Preloading To Accelerate Slow-Crack-Growth Testing
NASA Technical Reports Server (NTRS)
Gyekenyesi, John P.; Choi, Sung R.; Pawlik, Ralph J.
2004-01-01
An accelerated-testing methodology has been developed for measuring the slow-crack-growth (SCG) behavior of brittle materials. Like the prior methodology, the accelerated-testing methodology involves dynamic fatigue ( constant stress-rate) testing, in which a load or a displacement is applied to a specimen at a constant rate. SCG parameters or life prediction parameters needed for designing components made of the same material as that of the specimen are calculated from the relationship between (1) the strength of the material as measured in the test and (2) the applied stress rate used in the test. Despite its simplicity and convenience, dynamic fatigue testing as practiced heretofore has one major drawback: it is extremely time-consuming, especially at low stress rates. The present accelerated methodology reduces the time needed to test a specimen at a given rate of applied load, stress, or displacement. Instead of starting the test from zero applied load or displacement as in the prior methodology, one preloads the specimen and increases the applied load at the specified rate (see Figure 1). One might expect the preload to alter the results of the test and indeed it does, but fortunately, it is possible to account for the effect of the preload in interpreting the results. The accounting is done by calculating the normalized strength (defined as the strength in the presence of preload the strength in the absence of preload) as a function of (1) the preloading factor (defined as the preload stress the strength in the absence of preload) and (2) a SCG parameter, denoted n, that is used in a power-law crack-speed formulation. Figure 2 presents numerical results from this theoretical calculation.
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.
Accelerated fatigue crack growth behavior of PWA 1480
NASA Technical Reports Server (NTRS)
Telesman, Jack; Ghosn, Louis J.
1988-01-01
An investigation of the fatigue crack growth (FCG) behavior of PWA 1480 single crystal nickel base superalloy was conducted. Typical Paris region behavior was observed above a delta K of 8 MPa sq rt of m. However, below that stress intensity range, the alloy exhibited highly unusual behavior. This behavior consisted of a region where the crack growth rate became essentially independent of the applied stress intensity. The transition in the FCG behavior was related to a change in the observed crack growth mechanisms. In the Paris region, fatigue failure occurred along (111) facets; however, at the lower stress intensities, (001) fatigue failure was observed. A mechanism was proposed, based on barriers to dislocation motion, to explain the changes in the observed FCG behavior. The FCG data were also evaluated in terms of a recently proposed stress intensity parameter, K sub rss. This parameter, based on the resolved shear stresses on the slip planes, quantified the crack driving force as well as the mode I delta K, and at the same time was also able to predict the microscopic crack path under different stress states.
Accelerated Threshold Fatigue Crack Growth Effect-Powder Metallurgy Aluminum Alloy
NASA Technical Reports Server (NTRS)
Piascik, R. S.; Newman, J. 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 (Delta) K, an abrupt acceleration in room temperature FCG rate occurs at high stress ratio (R = K(sub min)/K(sub max)). The near threshold accelerated FCG rates are exacerbated by increased levels of K(sub max) (K(sub max) = 0.4 K(sub IC)). 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 K(sub max) influenced accelerated crack growth is time and temperature dependent.
Accelerated crack growth, residual stress, and a cracked zinc coated pressure shell
NASA Technical Reports Server (NTRS)
Dittman, Daniel L.; Hampton, Roy W.; Nelson, Howard G.
1987-01-01
During a partial inspection of a 42 year old, operating, pressurized wind tunnel at NASA-Ames Research Center, a surface connected defect 114 in. long having an indicated depth of a 0.7 in. was detected. The pressure shell, constructed of a medium carbon steel, contains approximately 10 miles of welds and is cooled by flowing water over its zinc coated external surface. Metallurgical and fractographic analysis showed that the actual detect was 1.7 in. deep, and originated from an area of lack of weld penetration. Crack growth studies were performed on the shell material in the laboratory under various loading rates, hold times, and R-ratios with a simulated shell environment. The combination of zinc, water with electrolyte, and steel formed an electrolytic cell which resulted in an increase in cyclic crack growth rate by as much as 500 times over that observed in air. It was concluded that slow crack growth occurred in the pressure shell by a combination of stress corrosion cracking due to the welding residual stress and corrosion fatigue due to the cyclic operating stress.
Acceleration and localization of subcritical crack growth in a natural composite material.
Lennartz-Sassinek, S; Main, I G; Zaiser, M; Graham, C C
2014-11-01
Catastrophic failure of natural and engineered materials is often preceded by an acceleration and localization of damage that can be observed indirectly from acoustic emissions (AE) generated by the nucleation and growth of microcracks. In this paper we present a detailed investigation of the statistical properties and spatiotemporal characteristics of AE signals generated during triaxial compression of a sandstone sample. We demonstrate that the AE event amplitudes and interevent times are characterized by scaling distributions with shapes that remain invariant during most of the loading sequence. Localization of the AE activity on an incipient fault plane is associated with growth in AE rate in the form of a time-reversed Omori law with an exponent near 1. The experimental findings are interpreted using a model that assumes scale-invariant growth of the dominating crack or fault zone, consistent with the Dugdale-Barenblatt "process zone" model. We determine formal relationships between fault size, fault growth rate, and AE event rate, which are found to be consistent with the experimental observations. From these relations, we conclude that relatively slow growth of a subcritical fault may be associated with a significantly more rapid increase of the AE rate and that monitoring AE rate may therefore provide more reliable predictors of incipient failure than direct monitoring of the growing fault. PMID:25493797
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.
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.
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.
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 %.
NASA Technical Reports Server (NTRS)
Bianca, C.; Creager, M.
1976-01-01
Flexible, adaptable, integrative routine, computer program incorporates Collipriest-Ehret and Paris-Forman equations. Calculates growth from initial defect size and terminates calculation when crack is sufficiently large for critical condition. Wheeler, Willenborg, and Grumman Closure models are available.
Elevated temperature crack growth
NASA Technical Reports Server (NTRS)
Kim, K. S.; Vanstone, R. H.; Malik, S. N.; Laflen, J. H.
1988-01-01
A study was performed to examine the applicability of path-independent (P-I) integrals to crack growth problems in hot section components of gas turbine aircraft engines. Alloy 718 was used and the experimental parameters included combined temperature and strain cycling, thermal gradients, elastic-plastic strain levels, and mean strains. A literature review was conducted of proposed P-I integrals, and those capable of analyzing hot section component problems were selected and programmed into the postprocessor of a finite element code. Detailed elastic-plastic finite element analyses were conducted to simulate crack growth and crack closure of the test specimen, and to evaluate the P-I integrals. It was shown that the selected P-I integrals are very effective for predicting crack growth for isothermal conditions.
A theory for accelerated slow crack growth in medium-density polyethylene fuel-gas pipes
Chaoui, K.
1989-01-01
In the present work, a new testing procedure is developed for medium density polyethylene (MDPE) fuel gas pipes to produce brittle fracture at laboratory scale within a shorter period of time as compared to existing procedures. In the proposed procedure, the intrinsic resistance of (MDPE) pipes is studied under fatigue mode and ambient environment. Because of the lack of theoretical foundations in the existing procedures, brittle fracture in such structural components is addressed using the crack layer formalism in order to assess the controlling damage mechanisms and extract characteristic parameters representative of the phenomena occurring as a result of failure. It is found that crack propagation behavior is split into a brittle regime which extends up to half of the specimen width and a ductile regime which controls most of the second half. Damage analysis revealed that the brittle regime is led by a craze zone which becomes diffuse and larger as ductility increases. Crack and its surrounding damage are treated as a single macroscopic entity called a crack layer (CL). The portion where damage accumulates under the effects of the stress field at the crack tip is defined as the active zone. This zone is a major energy sink and thus, controls the crack propagation rate. The driving force and the instability conditions for CL propagation are commonly given in terms of the energy available and the energy required for propagation which is expressed as the product of the resistance moment R{sub 1} and the specific enthalpy of damage. In the present case, the analysis is limited to the brittle regime which is of a great interest in the long-term failure of MDPE pipes under service conditions. Also, the analysis uses the entire brittle fracture history to evaluate the resistance moment for the crazing mechanism and to extract the specific enthalpy of damage {Psi}*.
NASA Technical Reports Server (NTRS)
Telesman, Jack; Ghosn, Louis J.
1988-01-01
An investigation of the fatigue crack growth (FCG) behavior of PWA 1480 single crystal nickel base superalloy was conducted. Typical Paris region behavior was observed above a delta K of 8 MPa sq rt of m. However, below that stress intensity range, the alloy exhibited highly unusual behavior. This behavior consisted of a region where the crack growth rate became essentially independent of the applied stress intensity. The transition in the FCG behavior was related to a change in the observed crack growth mechanisms. In the Paris region, fatigue failure occurred along (111) facets; however, at the lower stress intensities, (001) fatigue failure was observed. A mechanism was proposed, based on barriers to dislocation motion, to explain the changes in the observed FCG behavior. The FCG data were also evaluated in terms of a recently proposed stress intensity parameter, K sub rss. This parameter, based on the resolved shear stresses on the slip planes, quantified the crack driving force as well as the mode I delta K, and at the same time was also able to predict the microscopic crack path under different stress states.
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.
Elevated temperature crack growth
NASA Technical Reports Server (NTRS)
Yau, J. F.; Malik, S. N.; Kim, K. S.; Vanstone, R. H.; Laflen, J. H.
1985-01-01
The objective of the Elevated Temperature Crack Growth Project is to evaluate proposed nonlinear fracture mechanics methods for application to combustor liners of aircraft gas turbine engines. During the first year of this program, proposed path-independent (P-I) integrals were reviewed for such applications. Several P-I integrals were implemented into a finite-element postprocessor which was developed and verified as part of the work. Alloy 718 was selected as the analog material for use in the forthcoming experimental work. A buttonhead, single-edge notch specimen was designed and verified for use in elevated-temperature strain control testing with significant inelastic strains. A crack mouth opening displacement measurement device was developed for further use.
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
It is essential to study time-dependent deformation and fracturing in various rock materials to prevent natural hazards related to the failure of a rock mass. In addition, information of time-dependent fracturing is essential to ensure the long-term stability of a rock mass surrounding various structures. Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. It is known that subcritical crack growth is influenced by not only stress but also surrounding environment. Studies of subcritical crack growth have been widely conducted for silicate rocks such as igneous rocks and sandstones. By contrast, information of subcritical crack growth in carbonate rocks is not enough. Specifically, influence of surrounding environment on subcritical crack growth in carbonate rock should be clarified to ensure the long-term stability of a rock mass. In this study, subcritical crack growth in marble was investigated. Especially, the influence of the temperature, relative humidity and water on subcritical crack growth in marble is investigated. As rock samples, marbles obtained in Skopje-City in Macedonia and Carrara-City in Italy were used. To measure subcritical crack growth, we used the load relaxation method of the double-torsion (DT) test. All measurements by DT test were conducted under controlled temperature and relative humidity. For both marbles, it was shown that the crack velocity in marble in air increased with increasing relative humidity at a constant temperature. Additionally, the crack velocity in water was much higher than that in air. It was also found that the crack velocity increased with increasing temperature. It is considered that temperature and water have significant influences on subcritical crack growth in marble. For Carrara marble in air, it was recognized that the value of subcritical crack growth index became low when the crack velocity was higher than 10-4 m/s. This is similar to Region II of subcritical crack growth
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.
Grain boundary oxidation and fatigue crack growth at elevated temperatures
NASA Technical Reports Server (NTRS)
Liu, H. W.; Oshida, Y.
1986-01-01
Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.
Nonlinear structural crack growth monitoring
Welch, Donald E.; Hively, Lee M.; Holdaway, Ray F.
2002-01-01
A method and apparatus are provided for the detection, through nonlinear manipulation of data, of an indicator of imminent failure due to crack growth in structural elements. The method is a process of determining energy consumption due to crack growth and correlating the energy consumption with physical phenomena indicative of a failure event. The apparatus includes sensors for sensing physical data factors, processors or the like for computing a relationship between the physical data factors and phenomena indicative of the failure event, and apparatus for providing notification of the characteristics and extent of such phenomena.
Slow Crack Growth of Germanium
NASA Technical Reports Server (NTRS)
Salem, Jon
2016-01-01
The fracture toughness and slow crack growth parameters of germanium supplied as single crystal beams and coarse grain disks were measured. Although germanium is anisotropic (A=1.7), it is not as anisotropic as SiC, NiAl, or Cu, as evidence by consistent fracture toughness on the 100, 110, and 111 planes. Germanium does not exhibit significant slow crack growth in distilled water. (n=100). Practical values for engineering design are a fracture toughness of 0.7 MPam and a Weibull modulus of m=6+/-2. For well ground and reasonable handled coupons, fracture strength should be greater than 30 MPa.
Random loading fatigue crack growth: Crack closure considerations
NASA Technical Reports Server (NTRS)
Ortiz, Keith
1987-01-01
The prediction of fatigue crack growth is an important element of effective fracture control for metallic structures and mechanical components, especially in the aerospace industry. The prediction techniques available and applied today are mostly based on fatigue crack growth measurements determined in constant amplitude testing. However, while many service loadings are constant amplitude, many more loadings are random amplitude. An investigation to determine which statistics of random loadings are relevant to fatigue crack closure was conducted. The fundamentals of random processes and crack closure are briefly reviewed, then the relevance of certain random process parameters to the crack closure calculation are discussed qualitatively. A course for further research is outlined.
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.
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.
Fatigue Crack Growth in Peened Friction Stir Welds
NASA Technical Reports Server (NTRS)
Forth, Scott C.; Hatamleh, Omar
2008-01-01
Friction stir welding induces residual stresses that accelerates fatigue crack growth in the weld nugget. Shot peening over the weld had little effect on growth rate. Laser peening over the weld retarded the growth rate: Final crack growth rate was comparable to the base, un-welded material. Crack tunneling evident from residual compressive stresses. 2195-T8 fracture surfaces were highly textured. Texturing makes comparisons difficult as the material system is affecting the data as much as the processing. Material usage becoming more common in space applications requiring additional work to develop useful datasets for damage tolerance analyses.
Crack propagation driven by crystal growth
A. Royne; Paul Meaking; A. Malthe-Sorenssen; B. Jamtveit; D. K. Dysthe
2011-10-01
Crystals that grow in confinement may exert a force on their surroundings and thereby drive crack propagation in rocks and other materials. We describe a model of crystal growth in an idealized crack geometry in which the crystal growth and crack propagation are coupled through the stress in the surrounding bulk solid. Subcritical crack propagation takes place during a transient period, which may be very long, during which the crack velocity is limited by the kinetics of crack propagation. When the crack is sufficiently large, the crack velocity becomes limited by the kinetics of crystal growth. The duration of the subcritical regime is determined by two non-dimensional parameters, which relate the kinetics of crack propagation and crystal growth to the supersaturation of the fluid and the elastic properties of the surrounding material.
On the Crack Bifurcation and Fanning of Crack Growth Data
NASA Technical Reports Server (NTRS)
Forman, Royce G.; Zanganeh, Mohammad
2015-01-01
Crack growth data obtained from ASTM load shedding method for different R values show some fanning especially for aluminum alloys. It is believed by the authors and it has been shown before that the observed fanning is due to the crack bifurcation occurs in the near threshold region which is a function of intrinsic properties of the alloy. Therefore, validity of the ASTM load shedding test procedure and results is confirmed. However, this position has been argued by some experimentalists who believe the fanning is an artifact of the test procedure and thus the obtained results are invalid. It has been shown that using a special test procedure such as using compressively pre-cracked specimens will eliminate the fanning effect. Since not using the fanned data fit can result in a significantly lower calculated cyclic life, design of a component, particularly for rotorcraft and propeller systems will considerably be impacted and therefore this study is of paramount importance. In this effort both test procedures i.e. ASTM load shedding and the proposed compressive pre-cracking have been used to study the fatigue crack growth behavior of compact tension specimens made of aluminum alloy 2524-T3. Fatigue crack growth paths have been closely observed using SEM machines to investigate the effects of compression pre-cracking on the crack bifurcation behavior. The results of this study will shed a light on resolving the existing argument by better understanding of near threshold fatigue crack growth behavior.
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.
Jumplike fatigue crack growth in compressor blades
NASA Astrophysics Data System (ADS)
Limar', L. V.; Demina, Yu. A.; Botvina, L. R.
2014-04-01
It is shown that power relations between the two main fractographic characteristics of fracture surfaces forming during jumplike fatigue crack growth, namely, the crack depth and the corresponding crack front length, can be used to estimate the fracture stress during vibration tests of the compressor blades of an aviation gas turbine engine, which are made of VT3-1 titanium alloy.
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.
Fatigue-Crack-Growth Structural Analysis
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1986-01-01
Elastic and plastic deformations calculated under variety of loading conditions. Prediction of fatigue-crack-growth lives made with FatigueCrack-Growth Structural Analysis (FASTRAN) computer program. As cyclic loads are applied to initial crack configuration, FASTRAN predicts crack length and other parameters until complete break occurs. Loads are tensile or compressive and of variable or constant amplitude. FASTRAN incorporates linear-elastic fracture mechanics with modifications of load-interaction effects caused by crack closure. FASTRAN considered research tool, because of lengthy calculation times. FASTRAN written in FORTRAN IV for batch execution.
Fracture toughness and crack growth of Zerodur
NASA Technical Reports Server (NTRS)
Viens, Michael J.
1990-01-01
The fracture toughness and crack growth parameters of Zerodur, a low expansion glass ceramic material, were determined. The fracture toughness was determined using indentation techniques and was found to be 0.9 MPa x m(sup 1/2). The crack growth parameters were determined using indented biaxial specimens subjected to static and dynamic loading in an aqueous environment. The crack growth parameters n and 1n(B) were found to be 30.7 and -6.837, respectively. The crack growth parameters were also determined using indented biaxial specimens subjected to dynamic loading in an ambient 50 percent relative humidity environment. The crack growth parameters n and 1n(B) at 50 percent relative humidity were found to be 59.3 and -17.51, respectively.
Prediction of fatigue crack-growth patterns and lives in three-dimensional cracked bodies
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Raju, I. S.
1984-01-01
Fatigue crack growth patterns and lives for surface cracks, surface cracks at holes, and corner cracks at holes in three dimensional bodies were predicted using linear-elastic fracture mechanics concepts that were modified to account for crack-closure behavior. The predictions were made by using stress intensity factor equations for these crack configurations and the fatigue crack-growth (delta K against rate) relationship for the material of interest. The crack configurations were subjected to constant-amplitude fatigue loading under either remote tension or bending loads. The predicted crack growth patterns and crack growth lives for aluminum alloys agreed well with test data from the literature.
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.
Fatigue crack growth automated testing method
Hatch, P.W.; VanDenAvyle, J.A.; Laing, J.
1989-06-01
A computer controlled servo-hydraulic mechanical test system has been configured to conduct automated fatigue crack growth testing. This provides two major benefits: it allows continuous cycling of specimens without operator attention over evenings and weekends; and complex load histories, including random loading and spectrum loading, can be applied to the specimens to simulate cyclic loading of engineering structures. The software is written in MTS Multi-User Basic to control test machine output and acquire data at predetermined intervals. Compact tension specimens are cycled according to ASTM specification E647-86. Fatigue crack growth is measured via specimen compliance during the test using a compliance/crack length calibration determined earlier by visual crack length measurements. This setup was used to measure crack growth rates in 6063 aluminum alloy for a variety of cyclic loadings, including spectrum loads. Data collected compared well with tests run manually. 13 figs.
