diameters and crack length data. The original model predictions did not compare favorably with the experimental data, especially for cases in which pressure wall petalling did not occur. Several modifications were made to the original model to bring its predictions closer in line with the experimental results. Following the adjustment of several empirical constants, the predictions of the modified analytical model were in much closer agreement with the experimental results.
A generalized DGS method for studying the deformation field around a crack tip
NASA Astrophysics Data System (ADS)
Zhang, Rui; Guo, Ran; Cheng, Heming
2016-04-01
A generalized method for calculating the stress intensity factor from angular deflection of light rays is proposed. The method is based on 2D digital image correlation (DIC) to measure angular deflection of light rays, however, when a specimen is subjected to loading, deformation measurement from DIC is not perfect because of the existence of small in-plane and out-of-plane motions of the test sample surface that occurred after loading. These disadvantages will lead to errors in the measured angular deflections and fracture parameters. The influence of unavoidable in-plane and out-of-plane motions is discussed, and a generalized method to eliminate them to show the pure stress gradient of the crack tip of Polymethyl Methacrylate (PMMA) is demonstrated. At the same time, the fracture parameter of stress intensity was calculated. The experimental angular deflection of light rays was compared with the theoretical angular deflection predicted by generalized model using data points in an array around the crack tip region. To show the potential and efficacy of the method, K values were obtained from experimental data during tests conducted in a PMMA three-point-bend specimen. Results show an excellent level of agreement with K values predicted from FEM, highlighting the potential of the proposed methodology.
Detecting Gear Tooth Fatigue Cracks in Advance of Complete Fracture
NASA Technical Reports Server (NTRS)
Zakrajsek, James J.; Lewicki, David G.
1996-01-01
Results of using vibration-based methods to detect gear tooth fatigue cracks are presented. An experimental test rig was used to fail a number of spur gear specimens through bending fatigue. The gear tooth fatigue crack in each test was initiated through a small notch in the fillet area of a tooth on the gear. The primary purpose of these tests was to verify analytical predictions of fatigue crack propagation direction and rate as a function of gear rim thickness. The vibration signal from a total of three tests was monitored and recorded for gear fault detection research. The damage consisted of complete rim fracture on the two thin rim gears and single tooth fracture on the standard full rim test gear. Vibration-based fault detection methods were applied to the vibration signal both on-line and after the tests were completed. The objectives of this effort were to identify methods capable of detecting the fatigue crack and to determine how far in advance of total failure positive detection was given. Results show that the fault detection methods failed to respond to the fatigue crack prior to complete rim fracture in the thin rim gear tests. In the standard full rim gear test all of the methods responded to the fatigue crack in advance of tooth fracture; however, only three of the methods responded to the fatigue crack in the early stages of crack propagation.
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.
NASA Astrophysics Data System (ADS)
Hong, Soonsung; Chew, Huck Beng; Kim, Kyung-Suk
2009-08-01
A hybrid framework for inverse analysis of crack-tip cohesive-zone model is developed in this two-part paper to measure cohesive-zone laws of void growth in polymers by combining analytical, experimental, and numerical approaches. This paper focuses on experimental measurements of the cohesive-zone laws for two nonlinear fracture processes in glassy polymers, namely multiple crazing in crack-growth toughening of rubber-toughened high-impact polystyrene (HIPS) and crazing of steady-state crack growth in polymethylmethacrylate (PMMA) under a methanol environment. To this end, electronic speckle pattern interferometry (ESPI) is first applied to measure the crack-tip displacement fields surrounding the fracture process zones in these polymers. These fields are subsequently equilibrium smoothed and used in the extraction of the cohesive-zone laws via an analytical solution method of the inverse problem, the planar field projection method (P-FPM) [Hong, S., Kim, K.-S., 2003. Extraction of cohesive-zone laws from elastic far-fields of a cohesive crack tip: a field projection method. Journal of the Mechanics and Physics of Solids 51, 1267-1286]. Results show that the proposed framework of the P-FPM could provide a systematic way of finding the shape of the cohesive-zone laws governed by the different micro-mechanisms in the fracture processes. In HIPS, inter-particle multiple crazing develops and the craze zone broadens ahead of a crack-tip under mechanical loading. The corresponding cohesive-zone relationship of the multiple-craze zone is found to be highly convex, which indicates effectiveness of rubber particle toughening. It is also observed that the effective peak traction, 7 MPa, in the crack-tip cohesive zone of HIPS (30% rubber content) is lower than the uniaxial yield stress of 9 MPa, presumably due to stress multi-axiality effects. In contrast, in PMMA, methanol localizes the crack-tip craze, weakening the craze traction for craze-void initiation to about 9 MPa
NASA Astrophysics Data System (ADS)
Neville, D. J.
T HE PRESENT paper describes a distance criterion for failure at the tips of cracks in materials showing plasticity which, for stress-controlled cleavage on the lower shelf of toughness, is based on consideration of plasticity local to individual crack-initiation sites rather than on the distance from the remote crack tip to a crack-initiation site. For the upper shelf, consideration is of conditions local to voids, but the thermodynamic balance determining the moment of failure is for the macrocrack rather than microcracks, and a criterion of strain is used rather than a criterion of stress. This new criterion of distance or critical distance leads to a good description of temperature-dependent changes in toughness, on the lower shelf where failure is caused by stress-controlled cleavage and on the upper shelf where fracture runs by a strain-controlled ductile mechanism as well as in the region of transition in between, and always on the basis that these changes are due to temperature-dependent changes in yield strength and Young's modulus. A non-dimensional toughness parameter is observed. The present work is in agreement with many accepted ideas about fracture. The significance of the self-similarity of the distributions of stress and strain at the tips of cracks and the over-riding importance of crack-opening displacement in the determination of conditions for propagation are emphasised.
NASA Astrophysics Data System (ADS)
Hong, Soonsung
A framework for an inverse analysis of the crack-tip cohesive zone model has been developed to measure the cohesive zone laws by analytical, experimental and numerical approaches. An analytical solution method of the inverse problem is developed to extract cohesive-zone laws from elastic far-fields surrounding a crack-tip cohesive zone. A general form of cohesive-crack-tip fields is obtained and used for eigenfunction expansions of the plane elastic field in a complex variable representation. The closing tractions and the separation-gradients at the cohesive zone are expressed in terms of orthogonal polynomial series expansions of the general-form complex functions. The coefficients of the eigenfunctions in J-orthogonal representation are extracted directly, using interaction J-integrals at far field between the physical field and auxiliary fields. The results from numerical experiments suggest that the proposed algorithms are well suited for extracting cohesive-zone laws from the far-field data. An experimental measurement technique called the Electronic Speckle Pattern Interferometry has been applied to measure whole-field crack-tip displacement near fracture process zones. A 4-beam laser speckle interferometer has been constructed to measure 2-dimensional displacement field around a crack tip in glassy polymers. The experimentally measured deformation fields around a crack-tip are used later as input data for the inverse analysis of the crack-tip cohesive zone problem. A numerical noise reduction algorithm, called Equilibrium Smoothing Method, is developed to extract smooth equilibrium fields from the experimentally measured displacement fields. The obtained smooth displacement field is used in inverse analysis of the crack problem. Two nonlinear fracture processes in glassy polymers have been investigated by the framework for the inverse problem. The environmental crazing in PMMA under methanol environment and the nonlinear fracture process in the rubber
NASA Technical Reports Server (NTRS)
Wallhead, Ian R.; Edwards, Lyndon; Poole, Peter
1994-01-01
The optical method of caustics has been successfully extended to enable stress intensity factors as low as 1MPa square root of m to be determined accurately for central fatigue cracks in 2024-T3 aluminium alloy test panels. The feasibility of using this technique to study crack closure, and to determine the effective stress intensity factor range, Delta K(sub eff), has been investigated. Comparisons have been made between the measured values of stress intensity factor, K(sub caus), and corresponding theoretical values, K(sub theo), for a range of fatigue cracks grown under different loading conditions. The values of K(sub caus) and K(sub theo) were in good agreement at maximum stress, where the cracks are fully open, while K(sub caus) exceeded K(sub theo) at minimum stress, due to crack closure. However, the levels of crack closure and values of Delta K(sub eff) obtained could not account for the variations of crack growth rate with loading conditions. It is concluded that the values of Delta K(sub eff), based on caustic measurements in a 1/square root of r stress field well outside the plastic zone, do not fully reflect local conditions which control crack tip behavior.
NASA Astrophysics Data System (ADS)
Murphy, J. D.; Wilkinson, A. J.; Roberts, S. G.
2009-07-01
Plastic zones around crack-tips in tungsten were characterised by electron backscatter diffraction (EBSD). Pre-cracks were made in ~1mm square cross section beams of pure single-crystal tungsten using a spark erosion method. The beams were loaded at different temperatures to a range of stress intensity factors below the fracture toughness. High resolution EBSD patterns were recorded in a two dimensional array in the vicinity of the crack-tip on sectioned samples. Cross-correlation based analysis of these patterns was used to determine lattice rotations associated with the plastic deformation near the cracks. Crystal rotations in the plane of observation were found to dominate over rotations in other planes. For all specimens deformed above the brittle-to-ductile transition temperature, plastic zones were found to extend from the crack tips on along narrow bands along {110} planes. The sizes of the plastic zones and their associated lattice rotations were determined as a function of loading history.
Subcritical crack-growth behavior in advanced silicon nitride ceramics
NASA Astrophysics Data System (ADS)
Bhatnagar, Ajay
Advanced silicon nitride ceramics (Sisb3Nsb4) are leading candidates for structural components in gas turbine and reciprocating engines. However, widespread use of these materials has been deterred due to their low fracture toughness under tensile loads. In the last decade, novel processing techniques have allowed extrinsic toughening of this material through grain bridging processes. The extrinsic toughening mechanisms, however, are prone to subcritical crack-growth processes through environmental, mechanical and high temperature degradation mechanisms. Understanding these failure mechanisms is critical for long term reliability and design. In the first part of this study, fracture and environmentally-assisted subcritical crack-growth processes were examined in bulk Y-Si-Al-O-N oxynitride glasses with compositions typical of the grain boundary phase of silicon nitride ceramics. Both long crack as well as short crack behavior were investigated to establish a reliable fracture toughness value and to elucidate the anomalous densification behavior of the oxynitride glass under indentation loads. Environmentally assisted subcritical crack-growth processes were studied in inert, moist and wet environments under both cyclic and static loading conditions and compared to commercial soda lime and borosilicate glasses. The second part of this study involved the effect of loading, microstructure and temperature on subcritical crack-growth behavior in silicon nitride ceramics. Crack-growth rates under an alternating applied stress intensity were compared to those under static loads. The effect of microstructure on fatigue crack-growth rates was determined in silicon nitrides sintered using different processing techniques and with different grain sizes. Unique experimental techniques were used to determine subcritical crack-growth behavior from room temperature to elevated temperatures of 1250sp°C. Frictional wear models were used to explain the trends in experimental data at
Lam, P.S.; Sindelar, R.L.; Barnes, D.M.; Awadalla, N.G.
1992-01-01
The primary coolant piping system of the Savannah River Site (SRS) reactors contains twelve heat exchangers to remove the waste heat from the nuclear materials production. A large break at the inlet or outlet heads of the heat exchangers would occur if the restraint members of the heads become inactive. The heat exchanger head is attached to the tubesheet by 84 staybolts. The structural integrity of the heads is demonstrated by showing the redundant capacity of the staybolts to restrain the head at design conditions and under seismic loadings. The beat exchanger head is analyzed with a three- dimensional finite element model. The restraint provided by the staybolts is evaluated for several postulated cases of inactive or missing staybolts, that is, bolts that have a flaw exceeding the ultrasonic testing (UT) threshold depth of 25% of the bolt diameter. A limit of 6 inactive staybolts is reached with a fracture criterion based on the maximum allowable local displacement at the active staybolts which corresponds to the crack tip opening displacement (CTOD) of 0.032 inches. An acceptance criteria methodology has been developed to disposition flaws reported in the staybolt inspections while ensuring adequate restraint capacity of the staybolts to maintain integrity of the heat exchanger heads against collapse. The methodology includes an approach for the baseline and periodic inspections of the staybolts. A total of up to 6 staybolts, reported as containing flaws with depths at or exceeding 25% would be acceptable in the heat exchanger.
NASA Astrophysics Data System (ADS)
Saka, M.; Abé, H.; Tanaka, S.
1986-03-01
The blunting of the tip of a crack in a ductile material is analysed under the conditions of plane strain, small-scale yielding, and mixed mode loading of Modes I and II. The material is assumed to be an elastic-perfectly plastic solid with Poisson's ratio being 1/2. The stress and strain fields for a sharp crack under mixed mode loading are first determined by means of elastic-plastic finite element analysis. It is shown that only one elastic sector exists around the crack tip, in contrast with the possibility of existence of two elastic sectors as discussed by Gao. The results obtained for a sharp crack are used as the boundary conditions for the subsequent numerical analysis of crack tip blunting under mixed mode loading, based on slip line theory. The characteristic shapes of the blunted crack tip are obtained for a wide range of Mode I and Mode II combinations, and found to resemble the tip of Japanese sword. Also the stress field around the blunted crack tip is determined.
Zhang, T.Y.; Hack, J.E.
1999-01-01
Calculations of the equilibrium hydrogen concentration profiles about a mixed ode I-mode III crack in single crystal iron were performed. Both material anisotropy and the tetragonal nature of the distortion induced in the iron crystal structure by interstitial hydrogen were incorporated. Results show that, unlike the case of a spherical distortion, a strong coupling exists between the strain field of the interstitial hydrogen and the stress field of the crack for orientations of the crack plane that are not coincident with the cube axes of the lattice. As a result, the predicated enhancement of hydrogen in the crack tip region increases with increasing levels of mode III loading for those orientations. The results may help reconcile conflicting observations concerning the potential role of shear stresses in hydrogen embrittlement and preferential cracking of grains ahead of loaded crack tips in sustained load cracking experiments.
NASA Astrophysics Data System (ADS)
Sakagami, Takahide; Kubo, Shiro
A new thermographic NDT technique was proposed, in which singular electrothermal field near crack tips under the application of periodically modulated electric current was measured using an infrared thermography combined with lock-in data processing technique. Experimental investigations were made on the resolution and the applicability in the identification of through-thickness artificial cracks and fatigue cracks embedded in steel and aluminum alloy plate samples. Modulated electric current was applied to the cracked sample by an induction coli. Lock-in thermal images synchronized to the reference current modulation signal were taken by the lock-in thermography. Significant singular electrothermal field was observed at the crack tip in the lock-in thermal image. The fatigue cracks as well as artificial cracks were found to be sensitively identified by the proposed technique in good resolution compared with the singular method using a conventional thermographic temperature measurement.
Schembri, Philip E
2008-01-01
It is well known that dissolved hydrogen interacts with the stress field at a crack tip, with one result being an intensification of the hydrogen concentration in the region of maximum crack tip stress. The current paper presents recent calculations in ongoing efforts to use coupled stress-diffusion finite element analyses to aid in the structural integrity assessment of pressure vessels containing tritium. The focus of the current work is quantification of the effect of material properties (structural and diffusion) and temperature on the values of maximum stress and hydrogen concentration at the tip of a crack. A one-way-coupled finite element model of a compact tension specimen is used in which the effect of stress and trapping on the hydrogen diffusion is accounted for. Results show that, within the ranges of inputs considered, maximum stress varies approximately linearly with a material's room temperature yield stress but nonlinearly with temperature. Also, peak lattice hydrogen is shown to be a strong function of solubility parameters, a moderate function of yield stress, but only a weak function of trap binding energy and density when trap density is relatively low.
Prediction of stable tearing of 2024-T3 aluminum alloy using the crack-tip opening angle approach
NASA Technical Reports Server (NTRS)
Bakuckas, J. G., Jr.; Newman, J. C., Jr.
1993-01-01
In this study, the crack-tip opening angle (CTOA) approach was incorporated into a damage growth finite element program, MADGIC (Micromechanics Analysis and Damage Growth in Composites), and was used to predict stable tearing in a middle-crack tension 2024-T3 aluminum alloy specimen. The MADGIC code is a displacement based finite element program implemented with an incremental elastic-plastic algorithm used to model elastic-plastic behavior and a nodal splitting and nodal force relaxation algorithm used to generate crack surfaces. Predictions of the applied stress as a function of crack extension and applied stress as a function of load-line displacement were in good agreement with experiments and with similar predictions made using an existing finite element program, ZIP2D. In addition, path integrals, namely, the J-integral and T*-integral, were also evaluated and compared with the CTOA approach. There appears to be a weak relationship between the CTOA and the T*-integral evaluated on a specific integration path during crack extension beyond maximum applied stress. This study further verifies that the CTOA can be used as an effective elastic-plastic fracture mechanics parameter to predict crack growth.
NASA Astrophysics Data System (ADS)
Komaragiri, Uday; Agnew, Sean R.; Gangloff, Richard P.; Begley, Matthew R.
This paper quantifies the effect of strain gradient plasticity (SGP) on crack tip stress elevation for a broad range of applied loading conditions and constitutive model parameters, including both macroscopic hardening parameters and individual material length-scales controlling gradient effects. Finite element simulations incorporating the Fleck-Hutchinson SGP theory are presented for an asymptotically sharp stationary crack. Results identify fundamental scaling relationships describing (i) the physical length-scales over which strain gradients are prominent, and (ii) the degree of stress elevation over conventional Hutchinson-Rice-Rosengren (HRR) fields. Results illustrate that the three length-scale theory predicts much larger SGP effects than the single length-scale theory. Critically, the first length-scale parameter dominates SGP stress elevation: this suggests that SGP effects in fracture can be predicted using the length-scales extracted from nanoindentation, which exhibits similar behavior. Transitional loading/material parameters are identified that establish regimes of SGP relevance: this provides the foundation for the rational application of SGP when developing new micromechanical models of crack tip damage mechanisms and associated subcritical crack propagation behavior in structural alloys.
Advanced ceramic material for high temperature turbine tip seals
NASA Technical Reports Server (NTRS)
Vogan, J. W.; Solomon, N. G.; Stetson, A. R.
1980-01-01
Forty-one material systems were evaluated for potential use in turbine blade tip seal applications at 1370 C. Both ceramic blade tip inserts and abradable ceramic tip shoes were tested. Hot gas erosion, impact resistance, thermal stability, and dynamic rub performance were the criteria used in rating the various materials. Silicon carbide and silicon nitride were used, both as blade tips and abradables. The blade tip inserts were fabricated by hot pressing while low density and honeycomb abradables were sintered or reaction bonded.
Recent advances in capacitance type of blade tip clearance measurements
NASA Technical Reports Server (NTRS)
Barranger, John P.
1988-01-01
Two recent electronic advances at NASA-Lewis that meet the blade tip clearance needs of a wide class of fans, compressors, and turbines are described. The first is a frequency modulated (FM) oscillator that requires only a single low cost ultrahigh frequency operational amplifier. Its carrier frequency is 42.8 MHz when used with a 61 cm long hermetically sealed coaxial cable. The oscillator can be calibrated in the static mode and has a negative peak frequency deviation of 400 kHz for a typical rotor blade. High temperature performance tests of the probe and 13 cm of the adjacent cable show good accuracy up to 600 C, the maximum which produces a clearance error of + or - 10 microns at a clearance of 500 microns. In the second advance, a guarded probe configuration allows a longer cable capacitance. The capacitance of the probe is part of a small time constant feedback in a high speed operational amplifier. The solution of the governing differential equation is applied to a ramp type of input. The results show an amplifier output that contains a term which is proportional to the derivative of the feedback capacitance. The capacitance is obtained by subtracting a balancing reference channel followed by an integration stage.
Tip-enhanced Raman scattering microscopy: Recent advance in tip production
NASA Astrophysics Data System (ADS)
Fujita, Yasuhiko; Walke, Peter; De Feyter, Steven; Uji-i, Hiroshi
2016-08-01
Tip-enhanced Raman scattering (TERS) microscopy is a technique that combines the chemical sensitivity of Raman spectroscopy with the resolving power of scanning probe microscopy. The key component of any TERS setup is a plasmonically-active noble metal tip, which serves to couple far-field incident radiation with the near-field. Thus, the design and implementation of reproducible probes are crucial for the continued development of TERS as a tool for nanoscopic analysis. Here we discuss conventional methods for the fabrication of TERS-ready tips, highlighting the problems therein, as well as detailing more recent developments to improve reducibility. In addition, the idea of remote excitation-TERS is enlightened upon, whereby TERS sensitivity is further improved by using propagating surface plasmons to separate the incident radiation from the tip apex, as well as how this can be incorporated into the fabrication process.
Advances in crack-arrest technology for reactor pressure vessels
Bass, B.R.; Pugh, C.E.
1988-01-01
The Heavy-Section Steel Technology (HSST) Program at the Oak Ridge National Laboratory (ORNL) under the sponsorship of the US Nuclear Regulatory Commission is continuing to improve the understanding of conditions that govern the initiation, rapid propagation, arrest, and ductile tearing of cracks in reactor pressure vessel (RPV) steels. This paper describes recent advances in a coordinated effort being conducted under the HSST Program by ORNL and several subcontracting groups to develop the crack-arrest data base and the analytical tools required to construct inelastic dynamic fracture models for RPV steels. Large-scale tests are being carried out to generate crack-arrest toughness data at temperatures approaching and above the onset of Charpy upper-shelf behavior. Small- and intermediate-size specimens subjected to static and dynamic loading are being developed and tested to provide additional fracture data for RPV steels. Viscoplastic effects are being included in dynamic fracture models and computer programs and their utility validated through analyses of data from carefully controlled experiments. Recent studies are described that examine convergence problems associated with energy-based fracture parameters in viscoplastic-dynamic fracture applications. Alternative techniques that have potential for achieving convergent solutions for fracture parameters in the context of viscoplastic-dynamic models are discussed. 46 refs., 15 figs., 3 tabs.
NASA Astrophysics Data System (ADS)
Shin, Sang Yong
2013-06-01
The effects of microstructure on tensile, Charpy impact, and crack tip opening displacement (CTOD) properties of two API X80 pipeline steels were investigated in this study. Two API X80 pipeline steels consisting of acicular ferrite and granular bainite, and a small amount of hard phases such as martensite and secondary phases have elongated grains along the rolling direction, so that they show different mechanical properties as the specimens' directions change. The 90 deg specimens have high tensile strength due to the low stress concentration on the fine hard phases and the high loads for the deformation of the elongated grains. In contrast, the 30 deg specimens have less elongated grains and larger hard phases such as martensite, with the size of about 3 μm, than the 90 deg specimens. Hence, the 30 deg specimens have low tensile strength because of the high stress concentration on the large hard phases and the low loads to deform grains. In the 90 deg specimen, brittle crack propagation surfaces are even since cracks propagate in a straight line along the elongated grain structure. In the 30 deg specimen, however, brittle crack propagation surfaces are uneven, and secondary cracks are observed, because of the zigzag brittle crack propagation path. In the CTOD properties, the 90 deg specimens have maximum forces of higher magnitude than the 30 deg specimens, because of the elongated grain structure. However, CTODs of the 90 deg specimens are lower than those of the 30 deg specimens because of the low plastic deformation areas by the elongated grains in the 90 deg specimens.
Advanced ceramic material for high temperature turbine tip seals
NASA Technical Reports Server (NTRS)
Solomon, N. G.; Vogan, J. W.
1978-01-01
Ceramic material systems are being considered for potential use as turbine blade tip gas path seals at temperatures up to 1370 1/4 C. Silicon carbide and silicon nitride structures were selected for study since an initial analysis of the problem gave these materials the greatest potential for development into a successful materials system. Segments of silicon nitride and silicon carbide materials over a range of densities, processed by various methods, a honeycomb structure of silicon nitride and ceramic blade tip inserts fabricated from both materials by hot pressing were tested singly and in combination. The evaluations included wear under simulated engine blade tip rub conditions, thermal stability, impact resistance, machinability, hot gas erosion and feasibility of fabrication into engine components. The silicon nitride honeycomb and low-density silicon carbide using a selected grain size distribution gave the most promising results as rub-tolerant shroud liners. Ceramic blade tip inserts made from hot-pressed silicon nitride gave excellent test results. Their behavior closely simulated metal tips. Wear was similar to that of metals but reduced by a factor of six.
Thermally-induced interlaminar crack-tip singularities in laminated anisotropic composites
NASA Astrophysics Data System (ADS)
Choi, Hyung J.; Thangjitham, S.
1993-04-01
Thermally-induced stress singularities of an interlaminar crack in a fiber-reinforced composite laminate under a state of generalized plane deformation are examined within the framework of steady-state anisotropic thermoelasticity. The crack is assumed to be embedded within a matrix-rich interlaminar region of the composite. The Fourier integral transform technique and the flexibility/stiffness matrix method are introduced to formulate the current mixed boundary value problem. As a result, two sets of simultaneous Cauchy-type singular integral equations of the first kind are derived for the heat conduction and thermoelasticity. Within the context of linear elastic fracture mechanics, the mixed-mode thermal stress intensity factors are defined in terms of the solutions of the corresponding integral equations. Numerical results are presented, addressing the effects of laminate stacking sequence, crack 1ocation, and crack surface partial insulation on the values of thermal stress intensity factors.
Luo, P.F.; Wang, J.S.; Chao, Y.J.; Sutton, M.A.
1996-12-31
The stereo vision is used to study the fracture behavior in the compact tension (CT) specimen made from 304L stainless steel. During crack tip blunting, initiation, and growth in the CT specimen, both in-plane and out-of-plane displacement fields near the crack tip are measured by the stereo vision. Based on the plane stress assumption and the deformation theory of plasticity, the J integral is evaluated along several rectangular paths surrounding the crack tip by using the measured in-plane displacement field. Prior to crack growth, the J integral is path independent. For crack extension up to {Delta}a {approx} 3 mm, the near field J integral values are 6% to 10% lower than far field J integral values. For the crack extension of {Delta}a {approx} 4 mm, the J integral lost path independence. The far field J integral values are in good agreement with results obtained from Merkle-Corten`s formula. Both J-{Delta}a and CTOA-{Delta}a are obtained by computing the J integral value and crack tip opening angle (CTOA) at each {Delta}a. Results indicate that CTOA reached a nearly constant value at a crack extension of {Delta}a = 3 mm with a leveled resistance curve thereafter. Also, the J integral value is determined by the maximum transverse diameter of the shadow spots, which are generated by using the out-of-plane displacement field. Results indicate that for crack extension up to 0.25 mm, the J integral values evaluated by using the out-of- plane displacement are close to those obtained by using in-plane displacements and Merkle-Corten`s formula.
NASA Astrophysics Data System (ADS)
Yu, Hongjun; Wang, Jie; Shimada, Takahiro; Wu, Huaping; Wu, Linzhi; Kuna, Meinhard; Kitamura, Takayuki
2016-09-01
In the present study, an I-integral method is established for solving the crack-tip intensity factors of ferroelectric single-crystals. The I-integral combined with the phase field model is successfully used to investigate crack-tip intensity factor variations due to domain switching in ferroelectricity subjected to electromechanical loadings, which exhibits several advantages over previous methods based on small-scale switching. First, the shape of the switching zone around a crack tip is predicted by the time-dependent Ginzburg-Landau equation, which does not require preset energy-based switching criterion. Second, the I-integral can directly solve the crack-tip intensity factors and decouple the crack-tip intensity factors of different modes based on superimposing an auxiliary state onto an actual state. Third, the I-integral is area-independent, namely, the I-integral is not affected by the integral area size, the polarization distributions, or domain walls. This makes the I-integral applicable to large-scale domain switching. To this end, the electro-elastic field intensity factors of an impermeable crack in PbTiO3 ferroelectric single crystals are evaluated under electrical, mechanical, and combined loading. The intensity factors obtained by the I-integral agree well with those obtained by the extrapolation technique. From numerical results, the following conclusions can be drawn with respect to fracture behavior of ferroelectrics under large-scale switching. Under displacement controlled mechanical loading, the stress intensity factors (SIFs) decrease monotonically due to the domain switching process, which means a crack tip shielding or effective switching-induced toughening occurs. If an external electric field is applied, the electric displacement intensity factor (EDIF) increases in all cases, i.e., the formed domain patterns enhance the electric crack tip loading. The energy release rate, expressed by the crack-tip J-integral, is reduced by the domain
Tests of Full-Scale Helicopter Rotors at High Advancing Tip Mach Numbers and Advance Ratios
NASA Technical Reports Server (NTRS)
Biggers, James C.; McCloud, John L., III; Stroub, Robert H.
2015-01-01
As a continuation of the studies of reference 1, three full-scale helicopter rotors have been tested in the Ames Research Center 40- by SO-foot wind tunnel. All three of them were two-bladed, teetering rotors. One of the rotors incorporated the NACA 0012 airfoil section over the entire length of the blade. This rotor was tested at advance ratios up to 1.05. Both of the other rotors were tapered in thickness and incorporated leading-edge camber over the outer 20 percent of the blade radius. The larger of these rotors was tested at advancing tip Mach numbers up to 1.02. Data were obtained for a wide range of lift and propulsive force, and are presented without discussion.
NASA Technical Reports Server (NTRS)
Johnston, William M.; Newman, James C. (Technical Monitor)
2002-01-01
A series of fracture tests were conducted on Middle-crack tension M(T) and compact tension C(T) specimens to determine the effects of specimen type, specimen width, notch tip sharpness and buckling on the fracture behavior of cracked thin sheet (0.04 inch thick) 2024-T3 aluminum alloy material. A series of M(T) specimens were tested with three notch tip configurations: (1) a fatigue pre-cracked notch, (2) a 0.010-inch-diameter wire electrical discharge machined (EDM) notch, and (3) a EDM notch sharpened with a razor blade. The test procedures are discussed and the experimental results for failure stress, load vs. crack extension and the material stress-strain response are reported.
Corrosion fatigue crack propagation in metals
Gangloff, R.P.
1990-06-01
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.
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.
NASA Astrophysics Data System (ADS)
Gao, Huajian; Rice, James R.
T HE GENERAL weight function expressions given in G AO ( J. Mech. Phys. Solids37, 133, 1989), referred to here as part I, for combined-mode crack-dislocation interaction problems in the three-dimensional regime are applied to solve for the stress field and energy of a shear dislocation loop emerging from the tip of a half-plane crack. The results are compared to the previously proposed approximate estimates for shear loops by A NDERSON and R ICE ( J. Mech. Phys. Solids35, 743, 1987), who solved exactly for prismatic opening dislocation loops that are co-planar with the crack and also for the analogous 2-D cases of general crack tip-parallel line dislocations. The energy results are presented in terms of a correction factor m, following Anderson and Rice, to the usual estimate of energy for an emergent crack tip loop as half the energy of a full loop (identified as the emergent loop and its image relative to the crack front) in an uncracked solid. For a full circular shear loop the energy is U = [(2 - ν)μ b2r/4(1-ν)] In (8 r/e 2r0), where r0 denotes the core cut-off parameter and μ, ν are the shear modulus and Poisson ratio. Thus for a semicircular loop emerging from the crack tip, the energy is expressed as U = [(2 - ν)μ b2r/8(1-ν] In (8 mr/e 2r 0), where the constant m depends on the orientation angle Ψ of the Burgers vector relative to a line normal to the crack tip and the inclination angle φ of the dislocated plane relative to the crack plane. The m factors are calculated at selected angles φ for rectangular and semicircular loops. This involves multiple numerical integrations based on the weight functions of part I, first to obtain the stress field and then to integrate it over the dislocated area to get the energy, and requires a large amount of computing CPU time. An approximate formula for m is proposed for general inclined dislocation loops, based on known 2-D results for m factors for arbitrary angles φ calculated by A NDERSON and R ICE (1987
Modeling Near-Crack-Tip Plasticity from Nano- to Micro-Scales
NASA Technical Reports Server (NTRS)
Glaessgen, Edward H.; Saether, Erik; Hochhalter, Jake D.; Yamakov, Vesselin I.
2010-01-01
Several efforts that are aimed at understanding the plastic deformation mechanisms related to crack propagation at the nano-, meso- and micro-length scales including atomistic simulation, discrete dislocation plasticity, strain gradient plasticity and crystal plasticity are discussed. The paper focuses on discussion of newly developed methodologies and their application to understanding damage processes in aluminum and its alloys. Examination of plastic mechanisms as a function of increasing length scale illustrates increasingly complex phenomena governing plasticity
NASA Astrophysics Data System (ADS)
Xie, Hong-Xian; Wang, Chong-Yu; Yu, Tao; Du, Jun-Ping
2009-01-01
The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations. Owing to the unstable stacking energy, the [0bar 11] superdislocation is dissociated into partial dislocations separated by a stacking fault. The simulation results show that when the crack speed is larger than a critical speed, the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip; subsequently the super intrinsic stacking faults are formed in adjacent {111} planes, meanwhile the super extrinsic stacking faults and twinning also occur. Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning, which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.
Modeling and Characterization of Near-Crack-Tip Plasticity from Micro- to Nano-Scales
NASA Technical Reports Server (NTRS)
Glaessgen, Edward H.; Saether, Erik; Hochhalter, Jacob; Smith, Stephen W.; Ransom, Jonathan B.; Yamakov, Vesselin; Gupta, Vipul
2010-01-01
Methodologies for understanding the plastic deformation mechanisms related to crack propagation at the nano-, meso- and micro-length scales are being developed. These efforts include the development and application of several computational methods including atomistic simulation, discrete dislocation plasticity, strain gradient plasticity and crystal plasticity; and experimental methods including electron backscattered diffraction and video image correlation. Additionally, methodologies for multi-scale modeling and characterization that can be used to bridge the relevant length scales from nanometers to millimeters are being developed. The paper focuses on the discussion of newly developed methodologies in these areas and their application to understanding damage processes in aluminum and its alloys.
Modeling and Characterization of Near-Crack-Tip Plasticity from Micro- to Nano-Scales
NASA Technical Reports Server (NTRS)
Glaessgen, Edward H.; Saether, Erik; Hochhalter, Jacob; Smith, Stephen W.; Ransom, Jonathan B.; Yamakov, Vesselin; Gupta, Vipul
2011-01-01
Methodologies for understanding the plastic deformation mechanisms related 10 crack propagation at the nano, meso- and micro-length scales are being developed. These efforts include the development and application of several computational methods including atomistic simulation, discrete dislocation plasticity, strain gradient plasticity and crystal plasticity; and experimental methods including electron backscattered diffraction and video image correlation. Additionally, methodologies for multi-scale modeling and characterization that can be used to bridge the relevant length scales from nanometers to millimeters are being developed. The paper focuses on the discussion of newly developed methodologies in these areas and their application to understanding damage processes in aluminum and its alloys.
Advanced Finite Element Modeling of Low Cycle Fatigue Crack Growth
NASA Technical Reports Server (NTRS)
Gregg, Wayne; McGill, Preston; Swanson, Greg; Wells, Doug; Throckmorton, D. A. (Technical Monitor)
2001-01-01
This document (a viewgraph presentation) assumes a crack-like defect of a size which may be missed in inspection will exist in most critical location of any critical structure or component. Flaw existence assumption is usually, but not always, conservative based on past experiences in NASA and knowledge of manufacturing processes. Cyclic, environmental, and sustained loads used to generate stresses on models. Fracture Mechanics analysis used to predict crack growth and residual strength. Must show that defective structure will still provide four times required mission lifetime. Special exemptions cover redundant structures, low risk parts, etc. Assessments require specialized software tools, experienced analysts, and reliable material crack growth rate test database.
... sound the drug makes as it heats up. Short-Term Effects Crack is a stimulant that is absorbed through ... quickly, after about 5 or 10 minutes. Other short-term effects include: higher heart rate, breathing rate, blood pressure , ...
Aeroelastic behavior of composite helicopter rotor blades with advanced geometry tips
Friedmann, P.P.; Yuan, K.A.
1995-12-31
A new structural and aeroelastic model capable of representing the aeroelastic stability and response of composite helicopter rotor blades with advanced geometry tips is presented. Where it is understood that advanced geometry tips are blade tips having sweep, anhedral and taper in the outboard 10% segment of the blade. The blade is modeled by beam finite elements. A single element is used to represent the swept tip. The nonlinear equations of motion are derived using the Hamilton`s principle and are based on moderate deflection theory. Thus, the nonlinearities are of the geometric type. The important structural blade attributes captured by the model are arbitrary cross-sectional shape, general anisotropic material behavior, transverse shear and out-of-plane warping. The aerodynamic loads are based on quasi-steady Greenberg theory with reverse flow effects, using an implicit formulation. The nonlinear aeroelastic response of the blade is obtained from a fully coupled propulsive trim/aeroelastic response analysis. Aeroelastic stability is obtained from linearizing the equations of motion about the steady state response of the blade and using Floquet theory. Numerical results for the aeroelastic stability and response of a hingeless composite blade with two cell type cross section are presented, together with vibratory hub shears and moments. The influence of ply orientation and tip sweep is clearly illustrated by the results.
NASA Astrophysics Data System (ADS)
Shimamoto, A.; Zhao, H.; Azakami, T.
2007-06-01
The paper presented the effectiveness of a shape memory alloy hybrid composite. It was designed to actively suppress stress intensity in the vicinity of a crack-tip. A shape memory alloy (SMA) TiNi fiber reinforced epoxy composite was fabricated based on the proposed design concept and its material and mechanical properties were investigated by photoelastic examinations. The stress intensity factors, KI and KII, at a crack-tip decreased temperatures greater than Af under mixed mode. The phenomenon was caused by the recovery force of the TiNi fiber. The relationship of the stress intensity factors with the prestrain in the SMA fiber as well as with the ambient temperature in an isothermal furnace was clarified. On this basis, the active control for stress intensity by a shape memory composite was discussed.
Advanced fatigue-crack detection system in steel bridges
NASA Astrophysics Data System (ADS)
Fleming, Marvin F.; Hersh, S.; Chase, Steven B.
1995-05-01
The Federal Highway Administration has sponsored the development of a new system for fatigue crack detection and quantification of fatigue cracks in steel bridges. The NDE technology selected for the new system is based on earlier studies that have identified the best methods for this task. The new system that has been developed is based on previous work which produced two portable instruments that were field tested but were not widely accepted. The best characteristics from these systems have been integrated into a single instrument, using portable computer technology and adapted to the bridge inspection environment. The new system, which has come to be known as the New Ultrasonic-Magnetic Detection System (NUMAC), is configured as a backpack with a heads-up display that leaves the inspectors hands free to climb the structure and to view the inspection site simultaneously while viewing the ultrasonic or magnetic signals. The operation of the system controlled with a mouse or a keyboard. Importantly, the accuracy and repeatability of the NUMAC is combined with the ability to store inspection data. The stored data can be used to document condition, demonstrate and identity important trends, and efficiently channel resources. The flexibility of the portable computer based NDE system is intended to provide a basic, reliable and cost- effective instrument for steel bridge inspection.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
2002-01-01
The life prediction analysis based on an exponential crack velocity formulation was examined using a variety of experimental data on glass and advanced structural ceramics in constant stress-rate ("dynamic fatigue") and preload testing at ambient and elevated temperatures. The data fit to the strength versus In (stress rate) relation was found to be very reasonable for most of the materials. It was also found that preloading technique was equally applicable for the case of slow crack growth (SCG) parameter n > 30. The major limitation in the exponential crack velocity formulation, however, was that an inert strength of a material must be known priori to evaluate the important SCG parameter n, a significant drawback as compared to the conventional power-law crack velocity formulation.
NASA Technical Reports Server (NTRS)
Beil, R. J.
1982-01-01
A theoretical model representing blunting of a crack tip radius through diffusion of vacancies is presented. The model serves as the basis for a computer program which calculates changes, due to successive weld heat passes, in the ultimate tensile strength of 2219-T81 aluminum. In order for the model to yield changes of the same order in the ultimate tensile strength as that observed experimentally, a crack tip radius of the order of .001 microns is required. Such sharp cracks could arise in the fusion zone of a weld from shrinkage cavities or decohered phase boundaries between dendrites and the eutectic phase, or, possibly, from plastic deformation due to thermal stresses encountered during the welding process. Microstructural observations up to X2000 (resolution of about .1 micron) did not, in the fusion zone, show structural details which changed significantly under the influence of a heat pass, with the exception of possible small changes in the configuration of the interdendritic eutectic and in porosity build-up in the remelt zone.
Elevated temperature crack growth in advanced powder metallurgy aluminum alloys
NASA Technical Reports Server (NTRS)
Porr, William C., Jr.; Gangloff, Richard P.
