Prediction Of Fatigue Crack Growth Rate Research Articles

Effects of manufacturing plastic prestrains found on calendered and UOE pipes and pressure vessels on structural integrity assessments regarding fatigue crack growth and LBB

High responsibility components operating under cyclic loading can have their resistance against initiation and growth of fatigue cracks highly influenced by previous thermomechanical processing. Within the interest of the present work, different manufacturing processes (such as the fabrication of pipes using UOE) or installation procedures (such as pipe reeling) can lead to nonuniform plastic prestrain fields thus affecting lifetime and safety. Previous studies conducted by the authors revealed that prestraining up to 14% considerably reduced fatigue crack growth rates (da/dN-ΔK) for a hot-rolled (1/2”) ASTM-A36 steel as a result of strain hardening and its effects on plastic zone sizes. In addition, results also revealed two interesting trends: i) the larger is the imposed prestrain, the greater is the growth rate reduction in a nonlinear asymptotic relationship; ii) the larger is imposed ΔK, the more pronounced is the effect of prestraining. Within this scenario, this work explores the effects of those results on more realistic lifetime predictions applicable to pipelines and pressure vessels. First, a nonlinear model based on the experimental results was developed to predict crack growth rates (da/dN) as a function of ΔK and varying plastic prestrain levels. Second, refined nonlinear FE models were developed to quantify plastic prestrain fields caused by manufacturing sequence of the studied pressure vessels and pipes. Such fields proved to be remarkably nonuniform, in accordance to the literature. Finally, considering an idealized cyclic internal pressure and the obtained fields, a MatLab algorithm was implemented to predict fatigue crack growth rates until LBB (Leak Before Break) condition takes place. Best practices for implementing such evaluations are presented and results revealed that effects of plastic prestrains are not negligible and should be taken into account on recommended practices and structural integrity assessments to avoid excessive conservatism or lack of safety in applications.

Modeling of temperature influence on the fatigue crack growth behavior of superalloys

Fatigue crack growth (FCG) rate is an important material parameter in structure safety and reliability assessment. As temperature T has remarkable effects on FCG behaviors of superalloys, it is in urgent need to establish an accurate FCG model which can predict FCG rates of superalloys at different temperatures. This work aims to model the temperature influence on FCG rate for superalloys. Firstly, systematic FCG experiments of a Ni-base superalloy at different temperatures are conducted to obtain the experimental data. Test results show that the FCG rate increases significantly with increase of temperature. Then, based on the observed linear relationship between the ratio of FCG rate to T and the stress intensity factor (SIF) range ΔK, an empirical FCG model called temperature exponential model, in which there are four material parameters C1, C2, C3 and C4, is built up by means of fitting approach. Two different methods, which are based on the FCG rates data and Paris model parameters respectively, are developed to acquire the four material parameters. At last, the temperature exponential model is validated against a variety of superalloys, proving its high accuracy and wide applicability.

Using the strip-yield mechanics to model fatigue crack growth by damage accumulation ahead of the crack tip

Elber found in the early 70s that fatigue cracks can close under tensile loads, and assumed that fatigue crack growth (FCG) would be controlled by ΔKeff=Kmax−Kop, where Kmax and Kop are the maximum and opening values of the stress intensity factor. This hypothesis can rationalize many transient effects observed under service loads, but it cannot explain many other effects like FCG retardation or arrest after overloads under high R=Kmin/Kmax, when Kmin>Kop; FCG at constant rates under highly variable ΔKeff; cracks arrested at a given R that can reinitiate to grow at a lower R without changing their ΔKeff; or the R-insensitivity of FCG in inert environments. Nevertheless, strip-yield models (SYM) based on ΔKeff ideas are more used for FCG life predictions than alternative models based on any other principles. To verify whether SYMs are indeed intrinsically better, their mechanics is used to predict FCG rates based both on Elber’s ideas and on the alternative view that FCG is instead due to damage accumulation ahead of the crack tip, which does not need the ΔKeff hypothesis or arbitrary data-fitting parameters. Despite based on conflicting principles, both models can reproduce quite well FCG data obtained under quasi-constant ΔK loading, a somewhat surprising result that deserves to be carefully analyzed.

