Fatigue Crack Extension Research Articles

Constitutive creep–fatigue crack growth methodology in two steels using a strip yield model

A creep–fatigue crack extension model based on a strip-yield methodology is presented and model predictions compared to experimental data over a range of temperatures, stresses and materials. In addition, a constitutive creep damage model for AISI 316L is developed. An empirical model based on the C* parameter and a constitutive model will be used to predict crack extension and compared. Advantages and limitations of the empirical creep model and the constitutive model are highlighted. In view of these limitations and the experimental comparisons, future development actions are recommended.

Extension of Fatigue Cracks in a Low-Alloy Steel After Massive Plastic Deformation

Abstract Fatigue crack extension was studied in a ferritic–pearlitic steel in two different material conditions. The first one was the virgin state with a typical pearlitic microstructure containing comparatively large stacks of cementite lamellae. The second material contained predominantly globular cementite and stacks of very fine cementite lamellae and was found after massive plastic deformation during a thermo-mechanical forming process. These differences in the microstructure lead to significantly different mechanisms in the damage accumulation process under fatigue loading. Stacks of cementite lamellae pearlite were both crack initiation sites and microstructural barriers for small cracks in the virgin material, whereas the fine lamellae in the plastically deformed (cold-worked) material could cause crack deviation, but no crack initiation and no significant crack arrest or crack retardation.

再現HAZ鋼材の４点曲げ疲労き裂進展試験

To reduce the weight of connecting structures, welding is usually used. In the case of welding thin plates, fatigue cracks often propagate in the heat affected zone (HAZ) rather than base metals. Therefore, it is important to investigate the extension behavior of fatigue crack in HAZ of welding joint. In this study, four-point bending fatigue tests for two high strength steels, an ordinary shipbuilding steel K36A and a shipbuilding steel K36 with low carbon contents by TMCP, were carried out under the stress ratios (R) of 0.1 and 0.8. The specimen with HAZ introduced by thermal cycle simulating the temperature history of HAZ was prepared. The crack extension rate of K36A HAZ specimen is lower than that of the corresponding base metals, especially at small repeating range of stress intensity factor. For R = 0.8 tests imitating tensile residual stress, the crack extension rate much greater than that of R = 0.1.

OS0701 CNF強化型CFRP積層板のモードI疲労き裂進展特性

OS0701 CNF強化型CFRP積層板のモードI疲労き裂進展特性

Evaluation of Overloading and Crack Closure Effects on Fatigue Crack Growth in an Aircraft 7075-T7351 Al-Alloy

Al 7075 alloy is a high strength material usually used for highly stressed components in lightweight structures, typically in aircraft, aerospace and defence applications. It can be applied in different heat treatment conditions, but the T7351 temper state is most widely used because of improved stress-corrosion cracking resistance. An investigation of effects of overloads on fatigue crack growth (FCG) and retardation in Al 7075-T7351 alloy was carried out. FCG rates were measured at load asymmetry R = Fmin / Fmax = 0.1, in quite wide region of growth between 10-8 and 10-5 m/cycle (stress intensity factor range ΔK between 6 and 40 MPa m1/2). Retardation effects of overloads of the magnitudes 2.7-times and 3.0-times of the maximum load in the constant range fatigue loading were significant. Crack mouth opening displacement was evaluated at numerous stages of crack growth including pre-cracking with so called load shedding method. The overloads resulted in substantial crack closure effects, which, however, did not occur immediately after the overloading, but after further fatigue crack extension. Results are discussed considering both theoretically and experimentally estimated plastic zone size and considering crack closure issues recently published in the literature

Research on Commercial Vehicle Clutch Release Bearing Fatigue Life Prediction Model