1990-01-01
Rapidly solidified Al-Fe-V-Si powder metallurgy alloy FVS0812 is among the most promising of the elevated temperature aluminum alloys developed in recent years. The ultra fine grain size and high volume fraction of thermally stable dispersoids enable the alloy to maintain tensile properties at elevated temperatures. In contrast, this alloy displays complex and potentially deleterious damage tolerant and time dependent fracture behavior that varies with temperature. J-Integral fracture mechanics were used to determine fracture toughness (K sub IC) and crack growth resistance (tearing modulus, T) of extruded FVS0812 as a function of temperature. The alloy exhibits high fracture properties at room temperature when tested in the LT orientation, due to extensive delamination of prior ribbon particle boundaries perpendicular to the crack front. Delamination results in a loss of through thickness constraint along the crack front, raising the critical stress intensity necessary for precrack initiation. The fracture toughness and tensile ductility of this alloy decrease with increasing temperature, with minima observed at 200 C. This behavior results from minima in the intrinsic toughness of the material, due to dynamic strain aging, and in the extent of prior particle boundary delaminations. At 200 C FVS0812 fails at K levels that are insufficient to cause through thickness delamination. As temperature increases beyond the minimum, strain aging is reduced and delamination returns. For the TL orientation, K (sub IC) decreased and T increased slightly with increasing temperature from 25 to 316 C. Fracture in the TL orientation is governed by prior particle boundary toughness; increased strain localization at these boundaries may result in lower toughness with increasing temperature. Preliminary results demonstrate a complex effect of loading rate on K (sub IC) and T at 175 C, and indicate that the combined effects of time dependent deformation, environment, and strain aging
Simulations of Failure via Three-Dimensional Cracking in Fuel Cladding for Advanced Nuclear Fuels
Lu, Hongbing; Bukkapatnam, Satish; Harimkar, Sandip; Singh, Raman; Bardenhagen, Scott
2014-01-09
Enhancing performance of fuel cladding and duct alloys is a key means of increasing fuel burnup. This project will address the failure of fuel cladding via three-dimensional cracking models. Researchers will develop a simulation code for the failure of the fuel cladding and validate the code through experiments. The objective is to develop an algorithm to determine the failure of fuel cladding in the form of three-dimensional cracking due to prolonged exposure under varying conditions of pressure, temperature, chemical environment, and irradiation. This project encompasses the following tasks: 1. Simulate 3D crack initiation and growth under instantaneous and/or fatigue loads using a new variant of the material point method (MPM); 2. Simulate debonding of the materials in the crack path using cohesive elements, considering normal and shear traction separation laws; 3. Determine the crack propagation path, considering damage of the materials incorporated in the cohesive elements to allow the energy release rate to be minimized; 4. Simulate the three-dimensional fatigue crack growth as a function of loading histories; 5. Verify the simulation code by comparing results to theoretical and numerical studies available in the literature; 6. Conduct experiments to observe the crack path and surface profile in unused fuel cladding and validate against simulation results; and 7. Expand the adaptive mesh refinement infrastructure parallel processing environment to allow adaptive mesh refinement at the 3D crack fronts and adaptive mesh merging in the wake of cracks. Fuel cladding is made of materials such as stainless steels and ferritic steels with added alloying elements, which increase stability and durability under irradiation. As fuel cladding is subjected to water, chemicals, fission gas, pressure, high temperatures, and irradiation while in service, understanding performance is essential. In the fast fuel used in advanced burner reactors, simulations of the nuclear
Damage analysis of a crack layer
NASA Technical Reports Server (NTRS)
Botsis, J.
1989-01-01
Damage analysis of a crack layer in polystyrene is carried out by employing optical microscopy and principles of quantitative stereology. The results show that, within the quasistatic phase of crack layer propagation, the average crazing density, along the trailing edge of the active zone, is constant. This is consistent with a self-similarity hypothesis of damage evolution employed by the crack layer theory. The average crazing densities within the active zone and along its trailing edge are found to be practically equal. A layer of constant crazing density, adjacent to the crack planes, accompanies the crack during its quasi-static growth. This suggests that: (1) a certain level of crazing density should be reached, around the crack tip, prior to crack advance; (2) the specific energy, associated with this 'core' of damage, could be considered as a Griffith's type energy. The results are in favor of certain hypothesis adopted by the crack layer theory.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Salem, Jonathan A.
1998-01-01
The service life of structural ceramic components is often limited by the process of slow crack growth. Therefore, it is important to develop an appropriate testing methodology for accurately determining the slow crack growth design parameters necessary for component life prediction. In addition, an appropriate test methodology can be used to determine the influences of component processing variables and composition on the slow crack growth and strength behavior of newly developed materials, thus allowing the component process to be tailored and optimized to specific needs. At the NASA Lewis Research Center, work to develop a standard test method to determine the slow crack growth parameters of advanced ceramics was initiated by the authors in early 1994 in the C 28 (Advanced Ceramics) committee of the American Society for Testing and Materials (ASTM). After about 2 years of required balloting, the draft written by the authors was approved and established as a new ASTM test standard: ASTM C 1368-97, Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature. Briefly, the test method uses constant stress-rate testing to determine strengths as a function of stress rate at ambient temperature. Strengths are measured in a routine manner at four or more stress rates by applying constant displacement or loading rates. The slow crack growth parameters required for design are then estimated from a relationship between strength and stress rate. This new standard will be published in the Annual Book of ASTM Standards, Vol. 15.01, in 1998. Currently, a companion draft ASTM standard for determination of the slow crack growth parameters of advanced ceramics at elevated temperatures is being prepared by the authors and will be presented to the committee by the middle of 1998. Consequently, Lewis will maintain an active leadership role in advanced ceramics standardization within ASTM
Tips and tricks of ureteroscopy: consensus statement. Part II. Advanced ureteroscopy
Rukin, Nicholas J.; Patterson, Jake; Grey, Ben R.; Finch, William; McClinton, Sam; Parys, Bo; Young, Graham; Syed, Haider; Myatt, Andy; Samsudin, Azi; Inglis, John A.; Smith, Daron
2015-01-01
Our “tips and tricks” focuses on all aspects of upper tract endourology and we hope these will be of use to all trainees and consultants who perform ureteroscopy. We report an “expert consensus view” from experienced endourological surgeons, on all aspects of advanced ureteroscopic techniques, with a particular focus on avoiding and getting out of trouble while performing ureteroscopy. In this paper we provide a summary of placing ureteric access sheath, flexible ureteroscopy, intra renal stone fragmentation and retrieval, maintaining visual clarity and biopsy of ureteric and pelvicalyceal tumours. PMID:27123335
Crack Branching and Fracture Mirror Data of Glasses and Advanced Ceramics
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
1998-01-01
The fracture mirror and crack branching constants were determined from three glasses and nine advanced ceramics tested under various loading and specimen configurations in an attempt to use the constants as a data base for fractography. The ratios of fracture mirror or crack branching constant to fracture toughness were found to be approximately two for most ceramic materials tested. A demonstration of how to use the two constants as a tool for verifying stress measurements was presented for silicon nitride disk specimens subjected to high-temperature, constant stress-rate biaxial flexure testing.
Meng, W.G.; Wert, J.A. . Dept. of Materials Science and Engineering); Vaudin, M.D. . Ceramics Division); Bartholomeusz, M.F. )
1995-02-01
A potential explanation for the cleavage fracture of intermetallic alloys with low or moderate critical resolved shear stress (CRSS) is the existence of an energy barrier for crack-tip dislocation emission, as described by models that analyze the energetics of dislocation emission from crack tips. In the present study, an intermetallic alloy with the L1[sub 2] crystal structure, Al[sub 67]Cr[sub 8]Ti[sub 25], has been used to experimentally assess the predictions of the Rice-Thomson dislocation-emission model. The assessment is performed in two ways. First, model predictions of a fracture mode transition at elevated temperature are compared with experimental results. Bend tests performed at temperatures in the range of 293 to 1,061 K reveal that the fracture model of Al[sub 67]Cr[sub 8]Ti[sub 25] changes from predominantly cleavage fracture at room temperature to a mixed mode of cleavage and intergranular fracture at intermediate temperatures and then to predominantly intergranular fracture at high temperatures. The observed cleavage-to-intergranular fracture transition temperature is approximately 800 K, in good agreement with the model prediction. Second, model predictions of the effect of grain orientation on the fracture mode are compared with experimental results. Electron backscatter patterns and fractographic techniques were used to analyze the grain orientations and fracture modes of grains on the fracture surfaces of specimens fractured at four temperatures in the range 439 to 1,061 K. Experimental results reveal a correlation between fracture mode and slip system orientation relative to the crack, in good agreement with dislocation emission model predictions.
How fatigue cracks grow, interact with microstructure, and lose similitude
Davidson, D.L.
1997-12-01
This paper reviews the processes by which fatigue cracks grow and interact with applied load and microstructure. Fatigue crack growth processes are remarkably similar irrespective of microstructure, crack size, or nature of the loading. Large strains at fatigue crack tips applied over repeated cycles severely alter, or homogenize, microstructures, followed by crack advance. Microstructure affects fatigue crack growth kinetics more than growth processes. But, under marginal conditions, fatigue crack growth rates are also affected by microstructural features. Examples are small cracks growing under low stresses or large cracks growing near threshold. The prediction of safe lifetimes for machine parts, such as gas turbine components, requires that laboratory-generated fatigue crack growth rate data be transferred to field-operating conditions. This transfer depends on the maintenance of similitude: microstructurely, mechanically, and environmentally. However, for many industrially important conditions, similitude with large fatigue crack growth is lost, partially because of changes in fatigue crack closure. The effect of closure on similitude is discussed. New data are presented to illustrate the loss of similitude between applied loading and crack tip strain response. The resulting strain rates of material within the process zone are unexpected. Environmentally influenced fatigue crack growth rates are likely to be influenced by these strain rates.
Advanced tip design for liquid phase vibration mode atomic force microscopy.
Muramatsu, Hiroshi; Yamamoto, Yuji; Shigeno, Masatsugu; Shirakawabe, Yoshiharu; Inoue, Akira; Kim, Woo-Sik; Kim, Seung Jin; Chang, Sang-Mok; Kim, Jong Min
2008-03-24
We have fabricated polymer tips for atomic force microscopy in order to elucidate the effects of tip length and shape on cantilever vibration damping in liquids. The vibration damping is investigated by measuring the vibration amplitude of cantilevers as a function of tip-sample distance. The cantilever with a short tip provides a higher damping effect over long tip-sample distances. When the vibration amplitude was rescaled to show the effect of the cantilever width on oscillation damping, the vibration amplitude of cantilevers with various tip lengths was similarly obtained in a long distance range over 50 microm. This similarity is explained by an acoustic damping model in which an acoustic wave is generated by the cantilever. Finally, the results indicate a cantilever with a sufficiently long tip compared to the cantilever width can dramatically reduce the long-range damping effect in a liquid environment. PMID:18328326
Crack Propagation in Bamboo's Hierarchical Cellular Structure
Habibi, Meisam K.; Lu, Yang
2014-01-01
Bamboo, as a natural hierarchical cellular material, exhibits remarkable mechanical properties including excellent flexibility and fracture toughness. As far as bamboo as a functionally graded bio-composite is concerned, the interactions of different constituents (bamboo fibers; parenchyma cells; and vessels.) alongside their corresponding interfacial areas with a developed crack should be of high significance. Here, by using multi-scale mechanical characterizations coupled with advanced environmental electron microscopy (ESEM), we unambiguously show that fibers' interfacial areas along with parenchyma cells' boundaries were preferred routes for crack growth in both radial and longitudinal directions. Irrespective of the honeycomb structure of fibers along with cellular configuration of parenchyma ground, the hollow vessels within bamboo culm affected the crack propagation too, by crack deflection or crack-tip energy dissipation. It is expected that the tortuous crack propagation mode exhibited in the present study could be applicable to other cellular natural materials as well. PMID:24998298
Comparison of experiment and theory for elastic-plastic plane strain crack growth
Hermann, L; Rice, J R
1980-02-01
Recent theoretical results on elastic-plastic plane strain crack growth, and experimental results for crack growth in a 4140 steel in terms of the theoretical concepts are reviewed. The theory is based on a recent asymptotic analysis of crack surface opening and strain distributions at a quasi-statically advancing crack tip in an ideally-plastic solid. The analysis is incomplete in that some of the parameters which appear in it are known only approximately, especially at large scale yielding. Nevertheless, it suffices to derive a relation between the imposed loading and amount of crack growth, prior to general yielding, based on the assumption that a geometrically similar near-tip crack profile is maintained during growth. The resulting predictions for the variation of J with crack growth are found to fit well to the experimental results obtained on deeply cracked compact specimens.
NASA Astrophysics Data System (ADS)
Smith, E.
1984-11-01
During a hypothetical thermal shock event involving a water-cooled nuclear reactor pressure vessel, a crack can propagate deep into the reactor vessel thickness by a series of run-arrest-reinitiation events. Within the transition temperature regime, crack propagation and arrest in pressure vessel steels is associated with a combination of cleavage and dimpled rupture processes, the dimpled rupture regions being contained within ligaments that are normal to the crack plane and parallel to the direction of crack propagation. The present paper models the effect of ligaments on the reinitiation of fracture at the tip of an arrested crack, and the results of a theoretical analysis define the conditions under which ligaments might increase the reinitiation value above k IC, assuming that they fracture by a ductile rupture process. By comparing the predictions with experimental results for model vessels subject to thermal shock, it is shown that the ligaments, which are present at arrest, are unlikely to fail entirely by ductile rupture prior to the reinitiation of fracture at an arrested crack tip. Instead it is suggested that the ligaments fail by cleavage, whereupon they do not markedly affect the reinitiation K value, which thus correlates with K IC.
Recent advances in the modelling of crack growth under fatigue loading conditions
NASA Technical Reports Server (NTRS)
Dekoning, A. U.; Tenhoeve, H. J.; Henriksen, T. K.
1994-01-01
Fatigue crack growth associated with cyclic (secondary) plastic flow near a crack front is modelled using an incremental formulation. A new description of threshold behaviour under small load cycles is included. Quasi-static crack extension under high load excursions is described using an incremental formulation of the R-(crack growth resistance)- curve concept. The integration of the equations is discussed. For constant amplitude load cycles the results will be compared with existing crack growth laws. It will be shown that the model also properly describes interaction effects of fatigue crack growth and quasi-static crack extension. To evaluate the more general applicability the model is included in the NASGRO computer code for damage tolerance analysis. For this purpose the NASGRO program was provided with the CORPUS and the STRIP-YIELD models for computation of the crack opening load levels. The implementation is discussed and recent results of the verification are presented.
Focused ion beam 3D nano-patterned optical fiber tips for advanced beam profile engineering
NASA Astrophysics Data System (ADS)
Janeiro, Ricardo; Flores, Raquel; Ribeiro, Ana R.; Jorge, Pedro; Viegas, Jaime
2015-03-01
Focused ion beam (FIB) patterning of 3D topography on optical fiber tips for application in stand-alone, rugged and simplified setups for optical tweezers cell sorters, optical near-field lithography and optical beam profile engineering are reported. We demonstrate various configurations based on single-step FIB patterning, multiple-step FIB processing and hybrid approaches based on optical fiber pre- and post-FIB treatment with either etching, fusion splicing, photopolymerization or electroplating steps for optical fiber texture, topography and composition engineering. Different conductive coatings for minimal charge accumulation and beam drift are studied with the relative merits compared. Furthermore optimal beam parameters for accurate pattern replication and positioning are also presented. Measured experimental field profiles are compared with numerical simulations of fabricated optical fiber tips for fabrication accuracy evaluation. Applications employing these engineered fiber tips in the field of optical tweezers, optical vortex generation, photolithography, photo-polymerization and beam forming are presented.
Innovative Approach to Establish Root Causes for Cracking in Aggressive Reactor Environments
Bruemmer, Stephen M.; Thomas, Larry E.; Vetrano, John S.; Simonen, Edward P.
2003-10-31
The research focuses on the high-resolution characterization of degradation microstructures and microchemistries in specimens tested under controlled conditions for the environment and for the material where in-service complexities can be minimized. Thermodynamic and kinetic modeling of crack-tip processes is employed to analyze corrosion-induced structures and gain insights into degradation mechanisms. Novel mechanistic ''fingerprinting'' of crack-tip structures is used to isolate causes of environmental cracking in tandem with quantitative measurements of crack growth. Sample preparation methods and advanced analytical techniques are used to characterize corrosion/oxidation reactions and crack-tip structures at near atomic dimensions in order to gain insight into fundamental environmental cracking mechanisms. Reactions at buried interfaces, not accessible by conventional approaches, are being systematically interrogated. Crack-growth experiments in high-temperature water environments are evaluating and isolating the effects of material condition (matrix strength, grain boundary composition and precipitation) on stress corrosion cracking (SCC). The fundamental understanding of crack advance mechanisms will establish the basis to design new corrosion-resistant alloys for current light-water reactors and advanced reactor systems.
Stable Crack Growth During Thermal Actuation of Shape Memory Alloys
NASA Astrophysics Data System (ADS)
Jape, S.; Baxevanis, T.; Lagoudas, D. C.
2016-03-01
A finite element analysis of crack growth is carried out in shape memory alloys subjected to thermal variations under plane strain, mode I, constant applied loading. The crack is assumed to propagate at a critical level of the crack-tip energy release rate which is modeled using the virtual crack closure technique. The load level, applied at a high temperature at which the austenite phase is stable, is assumed sufficiently low so that the resulting crack-tip energy release rate is smaller than the critical value but sufficiently high so that the critical value is reached during cooling, initiating crack growth (Baxevanis and Lagoudas in Int J Fract 191:191-213, 2015). Stable crack growth is observed, mainly associated with the shielding effect of the transformed material left in the wake of the advancing crack. Results pertaining to the near-tip mechanical fields and fracture toughness are presented and their sensitivity to phase transformation metrics and bias load levels is investigated.
Advancing the Message: Tips and Suggestions for Communications People in Child Welfare Agencies.
ERIC Educational Resources Information Center
Layton, Mary J.
Nonprofit agencies involved in child welfare are increasingly interested in generating publicity for their work and improving their organization's public image. This booklet compiles information, tips, and resources helpful to public relations and communications professionals and agency personnel responsible for handling media relations for child…
Multiscale modeling of crack initiation and propagation at the nanoscale
NASA Astrophysics Data System (ADS)
Shiari, Behrouz; Miller, Ronald E.
2016-03-01
Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly "converting" the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region. The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
2000-01-01
Slow crack growth analysis was performed with three different loading histories including constant stress-rate/constant stress-rate testing (Case I loading), constant stress/constant stress-rate testing (Case II loading), and cyclic stress/constant stress-rate testing (Case III loading). Strength degradation due to slow crack growth arid/or damage accumulation was determined numerically as a Function of percentage of interruption time between the two loading sequences for a given loading history. The numerical solutions were examined with the experimental data determined at elevated temperatures using four different advanced ceramic materials, two silicon nitrides, one silicon carbide and one alumina for the Case I loading history, and alumina for the Case II loading history. The numerical solutions were in reasonable agreement with the experimental data, indicating that notwithstanding some degree of creep deformation presented for some test materials slow crack growth was a governing mechanism associated with failure for all the test material&
NASA Technical Reports Server (NTRS)
Choi, S. R.; Gyekenyesi, J. P.
2001-01-01
Slow crack growth analysis was performed with three different loading histories including constant stress- rate/constant stress-rate testing (Case I loading), constant stress/constant stress-rate testing (Case II loading), and cyclic stress/constant stress-rate testing (Case III loading). Strength degradation due to slow crack growth and/or damage accumulation was determined numerically as a function of percentage of interruption time between the two loading sequences for a given loading history. The numerical solutions were examined with the experimental data determined at elevated temperatures using four different advanced ceramic materials, two silicon nitrides, one silicon carbide and one alumina for the Case I loading history, and alumina for the Case II loading history. The numerical solutions were in reasonable agreement with the experimental data, indicating that notwithstanding some degree of creep deformation presented for some test materials slow crack growth was a governing mechanism associated with failure for all the rest materials.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
2000-01-01
Slow crack growth analysis was performed with three different loading histories including constant stress-rate/constant stress-rate testing (Case 1 loading), constant stress/constant stress-rate testing (Case 2 loading), and cyclic stress/constant stress-rate testing (Case 2 loading). Strength degradation due to slow crack growth and/or damage accumulation was determined numerically as a function of percentage of interruption time between the two loading sequences for a given loading history. The numerical solutions were examined with the experimental data determined at elevated temperatures using four different advanced ceramic materials, two silicon nitrides, one silicon carbide and one alumina for the Case 1 loading history, and alumina for the Case 3 loading history. The numerical solutions were in reasonable agreement with the experimental data, indicating that notwithstanding some degree of creep deformation presented for some test materials slow crack growth was a governing mechanism associated with failure for all the test materials.
A model to predict deflection of bevel-tipped active needle advancing in soft tissue.
Datla, Naresh V; Konh, Bardia; Honarvar, Mohammad; Podder, Tarun K; Dicker, Adam P; Yu, Yan; Hutapea, Parsaoran
2014-03-01
Active needles are recently being developed to improve steerability and placement accuracy for various medical applications. These active needles can bend during insertion by actuators attached to their bodies. The bending of active needles enables them to be steered away from the critical organs on the way to target and accurately reach target locations previously unachievable with conventional rigid needles. These active needles combined with an asymmetric bevel-tip can further improve their steerability. To optimize the design and to develop accurate path planning and control algorithms, there is a need to develop a tissue-needle interaction model. This work presents an energy-based model that predicts needle deflection of active bevel-tipped needles when inserted into the tissue. This current model was based on an existing energy-based model for bevel-tipped needles, to which work of actuation was included in calculating the system energy. The developed model was validated with needle insertion experiments with a phantom material. The model predicts needle deflection reasonably for higher diameter needles (11.6% error), whereas largest error was observed for the smallest needle diameter (24.7% error). PMID:24296105
McClung, R.C.; Newman, J.C. Jr.
1999-07-01
The discovery of the phenomenon of plasticity-induced fatigue crack closure by Elber was truly a landmark event in the study of fatigue crack growth (FCG) and the development of practical engineering methods for fatigue life management. Subsequent research identified other contributing mechanisms for crack closure, including crack surface roughness and oxide debris. Fatigue crack closure is now understood to be an intrinsic feature of crack growth behavior that must be considered to understand or treat many FCG problems, although closure may not be an issue in all problems and does not always provide a complete explanation of crack growth behavior. As the thirtieth anniversary of the Elber discovery approached, the strong, continuing international interest in crack closure prompted the organization of another ASTM symposium. An international audience numbering over sixty-five persons heard thirty papers contributed by authors from twelve different countries, with more than half of the papers originating from outside the United States. This STP volume contains peer-reviewed manuscripts for twenty-seven of those presentations, plus one peer-reviewed paper that could not be presented at the symposium. Topics covered are: Fundamental Studies; Experimental Characterization of Closure; Load History Effects; Surface Roughness Effects; and Closure Effects on Crack Behavior. Separate abstracts were prepared for all 28 papers.
NASA Astrophysics Data System (ADS)
Mueller, Arnold W.; Smith, Charles D.; Lemasurier, Philip
1990-07-01
During the design of a helicopter, the weight, engine, rotor speed, and rotor geometry are given significant attention when considering the specific operations for which the helicopter will be used. However, the noise radiated from the helicopter and its relationship to the design variables is currently not well modeled with only a limited set of full-scale field test data to study. In general, limited field data have shown that reduced main-rotor advancing blade-tip Mach numbers result in reduced far-field noise levels. The status of a recent helicopter noise research project is reviewed. It is designed to provide flight experimental data which may be used to further understand helicopter main-rotor advancing blade-tip Mach number effects on far-field acoustic levels. Preliminary results are presented relative to tests conducted with a Sikorsky S-76A helicopter operating with both the rotor speed and the flight speed as the control variable. The rotor speed was operated within the range of 107 to 90 percent NR at nominal forward speeds of 35, 100, and 155 knots.
NASA Technical Reports Server (NTRS)
Mueller, Arnold W.; Smith, Charles D.; Lemasurier, Philip
1990-01-01
During the design of a helicopter, the weight, engine, rotor speed, and rotor geometry are given significant attention when considering the specific operations for which the helicopter will be used. However, the noise radiated from the helicopter and its relationship to the design variables is currently not well modeled with only a limited set of full-scale field test data to study. In general, limited field data have shown that reduced main-rotor advancing blade-tip Mach numbers result in reduced far-field noise levels. The status of a recent helicopter noise research project is reviewed. It is designed to provide flight experimental data which may be used to further understand helicopter main-rotor advancing blade-tip Mach number effects on far-field acoustic levels. Preliminary results are presented relative to tests conducted with a Sikorsky S-76A helicopter operating with both the rotor speed and the flight speed as the control variable. The rotor speed was operated within the range of 107 to 90 percent NR at nominal forward speeds of 35, 100, and 155 knots.
Anderson, Michael T.; Crawford, Susan L.; Cumblidge, Stephen E.; Diaz, Aaron A.; Doctor, Steven R.
2007-01-01
Studies conducted at the Pacific N¬orthwest National Laboratory (PNNL) in Richland, Washington, have focused on developing and evaluating the reliability of nondestructive examination (NDE) approaches for inspecting coarse-grained, cast stainless steel reactor components. The objective of this work is to provide information to the United States Nuclear Regulatory Commission (NRC) on the utility, effec¬tiveness and limitations of ultrasonic testing (UT) inspection techniques as related to the in-service inspec¬tion of primary system piping components in pressurized water reactors (PWRs). Cast stainless steel pipe specimens were examined that contain thermal and mechanical fatigue cracks located close to the weld roots and have inside/outside surface geometrical conditions that simulate several PWR primary piping configurations. In addition, segments of vintage centrifugally cast piping were also examined to understand inherent acoustic noise and scattering due to grain structures and determine consistency of UT responses from different locations. The advanced UT methods were applied from the outside surface of these specimens using automated scanning devices and water coupling. The low-frequency ultrasonic method employed a zone-focused, multi-incident angle inspection protocol (operating at 250-450 kHz) coupled with a synthetic aperture focusing technique (SAFT) for improved signal-to-noise and advanced imaging capabilities. The phased array approach was implemented with a modified instrument operating at 500 kHz and composite volumetric images of the specimens were generated. Re¬sults from laboratory studies for assessing detection, localization and sizing effectiveness are discussed in this paper.
On the shielding effect of a multiligament zone of a crack in WC-Co
Kotoul, M.
1997-08-01
A crack tip shielding analysis of the fracture of cobalt reinforced tungsten carbide hardmetals is developed. It is shown that a multiligament zone formed by ductile binder ligaments behind the crack tip strongly affects the cleavage process in the WC matrix. This is due to a decrease in both the crack tip opening displacement and the plastic zone size ahead of a blunting crack tip. Consequently, normal stresses build up and, as a result, cleavage of the matrix is facilitated even under a low local stress intensity factor perceived by the crack tip. The cooperation of the cleavage of the matrix and the tearing of binder ligaments by dimple rupture results in a meandering crack path with a surprisingly high ligament area fraction. This ligament area fraction is about 15% higher than the volume fraction of the cobalt binder. It is suggested that the facilitated cleavage process promotes an advance of the crack front and maximizes the reduction of the potential energy of a body. The partition between carbide and binder phase related fracture is predicted.
NASA Astrophysics Data System (ADS)
Cooper, Kevin Richard
2001-11-01
The mechanism of environment-assisted cracking (EAC) of 7xxx-series alloys is unclear, involving uncertain contributions of hydrogen embrittlement (HE) and anodic dissolution (AD). Fundamental understanding of the EAC mechanism is lacking in part because the role of the crack environment is not well understood. The objective of this research was to characterize and understand the role of the crack chemistry and electrochemistry during aqueous EAC of AA 7050. The crack environment can differ significantly from bulk conditions. Cations, produced by AD, hydrolyze causing local acidification; anions from the bulk electrolyte concentrate within the crack to maintain charge neutrality; ohmic potential drop results from ion migration and diffusion. A positive correlation exists between da/dt and [Al3+]Tip in chromate-chloride electrolyte wherein tip dissolution dominates flank corrosion in establishing the crack chemistry. Tip pH was 2 to 4 and determined by the reaction Al3+ + H 2O = AlOH2+ + H+. The tip potential (ETip) was approximately -0.90 VSCE and independent of EApp . The low ETip and pH promote H+ reduction, generating atomic and molecular H. Hydrogen bubbles restrict ion movement, substantially increasing the effective crack resistance over bulk conditions. Absorbed atomic hydrogen facilitates HE. The spontaneous transition from slow, incubation to high-rate da/dt coincides with the establishment of a critical aggressive tip chemistry and tip depolarization. Development of the critical occluded chemistry necessary for accelerated da/dt is a competitive process between opposing forces: AD, hydrolysis and migration promote an aggressive environment whereas diffusion reduces concentration gradients, thereby retarding the formation of an aggressive chemistry. Quantitative assessment of the contribution of tip dissolution to crack advance is hindered by a lack of knowledge of two key parameters: the tip corrosion front height and the effective crack conductivity
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
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Dawicke, D. S.
1989-01-01
The present paper is concerned with the application of an analytical crack-closure model to study crack growth under various load histories. The model was based on a crack-tip plasticity concept like the Dugdale model, but modified to leave plastically deformed material in the wake of the advancing crack tip. The effect of material thickness on plasticity was accounted for by using a constraint factor on tensile yielding at the crack tip. The model was used to correlate crack-growth rates under constant-amplitude loading, and to predict crack growth under variable-amplitude loading on a high-strength aluminum alloy (7475-T7351) sheet material. The experimental data were obtained from Zhang et al. Predicted crack-growth lives agreed well with experimental data. For ten crack-growth tests subjected to various variable-amplitude load histories, the ratio of predicted-to-experimental lives ranged from 0.54 to 1.19. The mean value of the ratio of predicted-to-experimental lives was 0.95 and the standard error was 0.2 using a constraint factor of 1.9 in the model. Crack-opening stresses calculated from the model were significantly different from those determined by Zhang et al. using a striation-based experimental method.
Development of a Low Data Event Timer for Monitoring an Advancing Crack in Fracture
NASA Technical Reports Server (NTRS)
Macon, D. J.; Totman, P. D.; Bodily, M. L.; Everton, R. L.; Eggett, M. R.
2004-01-01
Monitoring the crack position and velocity in a fracture specimen can be difficult and laborious. In addition, the data storage requirements can be considerable depending upon the testing conditions. A low data event timer was developed to alleviate these problems. The test apparatus was applied to cantilever beams bonded with a structural epoxy and tested under different conditions such as stable to unstable transitions and different temperature extremes. The results indicate that the approach eliminates problems associated with other types of crack measurement and greatly simplifies the measuring process.
Environment enhanced fatigue of advanced aluminum alloys and metal matrix composites
NASA Technical Reports Server (NTRS)
Slavik, Donald C.; Gangloff, Richard P.
1991-01-01
The environmental fatigue crack propagation behavior of advanced Al-Li-Cu based alloys and metal matrix composites is being characterized. Aqueous NaCl and water vapor, which produce atomic hydrogen by reactions on clean crack surfaces, are emphasized. The effects of environment sensitive crack closure, stress ratio, and precipitate microstructure are assessed. Mechanistic models are sought for intrinsic crack tip damage processes to enable predictions of cracking behavior outside of the data, metallurgical improvements in material cracking resistance, and insight on hydrogen compatibility.
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.
NASA Astrophysics Data System (ADS)
Tian, Yingtao; Robson, Joseph D.; Riekehr, Stefan; Kashaev, Nikolai; Wang, Li; Lowe, Tristan; Karanika, Alexandra
2016-07-01
Laser welding of advanced Al-Li alloys has been developed to meet the increasing demand for light-weight and high-strength aerospace structures. However, welding of high-strength Al-Li alloys can be problematic due to the tendency for hot cracking. Finding suitable welding parameters and filler material for this combination currently requires extensive and costly trial and error experimentation. The present work describes a novel coupled model to predict hot crack susceptibility (HCS) in Al-Li welds. Such a model can be used to shortcut the weld development process. The coupled model combines finite element process simulation with a two-level HCS model. The finite element process model predicts thermal field data for the subsequent HCS hot cracking prediction. The model can be used to predict the influences of filler wire composition and welding parameters on HCS. The modeling results have been validated by comparing predictions with results from fully instrumented laser welds performed under a range of process parameters and analyzed using high-resolution X-ray tomography to identify weld defects. It is shown that the model is capable of accurately predicting the thermal field around the weld and the trend of HCS as a function of process parameters.
NASA Astrophysics Data System (ADS)
Tian, Yingtao; Robson, Joseph D.; Riekehr, Stefan; Kashaev, Nikolai; Wang, Li; Lowe, Tristan; Karanika, Alexandra
2016-04-01
Laser welding of advanced Al-Li alloys has been developed to meet the increasing demand for light-weight and high-strength aerospace structures. However, welding of high-strength Al-Li alloys can be problematic due to the tendency for hot cracking. Finding suitable welding parameters and filler material for this combination currently requires extensive and costly trial and error experimentation. The present work describes a novel coupled model to predict hot crack susceptibility (HCS) in Al-Li welds. Such a model can be used to shortcut the weld development process. The coupled model combines finite element process simulation with a two-level HCS model. The finite element process model predicts thermal field data for the subsequent HCS hot cracking prediction. The model can be used to predict the influences of filler wire composition and welding parameters on HCS. The modeling results have been validated by comparing predictions with results from fully instrumented laser welds performed under a range of process parameters and analyzed using high-resolution X-ray tomography to identify weld defects. It is shown that the model is capable of accurately predicting the thermal field around the weld and the trend of HCS as a function of process parameters.
NASA Astrophysics Data System (ADS)
Chen, Wei-Qiu
2015-10-01
Significant progress has been made in mixed boundary-value problems associated with three-dimensional (3D) crack and contact analyses of advanced materials featuring more complexities compared to the conventional isotropic elastic materials. These include material anisotropy and multifield coupling, two typical characteristics of most current multifunctional materials. In this paper we try to present a state-of-the-art description of 3D exact/analytical solutions derived for crack and contact problems of elastic solids with both transverse isotropy and multifield coupling in the latest decade by the potential theory method in the spirit of V. I. Fabrikant, whose ingenious breakthrough brings new vigor and vitality to the old research subject of classical potential theory. We are particularly interested in crack and contact problems with certain nonlinear features. Emphasis is also placed on the coupling between the temperature field (or the like) and other physical fields (e.g., elastic, electric, and magnetic fields). We further highlight the practical significance of 3D contact solutions, in particular in applications related to modern scanning probe microscopes.
Development of Advanced Life Prediction Tools for Elastic-Plastic Fatigue Crack Growth
NASA Technical Reports Server (NTRS)
Gregg, Wayne; McGill, Preston; Swanson, Greg; Wells, Doug; Throckmorton, D. A. (Technical Monitor)
2001-01-01
The objective of this viewgraph presentation is to develop a systematic approach to improving the fracture control process, including analytical tools, standards, guidelines, and awareness. Analytical tools specifically for elastic-plastic fracture analysis is a regime that is currently empirical for the Space Shuttle External Tank (ET) and is handled by simulated service testing of pre-cracked panels.
Environment enhanced fatigue of advanced aluminum alloys and composites
NASA Technical Reports Server (NTRS)
Slavik, Donald C.; Gangloff, Richard P.
1990-01-01
The objective is to characterize and understand the environmental fatigue crack propagation behavior of advanced, high stiffness and strength, aluminum alloys and metal matrix composites. Those gases and aqueous electrolytes which are capable of producing atomic hydrogen by reactions on clean crack surfaces are emphasized. Characterizations of the behavior of new materials are sought to provide data for damage tolerant component life prediction. Mechanistic models are sought for crack tip damage processes which are generally applicable to structural aluminum alloys. Such models will enable predictions of cracking behavior outside of the data, metallurgical improvements in material cracking resistance, and insight on hydrogen compatibility.
Hoffman, M.J. Sydney Univ., NSW . Dept. of Mechanical Engineering); Dauskardt, R.H.; Ritchie, R.O. ); Mai, Y.W. . Dept. of Mechanical Engineering)
1992-05-01
Damage and cyclic fatigue failure under alternating loading in transformation-toughened zirconia ceramics is reviewed and compared to corresponding behavior under quasi-static loading (static fatigue). Current understanding of the role of transformation toughening in influencing cyclic fatigue-crack propagation behavior is examined based on studies which altered the extent of the tetragonal-to-monoclinic phase transformation in MG-PSZ through subeutectoid aging. These studies suggest that near-tip computations of the crack-driving force (in terms of the local stress intensity) can be used to predict crack-growth behavior under constant amplitude and variable-amplitude (spectrum) loading, using spatially resolved Raman spectroscopy to measure the extent of the transformation zones. In addition, results are reviewed which rationalize distinctions between the crack-growth behavior of preexisting, long'' (> 2 mm), through-thickness cracks and naturally-occurring, small'' (1 to 100 [mu]m), surface cracks in terms of variations in crack-tip shielding with crack size. In the present study, the effect of grain size variations on crack-growth behavior under both monotonic (R-curve) and cyclic fatigue loading are examined. Such observations are used to speculate on the mechanisms associated with cyclic crack advance, involving such processes as alternating shear via transformation-band formation, cyclic modification of the degree of transformation toughening, and uncracked-ligament (or grain) bridging.
Hoffman, M.J. |; Dauskardt, R.H.; Ritchie, R.O.; Mai, Y.W.
1992-05-01
Damage and cyclic fatigue failure under alternating loading in transformation-toughened zirconia ceramics is reviewed and compared to corresponding behavior under quasi-static loading (static fatigue). Current understanding of the role of transformation toughening in influencing cyclic fatigue-crack propagation behavior is examined based on studies which altered the extent of the tetragonal-to-monoclinic phase transformation in MG-PSZ through subeutectoid aging. These studies suggest that near-tip computations of the crack-driving force (in terms of the local stress intensity) can be used to predict crack-growth behavior under constant amplitude and variable-amplitude (spectrum) loading, using spatially resolved Raman spectroscopy to measure the extent of the transformation zones. In addition, results are reviewed which rationalize distinctions between the crack-growth behavior of preexisting, ``long`` (> 2 mm), through-thickness cracks and naturally-occurring, ``small`` (1 to 100 {mu}m), surface cracks in terms of variations in crack-tip shielding with crack size. In the present study, the effect of grain size variations on crack-growth behavior under both monotonic (R-curve) and cyclic fatigue loading are examined. Such observations are used to speculate on the mechanisms associated with cyclic crack advance, involving such processes as alternating shear via transformation-band formation, cyclic modification of the degree of transformation toughening, and uncracked-ligament (or grain) bridging.
Zebrafish models in translational research: tipping the scales toward advancements in human health
Phillips, Jennifer B.; Westerfield, Monte
2014-01-01
Advances in genomics and next-generation sequencing have provided clinical researchers with unprecedented opportunities to understand the molecular basis of human genetic disorders. This abundance of information places new requirements on traditional disease models, which have the potential to be used to confirm newly identified pathogenic mutations and test the efficacy of emerging therapies. The unique attributes of zebrafish are being increasingly leveraged to create functional disease models, facilitate drug discovery, and provide critical scientific bases for the development of new clinical tools for the diagnosis and treatment of human disease. In this short review and the accompanying poster, we highlight a few illustrative examples of the applications of the zebrafish model to the study of human health and disease. PMID:24973743
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.
Automatic crack propagation tracking
NASA Technical Reports Server (NTRS)
Shephard, M. S.; Weidner, T. J.; Yehia, N. A. B.; Burd, G. S.
1985-01-01
A finite element based approach to fully automatic crack propagation tracking is presented. The procedure presented combines fully automatic mesh generation with linear fracture mechanics techniques in a geometrically based finite element code capable of automatically tracking cracks in two-dimensional domains. The automatic mesh generator employs the modified-quadtree technique. Crack propagation increment and direction are predicted using a modified maximum dilatational strain energy density criterion employing the numerical results obtained by meshes of quadratic displacement and singular crack tip finite elements. Example problems are included to demonstrate the procedure.
Shear fatigue crack growth - A literature survey
NASA Technical Reports Server (NTRS)
Liu, H. W.
1985-01-01
Recent studies of shear crack growth are reviewed, emphasizing test methods and data analyses. The combined mode I and mode II elastic crack tip stress fields are considered. The development and design of the compact shear specimen are described, and the results of fatigue crack growth tests using compact shear specimens are reviewed. The fatigue crack growth tests are discussed and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center-cracked panels and double edge-cracked plates under cyclic shear loading are examined and analyzed in detail.
R and D advances in corrosion and crack monitoring for oil and gas lines
Atherton, D.L.; Czura, W.; Krause, T.W.; Laursen, P.; Mergelas, B.; Hauge, C.
1996-12-31
Magnetic Flux Leakage (MFL) inspection techniques for in-line corrosion monitoring of pipelines continue to evolve rapidly. Current R and D is aimed at improving the accuracy and reliability of defect sizing. Major issues are the variability and consequent need to characterize the magnetic properties of the pipes and the effects of line pressure, residual and bending stresses on MFL signals. Magnetic Barkhausen Noise (MBN) measurements are being used to study the stress-induced changes in magnetic anisotropy. Remote Field Eddy Current (RFEC) techniques are being investigated for detection and measurement of stress corrosion cracking in gas pipelines. Anomalous defect source models have improved the detailed explanation of crack defect interactions greatly.
Sadananda, K.; Vasudevan, A.K.
1997-12-01
The authors have re-evaluated short crack growth behavior using concepts developed recently, and they show that these concepts provide a unified framework that can explain both short and long crack growth behavior without resorting to the crack closure effect. They consider that the behavior of long cracks, including the effects of load ratio, R, is fundamental. they had shown previously that, since fatigue is at least a two-parameter problem in that at least two load parameters are required for an unambiguous description, there are two critical driving forces required simultaneously for fatigue cracks to grow. In extending this analysis to the growth of short cracks, they reject the current notion of the lack of similitude for short cracks and express the similitude as a fundamental postulate that, for a given crack growth mechanism, equal crack tip driving forces result in equal crack growth rates. Short crack growth behavior confirms the concept that two parameters are required to define fatigue; consequently, for fatigue cracks to grow, two thresholds need to be satisfied simultaneously. The authors present examples from the literature to illustrate the concepts discussed.
Crack growth in single-crystal silicon
NASA Technical Reports Server (NTRS)
Chen, C. P.; Leipold, M. H.
1986-01-01
Crack growth in single-crystal silicon at room temperature in air was evaluated by double torsion (DT) load-relaxation method and monitored by acoustic emission (AE) technique. Both DT and AE methods indicated lack of subcritical crack growth in silicon. At the critical stress intensity factor, the crack front was found to be jumping several times in a 'mirror' region and then followed by fast crack growth in a 'hackle' region. Hackle marks were found to be associated with plastic deformation at the tip of the fast moving crack. No dislocation etch pits were found in the 'mirror' region, in which crack growth may result from interatomic bonds broken at the crack tip under stress without any plastic deformation. Acoustic emission appears to be spontaneously generated from both interatomic bonds broken and dislocation generation at the moving crack tip during the crack growth in single-crystal silicon.
Anderson, Michael T.; Crawford, Susan L.; Cumblidge, Stephen E.; Denslow, Kayte M.; Diaz, Aaron A.; Doctor, Steven R.