Evaluation of three current methods for including the mean stress effect in fatigue crack growth rate prediction

AbstractThe fatigue crack growth rates curves of engineering materials depend on two parameters. In addition to the dependence on the classical stress intensity factor (SIF) range ΔK, there is a dependence on the mean load (or mean SIF), mainly in the near‐threshold region. The present paper provides some useful suggestions and good practices for using three of the current available methods to reduce this second dependence through the use of tuning constants. The methods considered here are the Elber, Walker and Vasudevan (or unified approach). For each approach, multiple regression analyses are performed on experimental data from the literature, and the correlations in two and three dimensions are graphically analyzed. Numerical examples of crack growth analysis for cracks growing under nominal stresses of constant amplitude in single‐edge and notch/hole geometries are performed, assuming an identical material component to that of the available experimental data. The resulting curves of crack size versus number of cycles (a versus N) are then compared. All three models gave approximately the same (a versus N) curves in both geometries. Differences between the behaviors of the (a versus N) curves in both geometries are highlighted, and the reasons for these particular behaviors are discussed.

Fatigue modelling of aluminium plates reinforced with bonded fibre metal laminates

Purpose – This paper aims to present the implementation of a finite element (FE) model used to establish crack and delamination development in a Glare reinforced aluminium plate under fatigue loading. This model predicts the behaviour of bonded GLARE straps used as crack retarders for life extension of aircraft structures. In particular, it takes into account the interaction that exists between the substrate crack and the delamination crack at the interface with the reinforcement. Design/methodology/approach – In this work, a 3D FE model with three-layer continuum shell elements has been developed to calculate changes in substrate stress intensity and in fatigue crack growth (FCG) rate produced by bonded strap reinforcement. Both circular and elliptical strap delamination geometries were incorporated into the model. Calculated stress intensity factors (SIFs) were used together with measured FCG data for substrate material to predict FCG rates for the strapped condition. Findings – The model predicted a decrease in the SIF and a retardation of FCG rates. The SIF was predicted to vary through the thickness of the substrate due to the phenomenon of secondary bending and also the bridging effect caused by the presence of the strap. The influence of delamination shape and size on substrate crack stress intensity and delamination strain energy release rate has been calculated. Originality/value – This research aims at developing modelling techniques that could be used when studying larger reinforced structures found in aircraft.

A new method to predict fatigue crack growth rate of materials based on average cyclic plasticity strain damage accumulation

By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren (HRR) field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory—Miner law, a new model for predicting the fatigue crack growth (FCG) of the opening mode crack based on the low cycle fatigue (LCF) damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What’s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.

An improved constitutive model to predict fatigue crack growth rate under constant-amplitude loading with single and multiple overload

Fatigue crack growth under variable-amplitude loading is usually accompanied by the load interaction effect. One of the significant phenomena of load interaction effects is retardation induced by an overload or multiple overloads. To predict the fatigue crack growth rate under constant-amplitude loading with single and multiple overload, an improved constitutive model based on partial crack closure due to crack-tip plasticity is proposed in this paper. In this model, both the maximum stress intensity factor at crack opening level and the threshold value of effective stress intensity factor range are expressed as the explicit functions of stress ratio and the threshold stress intensity factor range when stress ratio is zero. It is assumed in the improved constitutive model that the crack closure level can rise due to a larger plastic zone resulting from the overload effect, and a modified coefficient based on the Wheeler model is introduced to correct the amount of the maximum stress intensity factor at crack opening level during the recovering period after an overload. By a cycle-by-cycle integration procedure, the fatigue crack growth rate under constant-amplitude loading with single and multiple overload is predicted quantitatively with this model. Comparison is made between the results of analytical predictions and experimental data and good agreements are found. This indicates that the proposed constitutive model is able to explain the load interaction effect under variable-amplitude loading.

Finite element computation of fatigue growth rates for mode I cracks subjected to welding residual stresses

This paper presents a finite element modeling procedure for predicting fatigue crack growth rate in butt welds subject to mode I loading condition. Sequentially coupled three-dimensional thermal–mechanical finite element model to simulate welding residual stress was first developed. The weld-induced residual stress effect on the fatigue crack growth rate was then modeled by calculating the stress intensity factor due to the residual stress field based on the superposition rule of the linear elastic fracture mechanics. The results demonstrated the significance of the residual stresses in assessment of the fatigue crack growth rate in the welds.