Commercial vehicle clutch release bearings working at high speed, strong vibration,high temperature, damp and easy pollution conditions. Fatigue life analysis is based on the release bearing rings or rolling body began to appear fatigue spalling, in which this kind of phenomenon is under cyclic stress. The contact stress distribution is not uniform, the contact stress is mainly concentrated near the surface; influenced by the geometry and physical properties and lubrication of the surface significantly. Contact between the two types of fatigue crack extension methods: fatigue crack surface under expansion and surface fatigue crack propagation. The surface crack growth mainly originated from two kinds of cases: crack caused by surface pre crack and contact between the two surface asperity each other. New life prediction model for the release bearing based on L-P theory and Tallian model ,in which influence factors of fatigue life is introduced on the smelting process, surface defect, surface roughness, residual stress, elastohydrodynamic lubrication oil film,environmental cleanliness, temperature, the effect of varying load characteristics and other factors of fatigue life. The results show that: the clutch release bearing life prediction model of new and more close to the real conditions of automobile clutch, provide the theory basis for the development of a new generation high-speed heavy-duty clutch release bearing of the commercial vehicle.

Fatigue Crack Growth in A 7075 Al-Alloy with Evaluation of Overloading Effects

Measurement of fatigue crack growth (FCG) in an Al 7075-T7351 alloy of a particularly high homogeneity was carried out as a part of the Proficiency Test Programme on fatigue crack growth measurement, organised by General Electric Aviation, USA, in collaboration with the company Exova in France. To pass the test programme successfully, a particular attention was paid to improve and verify DCPD (direct current potential drop) method being used for the crack length measurement. Additionally, an investigation of effects of overloads on fatigue crack growth and retardation was carried out. In the CT specimens used, the difference between actual and calculated crack length at the crack starting point was quite significant, approximately 4.3 mm. However, the difference as a function of crack length was almost constant. Results of FCG rates were very self consistent, likely due to the material homogeneity. Values of the parameters C and m measured at the SVÚM a.s. laboratory were evaluated as the third best precise from all the 16 worldwide participating laboratories. Retardation effects of overloads of the magnitudes 2.7-times and 3.0-times of the maximum load in the constant range fatigue loading were significant. The overloads resulted in substantial crack closure effects, which, however, did not occur immediately, but just after further fatigue crack extension, comparable with plastic zone size.

Study on the Inwall Crack Extension and Fatigue Life Evaluation for Deep-Sea Steel Catenary Riser under Wave and Current Action

Aimed at deep-sea steel catenary risers(SCRs) transporting supercritical or dense CO2, this paper uses the macroscopic pipeline-transporting finite element model and the microscopic pipewall crack growth model to numerically simulate corrosive-inwall-defect-induced crack extension along radial direction for the deep-sea riser which is subjected to wave and current loads. Meanwhile, based on linear elastic fracture mechanics and fatigue crack extension theory, the effect of wave height on the fatigue life of the pipewall at touchdown zone is discussed. The above-mentioned study in this paper is expected to give a good reference to the similar study for oil&gas SCRs.

The Analysis of Initial Welding Defect and Structure Improvement of Converter Loop

Aimed at the welding initial defect fatigue extension of 300t converter loop in a factory, the whole assembly finite-element (FE) model of converter loop with the welding initial defect is established. The converter loop FE model with different depth initial defect is computed by using 3D nonlinear finite element method at different tilting angles. And the stress intensity factor is extracted and analyzed under different working circumstances. The analysis results show that when the initial defect depth is greater than 11.715mm, the defect will result in fatigue crack extension at the complex alternating stress. To solve the cracking leakage problem of converter loop, the original welding manholes is replaced by the movable manhole. In the past over one year, the fatigue cracking and leaking phenomena did not occur in the converter loop, so the effect is obvious.

Statistical characterization of fatigue lifetime of polysilicon thin films

This paper discusses, from a statistical point of view, whether the fatigue behavior of polysilicon thin films can be formulated uniquely on the basis of initial defects which determine their strength distribution. The fatigue process determining the lifetime is modeled with a power law similar to the well-known Paris law as the fatigue crack extension process from the initial defects whose equivalent crack lengths were evaluated from the initial strength distribution. The parameters in the power law were here fit to the results of tensile fatigue tests performed on three groups of specimens patterned using three different conditions leading to different etching damage. The results showed that the obtained power law parameters of polysilicon are spread over a range where the calculated behavior changes insignificantly in spite of the different etching conditions leading to different initial strength distributions. They support a possibility that the fatigue fracture is caused by the accumulation of defects and that therefore the fatigue lifetimes can be practically predicted from their strength distributions by using the same values of the parameters in the power law.