2007-03-01
Studies conducted at the Pacific Northwest National Laboratory in Richland, Washington, have focused on assessing the effectiveness and reliability of novel approaches to nondestructive examination (NDE) for inspecting coarse-grained, cast stainless steel reactor components. The primary objective of this work is to provide information to the U.S. Nuclear Regulatory Commission on the effectiveness and reliability of advanced NDE methods as related to the inservice inspection of safety-related components in pressurized water reactors (PWRs). This report provides progress, recent developments, and results from an assessment of low frequency ultrasonic testing (UT) for detection of inside surface-breaking cracks in cast stainless steel reactor piping weldments as applied from the outside surface of the components. Vintage centrifugally cast stainless steel piping segments were examined to assess the capability of low-frequency UT to adequately penetrate challenging microstructures and determine acoustic propagation limitations or conditions that may interfere with reliable flaw detection. In addition, welded specimens containing mechanical and thermal fatigue cracks were examined. The specimens were fabricated using vintage centrifugally cast and statically cast stainless steel materials, which are typical of configurations installed in PWR primary coolant circuits. Ultrasonic studies on the vintage centrifugally cast stainless steel piping segments were conducted with a 400-kHz synthetic aperture focusing technique and phased array technology applied at 500 kHz, 750 kHz, and 1.0 MHz. Flaw detection and characterization on the welded specimens was performed with the phased array method operating at the frequencies stated above. This report documents the methodologies used and provides results from laboratory studies to assess baseline material noise, crack detection, and length-sizing capability for low-frequency UT in cast stainless steel piping.
Accelerated Testing Methodology Developed for Determining the Slow Crack Growth of Advanced Ceramics
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Gyekenyesi, John P.
1998-01-01
Constant stress-rate ("dynamic fatigue") testing has been used for several decades to characterize the slow crack growth behavior of glass and structural ceramics at both ambient and elevated temperatures. The advantage of such testing over other methods lies in its simplicity: strengths are measured in a routine manner at four or more stress rates by applying a constant displacement or loading rate. The slow crack growth parameters required for component design can be estimated from a relationship between strength and stress rate. With the proper use of preloading in constant stress-rate testing, test time can be reduced appreciably. If a preload corresponding to 50 percent of the strength is applied to the specimen prior to testing, 50 percent of the test time can be saved as long as the applied preload does not change the strength. In fact, it has been a common, empirical practice in the strength testing of ceramics or optical fibers to apply some preloading (<40 percent). The purpose of this work at the NASA Lewis Research Center is to study the effect of preloading on measured strength in order to add a theoretical foundation to the empirical practice.
Accelerated Testing Methodology for the Determination of Slow Crack Growth of Advanced Ceramics
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Salem, Jonathan A.; Gyekenyesi, John P.
1997-01-01
Constant stress-rate (dynamic fatigue) testing has been used for several decades to characterize slow crack growth behavior of glass and ceramics at both ambient and elevated temperatures. The advantage of constant stress-rate testing over other methods lies in its simplicity: Strengths are measured in a routine manner at four or more stress rates by applying a constant crosshead speed or constant loading rate. The slow crack growth parameters (n and A) required for design can be estimated from a relationship between strength and stress rate. With the proper use of preloading in constant stress-rate testing, an appreciable saving of test time can be achieved. If a preload corresponding to 50 % of the strength is applied to the specimen prior to testing, 50 % of the test time can be saved as long as the strength remains unchanged regardless of the applied preload. In fact, it has been a common, empirical practice in strength testing of ceramics or optical fibers to apply some preloading (less then 40%). The purpose of this work is to study the effect of preloading on the strength to lay a theoretical foundation on such an empirical practice. For this purpose, analytical and numerical solutions of strength as a function of preloading were developed. To verify the solution, constant stress-rate testing using glass and alumina at room temperature and alumina silicon nitride, and silicon carbide at elevated temperatures was conducted in a range of preloadings from O to 90 %.
Hot cracks in rubber: origin of the giant toughness of rubberlike materials.
Carbone, G; Persson, B N J
2005-09-01
We study crack propagation in rubberlike materials and show that the nonuniform temperature distribution which occurs in the vicinity of the crack tip has a profound influence on the crack propagation, and may strongly enhance the crack propagation energy G(v) for high crack velocities v. At very low crack-tip velocities, the heat produced at the crack tip can diffuse away, but already at moderate crack-tip velocities a very large temperature increase occurs close to the crack tip resulting in a "hot-crack" propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt and may result in unstable crack motion, e.g., stick-slip motion or catastrophic failure. PMID:16197007
Crack analysis of unfilled natural rubber using infrared microspectroscopy
Neumeister, L.A.; Koenig, J.L.
1996-10-01
The characterization and analysis of the crack tip region in unfilled natural rubber is crucial in developing a fatigue resistant material. Therefore, the molecular structure and orientation of the material in the crack tip region has been compared to the bulk. Cracks formed by different methods were evaluated using both transmission and reflection techniques of infrared microspectroscopy. The material around the crack tip and edges shows much higher absorbances than the bulk material for stressed rubber. This is due to more material and predominantly, from residual orientation effects. For unstressed material the crack tip region exhibits identical characteristics to the bulk material.
Automatic crack growth tracking of bimaterial interface cracks
NASA Technical Reports Server (NTRS)
Yehia, Nabil A. B.; Shephard, Mark S.
1988-01-01
The propagation process of an interfacial crack in composite material is studied using the modified maximum dilatational strain energy density criterion, NT-criterion. Some necessary assumptions have been adopted to facilitate the use of the NT-criterion in this case. The stress intensity factors at the crack tip are extracted from the complex displacement field and finite element results. A simple algorithm for automatic crack propagation is presented with an illustrative example.
Atomic simulation of cracks under mixed mode loading
NASA Technical Reports Server (NTRS)
Mullins, M.
1984-01-01
A discrete atomic model of a crack tip in iron under mixed mode loads is examined. The results indicate that the behavior of the crack at the atomic scale as a function of the ratio of mode I to mode II component of load is quite complex. In general, crack tip plasticity appears to increase as the mode II component of load increases.
Temperature evolution and heat dissipation during crack propagation
NASA Astrophysics Data System (ADS)
Lengliné, Olivier; Santucci, Stéphane; Jørgen Måløy, Knut; Vincent-Dospital, Tom; Toussaint, Renaud
2013-04-01
During a crack propagation, energy is dissipated in mainly three ways: creation of new fracture surface (possibly at microscopic scale in a process zone), emission of elastic waves that get dissipated in the far field, and local Joule heating during friction in a process zone. There is in addition some reversible elastic energy change associated to the crack advance.Since the temperature variations can have an important impact on the physics of the crack propagation, establishing properly this balance in different crack propagation scenarios is of great importance. Notably, the physics of fracture propagation has been shown to be strongly affected by thermally activated rupture, even when the heterogeneity of material properties determines strongly the microscopic fracture geometry and the intermittency in the fracture propagation. A natural question, in such kinetic crack propagation, is the temperature field during the cracks propagation. This question is also central in earth science, where a lot of attention has been set recently on thermal effects, with the possibility of thermo-pressurization of faults due to thermal expansion of fluids present in faults. Independently of thermo pressurization, the rise of temperature locally, at the zone enduring damage, could significantly affect the creep in this zone, as understood by statistical physics and Arrhenius law, and thus the crack propagation. We are interested in quantifying directly these different effects in an experimental situation. We present results based on infrared and optical imaging of the propagation of a crack in a sheet of paper. The temperature field shows local increases of the temperature of several degrees during the crack propagation. Optical images acquired with a fast video camera are correlated in order to extract the deformation field at each time step. We show how the temperature in our paper sample varies with the deformation rate at the tip of the crack. We also present some numerical
Crack motion in viscoelastic solids: the role of the flash temperature.
Carbone, G; Persson, B N J
2005-07-01
We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this "low-speed" regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a "hot-crack" propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold. PMID:15997339
Mode III fatigue crack propagation in low alloy steel
NASA Astrophysics Data System (ADS)
Ritchie, R. O.; McClintock, F. A.; Nayeb-Hashemi, H.; Ritter, M. A.
1982-01-01
To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects, showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 104 cycles at nominal yield to 106 cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to be slower than in Mode I, although still power-law related to the alternating stress intensity (△K III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements (△CTD III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTDIII per cycle.
Effect of Measured Welding Residual Stresses on Crack Growth
NASA Technical Reports Server (NTRS)
Hampton, Roy W.; Nelson, Drew; Doty, Laura W. (Technical Monitor)
1998-01-01
Welding residual stresses in thin plate A516-70 steel and 2219-T87 aluminum butt weldments were measured by the strain-gage hole drilling and X-ray diffraction methods. The residual stress data were used to construct 3D strain fields which were modeled as thermally induced strains. These 3D strain fields were then analyzed with the WARP31) FEM fracture analysis code in order to predict their effect on fatigue and on fracture. For analyses of fatigue crack advance and subsequent verification testing, fatigue crack growth increments were simulated by successive saw-cuts and incremental loading to generate, as a function of crack length, effects on crack growth of the interaction between residual stresses and load induced stresses. The specimen experimental response was characterized and compared to the WARM linear elastic and elastic-plastic fracture mechanics analysis predictions. To perform the fracture analysis, the plate material's crack tearing resistance was determined by tests of thin plate M(T) specimens. Fracture analyses of these specimen were performed using WARP31D to determine the critical Crack Tip Opening Angle [CTOA] of each material. These critical CTOA values were used to predict crack tearing and fracture in the weldments. To verify the fracture predictions, weldment M(T) specimen were tested in monotonic loading to fracture while characterizing the fracture process.
Fracture mechanics parameters for cracks on a slightly undulating interface
NASA Astrophysics Data System (ADS)
Yang, Lin; Qu, Jianmin
1993-11-01
Typical bimaterial interfaces are nonplanar due to surface facets or roughness. Crack-tip stress fields of an interface crack must be influenced by nonplanarity of the interface. Consequently, interface toughness is affected. The crack-tip fields of a finite crack on an elastic/rigid interface with periodic undulation are studied. Particular emphasis is given to the fracture mechanics parameters, such as the stress intensity factors, crack-tip energy release rate, and crack-tip mode mixity. When the amplitude of interface undulation is very small relative to the crack length (which is the case for rough interfaces), asymptotic analysis is used to convert the nonplanarity effects into distributed dislocations located on the planar interface. Then, the resulting stress fields near the crack tip are obtained by using the Fourier integral transform method. It is found that the stress fields at the crack tip are strongly influenced by nonplanarity of the interface. Generally speaking, nonplanarity of the interface tends to shield the crack tip by reducing the crack-tip stress concentration.
Library Management Tips that Work
ERIC Educational Resources Information Center
Smallwood, Carol, Ed.
2011-01-01
There's no shortage of library management books out there--but how many of them actually tackle the little details of day-to-day management, the hard-to-categorize things that slip through the cracks of a larger handbook? "Library Management Tips that Work" does exactly that, addressing dozens of such issues facing library managers, including: (1)…
Meisnar, Martina; Lozano-Perez, Sergio; Moody, Michael; Holland, James
2014-11-01
Intergranular stress corrosion cracking (IGSCC) in type SUS304 stainless steels, tested under pressurized water reactor (PWR) primary water conditions, has been characterized with unprecedented spatial resolution using scanning electron microscopy (SEM) and novel low-energy (∼3 kV) energy dispersive X-ray spectroscopy (EDX). An advancement of the large area silicon drift detector (SDD) has enhanced its sensitivity for X-rays in the low-energy part of the atomic spectrum. Therefore, it was possible to operate the SEM at lower accelerating voltages in order to reduce the interaction volume of the beam with the material and achieve higher spatial resolution and better signal-to-noise ratio. In addition to studying the oxide chemistry at the surface of intergranular stress corrosion cracks, the technique has proven capable of resolving Ni enrichment ahead of some crack tips. Active cracks could be distinguished from inactive ones due to the presence of oxides in the open crack and Ni-rich regions ahead of the crack tip. Furthermore, it has been established that SCC features can be better resolved with low-energy (3 kV) than high-energy (12 kV) EDX. The low effort in sample preparation, execution and data analysis makes SEM the ideal tool for initial characterization and selection of the most important SCC features such as dominant cracks and interesting crack tips, later to be studied by transmission electron microscopy (TEM) and atom probe tomography (APT). PMID:25080272
Formation and interpretation of dilatant echelon cracks.
Pollard, D.D.; Segall, P.; Delaney, P.T.
1982-01-01
The relative displacements of the walls of many veins, joints, and dikes demonstrate that these structures are dilatant cracks. We infer that dilatant cracks propagate in a principal stress plane, normal to the maximum tensile or least compressive stress. Arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks. Breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction. Near the parent-crack tip, a rotation of the local principal stresses is induced in the same sense, but not necessarily through the same angle. Incipient echelon cracks form at the parent-crack tip normal to the local maximum tensile stress. Further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane. The walls of these twisted cracks may be idealized as helicoidal surfaces. An array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle. Thus, many echelon cracks grow from a single parent because the work done in creating the array, as measured by its surface area decreases as the number of cracks increases. -from Authors
An analytically enriched finite element method for cohesive crack modeling.
Cox, James V.
2010-04-01
Meaningful computational investigations of many solid mechanics problems require accurate characterization of material behavior through failure. A recent approach to fracture modeling has combined the partition of unity finite element method (PUFEM) with cohesive zone models. Extension of the PUFEM to address crack propagation is often referred to as the extended finite element method (XFEM). In the PUFEM, the displacement field is enriched to improve the local approximation. Most XFEM studies have used simplified enrichment functions (e.g., generalized Heaviside functions) to represent the strong discontinuity but have lacked an analytical basis to represent the displacement gradients in the vicinity of the cohesive crack. As such, the mesh had to be sufficiently fine for the FEM basis functions to capture these gradients.In this study enrichment functions based upon two analytical investigations of the cohesive crack problem are examined. These functions have the potential of representing displacement gradients in the vicinity of the cohesive crack with a relatively coarse mesh and allow the crack to incrementally advance across each element. Key aspects of the corresponding numerical formulation are summarized. Analysis results for simple model problems are presented to evaluate if quasi-static crack propagation can be accurately followed with the proposed formulation. A standard finite element solution with interface elements is used to provide the accurate reference solution, so the model problems are limited to a straight, mode I crack in plane stress. Except for the cohesive zone, the material model for the problems is homogenous, isotropic linear elasticity. The effects of mesh refinement, mesh orientation, and enrichment schemes that enrich a larger region around the cohesive crack are considered in the study. Propagation of the cohesive zone tip and crack tip, time variation of the cohesive zone length, and crack profiles are presented. The analysis
ERIC Educational Resources Information Center
Mathematics Teacher, 2004
2004-01-01
Some inexpensive or free ways that enable to capture and use images in work are mentioned. The first tip demonstrates the methods of using some of the built-in capabilities of the Macintosh and Windows-based PC operating systems, and the second tip describes methods to capture and create images using SnagIt.
Elevated Temperature Crack Propagation
NASA Technical Reports Server (NTRS)
Orange, Thomas W.
1994-01-01
This paper is a summary of two NASA contracts on high temperature fatigue crack propagation in metals. The first evaluated the ability of fairly simple nonlinear fracture parameters to correlate crack propagation. Hastelloy-X specimens were tested under isothermal and thermomechanical cycling at temperatures up to 980 degrees C (1800 degrees F). The most successful correlating parameter was the crack tip opening displacement derived from the J-integral. The second evaluated the ability of several path-independent integrals to correlate crack propagation behavior. Inconel 718 specimens were tested under isothermal, thermomechanical, temperature gradient, and creep conditions at temperatures up to 650 degrees C (1200 degrees F). The integrals formulated by Blackburn and by Kishimoto correlated the data reasonably well under all test conditions.
On the location of crack closure and the threshold condition for fatigue crack growth
Zaiken, E.; Ritchie, R.O.
1984-08-01
These experiments on ingot aluminum alloys provide further confirmation that the development of a threshold for the growth of long fatigue cracks is primarily associated with a reduction in local crack driving force due to crack closure in the wake of the crack tip. Moreover, based on studies of the change in K /SUB c1/ during progressive removal of the wake at threshold levels, it appears that although such closure is fairly evenly distributed over most of the crack length, more than 40% of the closure is confined to the near-tip region.
Advances in Fatigue and Fracture Mechanics Analyses for Aircraft Structures
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1999-01-01
This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crack-closure concept. Residual strength of cracked panels with severe out-of-plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Dawicke, D. S.
1989-01-01
The present paper is concerned with the application of an analytical crack-closure model to study crack growth under various load histories. The model was based on a concept like the Dugdale model, but modified to leave plastically deformed material in the wake of the advancing crack tip. The thickness effect was accounted for by using a 'constraint' factor on tensile yielding at the crack tip. The model was used to correlate crack-growth rates under constant-amplitude loading, and to predict crack growth under variable-amplitude loading on a high-strength aluminum alloy (7475-T7351) sheet material. The experimental data was obtained from Zhang et al. Predicted crack-growth lives agreed well with experimental data. For ten crack-growth tests subjected to various variable-amplitude load histories, the ratio of predicted-to-experimental lives ranged from 0.54 to 1.19. The mean value of predicted-to-experimental lives was 0.95 with a standard error of 0.2 for a constraint factor of 1.9.
Crack-path effect on material toughness
NASA Technical Reports Server (NTRS)
Rubinstein, Asher A.
1990-01-01
The main features of a toughening mechanism associated with a curvilinear crack path are examined using a model consisting of a macrocrack in a brittle solid with a curvilinear segment at the crack tip. A numerical procedure for finite and semiinfinite cracks is formulated and evaluated using an example which has an exact solution (a finite crack in the form of a circular arc in a uniform stress field). It is shown that, for a relatively small amplitude of crack path oscillations, the toughening ratio can be taken equal to the ratio of the corresponding crack path lengths.
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.
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Early stages in the development of stress corrosion cracks
Jones, R.H.; Simonen, E.P.
1993-12-01
Processes in growth of short cracks and stage I of long stress corrosion cracks were identified and evaluated. There is evidence that electrochemical effects can cause short stress corrosion cracks to grow at rates faster or slower than long cracks. Short cracks can grow at faster rates than long cracks for a salt film dissolution growth mechanism or from reduced oxygen inhibition of hydrolytic acidification. An increasing crack growth rate with increasing crack length could result from a process of increasing crack tip concentration of a critical anion, such as Cl{sup {minus}}, with increasing crack length in a system where the crack velocity is dependent on the Cl{sup {minus}} or some other anion concentration. An increasing potential drop between crack tip and mouth would result in an increased anion concentration at the crack tip and hence an increasing crack velocity. Stage I behavior of long cracks is another early development stage in the life of a stress corrosion crack which is poorly understood. This stage can be described by da/dt = AK{sup m} where da/dt is crack velocity, A is a constant, K is stress intensity and m ranges from 2 to 24 for a variety of materials and environments. Only the salt film dissolution model was found to quantitatively describe this stage; however, the model was only tested on one material and its general applicability is unknown.
Goel, V.S.
1986-01-01
Various papers on corrosion cracking are presented. The topics addressed include: unique case studies on hydrogen embrittlement failures in components used in aeronautical industry; analysis of subcritical cracking in a Ti-5Al-2.5Sn liquid hydrogen control valve; corrosion fatigue and stress corrosion cracking of 7475-T7351 aluminum alloy; effects of salt water environment and loading frequency on crack initiation in 7075-T7651 aluminum alloy and Ti-6Al-4V; stress corrosion cracking of 4340 steel in aircraft ignition starter residues. Also discussed are: stress corrosion cracking of a titanium alloy in a hydrogen-free environment; automation in corrosion fatigue crack growth rate measurements; the breaking load method, a new approach for assessing resistance to growth of early stage stress corrosion cracks; stress corrosion cracking properties of 2090 Al-Li alloy; repair welding of cracked free machining Invar 36; radial bore cracks in rotating disks.
Visual simulation of fatigue crack growth
Wang, S.; Margolin, H.; Lin, F.B.
1998-07-01
An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an ear profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state 1 and 2 crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.
Further progress on the wavy-crack model of dynamic crack propagation
Gao, H.; Pawlikowski, K.
1995-12-31
The state-of-the-art theory of dynamic crack propagation has not been able to provide an unequivocal explanation for a number of experimental findings. An important observation is that the crack surfaces, as the trace of fracture path, tend to exhibit a rough surface morphology during rapid crack propagation. In a wavy-crack model proposed recently by the author, the crack surface roughening is attributed to an inherent instability which causes the tip of the crack to propagate along an oscillatory fracture path. It appears that the wavy-crack model is capable of explaining important discrepancies currently existing between theory and experiments. In particular, experimentally observed terminal fracture speeds are significantly lower than the theoretically predicted value, i.e. the Rayleigh wave speed CR. This may be attributed to the oscillatory fracture path which makes the measured crack velocity appear lower than the actual crack speed. Also, the wavy-crack model explains how the local crack tip motion can exhibit high inertia behaviors while the measurable crack motion remains in the low inertia domain. As a result of different inertia effects associated with local and apparent crack motion, the high inertia field near the crack tip tends to induce nucleation of microcrack branches while the low inertia apparent crack field tends to suppress the microbranching. This view of dynamic fracture is not inconsistent with relevant experimental observations (e.g. see and references therein) and recent numerical simulation of fast crack motion. A planar wavy motion of a 3D crack front has been analyzed by Pice et al.. The wavy-crack model has also been applied to dynamic crack propagation along a weak interface having lower fracture resistance than the adjacent material. Further analytical and numerical developments of this model will be discussed in this presentation.
Crack velocity jumps engendered by a transformational process zone
NASA Astrophysics Data System (ADS)
Boulbitch, A.; Korzhenevskii, A. L.
2016-06-01
We study a concerted propagation of a fast crack with the process zone where a rearrangement of the solid structure takes place. The latter is treated as a second-order local phase transformation. We demonstrate that the propagation of such a zone gives rise to a nonlinear frictionlike force exerted on the crack tip, resisting its propagation. Depending on the temperature, it produces three regimes of crack motion, which differ in the behavior of the crack tip process zone: (i) always existing, (ii) only emerging at a high crack speed, and (iii) flickering. We show that the latter regime exhibits crack velocity jumps.
Crack problems involving nonhomogeneous interfacial regions in bonded materials
NASA Technical Reports Server (NTRS)
Erdogan, F.
1990-01-01
Consideration is given to two classes of fracture-related solid mechanics problems in which the model leads to some physically anomalous results. The first is the interface crack problem associated with the debonding process in which the corresponding elasticity solution predicts severe oscillations of stresses and the crack surface displacements vary near the crack tip. The second deals with crack intersecting the interface. The nature of the solutions around the crack tips arising from these problems is reviewed. The rationale for introducing a new interfacial zone model is discussed, its analytical consequences within the context of the two crack-problem classes are described, and some examples are presented.
Fatigue Crack Closure Analysis Using Digital Image Correlation
NASA Technical Reports Server (NTRS)
Leser, William P.; Newman, John A.; Johnston, William M.
2010-01-01
Fatigue crack closure during crack growth testing is analyzed in order to evaluate the critieria of ASTM Standard E647 for measurement of fatigue crack growth rates. Of specific concern is remote closure, which occurs away from the crack tip and is a product of the load history during crack-driving-force-reduction fatigue crack growth testing. Crack closure behavior is characterized using relative displacements determined from a series of high-magnification digital images acquired as the crack is loaded. Changes in the relative displacements of features on opposite sides of the crack are used to generate crack closure data as a function of crack wake position. For the results presented in this paper, remote closure did not affect fatigue crack growth rate measurements when ASTM Standard E647 was strictly followed and only became a problem when testing parameters (e.g., load shed rate, initial crack driving force, etc.) greatly exceeded the guidelines of the accepted standard.
Application of the V(R) resistance curve method to fracture of various crack configurations
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Sutton, M. A.; Mcneill, S. R.
1988-01-01
The fracture resistance curve method of Newman (1985), based on the crack-tip-opening displacement, V(R), for a 'stationary' crack, was applied to various crack configurations in 2024-T351 and 7075-T651 aluminum alloys tested at room temperature. Using a stationary crack solution, the crack-tip-displacement was calculated at the current crack length for the crack configurations which included compact, middle-crack, single-edge-crack, and three-hole-crack tension specimens. The results showed that the V(R) resistance curves are insensitive to crack length, specimen width, and specimen type up to maximum load. After the maximum load is reached, the V(R) remains nearly constant; this constant depends only on specimen type, specimen width, and crack length. The V(R) resistance curve method can be used with the strip-yield analyses to accurately predict stable crack growth and instability of cracked metallic materials.
Frefer, Abdulbaset Ali; Raddad, Bashir S.
2013-12-16
In the present study, stress corrosion cracking (SCC) behavior of naturally aged advanced silver-bearing Al-Li alloy in NaCl solution was investigated using slow strain rate test (SSRT) method. The SSRT’s were conducted at different strain rates and applied potentials at room temperature. The results were discussed based on percent reductions in tensile elongation in a SCC-causing environment over those in air tended to express the SCC susceptbility of the alloy under study at T3. The SCC behavior of the alloy was also discussed based on the microstructural and fractographic examinations.
On Generating Fatigue Crack Growth Thresholds
NASA Technical Reports Server (NTRS)
Forth, Scott C.; Newman, James, Jr.; Forman, Royce G.
2003-01-01
The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. These experimental procedures can induce load history effects that result in crack closure. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake or blunt at the crack tip, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor range, Delta K, will increase, as will the crack growth rate. da/dN. A fatigue crack growth threshold test procedure is experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R. The authors have chosen to study a ductile aluminum alloy where the plastic deformations generated during testing may be of the magnitude to impact the crack opening.
Comparison of fatigue crack propagation in Modes I and III
Ritchie, R.O.
1985-06-01
The propagation behavior of fatigue cracks in Mode III (anti-plane shear), measured under cyclic torsion, is described and compared with more commonly encountered behavior under Mode I (tensile opening) loads. It is shown that a unique, global characterization of Mode III growth rates, akin to the Paris ''law'' in Mode I, is only possible if characterizating parameters appropriate to large-scale yielding are employed and allowance is made for crack tip shielding from sliding crack surface interference (i.e., friction and abrasion) between mating fracture surfaces. Based on the crack tip stress and deformation fields for Mode III stationary cracks, the cyclic crack tip displacement, (..delta..CTD/sub III/, and plastic strain intensity range ..delta..GAMMA/sub III/, have been proposed and are found to provide an adequate description of behavior in a range of steels, provided crack surface interference is minimized. The magnitude of this interference, which is somewhat analogous to crack closure in Mode I, is further examined in the light of the complex fractography of torsional fatigue failures and the question of a ''fatigue threshold'' for Mode III crack growth. Finally, micro-mechanical models for cyclic crack extension in anti-plane shear are briefly described, and the contrasting behavior between Mode III and Mode I cracks subjected to simple variable amplitude spectra is examined in terms of the differing role of crack tip blunting and closure in influencing shear, as opposed to tensile opening, modes of crack growth.
Ludwig, M E
2006-06-22
Subcontractors and vendors providing services, including the installation of purchased goods, are required to complete a TIP List. This list does not include every Environment, Safety, and Health (ES&H) related concern at LLNL. It is intended to highlight major concerns common to most on-site service activities.
ERIC Educational Resources Information Center
Santos-Trigo, Manuel
2004-01-01
A dynamic program for geometry called Cabri Geometry II is used to examine properties of figures like triangles and make connections with other mathematical ideas like ellipse. The technology tip includes directions for creating such a problem with technology and suggestions for exploring it.
Crack curving in a ductile pressurized fuselage
NASA Astrophysics Data System (ADS)
Lam, Paul W.
Moire interferometry was used to study crack tip displacement fields of a biaxially loaded cruciform type 0.8mm thick 2024-T3 aluminum specimen with various tearstrap reinforcement configurations: Unreinforced, Bonded, Bonded+Riveted, and Machined Pad-up. A program was developed using the commercially available code Matlab to derive strain, stress, and integral parameters from the experimental displacements. An FEM model of the crack tip area, with experimental displacements as boundary conditions, was used to validate FEM calculations of crack tip parameters. The results indicate that T*-integral parameter reaches a value of approximately 120 MPa-m0.5 during stable crack propagation which agrees with previously published values for straight cracks in the same material. The approximate computation method employed in this study uses a partial contour around the crack tip that neglects the contribution from the portion behind the crack tip where there is significant unloading. Strain distributions around the crack tip were obtained from experimental displacements and indicate that Maximum Principal Strain or Equivalent Strain can predict the direction of crack propagation, and is generally comparable with predictions using the Erdogan-Sih and Kosai-Ramulu-Kobayashi criteria. The biaxial tests to failure showed that the Machined Pad-up specimen carried the highest load, with the Bonded specimen next, at 78% of the Machined Pad-up value. The Bonded+Riveted specimen carried a lower load than the Bonded, at 67% of the Machined Pad-up value, which was the same as that carried by the Unreinforced specimen. The tearstraps of the bonded specimens remained intact after the specimen failed while the integrally machined reinforcement broke with the specimen. FEM studies were also made of skin flapping in typical Narrow and Wide-body fuselage sections, both containing the same crack path from a full-scale fatigue test of a Narrow-body fuselage. Results indicate that the
An elastic-plastic finite element analysis of crack initiation, stable crack growth, and instability
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1984-01-01
Studies have been conducted to develop efficient techniques to simulate crack extension and to examine various local and global fracture criteria. Of the considered criteria, the crack-tip-opening angle (CTOA) or displacement (CTOD) at a specified distance from the crack tip was shown to be most suited for modeling stable crack growth and instability during the fracture process. The results obtained in a number of studies show the necessity for studying different crack configurations when assessing the validity of any fracture criteria. One of the objectives of the present investigation is related to a critical evaluation of the CTOD growth criterion using an elastic-plastic finite element analysis under monotonic loading to failure. The analysis was found to predict three stages of crack growth behavior under monotonic loading to failure. Calculated CTOD values agreed well with experimental values for crack growth initiation.
Bonded orthotropic strips with cracks
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1978-01-01
The elastostatic problem for a nonhomogeneous plane which consists of two sets of periodically arranged dissimilar orthotropic strips is considered. First, the problem of cracks fully imbedded into the homogeneous strips is considered. Then, the singular behavior of the stresses for two special crack geometries is studied in some detail. The first is the case of a broken laminate in which the crack tips touch the interfaces. The second is the case of cracks crossing the interfaces. A number of numerical examples are worked out in order to separate the primary material parameters influencing the stress intensity factors and the powers of stress singularity, and to determine the trends regarding the influence of the secondary parameters. Finally, some numerical results are given for the stress intensity factors in certain basic crack geometries and for typical material combinations.
On the fracture toughness of advanced materials
Launey, Maximilien E.; Ritchie, Robert O.
2008-11-24
Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials, strength and toughness are almost mutually exclusive. In the first instance, such resistance to fracture is a function of bonding and crystal structure (or lack thereof), but can be developed through the design of appropriate nano/microstructures. However, the creation of tough microstructures in structural materials, i.e., metals, polymers, ceramics and their composites, is invariably a compromise between resistance to intrinsic damage mechanisms ahead of the tip of a crack (intrinsic toughening) and the formation of crack-tip shielding mechanisms which principally act behind the tip to reduce the effective 'crack-driving force' (extrinsic toughening). Intrinsic toughening is essentially an inherent property of a specific microstructure; it is the dominant form of toughening in ductile (e.g., metallic) materials. However, for most brittle (e.g., ceramic) solids, and this includes many biological materials, it is largely ineffective and toughening conversely must be developed extrinsically, by such shielding mechanisms as crack bridging. From a fracture mechanics perspective, this results in toughening in the form of rising resistance-curve behavior where the fracture resistance actually increases with crack extension. The implication of this is that in many biological and high-strength advanced materials, toughness is developed primarily during crack growth and not for crack initiation. This is an important realization yet is still rarely reflected in the way that toughness is measured, which is invariably involves the use of single-value (crack-initiation) parameters such as the
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.
Subcritical crack growth in marble
NASA Astrophysics Data System (ADS)
Nara, Yoshitaka; Nishida, Yuki; Toshinori, Ii; Harui, Tomoki; Tanaka, Mayu; Kashiwaya, Koki
2016-04-01
observed generally in glass. It is considered that Region II of subcritical crack growth in glass is controlled by the mass-transport to the crack tip. In general, Region II is not observed for subcritical crack growth in rock materials, because rocks contains water. Since the porosity of Carrara marble is very low, the contained amount of water is also very low. Therefore, it is considered that Region II is observed in Carrara marble. Since the crack velocity increased in the environment with higher temperature and humidity, it is concluded that the condition with low humidity in air is desirable for the long-term integrity of a carbonate rock mass.
Advances in Fatigue and Fracture Mechanics Analyses for Metallic Aircraft Structures
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
2000-01-01
This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked metallic structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crack-closure concept. Residual strength of cracked panels with severe out-of-plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.
An Evaluation of the Plasticity-Induced Crack-Closure Concept and Measurement Methods
NASA Technical Reports Server (NTRS)
Newman, James C., Jr.
1998-01-01
An assessment of the plasticity-induced crack-closure concept is made, in light of some of the questions that have been raised on the validity of the concept, and the assumptions that have been made concerning crack-dp damage below the crack-opening stress. The impact of using other crack-tip parameters, such as the cyclic crack-tip displacement, to model crack-growth rate behavior was studied. Crack-growth simulations, using a crack-closure model, showed a close relation between traditional Delta K eff, and the cyclic crack-tip displacement (Delta eff) for an aluminum alloy and a steel. Evaluations of the cyclic hysteresis energy demonstrated that the cyclic plastic damage below the crack-opening stress was negligible in the Paris crack-growth regime. Some of the standard and newly proposed remote measurement methods to determine the 'effective' crack-tip driving parameter were evaluated on middle-crack tension specimens. A potential source of the Kmax effect on crack-growth rates was studied on an aluminum alloy. Results showed that the ratio of Kmax to Kc had a strong effect on crack-growth rates at high stress ratios and at low stress ratios for very high stress levels. The crack-closure concept and the traditional crack-growth rate equations were able to correlate and predict crack-growth rates under these extreme conditions.
Crack interaction with 3-D dislocation loops
NASA Astrophysics Data System (ADS)
Gao, Huajian
CRACKS in a solid often interact with other crystal defects such as dislocation loops. The interaction effects are of 3-D character yet their analytical treatment has been mostly limited to the 2-D regime due to mathematical complications. This paper shows that distribution of the stress intensity factors along a crack front due to arbitrary dislocation loops may be expressed as simple line integrals along the loop contours. The method of analysis is based on the 3-D Bueckner-Rice weight function theory for elastic crack analysis. Our results have significantly simplified the calculations for 3-D dislocation loops produced in the plastic processes at the crack front due to highly concentrated crack tip stress fields. Examples for crack-tip 3-D loops and 2-D straight dislocations emerging from the crack tip are given to demonstrate applications of the derived formulae. The results are consistent with some previous analytical solutions existing in the literature. As further applications we also analyse straight dislocations that are parallel or perpendicular to the crack plane but are not parallel to the crack front.
Fatigue crack growth with single overload - Measurement and modeling
NASA Technical Reports Server (NTRS)
Davidson, D. L.; Hudak, S. J., Jr.; Dexter, R. J.
1987-01-01
This paper compares experiments with an analytical model of fatigue crack growth under variable amplitude. The stereoimaging technique was used to measure displacements near the tips of fatigue cracks undergoing simple variations in load amplitude-single overloads and overload/underload combinations. Measured displacements were used to compute strains, and stresses were determined from the strains. Local values of crack driving force (Delta-K effective) were determined using both locally measured opening loads and crack tip opening displacements. Experimental results were compared with simulations made for the same load variation conditions using Newman's FAST-2 model. Residual stresses caused by overloads, crack opening loads, and growth retardation periods were compared.
Crack growth direction in unidirectional off-axis graphite epoxy
NASA Technical Reports Server (NTRS)
Herakovich, C. T.; Gregory, M. A.; Beuth, J. L., Jr.
1984-01-01
An anisotropic elasticity crack tip stress analysis is implemented using three crack extension direction criteria (the normal stress ratio, the tensor polynominal and the strain energy density) to predict the direction of crack extension in unidirectional off axis graphite-epoxy. The theoretical predictions of crack extension direction are then compared with experimental results for 15 deg off axis tensile coupons with center cracks. Specimens of various aspect ratios and crack orientations are analyzed. It is shown that only the normal stress ratio criterion predicts the correct direction of crack growth.
Alcala, J.; Anglada, M.
1997-11-01
The influence of precracking techniques in the crack growth behavior of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) is investigated. Load-bridge and cyclic-compression precracking enhance subsequent tensile crack growth rates, in comparison to results that are found with precracks that are extended under four-point bending prior to testing. The actual influence of these precracking techniques in the near-threshold crack growth regime is remarkably different. Although load-bridge precracking produces a pattern of crack growth fluctuations for stress intensity factors, K, lower than the effective crack-growth threshold of the material, compression-fatigue precracks start to propagate under far-field tensile loads at very fast growth rates and for K values that are slightly higher than the effective threshold. Crack-tip shielding by tetragonal-to-monoclinic transformation develops gradually, influencing the crack growth behavior in Y-TZP. Proposed fatigue crack growth micromechanisms involve damage accumulation at the crack-tip region. For K{sub max} > 3 MPa{center_dot}m{sup 1/2}, fatigue crack growth rates are strongly affected by environmental interactions at the crack tip, and postulated fatigue micromechanisms include the cyclic degradation of crack-tip shielding.
Lee, S. Y.; Choo, Hahn; Liaw, Peter K; An, Ke; Hubbard, Camden R
2011-01-01
The combined effects of overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are suggested to be responsible for the observed changes in the crack opening load and resultant post-overload transient crack growth behavior [Lee SY, Liaw PK, Choo H, Rogge RB, Acta Mater 2010;59:485-94]. In this article, in situ neutron diffraction experiments were performed to quantify the influence of the combined effects by investigating the internal-stress evolution at various locations away from the crack tip. In the overload-retardation period, stress concentration occurs in the crack blunting region (an overload point) until a maximum crack arrest load is reached. The stress concentration is then transferred from the blunting region to the propagating crack tip (following the overload), requiring a higher applied load, as the closed crack is gradually opened. The transfer phenomena of the stress concentration associated with a crack opening process account for the nonlinearity of strain response in the vicinity of the crack tip. The delaying action of stress concentration at the crack tip is understood in conjunction with the concept of a critical stress (i.e. the stress required to open the closed crack behind the crack tip). A linear relationship between {Delta}{var_epsilon}{sub eff} and {Delta}K{sub eff} provides experimental support for the hypothesis that {Delta}K{sub eff} can be considered as the fatigue crack tip driving force.
NASA Astrophysics Data System (ADS)
Kim, Hyunok; Mohr, William; Yang, Yu-Ping; Zelenak, Paul; Kimchi, Menachem
2011-08-01
Numerical modeling of local formability, such as hole-edge cracking and shear fracture in bending of AHSS, is one of the challenging issues for simulation engineers for prediction and evaluation of stamping and crash performance of materials. This is because continuum-mechanics-based finite element method (FEM) modeling requires additional input data, "failure criteria" to predict the local formability limit of materials, in addition to the material flow stress data input for simulation. This paper presents a numerical modeling approach for predicting hole-edge failures during static bend tests of AHSS structures. A local-strain-based failure criterion and a stress-triaxiality-based failure criterion were developed and implemented in LS-DYNA simulation code to predict hole-edge failures in component bend tests. The holes were prepared using two different methods: mechanical punching and water-jet cutting. In the component bend tests, the water-jet trimmed hole showed delayed fracture at the hole-edges, while the mechanical punched hole showed early fracture as the bending angle increased. In comparing the numerical modeling and test results, the load-displacement curve, the displacement at the onset of cracking, and the final crack shape/length were used. Both failure criteria also enable the numerical model to differentiate between the local formability limit of mechanical-punched and water-jet-trimmed holes. The failure criteria and static bend test developed here are useful to evaluate the local formability limit at a structural component level for automotive crash tests.
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.
Bonded orthotropic strips with cracks
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1979-01-01
The elastostatic problem for a nonhomogeneous plane which consists of two sets of periodically arranged dissimilar orthotropic strips is considered. It is assumed that the plane contains a series of collinear cracks perpendicular to the interfaces and is loaded in tension away from and perpendicular to the cracks. The problem of cracks fully imbedded into the homogeneous strips is considered. The singular behavior of the stresses for two special crack geometries is studied. The first is the case of a broken laminate in which the crack tips touch the interfaces. The second is the case of cracks crossing the interfaces. An interesting result found from the analysis of the latter is that for certain orthotropic material combinations the stress state at the point of intersection of a crack and an interface may be bounded whereas in isotropic materials at this point stresses are always singular. A number of numerical examples are worked out to separate the primary material parameters influencing the stress intensity factors and the powers of stress singularity, and to determine the trends regarding the influence of the secondary parameters. Some numerical results are given for the stress intensity factors in certain basic crack geometries and for typical material combinations.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Salem, Jonathan A.; Holland, Frederic A.
1997-01-01
The two estimation methods, individual data and arithmetic mean methods, were used to determine the slow crack growth (SCG) parameters (n and D) of advanced ceramics and glass from a large number of room- and elevated-temperature constant stress-rate ('dynamic fatigue') test data. For ceramic materials with Weibull modulus greater than 10, the difference in the SCG parameters between the two estimation methods was negligible; whereas, for glass specimens exhibiting Weibull modulus of about 3, the difference was amplified, resulting in a maximum difference of 16 and 13 %, respectively, in n and D. Of the two SCG parameters, the parameter n was more sensitive to the estimation method than the other. The coefficient of variation in n was found to be somewhat greater in the individual data method than in the arithmetic mean method.
Combustion in cracks of PBX 9501
Berghout, H. L.; Son, S. F.; Bolme, C. A.; Hill, L. G.; Asay, B. W.; Dickson, P. M.; Henson, B. F.; Smilowitz, L. B.