Crack growth resistance of textured alumina
NASA Technical Reports Server (NTRS)
Salem, Jonathan A.; Shannon, John L., Jr.; Bradt, Richard C.
1989-01-01
The crack growth resistance of a textured, extruded alumina body was compared with that of anisotropic, isopressed body of similar grain size, density, and chemistry. R-curve levels reflected the preferred orientation; however, R-curve slopes were the same in all instances, implying a similar crack growth-resistive mechanism. Three orthogonal orientations of crack growth in the two structures exhibited similar forms of K(IR) versus Delta-a curves, for which a schematic diagram for polycrystalline ceramics is proposed.
Fatigue-Crack-Growth Computer Program
NASA Technical Reports Server (NTRS)
Forman, Royce G.; Shivakumar, V.; Newman, James C., Jr.
1991-01-01
Fatigue Crack Growth (NASA/FLAGRO) computer program developed as aid in predicting growth of preexisting flaws and cracks in structural components of space systems. Is enhanced version of FLAGRO4 and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. Provides fracture-mechanics analyst with computerized method of evaluating "safe-crack-growth-life" capabilities of structural components. Also used to evaluate tolerance to damage of structure of given design. Designed modular to facilitate revisions and operation on minicomputers. Written in FORTRAN 77.
Subcritical crack growth in two titanium alloys.
NASA Technical Reports Server (NTRS)
Williams, D. N.
1973-01-01
Measurement of subcritical crack growth during static loading of precracked titanium alloys in salt water using samples too thin for plane strain loading to predominate was examined as a method for determining the critical stress intensity for crack propagation in salt water. Significant internal crack growth followed by arrest was found at quite low stress intensities, but crack growth rates were relatively low. Assuming these techniques provided a reliable measurement of the critical stress intensity, the value for annealed Ti-4Al-1.5Mo-0.5V alloy was apparently about 35 ksi-in. to the 1/2 power, while that for annealed Ti-4Al-3Mo-1V was below 45 ksi-in. to the 1/2 power. Crack growth was also observed in tests conducted in both alloys in an air environment. At 65 ksi-in. to the 1/2 power, the extent of crack growth was greater in air than in salt water. Ti-4Al-3Mo-1V showed arrested crack growth in air at a stress intensity of 45 ksi-in. to the 1/2 power.
Controlled crack growth specimen for brittle systems
NASA Technical Reports Server (NTRS)
Calomino, Anthony M.; Brewer, David N.
1990-01-01
A pure Mode 1 fracture specimen and test procedure has been developed which provides extended, stable, through-thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson's expansion of a compressively loaded cylindrical pin. Stable cracks were successfully grown in soda-lime glass and monolithic Al2O3 for lengths in excess of 20 mm without uncontrollable catastrophic failure.
Controlled crack growth specimen for brittle systems
NASA Technical Reports Server (NTRS)
Calomino, Anthony M.; Brewer, David N.
1992-01-01
A pure Mode 1 fracture specimen and test procedure has been developed which provides extended, stable, through-thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson's expansion of a compressively loaded cylindrical pin. Stable cracks were successfully grown in soda-lime glass and monolithic Al2O3 for lengths in excess of 2O mm without uncontrollable catastrophic failure.
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.
Crack Growth in Single-Crystal Silicon
NASA Technical Reports Server (NTRS)
Chen, C. P.; Leipold, M. H.
1986-01-01
Report describes experiments on crack growth in single-crystal silicon at room temperature in air. Crack growth in (111) cleavage plane of wafers, 50 by 100 by 0.76 mm in dimension, cut from Czochralski singlecrystal silicon studied by double-torsion load-relaxation method and by acoustic-emission measurements. Scanning electron microscopy and X-ray topography also employed. Results aid in design and fabrication of silicon photovoltaic and microelectronic devices.
The Growth of Small Corrosion Fatigue Cracks in Alloy 7075
NASA Technical Reports Server (NTRS)
Piascik, R. S.
2001-01-01
The corrosion fatigue crack growth characteristics of small (less than 35 microns) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500X) crack length measurements in laboratory air and 1% NaCl environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.
The Growth of Small Corrosion Fatigue Cracks in Alloy 7075
NASA Technical Reports Server (NTRS)
Piascik, Robert S.
2015-01-01
The corrosion fatigue crack growth characteristics of small (greater than 35 micrometers) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.
Crack Growth Properties of Sealing Glasses
NASA Technical Reports Server (NTRS)
Salem, Jonathan A.; Tandon, R.
2008-01-01
The crack growth properties of several sealing glasses were measured using constant stress rate testing in 2% and 95% RH (relative humidity). Crack growth parameters measured in high humidity are systematically smaller (n and B) than those measured in low humidity, and velocities for dry environments are approx. 100x lower than for wet environments. The crack velocity is very sensitivity to small changes in RH at low RH. Confidence intervals on parameters that were estimated from propagation of errors were comparable to those from Monte Carlo simulation.
Slow crack growth in spinel in water
NASA Technical Reports Server (NTRS)
Schwantes, S.; Elber, W.
1983-01-01
Magnesium aluminate spinel was tested in a water environment at room temperature to establish its slow crack-growth behavior. Ring specimens with artificial flaws on the outside surface were loaded hydraulically on the inside surface. The time to failure was measured. Various precracking techniques were evaluated and multiple precracks were used to minimize the scatter in the static fatigue tests. Statistical analysis techniques were developed to determine the strength and crack velocities for a single flaw. Slow crack-growth rupture was observed at stress intensities as low as 70 percent of K sub c. A strengthening effect was observed in specimens that had survived long-time static fatigue tests.
On fatigue crack growth under random loading
NASA Astrophysics Data System (ADS)
Zhu, W. Q.; Lin, Y. K.; Lei, Y.
1992-09-01
A probabilistic analysis of the fatigue crack growth, fatigue life and reliability of a structural or mechanical component is presented on the basis of fracture mechanics and theory of random processes. The material resistance to fatigue crack growth and the time-history of the stress are assumed to be random. Analytical expressions are obtained for the special case in which the random stress is a stationary narrow-band Gaussian random process, and a randomized Paris-Erdogan law is applicable. As an example, the analytical method is applied to a plate with a central crack, and the results are compared with those obtained from digital Monte Carlo simulations.
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.
Crack growth monitoring at CFRP bond lines
NASA Astrophysics Data System (ADS)
Rahammer, M.; Adebahr, W.; Sachse, R.; Gröninger, S.; Kreutzbruck, M.
2016-02-01
With the growing need for lightweight technologies in aerospace and automotive industries, fibre-reinforced plastics, especially carbon-fibre (CFRP), are used with a continuously increasing annual growth rate. A promising joining technique for composites is adhesive bonding. While rivet holes destroy the fibres and cause stress concentration, adhesive bond lines distribute the load evenly. Today bonding is only used in secondary structures due to a lack of knowledge with regard to long-term predictability. In all industries, numerical simulation plays a critical part in the development process of new materials and structures, while it plays a vital role when it comes to CFRP adhesive bondings conducing the predictability of life time and damage tolerance. The critical issue with adhesive bondings is crack growth. In a dynamic tensile stress testing machine we dynamically load bonded CFRP coupon specimen and measure the growth rate of an artificially started crack in order to feed the models with the results. We also investigate the effect of mechanical crack stopping features. For observation of the bond line, we apply two non-contact NDT techniques: Air-coupled ultrasound in slanted transmission mode and active lockin-thermography evaluated at load frequencies. Both methods give promising results for detecting the current crack front location. While the ultrasonic technique provides a slightly higher accuracy, thermography has the advantage of true online monitoring, because the measurements are made while the cyclic load is being applied. The NDT methods are compared to visual inspection of the crack front at the specimen flanks and show high congruence. Furthermore, the effect of crack stopping features within the specimen on the crack growth is investigated. The results show, that not all crack fronts are perfectly horizontal, but all of them eventually come to a halt in the crack stopping feature vicinity.
NASA Technical Reports Server (NTRS)
Liu, H. W.
1988-01-01
Two quantitative models based on experimentally observed fatigue damage processes have been made: (1) a model of low cycle fatigue life based on fatigue crack growth under general-yielding cyclic loading; and (2) a model of accelerated fatigue crack growth at elevated temperatures based on grain boundary oxidation. These two quantitative models agree very well with the experimental observations.
Fatigue crack growth in lithium hydride
Healy, T.E.
1993-09-01
Subcritical fatigue crack growth, from cyclic tensile loading, was demonstrated in warm pressed Polycrystalline lithium hydride. Experiments were performed with cyclic tension-tension crack opening (mode I) loads applied to a pre-cracked compact type specimen in an argon environment at a temperature of 21C (70F). The fatigue crack growth was found to occur between 7.56 {times} 10{sup {minus}ll} M/cycle (2.98 {times} l0{sup {minus}9} in/cycle) and 2.35 {times} l0{sup {minus}8} m/cycle (9.24{times}10{sup {minus}7} in/cycle) for a range of stress intensity factors between 1.04 MPa{center_dot}{radical}m (0.95 ksi{center_dot}{radical}in) and 1.49 MPa{center_dot}{radical}m (1.36 ksi{center_dot}{radical}in). The rate of fatigue crack growth from cyclic tensile loading was found to be in excess of crack growth from sustained loading at an equivalent stress intensity factor. Furthermore, a fatigue threshold was not evident from the acquired data.
NASA Astrophysics Data System (ADS)
Kelestemur, Mehmet Halidun
1998-12-01
acceleration right after the overload application was observed. However, the acceleration was not detected on the interior of the material along the crack tip. Only the surface in which the plane stress was effective showed crack propagation in acceleration zone. This result were explained by strain hardening and plastic deformation of grains at the overload zone. Discontinuous crack closure occurred following the overload. In other words, at the overload position, fracture surfaces on which stretching occurred came into contact at high stress intensity factor values and far away from the overload position. As a result of discontinuous crack closure, crack opening or closing data were very high although the fatigue crack growth rate reached its original value.
On the stochastic fatigue crack growth problem
NASA Astrophysics Data System (ADS)
Enneking, Thomas Joseph
The research focuses on continuous and discrete stochastic models for fatigue crack growth which are based on Markov process theory. These models account for the random nature of fatigue crack growth which is not adequately explained by a deterministic approach. A hybrid finite element/finite difference solution methodology is developed and shown to be highly effective in determining the solution of the backward Kolmogorov equation and the Pontryagin-Vitt equation yielding the probabilistic description of the time to reach a critical crack size as a function of the initial crack size. Excellent comparisons are shown between this method, previous analytical studies, and experimental results. A significant reduction in computer processing time and storage is achieved with this approach. Alternatively, the forward Fokker-Planck-Kolmogorov equation is formulated, and a two-dimensional initial boundary value problem developed, to determine the distribution of crack sizes as a function of time. A two-dimensional finite element solution approach is used for problem solution. A major advantage of this problem formulation is that the entire probability density function is obtained as a function of cycle number. Studies of discrete Markov process models are also considered for the characterization of fatigue crack growth. A cell-to-cell mapping approach, which has been effectively utilized for other two-state problems in stochastic dynamics, is developed for the stochastic fatigue crack growth problem. In this approach the transitional probability matrix for crack transition from cell i to any other cell is determined using simulation with a two-state lognormal random process model. Repeated matrix multiplication is then used to determine the distribution of crack lengths at other times for a given initial flow size distribution. The effect of varying the initial fatigue quality may be evaluated without repeating the simulation of the probability transition matrix
FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (UNIX VERSION)
NASA Technical Reports Server (NTRS)
Newman, J. C.
1994-01-01
Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied
FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (IBM PC VERSION)
NASA Technical Reports Server (NTRS)
Newman, J. C.
1994-01-01
Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied
Crack growth in ASME SA-105 grade 2 steel in hydrogen at ambient temperature
NASA Technical Reports Server (NTRS)
Walter, R. J.
1975-01-01
Cyclic-load crack growth measurements were performed on ASME SA-105 Grade 2 steel specimens exposed to 10,000- and 15,000-psi hydrogen and to 5000-psi helium, all at ambient temperatures. The cyclic-load crack growth rate was found to be faster in high-pressure hydrogen than in helium. Cyclic-load crack growth rates in this steel were not reduced by preloading in air to a stress intensity of 1.5 times the cyclic K sub max in hydrogen. There are indications that holding under load in hydrogen, and loading and unloading in helium retards hydrogen-accelerated cyclic-load crack growth. Cyclic frequency and R (ratio of K sub min/k sub max) were important variables determining crack growth rate. The crack growth rate increased as a logarithm of the cycle duration and decreased with increasing R.
Fatigue crack growth under variable amplitude loading
NASA Astrophysics Data System (ADS)
Sidawi, Jihad A.
1994-09-01
Fatigue crack growth tests were conducted on an Fe 510 E C-Mn steel and a submerged arc welded joint from the same material under constant, variable, and random loading amplitudes. Paris-Erdogan's crack growth rate law was tested for the evaluation of m and C using the stress intensity factor K, the J-integral, the effective stress intensity factor K(sub eff), and the root mean square stress intensity factor K(sub rms) fracture mechanics concepts. The effect of retardation and residual stresses resulting from welding was also considered. It was found that all concepts gave good life predictions in all cases.
Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation
NASA Technical Reports Server (NTRS)
Liu, H. W.; Oshida, Y.
1986-01-01
Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.
Modelling and measurement of crack closure and crack growth following overloads and underloads
NASA Technical Reports Server (NTRS)
Dexter, R. J.; Hudak, S. J.; Davidson, D. L.
1989-01-01
Ignoring crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading. Crack closure is believed to contribute to the crack growth retardation, although the specific closure mechanism is dabatable. The delay period and corresponding crack growth rate transients following overload and overload/underload cycles were systematically measured as a function of load ratio and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth, i.e. the effective stress intensity factor range. Experimental results are compared with the predictions of a Dugdale-type (1960) crack closure model, and improvements in the model are suggested.
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.
Surface crack growth in fiber composites
NASA Technical Reports Server (NTRS)
Im, J.; Mandell, J. F.; Wang, S. S.; Mcgarry, F. J.
1976-01-01
The results of an experimental study of damage extension and failure in glass and graphite/epoxy laminates containing partially through-thickness surface cracks are presented. The laminates studied are divided between those containing four plies, 90/0/0/90, 15/-15/-15/15, and 45/-45/-45/45, and those containing 12-16 plies of the general configurations 0/90, + or - 45, and 0/+ or - 60. Most of the results are for surface cracks of various lengths and several depths. Stable damage extension in laminates containing surface cracks is predominantly delamination between plies, and tends to be much more extensive prior to failure than is the case with through-thickness cracks, resulting in approximately notch-insensitive behavior in most cases. A greater tendency for notch-sensitive behavior is found for 0/90 graphite/epoxy laminates for which stable damage extension is more limited. The rate of damage extension with increasing applied stress depends upon the composite system and ply configuration as well as the crack length and depth. An approximate semiempirical method is presented for estimating the growth rate of large damage-regions.
Environmental Effects on Fatigue Crack Growth in 7075 Aluminum Alloy
NASA Astrophysics Data System (ADS)
Bonakdar, A.; Wang, F.; Williams, J. J.; Chawla, N.
2012-08-01
The fatigue behavior of aluminum alloys is greatly influenced by the environmental conditions. In this article, fatigue crack growth rates were measured for 7075-T651 Al alloy under ultrahigh vacuum (UHV, ~10-10 Torr), dry air, and water vapor. Standard compact tension (CT) specimens were tested along the L-T orientation under various load ratios of 0.1, 0.5, and 0.8. Fracture surfaces and crack morphologies were studied using scanning electron microscopy and crack deflection analysis. The crack growth behavior under vacuum was affected by friction and possible rewelding of crack surfaces, causing an asymmetry in the crack growth behavior, from load shedding to constant load. The enhancement of crack growth at higher moisture levels was observed and is discussed in terms of moisture decreasing friction between the crack faces. The effect of crack deflection as a function of R ratio and environment is also presented.
A crack-closure model for predicting fatigue-crack growth under aircraft spectrum loading
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1981-01-01
The development and application of an analytical model of cycle crack growth is presented that includes the effects of crack closure. The model was used to correlate crack growth rates under constant amplitude loading and to predict crack growth under aircraft spectrum loading on 2219-T851 aluminum alloy sheet material. The predicted crack growth lives agreed well with experimental data. The ratio of predicted to experimental lives ranged from 0.66 to 1.48. These predictions were made using data from an ASTM E24.06.01 Round Robin.
Fatigue Crack Growth Analysis Models for Functionally Graded Materials
Dag, Serkan; Yildirim, Bora; Sabuncuoglu, Baris
2008-02-15
The objective of this study is to develop crack growth analysis methods for functionally graded materials (FGMs) subjected to mode I cyclic loading. The study presents finite elements based computational procedures for both two and three dimensional problems to examine fatigue crack growth in functionally graded materials. Developed methods allow the computation of crack length and generation of crack front profile for a graded medium subjected to fluctuating stresses. The results presented for an elliptical crack embedded in a functionally graded medium, illustrate the competing effects of ellipse aspect ratio and material property gradation on the fatigue crack growth behavior.
Short-crack growth behaviour in various aircraft materials
NASA Technical Reports Server (NTRS)
Edwards, P. R. (Compiler); Newman, James C., Jr. (Compiler)
1990-01-01
The results of the first phase of an AGARD Cooperative Test Program on the behavior and growth of short fatigue cracks are reviewed. The establishment of a common test method, means of data collection/analysis and crack growth modeling in an aircraft alloy AA 2024-T3 are described. The second phase allowed testing of various materials and loading conditions. The results of this second phase are described. All materials exhibited a short-crack effect to some extent. The effect was much less evident in 4340 steel than in the other materials. For the aluminum, aluminum-lithium, and titanium alloys, short cracks grew at stress-intensity factor ranges lower, in some cases much lower, than the thresholds obtained from long crack tests. Several laboratories used the same crack growth model to analyze the growth of short cracks. Reasonable agreement was found between measured and predicted short-crack growth rates and fatigue lives.
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.
Modelling short crack growth behaviour in nickel-base superalloys
NASA Astrophysics Data System (ADS)
Grabowski, L.; King, J. E.
1992-06-01
This paper provides a description of the features and mechanisms of facetted short crack growth in Ni-base superalloys and briefly reviews existing short crack growth models in terms of their application to Ni-base alloys. The concept of soft barriers is introduced to produce a new two-phase model for local microstructural effects on short crack growth in Waspaloy. This is derived from detailed observations of crack growth through individual grains. The model differs from all previous approaches in highlighting the importance of crack path perturbations within grains. Potential applications of the model in alloy development are discussed.
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.
Fatigue crack initiation and small crack growth in several airframe alloys
NASA Technical Reports Server (NTRS)
Swain, M. H.; Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A.