Effect of humidity and temperature on the fatigue behavior of polysilicon thin film

The fatigue behavior of polysilicon thin films was investigated under three different environmental conditions, i.e., air with relative humidity 80% at 22 °C, inert nitrogen gas at 22 °C and 180 °C. A new evaluation technique was utilized, where the experiment applying a stress amplitude gradually increased with the number of fatigue cycles was combined with a statistical analysis using an equivalent fatigue crack extension model on the basis of Paris’ well-known law. The parameter values in Paris’ law for relative humidity 80% to control fatigue behavior were found to lie close to those obtained previously with conventional fatigue tests with constant stress amplitudes, when a sufficiently slow increasing rate of stress amplitude was applied. The fracture stress level appeared to be smaller than the static strength even in nitrogen gas at 22 °C, which suggested that fatigue damage was introduced by repeated loading even under an inert environment. Fatigue damage was observed to be more significant at a higher temperature (180 °C) in the same nitrogen gas atmosphere, which is an evidence of the contribution of intrinsic defects on the fatigue behavior.

5-5 異なる試験手法により得られたシリコンの疲労挙動の相互比較(セッション5:OS1-2 マルチスケール・シミュレーションとナノ計測)

Fatigue behavior of silicon was compared among different testing techniques performed in 6 institutions in Japan. A variety of specimens was subjected to fatigue loading with different testing techniques under different conditions such as load ratio, humidity and temperature. In summary, although their initial strength under monotonically increasing load scattered significantly, the trend of fatigue lifetime was found to be well formulated consistently on the basis of Paris' law to describe equivalent fatigue crack extension process.

A model for the formation of fatigue striations and its relationship with small fatigue crack growth in an aluminum alloy

The fatigue crack growth process involves damage accumulation and crack extension. The two sub-processes that lead to fatigue crack extension were quantified separately in a recent model for small fatigue crack growth applicable to engineering alloys. Here, we report the results of an experimental investigation to assess the assumptions of that model. The fatigue striation formation in an aluminum alloy is modeled, and it is verified that the number of cycles required for striation formation is related to the cyclic crack-tip opening displacement. The striation spacing is related to the value of the monotonic crack-tip displacement. It is concluded that extensive crack-tip geometry changes due to plasticity in the aluminum alloy causes a reduction in the slope of the fatigue crack propagation curves. The implications of these results on the fatigue crack propagation lifetime calculations are identified.

Accuracy of the Fatigue Lifetime of Polysilicon Predicted from its Strength Distribution

AbstractThis paper discusses the accuracy of the distribution of the fatigue lifetime of polysilicon thin films predicted from their strength distribution. On the basis of the authors' previous studies, where the fatigue process determining the lifetime was formulated using the well-known fatigue crack extension Paris' law, prediction error ranges for polysilicon specimens with different levels of strength are determined. The errors of the predicted fatigue lifetime in the logarithmic scale, defined as △logN= |log10Nexp-log10Npred| whereNexpandNpredwere the experimental and predicted number of cycle, were found to be less than 1 in the range of the cumulative fracture probability F between 0.1 and 0.9. Therefore, based on the measured Paris' law parameters of polysilicon, the fatigue lifetimes of different polysilicon thin film structures can be predicted from their strength distributions with errors of roughly 10% in the logarithmic scale, which was average of percentages of △logNto log10Nof experimental data.

T0302-1-4 Statistical Evaluation of Fracture and Fatigue Behavior of Polysilicon Thin Films with Arbitrary Shapes

This paper presents the evaluation of strength distribution and fatigue behavior of polycrystalline silicon specimens with two different shapes of notches under monotonically increasing and cyclic loading. Static strength distribution was described by Weibull distribution with the two material parameters m and σ_0. The fatigue crack extension process determining fatigue lifetime was formulated by the well-known Paris law with two unknown parameters n and C. These were fit to the results of the tensile static and fatigue tests. It was found that the parameters were independent of the shapes of specimens under the same environment.