2002-01-01
Recent experiments involving the combustion of PBX 9501 explosive under confined conditions reveal the importance of crack and flaws in reaction violence. Experiments on room temperature confined disks of pristine and thermally damaged PBX 9501 reveal that crack ignition depends on hot gases entering existing or pressure induced cracks rather than on energy release at the crack tip. PBX 9501 slot combustion experiments show that the reaction propagation rate in the slot does not depend on the external pressure. We have observed 1500 d s in long slots of highly-confined PBX 9501. We present experiments that examine the combustion of mechanically and thermally damaged samples of PBX 9501.
A three-dimensional validation of crack curvature in muscovite mica
J. C. Hill; J. W. Foulk III; P. A. Klein; E. P. Chen
2001-01-07
Experimental and computational efforts focused on characterizing crack tip curvature in muscovite mica. Wedge-driven cracks were propagated under monochromatic light. Micrographs verified the subtle curvature of the crack front near the free surface. A cohesive approach was employed to model mixed-mode fracture in a three-dimensional framework. Finite element calculations captured the crack curvature observed in experiment.
NASA Technical Reports Server (NTRS)
Tweedt, Daniel L.
2014-01-01
Computational Aerodynamic simulations of an 840 ft/sec tip speed, Advanced Ducted Propulsor fan system were performed at five different operating points on the fan operating line, in order to provide detailed internal flow field information for use with fan acoustic prediction methods presently being developed, assessed and validated. The fan system is a sub-scale, lownoise research fan/nacelle model that has undergone extensive experimental testing in the 9- by 15- foot Low Speed Wind Tunnel at the NASA Glenn Research Center, resulting in quality, detailed aerodynamic and acoustic measurement data. Details of the fan geometry, the computational fluid dynamics methods, the computational grids, and various computational parameters relevant to the numerical simulations are discussed. Flow field results for three of the five operating conditions simulated are presented in order to provide a representative look at the computed solutions. Each of the five fan aerodynamic simulations involved the entire fan system, excluding a long core duct section downstream of the core inlet guide vane. As a result, only fan rotational speed and system bypass ratio, set by specifying static pressure downstream of the core inlet guide vane row, were adjusted in order to set the fan operating point, leading to operating points that lie on a fan operating line and making mass flow rate a fully dependent parameter. The resulting mass flow rates are in good agreement with measurement values. The computed blade row flow fields for all five fan operating points are, in general, aerodynamically healthy. Rotor blade and fan exit guide vane flow characteristics are good, including incidence and deviation angles, chordwise static pressure distributions, blade surface boundary layers, secondary flow structures, and blade wakes. Examination of the computed flow fields reveals no excessive boundary layer separations or related secondary-flow problems. A few spanwise comparisons between
Propagation of stress corrosion cracks in alpha-brasses
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.
The transition from subsonic to supersonic cracks
Behn, Chris; Marder, M.
2015-01-01
We present the full analytical solution for steady-state in-plane crack motion in a brittle triangular lattice. This allows quick numerical evaluation of solutions for very large systems, facilitating comparisons with continuum fracture theory. Cracks that propagate faster than the Rayleigh wave speed have been thought to be forbidden in the continuum theory, but clearly exist in lattice systems. Using our analytical methods, we examine in detail the motion of atoms around a crack tip as crack speed changes from subsonic to supersonic. Subsonic cracks feature displacement fields consistent with a stress intensity factor. For supersonic cracks, the stress intensity factor disappears. Subsonic cracks are characterized by small-amplitude, high-frequency oscillations in the vertical displacement of an atom along the crack line, while supersonic cracks have large-amplitude, low-frequency oscillations. Thus, while supersonic cracks are no less physical than subsonic cracks, the connection between microscopic and macroscopic behaviour must be made in a different way. This is one reason supersonic cracks in tension had been thought not to exist. PMID:25713443
NASA Astrophysics Data System (ADS)
Lee, Sang-Kwon; Kramer, Daniel; Macdonald, Digby D.
2014-11-01
Evolution of the shape of surface cracks in sensitized Type 304 SS in Boiling Water Reactor primary coolant circuit piping at the reactor operating temperature of 288 °C is explored as a function of various environmental variables, such as electrochemical potential (ECP), solution conductivity, flow velocity, and multiplier for the oxygen reduction reaction (ORR) standard exchange current density (SECD), using the coupled environment fracture model (CEFM). For this work, the CEFM was upgraded by incorporating Shoji's model for calculating the crack tip strain rate and more advanced expressions were used for estimating the stress intensity factor for semi-elliptical surface cracks. This revised CEFM accurately predicts the dependence of the crack growth rate on stress intensity factor and offers an alternative explanation for the development of semi-elliptical cracks than that provided by fracture mechanics alone. The evolution of surface crack semi-elliptical shape depends strongly upon various environmental variables identified above, and the CEFM predicts that the minor axis of the ellipse should be oriented perpendicular to the surface, in agreement with observation. The development of the observed semi-elliptical cracks with the minor axis perpendicular to the surface is therefore attributed to the dependence of the crack growth rate on the electrochemical crack length.
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.
Variation of the energy release rate as a crack approaches and passes through an elastic inclusion
NASA Technical Reports Server (NTRS)
Li, Rongshun; Chudnovsky, A.
1993-01-01
The variation of the energy release rate (ERP) at the tip of a crack penetrating an elastic inclusion is analyzed using an approach involving modeling the random array of microcracks or other defects by an elastic inclusion with effective elastic properties. Computations are carried out using a finite element procedure. The eight-noded isoparametric serendipity element with the shift of the midpoint to the quarter-point is used to simulate the singularity at the crack tip, and the crack growth is accommodated by implementing a mesh regeneration technique. The ERP values were calculated for various crack tip positions which simulate the process of the crack approaching and penetrating the inclusion.
Modeling of crack bridging in a unidirectional metal matrix composite
NASA Technical Reports Server (NTRS)
Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack
1992-01-01
The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches: the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, Delta K(eff), was calculated using both models to correlate the measure crack growth rate in the composite. The calculated Delta K(eff) from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.
Generating Fatigue Crack Growth Thresholds with Constant Amplitude Loads
NASA Technical Reports Server (NTRS)
Forth, Scott C.; Newman, James C., J.; Forman, Royce G.
2002-01-01
The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. Some experimental procedures tend to induce load history effects that result in remote crack closure from plasticity. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor, K, will increase, as will the crack growth rate, da/dN. A fatigue crack growth threshold test procedure is developed and experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R.
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. PMID:22113876
NASA Technical Reports Server (NTRS)
Bakuckas, J. G.; Tan, T. M.; Lau, A. C. W.; Awerbuch, J.
1993-01-01
A finite element-based numerical technique has been developed to simulate damage growth in unidirectional composites. This technique incorporates elastic-plastic analysis, micromechanics analysis, failure criteria, and a node splitting and node force relaxation algorithm to create crack surfaces. Any combination of fiber and matrix properties can be used. One of the salient features of this technique is that damage growth can be simulated without pre-specifying a crack path. In addition, multiple damage mechanisms in the forms of matrix cracking, fiber breakage, fiber-matrix debonding and plastic deformation are capable of occurring simultaneously. The prevailing failure mechanism and the damage (crack) growth direction are dictated by the instantaneous near-tip stress and strain fields. Once the failure mechanism and crack direction are determined, the crack is advanced via the node splitting and node force relaxation algorithm. Simulations of the damage growth process in center-slit boron/aluminum and silicon carbide/titanium unidirectional specimens were performed. The simulation results agreed quite well with the experimental observations.
Crack problems for bonded nonhomogeneous materials under antiplane shear loading
NASA Technical Reports Server (NTRS)
Erdogan, F.
1984-01-01
The singular nature of the crack tip stress field in a nonhomogeneous medium with a shear modulus with a discontinuous derivative was investigated. The simplest possible loading and geometry, the antiplane shear loading of two bonded half spaces in which the crack is perpendicular to the interface is considered. It is shown that the square root singularity of the crack tip stress field is unaffected by the discontinuity in the derivative of the shear modulus. The problem is solved for a finite crack and results for the stress intensity factors are presented.
Crack problems for bonded nonhomogeneous materials under antiplane shear loading
NASA Technical Reports Server (NTRS)
Erdogan, F.
1985-01-01
The singular nature of the crack tip stress field in a nonhomogeneous medium having a shear modulus with a discontinuous derivative was investigated. The problem is considered for the simplest possible loading and geometry, namely the antiplane shear loading of two bonded half spaces in which the crack is perpendicular to the interface. It is shown that the square-root singularity of the crack tip stress field is unaffected by the discontinuity in the derivative of the shear modulus. The problem is solved for a finite crack and extensive results are given for the stress intensity factors.
Peloquin, John M; Elliott, Dawn M
2016-04-01
Cracks in fibrous soft tissue, such as intervertebral disc annulus fibrosus and knee meniscus, cause pain and compromise joint mechanics. A crack concentrates stress at its tip, making further failure and crack extension (fracture) more likely. Ex vivo mechanical testing is an important tool for studying the loading conditions required for crack extension, but prior work has shown that it is difficult to reproduce crack extension. Most prior work used edge crack specimens in uniaxial tension, with the crack 90° to the edge of the specimen. This configuration does not necessarily represent the loading conditions that cause in vivo crack extension. To find a potentially better choice for experiments aiming to reproduce crack extension, we used finite element analysis to compare, in factorial combination, (1) center crack vs. edge crack location, (2) biaxial vs. uniaxial loading, and (3) crack-fiber angles ranging from 0° to 90°. The simulated material was annulus fibrosus fibrocartilage with a single fiber family. We hypothesized that one of the simulated test cases would produce a stronger stress concentration than the commonly used uniaxially loaded 90° crack-fiber angle edge crack case. Stress concentrations were compared between cases in terms of fiber-parallel stress (representing risk of fiber rupture), fiber-perpendicular stress (representing risk of matrix rupture), and fiber shear stress (representing risk of fiber sliding). Fiber-perpendicular stress and fiber shear stress concentrations were greatest in edge crack specimens (of any crack-fiber angle) and center crack specimens with a 90° crack-fiber angle. However, unless the crack is parallel to the fiber direction, these stress components alone are insufficient to cause crack opening and extension. Fiber-parallel stress concentrations were greatest in center crack specimens with a 45° crack-fiber angle, either biaxially or uniaxially loaded. We therefore recommend that the 45° center crack case be
The Dugdale crack on bimaterial interface
NASA Astrophysics Data System (ADS)
Wang, X.-M.; Shen, Y.-P.
1993-01-01
The hypothesis of the Dugdale model (which assumes that the cohesive zone appears ahead of the crack tip and the normal cohesive stress acting on the surface of the zone makes the stress intensity factor at the zone tip equal zero) is extended to cases where not only the small-scale cohesive zone is supposed to exist ahead of a semiinfinite crack tip but the cohesive zone is subject to undetermined normal and shear cohesive stres which meets the Mises yield condition. The cohesive zone is measured, and the relation between the cohesive zone and the contact zone is discussed. The results obtained are different from the results of the Dugdale model for homogeneous brittle crack problems.
Elastic-plastic analysis of growing cracks
Rice, J.R.; Drugan, W.J.; Sham, T.L.
1980-01-01
The elastic-plastic stress and deformation fields at the tip of a crack which grow in an ideally plastic solid under plane strain, small-scale yielding conditions is discussed. Asymptotic analysis suggests a crack-tip stress state similar to that of the classical Prandtl field, but containing elastic unloading between the centered fan region and the trailing constant stress plastic region. The near tip expression for the rate of opening displacement delta at distance r from the growing tip is found to have the same form suggested by Rice and Sorensen, delta = ..cap alpha..J/sigma/sub 0/ + ..beta..(sigma/sub 0//E)a ln (R/r), but now the presence of the elastic wedge causes ..beta.. to have the revised value of 5.08 (for Poisson ratio ..nu.. = 0.3). Here, a = crack length, sigma/sub 0/ = yield strength, E = elastic modulus, and J denotes the far-field value (1 - ..nu../sup 2/) K/sup 2//E for the small scale yielding conditions considered. The parameters ..cap alpha.. and R cannot be determined from the asymptotic analysis, but ..cap alpha.. is approximately the same for stationary and growing cracks, and R scales approximately with the size of the plastic zone, being about 15 to 30% larger. For large scale yielding, a similar form applies with possible variations in ..cap alpha.. and ..beta.., at least in cases which maintain triaxial constraint at the crack tip, but in the fully yielded case R is expected to be proportional to the dimension of the uncracked ligament. The model crack growth criterion of Rice and Sorensen, requiring a critical delta at some fixed r from the tip, is reexamined. Results suggest that the J versus ..delta..a relation describing growth will be dependent on the extent of yielding, although it is suggested that this dependency might be small for highly ductile materials, provided that a similar triaxial constraint is maintained in all cases.
Interface crack in a nonhomogeneous elastic medium
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1988-01-01
The linear elasticity problem for an interface crack between two bonded half planes is reconsidered. It is assumed that one of the half planes is homogeneous and the second is nonhomogeneous in such a way that the elastic properties are continuous throughout the plane and have discontinuous derivatives along the interface. The problem is formulated in terms of a system of integral equations and the asymptotic behavior of the stress state near the crack tip is determined. The results lead to the conclusion that the singular behavior of stresses in the nonhomogeneous medium is identical to that in a homogeneous material provided the spacial distribution of material properties is continuous near and at the crack tip. The problem is solved for various values of the nonhomogeneity parameter and for four different sets of crack surface tractions, and the corresponding stress intensity factors are tabulated.
The crack problem in bonded nonhomogeneous materials
NASA Technical Reports Server (NTRS)
Erdogan, F.; Joseph, P. F.; Kaya, A. C.
1991-01-01
The plane elasticity problem for two bonded half planes containing a crack perpendicular to the interface was considered. The effect of very steep variations in the material properties near the diffusion plane on the singular behavior of the stresses and stress intensity factors were studied. The two materials were thus, assumed to have the shear moduli mu(o) and mu(o) exp (Beta x), x=0 being the diffusion plane. Of particular interest was the examination of the nature of stress singularity near a crack tip termination at the interface where the shear modulus has a discontinuous derivative. The results show that, unlike the crack problem in piecewise homogeneous materials for which the singularity is of the form r/alpha, 0 less than alpha less than 1, in this problem the stresses have a standard square-root singularity regardless of the location of the crack tip. The nonhomogeneity constant Beta has, however, considerable influence on the stress intensity factors.
The crack problem in bonded nonhomogeneous materials
NASA Technical Reports Server (NTRS)
Erdogan, Fazil; Kaya, A. C.; Joseph, P. F.
1988-01-01
The plane elasticity problem for two bonded half planes containing a crack perpendicular to the interface was considered. The effect of very steep variations in the material properties near the diffusion plane on the singular behavior of the stresses and stress intensity factors were studied. The two materials were thus, assumed to have the shear moduli mu(o) and mu(o) exp (Beta x), x=0 being the diffusion plane. Of particular interest was the examination of the nature of stress singularity near a crack tip terminating at the interface where the shear modulus has a discontinuous derivative. The results show that, unlike the crack problem in piecewise homogeneous materials for which the singularity is of the form r/alpha, 0 less than alpha less than 1, in this problem the stresses have a standard square-root singularity regardless of the location of the crack tip. The nonhomogeneity constant Beta has, however, considerable influence on the stress intensity factors.
Dislocation shielding of a cohesive crack
NASA Astrophysics Data System (ADS)
Bhandakkar, Tanmay K.; Chng, Audrey C.; Curtin, W. A.; Gao, Huajian
2010-04-01
Dislocation interaction with a cohesive crack is of increasing importance to computational modelling of crack nucleation/growth and related toughening mechanisms in confined structures and under cyclic fatigue conditions. Here, dislocation shielding of a Dugdale cohesive crack described by a rectangular traction-separation law is studied. The shielding is completely characterized by three non-dimensional parameters representing the effective fracture toughness, the cohesive strength, and the distance between the dislocations and the crack tip. A closed form analytical solution shows that, while the classical singular crack model predicts that a dislocation can shield or anti-shield a crack depending on the sign of its Burgers vector, at low cohesive strengths a dislocation always shields the cohesive crack irrespective of the Burgers vector. A numerical study shows the transition in shielding from the classical solution of Lin and Thomson (1986) in the high strength limit to the solution in the low strength limit. An asymptotic analysis yields an approximate analytical model for the shielding over the full range of cohesive strengths. A discrete dislocation (DD) simulation of a large (>10 3) number of edge dislocations interacting with a cohesive crack described by a trapezoidal traction-separation law confirms the transition in shielding, showing that the cohesive crack does behave like a singular crack at very high cohesive strengths (˜7 GPa), but that significant deviations in shielding between singular and cohesive crack predictions arise at cohesive strengths around 1GPa, consistent with the analytic models. Both analytical and numerical studies indicate that an appropriate crack tip model is essential for accurately quantifying dislocation shielding for cohesive strengths in the GPa range.
... older obese people. Question: Can cracking knuckles / joints lead to arthritis? Answer: There is no evidence of ... or damaged joints due to arthritis could potentially lead more easily to ligament injury or acute trauma ...
Crack propagation, arrest and statistics in heterogeneous materials.
Kierfeld, J.; Vinokur, V.; Materials Science Division; Dortmund Univ. of Technology
2008-04-01
We investigate theoretically statistics and thermally activated dynamics of crack nucleation and propagation in a two-dimensional heterogeneous material containing quenched randomly distributed defects. We consider a crack tip dynamics accounting for dissipation, thermal noise and the random forces arising from the elastic interactions of the crack opening with the defects. The equation of motion is based on the generalized Griffith criterion and the dynamic energy release rate and gives rise to Langevin-type stochastic dynamics in a quenched disordered potential. For different types of quenched random forces, which are characterized (a) by the range of elastic interactions with the crack tip and (b) the range of correlations between defects, we derive a number of static and dynamic quantities characterizing crack propagation in heterogeneous materials both at zero temperature and in the presence of thermal activation. In the absence of thermal fluctuations we obtain the nucleation and propagation probabilities, typical arrest lengths, the distribution of crack lengths and of critical forces. For thermally activated crack propagation we calculate the mean time to fracture. Depending on the range of elastic interactions between crack tip and frozen defects, heterogeneous material exhibits brittle or ductile fracture. We find that aggregations of defects generating long-range interaction forces (e.g. clouds of dislocations) lead to anomalously slow creep of the crack tip or even to its complete arrest. We demonstrate that heterogeneous materials with frozen defects contain a large number of arrested microcracks and that their fracture toughness is enhanced to the experimentally accessible timescales.
Opening and closing of cracks at high cyclic strains
NASA Technical Reports Server (NTRS)
Iyyer, N. S.; Dowling, N. E.
1986-01-01
The closure behavior of cracks of different length and at different cyclic strain levels (ranging from predominantly elastic to grossly plastic strains) was studied to observe the effect of residual crack-tip plasticity on crack closure. Cracks were initiated either naturally or artificially (from electric discharge machining pits) in uniaxial test specimens of strengthened alloy steel AISI 4340 with a grain size of 0.016 mm. It was found that, at high strains, cracks closed only when the lowest stress level in the cycle was approached. The stress or the strain opening level depended upon the exact point along the crack length where the observations were made. As the plastic deformation increased, the relative crack opening level was found to decrease and approach the value of stress ratio R. The experimental results were compared with those of three analytical models of crack closure and opening, demonstrating the limitations of the currently available elastic-plastic crack growth analysis.
Crack initiation under generalized plane strain conditions
Shum, D.K.M.; Merkle, J.G.
1991-01-01
A method for estimating the decrease in crack-initiation toughness, from a reference plane strain value, due to positive straining along the crack front of a circumferential flaw in a reactor pressure vessel is presented in this study. This method relates crack initiation under generalized plane strain conditions with material failure at points within a distance of a few crack-tip-opening displacements ahead of a crack front, and involves the formulation of a micromechanical crack-initiation model. While this study is intended to address concerns regarding the effects of positive out-of- plane straining on ductile crack initiation, the approach adopted in this work can be extended in a straightforward fashion to examine conditions of macroscopic cleavage crack initiation. Provided single- parameter dominance of near-tip fields exists in the flawed structure, results from this study could be used to examine the appropriateness of applying plane strain fracture toughness to the evaluation of circumferential flaws, in particular to those in ring-forged vessels which have no longitudinal welds. In addition, results from this study could also be applied toward the analysis of the effects of thermal streaming on the fracture resistance of circumferentially oriented flaws in a pressure vessel. 37 refs., 8 figs., 1 tab.
NASA Technical Reports Server (NTRS)
Bassani, J. L.; Erdogan, F.
1979-01-01
The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks is considered. For the semi-infinite crack the problem is solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses are calculated. For finite cracks the problem is reduced to a pair of integral equations. Numerical results are obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles.
NASA Technical Reports Server (NTRS)
Bassani, J. L.; Erdogan, F.
1978-01-01
The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks was considered. For the semi-infinite crack the problem was solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses were calculated. For finite cracks the problem was reduced to a pair of integral equations. Numerical results were obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles.
A fracture criterion for widespread cracking in thin-sheet aluminum alloys
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Dawicke, D. S.; Sutton, M. A.; Bigelow, C. A.
1993-01-01
An elastic-plastic finite-element analysis was used with a critical crack-tip-opening angle (CTOA) fracture criterion to model stable crack growth in thin-sheet 2024-T3 aluminum alloy panels with single and multiple-site damage (MSD) cracks. Comparisons were made between critical angles determined from the analyses and those measured with photographic methods. Calculated load against crack extension and load against crack-tip displacement on single crack specimens agreed well with test data even for large-scale plastic deformations. The analyses were also able to predict the stable tearing behavior of large lead cracks in the presence of stably tearing MSD cracks. Small MSD cracks significantly reduced the residual strength for large lead cracks.
Observations on hydrogen induced delayed plasticity and cracking in 4340 steel
Wert, J.A.
1983-02-01
The crack growth rates measured by potential drop provided strong evidence that crack advance occurs continuously during hydrogen assisted cracking. If crack growth occurred by the stepwise HIDP-C mechanism, variations in growth rate would be expected to appear in the potential drop results. For example, during the period of plastic zone extension, a low crack growth rate would be expected, followed by a period of higher crack growth rate during actual extension of the crack. The results obtained in this investigation do not eliminate the possibility that stepwise crack growth occurred at different points along the crack front at different times, appearing as continuous average crack advance. Furthermore, these results do not provide evidence for the mechanism of crack extension. The results do show that the average crack front advance is continuous during hydrogen assisted cracking, not stepwise, as would be expected for the HIDP-C mechanism of crack growth. 13 references.
Cessation of environmentally-assisted cracking in a low-alloy steel: Theoretical analysis
Wire, G.L.
1997-02-01
Environmentally Assisted Cracking (EAC) can cause increases in fatigue crack growth rates of 40 to 100 times the rate in air for low alloy steels. The increased rates can lead to very large predicted crack growth. EAC is activated by a critical level of dissolved sulfides at the crack tip. Sulfide inclusions (MnS) in the steel produce corrosive sulfides in solution following exposure by a growing crack. In stagnant, low oxygen water conditions considered here, diffusion is the dominant mass transport mechanism acting to change the sulfide concentration within the crack. The average crack tip velocity is below the level required to produce the critical crack tip sulfide ion concentration required for EAC. Crack extension analyses also consider the breakthrough of large, hypothetical embedded defects with the attendant large freshly exposed sulfide inventory. Combrade et al. noted that a large inventory of undissolved metallurgical sulfides on crack flanks could trigger EAC, but did not quantify the effects. Diffusion analysis is extended herein to cover breakthrough of embedded defects with large sulfide inventories. The mass transport via diffusion is limited by the sulfide solubility. As a result, deep cracks in high sulfur steels are predicted to retain undissolved sulfides for extended but finite periods of time t{sub diss} which increase with the crack length and the metallurgical sulfide content in the steel. The analysis shows that the duration of EAC is limited to t{sub diss} providing V{sub eac}, the crack tip velocity associated with EAC is less than V{sub In}, the crack tip velocity below which EAC will not occur in an initially sulfide free crack. This condition on V{sub eac} need only be met for a short time following crack cleanup to turn off EAC. The predicted crack extension due to limited duration of EAC is a small fraction of the initial embedded defect size and would not greatly change calculated crack depths.
Determination of the crack resistance of a ceramic in bending of beams with a notch
Gogotsi, G.A.; Petrenko, V.P.; Zavada, V.P.
1985-06-01
These authors investigate crack resistance in new ceramic materials with particular regard to temperature and to the features of notches. Charts present data on the relationship of experimental values of crack resistance to crack depth for silicon carbide; on the influence of notch depth on crack resistance; on the relationship of stress intensity to notch tip radius; on characteristic load-time curves for various materials; and on the relationship of temperature to crack resistance.
Green's function for cracks of equal length from a circular hole in an infinite sheet
NASA Technical Reports Server (NTRS)
Shivakumar, V.; Forman, R. G.
1981-01-01
Stress-intensity factors are obtained for twin collinear cracks emanating from a hole in an infinite plate. Point forces that tend to make the crack surfaces separate and slide are applied to one of the cracks. A conformal mapping and the Muskelishvili formulation involving series representation of the Kolosov stress functions are used to derive the solutions. The variation of stress-intensity factor with crack length and with point of application of the loads is presented for both crack tips.
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Deformation fields near a steady fatigue crack with anisotropic plasticity
Gao, Yanfei
2015-11-30
In this work, from finite element simulations based on an irreversible, hysteretic cohesive interface model, a steady fatigue crack can be realized if the crack extension exceeds about twice the plastic zone size, and both the crack increment per loading cycle and the crack bridging zone size are smaller than the plastic zone size. The corresponding deformation fields develop a plastic wake behind the crack tip and a compressive residual stress field ahead of the crack tip. In addition, the Hill’s plasticity model is used to study the role of plastic anisotropy on the retardation of fatigue crack growth andmore » the elastic strain fields. It is found that for Mode-I cyclic loading, an enhanced yield stress in directions that are inclined from the crack plane will lead to slower crack growth rate, but this retardation is insignificant for typical degrees of plastic anisotropy. Furthermore, these results provide key inputs for future comparisons to neutron and synchrotron diffraction measurements that provide full-field lattice strain mapping near fracture and fatigue crack tips, especially in textured materials such as wrought or rolled Mg alloys.« less
Deformation fields near a steady fatigue crack with anisotropic plasticity
Gao, Yanfei
2015-11-30
In this work, from finite element simulations based on an irreversible, hysteretic cohesive interface model, a steady fatigue crack can be realized if the crack extension exceeds about twice the plastic zone size, and both the crack increment per loading cycle and the crack bridging zone size are smaller than the plastic zone size. The corresponding deformation fields develop a plastic wake behind the crack tip and a compressive residual stress field ahead of the crack tip. In addition, the Hill’s plasticity model is used to study the role of plastic anisotropy on the retardation of fatigue crack growth and the elastic strain fields. It is found that for Mode-I cyclic loading, an enhanced yield stress in directions that are inclined from the crack plane will lead to slower crack growth rate, but this retardation is insignificant for typical degrees of plastic anisotropy. Furthermore, these results provide key inputs for future comparisons to neutron and synchrotron diffraction measurements that provide full-field lattice strain mapping near fracture and fatigue crack tips, especially in textured materials such as wrought or rolled Mg alloys.
NASA Technical Reports Server (NTRS)
Boyce, Lola; Lovelace, Thomas B.
1989-01-01
FORTRAN program RANDOM2 is presented in the form of a user's manual. RANDOM2 is based on fracture mechanics using a probabilistic fatigue crack growth model. It predicts the random lifetime of an engine component to reach a given crack size. Details of the theoretical background, input data instructions, and a sample problem illustrating the use of the program are included.
A circumferential crack in a cylindrical shell under tension.
NASA Technical Reports Server (NTRS)
Duncan-Fama, M. E.; Sanders, J. L., Jr.
1972-01-01
A closed cylindrical shell under uniform internal pressure has a slit around a portion of its circumference. Linear shallow shell theory predicts inverse square-root-type singularities in certain of the stresses at the crack tips. This paper reports the computed strength of these singularities for different values of a dimensionless parameter based on crack length, shell radius and shell thickness.
Metal crack propagation monitoring by photoluminescence enhancement of quantum dots.
Zhao, Ziming; Luan, Weiling; Yin, Shaofeng; Brandner, Juergen J
2015-07-20
A visualization method for monitoring minor metal crack propagation is presented in this paper. Through CdS@ZnS core-shell quantum dots (QDs) enhanced emission of photoluminescence (PL), this crack detection method provides a visualization signal in real time and through a noncontact fashion. The crack of the CdS@ZnS core-shell QDs-epoxy resin kept a synchronous propagation with the metal crack. Detection of the tip growth in the film layers demonstrated that the actual crack propagation on the metal surface could be deduced from the tips in the film layers. The fluorescence peak tended to increase along the crack from the initial opening to the tip. Crack width as small as 10 μm can be detected with a precision of 0.1 μm and the minimum crack tip width of the QDs-epoxy resin was measured as 0.72 μm. PMID:26367834
An analysis of creep crack growth of interface cracks in layered/graded materials
Biner, S.B.
1997-07-01
In this study, the growth behavior of interface cracks in bimaterials and in layered materials resulting from the creep cavitation was studied. The growth model includes the effects of material deposition resulting from the growth of creep cavities on the crack tip stress fields. The results indicate that in layered materials under identical applied loading, the location of the interface crack strongly influence the amplitude of the stress field at steady-state. Due to large variation in the distribution of the stresses ahead of the interface cracks at creep regime, depending upon the crack location, the creep crack growth rates will be significantly different from each other under identical loading for a given layered material.
The crack problem for a nonhomogeneous plane
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1983-01-01
The plane elasticity problem for a nonhomogeneous medium containing a crack is considered. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy type kernel. Hence, its solution and the stresses around the crack tips have the conventional square root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible.
Crack problem for a nonhomogeneous plane
Delale, F.; Erdogan, F.
1983-09-01
This study considers the plane elasticity problem for a nonhomogeneous medium containing a crack. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then, the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy-type kernel. Hence, its solution and the stresses around the crack tips have the conventional square-root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible. 14 references.
The crack problem for a nonhomogeneous plane
NASA Technical Reports Server (NTRS)
Delale, F.; Erdogan, F.
1982-01-01
The plane elasticity problem for a nonhomogeneous medium containing a crack is considered. It is assumed that the Poisson's ratio of the medium is constant and the Young's modulus E varies exponentially with the coordinate parallel to the crack. First the half plane problem is formulated and the solution is given for arbitrary tractions along the boundary. Then the integral equation for the crack problem is derived. It is shown that the integral equation having the derivative of the crack surface displacement as the density function has a simple Cauchy type kernel. Hence, its solution and the stresses around the crack tips have the conventional square root singularity. The solution is given for various loading conditions. The results show that the effect of the Poisson's ratio and consequently that of the thickness constraint on the stress intensity factors are rather negligible.
NASA Astrophysics Data System (ADS)
Shang, Jian Ku; Ritchie, R. O.
1989-05-01
Micro-mechanisms of crack-tip shielding associated with the growth of fatigue cracks in metalmatrix composites are examined with specific emphasis on the role of crack bridging by uncracked ligaments. Simple analytical models are developed for such bridging induced by both overlapping cracks and by coplanar ligaments in the wake of the crack tip; the models are based on respective notions of a critical tensile strain or critical crack-opening displacement in the ligament. The predicted degree of shielding derived from these mechanisms is not large, but is found to be consistent with experimental observations in high-strength P/M aluminum alloys reinforced with 15 to 20 vol pct of SiC particulate.
Shyam, Amit; Lara-Curzio, Edgar
2010-01-01
The fatigue crack growth process involves damage accumulation and crack extension. The two sub-processes that lead to fatigue crack extension were quantified separately in a recent model for small fatigue crack growth applicable to engineering alloys. Here, we report the results of an experimental investigation to assess the assumptions of that model. The fatigue striation formation in an aluminum alloy is modeled and it is verified that the number of cycles required for striation formation is related to the cyclic crack tip opening displacement and that the striation spacing is related to the monotonic crack tip displacement. It is demonstrated that extensive cyclic crack tip plasticity in the aluminum alloy causes a reduction in the magnitude of the slope of the fatigue crack propagation curves. The implications of these results on the fatigue crack propagation lifetime calculations are identified.
Effective crack lengths by compliance measurement for ARALL-2 laminates
NASA Technical Reports Server (NTRS)
Wilson, Christopher D.; Wilson, Dale A.
1991-01-01
As a means of determining a stress intensity factor solution, the compliance properties of an ARALL-2 laminated-sheet composite were investigated. Fatigue crack growth rate (FCGR) tests were conducted on middle crack tension (MT) specimens fabricated from a layup consisting of three sheets of 2024-T3 aluminum bonded together with unidirectional aramid fibers embedded in epoxy. Excellent fatigue crack growth properties are obtained by the presence of unbroken aramid fibers in the wake of the crack tip. These unbroken fibers act as a bridging mechanism to inhibit further crack growth. To quantify the effect of maximum fatigue load on compliance, a series of FCGR tests were performed. Effective crack lengths were determined to be at least 10 mm shorter than surface measured crack lengths for a 76-mm-wide specimen. The bridging zone was estimated to be at least 5 mm. Compliance and stress intensity factor as functions of effective crack length were determined.
Crack Turning in Integrally Stiffened Aircraft Structures
NASA Technical Reports Server (NTRS)
Pettit, Richard Glen
2000-01-01
Current emphasis in the aircraft industry toward reducing manufacturing cost has created a renewed interest in integrally stiffened structures. Crack turning has been identified as an approach to improve the damage tolerance and fail-safety of this class of structures. A desired behavior is for skin cracks to turn before reaching a stiffener, instead of growing straight through. A crack in a pressurized fuselage encounters high T-stress as it nears the stiffener--a condition favorable to crack turning. Also, the tear resistance of aluminum alloys typically varies with crack orientation, a form of anisotropy that can influence the crack path. The present work addresses these issues with a study of crack turning in two-dimensions, including the effects of both T-stress and fracture anisotropy. Both effects are shown to have relation to the process zone size, an interaction that is central to this study. Following an introduction to the problem, the T-stress effect is studied for a slightly curved semi-infinite crack with a cohesive process zone, yielding a closed form expression for the future crack path in an infinite medium. For a given initial crack tip curvature and tensile T-stress, the crack path instability is found to increase with process zone size. Fracture orthotropy is treated using a simple function to interpolate between the two principal fracture resistance values in two-dimensions. An extension to three-dimensions interpolates between the six principal values of fracture resistance. Also discussed is the transition between mode I and mode II fracture in metals. For isotropic materials, there is evidence that the crack seeks out a direction of either local symmetry (pure mode I) or local asymmetry (pure mode II) growth. For orthotropic materials the favored states are not pure modal, and have mode mixity that is a function of crack orientation.
Cerkes, Nazim
2016-01-01
Nasal tip deficiency can be congenital or secondary to previous nasal surgeries. Underdeveloped medial crura usually present with underprojected tip and lack of tip definition. Weakness or malposition of lateral crura causes alar rim retraction and lateral nasal wall weakness. Structural grafting of alar cartilages strengthens the tip framework, reinforces the disrupted support mechanisms, and controls the position of the nasal tip. In secondary cases, anatomic reconstruction of the weakened or interrupted alar cartilages and reconstitution of a stable nasal tip tripod must be the goal for a predictable outcome. PMID:26616702
Micromechanisms of fatigue crack propagation in particulate-reinforced metal-matrix composites
Shang, Jianku.
1989-01-01
Consequences of the interaction of cracks with SiC particles are examined with emphasis on micromechanisms influencing fatigue crack propagation in high strength aluminum alloy matrix composites. Fatigue crack propagation is found to show three distinct regimes; each accompanied by growth mechanisms reflecting different roles of SiC particles. At near-threshold levels, SiC particles impeded fatigue crack growth by deflecting the crack to promote roughness-induced crack closure and by acting as crack traps along the crack front. A two-dimensional crack trapping analysis based on the interaction of a finite crack with a SiC particle indicates that a limiting criterion for fatigue crack growth in SiC{sub p}/Al composites can be established, which requires that the maximum plastic-zone size exceed the effective mean particle size or that the tensile stress in the matrix beyond the particle on the crack front exceed the yield strength of the material. Implications of crack closure and crack trapping to near-threshold crack growth, including load-ration and particle-size dependence of fatigue thresholds, are discussed in terms of contributions from each mechanism. At higher stress intensities, limited fracture of SiC particles ahead of the crack tip leads to the development of uncracked ligaments along the crack length, resulting in a reduced crack-tip stress intensity from ligament bridging. Micromechanical models are developed for such bridges induced by both overlapping cracks and co-planar ligaments, based on the notion of a limiting crack opening displacement or limiting strain in the ligament. The predicted reduction in crack tip stress intensity is shown to be consistent with experimental observations.
Growth rate models for short surface cracks in AI 2219-T851
NASA Astrophysics Data System (ADS)
Morris, W. L.; James, M. R.; Buck, O.
1981-01-01
Rates of fatigue propagation of short Mode I surface cracks in Al 2219-T851 are measured as a function of crack length and of the location of the surface crack tips relative to the grain boundaries. The measured rates are then compared to values predicted from crack growth models. The crack growth rate is modeled with an underlying assumption that slip responsible for early propagation does not extend in significant amounts beyond the next grain boundary in the direction of crack propagation. Two models that contain this assumption are combined: 1) cessation of propagation into a new grain until a mature plastic zone is developed; 2) retardation of propagation by crack closure stress, with closure stress calculated from the location of a crack tip relative to the grain boundary. The transition from short to long crack growth behavior is also discussed.
Constraint effects observed in crack initiation stretch
Lambert, D.M.; Ernst, H.A.
1995-12-31
The current paper characterizes constraint in fracture: J-modified resistance (Jr) curves were developed for two tough structural materials, 6061-T651 (aluminum) and IN718-STA1 (nickel-base superalloy). A wide variety of configurations was tested to consider load configurations from bending to tension including three specimen types (compact tension, center-crack tension, and single-edge notched tension), and a range of ligament lengths and thicknesses, as well as side-grooved and smooth-sided ligaments. The Jr curves exhibited an inflection point after some crack extension, and the data were excluded beyond the inflection. Qualified Jr curves for the two materials showed similar behavior, but R-curves were identical for equal ligament length-to-thickness ratio (RL), for the aluminum alloy, with increasing slope for increasing RL, while for the nickel, the resistance curves aligned for equal ligament thickness, B, and the slope increased for decreasing B. Displacements at the original crack tip (CToD) were recorded throughout the test for several specimens. CToD-versus-crack extension curves were developed, and data were excluded beyond the inflection point (as with the Jr curves). The data collapsed into two distinct curves, thought to represent the surface, plane stress effect and the central, plane strain effect. This was observed for both materials. A technique called profiling is presented for the aluminum alloy only, where the crack face displacements are recorded at the final point of the test as a function of the position throughout the crack cavity, along with an effort to extract the observations in a usable form. Displacements were consistent throughout the cross-section at and behind the original crack tip. In the region where the crack grew, this displacement was developed by a combination of stretch and crack growth. The stretch required to initiate crack extension was a function of the depth beneath the surface into the cross-section.
Crack Healing in Quartz: Influence of Crack Morphology and pOH-
NASA Astrophysics Data System (ADS)
Fallon, J. A.; Kronenberg, A. K.; Popp, R. K.; Lamb, W. M.
2004-12-01
Crack healing in quartz has been investigated by optical microscopy and interferometry of rhombohedral r-cleavage cracks in polished Brazilian quartz prisms that were hydrothermally annealed. Quartz prisms were pre-cracked at room temperature and then annealed at temperatures T of 250° and 400° C for 2.4 to 240 hours, fluid pressure Pf = 41 MPa (equal to confining pressure Pc), and varying pOH- (from 5.4 to 1.2 at 250° C for fluids consisting of distilled water and NaOH solutions). Crack morphologies before and after annealing were recorded for each sample in plane light digital images and apertures were determined from interference fringes recorded using transmitted monochromatic light (λ = 598 nm). As documented in previous studies (Smith and Evans, 1984; Brantley et al., 1990; Beeler and Hickman, 1996), crack healing of quartz is driven by reductions in surface energy and healing rates appear to be limited by diffusional solute transport; sharply defined crack tips become blunted and break up into fluid-filled tubes and inclusions. However, fluid inclusion geometries are also observed with nonequilibrium shapes that depend on initial surface roughness. Crack healing is significant at 400° C after short run durations (24 hr) with healing rates reaching 10-5 mm/s. Crack healing is also observed at T=250° C, but only for smooth cracks with apertures < 0.6 μ m or for cracks subject to low pOH-. The extent of crack healing is sensitive to crack aperture and to hackles formed by fine-scale crack branching during crack growth. Initial crack apertures appear to be governed by the presence of fine particles, often found in the vicinity of hackles, which maintain the separation of crack surfaces. Where rough cracks exhibit healing, hackles are sites of either enhanced or reduced loss of fluid-solid interface depending on slight mismatches and sense of twist of opposing crack surfaces. Hackles of open r-cleavage cracks are replaced either by (1) healed curvilinear
Dynamics of cracking in drying colloidal sheets.
Sengupta, Rajarshi; Tirumkudulu, Mahesh S
2016-04-01
Colloidal dispersions are known to display a fascinating network of cracks on drying. We probe the fracture mechanics of free-standing films of aqueous polymer-particle dispersions. Thin films of the dispersion are cast between a pair of plain steel wires and allowed to dry under ambient conditions. The strain induced on the particle network during drying is relieved by cracking. The stress which causes the films to crack has been calculated by measuring the deflection of the wires. The critical cracking stress varied inversely to the two-thirds' power of the film thickness. We also measure the velocity of the tip of a moving crack. The motion of a crack has been modeled as a competition between the release of the elastic energy stored in the particle network, the increase in surface energy as a result of the growth of a crack, the rate of viscous dissipation of the interstitial fluid and the kinetic energy associated with a moving crack. There is fair agreement between the measured crack velocities and predictions. PMID:26924546
The role of cyclic plastic zone size on fatigue crack growth behavior in high strength steels
NASA Astrophysics Data System (ADS)
Korda, Akhmad A.; Miyashita, Y.; Mutoh, Y.