1990-01-01
The growth of naturally-initiated small cracks under a variety of constant amplitude and variable amplitude load sequences is examined for several airframe materials: the conventional aluminum alloys, 2024-T3 and 7075-T6, the aluminum-lithium alloy, 2090-T8E41 and 4340 steel. Loading conditions investigated include constant amplitude loading at R = 0.5, 0, -1 and -2 and the variable amplitude sequences FALSTAFF, Mini-TWIST, and FELIX/28. Crack growth was measured at the root of semicircular edge notches using acetate replicas. Crack growth rates are compared on a stress intensity factor basis, to those for large cracks to evaluate the extent of the small crack effect in each alloy. In addition, the various alloys are compared on a crack initiation and crack growth morphology basis.
Fatigue crack initiation and small crack growth in several airframe alloys
NASA Technical Reports Server (NTRS)
Swain, M. H.; Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.
1990-01-01
The growth of naturally-initiated small cracks under a variety of constant amplitude and variable amplitude load sequences is examined for several airframe materials: the conventional aluminum alloys, 2024-T3 and 7075-T6, the aluminum-lithium alloy, 2090-T8E41, and 4340 steel. Loading conditions investigated include constant amplitude loading at R = 0.5, 0, -1 and -2 and the variable amplitude sequences FALSTAFF, Mini-TWIST and FELIX/28. Crack growth was measured at the root of semicircular edge notches using acetate replicas. Crack growth rates are compared on a stress intensity factor basis, to those for large cracks to evaluate the extent of the small crack effect in each alloy. In addition, the various alloys are compared on a crack initiation and crack growth morphology basis.
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.
Thermoelastic analysis of matrix crack growth in particulate composites
Sridhar, N.; Rickman, J.M.; Srolovitz, D.J.
1995-04-01
The authors examine the conditions under which differences in thermal expansion between a particle and the matrix lead to crack growth within the matrix. Using linear elasticity fracture mechanics, they obtain closed-form, analytical results for the case of a penny shaped crack present in the matrix interacting with a spherical inclusion which is misfitting with respect to the matrix. A simple and direct relationship is established between the strain energy release rate, the crack size, the crack orientation with respect to the inclusion, the crack/inclusion separation, the degree of thermal expansion mismatch and the elastic properties of the medium. The authors also analyze the size to which these cracks can grow and find that for a given misfit strain and material properties, crack growth is inhibited beyond a certain critical crack size. They find that beyond this critical size, the elastic strain energy released upon crack growth is no longer sufficient to compensate for the energy expended in extending the crack, since the crack is growing into the rapidly decreasing stress field. The modification of the above conditions for crack growth due to the superposition of an external stress field has also been analyzed. The preferred orientation of these cracks as a function of misfit strain is predicted. The implication of these results for thermal cycling are analyzed.
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.
NASA Technical Reports Server (NTRS)
Mahishi, J. M.; Adams, D. F.
1982-01-01
An elastoplastic, axisymmetric finite element model has been used to predict the initiation and propagation of a crack in a composite model consisting of a single broken boron fiber embedded in an annular sheath of aluminum matrix. The accuracy of the axisymmetric finite element model for crack problems has been established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. Also, the stress intensity factors predicted by the present numerical model are compared with continuum results. A constant displacement boundary condition applied during an increment of crack growth permits a substantial amount of stable crack growth in the matrix material. The concept of Crack Growth Resistance Curves (KR-curves) has been used to determine the point of crack instability
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.
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.
Data base for crack growth properties of materials
NASA Technical Reports Server (NTRS)
Forman, Royce G.; Lawrence, Victor B.; Nguy, Henry L.
1988-01-01
A computerized data base of crack growth properties of materials was developed for use in fracture control analysis of rocket engine components and other NASA space hardware. The software system has files of basic crack growth rate data, other fracture mechanics material properties such as fracture toughness and environmental crack growth threshold values, and plotting and fitting routines for deriving material properties for use in fracture control analysis. An extensive amount of data was collected and entered, and work is continuing on compiling additional data. The data base and software codes are useful both for fracture control analysis and for evaluation or development of improved crack growth theories.
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.
Considerations of crack growth and plasticity in finite element analysis
NASA Technical Reports Server (NTRS)
Lee, J. D.; Liebowitz, H.
1978-01-01
A finite-element analysis was made of crack growth in a center-cracked specimen subjected to monotonically increasing load until the point of fast fracture. Since part of the specimen experienced unloading, the boundary value problem which was formulated was based upon incremental theory of plasticity. Experimental load and crack size records were utilized. Linear relations between plastic energy and crack growth were observed. Fracture toughness parameters, which were evaluated at the onset of unstable crack propagation from finite-element analysis, were in good agreement with those determined experimentally.
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 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.
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.
Updated Fatigue-Crack-Growth And Fracture-Mechanics Software
NASA Technical Reports Server (NTRS)
Forman, Royce G.; Shivakumar, Venkataraman; Newman, James C., Jr.
1995-01-01
NASA/FLAGRO 2.0 developed as analytical aid in predicting growth and stability of preexisting flaws and cracks in structural components of aerospace systems. Used for fracture-control analysis of space hardware. Organized into three modules to maximize efficiency in operation. Useful in: (1) crack-instability/crack-growth analysis, (2) processing raw crack-growth data from laboratory tests, and (3) boundary-element analysis to determine stresses and stress-intensity factors. Written in FORTRAN 77 and ANSI C.
Aircraft fatigue and crack growth considering loads by structural component
NASA Technical Reports Server (NTRS)
Yost, J. D.
1994-01-01
The indisputable 1968 C-130 fatigue/crack growth data is reviewed to obtain additional useful information on fatigue and crack growth. The proven Load Environment Model concept derived empirically from F-105D multichannel recorder data is refined to a simpler method by going from 8 to 5 variables in the spectra without a decrease in accuracy. This approach provides the true fatigue/crack growth and load environment by structural component for both fatigue and strength design. Methods are presented for defining fatigue scatter and damage at crack initiation. These design tools and criteria may be used for both metal and composite aircraft structure.
Creep crack growth behavior of several structural alloys
NASA Astrophysics Data System (ADS)
Sadananda, K.; Shahinian, P.
1983-07-01
Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.
Crack-closure and crack-growth measurements in surface-flawed titanium alloy Ti6Al-4V
NASA Technical Reports Server (NTRS)
Elber, W.
1975-01-01
The crack-closure and crack-growth characteristics of the titanium alloy Ti-6Al-4V were determined experimentally on surface-flawed plate specimens. Under cyclic loading from zero to tension, cracks deeper than 1 mm opened at approximately 50 percent of the maximum load. Cracks shallower than 1 mm opened at higher loads. The correlation between crack-growth rate and the total stress-intensity range showed a lower threshold behavior. This behavior was attributed to the high crack-opening loads at short cracks because the lower threshold was much less evident in correlations between the crack-growth rates and the effective stress-intensity range.
Stress Ratio Effects on Crack Opening Loads and Crack Growth Rates in Aluminum Alloy 2024
NASA Technical Reports Server (NTRS)
Riddell, William T.; Piascik, Robert S.
1998-01-01
The effects of stress ratio (R) and crack opening behavior on fatigue crack growth rates (da/dN) for aluminum alloy (AA) 2024-T3 were investigated using constant-delta K testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of delta K and stress ratios. Results show that constant delta K fatigue crack growth for R ranging from near 0 to 1 is divided into three regions. In Region 1, at low R, da/dN increases with increasing R. In Region 2, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region 3, at high R, further increases in da/dN are observed with increasing R.
Precision compliance techniques for slow crack growth measurements
NASA Technical Reports Server (NTRS)
Noronha, P. J.
1975-01-01
A method is presented for using simple electronic components to obtain the high sensitivity needed to measure very slow crack growth rates. The technique presented can reduce the experimental time considerably and also yield a greater amount of data more accurately than optical techniques for measuring crack growth rates.
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.
NASA Astrophysics Data System (ADS)
Khan, Z.; Ahmed, M.
1996-04-01
This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α' and γ-ɛ martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α' or ɛ transformation.
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.
Fatigue reliability of deck structures subjected to correlated crack growth
NASA Astrophysics Data System (ADS)
Feng, G. Q.; Garbatov, Y.; Guedes Soares, C.
2013-12-01
The objective of this work is to analyse fatigue reliability of deck structures subjected to correlated crack growth. The stress intensity factors of the correlated cracks are obtained by finite element analysis and based on which the geometry correction functions are derived. The Monte Carlo simulations are applied to predict the statistical descriptors of correlated cracks based on the Paris-Erdogan equation. A probabilistic model of crack growth as a function of time is used to analyse the fatigue reliability of deck structures accounting for the crack propagation correlation. A deck structure is modelled as a series system of stiffened panels, where a stiffened panel is regarded as a parallel system composed of plates and are longitudinal. It has been proven that the method developed here can be conveniently applied to perform the fatigue reliability assessment of structures subjected to correlated crack growth.
NASA Technical Reports Server (NTRS)
Kitamura, Takayuki; Ghosn, Louis J.; Ohtani, Ryuichi
1992-01-01
A simplified stochastic model is proposed for crack initiation and short-crack growth under creep and creep-fatigue conditions. Material inhomogeneity provides the random nature of crack initiation and early growth. In the model, the influence of microstructure is introduced by the variability of: (1) damage accumulation along grain boundaries, (2) critical damage required for crack initiation or growth, and (3) the grain-boundary length. The probabilities of crack initiation and growth are derived by using convolution integrals. The model is calibrated and used to predict the crack density and crack-growth rate of short cracks of 304 stainless steel under creep and creep-fatigue conditions. The mean-crack initiation lives are predicted to be within an average deviation of about 10 percent from the experimental results. The predicted comulative distributions of crack-growth rate follow the experimental data closely. The applicability of the simplified stochastic model is discussed and the future research direction is outlined.
NASA Technical Reports Server (NTRS)
Kitamura, Takayuki; Ghosn, Louis J.; Ohtani, Ryuichi
1989-01-01
A simplified stochastic model is proposed for crack initiation and short-crack growth under creep and creep-fatigue conditions. Material inhomogeneity provides the random nature of crack initiation and early growth. In the model, the influence of microstructure is introduced by the variability of: (1) damage accumulation along grain boundaries, (2) critical damage required for crack initiation or growth, and (3) the grain-boundary length. The probabilities of crack initiation and growth are derived by using convolution integrals. The model is calibrated and used to predict the crack density and crack-growth rate of short cracks of 304 stainless steel under creep and creep-fatigue conditions. The mean-crack initiation lives are predicted to be within an average deviation of about 10 percent from the experimental results. The predicted cumulative distributions of crack-growth rate follow the experimental data closely. The applicability of the simplified stochastic model is discussed and the future research direction is outlined.
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.
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
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.
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.
Elevated Temperature Crack Growth Behavior in HSCT Structural Materials
NASA Technical Reports Server (NTRS)
Saxena, Ashok
1998-01-01
Structures in super-sonic aircraft are subjected to conditions of high temperature and cyclic and sustained loading for extended periods of time. The durability of structures fabricated from aluminum and certain titanium alloys in such demanding conditions is of primary concern to the designers and manufacturers of futuristic transport aircraft. Accordingly, the major goal of this project was to evaluate the performance and durability of high temperature aluminum and titanium alloys for use in high speed civil transport (HSCT) structures. Additional goals were to develop time-dependent fracture mechanics methodology and test methods for characterizing and predicting elevated temperature crack growth behavior in creep-brittle materials such as ones being considered for use in HSCT structures and to explore accelerated methods of simulating microstructural degradation during service and measuring degraded properties in these materials.
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.
James, L.A.; Moshier, W.C.
1997-04-01
Corrosion-fatigue crack propagation experiments were conducted on several low-alloy steels in elevated temperature aqueous environments, and experimental parameters included temperature, sulfur content of the steel, applied potential level, and dissolved hydrogen (and in one case, dissolved oxygen) concentration in the water. Specimen potentials were controlled potentiostatically, and the observation (or non-observation) of accelerated fatigue crack growth rates was a complex function of the above parameters. Electrochemical results and the postulated explanation for the complex behavior are given in Part II.
Fatigue crack growth behavior in equine cortical bone
NASA Astrophysics Data System (ADS)
Shelton, Debbie Renee
2001-07-01
Objectives for this research were to experimentally determine crack growth rates, da/dN, as a function of alternating stress intensity factor, DeltaK, for specimens from lateral and dorsal regions of equine third metacarpal cortical bone tissue, and to determine if the results were described by the Paris law. In one set of experiments, specimens were oriented for crack propagation in the circumferential direction with the crack plane transverse to the long axis of the bone. In the second set of experiments, specimens were oriented for radial crack growth with the crack plane parallel to the long axis of the bone. Results of fatigue tests from the latter specimens were used to evaluate the hypothesis that crack growth rates differ regionally. The final experiments were designed to determine if crack resistance was dependent on region, proportion of hooped osteons (those with circumferentially oriented collagen fibers in the outer lamellae) or number of osteons penetrated by the crack, and to address the hypothesis that hooped osteons resist invasion by cracks better than other osteonal types. The transverse crack growth data for dorsal specimens were described by the Paris law with an exponent of 10.4 and suggested a threshold stress intensity factor, DeltaKth, of 2.0 MPa·m1/2 and fracture toughness of 4.38 MPa·m 1/2. Similar results were not obtained for lateral specimens because the crack always deviated from the intended path and ran parallel to the loading direction. Crack growth for the dorsal and lateral specimens in the radial orientation was described by the Paris law with exponents of 8.7 and 10.2, respectively, and there were no regional differences in the apparent DeltaK th (0.5 MPa·m1/2) or fracture toughness (1.2 MPa·m 1/2). Crack resistance was not associated with cortical region, proportion of hooped osteons or the number of osteons penetrated by the crack. The extent to which cracks penetrate osteons was influenced by whether the collagen fiber
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.
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.
Fatigue crack growth theory and experiment: A comparative analysis
Sananda, K.
1983-12-01
A number of theoretical models have been proposed in the literature which explain the second or the fourth power dependence of fatigue crack growth rate on ..delta..K, the stress intensity factor range in the Paris-Erdogan relation da/dN = C ..delta..K /SUP m/ . All of these models pertain to the intermediate range of crack growth rates where the m values are relatively low in the range of 2 to 4. The values of m for many metals and alloys can be much larger than 4 at near threshold crack growth rates or at stress intensities close to the fast fracture, and in some cases throughout the range of ..delta..K when the faceted mode of crack growth occurs. For such cases, the models appear to have no relevance. In this report predictions of different theoretical models are critically examined in comparison to experimentally determined crack growth rates in a MA 956, oxide dispersion strengthened alloy. Cumulative damage models predict crack growth rates reasonably well except in the range where ductile striations are observed. Lack of agreement with any particular model in this range is related to the fact that at different regions across the specimen thickness different mechanisms, either plastic blunting or cumulative damage, control the crack growth.
Modeling growth paths of interacting crack pairs in elastic media.
Ghelichi, Ramin; Kamrin, Ken
2015-10-28
The problem of predicting the growth of a system of cracks, each crack influencing the growth of the others, arises in multiple fields. We develop an analytical framework toward this aim, which we apply to the 'En-Passant' family of crack growth problems, in which a pair of initially parallel, offset cracks propagate nontrivially toward each other under far-field opening stress. We utilize boundary integral and perturbation methods of linear elasticity, linear elastic fracture mechanics, and common crack opening criteria to calculate the first analytical model for curved En-Passant crack paths. The integral system is reduced under a hierarchy of approximations, producing three methods of increasing simplicity for computing crack paths. The last such method is a major highlight of this work, using an asymptotic matching argument to predict crack paths based on superposition of simple, single-crack fields. Within the corresponding limits of the three methods, all three are shown to agree with each other. We provide comparisons to exact results and existing experimental data to verify certain approximation steps. PMID:26330342
Predicting crack growth in continuous-fiber composite materials
Cordes, J.A.; Yazici, R.
1995-12-31
Pre-notched composite lamina with unidirectional fibers were studied experimentally and using finite element analysis. Experiments were conducted on notched graphite/aluminum and glass/epoxy panels and the results were compared to a finite element method. Under remote tensile loading, cracks in the graphite/aluminum panels propagated perpendicular to the applied load without stable crack growth. In the glass/epoxy panels, crack propagation was initially stable and parallel to the fibers. A nonlinear damage zone method (DZM) was used to predict the crack growth directions, estimate damages, model stable and unstable crack growths, and predict the loads at failure. For both materials, the predicted loads at failure were within 20% of experimental loads.
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.
A study of creep crack growth in 2219-T851
NASA Astrophysics Data System (ADS)
Bensussan, Philippe L.; Jablonski, David A.; Pelloux, Regis M.
1984-01-01
Creep crack growth rates were measured in high strength 2219-T851 aluminum alloy with a computerized fully automated test procedure. Crack growth tests were performed on CT specimens with side grooves. The experimental set-up is described. During a test, the specimen is cyclically loaded on a servohydraulic testing machine under computer control, maintained at maximum load for a given hold time at each cycle, unloaded, and then reloaded. Crack lengths are obtained from compliance measurements recorded during each unloading. It is shown that the measured crack growth rates per cycle do represent creep crack growth rates per unit time for hold times longer than 10 seconds. The validity of LEFM concepts for side-grooved specimens is reviewed, and compliance and stress intensity factor calibrations for such specimens are reported. For the range of testing conditions of this study, 2219-T851 is shown to be creep brittle in terms of concepts of fracture mechanics of creeping solids. It is found that, under these testing conditions, a correlation exists between the creep crack growth rates under plane strain conditions and the stress intensity factor ( da/dt = A K 3.8 at 175 °C) for simple K histories in a regime of steady or quasi-steady state crack growth. The micromechanisms of fracture are determined to be of complex nature. The fracture mode is observed to be mixed inter- and transgranular, the relative amount of intergranular fracture decreasing as K and da/dt increase.
Dill, S.J.; Dauskardt, R.H.; Bennison, S.J.
1997-03-01
Amorphous glasses are generally considered immune to mechanical fatigue effects associated with cyclic loading. In this study surprising new evidence is presented for a mechanical fatigue effect in borosilicate glass, in both moist air and dry nitrogen environments. The fatigue effect occurs at near threshold subcritical crack-growth rates (da/dt < 3 {times} 10{sup {minus}8} m/s) as the crack extension per cycle approaches the dimensions of the borosilicate glass network. While subcritical crack growth under cyclic loads at higher load levels is entirely consistent with environmentally assisted crack growth, lower growth rates actually exceed those measured under monotonic loads. This suggests a mechanical fatigue effect which accelerates subcritical crack-growth rates. Likely mechanisms for the mechanical fatigue effect are presented.
Simulating Fatigue Crack Growth in Spiral Bevel Pinion
NASA Technical Reports Server (NTRS)
Ural, Ani; Wawrzynek, Paul A.; Ingraffe, Anthony R.