S0301-1-4 低サイクル疲労を受ける切欠付オーステナイトステンレス鋼のき裂先端開口角解析(疲労特性に及ぼす各種因子の影響(1))

The crack tip opening angle (CTOA) is a viable ductile fracture criterion and could be used for elastic-plastic, crack propagation analysis without the constraint of LEFM. This paper reports on our investigation to correlate ΔCTOA with the crack propagation rate, da/dN, in stainless steel (SUS 316), single edged notch bend (SENB) specimens. The SENB specimens were cyclically loaded at various constant load amplitudes with a load ratio, R〜0.01. ΔCTOA at the initial phase of fatigue crack extension was measured by moire interferometry and inferred from a ΔCTOA versus ΔCMOD master curve, which was constructed through CMOD and crack profile measurements, during the latter phase of fatigue crack extension. The fatigue data was then used to construct a da/dN versus ΔCTOA relation, similar to Paris' law of LEFM, in the presence of small to large crack tip deformation and net section yielding.

J0103-2-4 ポリシリコン薄膜の疲労挙動の評価(電子情報機器,電子デバイスの強度・信頼性評価と熱制御(2))

This paper presents the possibility of fatigue lifetime prediction of polycrystalline silicon films under cyclic loading. Fatigue crack extension process determining fatigue lifetime was formulated by the well-known Paris law with two unknown parameters. These parameters were fit to the results of the tensile fatigue tests performed on specimens with three different conditions of etching damage. The optimum values and the average values of them were applied to predict fatigue lifetime distribution, which showed that fatigue behavior of polysilicon can be approximately estimated with common values of two unknown parameters of the Paris law.

Seamless interpretation of the strength and fatigue lifetime of polycrystalline silicon thin films

Thin film polycrystalline silicon was subjected to strength and fatigue tests. A model is proposed which correlates the strength of this material under monotonic loading and its fatigue lifetime under cyclic loading in a seamless manner on a three-dimensional surface. This surface describes the cumulative failure probability as a function of both applied stress and fatigue cycles. In the first step, the statistical distribution of equivalent initial crack lengths corresponding to the defects introduced into the specimens in the etching process is deduced from the strength distribution of the specimens fabricated on a silicon wafer. From this distribution, the fatigue lifetime under different stress levels is estimated by applying the well-known Paris law of fatigue crack extension. The Paris law exponent determining the crack extension behavior as a function of the applied stress intensity factor was obtained by solving an inverse problem by fitting the estimated lifetime to the experimental results.

The problem of adequate yield load solutions in the context of proof tests on a damaged subsea umbilical

The residual lifetime and fitness for service of a pre-damaged tube of a subsea umbilical was determined. The tube had experienced large ovalisation and re-rounding by applying a boost pressure prior to service. The aim of this paper was to describe and to critically discuss the methodology of the fracture mechanics analysis required in addition to leak tightness testing. Special emphasis was put on the reversal of conservatism between the two steps: (a) Determination of the maximum crack size in the component which just would not have caused component failure under the boost loading applied for re-rounding the tubes, and (b) Prediction of the residual lifetime of the component under in-service loading assuming the existence of the maximum possible crack size of step (a). A number of yield load solutions for axial cracks in pressurised hollow cylinders was applied which yielded quite different results. The fatigue crack extension analyses were performed for various crack depth-crack length combinations which just would have survived the boost pressure. For both the simulation of crack propagation under service load and the prediction of the end-of-life state residual stresses due to the pre-history of the tube were considered. The information on these stresses was provided by finite element simulation.

A Generalization of the Half-Normal Distribution with Applications to Lifetime Data

A two-parameter family of lifetime distribution which is derived from a model for static fatigue is presented. This derivation follows from considerations of the relationship between static fatigue crack extension and the failure time of a certain specimen. The cumulative distribution function (cdf) of this new family is quite similar to the cdf of the half-normal distribution, and therefore this density is referred to as the generalized half-normal distribution (GHN). Furthermore, this GHN family is a special case of the three-parameter generalized gamma distribution. Even though the GHN distribution is a two-parameter distribution, the hazard rate function can form variety of shapes such as monotonically increasing, monotonically decreasing, and bathtub shapes. Some properties of this family are given, and examples are cited to compare with other commonly used failure time distributions such as Weibull, gamma, lognormal, and Birnbaum-Saunders.