2015-09-01
The role of cyclic plastic zone in front of the crack tip was studied in high strength steels. Estimated plastic zone size would be compared with actual observation. Strain controlled fatigue tests of the steels were carried out to obtain cyclic stress-strain curves for plastic zone estimation. Observations of plastic zone were carried out using in situ SEM fatigue crack growth tests under a constant-ΔK. Hard microstructures in structural steels showed to inhibit the extent of plastic deformation around the crack tip. The rate of crack growth can be correlated with the size of plastic zone. The smaller the plastic zone size, the slower the fatigue crack growth.
Transient and intermittent crack growth during embrittlement of 7075-T651 aluminum by mercury
Liu, Y.; Hoagland, R.G.
1989-03-01
This paper presents some results which show intermittent and transient crack growth behavior of a high strength aluminum alloy embrittled by liquid mercury. Run-arrest-run type of cracking behavior resulted in ''n'' and ''m''-shaped curves on the crack velocity (da/dt) vs. stress intensity (K) diagram. While variations in oxygen activity in the crack tip environment may play a role in producing non-unique /nu/-K behavior, the results reported here suggest that formation and rupture of ligaments during crack extension may also be at work in a way that causes the stress intensity associated with the field local to the crack tip (K/sub tip/) to vary stochastically as the crack extends.
Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels
Ronevich, Joseph A.; Somerday, Brian P.; San Marchi, Chris W.
2015-09-10
Banded ferrite-pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite-pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. Thus the reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impeded hydrogen diffusion across the banded pearlite.
Fatigue crack propagation in ceria-partially-stabilized zirconia (Ce-TZP)-alumina composites
Tsai, J.F.; Yu, C.S.; Shetty, D.K.
1990-10-10
Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO{sub 2}-12 mol% CeO{sub 2}-10 wt% Al{sub 2}O{sub 3} ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs. The fatigue crack growth behavior was strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates.
NASA Astrophysics Data System (ADS)
Moore, John
The flow development within the tip gap and the flow tip leakage, applying Navier-Stokes codes, are discussed. The loss production, the turbine inefficiency and the heat transfer to the blade tip, are considered. The measurements and calculations used demonstrate features of the flow, such as separation and reattachment on the blade tip, shock formation in the tip gap, and formation and dissipation of tip gap secondary kinetic energy. A procedure for calculating turbine blade tip temperatures is included. The results for a centrifugal compressor show the interaction of the tip leakage and passage flows. The radial blackflow near the shroud wall at low off-design flow rates is considered. The calculations demonstrate the potential use of a computational fluid dynamics code for predicting a centrifugal compressor map.
... Health News & Publications Annual Meeting Calendar Diet and Exercise Tips Diet and Exercise Tips News media interested in covering the latest ... Health Statistics concludes that 35 percent of adults exercise regularly (more than 6 of 10 dont), ...
Veligdan, James T.
2004-01-06
A laser scalpel includes a ribbon optical waveguide extending therethrough and terminating at a slender optical cutting tip. A laser beam is emitted along the height of the cutting tip for cutting tissue therealong.
Transient Elastodynamic Crack Growth in Functionally Graded Materials
Chalivendra, Vijaya B.
2008-02-15
A generalized elastic solution for an arbitrarily propagating transient crack in Functionally Graded Materials (FGMs) is obtained through an asymptotic analysis. The shear modulus and mass density of the FGM are assumed to vary exponentially along the gradation direction. The mode-mixity due to the inclination of property gradient with respect to the propagating crack tip is accommodated in the analysis through superposition of the opening and shear modes. First three terms of out of plane displacement field and its gradients about the crack tip are obtained in powers of radial coordinates, with the coefficients depending on the time rate of change of crack tip speed and stress intensity factors. Using these displacement fields, the effect of transient stress intensity factors and acceleration on synthetic contours of constant out of plane displacement under both opening and mixed mode loading conditions has been studied. These contours show that the transient terms cause significant spatial variation on out of plane displacements around the crack tip. Therefore, in studying dynamic fracture of FGMs, it is appropriate to include the transient terms in the field equations for the situations of sudden variation of stress intensity factor or crack tip velocity.
A review of fatigue crack growth analyses
NASA Technical Reports Server (NTRS)
Liu, H. W.
1991-01-01
Stress intensity factor range, Delta K, has been shown to correlate well with fatigue crack growth rate, da/dN. A number of fatigue crack growth theories have been developed for such correlations. Often, conjectory theories of fatigue crack growth are constructed from experimental data. On the other hand, fatigue crack growth theories can also be derived rigorously with deductive logic. Four such deductive theories are reviewed: (1) that for the growth of a small crack in a very wide homogeneous plate, (2) the theory of similitude for the correlation of da/dN with Delta K, (3) a theory of crack growth in homogeneous materials in small-scale yielding, and (4) the unzipping theory of fatigue crack growth. This paper synthesizes these four theories into a logic framework useful for fatigue crack growth analysis. The deductive theories and the conjectory theories complement each other in the advances of the understanding of fatigue crack growth. The applications of logic framework to formulating an overview of fatigue crack growth behavior and to defining the complex issues of the growth of small cracks and crack growth in composites are illustrated.
ERIC Educational Resources Information Center
Department of Education, Washington, DC.
Noting that involved parents can improve their children's chances of succeeding in school, this packet of cards offers 100 tips created to help parents become more involved in their child's education. Following a card of general tips on becoming involved, tips are offered in the following topic areas: helping a child stay alcohol, tobacco, and…
Child Transportation Safety Tips.
ERIC Educational Resources Information Center
National Highway Traffic Safety Administration (DOT), Washington, DC.
This document presents nine tips regarding safe infant and child transportation, each tip explained in one to two pages. The tips are as follows: (1) quick safety seat checkup; (2) where should your child ride? (3) how to protect your new baby in the car; (4) what safety seat to use for a big baby or toddler? (5) how should preschool and school…
Warner, Jeremy; Adamson, Peter
2011-10-01
Successful treatment of the crooked nasal tip includes proper analysis and assessment, employment of the proper techniques, reaching ideal tip dynamics, and close follow-up. Both the caudal septum and the nasal tip cartilages must be addressed. When executed properly, satisfaction should be high for both the patient and the surgeon. PMID:22028009
User's manual for PEPSIG NASA tip vortex version
NASA Technical Reports Server (NTRS)
Tsai, Tommy M.; Dejong, Frederick J.; Levy, Ralph
1988-01-01
The tip vortex flowfield plays a significant role in the performance of advanced aircraft propellers. The flowfield in the tip region is complex, three-dimensional and viscous with large secondary velocities. A computer code was developed to predict the tip vortex flowfield of advanced aircraft propellers. This document is the user's manual. The analysis and a series of test cases are presented in NASA-CR-182179.
The Effect of Water on Crack Interaction
NASA Astrophysics Data System (ADS)
Gaede, O.; Regenauer-Lieb, K.
2009-04-01
While the mechanical coupling between pore fluid and solid phase is relatively well understood, quantitative studies dealing with chemical-mechanical weakening in geological materials are rare. Many classical poroelastic problems can be addressed with the simple law of effective stress. Experimental studies show that the presence of a chemically active fluid can have effects that exceed the predictions of the law of effective stress. These chemical fluid-rock interactions alter the mechanical properties of the solid phase. Especially chemical-mechanical weakening has important ramifications for many areas of applied geosciences ranging from nuclear waste disposal over reservoir enhancement to fault stability. In this study, we model chemically induced changes of the size of the process zone around a crack tip. The knowledge of the process zone size is used to extend existing effective medium approximations of cracked solids. The stress distribution around a crack leads to a chemical potential gradient. This gradient will be a driver for mass diffusion through the solid phase. As an example, mass diffusion is towards the crack tip for a mode I crack. In this case a chemical reaction, that weakens the solid phase, will increase the size of the process zone around the crack tip. We apply our model to the prominent hydrolytic weakening effect observed in the quartz-water system (Griggs and Blacic, 1965). Hydrolytic weakening is generally attributed to water hydrolyzing the strong Si-O bonds of the quartz crystal. The hydrolysis replaces a Si-O-Si bridge with a relatively weak hydrogen bridge between two silanol groups. This enhances dislocation mobility and hence the yield stress is reduced. The plastic process zone around a crack tip is therefore larger in a wet crystal than in a dry crystal. We calculate the size of the process zone by solving this coupled mechanical-chemical problem with the Finite Element code ABAQUS. We consider single crack, collinear crack and
Crack instability of ferroelectric solids under alternative electric loading
NASA Astrophysics Data System (ADS)
Chen, Hao-Sen; Wang, He-Ling; Pei, Yong-Mao; Wei, Yu-Jie; Liu, Bin; Fang, Dai-Ning
2015-08-01
The low fracture toughness of the widely used piezoelectric and ferroelectric materials in technological applications raises a big concern about their durability and safety. Up to now, the mechanisms of electric-field induced fatigue crack growth in those materials are not fully understood. Here we report experimental observations that alternative electric loading at high frequency or large amplitude gives rise to dramatic temperature rise at the crack tip of a ferroelectric solid. The temperature rise subsequently lowers the energy barrier of materials for domain switch in the vicinity of the crack tip, increases the stress intensity factor and leads to unstable crack propagation finally. In contrast, at low frequency or small amplitude, crack tip temperature increases mildly and saturates quickly, no crack growth is observed. Together with our theoretical analysis on the non-linear heat transfer at the crack tip, we constructed a safe operating area curve with respect to the frequency and amplitude of the electric field, and validated the safety map by experiments. The revealed mechanisms about how electro-thermal-mechanical coupling influences fracture can be directly used to guide the design and safety assessment of piezoelectric and ferroelectric devices.
Arctic climate tipping points.
Lenton, Timothy M
2012-02-01
There is widespread concern that anthropogenic global warming will trigger Arctic climate tipping points. The Arctic has a long history of natural, abrupt climate changes, which together with current observations and model projections, can help us to identify which parts of the Arctic climate system might pass future tipping points. Here the climate tipping points are defined, noting that not all of them involve bifurcations leading to irreversible change. Past abrupt climate changes in the Arctic are briefly reviewed. Then, the current behaviour of a range of Arctic systems is summarised. Looking ahead, a range of potential tipping phenomena are described. This leads to a revised and expanded list of potential Arctic climate tipping elements, whose likelihood is assessed, in terms of how much warming will be required to tip them. Finally, the available responses are considered, especially the prospects for avoiding Arctic climate tipping points. PMID:22270703
Fatigue life and crack growth prediction methodology
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.
1993-01-01
The capabilities of a plasticity-induced crack-closure model and life-prediction code to predict fatigue crack growth and fatigue lives of metallic materials are reviewed. Crack-tip constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta(K(sub eff))) under constant-amplitude loading. Some modifications to the delta(K(sub eff))-rate relations were needed in the near threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constant-amplitude, variable-amplitude, and spectrum loading. Fatigue lives were calculated using the crack growth relations and microstructural features like those that initiated cracks. Results from the tests and analyses agreed well.
NASA Astrophysics Data System (ADS)
Kelestemur, Mehmet Halidun
1998-12-01
AISI 304 stainless steel shows strain induced martensitic transformation at the crack tip. Such transformation may have effects on crack closure during fatigue crack propagation. Due to importance of AISI 304 in structural applications, the fatigue crack propagation and martensitic transformation in this material have to be investigated thoroughly. Fatigue crack growth behavior, overload retardation and characterization of martensitic transformation at the crack tip upon fatigue loading were investigated in 304 stainless steel at three different atmospheres, namely dry argon, moist air (75% relative humidity) and hydrogen. Comparison in various atmospheres showed that moist air did not influence that fatigue crack growth rate. However, in hydrogen atmosphere the material did not show threshold behavior and the crack growth rate was considerably higher. It was found that roughness-induced crack closure was the primary mechanism in the threshold region. Fractographic pictures taken by SEM and direct observation of crack profile showed that crack deflection and branching occurred during the fatigue crack propagation and plasticity-induced crack closure was not the primary closure mechanism. The influence of fatigue crack propagation on the rate and size of martensitic transformation at the crack tip was investigated. The overload retardation of the material was lower at hydrogen atmosphere. This low degree of retardation was explained by hydrogen embrittlement mechanism. Fractographic observations show striations at the overload zone in argon atmosphere indicating ductile fracture. In hydrogen atmosphere, overload area shows secondary cracks which represent brittle fracture. Crack closure measurements and modified Paris law did not show evidence for different retardation mechanisms at different atmospheres. It is found that primary retardation mechanisms were crack deflection, crack blunting and roughness-induced crack closure after application of overload(s). An
Experimental study of thermodynamics propagation fatigue crack in metals
Vshivkov, A. Iziumova, A. Plekhov, O.
2015-10-27
This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.
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.
Finite-element analysis of crack growth under monotonic and cyclic loading
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1977-01-01
An elastic-plastic (incremental) finite-element analysis, in conjunction with a crack-growth criterion, was used to study crack-growth behavior under monotonic and cyclic loading. The crack-growth criterion was based on crack-tip strain. Whenever the crack-tip strain equals or exceeds a critical strain value, the crack grows. The effects of element-mesh size, critical strain, strain hardening, and specimen type (tension or bending) on crack growth under monotonic loading were investigated. Crack growth under cyclic loading (constant amplitude and simple variable amplitude) were also studied. A combined hardening theory, which incorporates features of both isotropic and kinematic hardening under cyclic loading, was also developed for smooth yield surfaces and was used in the analysis.
Comninou contact zones for a crack parallel to an interface
Joseph, P.F.; Gadi, K.S.; Erdogen, F.
1995-12-31
One of the interesting features in studying the state of stress in elastic solids near singular points, is the so called complex singularity that gives rise to an apparent local oscillatory behavior in the stress and displacement fields. The region in which this occurs is very small, much smaller than any plastic zone would be, and therefore the oscillations can be ignored in practical applications. Nevertheless, it is a matter of interesting theoretical investigation. The Comninou model of a small contact zone near the crack tip appears to correct for this anomaly within the framework of the linear theory. This model seems to make sense out of a {open_quotes}solution{close_quotes} that violates the boundary conditions. Erdogan and Joseph, showed (to themselves anyway) that the Comninou model actually has a physical basis. They considered a crack parallel to an interface where the order of the singularity is always real. With great care in solving the singular integral equations, it was shown that as the crack approaches the interface, a pinching effect is observed at the crack tip. This pinching effect proves that in the limit as the crack approaches the interface, the correct way to handle the problem is to consider crack surface contact. In this way, the issue of {open_quotes}oscillations{close_quotes} is never encountered for the interface crack problem. In the present study, the value of h/a that corresponds to crack closure (zero value of the stress intensity factor) will be determined for a given material pair for tensile loading. An asymptotic numerical method for the solution of singular integral equations making use of is used to obtain this result. Results for the crack opening displacement near the tip of the crack and the behavior of the stress intensity factor for cracks very close to the interface are presented. Among other interesting issues to be discussed, this solution shows that the semi-infinite crack parallel to an interface is closed.
Blade Tip Rubbing Stress Prediction
NASA Technical Reports Server (NTRS)
Davis, Gary A.; Clough, Ray C.
1991-01-01
An analytical model was constructed to predict the magnitude of stresses produced by rubbing a turbine blade against its tip seal. This model used a linearized approach to the problem, after a parametric study, found that the nonlinear effects were of insignificant magnitude. The important input parameters to the model were: the arc through which rubbing occurs, the turbine rotor speed, normal force exerted on the blade, and the rubbing coefficient of friction. Since it is not possible to exactly specify some of these parameters, values were entered into the model which bracket likely values. The form of the forcing function was another variable which was impossible to specify precisely, but the assumption of a half-sine wave with a period equal to the duration of the rub was taken as a realistic assumption. The analytical model predicted resonances between harmonics of the forcing function decomposition and known harmonics of the blade. Thus, it seemed probable that blade tip rubbing could be at least a contributor to the blade-cracking phenomenon. A full-scale, full-speed test conducted on the space shuttle main engine high pressure fuel turbopump Whirligig tester was conducted at speeds between 33,000 and 28,000 RPM to confirm analytical predictions.
Cracked shells under skew-symmetric loading
NASA Technical Reports Server (NTRS)
Lelale, F.
1982-01-01
A shell containing a through crack in one of the principal planes of curvature and under general skew-symmetric loading is considered. By employing a Reissner type shell theory which takes into account the effect of transverse shear strains, all boundary conditions on the crack surfaces are satisfied separately. Consequently, unlike those obtained from the classical shell theory, the angular distributions of the stress components around the crack tips are shown to be identical to the distributions obtained from the plane and antiplane elasticity solutions. Extensive results are given for axially and circumferentially cracked cylindrical shells, spherical shells, and toroidal shells under uniform inplane shearing, out of plane shearing, and torsion. The effect of orthotropy on the results is also studied.
NURBS distance fields for extremely curved cracks
NASA Astrophysics Data System (ADS)
Sevilla, Ruben; Barbieri, Ettore
2014-12-01
This paper proposes for the first time an intrinsic enrichment for extremely curved cracks in a meshfree framework. The unique property of the proposed method lies in the exact geometric representation of cracks using non-uniform rational B-splines (NURBS). A distance function algorithm for NURBS is presented, resulting in a spatial field which is simultaneously discontinuous over the (finite) curved crack and continuous all around the crack tips. Numerical examples show the potential of the proposed approach and illustrate its advantages with respect to other techniques usually employed to model fracture, including standard finite elements with remeshing and the extended finite element method. This work represents a further step in an ongoing effort in the community to integrate computer aided design with numerical simulations.
Kinetics of fatigue cracks in iron in electrolytic hydrogen impregnation
Pokhmurskii, V.I.; Bilyi, L.M.
1985-05-01
Fatigue failure of metals is localized in the zone of plastic deformation at the tip of the developing crack. Crack development depends to a large extent upon the parameters of the deformed volume, the loading conditions, and features of the material microstructure. It may be assumed that the medium, especially a hydrogen-impregnating medium, leads to a change in the zone of plastic deformation and thereby influences the rate of fatigue crack growth. This work is devoted to a study of cyclic crack resistance and determination of the zone of plastic deformation of failure specimens of Armco iron under conditions of the action of a hydrogen-impregnating medium.
Rock Failure and Crack Propagation Beneath Disc Cutters
NASA Astrophysics Data System (ADS)
Entacher, Martin; Schuller, E.; Galler, R.
2015-07-01
Analyses of rock failure mechanisms beneath disc cutters are presented. Full-scale cutting tests are conducted to assess the global energy input in comparison with rock chips and excavated volume. Small-scale cutting tests are subsequently used for macro- and microscopic analyses of rupture modes and crack propagation. A high spatial resolution allows to obtain pictures of crack networks in different rock types. It is shown that all specimens develop lateral cracks in sufficiently confined areas whereas median cracks typically develop in boundary regions. Regarding cutting forces, a hypothesis is proposed that associates sudden force drops accompanied by sudden sound emission with grain crushing in the proximity of the cutter tip.
Fatigue crack growth in unidirectional metal matrix composite
NASA Technical Reports Server (NTRS)
Ghosn, Louis J.; Telesman, Jack; Kantzos, Peter
1990-01-01
The weight function method was used to determine the effective stress intensity factor and the crack opening profile for a fatigue tested composite which exhibited fiber bridging. The bridging mechanism was modeled using two approaches; the crack closure approach and the shear lag approach. The numerically determined stress intensity factor values from both methods were compared and correlated with the experimentally obtained crack growth rates for SiC/Ti-15-3 (0)(sub 8) oriented composites. The near crack tip opening profile was also determined for both methods and compared with the experimentally obtained measurements.
Discrete crack growth analysis methodology for through cracks in pressurized fuselage structures
NASA Astrophysics Data System (ADS)
Potyondy, David O.; Wawrzynek, Paul A.; Ingraffea, Anthony R.
1995-05-01
A methodology for simulating the growth of long through cracks in the skin of pressurized aircraft fuselage structures is described. Crack trajectories are allowed to be arbitrary and are computed as part of the simulation. The interaction between the mechanical loads acting on the superstructure and the local structural response near the crack tips is accounted for by employing a hierarchical modelling strategy. The structural response for each cracked configuration is obtained using a geometrically non-linear shell finite element analysis procedure. Four stress intensity factors, two for membrane behavior and two for bending using Kirchhoff plate theory, are computed using an extension of the modified crack closure integral method. Crack trajectories are determined by applying the maximum tangential stress criterion. Crack growth results in localized mesh deletion, and the deletion regions are remeshed automatically using a newly developed all-quadrilateral meshing algorithm. The effectiveness of the methodology, and its applicability to performing practical analyses of realistic structures, is demonstrated by simulating curvilinear crack growth in a fuselage panel that is representative of a typical narrow-body aircraft. The predicted crack trajectory and fatigue life compare well with measurements of these same quantities from a full-scale pressurized panel test.
Discrete crack growth analysis methodology for through cracks in pressurized fuselage structures
NASA Astrophysics Data System (ADS)
Potyondy, David O.; Wawrzynek, Paul A.; Ingraffea, Anthony R.
1994-09-01
A methodology for simulating the growth of long through cracks in the skin of pressurized aircraft fuselage structures is described. Crack trajectories are allowed to be arbitrary and are computed as part of the simulation. The interaction between the mechanical loads acting on the superstructure and the local structural response near the crack tips is accounted for by employing a hierarchical modeling strategy. The structural response for each cracked configuration is obtained using a geometrically nonlinear shell finite element analysis procedure. Four stress intensity factors, two for membrane behavior and two for bending using Kirchhoff plate theory, are computed using an extension of the modified crack closure integral method. Crack trajectories are determined by applying the maximum tangential stress criterion. Crack growth results in localized mesh deletion, and the deletion regions are remeshed automatically using a newly developed all-quadrilateral meshing algorithm. The effectiveness of the methodology and its applicability to performing practical analyses of realistic structures is demonstrated by simulating curvilinear crack growth in a fuselage panel that is representative of a typical narrow-body aircraft. The predicted crack trajectory and fatigue life compare well with measurements of these same quantities from a full-scale pressurized panel test.
Discrete crack growth analysis methodology for through cracks in pressurized fuselage structures
NASA Technical Reports Server (NTRS)
Potyondy, David O.; Wawrzynek, Paul A.; Ingraffea, Anthony R.
1994-01-01
A methodology for simulating the growth of long through cracks in the skin of pressurized aircraft fuselage structures is described. Crack trajectories are allowed to be arbitrary and are computed as part of the simulation. The interaction between the mechanical loads acting on the superstructure and the local structural response near the crack tips is accounted for by employing a hierarchical modeling strategy. The structural response for each cracked configuration is obtained using a geometrically nonlinear shell finite element analysis procedure. Four stress intensity factors, two for membrane behavior and two for bending using Kirchhoff plate theory, are computed using an extension of the modified crack closure integral method. Crack trajectories are determined by applying the maximum tangential stress criterion. Crack growth results in localized mesh deletion, and the deletion regions are remeshed automatically using a newly developed all-quadrilateral meshing algorithm. The effectiveness of the methodology and its applicability to performing practical analyses of realistic structures is demonstrated by simulating curvilinear crack growth in a fuselage panel that is representative of a typical narrow-body aircraft. The predicted crack trajectory and fatigue life compare well with measurements of these same quantities from a full-scale pressurized panel test.
NASA Astrophysics Data System (ADS)
Baltazart, Vincent; Moliard, Jean-Marc; Amhaz, Rabih; Wright, Dean; Jethwa, Manish
2015-04-01
Monitoring road surface conditions is an important issue in many countries. Several projects have looked into this issue in recent years, including TRIMM 2011-2014. The objective of such projects has been to detect surface distresses, like cracking, raveling and water ponding, in order to plan effective road maintenance and to afford a better sustainability of the pavement. The monitoring of cracking conventionally focuses on open cracks on the surface of the pavement, as opposed to reflexive cracks embedded in the pavement materials. For monitoring surface condition, in situ human visual inspection has been gradually replaced by automatic image data collection at traffic speed. Off-line image processing techniques have been developed for monitoring surface condition in support of human visual control. Full automation of crack monitoring has been approached with caution, and depends on a proper manual assessment of the performance. This work firstly presents some aspects of the current state of monitoring that have been reported so far in the literature and in previous projects: imaging technology and image processing techniques. Then, the work presents the two image processing techniques that have been developed within the scope of the TRIMM project to automatically detect pavement cracking from images. The first technique is a heuristic approach (HA) based on the search for gradient within the image. It was originally developed to process pavement images from the French imaging device, Aigle-RN. The second technique, the Minimal Path Selection (MPS) method, has been developed within an ongoing PhD work at IFSTTAR. The proposed new technique provides a fine and accurate segmentation of the crack pattern along with the estimation of the crack width. HA has been assessed against the field data collection provided by Yotta and TRL with the imaging device Tempest 2. The performance assessment has been threefold: first it was performed against the reference data set
Computational two-dimensional modeling of the stress intensity factor in a cracked metallic material
NASA Astrophysics Data System (ADS)
Rolón, J. E.; Cendales, E. D.; Cruz, I. M.
2016-02-01
Cracking of metallic engineering materials is of great importance due cost of replacing mechanical elements cracked and the danger of sudden structural failure of these elements. One of the most important parameters during consideration of the mechanical behavior of machine elements having cracking and that are subject to various stress conditions is the stress intensity factor near the crack tip called factor Kic. In this paper a computational model is developed for the direct assessment of stress concentration factor near to the crack tip and compared with the results obtained in the literature in which other models have been established, which consider continuity of the displacement of the crack tip (XBEM). Based on this numerical approximation can be establish that computational XBEM method has greater accuracy in Kic values obtained than the model implemented by the method of finite elements for the virtual nodal displacement through plateau function.
A thermomechanical cohesive zone model for bridged delamination cracks
NASA Astrophysics Data System (ADS)
Hattiangadi, Ashwin; Siegmund, Thomas
2004-03-01
The coupled thermomechanical numerical analysis of composite laminates with bridged delamination cracks loaded by a temperature gradient is described. The numerical approach presented is based on the framework of a cohesive zone model. A traction-separation law is presented which accounts for breakdown of the micromechanisms responsible for load transfer across bridged delamination cracks. The load transfer behavior is coupled to heat conduction across the bridged delamination crack. The coupled crack-bridging model is implemented into a finite element framework as a thermomechanical cohesive zone model (CZM). The fundamental response of the thermomechanical CZM is described. Subsequently, bridged delamination cracks of fixed lengths are studied. Values of the crack tip energy release rate and of the crack heat flux are computed to characterize the loading of the structure. Specimen geometries are considered that lead to crack opening through bending deformation and buckling delamination. The influence of critical mechanical and thermal parameters of the bridging zone on the thermomechanical delamination behavior is discussed. Bridging fibers not only contribute to crack conductance, but by keeping the crack opening small they allow heat flux across the delamination crack to be sustained longer, and thereby contribute to reduced levels of thermal stresses. The micro-mechanism based cohesive zone model allows the assessment of the effectiveness of the individual mechanisms contributing to the thermomechanical crack bridging embedded into the structural analysis.
Crack-Defined Electronic Nanogaps.
Dubois, Valentin; Niklaus, Frank; Stemme, Göran
2016-03-01
Achieving near-atomic-scale electronic nanogaps in a reliable and scalable manner will facilitate fundamental advances in molecular detection, plasmonics, and nanoelectronics. Here, a method is shown for realizing crack-defined nanogaps separating TiN electrodes, allowing parallel and scalable fabrication of arrays of sub-10 nm electronic nanogaps featuring individually defined gap widths. PMID:26784270
Crack diffusion coefficient - A candidate fracture toughness parameter for short fiber composites
NASA Technical Reports Server (NTRS)
Mull, M. A.; Chudnovsky, A.; Moet, A.
1987-01-01
In brittle matrix composites, crack propagation occurs along random trajectories reflecting the heterogeneous nature of the strength field. Considering the crack trajectory as a diffusive process, the 'crack diffusion coefficient' is introduced. From fatigue crack propagation experiments on a set of identical SEN polyester composite specimens, the variance of the crack tip position along the loading axis is found to be a linear function of the effective 'time'. The latter is taken as the effective crack length. The coefficient of proportionality between variance of the crack trajectory and the effective crack length defines the crack diffusion coefficient D which is found in the present study to be 0.165 mm. This parameter reflects the ability of the composite to deviate the crack from the energetically most efficient path and thus links fracture toughness to the microstructure.
Prediction of stable crack growth and instability using the V sub R-curve method
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1985-01-01
A methodology is presented for predicting stable crack growth and instability of cracked structural components from results of laboratory tests on metallic materials under plane-stress conditions. The methodology is based on the displacement V sub R at the tip of a stably tearing crack. Basically, the V sub R-curve method is a resistance curve approach, such as K sub R and J sub R, except that the 'crack drive' is written in terms of crack-tip displacement instead of K or J. The relationship between crack-tip-opening displacement, crack length, specimen type, and tensile properties is derived from the Dugdale model for the cracked structure of interest. This report describes the laboratory test procedure and calculations used to obtain the V sub R resistance curve from fracture tests of compact or of middle-crack tension (formally center-crack) specimens. The analysis procedure used to predict stable crack growth and instability of any through-the-thickness crack configuration made of the same material and thickness, and tested under the same environmental conditions, is presented. The various limitations of the present V sub R curve method are given. Four example calculations and predictions are shown.
NASA Astrophysics Data System (ADS)
Li, Qun; Pan, Suxin; Liu, Qida; Wang, Jie
2016-07-01
The spatial and temporal evolution of domain switching near the tip of a mixed-mode crack (e.g., an inclined crack) is observed in ferroelectrics. The birefringence technique is used to measure the optical quantities to demonstrate the domain switching near the crack tip. The results show an intriguing feature that there appears electrical creep and domain switching emission from the crack tip. The actual time-dependence of domain switching emission and its anisotropic velocity is approximately measured. Moreover, the phase field modeling is developed to simulate polarization distribution and domain switching near the crack tip where the time-dependent Ginzburg–Landau equation is used to describe the change of polarization. The phase field results indicate the same features of domain switching emission from the mixed-mode crack. A good agreement between phase field simulation and birefringence measurement is concluded by setting the appropriate kinetic coefficient in the time-dependent Ginzburg–Landau equation.
Creep crack growth behavior of aluminum alloy 2519. Part 2: Numerical analysis
Hall, D.E.; Hamilton, B.C.; McDowell, D.L.; Saxena, A.
1997-12-31
The experimental analysis of high temperature fracture in Aluminum Alloy 2519-T87 presented in Part 1 of this paper highlighted the creep-brittle fracture characteristics of the material and showed reasonable correlation of crack growth rates with the stress intensity factor K. Part 2 continues this investigation numerically using growing crack finite element analyses. Experimentally observed crack growth histories of four aluminum 2519-T87 compact specimens are enforced by controlling the rate of release of finite element nodes along the crack growth path to gain insight into the relation of the crack tip fields to far field fracture parameters and to crack growth rates. A variable time-step, nodal-release algorithm is presented to model the high strain rates that occur during the initial stages of crack growth. The numerical results indicate an initial transient period of crack growth followed by a quasi-steady-state crack growth regime in which the crack tip fields change slowly with increasing crack length. Transition of crack growth to the quasi-steady-state regime, where similitude and small-scale creep conditions roughly exist, is given by a transition time t{sub g} that depends on the crack growth history and material properties. Excellent correlation of the stress intensity factor K with the crack growth rates is observed after time t{sub g}. Experimental difficulties in measuring the creep component of the load-line deflection rate are also discussed.
Modelling microstructurally sensitive fatigue short crack growth
NASA Astrophysics Data System (ADS)
de Los Rios, E. R.; Xin, X. J.; Navarro, A.
1994-10-01
Microstructurally sensitive fatigue short crack growth can occur in many engineering components devoid of large defects. Continuum mechanics principles, including linear elastic fracture mechanics, used in damage tolerance design and life prediction methods are not applicable in these situations and therefore new concepts need to be developed to characterize this type of growth. A microstructurally sensitive model of fatigue crack growth is presented in which the effect of microstructure is dominant in the early stage of growth but plays a negligible role after the crack has gone through the transition from structure-sensitive to structure-insensitive growth. The effect of both microstructure and structure sensitive variables on the transition from short cracks to continuum mechanics and the conditions for crack instability leading to final failure are examined. The microstructural variables incorporated in the equations that describe the model are those controlling the extent and intensity of crack tip plasticity such as grain size, precipitation and dispersion hardening, strain hardening and mis-orientation between grains. It is expected that the concepts developed within the model will form the basis for the design of new crack-resistant materials.
Contact of nonflat crack surfaces during fatigue
Sehitoglu, H.; Garcia, A.M.
1999-07-01
A model has been developed to predict crack opening and closure behavior for propagating fatigue cracks which are nonflat and undergo significant sliding displacements. Crack surfaces were characterized by a random distribution of asperity heights, density of asperities, and asperity radii. The propagating crack was subdivided into ligaments and each ligament was treated as a contact problem between two randomly rough surfaces. The far-field tensile stresses were varied in a cyclic manner for R = 0.1 and {minus}1 loading conditions. The contact stresses at the minimal load were determined by analyzing the local crushing of the asperities. Then, upon loading the crack opening, stresses were computed when the contact stresses were overcome. The results of crack opening stresses were correlated with CTOD/{sigma}{sub 0} where CTOD is the crack-tip opening displacement and {sigma}{sub 0} is the average asperity height. The asperity effects on closure were compared with plasticity-induced closure results from the literature for identification of conditions when one mechanism dominates the other.
Fatigue life and crack growth prediction methodology
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Phillips, E. P.; Everett, Richard A., Jr.
1994-01-01
This paper reviews the capabilities of a plasticity-induced crack-closure model and life-prediction code to predict fatigue crack growth and fatigue lives of metallic materials. Crack-tip constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (Delta K(sub eff)) under constant amplitude loading. Some modifications to the Delta K(sub eff)-rate relations were needed in the near threshold regime to fit small-crackgrowth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constant-amplitude, variable-amplitude, and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks. Results from the tests and analyses agreed well.
Life prediction for bridged fatigue cracks
Cox, B.N.
1994-08-01
One of the more promising classes of composites touted for high temperature applications, and certainly the most available, is that of relatively brittle matrices, either ceramic or intermetallic, reinforced by strong, aligned, continuous fibers. Under cyclic loading in the fiber direction, these materials develop matrix cracks that often run perpendicular to the fibers, while the fibers remain intact in the crack wake, supplying bridging tractions across the fracture surfaces. The bridging tractions shield the crack tip from the applied load, dramatically reducing the crack velocity from that expected in an unreinforced material subjected to the same value, {Delta}K{sub a}, of the cyclic applied stress intensity factor. An important issue in reliability is the prediction of the growth rates of the bridged cracks. The growth rates of matrix fatigue cracks bridged by sliding fibers are now commonly predicted by models based on the micromechanics of frictional interfaces. However, there exist many reasons, both theoretical and experimental, for suspecting that the most popular micromechanical models are probably wrong in detail in the context of fatigue cracks. Furthermore, a review of crack growth data reveals that the validity of the micromechanics-based predictive model has never been tested and may never be tested. In this paper, two alternative approaches are suggested to the engineering problem of predicting the growth rates of bridged cracks without explicit recourse to micromechanics. Instead, it is shown that the material properties required to analyze bridging effects can be deduced directly from crack growth data. Some experiments are proposed to test the validity of the proposals.
NASA Astrophysics Data System (ADS)
Niu, Yong; Su, Weiguo
2016-06-01
A line spring model is developed for analyzing the fracture problem of cracked metallic plate repaired with the double-sided adhesively bonded composite patch. The restraining action of the bonded patch is modeled as continuous distributed linear springs bridging the crack faces provided that the cracked plate is subjected to extensional load. The effective spring constant is determined from 1-D bonded joint theory. The hyper-singular integral equation (HSIE), which can be solved using the second kind Chebyshev polynomial expansion method, is applied to determine the crack opening displacements (COD) and the crack tip stress intensity factors (SIF) of the repaired cracked plate. The numerical result of SIF for the crack-tip correlates very well with the finite element (FE) computations based on the virtual crack closure technique (VCCT). The present analysis approaches and mathematical techniques are critical to the successful design, analysis and implementation of crack patching.
Velocity-Dependent Fatigue Crack Paths in Nanograined Pt Films
NASA Astrophysics Data System (ADS)
Meirom, R. A.; Clark, T.; Polcawich, R.; Pulskamp, J.; Dubey, M.; Muhlstein, C. L.
2008-08-01
Studies of crack growth in nanograined films assert that mechanical damage accumulates at grain boundaries irrespective of the crack velocity and loading conditions. This work shows that crack advance in nanograined Pt films involves a dislocation-slip mechanism that is a function of the crack growth rate and mode of loading. Crack paths in Pt were initially intergranular, but transitioned to a transgranular mode that persisted until catastrophic failure. This research demonstrates that crack growth mechanisms modeled for nanograined Ni cannot be generalized to other pure, metallic systems.
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.
The crack problem for bonded nonhomogeneous materials under antiplane shear loading
NASA Technical Reports Server (NTRS)
Erdogan, F.
1985-01-01
The singular nature of the crack tip stress field in a nonhomogeneous medium having a shear modulus with a discontinuous derivative was investigated. The problem is considered for the simplest possible loading and geometry, namely the antiplane shear loading of two bonded half spaces in which the crack is perpendicular to the interface. It is shown that the square-root singularity of the crack tip stress field is unaffected by the discontinuity in the derivative of the shear modulus. The problem is solved for a finite crack and extensive results are given for the stress intensity factors.
NASA Technical Reports Server (NTRS)
James, Mark Anthony
1999-01-01
A finite element program has been developed to perform quasi-static, elastic-plastic crack growth simulations. The model provides a general framework for mixed-mode I/II elastic-plastic fracture analysis using small strain assumptions and plane stress, plane strain, and axisymmetric finite elements. Cracks are modeled explicitly in the mesh. As the cracks propagate, automatic remeshing algorithms delete the mesh local to the crack tip, extend the crack, and build a new mesh around the new tip. State variable mapping algorithms transfer stresses and displacements from the old mesh to the new mesh. The von Mises material model is implemented in the context of a non-linear Newton solution scheme. The fracture criterion is the critical crack tip opening displacement, and crack direction is predicted by the maximum tensile stress criterion at the crack tip. The implementation can accommodate multiple curving and interacting cracks. An additional fracture algorithm based on nodal release can be used to simulate fracture along a horizontal plane of symmetry. A core of plane strain elements can be used with the nodal release algorithm to simulate the triaxial state of stress near the crack tip. Verification and validation studies compare analysis results with experimental data and published three-dimensional analysis results. Fracture predictions using nodal release for compact tension, middle-crack tension, and multi-site damage test specimens produced accurate results for residual strength and link-up loads. Curving crack predictions using remeshing/mapping were compared with experimental data for an Arcan mixed-mode specimen. Loading angles from 0 degrees to 90 degrees were analyzed. The maximum tensile stress criterion was able to predict the crack direction and path for all loading angles in which the material failed in tension. Residual strength was also accurately predicted for these cases.
A plane stress finite element model for elastic-plastic mode I/II crack growth
NASA Astrophysics Data System (ADS)
James, Mark Anthony
A finite element program has been developed to perform quasi-static, elastic-plastic crack growth simulations. The model provides a general framework for mixed-mode I/II elastic-plastic fracture analysis using small strain assumptions and plane stress, plane strain, and axisymmetric finite elements. Cracks are modeled explicitly in the mesh. As the cracks propagate, automatic remeshing algorithms delete the mesh local to the crack tip, extend the crack, and build a new mesh around the new tip. State variable mapping algorithms transfer stresses and displacements from the old mesh to the new mesh. The von Mises material model is implemented in the context of a non-linear Newton solution scheme. The fracture criterion is the critical crack tip opening displacement, and crack direction is predicted by the maximum tensile stress criterion at the crack tip. The implementation can accommodate multiple curving and interacting cracks. An additional fracture algorithm based on nodal release can be used to simulate fracture along a horizontal plane of symmetry. A core of plane strain elements can be used with the nodal release algorithm to simulate the triaxial state of stress near the crack tip. Verification and validation studies compare analysis results with experimental data and published three-dimensional analysis results. Fracture predictions using nodal release for compact tension, middle-crack tension, and multi-site damage test specimens produced accurate results for residual strength and link-up loads. Curving crack predictions using remeshing/mapping were compared with experimental data for an Arcan mixed-mode specimen. Loading angles from 0 degrees to 90 degrees were analyzed. The maximum tensile stress criterion was able to predict the crack direction and path for all loading angles in which the material failed in tension. Residual strength was also accurately predicted for these cases.
NASA Astrophysics Data System (ADS)
Husseini, Naji Sami
openings were very small, as determined by absorption and phase contrast, and suggested multiple fracture modes for propagation along {111} planes at room temperature, which was verified by finite element analysis. With increasing temperature, cracks became Mode I (perpendicular to the loading axis) in character and more sensitive to the microstructure. Advancing plastic zones ahead of crack tips altered the crystallographic quality, from which diffraction contrast anticipated initiation and propagation. These studies demonstrate the extreme sensitivity of x-ray radiography for detailed studies of superalloys and crack growth processes.
ERIC Educational Resources Information Center
Lee, Helen C.
The paper describes a current events lesson based on an editorial which quoted 10 tips from George Washington and suggested that a review of Washington's statements might be useful in making judgments about current national issues. Used in United States history, world history, and government classes, adaptation and revision of the tips lead…
... How Can I Help a Friend Who Cuts? ADHD: Tips to Try KidsHealth > For Teens > ADHD: Tips to Try Print A A A Text Size en español TDAH: Consejos que puedes probar ADHD , or attention deficit hyperactivity disorder, is a medical ...