2003-01-01
This project investigates computational modeling of fatigue crack growth in spiral bevel gears. Current work is a continuation of the previous efforts made to use the Boundary Element Method (BEM) to simulate tooth-bending fatigue failure in spiral bevel gears. This report summarizes new results predicting crack trajectory and fatigue life for a spiral bevel pinion using the Finite Element Method (FEM). Predicting crack trajectories is important in determining the failure mode of a gear. Cracks propagating through the rim may result in catastrophic failure, whereas the gear may remain intact if one tooth fails and this may allow for early detection of failure. Being able to predict crack trajectories is insightful for the designer. However, predicting growth of three-dimensional arbitrary cracks is complicated due to the difficulty of creating three-dimensional models, the computing power required, and absence of closed- form solutions of the problem. Another focus of this project was performing three-dimensional contact analysis of a spiral bevel gear set incorporating cracks. These analyses were significant in determining the influence of change of tooth flexibility due to crack growth on the magnitude and location of contact loads. This is an important concern since change in contact loads might lead to differences in SIFs and therefore result in alteration of the crack trajectory. Contact analyses performed in this report showed the expected trend of decreasing tooth loads carried by the cracked tooth with increasing crack length. Decrease in tooth loads lead to differences between SIFs extracted from finite element contact analysis and finite element analysis with Hertz contact loads. This effect became more pronounced as the crack grew.
Effects of constraint on crack growth under aircraft spectrum loading
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1992-01-01
The objective is to study the effects of constraint on fatigue crack growth under aircraft spectrum loading. A plasticity-induced crack-closure model that accounts for constraint variations during the transition from flat-to-slant crack growth was used to correlate crack-growth rate data under constant-amplitude loading and to calculate crack growth under simulated aircraft spectrum loading. The model was applied to several thin-sheet aluminum alloy materials. Under laboratory air conditions, the transition was shown to be related to the size of the cyclic plastic zone based on the effective stress-intensity factor range for several sheet materials and thicknesses. Results from three-dimensional, elastic-plastic, finite-element analyses of a flat, straight-through crack in a thin-sheet aluminum alloy specimen showed a constraint loss similar to that assumed in the model. Using test data and the closure model, the location of the constraint-loss regime in terms of growth rate and the value of the constraint factor at these rates were determined by trial and error. The model was then used to calculate crack growth under the TWIST spectrum. The calculated results agreed reasonably well with test data. In general, the model predicted shorter crack-growth lives than tests under the TWIST spectrum by about 40 percent. For the TWIST spectrum clipped at Level 3, the calculated lives were within about 20 percent. The results demonstrated that constraint variations, especially for thin-sheet alloys, should be accounted for to predict crack growth under typical aircraft spectra.
Cohesive models of fatigue crack growth and stress-corrosion cracking
NASA Astrophysics Data System (ADS)
Nguyen, Olivier T.
The aim of this dissertation was to develop models of fatigue crack growth and stress-corrosion cracking by investigating cohesive theories of fracture. These models were integrated in a finite-element framework embedding a contact algorithm and techniques of remeshing and adaptive meshing.For the fatigue model, we developed a phenomenological cohesive law which exhibits unloading-reloading hysteresis. This model qualitatively predicts fatigue crack growth rates in metals under constant amplitude regime for short and long cracks, as well as growth retardation due to overload. Quantitative predictions were obtained in the case of long cracks.We developed a chemistry-dependent cohesive law which serves as a basis for the stress-corrosion cracking model. In order to determine this cohesive law, two approaches, based on energy relaxation and the renormalization group, were used for coarse-graining interplanar potentials. We analyzed the cohesive behavior of a large--but finite--number of interatomic planes and found that the macroscopic cohesive law adopts a universal asymptotic form. The resulting stress-corrosion crack growth rates agreed well with those observed experimentally in 'static' fatigue tests given in the literature.
Crack initiation and crack growth behavior of carbon and low-alloy steels
Gavenda, D.J.; Luebbers, P.R.; Chopra, O.K.
1997-01-01
Section III of the ASME Boiler and Pressure Vessel Code specifies fatigue design curves for structural materials. These curves were based on tests of smooth polished specimens at room temperature in air. The effects of reactor coolant environments are not explicitly addressed by the Code design curves, but recent test data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of carbon and low-alloy steels. Under certain loading and environmental conditions, fatigue lives of test specimens may be a factor of {approx}70 shorter than in air. Results of fatigue tests that examine the influence of reactor environment on crack imitation and crack growth of carbon and low-alloy steels are presented. Crack lengths as a function of fatigue cycles were determined in air by a surface replication technique, and in water by block loading that leaves marks on the fracture surface. Decreases in fatigue life of low-alloy steels in high-dissolved-oxygen (DO) water are primarily caused by the effects of environment during early stages of fatigue damage, i.e., growth of short cracks <100 {micro}m in depth. For crack sizes of >100 {micro}m, crack growth rates in high-DO water are higher than in air by one order of magnitude. The effects of LWR environments on growth of short cracks are discussed.
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.
Sub-critical crack growth in a sheet of paper
NASA Astrophysics Data System (ADS)
Vanel, L.; Santucci, S.; Mallick, N.; Cortet, P.-P.; Roux, S. G.; Ciliberto, S.
2007-03-01
We present experiments on the slow growth of a single crack in a fax paper sheet submitted to a constant force F. The non-averaged crack growth curves present a stepwise growth dynamics. Modelling the material as a lattice where the crack is pinned by elastic traps and grows due to thermal noise, we find that, in agreement with experiments, the distribution of step sizes follows subcritical point statistics with a power law (exponent 3/2) and a stress-dependent exponential cutoff diverging at the critical rupture threshold [1]. Taking into account the microstructure of cellulose fibers, the model is able to reproduce the shape of the statistically averaged crack growth curves, the dependence of the characteristic growth length on F as well as the effect of temperature on the rupture time. Finally, roughness of the crack interface is shown to depend on whether the crack grows in the subcritical regime, or in the rapid regime, over the critical rupture threshold. We analyze this roughness difference using a new approach based on the cumulants of the statistical distribution of the crack front height variations. [1] S. Santucci, L. Vanel and S. Ciliberto, Phys. Rev. Lett. 93, 095505 (2004).
Zhang, Z.; Shang, J.K.
1996-01-01
Fatigue crack growth along an Al/epoxy interface was examined under different combinations of mode-I and mode-II loadings using the flexural peel technique. Fatigue crack growth rates were obtained as a function of the total strain energy rate for G{sub II}/G{sub I} ratios of 0.3 to 1.4, achieved by varying the relative thickness of the outerlayers for the flexural peel specimen. Fatigue crack growth resistance of the interface was found to increase with increasing G{sub II}/G{sub I} ratio. Such a shear-enhanced crack growth resistance of the interface resulted in a gradual transition of crack growth mechanism from interfacial at the low G{sub II}/G{sub I} ratio to cohesive at the high G{sub II}/G{sub I} ratio. Under predominantly mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under highly shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance.
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.
Simulating Fatigue Crack Growth in Spiral Bevel Gears
NASA Technical Reports Server (NTRS)
Spievak, Lisa E.; Wawrzynek, Paul A.; Ingraffea, Anthony R.
2000-01-01
The majority of helicopter transmission systems utilize spiral bevel gears to convert the horizontal power from the engine into vertical power for the rotor. Due to the cyclical loading on a gear's tooth, fatigue crack propagation can occur. In rotorcraft applications, a crack's trajectory determines whether the gear failure will be benign or catastrophic for the aircraft. As a result, the capability to predict crack growth in gears is significant. A spiral bevel gear's complex shape requires a three dimensional model of the geometry and cracks. The boundary element method in conjunction with linear elastic fracture mechanics theories is used to predict arbitrarily shaped three dimensional fatigue crack trajectories in a spiral bevel pinion under moving load conditions. The predictions are validated by comparison to experimental results. The sensitivity of the predictions to variations in loading conditions and crack growth rate model parameters is explored. Critical areas that must be understood in greater detail prior to predicting more accurate crack trajectories and crack growth rates in three dimensions are identified.
Time-dependent crack growth behavior of alloy 617 and alloy 230 at elevated temperatures
NASA Astrophysics Data System (ADS)
Roy, Shawoon Kumar
2011-12-01
Two Ni-base solid-solution-strengthened superalloys: INCONEL 617 and HAYNES 230 were studied to check sustained loading crack growth (SLCG) behavior at elevated temperatures appropriate for Next Generation Nuclear Plant (NGNP) applictaions with constant stress intensity factor (Kmax= 27.75 MPa✓m) in air. The results indicate a time-dependent rate controlling process which can be characterized by a linear elastic fracture mechanics (LEFM) parameter -- stress intensity factor (K). At elevated temperatures, the crack growth mechanism was best described using a damage zone concept. Based on results and study, SAGBOE (stress accelerated grain boundary oxidation embrittlement) is considered the primary reason for time-dependent SLCG. A thermodynamic equation was considered to correlate all the SLCG results to determine the thermal activation energy in the process. A phenomenological model based on a time-dependent factor was developed considering the previous researcher's time-dependent fatigue crack propagation (FCP) results and current SLCG results to relate cycle-dependent and time-dependent FCP for both alloys. Further study includes hold time (3+300s) fatigue testing and no hold (1s) fatigue testing with various load ratios (R) at 700°C with a Kmax of 27.75 MPa✓m. Study results suggest an interesting point: crack growth behavior is significantly affected with the change in R value in cycle-dependent process whereas in time-dependent process, change in R does not have any significant effect. Fractography study showed intergranular cracking mode for all time-dependent processes and transgranular cracking mode for cycle-dependent processes. In Alloy 230, SEM images display intergranular cracking with carbide particles, dense oxides and dimple mixed secondary cracks for time-dependent 3+300s FCP and SLCG test. In all cases, Alloy 230 shows better crack growth resistance compared to Alloy 617.
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.
Stable crack growth during actuation in shape memory alloys
NASA Astrophysics Data System (ADS)
Jape, Sameer; Baxevanis, Theocharis; Lagoudas, Dimitris C.
2014-03-01
A finite element analysis of crack growth is carried out in an in nite center-cracked shape memory alloy plate subjected to thermal variations under plane strain mode I constant applied loading. Crack is assumed to propagate when the energy release rate reaches a material specific critical value. The virtual crack growth technique is employed to calculate the energy release rate, which was shown to increase an order of magnitude at constant applied loading as a result of phase transformation induced by thermal variations.1 A fracture toughening is observed associated with the energy dissipated by the transformed material in the wake of the growing crack and its sensitivity over key thermomechanical parameters is presented.
Thermal-mechanical fatigue crack growth in Inconel X-750
NASA Technical Reports Server (NTRS)
Marchand, N.; Pelloux, R. M.
1984-01-01
Thermal-mechanical fatigue crack growth (TMFCG) was studied in a gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.
Effects of a Hydrogen Gas Environment on Fatigue Crack Growth of a Stable Austenitic Stainless Steel
NASA Astrophysics Data System (ADS)
Kawamoto, Kyohei; Oda, Yasuji; Noguchi, Hiroshi; Higashida, Kenji
In order to clarify the effects of a hydrogen gas environment on the fatigue crack growth characteristics of stable austenitic stainless steels, bending fatigue tests were carried out in a hydrogen gas, in a nitrogen gas at 1.0 MPa and in air on a SUS316L using the Japanese Industrial Standards (type 316L). Also, in order to discuss the difference in the hydrogen sensitivity between austenitic stainless steels, the fatigue tests were also carried out on a SUS304 using the Japanese Industrial Standards (type 304) metastable austenitic stainless steel as a material for comparison. The main results obtained are as follows. Hydrogen gas accelerates the fatigue crack growth rate of type 316L. The degree of the fatigue crack growth acceleration is low compared to that in type 304. The fracture surfaces of both the materials practically consist of two parts; the faceted area seemed to be brittle and the remaining area occupying a greater part of the fracture surface and seemed to be ductile. The faceted area does not significantly contribute to the fatigue crack growth rate in both austenitic stainless steels. The slip-off mechanism seems to be valid not only in air and in nitrogen, but also in hydrogen. Also, the main cause of the fatigue crack growth acceleration of both materials occurs by variation of the slip behaviour. The difference in the degree of the acceleration, which in type 316L is lower than in type 304, seems to be caused by the difference in the stability of the γ phase.
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.
The growth of small cracks in 4340 steel
NASA Technical Reports Server (NTRS)
Swain, M. H.; Everett, R. A., Jr.; Newman, J. C., Jr.; Phillips, E. P.
1988-01-01
A semiempirical crack-growth model incorporating crack-closure effects has been used to predict crack growth rates and total fatigue life in notched specimens of 4340 steel having an initial defect typical of those previously noted in this steel upon subjection to both constant-amplitude and spectrum loadings. Model predictions for fatigue life are found to be in reasonable agreement with experiments and, in particular, the model was able to accurately predict total fatigue life for specimens subjected to the 'Felix/28' spectrum loading sequence on the sole basis of constant-amplitude test test results.
Anomolous Fatigue Crack Growth Phenomena in High-Strength Steel
NASA Technical Reports Server (NTRS)
Forth, Scott C.; James, Mark A.; Johnston, William M., Jr.; Newman, James C., Jr.
2004-01-01
The growth of a fatigue crack through a material is the result of a complex interaction between the applied loading, component geometry, three-dimensional constraint, load history, environment, material microstructure and several other factors. Previous studies have developed experimental and computational methods to relate the fatigue crack growth rate to many of the above conditions, with the intent of discovering some fundamental material response, i.e. crack growth rate as a function of something. Currently, the technical community uses the stress intensity factor solution as a simplistic means to relate fatigue crack growth rate to loading, geometry and all other variables. The stress intensity factor solution is a very simple linear-elastic representation of the continuum mechanics portion of crack growth. In this paper, the authors present fatigue crack growth rate data for two different high strength steel alloys generated using standard methods. The steels exhibit behaviour that appears unexplainable, compared to an aluminium alloy presented as a baseline for comparison, using the stress intensity factor solution.
High Temperature Fatigue Crack Growth Behavior of Alloy 10
NASA Technical Reports Server (NTRS)
Gayda, John
2001-01-01
Methods to improve the high temperature, dwell crack growth resistance of Alloy 10, a high strength, nickel-base disk alloy, were studied. Two approaches, heat treat variations and composition modifications, were investigated. Under the heat treat approach, solution temperature, cooling rates, and stabilization, were studied. It was found that higher solution temperatures, which promote coarser grain sizes, coupled with a 1550 F stabilization treatment were found to significantly reduce dwell crack growth rates at 1300 F Changes in the niobium and tantalum content were found to have a much smaller impact on crack growth behavior. Lowering the niobium:tantalum ratio did improve crack growth resistance and this effect was most pronounced for coarse grain microstructures. Based on these findings, a coarse grain microstructure for Alloy 10 appears to be the best option for improving dwell crack growth resistance, especially in the rim of a disk where temperatures can reach or exceed 1300 T. Further, the use of advanced processing technologies, which can produce a coarse grain rim and fine grain bore, would be the preferred option for Alloy 10 to obtain the optimal balance between tensile, creep, and crack growth requirements for small gas turbine engines.
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.
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
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.
Fatigue crack growth simulations of interfacial cracks in bi-layered FGMs using XFEM
NASA Astrophysics Data System (ADS)
Bhattacharya, S.; Singh, I. V.; Mishra, B. K.; Bui, T. Q.
2013-10-01
An investigation of fatigue crack growth of interfacial cracks in bi-layered materials using the extended finite element method is presented. The bi-material consists of two layers of dissimilar materials. The bottom layer is made of aluminium alloy while the upper one is made of functionally graded material (FGM). The FGM layer consists of 100 % aluminium alloy on the left side and 100 % ceramic (alumina) on the right side. The gradation in material property of the FGM layer is assumed to be exponential from the alloy side to the ceramic side. The domain based interaction integral approach is extended to obtain the stress intensity factors for an interfacial crack under thermo-mechanical load. The edge and centre cracks are taken at the interface of bi-layered material. The fatigue life of the interface crack plate is obtained using the Paris law of fatigue crack growth under cyclic mode-I, mixed-mode and thermal loads. This study reveals that the crack propagates into the FGM layer under all types of loads.
Fatigue crack growth in metastable austenitic stainless steels
Mei, Z.; Chang, G.; Morris, J.W. Jr.
1988-06-01
The research reported here is an investigation of the influence of the mechanically induced martensitic transformation on the fatigue crack growth rate in 304-type steels. The alloys 304L and 304LN were used to test the influence of composition, the testing temperatures 298 K and 77 K were used to study the influence of test temperature, and various load ratios (R) were used to determine the influence of the load ratio. It was found that decreasing the mechanical stability of the austenite by changing composition or lowering temperature decreases the fatigue crack growth rate. The R-ratio effect is more subtle. The fatigue crack growth rate increases with increasing R-ratio, even though this change increases the martensite transformation. Transformation-induced crack closure can explain the results in the threshold regime, but cannot explain the R-ratio effect at higher cyclic stress intensities. 26 refs., 6 figs.
Fracture and crack growth in orthotropic laminates
NASA Technical Reports Server (NTRS)
Goree, J. G.; Gross, R. S.
1978-01-01
An approximate solution is developed for the determination of the interlaminar normal and shear stresses in the vicinity of a crack in a three dimensional composite containing unidirectional linearly elastic fibers in an infinite linearly elastic matrix. In order to reduce the complexity of the formulation, certain assumptions are made as to the physically significant stresses to be retained. These simplifications reduce the partial differential equations of elasticity to differential-difference equations which are tractable using Fourier transform techniques. The potential for damaged or debonded zones to be generated by an embedded crack is discussed, and stress concentration factors for fibers near the crack are given. Detailed comparisons are made between the present solution, the analogous two dimensional problem, and corresponding shear-lag models.
NASA/FLAGRO - FATIGUE CRACK GROWTH COMPUTER PROGRAM
NASA Technical Reports Server (NTRS)
Forman, R. G.
1994-01-01
Structural flaws and cracks may grow under fatigue inducing loads and, upon reaching a critical size, cause structural failure to occur. The growth of these flaws and cracks may occur at load levels well below the ultimate load bearing capability of the structure. The Fatigue Crack Growth Computer Program, NASA/FLAGRO, was developed as an aid in predicting the growth of pre-existing flaws and cracks in structural components of space systems. The earlier version of the program, FLAGRO4, was the primary analysis tool used by Rockwell International and the Shuttle subcontractors for fracture control analysis on the Space Shuttle. NASA/FLAGRO is an enhanced version of the program and incorporates state-of-the-art improvements in both fracture mechanics and computer technology. NASA/FLAGRO provides the fracture mechanics analyst with a computerized method of evaluating the "safe crack growth life" capabilities of structural components. NASA/FLAGRO could also be used to evaluate the damage tolerance aspects of a given structural design. The propagation of an existing crack is governed by the stress field in the vicinity of the crack tip. The stress intensity factor is defined in terms of the relationship between the stress field magnitude and the crack size. The propagation of the crack becomes catastrophic when the local stress intensity factor reaches the fracture toughness of the material. NASA/FLAGRO predicts crack growth using a two-dimensional model which predicts growth independently in two directions based on the calculation of stress intensity factors. The analyst can choose to use either a crack growth rate equation or a nonlinear interpolation routine based on tabular data. The growth rate equation is a modified Forman equation which can be converted to a Paris or Walker equation by substituting different values into the exponent. This equation provides accuracy and versatility and can be fit to data using standard least squares methods. Stress
Fatigue Crack Growth Rate and Stress-Intensity Factor Corrections for Out-of-Plane Crack Growth
NASA Technical Reports Server (NTRS)
Forth, Scott C.; Herman, Dave J.; James, Mark A.