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ERIC Educational Resources Information Center
Corder, Lloyd E.; And Others
This manual of telephone behavior tips for business and sales professionals offers ways to handle the disgruntled caller and makes suggestions on topics relevant to the telephone. The manual is divided into the following sections and subsections: (1) Common Courtesy (staff tips, answering the telephone, screening calls, transferring calls, taking…
Interlaminar crack growth in fiber reinforced composites during fatigue, part 3
NASA Technical Reports Server (NTRS)
Wang, S. S.; Wang, H. T.
1981-01-01
Interlaminar crack growth behavior in fiber-reinforced composites subjected to fatigue loading was investigated experimentally and theoretically. In the experimental phase, inter-laminar crack propagation rates and mechanisms were determined for the cases of various geometries, laminate parameters and cyclic stress levels. A singular hybrid-stress finite element method was used in conjuction with the experimental results to examine the local crack-tip behavior and to characterize the crack propagation during fatigue. Results elucidate the basic nature of the cyclic delamination damage, and relate the interlaminar crack growth rate to the range of mixed-mode crack-tip stress intensity factors. The results show that crack growth rates are directly related to the range of the mixed-mode cyclic stress intensity factors by a power law relationship.
Yi, K.S.; Dill, S.J.; Dauskardt, R.H.
1997-07-01
The effect of hydrodynamic pressure developed in the wake of a crack growing in a brittle material under cyclic loads in an aqueous environment is considered. The pressure acts in opposition to the movement of the crack faces, thus shielding the crack up from the applied loads. A general hydrodynamic fluid pressure relation based on a one-dimensional Reynolds equation, which applicable to a crack with an arbitrary crack opening profile, is developed. The model is modified to account for side flow through the thickness of the sample and cavitation near the crack tip. Both effects significantly modify the hydrodynamic pressure distribution. Finally, the resulting hydrodynamic pressure relations are combined with a fracture mechanics model to account for the change in the near-tip stress intensity. Resulting predictions of the cyclic crack-growth rate are found to be in good agreement with measured values for a borosilicate glass tested at various frequencies in a water environment.
On the driving force for crack growth during thermal actuation of shape memory alloys
NASA Astrophysics Data System (ADS)
Baxevanis, T.; Parrinello, A. F.; Lagoudas, D. C.
2016-04-01
The effect of thermomechanically induced phase transformation on the driving force for crack growth in polycrystalline shape memory alloys is analyzed in an infinite center-cracked plate subjected to a thermal actuation cycle under mechanical load in plain strain. Finite element calculations are carried out to determine the mechanical fields near the static crack and the crack-tip energy release rate using the virtual crack closure technique. A substantial increase of the energy release rate - an order of magnitude for some material systems - is observed during the thermal cycle due to the stress redistribution induced by large scale phase transformation. Thus, phase transformation occurring due to thermal variations under mechanical load may result in crack growth if the crack-tip energy release rate reaches a material specific critical value.
NASA Astrophysics Data System (ADS)
Matsuda, Yusuke; King, Sean W.; Oliver, Mark; Dauskardt, Reinhold H.
2013-02-01
Moisture-assisted cracking of silica-derived materials results from a stress-enhanced reaction between water molecules and moisture-sensitive SiOSi bonds at the crack tip. We report the moisture-assisted cracking of oxidized hydrogenated amorphous silicon carbide films (a-SiCO:H) consisting of both moisture-sensitive SiOSi bonds and moisture-insensitive bonds. The sensitivity of the films to moisture-assisted cracking was observed to increase with the SiOSi bond density, ρSiOSi. This sensitivity was correlated with the number of SiOSi bonds ruptured, NSiOSi, through an atomistic kinetic fracture model. By comparing these correlated NSiOSi values with those estimated by a planar crack model, we demonstrated that at the atomistic scale the crack path meanders three-dimensionally so as to intercept the most SiOSi bonds. This atomistic crack path meandering was verified by a computational method based on graph theory and molecular dynamics. Our findings could provide a basis for better understanding of moisture-assisted cracking in materials consisting of other types of moisture-sensitive and moisture-insensitive bonds.
A cylindrical shell with an arbitrarily oriented crack
NASA Technical Reports Server (NTRS)
Yahsi, O. S.; Erdogan, F.
1982-01-01
The general problem of a shallow shell with constant curvatures is considered. It is assumed that the shell contains an arbitrarily oriented through crack and the material is specially orthotropic. The nonsymmetric problem is solved for arbitrary self equilibrating crack surface tractions, which, added to an appropriate solution for an uncracked shell, would give the result for a cracked shell under most general loading conditions. The problem is reduced to a system of five singular integral equations in a set of unknown functions representing relative displacements and rotations on the crack surfaces. The stress state around the crack tip is asymptotically analyzed and it is shown that the results are identical to those obtained from the two dimensional in plane and antiplane elasticity solutions. The numerical results are given for a cylindrical shell containing an arbitrarily oriented through crack. Some sample results showing the effect of the Poisson's ratio and the material orthotropy are also presented.
Mechanics of fatigue crack closure
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr. (Editor); Elber, Wolf (Editor)
1988-01-01
Papers are presented on plasticity induced crack closure, crack closure in fatigue crack growth, the dependence of crack closure on fatigue loading variables, and a procedure for standardizing crack closure levels. Also considered are a statistical approach to crack closure determination, the crack closure behavior of surface cracks under pure bending, closure measurements on short fatigue cracks, and crack closure under plane strain conditions. Other topics include fatigue crack closure behavior at high stress ratios, the use of acoustic waves for the characterization of closed fatigue cracks, and the influence of fatigue crack wake length and state of stress on crack closure.
Crack, crack house sex, and HIV risk.
Inciardi, J A
1995-06-01
Limited attention has been focused on HIV risk behaviors of crack smokers and their sex partners, yet there is evidence that the crack house and the crack-using life-style may be playing significant roles in the transmission of HIV and other sexually transmitted diseases. The purposes of this research were to study the attributes and patterns of "sex for crack" exchanges, particularly those that occurred in crack houses, and to assess their potential impact on the spread of HIV. Structured interviews were conducted with 17 men and 35 women in Miami, Florida, who were regular users of crack and who had exchanged sex for crack (or for money to buy crack) during the past 30 days. In addition, participant observation was conducted in 8 Miami crack houses. Interview and observational data suggest that individuals who exchange sex for crack do so with considerable frequency, and through a variety of sexual activities. Systematic data indicated that almost a third of the men and 89% of the women had had 100 or more sex partners during the 30-day period prior to study recruitment. Not only were sexual activities anonymous, extremely frequent, varied, uninhibited (often undertaken in public areas of crack houses), and with multiple partners but, in addition, condoms were not used during the majority of contacts. Of the 37 subjects who were tested for HIV and received their test results 31% of the men and 21% of the women were HIV seropositive. PMID:7611845
Panontin, T.L.; Sheppard, S.D.
1997-12-01
Large-strain, 3-D finite element analyses with incremental plasticity were performed for a variety of crack geometries to study local crack-tip stress-strain fields and associated global fracture parameters under conditions of large-scale yielding. The geometries analyzed include thin, single-edge crack tension, single-edge crack bending, and center-crack tension fracture specimens with varying crack depth (a/W) ratios. Two materials were investigated: a high-hardening, low-strength steel and a moderate-hardening, high-strength steel. Mesh refinement studies were performed to ensure convergence of the finite element predictions. The studies examine the effects of in-plane crack-tip element size, initial crack-tip radius size, and number of through-thickness layers on predicted distributions of crack-tip stress and plastic strain and predicted values of the J-integral and CTOD. In addition, the finite element predictions of specimen behavior were verified experimentally by direct measurements, namely load displacement, load longitudinal strain, and load CTOS, made during and following testing of the fracture specimens. Representative results of the finite element analyses are presented and compared to previously published data where pertinent. Results from the mesh refinement studies and the verification testing are shown. Predicted trends among the specimens and materials in local distributions of crack-tip plastic strain, triaxiality, and opening stress as well as in global parameters, J-integral and m-factor, are discussed.
Anomalous mechanical behavior and crack growth of oxide glasses
NASA Astrophysics Data System (ADS)
Seaman, Jared Hilliard
This thesis is concerned with analytically describing anomalous mechanical behaviors of glass. A new slow crack growth model is presented that considers a semi-elliptical crack in a cylindrical glass rod subjected to 4-point bending that is both loaded statically and under a time-dependent load. This model is used to explain a suppression of the loading-rate dependency of ion-exchanged strengthened glass. The stress relaxation behavior of an ion-exchanged strengthened glass is then analyzed in view of a newly observed water-assisted surface stress relaxation mechanism. By making refinements to a time-dependent Maxwell material model for stress buildup and relaxation, the anomalous subsurface compressive stress peak in ion-exchanged strengthened glass is explained. The notion of water-assisted stress relaxation is extended to the crack tip, where high tensile stresses exist. A toughening effect has historically been observed for cracks aged at subcritical stress intensity factors, where crack tip stress relaxation is hypothesized. A simple fracture mechanics model is developed that estimates a shielding stress intensity factor that is then superimposed with the far-field stress intensity factor. The model is used to estimate anomalous "restart" times for aged cracks. The same model predicts a non-linear crack growth rate for cracks loaded near the static fatigue limit. Double cantilever beam slow crack growth experiments were performed and new slow crack growth data for soda-lime silicate glass was collected. Interpretation of this new experimental slow crack growth data suggests that the origin of the static fatigue limit in glass is due to water-assisted stress relaxation. This thesis combines a number of studies that offer a new unified understanding of historical anomalous mechanical behaviors of glass. These anomalies are interpreted as simply the consequence of slow crack growth and water-assisted surface stress relaxation.
A Continuum-Atomistic Analysis of Transgranular Crack Propagation in Aluminum
NASA Technical Reports Server (NTRS)
Yamakov, V.; Saether, E.; Glaessgen, E.
2009-01-01
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [111] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.
Fracture mechanics and surface chemistry investigations of environment-assisted crack growth
NASA Technical Reports Server (NTRS)
Wei, R. P.; Klier, K.; Simmons, G. W.; Chou, Y. T.
1984-01-01
It is pointed out that environment-assisted subcritical crack growth in high-strength steels and other high-strength alloys (particularly in hydrogen and in hydrogenous environments) is an important technological problem of long standing. This problem is directly related to issues of structural integrity, durability, and reliability. The terms 'hydrogen embrittlement' and 'stress corrosion cracking' have been employed to describe the considered phenomenon. This paper provides a summary of contributions made during the past ten years toward the understanding of environmentally assisted crack growth. The processes involved in crack growth are examined, and details regarding crack growth and chemical reactions are discussed, taking into account crack growth in steels exposed to water/water vapor, the effect of hydrogen, reactions involving hydrogen sulfide, and aspects of fracture surface morphology and composition. Attention is also given to the modeling of crack growth response, crack growth in gas mixtures, and the interaction of solute atoms with the crack-tip stress field.
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.
Mechanisms of time-dependent crack growth at elevated temperature
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.
SIF-based fracture criterion for interface cracks
NASA Astrophysics Data System (ADS)
Ji, Xing
2016-01-01
The complex stress intensity factor K governing the stress field of an interface crack tip may be split into two parts, i.e., hat{K} and s^{-i\\varepsilon } , so that K=hat{K}s^{-i\\varepsilon }, s is a characteristic length and \\varepsilon is the oscillatory index. hat{K} has the same dimension as the classical stress intensity factor and characterizes the interface crack tip field. That means a criterion for interface cracks may be formulated directly with hat{K} , as Irwin (ASME J. Appl. Mech. 24:361-364, 1957) did in 1957 for the classical fracture mechanics. Then, for an interface crack, it is demonstrated that the quasi Mode I and Mode II tip fields can be defined and distinguished from the coupled mode tip fields. Built upon SIF-based fracture criteria for quasi Mode I and Mode II, the stress intensity factor (SIF)-based fracture criterion for mixed mode interface cracks is proposed and validated against existing experimental results.
SIF-based fracture criterion for interface cracks
NASA Astrophysics Data System (ADS)
Ji, Xing
2016-06-01
The complex stress intensity factor K governing the stress field of an interface crack tip may be split into two parts, i.e., hat{K} and s^{-iɛ}, so that K=hat{K}s^{-iɛ}, s is a characteristic length and ɛ is the oscillatory index. hat{K} has the same dimension as the classical stress intensity factor and characterizes the interface crack tip field. That means a criterion for interface cracks may be formulated directly with hat{K}, as Irwin (ASME J. Appl. Mech. 24:361-364, 1957) did in 1957 for the classical fracture mechanics. Then, for an interface crack, it is demonstrated that the quasi Mode I and Mode II tip fields can be defined and distinguished from the coupled mode tip fields. Built upon SIF-based fracture criteria for quasi Mode I and Mode II, the stress intensity factor (SIF)-based fracture criterion for mixed mode interface cracks is proposed and validated against existing experimental results.
Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li
NASA Astrophysics Data System (ADS)
Ro, Yunjo; Agnew, Sean R.; Gangloff, Richard P.
2007-12-01
A combined electron backscatter diffraction (EBSD)/stereology method successfully quantifies the orientation of fatigue crack surfaces for Al-Li-Cu and Al-Cu-Mg alloys stressed at low Δ K, in which deformation is localized in slip bands and cracking is highly faceted. The method orients features as small as ˜1 μm in complex microstructures. Vacuum fatigue facets align within 15 deg of up to four variants of {111} slip planes, governed by the distribution of crack tip resolved shear stress. The small fraction of precisely oriented {111} facets suggests that cracking involves complex intraband and multiple-band interface paths. Water vapor and NaCl solution affect a similar dramatic change in the crack path; near-{111} facets are never observed, at odds with mechanisms for H-enhanced slip localization and associated slip band cracking. Rather, two environmental crack facet morphologies, broad flat and repeating step, exhibit a wide range of orientations between {001} and {110}, as governed by crack tip resolved normal stresses. The repetitive stepped facets appear to contain areas parallel to {100}/{110} on the ˜1- μm scale, coupled with surface curvature consistent with a mechanism of discontinuous fatigue crack growth involving H-enhanced {100}/{110} cleavage and intermingled crack tip plasticity. Broad-flat faceted regions are parallel to a variety of planes, consistent with a mechanism combining high crack tip tensile stresses and H trapped at the dislocation structure from cyclic deformation, within 1 μm of the crack tip.
Cessation of environmentally-assisted cracking in a low-alloy steel: Experimental results
Li, Y.Y.
1997-01-01
The presence of dissolved metallurgical sulfides in pressure vessel and piping steels has been linked to Environmentally-Assisted Cracking (EAC), a phenomenon observed in laboratory tests that results in fatigue crack growth rates as high as 100 times that in air. Previous experimental and analytical work based on diffusion as the mass transport process has shown that surface cracks that are initially clean of sulfides will not initiate EAC in most applications. This is because the average crack tip velocity would not be sufficiently high to expose enough metallurgical sulfides per unit time and produce the sulfide concentration required for EAC. However, there is a potential concern for the case of a relatively large embedded crack breaking through to the wetted surface. Such a crack would not be initially clean of sulfides, and EAC could initiate. This paper presents the results of a series of experiments conducted on two heats of an EAC susceptible, high-sulfur, low-alloy steel in 243{degrees}C low-oxygen water to further study the phenomenon of EAC persistence at low crack tip velocities. A load cycle profile that incorporated a significant load dwell period at minimum load was used. In one experiment, the fatigue cycling history was such that relatively high crack tip velocities at the start of the experiment produced a persistent case of EAC even when crack tip velocities were later reduced to levels below the EAC initiation velocity. The other series of experiments used initial crack tip velocities that were much lower and probably more realistic. Air precracking of the compact tension specimens produced an initial inventory of undissolved sulfides on the crack flanks that directly simulates the array of sulfides expected from the breakthrough of an embedded crack. In all cases, results showed EAC ceased after several hundred hours of cycling.
NASA Astrophysics Data System (ADS)
Zhao, Yanlin; Zhang, Lianyang; Wang, Weijun; Pu, Chengzhi; Wan, Wen; Tang, Jingzhou
2016-07-01
This paper investigates the cracking and stress-strain behavior, especially the local strain concentration near the flaw tips, of rock-like material containing two flaws. A series of uniaxial compression tests were carried out on rock-like specimens containing two flaws, with strain gauges mounted near the flaw tips to measure the local strain concentration under the uniaxial compressive loading. Four different types of cracks (wing cracks, anti-wing cracks, coplanar shear cracks and oblique shear cracks) and seven patterns of crack coalescences (T1 and T2; S1 and S2; and TS1, TS2 and TS3) are observed in the experiments. The type of crack coalescence is related to the geometry of the flaws. In general, the crack coalescence varies from the S-mode to the TS-mode and then to the T-mode with the increase of the rock bridge ligament angle. The stress-strain curves of the specimens containing two flaws are closely related to the crack development and coalescence process. The strain measurements indicate that the local tensile strain concentration below or above the pre-existing flaw tip causes wing or anti-wing cracks, while the local compressive strain concentration near the flaw tip is related to the shear crack. The measured local tensile strain shows a jump at the initiation of wing- and anti-wing cracks, reflecting the instant opening of the wing- and anti-wing crack propagating through the strain gauge. During the propagation of wing- and anti-wing cracks, the measured local tensile strain gradually increases with few jumps, implying that the opening deformation of wing- and anti-wing cracks occurs in a stable manner. The shear cracks initiate followed by a large and abrupt compressive strain jump and then quickly propagate in an unstable manner resulting in the failure of specimens.
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.
Fatigue crack propagation analysis of plaque rupture.
Pei, Xuan; Wu, Baijian; Li, Zhi-Yong
2013-10-01
Rupture of atheromatous plaque is the major cause of stroke or heart attack. Considering that the cardiovascular system is a classic fatigue environment, plaque rupture was treated as a chronic fatigue crack growth process in this study. Fracture mechanics theory was introduced to describe the stress status at the crack tip and Paris' law was used to calculate the crack growth rate. The effect of anatomical variation of an idealized plaque cross-section model was investigated. The crack initiation was considered to be either at the maximum circumferential stress location or at any other possible locations around the lumen. Although the crack automatically initialized at the maximum circumferential stress location usually propagated faster than others, it was not necessarily the most critical location where the fatigue life reached its minimum. We found that the fatigue life was minimum for cracks initialized in the following three regions: the midcap zone, the shoulder zone, and the backside zone. The anatomical variation has a significant influence on the fatigue life. Either a decrease in cap thickness or an increase in lipid pool size resulted in a significant decrease in fatigue life. Comparing to the previously used stress analysis, this fatigue model provides some possible explanations of plaque rupture at a low stress level in a pulsatile cardiovascular environment, and the method proposed here may be useful for further investigation of the mechanism of plaque rupture based on in vivo patient data. PMID:23897295
Crack analysis of unfilled natural rubber using infrared microspectroscopy
Neumeister, L.A.; Koenig, J.L.
1996-12-31
Several spectroscopic methods are available for characterizing the crack tip region of natural rubber on the molecular scale to develop a material with fatigue resistant properties. Attenuated total reflectance or ATR-IR has been used to characterize the structure of many different polymers, including rubbers. Transmission has also been well established as a viable technique for the molecular characterization of transparent materials including thin films of unfilled natural rubber. Unfilled natural rubber was stretched to 629% elongation until cracks in the surface. A cross section of the sample containing a crack was then microtomed at -85{degrees}C into slices approximately 0.5 {mu}m thick. Similarly, samples were cut with a razor and microtomed under the same conditions. The crack tip region was mapped using the IR{mu}s{trademark}/SIRM Molecular Microanalysis System. The map consisted of ten spectra taken in the x and y directions. The same region was analyzed for orientation of molecular structures. Points were selected along the crack tip, crack edges, and the bulk. Dichroic ratios of all prominent peaks were calculated. ATR was used to verify the results of the mapping experiments for both stressed unstressed material.
NASA Astrophysics Data System (ADS)
Tennant, Jill
2012-01-01
Attending an educator conference and its associated exhibit hall can be a rewarding experience for your brain. But if you keep in mind these insider's tips, your feet, arms, stomach, and wallet will also thank you.
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Feijó, José A; Costa, Sílvia S; Prado, Ana Margarida; Becker, Jörg D; Certal, Ana Catarina
2004-10-01
New molecules, including protein kinases, lipids and molecules that have neurotransmitter activities in animals have emerged as important players in tip-growing cells. Transcriptomics analysis reveals that the largest single class of genes expressed in pollen tubes encode signal transducers, reflecting the necessity to decode complex and diverse pathways that are associated with tip growth. Many of these pathways may use common intracellular second messengers, with ions and reactive oxygen species emerging as two major common denominators in many of the processes involved in tip growth. These second messengers might influence the actin cytoskeleton through known interactions with actin-binding proteins. In turn, changes in the dynamic properties of the cytoskeleton would define the basic polarity events needed to shape and modify tip-growing cells. PMID:15337103
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ERIC Educational Resources Information Center
Cuoco, Albert A.; And Others, Eds.
1994-01-01
Contains tips from readers about using technology in the classroom, including notebook computers, classroom sets of calculators, geometry software, LOGO software, publisher discounts, curriculum materials in CD-ROM, and volunteer help in computers and computer networking for schools. (MKR)
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On the variation in crack-opening stresses at different locations in a three-dimensional body
NASA Technical Reports Server (NTRS)
Chermahini, R. G.; Blom, Anders F.
1990-01-01
Crack propagation and closure behavior of thin, and thick middle crack tension specimens under constant amplitude loading were investigated using a three dimensional elastic plastic finite element analysis of fatigue crack propagation and closure. In the thin specimens the crack front closed first on the exterior (free) surface and closed last in the interior during the unloading portion of cyclic loading; a load reduced displacement technique was used to determine crack opening stresses at specified locations in the plate from the displacements calculated after the seven cycle. All the locations were on the plate external surface and were located near the crack tip, behind the crack tip, at the centerline of the crack. With this technique, the opening stresses at the specified points were found to be 0.52, 0.42, and 0.39 times the maximum applied stress.
ERIC Educational Resources Information Center
Mahon, Robert Lee
2005-01-01
In this article, the author shares some tips for teachers. His tips are as follows: (1) a teacher should forget his or her education; (2) a teacher should forget the theory (3) a teacher should remember that he or she is a translator, not an originator; (4) a teacher should respect his or her students; (5) a teacher should be true to his or her…
Hydrogen enhanced crack growth in 18 Ni maraging steels
NASA Technical Reports Server (NTRS)
Hudak, S. J., Jr.; Wei, R. P.
1976-01-01
The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high-purity hydrogen are examined using the crack-tip stress intensity factor K as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly defined K-independent stage (Stage II). Crack growth rates in an 18 Ni (grade 250) maraging steel are examined for temperatures from -6 to +100 C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures the activation energy for Stage II crack growth was found to be 16.7 plus or minus 3.3 kJ/mole. Temperature and hydrogen partial pressure are shown to interact in a complex manner to determine the apparent Kth (stress intensity level below which no observable crack growth occurs) and the crack growth behavior. Comparison of results on '250' and '300' grades of 18 Ni maraging steel indicate a significant influence of alloy composition and/or strength level on the crack growth behavior.
Identifying fatigue crack geometric features from acoustic emission signals
NASA Astrophysics Data System (ADS)
Bao, Jingjing; Poddar, Banibrata; Giurgiutiu, Victor
2016-04-01
Acoustic emission (AE) caused by the growth of fatigue crack were well studied by researchers. Conventional approaches predominantly are based on statistical analysis. In this study we focus on identifying geometric features of the crack from the AE signals using physics based approach. One of the main challenges of this approach is to develop a physics of materials based understanding of the generation and propagation of acoustic emissions due to the growth of a fatigue crack. As the geometry changes due to the crack growth, so does the local vibration modes around the crack. Our aim is to understand these changing local vibration modes and find possible relation between the AE signal features and the geometric features of the crack. Finite element (FE) analysis was used to model AE events due to fatigue crack growth. This was done using dipole excitation at the crack tips. Harmonic analysis was also performed on these FE models to understand the local vibration modes. Experimental study was carried out to verify these results. Piezoelectric wafer active sensors (PWAS) were used to excite cracked specimen and the local vibration modes were captured using laser Doppler vibrometry. The preliminary results show that the AE signals do carry the information related to the crack geometry.
Modeling of crack bridging in a unidirectional metal matrix composite
NASA Technical Reports Server (NTRS)
Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack
1991-01-01
The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, Delta K(sup eff), was calculated using both models to correlate the measure crack growth rate in the composite. The calculated Delta K(sup eff) from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.
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
Effect of multi-functional inhibitors on the electrochemistry within a corrosion crack
Omura, H.
1984-01-01
The electrochemical and mass transport mechanisms in stress corrosion cracking, which depend on the rate of metal dissolution and production of hydrogen, have been used to establish analytically the electrode potential distribution within the crack. When crack growth occurs by enhanced anodic dissolution of the plastically strained tip, the electrode potential at the crack tip always is more active than at the crack mouth because of the electric potential gradient that exists in the electrolyte within the crack. This also gives rise to additional or alternative electrochemical reactions such as hydrogen evolution and anodic dissolution at the crack tip. Furthermore, because of the potential difference from the crack mouth, the electrochemical driving force becomes more favorable for the development of corrosion inside the crack. The analysis predicts the distribution of electrode potential within a crack, and theoretical results have been compared with experimental measurements recorded from a model electrode system. Under free corrosion, a small potential difference may cause a concentration change of Cl/sup -/ ion and increase the chloride attack. In order to reduce the chloride and hydrogen attack, multifunctional inhibitors, such as borax-nitrite with small amounts of surfactant such as MBT or amino-methyl-propanol, are excellent inhibitors. The surfactant interferes in the dissolution reaction and blocks active chloride ion and hydrogen ion by interacting synergistically with the passive film produced by the borax-nitrite, which results in development of a stronger and thicker protective film.
NASA Astrophysics Data System (ADS)
Suzuki, Shinichi; Homma, Hiroomi; Kusaka, Riichiro
A METHOD OF pulsed holographic microscopy is applied to take instantaneous microscopic photographs of the neighborhoods of crack tips propagating through PMMA or through AISI 4340 steel specimens at a speed of several hundred meters per second. The cracks are in the opening mode. A fast propagating crack is recorded as a hologram at an instant during its propagation. A microscopic photograph of the crack is taken with a conventional microscope to magnify the reconstructed image from the hologram. From the microscopic photograph, crack opening displacement (COD) is measured along the crack in the vicinity of the crack tip. The COD is of the order often to one hundred microns, and in proportion to the square root of the distance from the crack tip. The dynamic fracture toughness KID is obtained using the formula for COD in the singular stress field of a fast propagating crack. Simultaneous KID measurement both through pulsed holographic microscopy and through the caustic method is furthermore carried out with PMMA specimens. The values of KID obtained through pulsed holographic microscopy are in agreement with those through the caustic method. Microcracks accompanied by a main crack are also photographed with the method of pulsed holographic microscopy.
Dynamic crack propagation in a 2D elastic body: The out-of-plane case
NASA Astrophysics Data System (ADS)
Nicaise, Serge; Sandig, Anna-Margarete
2007-05-01
Already in 1920 Griffith has formulated an energy balance criterion for quasistatic crack propagation in brittle elastic materials. Nowadays, a generalized energy balance law is used in mechanics [F. Erdogan, Crack propagation theories, in: H. Liebowitz (Ed.), Fracture, vol. 2, Academic Press, New York, 1968, pp. 498-586; L.B. Freund, Dynamic Fracture Mechanics, Cambridge Univ. Press, Cambridge, 1990; D. Gross, Bruchmechanik, Springer-Verlag, Berlin, 1996] in order to predict how a running crack will grow. We discuss this situation in a rigorous mathematical way for the out-of-plane state. This model is described by two coupled equations in the reference configuration: a two-dimensional scalar wave equation for the displacement fields in a cracked bounded domain and an ordinary differential equation for the crack position derived from the energy balance law. We handle both equations separately, assuming at first that the crack position is known. Then the weak and strong solvability of the wave equation will be studied and the crack tip singularities will be derived under the assumption that the crack is straight and moves tangentially. Using the energy balance law and the crack tip behavior of the displacement fields we finally arrive at an ordinary differential equation for the motion of the crack tip.
Analyses of Fatigue Crack Growth and Closure Near Threshold Conditions for Large-Crack Behavior
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1999-01-01
A plasticity-induced crack-closure model was used to study fatigue crack growth and closure in thin 2024-T3 aluminum alloy under constant-R and constant-K(sub max) threshold testing procedures. Two methods of calculating crack-opening stresses were compared. One method was based on a contact-K analyses and the other on crack-opening-displacement (COD) analyses. These methods gave nearly identical results under constant-amplitude loading, but under threshold simulations the contact-K analyses gave lower opening stresses than the contact COD method. Crack-growth predictions tend to support the use of contact-K analyses. Crack-growth simulations showed that remote closure can cause a rapid rise in opening stresses in the near threshold regime for low-constraint and high applied stress levels. Under low applied stress levels and high constraint, a rise in opening stresses was not observed near threshold conditions. But crack-tip-opening displacement (CTOD) were of the order of measured oxide thicknesses in the 2024 alloy under constant-R simulations. In contrast, under constant-K(sub max) testing the CTOD near threshold conditions were an order-of-magnitude larger than measured oxide thicknesses. Residual-plastic deformations under both constant-R and constant-K(sub max) threshold simulations were several times larger than the expected oxide thicknesses. Thus, residual-plastic deformations, in addition to oxide and roughness, play an integral part in threshold development.
Rodríguez-Martos, Ramón; Castellanos-Cosano, Lizett; Serrera-Figallo, María A.; Segura-Egea, Juan J.; Gutierrez-Perez, Jose L.
2012-01-01
Objective: The objective of our study is to analyse (with the help of scanning electron microscopes) the quality of the dental root surface and the appearance of dental cracks after performing apical preparations using two diffe-rent types of ultrasonic tips. Study design: We used 32 single-rooted teeth that underwent a root canal and apical resection. Afterwards, the teeth were divided into 4 groups of 8 teeth each, with preparations of the apical cavities in the following manner: Group 1: stainless steel ultrasonic tip at 33KHz. Group 2: stainless steel ultrasonic tip at 30KHz. Group 3: diamond ultrasonic tip at 30KHz. Group 4: diamond ultrasonic tip at 33 KHz. The quality of the root surface and the presence of cracks were evaluated by one single observer using a scanning electron microscope. Results: All of the teeth in our study had cracks after the apical preparations. The mean number of cracks per tooth ranged between 6.1±1.9 (group 1) and 3.5±2.4 (group 4), with a significantly higher number found in the groups that used stainless steel tips (P=.03). The types of cracks produced involved: 8 complete cracks (4.5%), 167 incomplete cracks (94.4%), and 2 intradentinal cracks (1.1%), with no significant differences observed between the different frequencies used for each group. Conclusions: Stainless steel ultrasonic tips provoked a larger number of cracks than diamond tips. The frequency of vibration used did not have any effect on the number of cracks found. Key words:Apicoectomy, scanning electron microscope, therapeutic ultrasound, endodontic surgery, dentinal crack. PMID:22926471
Resolved shear stress intensity coefficient and fatigue crack growth in large crystals
NASA Technical Reports Server (NTRS)
Chen, QI; Liu, Hao-Wen
1988-01-01
Fatigue crack growth in large grain Al alloy was studied. Fatigue crack growth is caused primarily by shear decohesion due to dislocation motion in the crack tip region. The crack paths in the large crystals are very irregular and zigzag. The crack planes are often inclined to the loading axis both in the inplane direction and the thickness direction. The stress intensity factors of such inclined cracks are approximated from the two dimensional finite element calculations. The plastic deformation in a large crystal is highly anisotropic, and dislocation motion in such crystals are driven by the resolved shear stress. The resolved shear stress intensity coefficient in a crack solid, RSSIC, is defined, and the coefficients for the slip systems at a crack tip are evaluated from the calculated stress intensity factors. The orientations of the crack planes are closely related to the slip planes with the high RSSIC values. If a single slip system has a much higher RSSIC than all the others, the crack will follow the slip plane, and the slip plane becomes the crack plane. If two or more slip systems have a high RSSIC, the crack plane is the result of the decohesion processes on these active slip planes.
NASA Astrophysics Data System (ADS)
Kroon, Martin
2012-01-01
In the present study, a computational framework for studying high-speed crack growth in rubber-like solids under conditions of plane stress and steady-state is proposed. Effects of inertia, viscoelasticity and finite strains are included. The main purpose of the study is to examine the contribution of viscoelastic dissipation to the total work of fracture required to propagate a crack in a rubber-like solid. The computational framework builds upon a previous work by the present author (Kroon in Int J Fract 169:49-60, 2011). The model was fully able to predict experimental results in terms of the local surface energy at the crack tip and the total energy release rate at different crack speeds. The predicted distributions of stress and dissipation around the propagating crack tip are presented. The predicted crack tip profiles also agree qualitatively with experimental findings.
Lee, S. Y.; Sun, Yinan; An, Ke; Choo, Hahn; Hubbard, Camden R; Liaw, Peter K
2010-01-01
Neutron diffraction was employed to investigate the crack-growth retardation phenomenon after a single tensile overload by mapping both one-dimensional and two-dimensional residual-strain distributions around the crack tip in a series of compact-tension specimens representing various crack-growth stages through an overload-induced retardation period. The results clearly show a large compressive residual-strain field near the crack tip immediately after the overload. As the fatigue crack propagates through the overload-induced plastic zone, the compressive residual strains are gradually relaxed, and a new compressive residual-strain field is developed around the propagating crack tip, illustrating that the subsequent fatigue-induced plastic zone grows out of the large plastic zone caused by the overloading. The relationship between the overload-induced plastic zone and subsequent fatigue-induced plastic zone, and its influence on the residual-strain distributions in the perturbed plastic zone are discussed.
High-Resolution Crack Imaging Reveals Degradation Processes in Nuclear Reactor Structural Materials
Olszta, Matthew J.; Schreiber, Daniel K.; Thomas, Larry E.; Bruemmer, Stephen M.
2012-04-01
Corrosion and cracking represent critical failure mechanisms for structural materials in many applications. Although a crack can often be seen with the unaided eye, higher resolution imaging techniques are required to understand the nature of the crack tips and underlying degradation processes. Researchers at Pacific Northwest National Laboratory (PNNL) employ a suite of microscopy techniques and site-specific material sampling to analyze corrosion and crack structures, producing images and compositional analyses with near-atomic spatial resolution. The samples are cracked components removed from commercial light-water reactor service or laboratory samples tested in simulated reactor environments.
Through-the-thickness fatigue crack closure behavior in an aluminum alloy
NASA Technical Reports Server (NTRS)
Dawicke, D. S.; Newman, J. C., Jr.; Grandt, A. F., Jr.
1990-01-01
The variation in fatigue crack closure behavior across the thickness of aluminum alloy specimens was investigated. The specimen geometries examined were the middle crack tension M(T) and compact tension C(T). The fatigue crack closure behavior was determined using remote displacement and strain gages, near tip strain gages, and fatigue striations. A hybrid experimental/numerical method was also used to infer the crack opening loads. The results indicate a variation in crack opening load, of 0.2 in the specimen interior to 0.4 to 0.5 at the surface.
Tiernan, M.
1980-09-01
Crack spectra derived from velocity data have been shown to exhibit systematics which reflect microstructural and textural differences between samples (Warren and Tiernan, 1980). Further research into both properties and information content of crack spectra have yielded the following: Spectral features are reproducible even at low pressures; certain observed spectral features may correspond to non-in-situ crack populations created during sample retrieval; the functional form of a crack spectra may be diagnostic of the sample's grain texture; hysteresis is observed in crack spectra between up and down pressure runs - it may be due to friction between the faces of closed crack populations.
A note on the cracked plates reinforced by a line stiffener
NASA Technical Reports Server (NTRS)
Yahsi, O. S.; Erdogan, F.
1983-01-01
The problem of a cracked plate reinforced by a line stiffener is reconsidered. The original solution of this problem was given in the literature. Also, a variation of the problem with debonding between the plate and the stiffener near the cracked region was reported in the literature. However, the special case of the problem in which the crack tip terminates at the stiffener does not appear to have been studied. In practice, the solution may be necessary in order to assess the crack arrest effectiveness of the stiffener. The problem of a stiffened plate with a crack is reformulated, the asymptotic stress state near the crack tip terminating at the stiffener is examined, and numerical results are given for various stiffness constants.
A note on the cracked plates reinforced by a line stiffener
NASA Technical Reports Server (NTRS)
Yahsi, O. S.; Erdogan, F.
1983-01-01
The problem of a cracked plate reinforced by a line stiffener is reconsidered. The original solution of this problem was given in the literature. Also, a variation of the problem with debonding between the plate and the stiffener near the cracked region was reported in the literature. However, the special case of the problem in which the crack tip terminates at the stiffener does not appear to have been studied. In practice, the solution may be necessary in order to assess the crack arrest effectiveness of the stiffener. The problem of a stiffened plate with a crack is reformulated, the asymptotic stress state near the crack tip terminating at the stiffener is examined, and numerical results are given for various stiffness constants. Previously announced in STAR as N83-21388
Nonclassical nucleation and growth of cohesive tensile cracks
Rundle, J. B.; Klein, W.
1989-07-10
We analyze the nucleation and growth of cohesive tensile cracks using a field-theoretic formulation in which the free energy is written as a functional of the crack separation (offset field). Our results indicate that for certain materials, crack nucleation and growth proceed through the formation and extension of a diffuse ''halo'' surrounding the classical portion of the crack. This is similar to nonclassical nucleation in magnetic systems. Theoretical considerations and numerical calculations strongly suggest that the diffuse halo can be identified with the fracture ''process zone'' seen in laboratory studies of advancing cracks.
Crack propagation induced heating in crystalline energetic materials
NASA Astrophysics Data System (ADS)
Holmes, W.; Francis, R. S.; Fayer, M. D.
1999-02-01
A model is presented for time and spatial dependences of the heating of molecular vibrations and the possible initiation of chemical reaction from heat dissipated in the vicinity of a propagating crack in a molecular crystal. In the model, energy from a moving crack tip is released as phonons in proximity to the crack. Initially the phonons and the molecular vibrations are not in thermal equilibrium. Subsequently, there is a competition between excitation of molecular vibrations by multiphonon up-pumping and diffusion of phonons from the crack region. If the coupling between the locally hot phonon bath and the molecular vibrations is sufficiently large, a transitory high vibrational temperature will be achieved prior to eventual thermal equilibration with the bulk of the crystal. It is found that the peak vibrational temperature can be sufficiently high for a significant time period for chemical reactions to occur. The model calculates the local time-dependent vibrational temperature using reasonable values of the physical input parameters. For a crack tip moving near the speed of sound, the calculations show that vibrational temperatures can reach ˜800 K in 55 ps and exceed 550 K for ˜1 ns after the initial heating. This temperature change is sufficient to produce chemical reaction in a secondary explosive such as HMX, but given the duration and size of the heated region, a single crack should not result in self-sustaining chemical reaction. The role that cracks may play in shock sensitivity is discussed.
Deformation localization at the tips of shear fractures: An analytical approach
NASA Astrophysics Data System (ADS)
Misra, Santanu
2011-04-01
Mechanical heterogeneities are important features in rocks which trigger deformation localization in brittle, ductile or brittle-ductile modes during deformation. In a recent study Misra et al. (2009) have investigated these different processes of deformation localization at the tips of pre-existing planar shear fractures. The authors identified four mechanisms of deformation, ranging from brittle to ductile, operating at the crack tips. Mechanism A: brittle deformation is the dominant process that forms a pair of long tensile fractures at the two crack tips. Mechanism B: nature of deformation is mixed where the tensile fractures grow to a finite length with incipient plastic deformation at the tips. Mechanism C: mixed mode deformation characterized by dominating macro-scale shear bands, and short, opened-out tensile fissures. Mechanism D: localization of plastic bands in the form of a pair of shear bands at each tip without any discernible brittle fracturing. The transition of the mechanisms is a function of orientation ( α) of the crack with respect to the bulk compression direction and the finite length ( l) of the crack. The aim of this study is to present a theoretical analysis to account for the variability of deformation localization in the vicinity of pre-existing shear cracks considering an elastic-plastic rheological model. The analysis calculates the principal tensile stress ( σ1) and the second stress invariant ( I2) of the stress field at the fracture tip to explain the transition from Mechanism A (tensile fracturing) to Mechanism D (ductile strain). The results show that σ1 at the fracture tip increases non-linearly with increasing α and Ar (aspect ratio of the shear crack), and assumes a large value when α > 50 o, promoting tensile fractures. On the other hand, I2 is a maximum at α < 45°, resulting in nucleation of ductile shear bands.
Double noding technique for mixed mode crack propagation studies
NASA Technical Reports Server (NTRS)
Liaw, B. M.; Kobayashi, A. S.; Emery, A. F.
1984-01-01
A simple dynamic finite element algorithm for analyzing a propagating mixed mode crack tip is presented. A double noding technique, which can be easily incorporated into existing dynamic finite element codes, is used together with a corrected J integral to extract modes I and II dynamic stress intensity factors of a propagating crack. The utility of the procedure is demonstrated by analyzing test problems involving a mode I central crack propagating in a plate subjected to uniaxial tension, a mixed mode I and II stationary, slanted central crack in a plate subjected to uniaxial impact loading, and a mixed mode I and II extending, slanted single edge crack in a plate subjected to uniaxial tension. Previously announced in STAR as N83-13491
Material grain size and crack size influences on cleavage fracturing.
Armstrong, Ronald W
2015-03-28
A review is given of the analogous dependence on reciprocal square root of grain size or crack size of fracture strength measurements reported for steel and other potentially brittle materials. The two dependencies have much in common. For onset of cleavage in steel, attention is focused on relationship of the essentially athermal fracture stress compared with a quite different viscoplastic yield stress behaviour. Both grain-size-dependent stresses are accounted for in terms of dislocation pile-up mechanics. Lowering of the cleavage stress occurs in steel because of carbide cracking. For crack size dependence, there is complication of localized crack tip plasticity in fracture mechanics measurements. Crack-size-dependent conventional and indentation fracture mechanics measurements are described also for results obtained on the diverse materials: polymethylmethacrylate, silicon crystals, alumina polycrystals and WC-Co (cermet) composites. PMID:25713456
Fatigue crack closure behavior at high stress ratios
NASA Technical Reports Server (NTRS)
Turner, C. Christopher; Carman, C. Davis; Hillberry, Ben M.
1988-01-01
Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.
Fatigue crack growth under general-yielding cyclic-loading
NASA Technical Reports Server (NTRS)
Minzhong, Z.; Liu, H. W.
1986-01-01
In low cycle fatigue, cracks are initiated and propagated under general yielding cyclic loading. For general yielding cyclic loading, Dowling and Begley have shown that fatigue crack growth rate correlates well with the measured delta J. The correlation of da/dN with delta J was also studied by a number of other investigators. However, none of thse studies have correlated da/dN with delta J calculated specifically for the test specimens. Solomon measured fatigue crack growth in specimens in general yielding cyclic loading. The crack tips fields for Solomon's specimens are calculated using the finite element method and the J values of Solomon's tests are evaluated. The measured crack growth rate in Solomon's specimens correlates very well with the calculated delta J.
The application of a logic framework for fatigue crack growth analyses to microstructural effects
Xu, J.G.; Liu, H.W.
1995-12-31
{Delta}K has been widely used to correlate da/dN data. The relation between da/dN and {Delta}K is usually found empirically. However, fatigue crack growth relations can also be derived theoretically. Three fatigue crack growth theories are derived for the state of small scale yielding and plane strain. These three theories constitute a logic framework useful for fatigue crack growth analyses. The application of the logic framework to the analyses of microstructural effects on fatigue crack growth is illustrated. The fatigue crack growth curve of 7075-T651 aluminum alloy has five distinct regions. A fatigue crack grows by crack-tip shear decohesion forming striations and by brittle fractures of particles followed by localized shear decohesion at these microcracks forming dimples. The logic framework helps to relate the fatigue crack growth behaviors in these five regions to the fractures of inclusions and to the resistance of grain boundaries and dispersoids to shear decohesion.
Short fatigue crack characterization and detection using confocal scanning laser microscopy (CSLM)
Varvani-Farahani, A.; Topper, T.H.
1997-12-31
This paper presents a new technique for studying the growth and morphology of fatigue cracks. The technique allows short fatigue crack growth, crack depth, aspect ratio (crack depth/half crack length), and crack front configuration to be measured using a Confocal Scanning Laser Microscope (CSLM). CSLM measurements of the initial stage of crack growth in Al 2024-T351 revealed that microstructurally short fatigue cracks grew initially along a plane inclined to the applied stress. The angle of the inclined plane (Stage I crack growth) was found to be about 45 degrees to the axis of the applied tensile load. Aspect ratio and the angle of maximum shear plane (Mode II), obtained using the CSLM technique, showed a good agreement with those obtained using a Surface Removal (SR) technique. The aspect ratios obtained using the CSLM technique were found to remain constant with increasing crack length in Al 2024-T351 and SAE 1045 Steel at 0.83 and 0.80, respectively. Optical sectioning along the length of a crack revealed that the crack front in the interior of the materials has a semi-elliptical shape. These results are in good agreement with results obtained using the SR technique. The CSLM technique was employed to characterize the fracture surface of fatigue cracks in an SAE 1045 Steel. CSLM image processing of the fracture surface near the crack tip constructed a three dimensional profile of fracture surface asperities. The heights of asperities were obtained from this profile. Optical sectioning from a post-image-processed crack provided crack depth and crack mouth width at every point along the crack length for each load level. The crack opening stress was taken as the stress level at which the crack depth stopped increasing with increases in a lied stress. 6 refs., 9 figs., 1 tab.
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NASA Astrophysics Data System (ADS)
Budarapu, P. R.; Javvaji, B.; Sutrakar, V. K.; Roy Mahapatra, D.; Zi, G.; Rabczuk, T.
2015-08-01
The crack initiation and growth mechanisms in an 2D graphene lattice structure are studied based on molecular dynamics simulations. Crack growth in an initial edge crack model in the arm-chair and the zig-zag lattice configurations of graphene are considered. Influence of the time steps on the post yielding behaviour of graphene is studied. Based on the results, a time step of 0.1 fs is recommended for consistent and accurate simulation of crack propagation. Effect of temperature on the crack propagation in graphene is also studied, considering adiabatic and isothermal conditions. Total energy and stress fields are analyzed. A systematic study of the bond stretching and bond reorientation phenomena is performed, which shows that the crack propagates after significant bond elongation and rotation in graphene. Variation of the crack speed with the change in crack length is estimated.
Thorn, C.E.; Chasman, C.; Baltz, A.J.
1981-11-19
An improved magnet more easily provides a radially increasing magnetic field, as well as reduced fringe field and requires less power for a given field intensity. The subject invention comprises a pair of spaced, opposed magnetic poles which further comprise a pair of pole roots, each having a pole tip attached to its center. The pole tips define the gap between the magnetic poles and at least a portion of each pole tip is separated from its associated pole root. The separation begins at a predetermined distance from the center of the pole root and increases with increasing radial distance while being constant with azimuth within that portion. Magnets in accordance with the subject invention have been found to be particularly advantageous for use in large isochronous cyclotrons.
Thorn, Craig E.; Chasman, Chellis; Baltz, Anthony J.
1984-04-24
An improved magnet which more easily provides a radially increasing magnetic field, as well as reduced fringe field and requires less power for a given field intensity. The subject invention comprises a pair of spaced, opposed magnetic poles which further comprise a pair of pole roots, each having a pole tip attached to its center. The pole tips define the gap between the magnetic poles and at least a portion of each pole tip is separated from its associated pole root. The separation begins at a predetermined distance from the center of the pole root and increases with increasing radial distance while being constant with azimuth within that portion. Magnets in accordance with the subject invention have been found to be particularly advantageous for use in large isochronous cyclotrons.
Three-dimensional elastic-plastic analysis of shallow cracks in single-edge-crack-tension specimens
NASA Technical Reports Server (NTRS)
Shivakumar, Kunigal N.; Newman, James C., Jr.
1990-01-01
Three dimensional, elastic-plastic, finite element results are presented for single-edge crack-tension specimens with several shallow crack-length-to-width ratios (0.05 less than or equal to a/W less than or equal to 0.5). Results showed the need to model the initial yield plateau in the stress-strain behavior to accurately model deformation of the A36 steel specimens. The crack-tip-opening-displacement was found to be linearly proportional to the crack-mouth-opening displacement. A new deformation dependent plastic-eta factor equation is presented for calculating the J-integral from test load-displacement records. This equation was shown to be accurate for all crack lengths considered.
Separation of crack extension modes in orthotropic delamination models
NASA Technical Reports Server (NTRS)
Beuth, Jack L.
1995-01-01
In the analysis of an interface crack between dissimilar elastic materials, the mode of crack extension is typically not unique, due to oscillatory behavior of near-tip stresses and displacements. This behavior currently limits the applicability of interfacial fracture mechanics as a means to predict composite delamination. The Virtual Crack Closure Technique (VCCT) is a method used to extract mode 1 and mode 2 energy release rates from numerical fracture solutions. The mode of crack extension extracted from an oscillatory solution using the VCCT is not unique due to the dependence of mode on the virtual crack extension length, Delta. In this work, a method is presented for using the VCCT to extract Delta-independent crack extension modes for the case of an interface crack between two in-plane orthotropic materials. The method does not involve altering the analysis to eliminate its oscillatory behavior. Instead, it is argued that physically reasonable, Delta-independent modes of crack extension can be extracted from oscillatory solutions. Knowledge of near-tip fields is used to determine the explicit Delta dependence of energy release rate parameters. Energy release rates are then defined that are separated from the oscillatory dependence on Delta. A modified VCCT using these energy release rate definitions is applied to results from finite element analyses, showing that Delta-independent modes of crack extension result. The modified technique has potential as a consistent method for extracting crack extension modes from numerical solutions. The Delta-independent modes extracted using this technique can also serve as guides for testing the convergence of finite element models. Direct applications of this work include the analysis of planar composite delamination problems, where plies or debonded laminates are modeled as in-plane orthotropic materials.
Numerical Analysis of Surface Cracks at Regions of Curvature in Oxide Scales
Williamson, Richard L; Wright, Julie Knibloe; Steffler, Eric Darwin; Cannon, R. M.
2003-02-01
Finite element simulations are used to examine surface cracks at regions of local curvature (corners or convolutions) in protective oxide scales. Stresses are generated during cooling from oxide formation temperatures. Three different modeling approaches are employed, since each adds some insight to crack behavior. For the first, a series of standard static analyses with varying crack lengths is used to approximate crack motion. Next, a simple node-release technique is used, permitting dynamic crack growth along an assumed path. Finally, a model based on an arbitrary crack path is employed, wherein the crack path is included as an unknown and is part of the solution. To quantify geometric effects, three different ratios of corner radii to scale thickness are considered. Further, the influence of the substrate material is investigated by considering both perfectly-plastic and work-hardening behavior. The computed stress-intensity factor at the crack tip is compared to the fracture toughness of the scale material to predict crack growth. Simulations indicate that sharper corners and lower substrate yield strengths increase crack growth potential. Reductions in the stress-intensity factor with increasing crack length are observed that result from the constraining effects of the substrate. Predictions of crack trajectory indicate initial crack motion perpendicular to the free surface of the scale, followed by a near 90° turn, resulting in a crack path nearly parallel to the free surface.
Hard-tip, soft-spring lithography.
Shim, Wooyoung; Braunschweig, Adam B; Liao, Xing; Chai, Jinan; Lim, Jong Kuk; Zheng, Gengfeng; Mirkin, Chad A
2011-01-27
Nanofabrication strategies are becoming increasingly expensive and equipment-intensive, and consequently less accessible to researchers. As an alternative, scanning probe lithography has become a popular means of preparing nanoscale structures, in part owing to its relatively low cost and high resolution, and a registration accuracy that exceeds most existing technologies. However, increasing the throughput of cantilever-based scanning probe systems while maintaining their resolution and registration advantages has from the outset been a significant challenge. Even with impressive recent advances in cantilever array design, such arrays tend to be highly specialized for a given application, expensive, and often difficult to implement. It is therefore difficult to imagine commercially viable production methods based on scanning probe systems that rely on conventional cantilevers. Here we describe a low-cost and scalable cantilever-free tip-based nanopatterning method that uses an array of hard silicon tips mounted onto an elastomeric backing. This method-which we term hard-tip, soft-spring lithography-overcomes the throughput problems of cantilever-based scanning probe systems and the resolution limits imposed by the use of elastomeric stamps and tips: it is capable of delivering materials or energy to a surface to create arbitrary patterns of features with sub-50-nm resolution over centimetre-scale areas. We argue that hard-tip, soft-spring lithography is a versatile nanolithography strategy that should be widely adopted by academic and industrial researchers for rapid prototyping applications. PMID:21270890
Moshier, W.C.; James, L.A.
1997-04-01
Environmentally assisted cracking (EAC) in low alloy steels was found to be dependent on externally applied potential in low sulfur steels in high temperature water. EAC could be turned on when the specimen was polarized anodically above a critical potential. However, hydrogen (H) additions inhibited the ability of potential to affect EAC. The behavior was related to formation of H ions during H oxidation at the crack mouth. A mechanism based on formation of H sulfide at the crack tip and H ions at the crack mouth is presented to describe the process by which sulfides and H ions affect the critical sulfide concentration at the crack tip.
The application of Newman crack-closure model to predicting fatigue crack growth
NASA Technical Reports Server (NTRS)
Si, Erjian
1994-01-01
Newman crack-closure model and the relevant crack growth program were applied to the analysis of crack growth under constant amplitude and aircraft spectrum loading on a number of aluminum alloy materials. The analysis was performed for available test data of 2219-T851, 2024-T3, 2024-T351, 7075-T651, 2324-T39, and 7150-T651 aluminum materials. The results showed that the constraint factor is a significant factor in the method. The determination of the constraint factor is discussed. For constant amplitude loading, satisfactory crack growth lives could be predicted. For the above aluminum specimens, the ratio of predicted to experimental lives, Np/Nt, ranged from 0.74 to 1.36. The mean value of Np/Nt was 0.97. For a specified complex spectrum loading, predicted crack growth lives are not in very good agreement with the test data. Further effort is needed to correctly simulate the transition between plane strain and plane stress conditions, existing near the crack tip.
The application of Newman crack-closure model to predicting fatigue crack growth
NASA Astrophysics Data System (ADS)
Si, Erjian
1994-09-01
Newman crack-closure model and the relevant crack growth program were applied to the analysis of crack growth under constant amplitude and aircraft spectrum loading on a number of aluminum alloy materials. The analysis was performed for available test data of 2219-T851, 2024-T3, 2024-T351, 7075-T651, 2324-T39, and 7150-T651 aluminum materials. The results showed that the constraint factor is a significant factor in the method. The determination of the constraint factor is discussed. For constant amplitude loading, satisfactory crack growth lives could be predicted. For the above aluminum specimens, the ratio of predicted to experimental lives, Np/Nt, ranged from 0.74 to 1.36. The mean value of Np/Nt was 0.97. For a specified complex spectrum loading, predicted crack growth lives are not in very good agreement with the test data. Further effort is needed to correctly simulate the transition between plane strain and plane stress conditions, existing near the crack tip.
Fatigue Crack Growth and Crack Bridging in SCS-6/Ti-24-11
NASA Technical Reports Server (NTRS)
Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack
1995-01-01
Interfacial damage induced by relative fiber/matrix sliding was found to occur in the bridged zone of unidirectional SCS-6/Ti-24Al-11Nb intermetallic matrix composite specimens subjected to fatigue crack growth conditions. The degree of interfacial damage was not uniform along the bridged crack wake. Higher damage zones were observed near the machined notch in comparison to the crack tip. The interfacial friction shear strength tau(sub f) measured in the crack wake using pushout testing revealed lower values than the as-received interface. Interfacial wear also reduced the strength of the bridging fibers. The reduction in fiber strength is thought to be a function of the magnitude of relative fiber/matrix displacements ind the degree of interfacial damage. Furthermore, two different fiber bridging models were used to predict the influence of bridging on the fatigue crack driving force. The shear lag model required a variable tau(sub f) in the crack wake (reflecting the degradation of the interface) before its predictions agreed with trends exhibited by the experimental data. The fiber pressure model did an excellent job in predicting both the FCG data and the DeltaCOD in the bridged zone even though it does not require a knowledge of tau(sub f).
Atomistic aspects of crack propagation along high angle grain boundaries
Farkas, D.
1997-12-31
The author presents atomistic simulations of the crack tip configuration near a high angle {Sigma} = 5 [001](210) symmetrical tilt grain boundary in NiAl. The simulations were carried out using molecular statics and embedded atom (EAM) potentials. The cracks are stabilized near a Griffith condition involving the cohesive energy of the grain boundary. The atomistic configurations of the tip region are different in the presence of the high angle grain boundary than in the bulk. Three different configurations of the grain boundary were studied corresponding to different local compositions. It was found that in ordered NiAl, cracks along symmetrical tilt boundaries show a more brittle behavior for Al rich boundaries than for Ni-rich boundaries. Lattice trapping effects in grain boundary fracture were found to be more significant than in the bulk.
Frequency domain stress intensity calibration of damped cracked panels
NASA Technical Reports Server (NTRS)
Doyle, James F.; Rizzi, Stephen A.
1993-01-01
This paper discusses two schemes for doing finite element K calibration in the frequency domain. The baseline scheme uses the definition of K as a limit toward the crack tip. The limiting process requires using a very fine mesh around the crack tip making the scheme computationally very expensive. It is shown that the behavior of K as a function of frequency is very similar to a modal response. Taking advantage of this, a more efficient scheme involves a modal analysis of the cracked sheet and scaling the response to that of the static stress intensity. In this way, only a static K calibration need be performed. All the examples shown are for a frequency range spanning multiple resonances and with two levels of damping.
ERIC Educational Resources Information Center
Department of Energy, Washington, DC.
According to 1986 U.S. Department of Energy data, 48% of our residential energy is used to heat and cool our homes, 16% goes for heating water, 12% is used to refrigerators and freezers, and the remaining 24% goes into lighting, cooking, and running appliances. This booklet contains tips for saving energy, including sections on: (1) draft-proof…
ERIC Educational Resources Information Center
Kriesberg, Daniel
2001-01-01
Offers reminders and tips for improving interpretive walks, including having a theme, drawing on basic teaching methods, drawing on all senses rather than just talking, being a role model to show how learning can be fun, using picture books, using tools of the trade to encourage visitors to learn for themselves, and playing games. (PVD)
... Griffin Rodgers, Director of the NIDDK Clinical Trials Current research studies and how you can volunteer Community Outreach and Health Fairs Science-based information and tips for planning an outreach effort or community event For Health Care Professionals Patient and provider resources ...
... PDF, 345 KB) Alternate Language URL Español Kegel Exercise Tips Page Content What are Kegel exercises? To do Kegel exercises, you just squeeze your ... help with your bladder control. How do you exercise your pelvic muscles? Find the right muscles. Try ...
ERIC Educational Resources Information Center
O'Donnell, Nina Sazer
In response to requests for information from people and organizations all over the United States on how to contribute to the healthy development of young children, the Families and Work Institute has gathered concrete suggestions from leaders in diverse fields into this booklet of action tips. This effort was undertaken to support the "I Am Your…
ERIC Educational Resources Information Center
Dishon, Dee; And Others
1989-01-01
This section includes (1) "Time Saver Options" (Dishon and P. W. O'Leary), which outlines ways to teach cooperative skills; (2) "Getting Started--Tips from Teachers," a collection of ideas for all levels; and (3) "Cooperative Mathematics Lesson Plans" for developing problem-solving skills, fractions, word problems, and number concepts. (JD)
ERIC Educational Resources Information Center
Marten, Gerald G.; Matthews, Catherine E.
2009-01-01
Contrary to what we often hear and teach, there is good news to be found on the environmental front. Environmental success stories show us not only that sustainability is possible, but also how people have made it happen. We can make these stories and their lessons accessible to students with help from the EcoTipping Points Project, which has…
TIPs for Technology Integration.
ERIC Educational Resources Information Center
Mandell, Susan; Sorge, Dennis H.; Russell, James D.
2002-01-01
Discusses the role of the teacher in effectively using technology in education based on the Technology Integration Project (TIP). Topics include why use technology; types of computer software; how to select software; software integration strategies; and effectively planning lessons that integrate the chosen software and integration strategy. (LRW)
NASA Technical Reports Server (NTRS)
Liebers, Fritz
1932-01-01
The present report is limited to a case of tip flutter recognized by experience as being important. It is the case where outside interferences force vibrations upon the propeller. Such interferences may be set up by the engine, or they may be the result of an unsymmetrical field of flow.
ERIC Educational Resources Information Center
Hart, R. Kay; And Others
1994-01-01
Tips for English-as-a-Second-Language classes include collecting passport stamps in an oral skills class (R. Kay Hart); turning process essays into treasure hunts (Margaret Moulton); using icebreakers (Beverly Williams, David Rutledge, Brent Green); and techniques for understanding course syllabi (Ruth Overman Fischer). (LB)
Ulaganathan, Jaganathan Newman, Roger C.
2014-06-01
The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously within the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work. - Highlights: • Plastic strain localization has a complex relationship with SCC susceptibility. • Surface relief created by cold work creates its own granular strain localization. • Cold work promotes crack growth but several other factors are involved.
Atomistic simulations of brittle crack growth.
Hoyt, Jeffrey John
2007-04-01
Ceramic materials such as lead zirconium titanates (PZT), low temperature co-fired ceramics and silica glasses are used in several of Sandia's mission critical components. Brittle fracture, either during machining and processing or after many years in service, remains a serious reliability and cost issue. Despite its technological importance, brittle fracture remains poorly understand, especially the onset and propagation of sub-critical cracks. However, some insights into the onset of fracture can be gleaned from the atomic scale structure of the amorphous material. In silica for example, it is well known [1] that the Si-O-Si bonds are relatively weak and, in angle distribution functions determined from scattering experiments, the bonds exhibit a wide spread around a peak at 150. By contrast the O-Si-O bonds are strong with a narrow peak in the distribution around the 109 dictated by the SiO{sub 4} tetrahedron. In addition, slow energy release in silica, as deduced from dissolution experiments, depends on the distribution of 3-fold and higher rings in the amorphous structure. The purpose of this four month LDRD project was to investigate the atomic structure of silica in the bulk and in the vicinity of a crack tip using molecular dynamics simulations. Changes in the amorphous structure in the neighborhood of an atomically sharp tip may provide important clues as to the initiation sites and the stress intensity required to propagate a sub-critical crack.
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.
Analysis of delamination in cross-ply laminates initiating from impact induced matrix cracking
NASA Technical Reports Server (NTRS)
Salpekar, S. A.
1993-01-01
Two-dimensional finite element analyses of (02/90(8)/02) glass/epoxy and graphite/epoxy composite laminates were performed to investigate some of the characteristics of damage development due to an impact load. A cross section through the thickness of the laminate with fixed ends, and carrying a transverse load in the center, was analyzed. Inclined matrix cracks, such as those produced by a low-velocity impact, were modeled in the 90 deg ply group. The introduction of the matrix cracks caused large interlaminar tensile and shear stresses in the vicinity of both crack tips in the 0/90 and 90/0 interfaces, indicating that matrix cracking may give rise to delamination. The ratio of Mode I to total strain energy release rate, G(I)/G(total), at the beginning of delamination, calculated at the two (top and bottom) matrix crack tips was 60 and 28 percent, respectively, in the glass/epoxy laminate. The corresponding ratio was 97 and 77 percent in the graphite/epoxy laminate. Thus, a significant Mode I component of strain energy release rate may be present at the delamination initiation due to an impact load. The value of strain energy release rate at either crack tip increased due to an increase in the delamination length at the other crack tip and may give rise to an unstable delamination growth under constant load.
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.
Research of infrared laser based pavement imaging and crack detection
NASA Astrophysics Data System (ADS)
Hong, Hanyu; Wang, Shu; Zhang, Xiuhua; Jing, Genqiang
2013-08-01
Road crack detection is seriously affected by many factors in actual applications, such as some shadows, road signs, oil stains, high frequency noise and so on. Due to these factors, the current crack detection methods can not distinguish the cracks in complex scenes. In order to solve this problem, a novel method based on infrared laser pavement imaging is proposed. Firstly, single sensor laser pavement imaging system is adopted to obtain pavement images, high power laser line projector is well used to resist various shadows. Secondly, the crack extraction algorithm which has merged multiple features intelligently is proposed to extract crack information. In this step, the non-negative feature and contrast feature are used to extract the basic crack information, and circular projection based on linearity feature is applied to enhance the crack area and eliminate noise. A series of experiments have been performed to test the proposed method, which shows that the proposed automatic extraction method is effective and advanced.
14. TIP TOP MINE. TAILINGS LOCATED DIRECTLY WEST FROM TIP ...
14. TIP TOP MINE. TAILINGS LOCATED DIRECTLY WEST FROM TIP TOP HOUSE. ID-31-C-12 WOODEN STRUCTURE IS VISIBLE IN TOP LEFT. CABLES VISIBLE LEFT AND CENTER OF TAILINGS. HOUSE IS JUST OVER APEX OF TAILINGS. CAMERA POINTED EAST. - Florida Mountain Mining Sites, Tip Top Mine, West face Florida Mountain, approximately 150 feet below summit, Silver City, Owyhee County, ID
Tipping off endothelial tubes: nitric oxide drives tip cells.
Priya, Mani Krishna; Sahu, Giriraj; Soto-Pantoja, David R; Goldy, Naga; Sundaresan, Abaya Meenakshi; Jadhav, Vivek; Barathkumar, T R; Saran, Uttara; Jaffar Ali, B M; Roberts, David D; Bera, Amal Kanti; Chatterjee, Suvro
2015-04-01
Angiogenesis, the formation of new blood vessels from pre-existing vessels, is a complex process that warrants cell migration, proliferation, tip cell formation, ring formation, and finally tube formation. Angiogenesis is initiated by a single leader endothelial cell called "tip cell," followed by vessel elongation by "stalk cells." Tip cells are characterized by their long filopodial extensions and expression of vascular endothelial growth factor receptor-2 and endocan. Although nitric oxide (NO) is an important modulator of angiogenesis, its role in angiogenic sprouting and specifically in tip cell formation is poorly understood. The present study tested the role of endothelial nitric oxide synthase (eNOS)/NO/cyclic GMP (cGMP) signaling in tip cell formation. In primary endothelial cell culture, about 40% of the tip cells showed characteristic sub-cellular localization of eNOS toward the anterior progressive end of the tip cells, and eNOS became phosphorylated at serine 1177. Loss of eNOS suppressed tip cell formation. Live cell NO imaging demonstrated approximately 35% more NO in tip cells compared with stalk cells. Tip cells showed increased level of cGMP relative to stalk cells. Further, the dissection of NO downstream signaling using pharmacological inhibitors and inducers indicates that NO uses the sGC/cGMP pathway in tip cells to lead angiogenesis. Taken together, the present study confirms that eNOS/NO/cGMP signaling defines the direction of tip cell migration and thereby initiates new blood vessel formation. PMID:25510468
Hall, M.M. Jr.; Symons, D.M.
1996-05-01
A strain energy density-distance criterion was previously developed and used to correlate rising-load K{sub c} initiation data for notched and fatigue precracked specimens of hydrogen precharged Alloy X-750. This criterion, which was developed for hydrogen embrittlement (HE) cracking, is used here to correlate static-load stress corrosion cracking (SCC) initiation times obtained for smooth geometry, notched and fatigue precracked specimens. The onset of SCC crack growth is hypothesized to occur when a critical strain, which is due to environment-enhanced creep, is attained within the specimen interior. For notched and precracked specimens, initiation is shown by analysis to occur at a variable distance from notch and crack tips. The initiation site varies from very near the crack tip, for highly loaded sharp cracks, to a site that is one grain diameter from the notch, for lower loaded, blunt notches. The existence of hydrogen gradients, which are due to strain-induced hydrogen trapping in the strain fields of notch and crack tips, is argued to be controlling the site for initiation of cracking. By considering the sources of the hydrogen, these observations are shown to be consistent with those from the previous HE study, in which the characteristic distance for crack initiation was found to be one grain diameter from the notch tip, independent of notch radius, applied stress intensity factor and hydrogen level.
Environmentally assisted cracking in light water reactors.
Chopra, O. K.; Chung, H. M.; Clark, R. W.; Gruber, E. E.; Shack, W. J.; Soppet, W. K.; Strain, R. V.
2007-11-06
indicate that IASCC in 289 C water is dominated by a crack-tip grain-boundary process that involves S. An initial IASCC model has been proposed. A crack growth test was completed on mill annealed Alloy 600 in high-purity water at 289 C and 320 C under various environmental and loading conditions. The results from this test are compared with data obtained earlier on several other heats of Alloy 600.
Transjugular intrahepatic portosystemic shunt (TIPS)
... gov/ency/article/007210.htm Transjugular intrahepatic portosystemic shunt (TIPS) To use the sharing features on this page, please enable JavaScript. Transjugular intrahepatic portosystemic shunt (TIPS) is a procedure to create new connections ...
Origin of moisture effects on crack propagation in composites
NASA Technical Reports Server (NTRS)
Mandell, J. F.
1978-01-01
A study has been made of the origin of unexpected moisture effects on crack extension in fiberglass laminates. Water immersion has been found to greatly reduce the rate of crack growth under constant loading, while increasing the rate under cyclic loading, the latter effect being the expected one. Observations were made of the extension of the stable damage zone at the tip of precut notches in wet and dry environments. The damage zone size is postulated as a critical element in the relaxation of high stress concentrations in composites, such as those at notch or crack tips. Under constant load, moisture is shown to greatly expand the interply delamination region in the damage zone, thus reducing the local fiber stresses and increasing crack resistance. Under cyclic loading moisture has little effect on the delamination region, which is large even for dry environments, and the only effect is weakening of the material and acceleration of cracks. Severe hygrothermal conditions can so weaken the material that the crack resistance is reduced under constant loading as well.
The equivalence between dislocation pile-ups and cracks
NASA Technical Reports Server (NTRS)
Liu, H. W.; Gao, Q.
1990-01-01
Cracks and dislocation pile-ups are equivalent to each other. In this paper, the physical equivalence between cracks and pile-ups is delineated, and the relationshps between crack-extension force, force on the leading dislocation, stress-intensity factor, and dislocation density are reviewed and summarized. These relations make it possible to extend quantitatively the recent advances in the concepts and practices of fracture mechanics to the studies of microfractures and microplastic deformations.
Inspection of metal surface containing cracks by small antennas
NASA Astrophysics Data System (ADS)
Yagi, K.; Tamakawa, K.; Minkov, D.; Sato, Y.; Shoji, T.
2001-04-01
In order to realize advanced nondestructive inspection technique characterized by high sensitivity and high accuracy, a new technique has been developed for detection and characterization of surface cracks in metals, using small loop antenna or dipole antenna. The principle of the new technique is presented. Experiments are performed on detection and characterization of artificial surface cracks on the paramagnetic and ferromagnetic materials, and a fatigue crack is detected in a 316 stainless steel specimen.
Fatigue crack propagation in aerospace aluminum alloys
NASA Technical Reports Server (NTRS)
Gangloff, R. P.; Piascik, R. S.; Dicus, D. L.; Newman, J. C., Jr.
1990-01-01
This paper reviews fracture mechanics based, damage tolerant characterizations and predictions of fatigue crack growth in aerospace aluminum alloys. The results of laboratory experimentation and modeling are summarized in the areas of: (1) fatigue crack closure, (2) the wide range crack growth rate response of conventional aluminum alloys, (3) the fatigue behavior of advanced monolithic aluminum alloys and metal matrix composites, (4) the short crack problem, (5) environmental fatigue, and (6) variable amplitude loading. Remaining uncertainties and necessary research are identified. This work provides a foundation for the development of fatigue resistant alloys and composites, next generation life prediction codes for new structural designs and extreme environments, and to counter the problem of aging components.
Crack growth in a welded microalloyed steel under sulfide stress cracking conditions
Albarran, J.L.; Martinez, L.; Lopez, H.F.
1998-12-01
In this work, the hydrogen sulfide stress-corrosion cracking (SSC) susceptibility of a welded API X-80 pipeline was investigated. For this purpose, steel welding was carried out normal to the rolling direction using a 60{degree} single V-joint design. After welding, compact modified-wedge opening loading (M-WOL) fracture mechanics specimens were machined and loaded to an applied stress intensity factor, K{sub 1}, of 27 to 53 MPa {radical}m. This was followed by specimen exposure to H{sub 2}S saturated synthetic seawater. Each of the M-WOL specimens contained the typical microstructures developed during welding, such as the weld metal (WM), base metal (BM), and heat affected zone (HAZ). No attempt was made to establish a unique K{sub ISCC} for crack arrest because its significance was not clear. Qualitatively, the experimental outcome indicated that in mode I loading under a K{sub 1} of 40.3 MPa {radical}m only the base metal region underwent SSC. Apparently, active anodic dissolution of the crack tip started the growth process, but it was followed by a transition to hydrogen induced cracking. At an applied K{sub 1} of 55 MPa {radical}m and under similar exposure times, crack growth in the base metal was discontinuous and tended to follow the grain boundaries. Moreover, the HAZ exhibited the least SSC susceptibility as inferred from the relatively short crack propagation lengths (0.829 mm). In this case, it was found that the crack lengths of up to 4.2 mm developed. In this case, the presence of a relatively coarse dendritic structure coupled with interdendritic segregation provided a weak path for crack propagation.
Investigation of Helicopter Longeron Cracks
NASA Technical Reports Server (NTRS)
Newman, John A.; Baughman, James; Wallace, Terryl A.
2009-01-01
Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurgical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 inches (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.
Crack Growth Simulation and Residual Strength Prediction in Airplane Fuselages
NASA Technical Reports Server (NTRS)
Chen, Chuin-Shan; Wawrzynek, Paul A.; Ingraffea, Anthony R.
1999-01-01
The objectives were to create a capability to simulate curvilinear crack growth and ductile tearing in aircraft fuselages subjected to widespread fatigue damage and to validate with tests. Analysis methodology and software program (FRANC3D/STAGS) developed herein allows engineers to maintain aging aircraft economically, while insuring continuous airworthiness, and to design more damage-tolerant aircraft for the next generation. Simulations of crack growth in fuselages were described. The crack tip opening angle (CTOA) fracture criterion, obtained from laboratory tests, was used to predict fracture behavior of fuselage panel tests. Geometrically nonlinear, elastic-plastic, thin shell finite element crack growth analyses were conducted. Comparisons of stress distributions, multiple stable crack growth history, and residual strength between measured and predicted results were made to assess the validity of the methodology. Incorporation of residual plastic deformations and tear strap failure was essential for accurate residual strength predictions. Issue related to predicting crack trajectory in fuselages were also discussed. A directional criterion, including T-stress and fracture toughness orthotropy, was developed. Curvilinear crack growth was simulated in coupon and fuselage panel tests. Both T-stress and fracture toughness orthotropy were essential to predict the observed crack paths. Flapping of fuselages were predicted. Measured and predicted results agreed reasonable well.
On Modeling Hydrogen-Induced Crack Propagation Under Sustained Load
NASA Astrophysics Data System (ADS)
Dadfarnia, Mohsen; Somerday, Brian p.; Schembri, Philip E.; Sofronis, Petros; Foulk, James W.; Nibur, Kevin A.; Balch, Dorian K.
2014-08-01
The failure of hydrogen containment components is generally associated with subcritical cracking. Understanding subcritical crack growth behavior and its dependence on material and environmental variables can lead to methods for designing structural components in a hydrogen environment and will be beneficial in developing materials resistant to hydrogen embrittlement. In order to identify the issues underlying crack propagation and arrest, we present a model for hydrogen-induced stress-controlled crack propagation under sustained loading. The model is based on the assumptions that (I) hydrogen reduces the material fracture strength and (II) crack propagation takes place when the opening stress over the characteristic distance ahead of a crack tip is greater than the local fracture strength. The model is used in a finite-element simulation of crack propagation coupled with simultaneous hydrogen diffusion in a model material through nodal release. The numerical simulations show that the same physics, i.e., diffusion-controlled crack propagation, can explain the existence of both stages I and II in the velocity versus stress intensity factor ( V- K) curve.
Fatigue-Life Prediction Methodology Using Small-Crack Theory
NASA Technical Reports Server (NTRS)
Newmann, James C., Jr.; Phillips, Edward P.; Swain, M. H.
1997-01-01
This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using 'small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta K(eff)) under constant-amplitude loading. Some modifications to the delta k(eff)-rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.
Variations of a global constraint factor in cracked bodies under tension and bending loads
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Crews, J. H., Jr.; Bigelow, C. A.; Dawicke, D. S.
1994-01-01
Elastic-plastic finite-element analyses were used to calculate stresses and displacements around a crack in finite-thickness plates for an elastic-perfectly plastic material. Middle- and edge-crack specimens were analyzed under tension and bending loads. Specimens were 1.25 to 20 mm thick with various widths and crack lengths. A global constraint factor alpha(sub g), an averaged normal-stress to flow-stress ratio over the plastic region, was defined to simulate three-dimensional (3D) effects in two-dimensional (2D) models. For crack lengths and uncracked ligament lengths greater than four times the thickness, the global constraint factor was found to be nearly a unique function of a normalized stress-intensity factor (related to plastic-zone size to thickness ratio) from small- to large-scale yielding conditions for various specimen types and thickness. For crack length-to-thickness ratios less than four, the global constraint factor was specimen type, crack length and thickness dependent. Using a 2D strip-yield model and the global constraint factors, plastic-zone sizes and crack-tip displacements agreed reasonably well with the 3D analyses. For a thin sheet aluminum alloy, the critical crack-tip-opening angle during stable tearing was found to be independent of specimen type and crack length for crack length-to-thickness ratios greater than 4.
Variations of a global constraint factor in cracked bodies under tension and bending loads
NASA Astrophysics Data System (ADS)
Newman, J. C., Jr.; Crews, J. H., Jr.; Bigelow, C. A.; Dawicke, D. S.
1994-05-01
Elastic-plastic finite-element analyses were used to calculate stresses and displacements around a crack in finite-thickness plates for an elastic-perfectly plastic material. Middle- and edge-crack specimens were analyzed under tension and bending loads. Specimens were 1.25 to 20 mm thick with various widths and crack lengths. A global constraint factor alpha(sub g), an averaged normal-stress to flow-stress ratio over the plastic region, was defined to simulate three-dimensional (3D) effects in two-dimensional (2D) models. For crack lengths and uncracked ligament lengths greater than four times the thickness, the global constraint factor was found to be nearly a unique function of a normalized stress-intensity factor (related to plastic-zone size to thickness ratio) from small- to large-scale yielding conditions for various specimen types and thickness. For crack length-to-thickness ratios less than four, the global constraint factor was specimen type, crack length and thickness dependent. Using a 2D strip-yield model and the global constraint factors, plastic-zone sizes and crack-tip displacements agreed reasonably well with the 3D analyses. For a thin sheet aluminum alloy, the critical crack-tip-opening angle during stable tearing was found to be independent of specimen type and crack length for crack length-to-thickness ratios greater than 4.
Propeller tip vortex interactions
NASA Technical Reports Server (NTRS)
Johnston, Robert T.; Sullivan, John P.
1990-01-01
Propeller wakes interacting with aircraft aerodynamic surfaces are a source of noise and vibration. For this reason, flow visualization work on the motion of the helical tip vortex over a wing and through the second stage of a counterrotation propeller (CRP) has been pursued. Initially, work was done on the motion of a propeller helix as it passes over the center of a 9.0 aspect ratio wing. The propeller tip vortex experiences significant spanwise displacements when passing across a lifting wing. A stationary propeller blade or stator was installed behind the rotating propeller to model the blade vortex interaction in a CRP. The resulting vortex interaction was found to depend on the relative vortex strengths and vortex sign.
Servodidio, C A
1998-03-01
Writing an article for publication may seem intimidating to you at first, but believe it or not, it can be exciting and fun. When you initially accept a writing assignment, research a clinical "gut feeling," or describe a nursing scenario, it may seem like a gargantuan task, but when you break it into small segments or steps you may be surprised how quickly you will complete your project. This article will offer some helpful hints or tips to get you started. Many of the tips will apply specifically for submission of an article to insight; absorb all clues that might be helpful and disregard anything that won't help you. Remember, you can get your article published, and the insight editorial board and peer reviewers only want to facilitate your success! PMID:9866524
Rössle, Martin
2013-11-01
In the 25 years since the first TIPS intervention has been performed, technical standards, indications, and contraindications have been set up. The previous considerable problem of shunt failure by thrombosis or intimal proliferation in the stent or in the draining hepatic vein has been reduced considerably by the availability of polytetrafluoroethylene (PTFE)-covered stents resulting in reduced rebleeding and improved survival. Unfortunately, most clinical studies have been performed prior to the release of the covered stent and, therefore, do not represent the present state of the art. In spite of this, TIPS has gained increasing acceptance in the treatment of the various complications of portal hypertension and vascular diseases of the liver. PMID:23811307
Fatigue Crack Growth Analysis Under Spectrum Loading in Various Environmental Conditions
NASA Astrophysics Data System (ADS)
Mikheevskiy, S.; Glinka, G.; Lee, E.
2013-03-01
The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic-plastic stresses-strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress-strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith-Watson-Topper (SWT) fatigue damage parameter, D = σmaxΔɛ/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, Δ K p K {max/1- p }. The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth
Effect of Microstructure on the Fatigue Crack Propagation Behavior of TC4-DT Titanium Alloy
NASA Astrophysics Data System (ADS)
Guo, Ping; Zhao, Yongqing; Zeng, Weidong; Liu, Jianglin
2015-05-01
This paper focused on the fatigue crack growth behavior of TC4-DT titanium alloy with different microstructures. Heat treatments were performed to produce different microstructures, which varied in α lamella width and cluster size. The fatigue crack propagation route was observed for different microstructures. The deformation characteristic of the crack tip plastic zone was analyzed. The results demonstrated that, for adequate mechanical properties of the alloy, the microstructure formed after performing two treatments (first, air cooling from the β-phase field, and then annealing at 550 °C for 4 h) exhibited a better fatigue anti-crack propagation ability. This result was related to the existing higher plastic deformation field in the crack tip. Wide α lamellae and coarse α colonies were found to contribute to the improvement of the fracture toughness.
Crack nucleation criterion and its application to impact indentation in glasses.
Luo, Jian; Vargheese, K Deenamma; Tandia, Adama; Hu, Guangli; Mauro, John C
2016-01-01
Molecular dynamics (MD) simulations are used to directly observe nucleation of median cracks in oxide glasses under indentation. Indenters with sharp angles can nucleate median cracks in samples with no pre-existing flaws, while indenters with larger indenter angles cannot. Increasing the tip radius increases the critical load for nucleation of the median crack. Based upon an independent set of simulations under homogeneous loading, the fracture criterion in the domain of the principal stresses is constructed. The fracture criterion, or "fracture locus", can quantitatively explain the observed effects of indenter angle and indenter tip radius on median crack nucleation. Our simulations suggest that beyond the maximum principal stress, plasticity and multi-axial stresses should also be considered for crack nucleation under indentation, even for brittle glassy systems. PMID:27079431
Crack nucleation criterion and its application to impact indentation in glasses
Luo, Jian; Vargheese, K. Deenamma; Tandia, Adama; Hu, Guangli; Mauro, John C
2016-01-01
Molecular dynamics (MD) simulations are used to directly observe nucleation of median cracks in oxide glasses under indentation. Indenters with sharp angles can nucleate median cracks in samples with no pre-existing flaws, while indenters with larger indenter angles cannot. Increasing the tip radius increases the critical load for nucleation of the median crack. Based upon an independent set of simulations under homogeneous loading, the fracture criterion in the domain of the principal stresses is constructed. The fracture criterion, or “fracture locus”, can quantitatively explain the observed effects of indenter angle and indenter tip radius on median crack nucleation. Our simulations suggest that beyond the maximum principal stress, plasticity and multi-axial stresses should also be considered for crack nucleation under indentation, even for brittle glassy systems. PMID:27079431
T{sub {epsilon}}{sup *} integral under plane stress crack growth
Omori, Yoshika; Ma, Leong; Kobayashi, A.S.; Okada, Hiroshi; Atluri, S.N.