2003-01-01
Fatigue crack growth rate testing is performed by automated data collection systems that assume straight crack growth in the plane of symmetry and use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which for cracks that propagate out-of-plane, under-estimates the crack growth rates and over-estimates the stress-intensity factors. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on two-dimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys. Results indicate the correction process works well for high DeltaK levels but fails to capture the mixed-mode effects at DeltaK levels approaching threshold (da/dN approximately 10(exp -10) meter/cycle).
Estimating crack growth in temperature damaged concrete
NASA Astrophysics Data System (ADS)
Recalde, Juan Jose
2009-12-01
Evaluation of the structural condition of deteriorated concrete infrastructure and evaluation of new sustainable cementitious materials require an understanding of how the material will respond to applied loads and environmental exposures. A fundamental understanding of how microstructural changes in these materials relate to changes in mechanical properties and changes in fluid penetrability is needed. The ability to provide rapid, inexpensive assessment of material characteristics and relevant engineering properties is valuable for decision making and asset management purposes. In this investigation, the effects of changes in dynamic elastic properties with water content and fluid penetrability properties before and after a 300°C exposure were investigated based on estimates of the crack density parameter from dry and saturated cracked media. The experimental and analytical techniques described in this dissertation allow calculation of a value for the crack density parameter using nondestructive determination of wet and dry dynamic shear modulus of relatively thin disks. The techniques were used to compare a conventional concrete mixture to several mixtures with enhanced sustainability characteristics. The three enhanced sustainable materials investigated were a very high fly ash mixture, a magnesium phosphate cement based mortar, and a magnesium phosphate cement based concrete, and were compared to a conventional concrete mixture. The analysis provided both quantitative assessment of changes with high temperature damage and autogenous healing, and estimates of changes in mean crack trace lengths. The results showed that water interaction, deterioration due to damage, and autogenous healing recovery were different for the magnesium phosphate cement based mixtures than the portland cement based concrete mixtures. A strong correlation was found between log-transformed Air Permeability Index, dynamic shear modulus, and crack density parameter. The findings imply
Near-threshold fatigue crack growth in aluminum composite laminates
Hoffman, P.B.; Gibeling, J.C.
1995-03-15
One promising method for improving the mechanical properties of particulate MMCs is to laminate the brittle composite with a more ductile component. A system currently being developed at Lawrence Livermore National Laboratory (LLNL) is a multilayer laminate consisting of alternating layers of AA6090/SiC/25p and more ductile AA5182. In order to further examine the effects of lamination on fatigue crack propagation mechanics and mechanisms, the fatigue crack growth behavior of the aluminum composite laminate developed at LLNL was examined. The laminate and the AA6090/SiC/25p component were studied in the T6 heat treatment condition for subsequent comparison. Fatigue crack surfaces were examined using scanning electron microscopy for further insight into crack growth mechanisms.
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.
Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth.
Mughrabi, Haël
2015-03-28
In this survey, the origin of fatigue crack initiation and damage evolution in different metallic materials is discussed with emphasis on the responsible microstructural mechanisms. After a historical introduction, the stages of cyclic deformation which precede the onset of fatigue damage are reviewed. Different types of cyclic slip irreversibilities in the bulk that eventually lead to the initiation of fatigue cracks are discussed. Examples of trans- and intercrystalline fatigue damage evolution in the low cycle, high cycle and ultrahigh cycle fatigue regimes in mono- and polycrystalline face-centred cubic and body-centred cubic metals and alloys and in different engineering materials are presented, and some microstructural models of fatigue crack initiation and early crack growth are discussed. The basic difficulties in defining the transition from the initiation to the growth of fatigue cracks are emphasized. In ultrahigh cycle fatigue at very low loading amplitudes, the initiation of fatigue cracks generally occupies a major fraction of fatigue life and is hence life controlling. PMID:25713457
Fatigue crack growth behavior of Al-Li alloy 1441
Prakash, R.V.; Parida, B.K.
1995-12-31
Fatigue crack growth behavior of Al-Li alloy 1441 having a marginally lower lithium content, compared to 80xx and 20xx series Al-Li alloys is presented in this paper. This investigation was conducted on single edge tension--SE(T)--specimens, under constant amplitude as well as under MiniLCA flight spectrum loading with the specific objective of determining the effects of stress ratio, orientation, thickness and cladding. Three thicknesses were considered: 1.2 mm(clad and unclad), 2.0 mm(clad and unclad) and 8.0 mm unclad. Constant amplitude fatigue tests were conducted at stress ratios of {minus}0.3, 0.1 and 0.7. Testing was performed under ambient conditions and along three orientations, namely L-T, T-L and L+45 degrees. Crack growth characteristics of this alloy are compared with that of BS:L73 (2014-T4 equivalent) for assessing the possibility of replacing BS:L73. Significant effect of stress ratio on crack growth rate was observed in all thicknesses. However, in case of 1.2 and 2.0 mm thick sheets, the effect was minimal at intermediate-crack growth regime. The orientation of the specimen does not adversely affect the fatigue crack growth behavior of 8.0 mm and 2.0 mm thick specimens. However, for 1.2 mm unclad sheet crack growth resistance in L-T direction was found to be superior to that along T-L direction. In majority of test cases considered, no significant effect was observed on crack growth rate due to thickness or cladding. Crack growth characteristics of Al-Li alloy 1441 and Al-Cu alloy BS:L73 under constant amplitude as well as MiniLCA spectrum loading are similar in the low and intermediate-crack growth rate regime. Based on these observations, it is felt that this Al-Li alloy has the potential for future aerospace applications.
Mode 2 fatigue crack growth specimen development
NASA Technical Reports Server (NTRS)
Buzzard, R. J.; Gross, B.; Srawley, J. E.
1983-01-01
A Mode II test specimen was developed which has potential application in understanding phemonena associated with mixed mode fatigue failures in high performance aircraft engine bearing races. The attributes of the specimen are: it contains one single ended notch, which simplifiers data gathering and reduction; the fatigue crack grous in-line with the direction of load application; a single axis test machine is sufficient to perform testing; and the Mode I component is vanishingly small.
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.
Fatigue crack growth in the highly plastic regime
Kim, K.S.; Baik, Y.M.
1997-12-31
This paper evaluates the performance of {Delta}J{sup {star}}, {Delta}J[superscript karet] and {Delta}K as fracture parameters for center-cracked plate specimens of Alloy 718 under R{sub {sigma}} = 0, nominally elastic and plastic loading at elevated temperatures. The parameters {Delta}J{sup {star}} and {Delta}J[superscript caret] are computed from the results of an elastic-plastic finite element analysis of crack growth. At 538 C the results show, in contrast to the previous results of R{sup {var_epsilon}} = {minus}1 strain control tests, that the correlation of crack growth rates with {Delta}J{sup {star}} or {Delta}J[superscript caret] deviates from the relation of the Paris law type while {Delta}K gives a satisfactory correlation. The correlation at 649 C is poor for all three parameters. The finite element analysis shows that the crack closing behavior diminishes and eventually disappears as the crack tip plasticity increases due to crack growth or increased applied stress.
Fatigue crack growth testing of sub-clad defects
Jones, D.P.; Leax, T.R.
1998-04-01
Fatigue crack growth tests were performed on four point bend specimens with crack like defects intentionally placed in A302B low-alloy pressure vessel steel clad with 308/309L weld deposited stainless steel. The defects were placed in the base metal under the cladding by machining a cavity from the side opposite the cladding, electric-discharge machining a very sharp flaw, fatigue pre-cracking the flaw, and then filling up the cavity by a weld repair process. The specimens were stress relieved before fatigue testing. The specimens were fatigue cycled at positive load ratios until the defects broke through to the surface. The specimens were then fractured at liquid nitrogen temperatures to reveal the fracture surfaces. Seven different sub-clad flaw specimens were tested in room temperature air and each test provides a record of cycles to defect break-through. Changes in defect size and shape as a function of applied load cycles were obtained by benchmarking the crack at various stages of the load history. The results provide a set of embedded defect data which can be used for qualifying fatigue crack growth analysis procedures such as those in Section XI of the ASME Boiler and Pressure Vessel Code. A comparison between calculated and measured values shows that the ASME B and PV Section XI fatigue crack growth procedures conservatively predict cycles to defect break-through for small sub-clad defects.
Fatigue Crack Growth Database for Damage Tolerance Analysis
NASA Technical Reports Server (NTRS)
Forman, R. G.; Shivakumar, V.; Cardinal, J. W.; Williams, L. C.; McKeighan, P. C.
2005-01-01
The objective of this project was to begin the process of developing a fatigue crack growth database (FCGD) of metallic materials for use in damage tolerance analysis of aircraft structure. For this initial effort, crack growth rate data in the NASGRO (Registered trademark) database, the United States Air Force Damage Tolerant Design Handbook, and other publicly available sources were examined and used to develop a database that characterizes crack growth behavior for specific applications (materials). The focus of this effort was on materials for general commercial aircraft applications, including large transport airplanes, small transport commuter airplanes, general aviation airplanes, and rotorcraft. The end products of this project are the FCGD software and this report. The specific goal of this effort was to present fatigue crack growth data in three usable formats: (1) NASGRO equation parameters, (2) Walker equation parameters, and (3) tabular data points. The development of this FCGD will begin the process of developing a consistent set of standard fatigue crack growth material properties. It is envisioned that the end product of the process will be a general repository for credible and well-documented fracture properties that may be used as a default standard in damage tolerance analyses.
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.
The effect of fatigue cracks on fastener flexibility, load distribution, and fatigue crack growth
NASA Astrophysics Data System (ADS)
Whitman, Zachary Layne
Fatigue cracks typically occur at stress risers such as geometry changes and holes. This type of failure has serious safety and economic repercussions affecting structures such as aircraft. The need to prevent catastrophic failure due to fatigue cracks and other discontinuities has led to durability and damage tolerant methodologies influencing the design of aircraft structures. Holes in a plate or sheet filled with a fastener are common fatigue critical locations in aircraft structure requiring damage tolerance analysis (DTA). Often, the fastener is transferring load which leads to a loading condition involving both far-field stresses such as tension and bending, and localized bearing at the hole. The difference between the bearing stress and the tensile field at the hole is known as load transfer. The ratio of load transfer as well as the magnitude of the stresses plays a significant part in how quickly a crack will progress to failure. Unfortunately, the determination of load transfer in a complex joint is far from trivial. Many methods exist in the open literature regarding the analysis of splices, doublers and attachment joints to determine individual fastener loads. These methods work well for static analyses but greater refinement is needed for crack growth analysis. The first fastener in a splice or joint is typically the most critical but different fastener flexibility equations will all give different results. The constraint of the fastener head and shop end, along with the type of fastener, affects the stiffness or flexibility of the fastener. This in turn will determine the load that the fastener will transfer within a given fastener pattern. However, current methods do not account for the change in flexibility at a fastener as the crack develops. It is put forth that a crack does indeed reduce the stiffness of a fastener by changing its constraint, thus lessening the load transfer. A crack growth analysis utilizing reduced load transfer will result in
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
This is the final report for the NASA funded project entitled "Crack Growth Prediction Methodology for Multi-Site Damage." The primary objective of the project was to create a capability to simulate curvilinear fatigue crack growth and ductile tearing in aircraft fuselages subjected to widespread fatigue damage. The second objective was to validate the capability by way of comparisons to experimental results. Both objectives have been achieved and the results are detailed herein. In the first part of the report, the crack tip opening angle (CTOA) fracture criterion, obtained and correlated from coupon tests to predict fracture behavior and residual strength of built-up aircraft fuselages, is discussed. Geometrically nonlinear, elastic-plastic, thin shell finite element analyses are used to simulate stable crack growth and to predict residual strength. Both measured and predicted results of laboratory flat panel tests and full-scale fuselage panel tests show substantial reduction of residual strength due to the occurrence of multi-site damage (MSD). Detailed comparisons of n stable crack growth history, and residual strength between the predicted and experimental results are used to assess the validity of the analysis methodology. In the second part of the report, issues related to crack trajectory prediction in thin shells; an evolving methodology uses the crack turning phenomenon to improve the structural integrity of aircraft structures are discussed, A directional criterion is developed based on the maximum tangential stress theory, but taking into account the effect of T-stress and fracture toughness orthotropy. Possible extensions of the current crack growth directional criterion to handle geometrically and materially nonlinear problems are discussed. The path independent contour integral method for T-stress evaluation is derived and its accuracy is assessed using a p- and hp-version adaptive finite element method. Curvilinear crack growth is simulated in
NASA Technical Reports Server (NTRS)
Smith, Stephen W.; Seshadri, Banavara R.; Newman, John A.
2015-01-01
The experimental methods to determine near-threshold fatigue crack growth rate data are prescribed in ASTM standard E647. To produce near-threshold data at a constant stress ratio (R), the applied stress-intensity factor (K) is decreased as the crack grows based on a specified K-gradient. Consequently, as the fatigue crack growth rate threshold is approached and the crack tip opening displacement decreases, remote crack wake contact may occur due to the plastically deformed crack wake surfaces and shield the growing crack tip resulting in a reduced crack tip driving force and non-representative crack growth rate data. If such data are used to life a component, the evaluation could yield highly non-conservative predictions. Although this anomalous behavior has been shown to be affected by K-gradient, starting K level, residual stresses, environmental assisted cracking, specimen geometry, and material type, the specifications within the standard to avoid this effect are limited to a maximum fatigue crack growth rate and a suggestion for the K-gradient value. This paper provides parallel experimental and computational simulations for the K-decreasing method for two materials (an aluminum alloy, AA 2024-T3 and a titanium alloy, Ti 6-2-2-2-2) to aid in establishing clear understanding of appropriate testing requirements. These simulations investigate the effect of K-gradient, the maximum value of stress-intensity factor applied, and material type. A material independent term is developed to guide in the selection of appropriate test conditions for most engineering alloys. With the use of such a term, near-threshold fatigue crack growth rate tests can be performed at accelerated rates, near-threshold data can be acquired in days instead of weeks without having to establish testing criteria through trial and error, and these data can be acquired for most engineering materials, even those that are produced in relatively small product forms.
Magma-driven subcritical crack growth and implications for dike initiation from a magma chamber
NASA Astrophysics Data System (ADS)
Chen, Zuan; Jin, Z.-H.
2006-10-01
The purpose of this paper is to explore a viscoelastic energy dissipation theory for subcritical dike growth from a magma chamber. The theoretical relationship between the dike growth velocity and dike length is established using the viscoelastic subcritical crack growth theory proposed by the first author and the solutions of stress intensity factor at the crack tip derived by a perturbation method. Effects of magma chamber over-pressure, buoyancy and viscoelastic properties of the host rock on the subcritical growth rate are included in the model. The numerical results indicate that the viscous energy dissipation of the host rock could allow a short dike to slowly grow on the order of 10-7-10-5 m/s under modest over-pressure and to accelerate when the stress intensity factor increases close to the fracture toughness, followed by the unstable dike propagation. The proposed theory provides a reasonable understanding of dike initiation process from a magma chamber.
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.
Deformation and crack growth response under cyclic creep conditions
Brust, F.W. Jr.
1995-12-31
To increase energy efficiency, new plants must operate at higher and higher temperatures. Moreover, power generation equipment continues to age and is being used far beyond its intended original design life. Some recent failures which unfortunately occurred with serious consequences have clearly illustrated that current methods for insuring safety and reliability of high temperature equipment is inadequate. Because of these concerns, an understanding of the high-temperature crack growth process is very important and has led to the following studies of the high temperature failure process. This effort summarizes the results of some recent studies which investigate the phenomenon of high temperature creep fatigue crack growth. Experimental results which detail the process of creep fatigue, analytical studies which investigate why current methods are ineffective, and finally, a new approach which is based on the T{sup *}-integral and its ability to characterize the creep-fatigue crack growth process are discussed. The potential validity of this new predictive methodology is illustrated.
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).
Slow crack growth measurement using an electrical grid
NASA Technical Reports Server (NTRS)
Martin, D. J.; Davido, K. W.; Scott, W. D.
1986-01-01
Photolithography was used to produce a resistance grid on the surface of a DCB fracture specimen. The grid line spacings were 10 microns over a distance of 2 cm. Slow crack growth was measured on soda-lime-silica glass. At low values of K(I) (0.3 to 0.4 MPa.sq r + m, increased. Equations are given for the design of grids. The grid technique could be used to measure very slow crack growth at high temperature with appropriate compatible metal-ceramic materials.
R-Curve Instability Calculations Of Crack Growth
NASA Technical Reports Server (NTRS)
Orange, Thomas W.
1989-01-01
Report discusses use of instability method of calculation and R-curve mathematical models to analyze growth of cracks in fracture-mechanics specimens. In case of single material and structure, such analysis sometimes simple enough to be done on pocket calculator. Where microcomputer or larger computer available, comprehensive program includes libraries of driving-force equations for various configurations and R-curve mathematical models for different materials. Author concludes instability method simple and effective and model equations studied all viable in sense at lease one of them should fit almost any applicable set of crack-growth data. Method and models constitute powerful mathematical tools for analysis of fractures.
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.
Slow Crack Growth of Brittle Materials With Exponential Crack-Velocity Formulation. Part 1; Analysis
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Nemeth, Noel N.; Gyekenyesi, John P.
2002-01-01
Extensive slow-crack-growth (SCG) analysis was made using a primary exponential crack-velocity formulation under three widely used load configurations: constant stress rate, constant stress, and cyclic stress. Although the use of the exponential formulation in determining SCG parameters of a material requires somewhat inconvenient numerical procedures, the resulting solutions presented gave almost the same degree of simplicity in both data analysis and experiments as did the power-law formulation. However, the fact that the inert strength of a material should be known in advance to determine the corresponding SCG parameters was a major drawback of the exponential formulation as compared with the power-law formulation.
Simulation of Crack Growth Rate in Martensitic Steel
NASA Astrophysics Data System (ADS)
Odukwe, A. O.; Ajayi, O. O.; Oluwadare, G. O.
This research used the stress intensity factor with rate of crack growth per cycle of loading to model and simulates the crack growth in Martensitic steel in air environment. The basic parameters used were da/dN and ΔK, log (da/dN) was analyzed against log (ΔK) and a regression analysis using data from log (da/dN) vs log (ΔK) was carried out and the outcome employed to develop a model and simulation which gave rise to interactive software that can be used to predict the behavior of a structural member under conditions of certain loading. Additionally, it can be employed to have quick access to data and design considerations, when input data are supplied. This became useful in monitoring the point at which crack can initiate and the rate at which it would grow in a particular structural member of interest. The software has been tested with theoretical and experimental data.