1997-12-01
The T{sub {epsilon}}{sup *} integral values associated with stable crack growth in A606 HSLA steel single-edge-notched (SEN) specimens were determined experimentally and numerically. The displacement fields obtained through Moire interferometry and elastic-plastic finite element analysis were used to evaluate T{sub {epsilon}}{sup *} associated with the moving crack tip. T{sub {epsilon}}{sup *} decreased with the reduction in the size of the integration contour, {Gamma}{sub {epsilon}}, and continually increased with stable crack growth in this specimen. The measured and computed crack-tip opening angle (CTOA), on the other hand, was a constant 15{degree} during stable crack growth after dropping from the measured and computed initial high values of 35 and 57{degree}, respectively.
Crack nucleation criterion and its application to impact indentation in glasses
NASA Astrophysics Data System (ADS)
Luo, Jian; Vargheese, K. Deenamma; Tandia, Adama; Hu, Guangli; Mauro, John C.
2016-04-01
Molecular dynamics (MD) simulations are used to directly observe nucleation of median cracks in oxide glasses under indentation. Indenters with sharp angles can nucleate median cracks in samples with no pre-existing flaws, while indenters with larger indenter angles cannot. Increasing the tip radius increases the critical load for nucleation of the median crack. Based upon an independent set of simulations under homogeneous loading, the fracture criterion in the domain of the principal stresses is constructed. The fracture criterion, or “fracture locus”, can quantitatively explain the observed effects of indenter angle and indenter tip radius on median crack nucleation. Our simulations suggest that beyond the maximum principal stress, plasticity and multi-axial stresses should also be considered for crack nucleation under indentation, even for brittle glassy systems.
Numerical method of crack analysis in 2D finite magnetoelectroelastic media
NASA Astrophysics Data System (ADS)
Zhao, Minghao; Xu, Guangtao; Fan, Cuiying
2010-04-01
The present paper extends the hybrid extended displacement discontinuity fundamental solution method (HEDD-FSM) (Eng Anal Bound Elem 33:592-600, 2009) to analysis of cracks in 2D finite magnetoelectroelastic media. The solution of the crack is expressed approximately by a linear combination of fundamental solutions of the governing equations, which includes the extended point force fundamental solutions with sources placed at chosen points outside the domain of the problem under consideration, and the extended Crouch fundamental solutions with extended displacement discontinuities placed on the crack. The coefficients of the fundamental solutions are determined by letting the approximated solution satisfy the prescribed boundary conditions on the boundary of the domain and on the crack face. The Crouch fundamental solution for a parabolic element at the crack tip is derived to model the square root variations of near tip fields. The extended stress intensity factors are calculated under different electric and magnetic boundary conditions.
Fatigue crack growth in 2024-T3 aluminum under tensile and transverse shear stresses
NASA Technical Reports Server (NTRS)
Viz, Mark J.; Zehnder, Alan T.
1994-01-01
The influence of transverse shear stresses on the fatigue crack growth rate in thin 2024-T3 aluminum alloy sheets is investigated experimentally. The tests are performed on double-edge cracked sheets in cyclic tensile and torsional loading. This loading generates crack tip stress intensity factors in the same ratio as the values computed for a crack lying along a lap joint in a pressurized aircraft fuselage. The relevant fracture mechanics of cracks in thin plates along with the details of the geometrically nonlinear finite element analyses used for the test specimen calibration are developed and discussed. Preliminary fatigue crack growth data correlated using the fully coupled stress intensity factor calibration are presented and compared with fatigue crack growth data from pure delta K(sub I)fatigue tests.
Intrinsic fatigue crack propagation in aluminum-lithium alloys - The effect of gaseous environments
NASA Technical Reports Server (NTRS)
Piascik, Robert S.; Gangloff, Richard P.
1989-01-01
Gaseous environmental effects on intrinsic fatigue crack growth are significant for the Al-Li-Cu alloy 2090, peak aged. For both moderate Delta K-low R and low Delta K-high R regimes, crack growth rates decrease according to the environment order: purified water vapor, moist air, helium and oxygen. Gaseous environmental effects are pronounced near threshold and are not closure dominated. Here, embrittlement by low levels of H2O (ppm) supports hydrogen embrittlement and suggests that molecular transport controlled cracking, established for high Delta K-low R, is modified near threshold. Localized crack tip reaction sites or high R crack opening shape may enable the strong, environmental effect at low levels of Delta K. Similar crack growth in He and O2 eliminates the contribution of surface films to fatigue damage in alloy 2090. While 2090 and 7075 exhibit similar environmental trends, the Al-Li-Cu alloy is more resistant to intrinsic corrosion fatigue crack growth.
Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels
Ronevich, Joseph A.; Somerday, Brian P.; San Marchi, Chris W.
2015-09-10
Banded ferrite-pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite-pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. Thus the reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impededmore » hydrogen diffusion across the banded pearlite.« less
The mode III crack problem in bonded materials with a nonhomogeneous interfacial zone
NASA Technical Reports Server (NTRS)
Erdogan, F.; Joseph, P. F.; Kaya, A. C.
1991-01-01
The mode 3 crack problem for two bonded homogeneous half planes was considered. The interfacial zone was modelled by a nonhomogeneous strip in such a way that the shear modulus is a continuous function throughout the composite medium and has discontinuous derivatives along the boundaries of the interfacial zone. The problem was formulated for cracks perpendicular to the nominal interface and was solved for various crack locations in and around the interfacial region. The asymptotic stress field near the tip of a crack terminating at an interface was examined and it was shown that, unlike the corresponding stress field in piecewise homogeneous materials, in this case the stresses have the standard square root singularity and their angular variation was identical to that of a crack in a homogeneous medium. With application to the subcritical crack growth process in mind, the results given include mostly the stress intensity factors for some typical crack geometries and various material combinations.
The mode 3 crack problem in bonded materials with a nonhomogeneous interfacial zone
NASA Technical Reports Server (NTRS)
Erdogan, Fazil; Kaya, A. C.; Joseph, P. F.
1988-01-01
The mode 3 crack problem for two bonded homogeneous half planes was considered. The interfacial zone was modelled by a nonhomogeneous strip in such a way that the shear modulus is a continuous function throughout the composite medium and has discontinuous derivatives along the boundaries of the interfacial zone. The problem was formulated for cracks perpendicular to the nominal interface and was solved for various crack locations in and around the interfacial region. The asymptotic stress field near the tip of a crack terminating at an interface was examined and it was shown that, unlike the corresponding stress field in piecewise homogeneous materials, in this case the stresses have the standard square root singularity and their angular variation was identical to that of a crack in a homogeneous medium. With application to the subcritical crack growth process in mind, the results given include mostly the stress intensity factors for some typical crack geometries and various material combinations.
A metallurgical evaluation of stress corrosion cracking in large diameter stainless steel piping
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.
Ma, Longzhou
2012-11-30
The nickel-based superalloy INCONEL 617 is a candidate material for heat exchanger applications in the next-generation nuclear plant (NGNP) system. This project will study the crack propagation process of alloy 617 at temperatures of 650°C-950°C in air under static/cyclic loading conditions. The goal is to identify the environmental and mechanical damage components and to understand in-depth the failure mechanism. Researchers will measure the fatigue crack propagation (FCP) rate (da/dn) under cyclic and hold-time fatigue conditions, and sustained crack growth rates (da/dt) at elevated temperatures. The independent FCP process will be identified and the rate-controlled sustained loading crack process will be correlated with the thermal activation equation to estimate the oxygen thermal activation energy. The FCP-dependent model indicates that if the sustained loading crack growth rate, da/dt, can be correlated with the FCP rate, da/dn, at the full time dependent stage, researchers can confirm stress-accelerated grain-boundary oxygen embrittlement (SAGBOE) as a predominate effect. Following the crack propagation tests, the research team will examine the fracture surface of materials in various cracking stages using a scanning electron microscope (SEM) and an optical microscope. In particular, the microstructure of the crack tip region will be analyzed in depth using high resolution transmission electron microscopy (TEM) and electron energy loss spectrum (EELS) mapping techniques to identify oxygen penetration along the grain boundary and to examine the diffused oxygen distribution profile around the crack tip. The cracked sample will be prepared by focused ion beam nanofabrication technology, allowing researchers to accurately fabricate the TEM samples from the crack tip while minimizing artifacts. Researchers will use these microscopic and spectroscopic results to interpret the crack propagation process, as well as distinguish and understand the environment or
NASA Astrophysics Data System (ADS)
Prasad, Kartik; Kumar, Vikas; Bhanu Sankara Rao, K.; Sundararaman, M.
2016-04-01
In this study, closure corrected in-phase (IP) and out-of-phase (OP) thermomechanical fatigue crack growth rates at two temperature intervals viz. 573 K to 723 K (300 °C to 450 °C) and 723 K to 873 K (450 °C to 600 °C) of Timetal 834 near α titanium alloy are presented. It is found that closure mechanisms significantly influence the stage I crack growth behavior. Surface roughness-induced crack closure (RICC) predominantly modifies the crack growth rate of near-threshold region at 573 K to 723 K (300 °C to 450 °C) test conditions. However, oxide-induced crack closure further strengthens RICC at 723 K to 873 K (450 °C to 600 °C) TMF loading. In stage II crack growth behavior, the alloy shows higher crack growth rates at 723 K to 873 K (450 °C to 600 °C) OP-TMF loading which is attributed to the combined effect of cyclic hardening occurring at the crack tip and weakening of interlamellar regions due to oxidation.
NASA Astrophysics Data System (ADS)
Prasad, Kartik; Kumar, Vikas; Bhanu Sankara Rao, K.; Sundararaman, M.
2016-07-01
In this study, closure corrected in-phase (IP) and out-of-phase (OP) thermomechanical fatigue crack growth rates at two temperature intervals viz. 573 K to 723 K (300 °C to 450 °C) and 723 K to 873 K (450 °C to 600 °C) of Timetal 834 near α titanium alloy are presented. It is found that closure mechanisms significantly influence the stage I crack growth behavior. Surface roughness-induced crack closure (RICC) predominantly modifies the crack growth rate of near-threshold region at 573 K to 723 K (300 °C to 450 °C) test conditions. However, oxide-induced crack closure further strengthens RICC at 723 K to 873 K (450 °C to 600 °C) TMF loading. In stage II crack growth behavior, the alloy shows higher crack growth rates at 723 K to 873 K (450 °C to 600 °C) OP-TMF loading which is attributed to the combined effect of cyclic hardening occurring at the crack tip and weakening of interlamellar regions due to oxidation.
Corrosion pitting and environmentally assisted small crack growth
Turnbull, Alan
2014-01-01
In many applications, corrosion pits act as precursors to cracking, but qualitative and quantitative prediction of damage evolution has been hampered by lack of insights into the process by which a crack develops from a pit. An overview is given of recent breakthroughs in characterization and understanding of the pit-to-crack transition using advanced three-dimensional imaging techniques such as X-ray computed tomography and focused ion beam machining with scanning electron microscopy. These techniques provided novel insights with respect to the location of crack development from a pit, supported by finite-element analysis. This inspired a new concept for the role of pitting in stress corrosion cracking based on the growing pit inducing local dynamic plastic strain, a critical factor in the development of stress corrosion cracks. Challenges in quantifying the subsequent growth rate of the emerging small cracks are then outlined with the potential drop technique being the most viable. A comparison is made with the growth rate for short cracks (through-thickness crack in fracture mechanics specimen) and long cracks and an electrochemical crack size effect invoked to rationalize the data. PMID:25197249
Corrosion pitting and environmentally assisted small crack growth.
Turnbull, Alan
2014-09-01
In many applications, corrosion pits act as precursors to cracking, but qualitative and quantitative prediction of damage evolution has been hampered by lack of insights into the process by which a crack develops from a pit. An overview is given of recent breakthroughs in characterization and understanding of the pit-to-crack transition using advanced three-dimensional imaging techniques such as X-ray computed tomography and focused ion beam machining with scanning electron microscopy. These techniques provided novel insights with respect to the location of crack development from a pit, supported by finite-element analysis. This inspired a new concept for the role of pitting in stress corrosion cracking based on the growing pit inducing local dynamic plastic strain, a critical factor in the development of stress corrosion cracks. Challenges in quantifying the subsequent growth rate of the emerging small cracks are then outlined with the potential drop technique being the most viable. A comparison is made with the growth rate for short cracks (through-thickness crack in fracture mechanics specimen) and long cracks and an electrochemical crack size effect invoked to rationalize the data. PMID:25197249
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.
CRACK MODELLING FOR RADIOGRAPHY
Chady, T.; Napierala, L.
2010-02-22
In this paper, possibility of creation of three-dimensional crack models, both random type and based on real-life radiographic images is discussed. Method for storing cracks in a number of two-dimensional matrices, as well algorithm for their reconstruction into three-dimensional objects is presented. Also the possibility of using iterative algorithm for matching simulated images of cracks to real-life radiographic images is discussed.
Crack Modelling for Radiography
NASA Astrophysics Data System (ADS)
Chady, T.; Napierała, L.
2010-02-01
In this paper, possibility of creation of three-dimensional crack models, both random type and based on real-life radiographic images is discussed. Method for storing cracks in a number of two-dimensional matrices, as well algorithm for their reconstruction into three-dimensional objects is presented. Also the possibility of using iterative algorithm for matching simulated images of cracks to real-life radiographic images is discussed.
Duisters, H.A.M. )
1994-01-01
This paper presents experimental data on the thermal cracking of butadiene in a pilot plant, under conditions representative of industrial operation. The product distribution of pure-butadiene cracking is shown. Results from cocracking experiments in naphtha and C[sub 4]-raffinate are also presented. It is shown that butadiene cracking can be an interesting outlet for the increasing butadiene overcapacity in steam crackers. Some aspects of coke formation during butadiene pyrolysis are addressed as well.
Three-Dimensional Gear Crack Propagation Studied
NASA Technical Reports Server (NTRS)
Lewicki, David G.
1999-01-01
Gears used in current helicopters and turboprops are designed for light weight, high margins of safety, and high reliability. However, unexpected gear failures may occur even with adequate tooth design. To design an extremely safe system, the designer must ask and address the question, "What happens when a failure occurs?" With gear-tooth bending fatigue, tooth or rim fractures may occur. A crack that propagates through a rim will be catastrophic, leading to disengagement of the rotor or propeller, loss of an aircraft, and possible fatalities. This failure mode should be avoided. A crack that propagates through a tooth may or may not be catastrophic, depending on the design and operating conditions. Also, early warning of this failure mode may be possible because of advances in modern diagnostic systems. One concept proposed to address bending fatigue fracture from a safety aspect is a splittooth gear design. The prime objective of this design would be to control crack propagation in a desired direction such that at least half of the tooth would remain operational should a bending failure occur. A study at the NASA Lewis Research Center analytically validated the crack-propagation failsafe characteristics of a split-tooth gear. It used a specially developed three-dimensional crack analysis program that was based on boundary element modeling and principles of linear elastic fracture mechanics. Crack shapes as well as the crack-propagation life were predicted on the basis of the calculated stress intensity factors, mixed-mode crack-propagation trajectory theories, and fatigue crack-growth theories. The preceding figures show the effect of the location of initial cracks on crack propagation. 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 was simulated in a case study to evaluate crack-propagation paths. Tooth
NASA Technical Reports Server (NTRS)
Seshadri, Banavara R.; Smith, Stephen W.; Newman, John A.
2013-01-01
Friction stir welding (FSW) fabrication technology is being adopted in aerospace applications. The use of this technology can reduce production cost, lead-times, reduce structural weight and need for fasteners and lap joints, which are typically the primary locations of crack initiation and multi-site fatigue damage in aerospace structures. FSW is a solid state welding process that is well-suited for joining aluminum alloy components; however, the process introduces residual stresses (both tensile and compressive) in joined components. The propagation of fatigue cracks in a residual stress field and the resulting redistribution of the residual stress field and its effect on crack closure have to be estimated. To insure the safe insertion of complex integral structures, an accurate understanding of the fatigue crack growth behavior and the complex crack path process must be understood. A life prediction methodology for fatigue crack growth through the weld under the influence of residual stresses in aluminum alloy structures fabricated using FSW will be detailed. The effects and significance of the magnitude of residual stress at a crack tip on the estimated crack tip driving force are highlighted. The location of the crack tip relative to the FSW and the effect of microstructure on fatigue crack growth are considered. A damage tolerant life prediction methodology accounting for microstructural variation in the weld zone and residual stress field will lead to the design of lighter and more reliable aerospace structures
A Mixed-Mode I/II Fracture Criterion and Its Application in Crack Growth Predictions
NASA Technical Reports Server (NTRS)
Sutton, Michael A.; Deng, Xiaomin; Ma, Fashang; Newman, James S., Jr.
1999-01-01
A crack tip opening displacement (CTOD)-based, mixed mode fracture criterion is developed for predicting the onset and direction of crack growth. The criterion postulates that crack growth occurs in either the Mode I or Mode II direction, depending on whether the maximum in either the opening or the shear component of CTOD, measured at a specified distance behind the crack tip, attains a critical value. For crack growth direction prediction, the proposed CTOD criterion is shown to be equivalent to seven commonly used crack growth criteria under linearly elastic and asymptotic conditions. Under elastic-plastic conditions the CTOD criterion's prediction of the dependence of the crack growth direction on the crack-up mode mixity is in excellent agreement with the Arcan test results. Furthermore, the CTOD criterion correctly predicts the existence of a crack growth transition from mode I to mode II as the mode mixity approaches the mode II loading condition. The proposed CTOD criterion has been implemented in finite element crack growth simulation codes Z1P2DL and FRANC2DL to predict the crack growth paths in (a) a modified Arcan test specimen and fixture made of AL 2024-T34 and (b) a double cantilever beam (DCB) specimen made of AL 7050. A series of crack growth simulations have been carried out for the crack growth tests in the Arcan and DCB specimens and the results further demonstrate the applicability of the mixed mode CTOD fracture criterion crack growth predictions and residual strength analyses for airframe materials.
Numerical investigation of tip clearance effects on the performance of ducted propeller
NASA Astrophysics Data System (ADS)
Yongle, Ding; Baowei, Song; Peng, Wang
2015-09-01
Tip clearance loss is a limitation of the improvement of turbomachine performance. Previous studies show the Tip clearance loss is generated by the leakage flow through the tip clearance, and is roughly linearly proportional to the gap size. This study investigates the tip clearance effects on the performance of ducted propeller. The investigation was carried out by solving the Navier-Stokes equations with the commercial Computational Fluid Dynamic (CFD) code CFX14.5. These simulations were carried out to determine the underlying mechanisms of the tip clearance effects. The calculations were performed at three different chosen advance ratios. Simulation results showed that the tip loss slope was not linearly at high advance due to the reversed pressure at the leading edge. Three type of vortical structures were observed in the tip clearance at different clearance size.
Popping balloons: formation of a crack network in rubber membranes
NASA Astrophysics Data System (ADS)
Moulinet, Sebastien; Adda-Bedia, Mokhtar; Equipe Morphogenèse et phénomènes multi-échelle Team
2015-03-01
Everyone can make the observation: a rubber balloon inflated until it spontaneously pop breaks into a large number of shreds. In contrast, a balloon pierced with a needle at an early stage of its inflation breaks into two large pieces. Using model latex balloons, we have experimentally investigated the transition between these two breaking regimes. We have showed that, above a threshold stress in the latex membrane, a single crack become unstable and separates into two new cracks. Then, a cascade of tip-splitting generates a network of cracks that eventually form a large number of fragments. We have observed that the instability of the crack occurs when it reaches a limit velocity that could the speed of sound. By studying the energy balance during the explosion, we can determine the intrinsic fracture energy of rubber, a measurement difficult to achieve with usual tensile testing.
Finite element techniques applied to cracks interacting with selected singularities
NASA Technical Reports Server (NTRS)
Conway, J. C.
1975-01-01
The finite-element method for computing the extensional stress-intensity factor for cracks approaching selected singularities of varied geometry is described. Stress-intensity factors are generated using both displacement and J-integral techniques, and numerical results are compared to those obtained experimentally in a photoelastic investigation. The selected singularities considered are a colinear crack, a circular penetration, and a notched circular penetration. Results indicate that singularities greatly influence the crack-tip stress-intensity factor as the crack approaches the singularity. In addition, the degree of influence can be regulated by varying the overall geometry of the singularity. Local changes in singularity geometry have little effect on the stress-intensity factor for the cases investigated.
The crack problem in a reinforced cylindrical shell
NASA Technical Reports Server (NTRS)
Yahsi, O. S.; Erdogan, F.
1986-01-01
In this paper a partially reinforced cylinder containing an axial through crack is considered. The reinforcement is assumed to be fully bonded to the main cylinder. The composite cylinder is thus modelled by a nonhomogeneous shell having a step change in the elastic properties at the z=0 plane, z being the axial coordinate. Using a Reissner type transverse shear theory the problem is reduced to a pair of singular integral equations. In the special case of a crack tip touching the bimaterial interface it is shown that the dominant parts of the kernels of the integral equations associated with both membrane loading and bending of the shell reduce to the generalized Cauchy kernel obtained for the corresponding plane stress case. The integral equations are solved and the stress intensity factors are given for various crack and shell dimensions. A bonded fiberglass reinforcement which may serve as a crack arrestor is used as an example.
Cracked shells under skew-symmetric loading. [Reissner theory
NASA Technical Reports Server (NTRS)
Delale, F.
1981-01-01
The general problem of a shell containing a through crack in one of the principal planes of curvature and under general skew-symmetric loading is considered. By employing a Reissner type shell theory which takes into account the effect of transverse shear strains, all boundary conditions on the crack surfaces are satisfied separately. Consequently, unlike those obtained from the classical shell theory, the angular distributions of the stress components around the crack tips are shown to be identical to the distributions obtained from the plane and anti-plane elasticity solutions. Results are given for axially and circumferentially cracked cylindrical shells, spherical shells, and toroidal shells under uniform in-plane shearing, out of plane shearing, and torsion. The problem is formulated for specially orthostropic materials, therefore, the effect of orthotropy on the results is also studied.
The crack problem in a reinforced cylindrical shell
NASA Technical Reports Server (NTRS)
Yahsi, O. S.; Erdogan, F.
1986-01-01
A partially reinforced cylinder containing an axial through crack is considered. The reinforcement is assumed to be fully bonded to the main cylinder. The composite cylinder is thus modelled by a nonhomogeneous shell having a step change in the elastic properties at the z = 0 plane, z being the axial coordinate. Using a Reissner type transverse shear theory the problem is reduced to a pair of singular integral equations. In the special case of a crack tip touching the bimaterial interface it is shown that the dominant parts of the kernels of the integral equations associated with both membrane loading and bending of the shell reduce to the generalized Cauchy kernel obtained for the corresponding plane stress case. The integral equations are solved and the stress intensity factors are given for various crack and shell dimensions. A bonded fiberglass reinforcement which may serve as a crack arrestor is used as an example.
Elastic-plastic analysis of crack in ductile adhesive joint
Ikeda, Toru; Miyazaki, Noriyuki; Yamashita, Akira; Munakata, Tsuyoshi
1995-11-01
The fracture of a crack in adhesive is important to the structural integrity of adhesive structures and composite materials. Though the fracture toughness of a material should be constant according to fracture mechanics, it is said that the fracture toughness of a crack in an adhesive joint depends on the bond thickness. In the present study, the elastic-plastic stress analyses of a crack in a thin adhesive layer are performed by the combination of the boundary element method and the finite element method. The effect of adhesive thickness on the J-integral, the Q`-factor which is a modified version of the Q-factor, and the crack tip opening displacement (CTOD) are investigated. It is found from the analyses that the CTOD begins to decrease at very thin bond thickness, the Q`-factor being almost constant. The decrease of the fracture toughness at very thin adhesive layer is expected by the present analysis.
Time-of-Flight Tip-Clearance Measurements
NASA Technical Reports Server (NTRS)
Dhadwal, H. S.; Kurkov, A. P.; Janetzke, D. C.
1999-01-01
In this paper a time-of-flight probe system incorporating the two integrated fiber optic probes which are tilted equally relative to the probe holder centerline, is applied for the first time to measure the tip clearance of an advanced fan prototype. Tip clearance is largely independent of the signal amplitude and it relies on timing measurement. This work exposes optical effects associated with the fan blade stagger angle that were absent during the original spin-rig experiment on the zero stagger rotor. Individual blade tip clearances were measured with accuracy of +/- 127-mm (+/- 0.005-in). Probe features are discussed and improvements to the design are suggested.
Estimation of crack closure stresses for in situ toughened silicon nitride with 8 wt pct scandia
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Salem, Jonathan A.; Sanders, William A.
1992-01-01
An 8-wt pct-scandia silicon nitride with an elongated grain structure was fabricated. The material exhibited high fracture toughness and a rising R-curve as measured by the indentation strength technique. The 'toughening' exponent m was found to be m about 0.1. The high fracture toughness and R-curve behavior was attributed mainly to bridging of the crack faces by the elongated grains. The crack closure (bridging) stress distribution in the wake region of the crack tip was estimated as a function of crack size from the R-curve data, with an arbitrarily assumed distribution function.
THERMAL-MECHANICAL RESPONSE OF CRACKED SATIN WEAVE CFRP COMPOSITES AT CRYOGENIC TEMPERATURES
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.
Accelerated Near-Threshold Fatigue Crack Growth Behavior of an Aluminum Powder Metallurgy Alloy
NASA Technical Reports Server (NTRS)
Piascik, Robert S.; Newman, John A.
2002-01-01
Fatigue crack growth (FCG) research conducted in the near threshold regime has identified a room temperature creep crack growth damage mechanism for a fine grain powder metallurgy (PM) aluminum alloy (8009). At very low DK, an abrupt acceleration in room temperature FCG rate occurs at high stress ratio (R = Kmin/Kmax). The near threshold accelerated FCG rates are exacerbated by increased levels of Kmax (Kmax less than 0.4 KIC). Detailed fractographic analysis correlates accelerated FCG with the formation of crack-tip process zone micro-void damage. Experimental results show that the near threshold and Kmax influenced accelerated crack growth is time and temperature dependent.
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.
Polycrystal orientation effects on microslip and mixed-mode behavior of microstructural small cracks
Bennett, V.; McDowell, D.L.
1999-07-01
There are two sources of mode mixity--on a macro level (combined loading situation), and on the micro level--that affect the propagation of small crystallographic cracks. This work explores mode mixity on the micro level by utilizing a computational model to simulate microstructural influences on driving forces for the formation and growth of small cracks. Two-dimensional computational cyclic crystal plasticity calculations are conducted to study the distribution of cyclic slip and critical plane-type fatigue parameters in a material with nominal stress-strain characteristics of 4340 steel. Cases of applied cyclic tension-compression and cyclic shear are analyzed at strain amplitudes below macroscopic yielding. Emphasis is placed on stress state and amplitude dependence of the distribution of these parameters among grains. The role of anisotropic plasticity is isolated by assuming the elastic behavior of grains to obey homogeneous, isotropic linear elasticity. All grains are of equal dimension and are assigned a random orientation distribution. It is found that the distribution of the Fatemi-Socie critical plane fatigue parameter among grains is Weibull-distributed, and it is argued that it forms an improved linkage to cyclic crack tip displacement for microstructurally small cracks. The authors also present computed crack tip opening and sliding displacements as a function of maximum applied tensile strain (from well below to just above nominal yielding) for small cracks within surface grains surrounded by a nearly random orientation distribution of grains. Multiple realizations of the local microstructure are examined for each crack length for sub-grain size cracks, with results normalized to the ratio of crack length to grain size. Key results include a very strong role of the free surface on crack tip displacement, with opening displacement being much greater than the sliding for suitably small crystallographic cracks in the surface grains. There is also a
Lewicki, D.G.
1996-05-01
Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. Gear tooth crack propagation was simulated using a finite element based computer program. Principles of linear elastic fracture mechanics were used. Quarter-point, triangular elements were used at the crack tip to represent the stress singularity. Crack tip stress intensity factors were estimated and used to determine crack propagation direction and fatigue crack growth rate. The computer program used had an automated crack propagation option in which cracks were grown numerically using an automated re-meshing scheme. In addition, experimental studies were performed in the NASA Lewis Spur Gear Fatigue Rig. Gears with various backup ratios were tested to validate crack path predictions. Also, specialized crack propagation gages were installed on the test gears to measure gear tooth crack growth rate. From both predictions and tests, gears with backup ratios (film thickness divided by tooth height) of 3.3 and 1.0 produced tooth fractures while a backup ratio of 0.3 produced rim fractures. For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial crack conditions. Good correlation between the predicted number of crack propagation cycles and measured number of cycles was achieved using both the Paris fatigue crack growth method and the Collipfiest crack growth equation when fatigue crack closure was considered.
Analysis of crack closure under plane strain conditions
NASA Technical Reports Server (NTRS)
Fleck, Norman A.; Newman, James C., Jr.
1988-01-01
The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.
Analysis of crack closure under plane strain conditions
NASA Technical Reports Server (NTRS)
Fleck, N. A.; Newman, J. C.
1986-01-01
The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.
Thermoelastic finite element analysis of subsurface cracking due to sliding surface traction
Cho, S.S.; Komvopoulos, K.
1997-01-01
A linear elastic fracture mechanics analysis of subsurface crack propagation in a half-space subjected to moving thermomechanical surface traction was performed using the finite element method. The effect of frictional heating at the sliding surface on the crack growth behavior is analyzed in terms of the coefficient of friction, crack length-to-depth ratio, and Peclet number. The crack propagation characteristics are interpreted in light of results for the directions and magnitudes of the maximum shear and tensile stress intensity factor ranges, respectively. It is shown that, while frictional heating exhibits a negligible effect on the crack propagation direction, it increases the in-plane crack growth rate and reduces the critical crack length at the onset of out-of-plane crack growth at the right tip due to the tensile mechanism (kink formation). The effect of frictional heating becomes more pronounced with increasing contact friction, crack length-to-depth ratio, and Peclet number. Crack mechanism maps showing the occurrence of opening, slip, and stick regions between the crack surfaces are presented for different values of crack length-to-depth ratio, coefficient of friction, and position of thermomechanical surface traction.
Kim, W Chan; Mauborgne, Renée
2003-04-01
When William Bratton was appointed police commissioner of New York City in 1994, turf wars over jurisdiction and funding were rife and crime was out of control. Yet in less than two years, and without an increase in his budget, Bratton turned New York into the safest large city in the nation. And the NYPD was only the latest of five law-enforcement agencies Bratton had turned around. In each case, he succeeded in record time despite limited resources, a demotivated staff, opposition from powerful vested interests, and an organization wedded to the status quo. Bratton's turnarounds demonstrate what the authors call tipping point leadership. The theory of tipping points hinges on the insight that in any organization, fundamental changes can occur quickly when the beliefs and energies of a critical mass of people create an epidemic movement toward an idea. Bratton begins by overcoming the cognitive hurdles that block organizations from recognizing the need for change. He does this by putting managers face-to-face with operational problems. Next, he manages around limitations on funds, staff, or equipment by concentrating resources on the areas that are most in need of change and that have the biggest payoffs. He meanwhile solves the motivation problem by singling out key influencers--people with disproportionate power due to their connections or persuasive abilities. Finally, he closes off resistance from powerful opponents. Not every CEO has the personality to be a Bill Bratton, but his successes are due to much more than his personality. He relies on a remarkably consistent method that any manager looking to turn around an organization can use to overcome the forces of inertia and reach the tipping point. PMID:12687920
NASA Technical Reports Server (NTRS)
Stonesifer, R. B.; Atluri, S. N.
1982-01-01
The physical meaning of (Delta T)c and its applicability to creep crack growth are reviewed. Numerical evaluation of (Delta T)c and C(asterisk) is discussed with results being given for compact specimen and strip geometries. A moving crack-tip singularity, creep crack growth simulation procedure is described and demonstrated. The results of several crack growth simulation analyses indicate that creep crack growth in 304 stainless steel occurs under essentially steady-state conditions. Based on this result, a simple methodology for predicting creep crack growth behavior is summarized.
Crack healing during molecular-beam-epitaxy growth of GaP/GaAs thin films
Li, Y.; Weatherly, G.C.; Niewczas, M.
2005-07-01
A crack-healing phenomenon occurring during epitaxial growth of GaP films on a GaAs substrate was studied by transmission electron microscopy. The process is driven by a decrease in the surface energy of the cracked film. The results indicate that the fundamental mechanism operating during healing is the deposition and diffusion of Ga and P atoms onto the crack surface in the GaP lattice, combined with self-diffusion of GaAs within the crack tip in the GaAs substrate. This process is not fully completed in the GaP/GaAs system; unhealed crack tips located in the GaAs substrate always remain in the structure. Development of cracks and subsequent crack healing during film growth lead to a decrease in residual stress in the film. New cracks are formed at an equilibrium spacing which increases with increasing film thickness. A modified expression for predicting the relation between crack spacing and film thickness in epitaxial films is proposed.
Dislocation mechanism based model for stage II fatigue crack propagation rate
NASA Technical Reports Server (NTRS)
Mazumdar, P. K.
1986-01-01
Repeated plastic deformation, which of course depends on dislocation mechanism, at or near the crack tip leads to the fatigue crack propagation. By involving the theory of thermally activated flow and the cumulative plastic strain criterion, an effort is made here to model the stage II fatigue crack propagation rate in terms of the dislocation mechanism. The model, therefore, provides capability to ascertain: (1) the dislocation mechanism (and hence the near crack tip microstructures) assisting the crack growth, (2) the relative resistance of dislocation mechanisms to the crack growth, and (3) the fracture surface characteristics and its interpretation in terms of the dislocation mechanism. The local microstructure predicted for the room temperature crack growth in copper by this model is in good agreement with the experimental results taken from the literature. With regard to the relative stability of such dislocation mechanisms as the cross-slip and the dislocation intersection, the model suggests an enhancement of crack growth rate with an ease of cross-slip which in general promotes dislocation cell formation and is common in material which has high stacking fault energy (produces wavy slips). Cross-slip apparently enhances crack growth rate by promoting slip irreversibility and fracture surface brittleness to a greater degree.
Key role of elastic vortices in the initiation of intersonic shear cracks.
Psakhie, Sergey G; Shilko, Evgeny V; Popov, Mikhail V; Popov, Valentin L
2015-06-01
Using the particle-based method of movable cellular automata, we analyze the initiation and propagation of intersonic mode II cracks along a weak interface. We show that the stress concentration in front of the crack tip, which is believed to be the mechanism of acceleration of the crack beyond the speed of shear waves, is due to the formation of an elastic vortex. The vortex develops in front of the crack during the short initial period of crack propagation. It expands and moves away from the crack tip and finally detaches from it. Maximum stress concentration in the vortex is achieved at the moment of detachment of the vortex. The crack can accelerate towards the longitudinal wave speed if the magnitude of shear stresses in the elastic vortex reaches the material shear strength before vortex detachment. We have found that for given material parameters, the condition for the unstable accelerated crack propagation depends only on the ratio of the initial crack length to its width (e.g., due to surface roughness). PMID:26172818
NASA Technical Reports Server (NTRS)
Chudnovsky, A.
1987-01-01
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.
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.
Near-Neutral pH SCC Crack Initiation Studies
NASA Astrophysics Data System (ADS)
Hiebert, John
4-Point bend studies on X-52 linepipe steel with ''natural'' surfaces (sigmamax = 95% SMYS, f = 0.001 Hz, R = 0.6) in synthetic soil solutions indicate that crack initiation and transgranular crack formation occurs more readily in C2 solutions than in C4 solutions. This increase is associated with solution compositions that increase general corrosion rates and that reduce the precipitation of carbonates. When it is assumed that relative differences in bulk solution properties are manifested at the crack tip, then these differences may promote a more favourable environment for crack tip dissolution, ion transport, and microplastic deformation. Although the results are not definitive, in these studies the development of longer and transgranular cracks appear to be associated more with differences in solution composition than with differences in surface finish. Increased corrosion and hydrogen permeation rates are associated with increased proton, carbonic acid, and bicarbonate ion concentrations and not explicitly with lower pH. Calculations show, at open circuit corrosion conditions, that FeCO3 precipitation can limit pH increases.
Nanometer voids prevent crack growth in polymer thin films
NASA Astrophysics Data System (ADS)
Yokoyama, Hideaki; Dutriez, Cedric; Satoh, Kotaro; Kamigaito, Masami
2007-03-01
Macroscopic voids initiate cracks and cause catastrophic failure in brittle materials. The effect of micrometer voids in the mechanical properties of polymeric materials was studied in 1980's and 90's with the expectation that such small voids may initiate crazing, the toughening mechanism in polymer solids, similar to dispersed rubber particles widely used in industry. However, the micrometer voids showed only limited resistance against crack growth, and it was concluded that much smaller voids are necessary for the drastic change in mechanical properties. We have recently succeeded the nondestructive introduction of nanometer voids (30--70 nm) in polymeric materials using block copolymer template and carbon dioxide (CO2) by partitioning CO2 in CO2-philic nanodomains of block copolymers. The reduction of Young's modulus with such nanometer voids was minimal (2 to 1 GPa) due to the (short-range) ordered spherical voids. While the unprocessed copolymer films failed in brittle manner at around 2 % of tensile strain, the processed copolymer films with nanometer voids did not break up to at least 60 %. A microscopic observation under strain of the crack tip revealed that the nanometer voids were deformed under strain and directly converted into the networked fibrils near the crack tip similar to crazing and thus prevented the crack growth.
The threshold stress intensity for hydrogen-induced crack growth
NASA Astrophysics Data System (ADS)
Akhurst, K. N.; Baker, T. J.
1981-06-01
The crack growth rates and threshold stress intensities, K TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth near K TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just below K TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen at K TH), a theoretical dependence of K TH on hydrogen pressure is derived which compares well with the experimental evidence.
Nonclassical Nucleation and Growth of Cohesive Tensile Cracks
NASA Astrophysics Data System (ADS)
Gran, Joseph; Rundle, John; Klein, William
2011-03-01
We analyze the nucleation and growth of cohesive tensile cracks using a Hamiltonian which is written as a functional of the crack separation (offset field). We simulate the nucleation events on a square lattice using a Metropolis Monte Carlo algorithm. Several modes of crack propagation are seen in the simulations. Our results indicate that for certain materials, crack nucleation and growth proceed through the formation and extension of a diffuse ``halo'' surrounding the classical portion of the crack. This is similar to nonclassical nucleation near the spinodal in magnetic systems. Theoretical considerations and numerical calculations strongly suggest that the diffuse halo can be identified with the fracture ``process zone'' seen in laboratory studies of advancing cracks. We are investigating scaling exponents associated with this apparent phase transition.
Fatigue crack growth behavior of Ti-1100 at elevated temperature
Maxwell, D.C.; Nicholas, T.
1995-12-31
Effects of temperature, frequency, and cycles with superimposed hold times are evaluated in Ti-1100 in order to study the complex creep-fatigue-environment interactions in this material. Crack growth rate tests conducted at cyclic loading frequency of 1.0 Hz show that raising the temperature from 593 to 650 C has only a slightly detrimental effect on crack growth rate, although these temperatures produce growth rates significantly higher than at room temperature. From constant {Delta}K tests, the effects of temperature at constant frequency show a minimum crack growth rate at 250 C. From the minimum crack growth rate at 250 C, the crack growth rate increases linearly with temperature. Increases in frequency at constant temperatures of 593 and 650 C produce a continuous decrease in growth rate in going from 0.001 to 1.0 Hz, although the behavior is primarily cycle dependent in this region. Tests at 1.0 Hz with superimposed hold times from 1 to 1,000 s are used to evaluate creep-fatigue-environment interactions. Hold times at maximum load are found to initially decrease and then increase the cyclic crack growth rate with increasing duration. This is attributed to crack-tip blunting during short hold times and environmental degradation at long hold times. Hold times at minimum load show no change in growth rates, indicating that there is no net environmental degradation to the bulk material beyond that experienced during the baseline 1 Hz cycling.
Three-dimensional crack growth assessment by microtopographic examination
Lloyd, W.R.; Piascik, R.S.
1995-12-31
The initial stage of the stable tearing process in two 2.3 mm sheet 2024-T3 aluminum alloy M(T) specimens are analyzed using fracture surface microtopography reconstruction techniques. The local crack tip opening angles (CTOA) in the interior of the specimens are determined relative to both crack extension and through-thickness position. The microtopographic analysis of cracks grown in the L-T and T-L orientations reveal that interior CTOA is comparable to those measured on the surface using standard optical analysis methods. Similar to surface CTOA results, interior (mid-thickness) CTOA exhibit a transient behavior; CTOA transitions from high angles, at near crack initiation, to a lower steady-state value of 5 deg. and 4.2 deg. for L-T and T-L, respectively, at crack lengths greater than 1.5mm. Fracture surface topographic projection maps are used to study the evolution of crack front tunneling during the initial stage of the fracture process. Stable tearing initiates at mid-thickness followed by a crack front tunneling process to a depth of approximately 2mm. A brief discussion of the basis of the fracture process reconstruction method is provided and comments on the general utility of microtopographic fracture surface examination for general assessment of elastic-plastic and fully-plastic fracture processes are made.