Brittle-tough transitions during crack growth in toughened adhesives
NASA Astrophysics Data System (ADS)
Thoules, Michael
2008-03-01
The use of structural adhesives in automotive applications relies on an effective understanding of their performance under crash conditions. In particular, there is considerable potential for mechanics-based modeling of the interaction between an adhesive layer and the adherends, to replace current empirical approaches to design. Since energy dissipation during a crash, mediated by plastic deformation of the structure, is a primary consideration for automotive applications, traditional approaches of fracture mechanics are not appropriate. Cohesive-zone models that use two fracture parameters - cohesive strength and toughness - have been shown to provide a method for quantitative mechanics analysis. Combined numerical and experimental techniques have been developed to deduce the toughness and strength parameters of adhesive layers, allowing qualitative modeling of the performance of adhesive joints. These techniques have been used to study the failure of joints, formed from a toughened adhesive and sheet metal, over a wide range of loading rates. Two fracture modes are observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth is associated with a toughened mode of failure; the dynamic crack growth is associated with a more brittle mode of failure. The results of the experiments and analyses indicate that the fracture parameters for quasi-static crack growth in this toughened system are essentially rate independent, and that quasi-static crack growth can occur even at the highest crack velocities. Effects of rate appear to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appears to be stochastic in nature, and it does not appear to be associated with the attainment of any critical value for crack velocity or loading rate. Fracture-mechanics models exist in the literature for brittle-ductile transitions in rate-dependent polymers, which rely on rate dependent values of toughness
Surface Fatigue Crack Growth Behavior from Small Notch in Waspaloy
NASA Astrophysics Data System (ADS)
Suh, Chang-Min; Kim, Seon-Gab
We investigated the surface fatigue crack behaviors including initial surface crack appearances depend on three artificial notch lengths applied with the axle load level of the maximum load, 1,103 MPa and minimum load 55.3 MPa at the stress ratio of 0.05. This load level is the F100 engine's maximum operation condition of Waspaloy. The initial cracking site in depth is started from multi-origin. The effectiveness of crack growth rate by ductile striation space measurement on the fractured surface is confirmed by the working load and the stress intensity factor range. The surface cracks of Waspaloy at room temperature in air follow the ΔK vs da/dN and db/dN relation, even though the crack length initiated early in notch size 1 mm and initiated very late in notch size 4 mm. And the ΔK vs da/dN and db/dN relation have similar slope at 3 kinds of notches.
A computational study of the time dependent crack growth process
Brust, F.W.; Krishnaswamy, P.
1992-01-01
Universal studies of creep crack growth for (1) constant load and (2) variable load cases are presented. Results of the constant load cases is compared to experiment. The behavior of displacements and integral creep for fracture parameters are discussed for both load cases. The need for using a constitutive law which can handle cyclic creep is discussed.
Modeling fatigue crack growth in cross ply titanium matrix composites
NASA Technical Reports Server (NTRS)
Bakuckas, J. G., Jr.; Johnson, W. S.
1993-01-01
In this study, the fatigue crack growth behavior of fiber bridging matrix cracks in cross-ply SCS-6/Ti-15-3 and SCS-6/Timetal-21S laminates containing center holes was investigated. Experimental observations revealed that matrix cracking was far more extensive and wide spread in the SCS-6/Ti-15-3 laminates compared to that in the SCS-6/Timetal-21S laminates. In addition, the fatigue life of the SCS-6/Ti-15-3 laminates was significantly longer than that of the SCS-6/Timetal-21S laminates. The matrix cracking observed in both material systems was analyzed using a fiber bridging (FB) model which was formulated using the boundary correction factors and weight functions for center hole specimen configurations. A frictional shear stress is assumed in the FB model and was used as a curve fitting parameter to model matrix crack growth data. The higher frictional shear stresses calculated in the SCS-6/Timetal-21S laminates resulted in lower stress intensity factors in the matrix and higher axial stresses in the fibers compared to those in the SCS-6/Ti-15-3 laminates at the same applied stress levels.
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.
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 influence of crack closure on fatigue crack growth thresholds in 2024-T3 aluminum alloy
NASA Technical Reports Server (NTRS)
Phillips, Edward P.
1988-01-01
Crack opening loads were determined in load-shedding fatigue crack growth threshold tests on 2024-T3 aluminum alloy at stress ratios R of -2, -1, 0, 0.33, 0.5, and 0.7. The effects of load-shedding procedure and rate were investigated. Values of threshold Delta-K were found to vary significantly with R, whereas values of threshold effective Delta-K did not. That is, the variation of threshold Delta-K with R was almost completely explained by accounting for the measured variation in crack opening load behavior with R. The load-shedding guidelines of ASTM Test Method for Measurement of Fatigue Crack Growth (E 647) produced a threshold Delta-K value for R = 0.7 that was in agreement with the value determined using a procedure that should minimize closure effects. At both R = 0 and R = 0.7, high load-shedding rates produced high values of threshold Delta-K caused by large closure effects.
NASA Astrophysics Data System (ADS)
Qu, Wenzhong
1994-12-01
Guided by the Schapery's theory on the viscoelastic fracture, a model crack growth experiment was conducted with the HTPB composite propellant. An analysis on the crack growth behavior was made which shows that at the beginning of crack growth, there exists a critical stress intensity factor K(sub IC) (beyond this threshold value, the crack expansion begins). The functional relationship between the crack growth rate da/dt and the stress intensity factor K(sub I) was determined to be of power type. In addition, the relationship between the fracture energy of this propellant and its crack growth rate was also discussed.
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.
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.
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.
Analysis and prediction of Multiple-Site Damage (MSD) fatigue crack growth
NASA Technical Reports Server (NTRS)
Dawicke, D. S.; Newman, J. C., Jr.
1992-01-01
A technique was developed to calculate the stress intensity factor for multiple interacting cracks. The analysis was verified through comparison with accepted methods of calculating stress intensity factors. The technique was incorporated into a fatigue crack growth prediction model and used to predict the fatigue crack growth life for multiple-site damage (MSD). The analysis was verified through comparison with experiments conducted on uniaxially loaded flat panels with multiple cracks. Configuration with nearly equal and unequal crack distribution were examined. The fatigue crack growth predictions agreed within 20 percent of the experimental lives for all crack configurations considered.
Modeling crack growth processes in fusion reactor materials
NASA Astrophysics Data System (ADS)
Jones, Russell H.; Wolfer, Wilhelm G.
1984-05-01
Models for the effect of the chemical environment on crack growth processes in austenitic and ferritic stainless were evaluated. The effect of impurity segregation, yield strength, and hydrogen on crack growth of HT-9 and radiation induced phosphorus segregation on the intergranular stress corrosion of 316SS have been evaluated. Moderate increases in impurity segregation and/or yield strength caused significant decreases in the K IC and K TH of HT-9, while less than a 10 fold increase in the intergranular stress corrosion crack growth rate of 316SS was predicted for a fluence of 100 dpa using the radiation induced phosphorus segregation data of Brimhall et al. and the stress corrosion model of Parkins. Therefore, while radiation induced impurity segregation is greater in 316SS than HT-9, the effect of impurity segregation may be more pronounced in HT-9. The effect of hydrogen on fatigue crack thresholds was evaluated using a model by Tien which describes the threshold as a function of surface energy. A reduction in the surface energy by hydrogen adsorption was found to cause a decrease in the fatigue threshold a small but comparable amount to that observed for 2-1/4Cr-lMo steel.
The growth of short cracks in 4340 steel and aluminum-lithium 2090
NASA Technical Reports Server (NTRS)
Swain, M. H.; Everett, R. A.; Newman, James C., Jr.; Phillips, E. P.
1990-01-01
The objectives were to investigate crack initiation characteristics and short crack growth behavior for Al-Li 2090 and for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total fatigue lives for the steel. Single-edge-notched tension specimens of each alloy were used to obtain the short crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests on the steel were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors). The short crack growth rates were compared to those for long cracks grown under similar loading conditions. Metallurgical features associated with crack initiation are discussed. For Al-Li 2090 under R = -1 loading, the short cracks grew well below the long crack threshold and grew at acute angles to the loading axis. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, short-crack growth rates agreed well with long-crack growth rates, even near the long-crack threshold. A slight short-crack effect, growth below the long-crack threshold, was observed at R = -1. Fatigue lives were found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. An initial defect size and shape typical of those identified in this steel was assumed for the life predictions. For all loading conditions, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue lives.
Crack growth measured on flat and curved surfaces at cryogenic temperatures
NASA Technical Reports Server (NTRS)
Orange, T. W.; Sullivan, T. L.
1967-01-01
Multiple element continuity gage measures plane stress crack growth plus surface crack growth under plane strain conditions. The gage measures flat and curved surfaces and operates at cryogenic temperatures.
Steady crack growth through ductile metals: Computational studies
NASA Astrophysics Data System (ADS)
Sobotka, James C.
This thesis examines the crack-front response during sustained ductile tearing in structural metals at quasistatic rates using high resolution finite element computations. At load levels approaching the steady-growth regime, well-established computational methods that model material damage break down numerically as vanishingly small load increments produce increasingly large amounts of crack extension. The computational model adopted here determines the deformation history of a steadily advancing crack directly without the need for a priori (transient) analysis that considers blunting of the pre-existing stationary crack and subsequent growth through the associated initial plastic zone. Crack extension occurs at the remotely applied, fixed loading without the need for a local growth criteria. This numerical scheme utilizes a streamline integration technique to determine the elastic-plastic fields, generalized from a two-dimensional to a fully three-dimensional setting and implemented within mixed Matlab/C++/F-90 based software. Modifications of the conventional finite element formulation lead to an efficient procedure -- readily parallelized -- and determine the invariant near-front fields, representative of steady-state growth, on a fixed mesh in a boundary-layer framework. In the small-scale yielding regime, the crack front does not sense the existence of remote boundaries, and computational results retain a strong transferability among various geometric configurations where near-front, plastic deformation remains entirely enclosed by the surrounding linear-elastic material. The global stress intensity factor (KI ) and imposed T-stress fully specify displacement constraints along the far-field boundary, and in a three-dimensional setting, the panel thickness reflects the only natural length scale. The initial studies in this work consider steady crack advance within the small-scale yielding context under plane-strain conditions and mode I loading. These analyses
Crack Growth in First Wall by Cyclic Thermal Stress
Nishimura, T.; Hatano, T.; Honda, T.; Saito, M.
2003-07-15
The long pulse operation is assumed in ITER and future reactors. If the first wall has a defect, the crack may be propagated by cyclic thermal loads. In addition, flattop of more than 300 sec during plasma burning is expected, therefore, an effect of transient creep must be included. In order to simulate a severe temperature gradient in the first wall, an experimental facility was designed using an electron beam (EB) as a heat source, which has a distinct feature that the various plasma burning scenarios can be simulated by controlling the beam power so as to make surface temperature of the specimen to be fixed. To clarify the crack growth mechanism and the effects of transient creep, elastic-plastic stress analysis and creep analysis were performed. It is concluded that the creep effect during the operation duration period enlarges the residual tensile stress in the cooling period, and that consequently the crack propagation length increases.
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.
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 method for the analysis of the growth of short fatigue cracks
McEvily, A.J.; Shin, Y.S.
1995-10-01
A method for the analysis of the fatigue crack growth rate for short cracks has been developed and is applied to the case of fatigue crack growth of short surface cracks in a 1045 carbon steel. The method entails three modifications to standard LEFM procedures. These modifications include the use of a material constant to bridge between smooth and cracked specimen behavior, consideration of the plastic zone size to crack length ratio, and incorporation of the development of crack closure. Comparisons are made between calculations based upon this approach and experimental data.
A Fracture-Mechanical Model of Crack Growth and Interaction: Application to Pre-eruptive Seismicity
NASA Astrophysics Data System (ADS)
Matthews, C.; Sammonds, P.; Kilburn, C.
2007-12-01
A greater understanding of the physical processes occurring within a volcano is a key aspect in the success of eruption forecasting. By considering the role of fracture growth, interaction and coalescence in the formation of dykes and conduits as well as the source mechanism for observed seismicity we can create a more general, more applicable model for precursory seismicity. The frequency of volcano-tectonic earthquakes, created by fracturing of volcanic rock, often shows a short-term increase prior to eruption. Using fracture mechanics, the model presented here aims to determine the conditions necessary for the acceleration in fracture events which produces the observed pre-eruptive seismicity. By focusing on the cause of seismic events rather than simply the acceleration patterns observed, the model also highlights the distinction between an accelerating seismic sequence ending with an eruption and a short-term increase which returns to background levels with no activity occurring, an event also observed in the field and an important capability if false alarms are to be avoided. This 1-D model explores the effects of a surrounding stress field and the distribution of multi-scale cracks on the interaction and coalescence of these cracks to form an open pathway for magma ascent. Similarly to seismic observations in the field, and acoustic emissions data from the laboratory, exponential and hyperbolic accelerations in fracturing events are recorded. Crack distribution and inter-crack distance appears to be a significant controlling factor on the evolution of the fracture network, dominating over the effects of a remote stress field. The generality of the model and its basis on fundamental fracture mechanics results makes it applicable to studies of fracture networks in numerous situations. For example looking at the differences between high temperature fracture processes and purely brittle failure the model can be similarly applied to fracture dynamics in the
A nonlinear fracture mechanics approach to the growth of small cracks
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.
1983-01-01
An analytical model of crack closure is used to study the crack growth and closure behavior of small cracks in plates and at notches. The calculated crack opening stresses for small and large cracks, together with elastic and elastic plastic fracture mechanics analyses, are used to correlate crack growth rate data. At equivalent elastic stress intensity factor levels, calculations predict that small cracks in plates and at notches should grow faster than large cracks because the applied stress needed to open a small crack is less than that needed to open a large crack. These predictions agree with observed trends in test data. The calculations from the model also imply that many of the stress intensity factor thresholds that are developed in tests with large cracks and with load reduction schemes do not apply to the growth of small cracks. The current calculations are based upon continuum mechanics principles and, thus, some crack size and grain structure exist where the underlying fracture mechanics assumptions become invalid because of material inhomogeneity (grains, inclusions, etc.). Admittedly, much more effort is needed to develop the mechanics of a noncontinuum. Nevertheless, these results indicate the importance of crack closure in predicting the growth of small cracks from large crack data.
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.
Microscopic image analysis of slow crack growth in vitreous materials
Smith, W.L.
1986-04-01
Automated computer control, advanced video techniques, and digital image processing have been integrated into a servo-hydraulic testing system to produce a state-of-the-art testing environment. This system, in combination with an ultra high vacuum controlled environmental chamber, is used to produce high accuracy, subcritical slow crack growth data in vitreous materials. This video presentation is a review of the technologies used to extract data from the double cleavage drilled compression (DCDC) sample geometry.
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.
Kinetics and mechanisms of creep crack growth in a creep-resisting steel
Vainshtok, V.A.; Baumshtein, M.V.; Makovetskaya, I.A.; Man'ko, V.D.
1986-02-01
This paper discusses the nature of kinetic diagrams of growth of fatigue cracks in the temperature range typical of operation of important components of power equipment and examines the proportion of the incubation period of crack growth in the total life. The relationship of the kinetic diagrams of crack growth with the fracture mechanisms are examined and the effect of running life on creep crack propagation is reviewed.
NASGRO(registered trademark): Fracture Mechanics and Fatigue Crack Growth Analysis Software
NASA Technical Reports Server (NTRS)
Forman, Royce; Shivakumar, V.; Mettu, Sambi; Beek, Joachim; Williams, Leonard; Yeh, Feng; McClung, Craig; Cardinal, Joe
2004-01-01
This viewgraph presentation describes NASGRO, which is a fracture mechanics and fatigue crack growth analysis software package that is used to reduce risk of fracture in Space Shuttles. The contents include: 1) Consequences of Fracture; 2) NASA Fracture Control Requirements; 3) NASGRO Reduces Risk; 4) NASGRO Use Inside NASA; 5) NASGRO Components: Crack Growth Module; 6) NASGRO Components:Material Property Module; 7) Typical NASGRO analysis: Crack growth or component life calculation; and 8) NASGRO Sample Application: Orbiter feedline flowliner crack analysis.
Subcritical crack growth of selected aerospace pressure vessel materials
NASA Technical Reports Server (NTRS)
Hall, L. R.; Bixler, W. D.
1972-01-01
This experimental program was undertaken to determine the effects of combined cyclic/sustained loads, stress level, and crack shape on the fatigue crack growth rate behavior of cracks subjected to plane strain conditions. Material/environment combinations tested included: 2219-T87 aluminum plate in gaseous helium, room air, and 3.5% NaCl solution at room temperature, liquid nitrogen, and liquid hydrogen; 5Al-2.5 Sn (ELI) titanium plate in liquid nitrogen and liquid hydrogen and 6AL-4V (ELI) STA titanium plate in gaseous helium and methanol at room temperature. Most testing was accomplished using surface flawed specimens instrumented with a clip gage to continuously monitor crack opening displacements at the specimen surface. Tapered double cantilever beam specimens were also tested. Static fracture and ten hour sustained load tests were conducted to determine fracture toughness and apparent threshold stress intensity values. Cyclic tests were performed using sinusoidal loading profiles at 333 MHz (20 cpm) and trapezoidal loading profiles at both 8.3 MHz (0.5 cpm) and 3.3 MHz (0.2 cpm). Data were evaluated using modified linear elastic fracture mechanics parameters.
Fatigue Crack Growth Fundamentals in Shape Memory Alloys
NASA Astrophysics Data System (ADS)
Wu, Y.; Ojha, A.; Patriarca, L.; Sehitoglu, H.
2015-03-01
In this study, based on a regression of the crack tip displacements, the stress intensity range in fatigue is quantitatively determined for the shape memory alloy Ni2FeGa. The results are compared to the calculated stress intensity ranges with a micro-mechanical analysis accounting for the transformation-induced tractions. The effective stress intensity ranges obtained with both methods are in close agreement. Also, the fatigue crack closure levels were measured as 30 % of the maximum load using virtual extensometers along the crack flanks. This result is also in close agreement with the regression and micro-mechanical modeling findings. The current work pointed to the importance of elastic moduli changes and the residual transformation strains playing a role in the fatigue crack growth behavior. Additional simulations are conducted for two other important shape memory alloys, NiTi and CuZnAl, where the reductions in stress intensity range were found to be lower than Ni2FeGa.
Crack growth sparse pursuit for wind turbine blade
NASA Astrophysics Data System (ADS)
Li, Xiang; Yang, Zhibo; Zhang, Han; Du, Zhaohui; Chen, Xuefeng
2015-01-01
One critical challenge to achieving reliable wind turbine blade structural health monitoring (SHM) is mainly caused by composite laminates with an anisotropy nature and a hard-to-access property. The typical pitch-catch PZTs approach generally detects structural damage with both measured and baseline signals. However, the accuracy of imaging or tomography by delay-and-sum approaches based on these signals requires improvement in practice. Via the model of Lamb wave propagation and the establishment of a dictionary that corresponds to scatters, a robust sparse reconstruction approach for structural health monitoring comes into view for its promising performance. This paper proposes a neighbor dictionary that identifies the first crack location through sparse reconstruction and then presents a growth sparse pursuit algorithm that can precisely pursue the extension of the crack. An experiment with the goal of diagnosing a composite wind turbine blade with an artificial crack is performed, and it validates the proposed approach. The results give competitively accurate crack detection with the correct locations and extension length.
A Strip-Yield Model for Predicting the Growth of Part-Through Cracks Under Cyclic Loading
NASA Technical Reports Server (NTRS)
Daniewicz, S. R.; Newman, J. C., Jr. (Technical Monitor)
2000-01-01
Flaws exist in aircraft structures due to manufacturing operations and material defects. Under variable amplitude cyclic loading, these flaws grow as part-through cracks reducing the residual strength of structural components. To meet damage tolerant design requirements, accurate flaw growth predictions are needed which account for continual changes in crack shape as well as crack growth retardation and acceleration. Predicting the growth of part-through cracks under cyclic loading using an innovative and computationally efficient model is the focus of the research summarized in this report. In this research effort, a slice synthesis methodology was developed and used to construct a modified strip-yield model for the part-through semi-elliptical surface flaw, enabling prediction of plasticity-induced closure along the crack front and subsequent fatigue crack growth under constant amplitude and variable amplitude loading. While modeling the plasticity-induced closure in a part-through flaw may be performed using three dimensional elastic-plastic finite element analysis, this type of effort is impractical from an engineering perspective. A modified strip-yield model similar to that used in FASTRAN for part-through flaws is a much needed engineering design tool, particularly when computational resources are limited.
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.
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.
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.
Three-dimensional crack growth with hp-generalized finite element and face offsetting methods
NASA Astrophysics Data System (ADS)
Pereira, J. P.; Duarte, C. A.; Jiao, X.
2010-08-01
A coupling between the hp-version of the generalized finite element method ( hp-GFEM) and the face offsetting method (FOM) for crack growth simulations is presented. In the proposed GFEM, adaptive surface meshes composed of triangles are utilized to explicitly represent complex three-dimensional (3-D) crack surfaces. By applying the hp-GFEM at each crack growth step, high-order approximations on locally refined meshes are automatically created in complex 3-D domains while preserving the aspect ratio of elements, regardless of crack geometry. The FOM is applied to track the evolution of the crack front in the explicit crack surface representation. The FOM provides geometrically feasible crack front descriptions based on hp-GFEM solutions. The coupling of hp-GFEM and FOM allows the simulation of arbitrary crack growth with concave crack fronts independent of the volume mesh. Numerical simulations illustrate the robustness and accuracy of the proposed methodology.
Micromechanical model of crack growth in fiber reinforced brittle materials
NASA Technical Reports Server (NTRS)
Rubinstein, Asher A.; Xu, Kang
1990-01-01
A model based on the micromechanical mechanism of crack growth resistance in fiber reinforced ceramics is presented. The formulation of the model is based on a small scale geometry of a macrocrack with a bridging zone, the process zone, which governs the resistance mechanism. The effect of high toughness of the fibers in retardation of the crack advance, and the significance of the fiber pullout mechanism on the crack growth resistance, are reflected in this model. The model allows one to address issues such as influence of fiber spacing, fiber flexibility, and fiber matrix friction. Two approaches were used. One represents the fracture initiation and concentrated on the development of the first microcracks between fibers. An exact closed form solution was obtained for this case. The second case deals with the development of an array of microcracks between fibers forming the bridging zone. An implicit exact solution is formed for this case. In both cases, a discrete fiber distribution is incorporated into the solution.
Micromechanical model of crack growth in fiber reinforced ceramics
NASA Technical Reports Server (NTRS)
Rubinstein, Asher A.; Xu, Kang
1990-01-01
A model based on the micromechanical mechanism of crack growth resistance in fiber reinforced ceramics is presented. The formulation of the model is based on a small scale geometry of a macrocrack with a bridging zone, the process zone, which governs the resistance mechanism. The effect of high toughness of the fibers in retardation of the crack advance, and the significance of the fiber pullout mechanism on the crack growth resistance, are reflected in this model. The model allows one to address issues such as influence of fiber spacing, fiber flexibility, and fiber matrix friction. Two approaches were used. One represents the fracture initiation and concentrated on the development of the first microcracks between fibers. An exact closed form solution was obtained for this case. The second case deals with the development of an array of microcracks between fibers forming the bridging zone. An implicit exact solution is formed for this case. In both cases, a discrete fiber distribution is incorporated into the solution.
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 Growth Threshold Testing of Metallic Rotorcraft Materials
NASA Technical Reports Server (NTRS)
Newman, John A.; James, Mark A.; Johnson, William M.; Le, Dy D.
2008-01-01
Results are presented for a program to determine the near-threshold fatigue crack growth behavior appropriate for metallic rotorcraft alloys. Four alloys, all commonly used in the manufacture of rotorcraft, were selected for study: Aluminum alloy 7050, 4340 steel, AZ91E Magnesium, and Titanium alloy Ti-6Al-4V (beta-STOA). The Federal Aviation Administration (FAA) sponsored this research to advance efforts to incorporate damage tolerance design and analysis as requirements for rotorcraft certification. Rotorcraft components are subjected to high cycle fatigue and are typically subjected to higher stresses and more stress cycles per flight hour than fixed-wing aircraft components. Fatigue lives of rotorcraft components are generally spent initiating small fatigue cracks that propagate slowly under near-threshold cracktip loading conditions. For these components, the fatigue life is very sensitive to the near-threshold characteristics of the material.
Application of constraint modelling to evaluation of crack growth experiments
Faleskog, J.; Nilsson, F.; Shehu, S.; Oeberg, H.
1997-12-01
A large number of fracture mechanics experiments were carried out using a variety of specimens in order to investigate the applicability of the J-Q approach to initiation and growth of cracks. The study was performed at different temperatures spanning the transition interval of a pressure vessel steel. A comparison of the cleavage initiation levels with the model by Ritchie et al. (RKR) was also conducted. The experiments did not show, although the scatter was large, any systematic geometry effects that could not be explained within the framework of the J-Q concept. This was also the case for surface-cracked plates subjected to nonproportional loading. The trends of the dependence of the initiation on constraint are in qualitative agreement with the RKR model, although this model seems to underestimate this dependence for the current material. The ductile initiation level seems to be fairly independent of constraint but showed a tendency of variation with temperature.
NASA Technical Reports Server (NTRS)
Willard, S. A.
1997-01-01
Groups of striations called marker bands generated on a fatigue fracture surface can be used to mark the position of an advancing fatigue crack at known intervals. A technique has been developed that uses the distance between multiple sets of marker bands to obtain a vs. N, crack front shape, and fatigue crack growth rate data for small cracks. This technique is particularly usefull for specimens that require crack length measurements during testing that cannot be obtained because corrosion obscures the surface of the specimen. It is also useful for specimens with unusual or non-symmetric shapes where it is difficult to obtain accurate crack lengths using traditional methods such as compliance or electric potential difference in the early stages of testing.
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.
A preliminary study of crack initiation and growth at stress concentration sites
NASA Technical Reports Server (NTRS)
Dawicke, D. S.; Gallagher, J. P.; Hartman, G. A.; Rajendran, A. M.
1982-01-01
Crack initiation and propagation models for notches are examined. The Dowling crack initiation model and the E1 Haddad et al. crack propagation model were chosen for additional study. Existing data was used to make a preliminary evaluation of the crack propagation model. The results indicate that for the crack sizes in the test, the elastic parameter K gave good correlation for the crack growth rate data. Additional testing, directed specifically toward the problem of small cracks initiating and propagating from notches is necessary to make a full evaluation of these initiation and propagation models.
Fatigue crack growth at elevated temperature 316 stainless steel and H-13 steel
NASA Technical Reports Server (NTRS)
Chen, W. C.; Liu, H. W.
1976-01-01
Crack growths were measured at elevated temperatures under four types of loading: pp, pc, cp, and cc. In H-13 steel, all these four types of loading gave nearly the same crack growth rates, and the length of hold time had negligible effects. In AISI 316 stainless steel, the hold time effects on crack growth rate were negligible if the loading was tension-tension type; however, these effects were significant in reversed bending load, and the crack growth rates under these four types of loading varied considerably. Both tensile and compressive hold times caused increased crack growth rate, but the compressive hold period was more deleterious than the tensile one. Metallographic examination showed that all the crack paths under different types of loading were largely transgranular for both CTS tension-tension specimens and SEN reversed cantilever bending specimens. In addition, an electric potential technique was used to monitor crack growth at elevated temperature.
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.
NASA Astrophysics Data System (ADS)
Keanini, Russell; Eppes, Martha-Cary
2016-04-01
Paris's law connects fatigue-induced subcritical crack growth and fatigue loading. Environmentally-driven subcritical crack growth, while a random process, can be decomposed into a spectrum of cyclic processes, where each spectral component is governed by Paris's law. Unfortunately, almost no data exists concerning the Paris law exponent, m; rather, the great majority of existing sub-critical crack growth measurements on rock have been carried out via Mode I tensile tests, where corresponding data are generally correlated using Charles' law, and where the latter, similar to Paris's law, exposes a power law relationship between crack growth rate and stress intensity. In this study, a statistical argument is used to derive a simple, rigorous relationship between the all-important Paris law and Charles law exponents, m and n. This result has a significant practical implication: subcritical fatigue crack growth in rock, driven by various random environmental weathering processes can now be predicted using available Mode I stress corrosion indices, n.
Corrosion fatigue crack growth behavior of titanium alloys in aqueous solutions
Shipilov, S.A.
1998-01-01
The corrosion fatigue crack growth (FCG) behavior, the effect of applied potential on corrosion FCG rates, and the fracture surfaces of VT20 (near-{alpha}) and TS6 (near-{beta}) titanium alloys were studied. Environments were aqueous solutions of sodium chloride (NaCl), sodium hydroxide (NaOH), potassium hydroxide (KOH), ferric chloride (FeCl{sub 3}), and chromic acid (H{sub 2}CrO{sub 4}) with and without NaCl. Depending upon solution composition, corrosion FCG rates were found to be higher or lower than those in air. Cathodic polarization retarded the corrosion FCG, while anodic polarization accelerated insignificantly or almost did not influence it in most of the solutions investigated. However, cathodic polarization accelerated corrosion FCG in 0.6 M FeCl{sub 3} and 0.5 M to 2 M H{sub 2}CrO{sub 4} + 0.01 M to 0.1 M NaCl solutions by a dozen times when the maximum stress intensity (K{sub max}) exceeded certain critical values. When K{sub max} was lower than the critical values, the same cathodic polarization (with all other /conditions being equal) retarded corrosion FCG. Results suggested the accelerated crack growth at cathodic potentials resulted from hydrogen-induced cracking (HIC). Therefore, critical values of K{sub max}, as well as the stress intensity range ({Delta}K) were regarded as corresponding to the beginning of corrosion FCG according to a HIC mechanism and designated as K{sub HIC} and {Delta}K{sub HIC}.
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.
Fracture processes and mechanisms of crack growth resistance in human enamel
NASA Astrophysics Data System (ADS)
Bajaj, Devendra; Park, Saejin; Quinn, George D.; Arola, Dwayne
2010-07-01
Human enamel has a complex micro-structure that varies with distance from the tooth’s outer surface. But contributions from the microstructure to the fracture toughness and the mechanisms of crack growth resistance have not been explored in detail. In this investigation the apparent fracture toughness of human enamel and the mechanisms of crack growth resistance were evaluated using the indentation fracture approach and an incremental crack growth technique. Indentation cracks were introduced on polished surfaces of enamel at selected distances from the occlusal surface. In addition, an incremental crack growth approach using compact tension specimens was used to quantify the crack growth resistance as a Junction of distance from the occlusal surface. There were significant differences in the apparent toughness estimated using the two approaches, which was attributed to the active crack length and corresponding scale of the toughening mechanisms.
Crack Growth Monitoring in Harsh Environments by Electric Potential Measurements
Lloyd, Wilson Randolph; Reuter, Walter Graham; Weinberg, David Michael
1999-09-01
Electric potential measurement (EPM) technology offers an attractive alternative to conventional nondestructive evaluation (NDE) for monitoring crack growth in harsh environments. Where conventional NDE methods typically require localized human interaction, the EPM technique developed at the Idaho National Engineering and Environmental Laboratory (INEEL) can be operated remotely and automatically. Once a crack-like defect is discovered via conventional means, EPM can be applied to monitor local crack size changes. This is of particular interest in situations where an identified structural defect is not immediately rejectable from a fitness-for-service viewpoint, but due to operational and environmental conditions may grow to an unsafe size with continuing operation. If the location is in a harsh environment where periodic monitoring by normal means is either too costly or not possible, a very expensive repair may be immediately mandated. However, the proposed EPM methodology may offer a unique monitoring capability that would allow for continuing service. INEEL has developed this methodology, supporting equipment, and calibration information to apply EPM in a field environment for just this purpose. Laboratory and pilot scale tests on full-size engineering structures (pressure vessels and piping) have been successfully performed. The technique applicable is many severe environments because the sensitive equipment (electronics, operators) can be situated in a remote location, with only current and voltage probe electrical leads entering into the harsh environment. Experimental results showing the utility of the methodology are presented, and unique application concepts that have been examined by multiple experiments are discussed.
Crack growth monitoring in harsh environments by electrical potential measurements
W. R. Lloyd; W. G. Reuter; D. M. Weinberg
1999-09-19
Electric potential measurement (EPM) technology offers an attractive alternative to conventional nondestructive evaluation (NDE) for monitoring crack growth in harsh environments. Where conventional NDE methods typically require localized human interaction, the EPM technique developed at the Idaho National Engineering and Environmental Laboratory (INEEL) can be operated remotely and automatically. Once a crack-like defect is discovered via conventional means, EPM can be applied to monitor local crack size changes. This is of particular interest in situations where an identified structural defect is not immediately rejectable from a fitness-for-service viewpoint, but due to operational and environmental conditions may grow to an unsafe size with continuing operation. If the location is in a harsh environment where periodic monitoring by normal means is either too costly or not possible, a very expensive repair may be immediately mandated. However, the proposed EPM methodology may offer a unique monitoring capability that would allow for continuing service. INEEL has developed this methodology, supporting equipment, and calibration information to apply EPM in a field environment for just this purpose. Laboratory and pilot scale tests on full-size engineering structures (pressure vessels and piping) have been successfully performed. The technique is applicable to many severe environments because the sensitive equipment (electronics, operators) can be situated in a remote location, with only current and voltage probe electrical leads entering into the harsh environment. Experimental results showing the utility of the methodology are presented, and unique application concepts that have been examined by multiple experiments are discussed.
MSFC crack growth analysis computer program, version 2 (users manual)
NASA Technical Reports Server (NTRS)
Creager, M.
1976-01-01
An updated version of the George C. Marshall Space Flight Center Crack Growth Analysis Program is described. The updated computer program has significantly expanded capabilities over the original one. This increased capability includes an extensive expansion of the library of stress intensity factors, plotting capability, increased design iteration capability, and the capability of performing proof test logic analysis. The technical approaches used within the computer program are presented, and the input and output formats and options are described. Details of the stress intensity equations, example data, and example problems are presented.
Prediction of Fatigue Crack Growth Using Regularized Numerical Models
NASA Technical Reports Server (NTRS)
Meade, Andrew J.
1999-01-01
Though it is known in the engineering community that successful analyses rest upon the proper balance of (1) theoretical analysis of mathematical models, (2) physical experimentation and (3) computational simulation, this balance is currently handled in sometimes unwieldy and inefficient manner. It is proposed to investigate and develop rigorous and computationally efficient method to effectively combine all available information, from both experimental measurements and mathematical models, in the emulation of physical systems. This will be specifically applied to fatigue crack growth in metallic structures of interest to NASA.
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.
Short-Crack Growth Behaviour in an Aluminum Alloy: An AGARD Cooperative Test Programme
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Edwards, P. R.
1988-01-01
An AGARD test program on the growth of short fatigue cracks was conducted to define the significance of the short-crack effect, to compare test results from various laboratories and to evaluate an existing analytical model to predict the growth of such cracks. The first phase of this program, the Core Program was aimed at test procedure and specimen standardization and calibration of the various laboratories. A detailed working document has been prepared and is included in this report. It describes the testing fundamentals and procedures and includes the analysis procedures used for handling the test data. The results from the test program showed good agreement among the participants on short-crack growth rates, on fatigue life to various crack sizes and breakthrough (surface- or corner-crack became a through crack), and on crack shapes.
Contributions of aging to the fatigue crack growth resistance of human dentin.
Ivancik, Juliana; Majd, Hessam; Bajaj, Devendra; Romberg, Elaine; Arola, Dwayne
2012-07-01
An evaluation of the fatigue crack resistance of human dentin was conducted to identify the degree of degradation that arises with aging and the dependency on tubule orientation. Fatigue crack growth was achieved in specimens of coronal dentin through application of Mode I cyclic loading and over clinically relevant lengths (0 ≤ a ≤ 2 mm). The study considered two directions of cyclic crack growth in which the crack was either in-plane (0°) or perpendicular (90°) to the dentin tubules. Results showed that regardless of tubule orientation, aging of dentin is accompanied by a significant reduction in the resistance to the initiation of fatigue crack growth, as well as a significant increase in the rate of incremental extension. Perpendicular to the tubules, the fatigue crack exponent increased significantly (from m=14.2 ± 1.5 to 24.1 ± 5.0), suggesting an increase in brittleness of the tissue with age. For cracks extending in-plane with the tubules, the fatigue crack growth exponent does not change significantly with patient age (from m=25.4 ± 3.03 to 22.9 ± 5.3), but there is a significant increase in the incremental crack growth rate. Regardless of age, coronal dentin exhibits the lowest resistance to fatigue crack growth perpendicular to the tubules. While there are changes in the cyclic crack growth rate and mechanisms of cyclic extension with aging, this tissue maintains its anisotropy. PMID:22484693
Contributions of Aging to the Fatigue Crack Growth Resistance of Human Dentin
Ivancik, Juliana; Majd, Hessam; Bajaj, Devendra; Romberg, Elaine; Arola, Dwayne
2012-01-01
An evaluation of the fatigue crack resistance of human dentin was conducted to identify the degree of degradation that arises with aging and the dependency on tubule orientation. Fatigue crack growth was achieved in specimens of coronal dentin through application of Mode I cyclic loading and over clinically relevant lengths (0 ≤ a ≤ 2 mm). The study considered two directions of cyclic crack growth in which the crack was either in-plane (0°) or perpendicular (90°) to the dentin tubules. Results showed that regardless of tubule orientation, aging of dentin is accompanied by a significant reduction in the resistance to the initiation of fatigue crack growth, as well as a significant increase in the rate of incremental extension. Perpendicular to the tubules, the fatigue crack exponent increased significantly (from m=14.2±1.5 to 24.1±5.0), suggesting an increase in brittleness of the tissue with age. For cracks extending in plane with the tubules, the fatigue crack growth exponent does not change significantly with patient age (from m=25.4±3.03 to 22.9±5.3), but there is a significant increase in the incremental crack growth rate. Regardless of age, coronal dentin exhibits the lowest resistance to fatigue crack growth perpendicular to the tubules. While there are changes in the cyclic crack growth rate and mechanisms of cyclic extension with aging, this tissue maintains its anisotropy. PMID:22484693
Elastic-Plastic Finite Element Analysis of Fatigue Crack Growth in Mode 1 and Mode 2 Conditions
NASA Technical Reports Server (NTRS)
Nakagaki, M.; Atluri, S. N.
1978-01-01
Presented is an alternate cost-efficient and accurate elastic-plastic finite element procedure to analyze fatigue crack closure and its effects under general spectrum loading. Both Modes 1 and 2 type cycling loadings are considered. Also presented are the results of an investigation, using the newly developed procedure, of various factors that cause crack growth acceleration or retardation and delay effects under high-to-low, low-to-high, single overload, and constant amplitude type cyclic loading in a Mode 1 situation. Further, the results of an investigation of a centercracked panel under external pure shear (Mode 2) cyclic loading, of constant amplitude, are reported.
Role of Prism Decussation on Fatigue Crack Growth and Fracture of Human Enamel
Bajaj, Devendra; Arola, Dwayne
2009-01-01
The role of prism decussation on the crack growth resistance of human enamel is evaluated. Miniature inset Compact Tension (CT) specimens embodying a section of cuspal enamel were subjected to Mode I cyclic or monotonic loads. Cracks were grown in either the forward (from outer enamel inwards) or reverse (from inner enamel outwards) direction and the responses were compared quantitatively. Results showed that the outer enamel exhibits lower resistance to the inception and growth of cracks. Regardless of the growth direction, the near threshold region of cyclic extension was typical of ‘short crack’ behavior (i.e. deceleration of growth with an increase in crack length). Cyclic crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. In response to the monotonic loads, a rising R-curve response was exhibited by growth in the forward direction only. The total energy absorbed in fracture for the forward direction was more than three times that in the reverse. The rise in crack growth resistance was largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which were induced by decussation in the inner enamel. An analysis of the responses distinguished that the microstructure of enamel appears optimized for resisting crack growth initiating from damage at the tooth’s surface. PMID:19433137
Effects of loading on the growth rates of deep stress-corrosion cracks
Beavers, J.A.; Christman, T.K.
1990-08-01
The goal of this research program was to determine the effects of loading on growth of stress-corrosion cracks (SCC) in line pipe steel and whether special loading procedures could actually inhibit crack growth. Of particular interest was the effect of hydrostatic retesting on the subsequent growth of existing cracks. The growth rate experiments showed that the slow-strain rate loading could successfully nucleate a group of fine cracks with depths up to 0.025 inches (0.64 mm). However, the subsequent cyclic loading at typical operating stress levels (lower than experienced during the slow- strain rate loading) produced minimal crack growth and stopped soon after the test was started. The limited growth is believed to be a real phenomenon which means this is not a suitable procedure for the measurement of average crack growth rates. These experiments indicate that cracks grown at high stress (as in the slow-strain rate phase) do not readily propagate at lower stress levels. This may be because of crack closure (compressive crack tip residual stress) induced by the initial higher stress level. If that is true, then hydrostatic retests could inhibit the growth of existing stress-corrosion cracks, especially if the hydrostatic tests are conducted at high stress levels. 15 figures, 3 tabs.
The Regularities of Fatigue Crack Growth in Airframes Elements at Real Operation Conditions
NASA Astrophysics Data System (ADS)
Pavelko, Igors; Pavelko, Vitalijs
The results of analytical and experimental researches concerning predicting of fatigue crack growth in the operating conditions are presented. First of all the main factors causing a fatigue damage initiation and growth are analyzed and divided to two groups. Common conditions of fatigue damage precise predicting are established. The problem of fatigue crack growth at the stresses of variable amplitude was analyzed and an approach of description of this process is performed. Two examples present the efficiency of this approach. Theory of fatigue crack growth indication and the crack growth indicator (CGI) are developed. There is planned and executed a flight experiment using CGI located on two aircraft An-24 and An-26. Results of crack growth in CGI at operational load allowed to evaluate the parameters of generalized Paris-Erdogan law and statistical properties of crack increment per flight.
Short-crack growth behaviour in an aluminum alloy: An AGARD cooperative test program
NASA Technical Reports Server (NTRS)
Newman, J. C., Jr.; Edwards, P. R.
1988-01-01
An AGARD Cooperative Test Program on the growth of short fatigue cracks was conducted to define the significance of the short-crack effect, to compare test results from various laboratories, and to evaluate an existing analytical crack-growth prediction model. The initiation and growth of short fatigue cracks (5 micrometer to 2 mm) from the surface of a semi-circular notch in 2024-T3 aluminum alloy sheet material were monitored under various load histories. The cracks initiated from inclusion particle clusters or voids on the notch surface and generally grew as surface cracks. Tests were conducted under several constant-amplitude (stress ratios of -2, -1, 0, and 0.5) and spectrum (FALSTAFF and Gaussian) loading conditions at 3 stress levels each. Short crack growth was recorded using a plastic-replica technique. Over 250 edge-notched specimens were fatigue tested and nearly 950 cracks monitored by 12 participants from 9 countries. Long crack-growth rate data for cracks greater than 2 mm in length were obtained over a wide range in rates (10 to the -8 to 10 to the -1 mm/cycle) for all constant-amplitude loading conditions. Long crack-growth rate data for the FALSTAFF and Gaussian load sequences were also obtained.
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
Growth of Crack-Free 3-Inch-Diameter Lithium Tetraborate Single Crystals by Czochralski Method
NASA Astrophysics Data System (ADS)
Komatsu, Ryuichi; Sugihara, Tadashi; Uda, Satoshi
1994-09-01
The growth of crack-free 3-inch-diameter lithium tetraborate ( Li2B4O7) single crystals by the Czochralski method has been studied. The relationships between crystal cracking rate during growth and the crystal rotation rate and the position of the work-coil have been examined. It is concluded that crystal cracking at a later stage of growth is related to the temperature fluctuation in melt.
Accelerated transport and growth with symmetrized dynamics
NASA Astrophysics Data System (ADS)
Merikoski, Juha
2013-12-01
In this paper we consider a model of accelerated dynamics with the rules modified from those of the recently proposed [Dong et al., Phys. Rev. Lett. 109, 130602 (2012), 10.1103/PhysRevLett.109.130602] accelerated exclusion process (AEP) such that particle-vacancy symmetry is restored to facilitate a mapping to a solid-on-solid growth model in 1+1 dimensions. In addition to kicking a particle ahead of the moving particle, as in the AEP, in our model another particle from behind is drawn, provided it is within the "distance of interaction" denoted by ℓmax. We call our model the doubly accelerated exclusion process (DAEP). We observe accelerated transport and interface growth and widening of the cluster size distribution for cluster sizes above ℓmax, when compared with the ordinary totally asymmetric exclusion process (TASEP). We also characterize the difference between the TASEP, AEP, and DAEP by computing a "staggered" order parameter, which reveals the local order in the steady state. This order in part explains the behavior of the particle current as a function of density. The differences of the steady states are also reflected by the behavior of the temporal and spatial correlation functions in the interface picture.
Effects of Different R ratios on Fatigue Crack Growth in Laser Peened Friction Stir Welds
NASA Technical Reports Server (NTRS)
Hatamleh, Omar; Hackel, Lloyd; Forth, Scott
2007-01-01
The influence of laser peening on the fatigue crack growth behavior of friction stir welded (FSW) Aluminum Alloy (AA) 7075-T7351 sheets was investigated. The surface modification resulting from the peening process on the fatigue crack growth of FSW was assessed for two different R ratios. The investigation indicated a significant decrease in fatigue crack growth rates resulting from using laser shock peening compared with unpeened, welded and unwelded specimens. The slower fatigue crack growth rate was attributed to the compressive residual stresses induced by the peening.
Near-Threshold Fatigue Crack Growth Behavior of Fine-Grain Nickel-Based Alloys
NASA Technical Reports Server (NTRS)
Newman, John A.; Piascik, Robert S.
2003-01-01
Constant-Kmax fatigue crack growth tests were performed on two finegrain nickel-base alloys Inconel 718 (DA) and Ren 95 to determine if these alloys exhibit near-threshold time-dependent crack growth behavior observed for fine-grain aluminum alloys in room-temperature laboratory air. Test results showed that increases in K(sub max) values resulted in increased crack growth rates, but no evidence of time-dependent crack growth was observed for either nickel-base alloy at room temperature.
Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy
Mercer, C.; Soboyejo, A.B.O.; Soboyejo, W.O. )
1999-07-09
The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K[sub max], and the stress intensity factor range, [Delta]K. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of [Delta]K and K[sub max] on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.
The effect of environment on the sustained load crack growth rates of forged WASPALOY
NASA Astrophysics Data System (ADS)
Vasatis, Ioannis P.; Pelloux, Regis M.
1985-08-01
The sustained load crack growth rates of wrought WASPALOY* were measured in air and in high purity argon at 650 °C using single edge notched (SEN) and compact tension (CT) specimens machined out of a turbine disk. The crack growth rates measured in air exhibited great variability across the WASPALOY disk, while the crack growth rates measured in purified argon were of the same order of magnitude. This difference in crack growth rates is attributed to local variations in oxidation resistance at the tip of the growing crack. The density and the distribution of carbides in different locations of the WASPALOY disk accounts for the variability in crack growth resistance in air.
Comparison of two computer codes for crack growth analysis: NASCRAC Versus NASA/FLAGRO
NASA Technical Reports Server (NTRS)
Stallworth, R.; Meyers, C. A.; Stinson, H. C.
1989-01-01
Results are presented from the comparison study of two computer codes for crack growth analysis - NASCRAC and NASA/FLAGRO. The two computer codes gave compatible conservative results when the part through crack analysis solutions were analyzed versus experimental test data. Results showed good correlation between the codes for the through crack at a lug solution. For the through crack at a lug solution, NASA/FLAGRO gave the most conservative results.
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.
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.
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
Creep crack growth behavior of aluminum alloy 2519. Part 1: Experimental analysis
Hamilton, B.C.; Saxena, A.; McDowell, D.L.; Hall, D.E.
1997-12-31
The discipline of time-dependent fracture mechanics has traditionally focused on the creep crack growth behavior of high-temperature materials that display creep-ductile behavior, such as stainless steels and chromium-molybdenum steels. Elevated temperature aluminum alloys, however, have been developed that exhibit creep-brittle behavior; in this case, the creep crack growth rate correlates with the stress intensity factor, K. The fracture characteristics of aluminum alloy 2519-T87 were studied at 135 C, and the creep and creep crack growth behavior were characterized utilizing experimental and numerical methods. The strain to failure for creep deformation specimens was limited to only 1.2 to 2.0%. Creep crack growth tests revealed a unique correlation between the creep crack growth rate and K, a result consistent with creep-brittle behavior. No experimental correlation was found between the creep crack growth rate and the C{sub t} parameter. Microscopy of fracture surfaces revealed distinct regions of intergranular and transgranular fracture, and the transition between the fracture regions was found to occur at a critical K-level. Experimental results also appeared to show that initiation of crack growth (incubation) is controlled by the accumulation of a critical amount of damage ahead of the crack tip and that a correlation exists between the incubation time and K. Total time to failure is viewed as a summation of the incubation period and the crack growth period, and the design importance of incubation time is discussed.
Vibration-Based Method Developed to Detect Cracks in Rotors During Acceleration Through Resonance
NASA Technical Reports Server (NTRS)
Sawicki, Jerzy T.; Baaklini, George Y.; Gyekenyesi, Andrew L.
2004-01-01
In recent years, there has been an increasing interest in developing rotating machinery shaft crack-detection methodologies and online techniques. Shaft crack problems present a significant safety and loss hazard in nearly every application of modern turbomachinery. In many cases, the rotors of modern machines are rapidly accelerated from rest to operating speed, to reduce the excessive vibrations at the critical speeds. The vibration monitoring during startup or shutdown has been receiving growing attention (ref. 1), especially for machines such as aircraft engines, which are subjected to frequent starts and stops, as well as high speeds and acceleration rates. It has been recognized that the presence of angular acceleration strongly affects the rotor's maximum response to unbalance and the speed at which it occurs. Unfortunately, conventional nondestructive evaluation (NDE) methods have unacceptable limits in terms of their application for online crack detection. Some of these techniques are time consuming and inconvenient for turbomachinery service testing. Almost all of these techniques require that the vicinity of the damage be known in advance, and they can provide only local information, with no indication of the structural strength at a component or system level. In addition, the effectiveness of these experimental techniques is affected by the high measurement noise levels existing in complex turbomachine structures. Therefore, the use of vibration monitoring along with vibration analysis has been receiving increasing attention.
Test Method Variability in Slow Crack Growth Properties of Sealing Glasses
NASA Technical Reports Server (NTRS)
Salem, J. A.; Tandon, R.
2010-01-01
The crack growth properties of several sealing glasses were measured by using constant stress rate testing in 2 and 95 percent RH (relative humidity). Crack growth parameters measured in high humidity are systematically smaller (n and B) than those measured in low humidity, and crack velocities for dry environments are 100x lower than for wet environments. The crack velocity is very sensitive to small changes in RH at low RH. Biaxial and uniaxial stress states produced similar parameters. Confidence intervals on crack growth parameters that were estimated from propagation of errors solutions were comparable to those from Monte Carlo simulation. Use of scratch-like and indentation flaws produced similar crack growth parameters when residual stresses were considered.
Subcritical crack growth in soda-lime glass in combined mode I and mode II loading
NASA Technical Reports Server (NTRS)
Singh, Dileep; Shetty, Dinesh K.
1990-01-01
Subcritical crack growth under mixed-mode loading was studied in soda-lime glass. Pure mode I, combined mode I and mode II, and pure mode II loadings were achieved in precracked disk specimens by loading in diametral compression at selected angles with respect to the symmetric radial crack. Crack growth was monitored by measuring the resistance changes in a microcircuit grid consisting of parallel, electrically conducting grid lines deposited on the surface of the disk specimens by photolithography. Subcritical crack growth rates in pure mode I, pure mode II, and combined mode I and mode II loading could be described by an exponential relationship between crack growth rate and an effective crack driving force derived from a mode I-mode II fracture toughness envelope. The effective crack driving force was based on an empirical representation of the noncoplanar strain energy release rate. Stress intensities for kinked cracks were assessed using the method of caustics and an initial decrease and a subsequent increase in the subcritical crack growth rates of kinked cracks were shown to correlate with the variations of the mode I and the mode II stress intensities.
Rubber Characterization with Fatigue and Crack Growth Analysis
NASA Astrophysics Data System (ADS)
Perier, Laurent; Favier, Arnaud
2010-06-01
Dynamic Mechanical Analysis brings advanced capabilities to generate quantitative dynamic property data of rubber compounds for modelers and end users. Additionally to the recognized DMA testing benefits, a new METRAVIB Analyzer DMA+300 includes new testing solutions for analyzing rubbers mechanical property data under extended testing conditions closer to the real product's life conditions. The DMA+300 is specifically designed and dimensioned for fatigue tests on elastomers and crack growth tests. Combined with new MULTITEST software, DMA+300 makes possible to generate multi harmonics excitation and to control the application of a specific wave form to the specimen of rubber. This paper presents the main benefits available with this machine in the domain of rubber and elastomer applications.
Fracture mechanics applied to nonisothermal fatigue crack growth
NASA Technical Reports Server (NTRS)
Jordan, E. H.; Meyers, G. J.
1986-01-01
Twelve nonisothermal fatigue crack growth tests were performed on Hastelloy-X tubular specimens in which strain and temperature varied simultaneously. Conditions were selected to include nominally elastic and nominally plastic conditions and temperatures up to 982 C. A number of parameters, including the stress intensity factor, strain intensity factor, and J-integral, were examined for their ability to correlate the data. There was no decisive difference between the success of the three parameters. Each parameter correlated data from different strain ranges to within no worse than a factor of 2.1 on da/dn. The effect of strain temperature cycle shape was investigated and found to be moderate, while a strain hold of 1 min had very little effect. An attempt was made to predict nonisothermal test results from isothermal data. These predictions were better than those made by using peak test temperature isothermal data but still not within scatter.
CRACK GROWTH ANALYSIS OF SOLID OXIDE FUEL CELL ELECTROLYTES
S. Bandopadhyay; N. Nagabhushana
2003-10-01
Defects and Flaws control the structural and functional property of ceramics. In determining the reliability and lifetime of ceramics structures it is very important to quantify the crack growth behavior of the ceramics. In addition, because of the high variability of the strength and the relatively low toughness of ceramics, a statistical design approach is necessary. The statistical nature of the strength of ceramics is currently well recognized, and is usually accounted for by utilizing Weibull or similar statistical distributions. Design tools such as CARES using a combination of strength measurements, stress analysis, and statistics are available and reasonably well developed. These design codes also incorporate material data such as elastic constants as well as flaw distributions and time-dependent properties. The fast fracture reliability for ceramics is often different from their time-dependent reliability. Further confounding the design complexity, the time-dependent reliability varies with the environment/temperature/stress combination. Therefore, it becomes important to be able to accurately determine the behavior of ceramics under simulated application conditions to provide a better prediction of the lifetime and reliability for a given component. In the present study, Yttria stabilized Zirconia (YSZ) of 9.6 mol% Yttria composition was procured in the form of tubes of length 100 mm. The composition is of interest as tubular electrolytes for Solid Oxide Fuel Cells. Rings cut from the tubes were characterized for microstructure, phase stability, mechanical strength (Weibull modulus) and fracture mechanisms. The strength at operating condition of SOFCs (1000 C) decreased to 95 MPa as compared to room temperature strength of 230 MPa. However, the Weibull modulus remains relatively unchanged. Slow crack growth (SCG) parameter, n = 17 evaluated at room temperature in air was representative of well studied brittle materials. Based on the results, further work
The NT-criterion for predicting crack growth increments
NASA Technical Reports Server (NTRS)
Yehia, Nabil A. B.; Shephard, Mark S.
1987-01-01
A new approach is presented to determine the crack propagation increment after the direction of crack propagation has been predicted. The maximum dilatational strain energy density (NT-criterion) is employed in the derivation for predicting both direction and increment of the propagating crack. The crack propagation path predicted by the NT-criterion is compared to the one predicted by the S-criterion and to some available experimental data.
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.
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 Technical Reports Server (NTRS)
Choi, Sung R.; Nemeth, Noel N.; Gyekenyesi, John P.
2002-01-01
The previously determined 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 and preload testing at ambient and elevated temperatures. The data fit to the relation of strength versus the log of the stress rate was very reasonable for most of the materials. Also, the preloading technique was determined equally applicable to the case of slow-crack-growth (SCG) parameter n greater than 30 for both the power-law and exponential formulations. The major limitation in the exponential crack-velocity formulation, however, was that the inert strength of a material must be known a priori to evaluate the important SCG parameter n, a significant drawback as compared with the conventional power-law crack-velocity formulation.
NASA Technical Reports Server (NTRS)
Choi, Sung R.; Nemeth, Noel N.; Gyekenyesi, John P.
2002-01-01
The previously determined life prediction analysis based on an exponential crack-velocity formulation was examined using a variety of experimental data on advanced structural ceramics tested under constant stress and cyclic stress loading at ambient and elevated temperatures. The data fit to the relation between the time to failure and applied stress (or maximum applied stress in cyclic loading) was very reasonable for most of the materials studied. It was also found that life prediction for cyclic stress loading from data of constant stress loading in the exponential formulation was in good agreement with the experimental data, resulting in a similar degree of accuracy as compared with the power-law formulation. The major limitation in the exponential crack-velocity formulation, however, was that the inert strength of a material must be known a priori to evaluate the important slow-crack-growth (SCG) parameter n, a significant drawback as compared with the conventional power-law crack-velocity formulation.