Alloy induced inhibition of fatigue crack growth in age-hardenable Al–Cu Alloys

Environmental fatigue crack propagation (EFCP) is alloy-inhibited in under aged Al–Cu–Mg and peak aged Al–Cu–Li alloys stressed in pure aqueous chloride solution. Counter to H diffusion and H-embrittlement rate limited step considerations which predict fatigue crack growth rate (da/dN) to be either independent of fatigue loading frequency (f) or increase with decreasing f, da/dN declines with decreasing f. The mechanism for such alloy induced inhibition and decreasing da/dN with decreasing f is reduced crack tip H production and uptake due to stabilization of the native aluminum passive film resulting from: (1) dissolution of anodic Cu-containing GP zones or precipitates (Al2CuLi or Al2CuMg) by dealloying, (2) crack surface Cu enrichment, and (3) enhanced crack wake cathodic reaction kinetics on Cu enriched sites that increase crack solution pH. Peak aged Al–Zn–Mg–Cu alloy 7075 does not exhibit alloy induced inhibition because the predominant anodic phase, Mg(Cu,Zn)2, does not provide a source for Cu surface enrichment. Alloy induced inhibition is similar to ion-assisted inhibition by molybdate or chromate addition into bulk chloride solution which provides an alternate path to stabilize a crack tip passive film. Both alloy-induced and ion-assisted inhibition of EFCP are promoted by reduced f and crack tip strain rate which favor repassivation of the crack tip passive film over film rupture.

Comparison of the simulation and experimental fatigue crack behaviors in the nanoseconds laser shocked aluminum alloy

This investigation was performed to compare the simulation and experimental results of the fatigue crack growth rates and behaviors of the 7050-T7451 aluminum alloy by nanoseconds laser shock processing (LSP). Forman–Newman–deKoning (FNK) model embedded in the Franc2D/L software was utilized to predict fatigue crack growth rate, which was conducted to weigh the stress intensity factor (SIF) changing on the surface cracks. LSP induced high compressive residual stresses that served to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation. The circulating times of crack growth obtained from the simulation and experimental values indicated a slower fatigue crack growth rates after LSP. The relationships between the elastic–plastic materials crack growth rates and the SIF changing after LSP are resolved.

Analysis of Material Properties for the Numerical Simulation of Fatigue Crack Growth under Variable Amplitude Loading

The numerical simulation of crack closure is employed to assist on the prediction of crack growth rate. Under fatigue load, the stress-strain response of metals is altered due to cyclic loading. For this reason, the material properties characterization is of prime concern as an input parameter to obtain reliable results. From numerical simulations, it was observed that simple material models do not provide accurate data for long crack lengths. In this paper, the effect that different hardening models have on the opening response of a cracked component when it is subject to variable amplitude loading is analyzed. The interaction effects (crack arrest/acceleration) for long crack length simulation are specially highlighted. For this purpose, a 6082-T6 aluminium alloy was analyzed experimentally and numerically in order to measure crack closure, and then, those data were used to predict fatigue crack growth rate under different patterns of overload. The Paris equation and the Elber crack closure concept were employed. The results showed that small variations in the opening stresses obtained from different material models produce high overestimated simulations of crack growth rate. Also, it was proved that the crack closure mechanism is able to take into account interaction effects due to variable amplitude loading.

Deterministic Approach to Predicting the Fatigue Crack Growth in the 2024-T3 Aluminum Alloy Under Variable Amplitude Loading

Deterministic Approach to Predicting the Fatigue Crack Growth in the 2024-T3 Aluminum Alloy Under Variable Amplitude LoadingThe paper presents the attempt to predict fatigue crack growth rate in a component subjected to variable amplitude loading containing overload-underload cycles. For this goal in a deterministic approach the modified Willenborg retardation model was applied. To provide experimental data the research into fatigue crack growth for 2024-T3 aluminum alloy sheet CCT specimens under LHL type block program loading with multiple overload-underload cycles was developed. The microfractographic analysis of fatigue fractures with the use of the transmission electron microscope (TEM) made it possible to trace the effect of block program loading on the crack growth rate. The knowledge of the affection of a particular overload-underload cycle or a block of these cycles on crack rate on the basis of microfractographic analysis was utilized for assessing the equivalent loading for the LHL block program. The diagrams that presented the crack growth rate both on the surface and inside the aluminum alloy sheet was performed. The crack growth rate inside the sheet was estimated on the basis of the striation spacing analysis.

Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect

A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (Reff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved.

Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds

In this work, results of a study conducted on fatigue crack propagation in friction stir welded AA2050 and the effects of FSW induced residual stresses, as well as changes in the microstructure, are presented. Longitudinal residual stress profiles across butt welded 2050 plates were determined using the cut compliance technique and fatigue crack growth testing was conducted on compact tension specimens machined from the friction stir welds. Tests were performed with the crack propagating nominally perpendicular to the weld and with a constant, applied, cyclic, stress intensity factor. Two different material tempers were investigated and in both cases residual stresses were found to have a major effect on the fatigue crack propagation. It was shown that compressive residual stresses are present in the vicinity of the weld, leading to crack closure and a decrease in the fatigue crack growth rate as the crack approaches the weld. Once in the weld nugget, the crack propagation rate increases. This increase is believed to be linked in part to tensile residual stresses but also to a microstructural effect present when the crack grows through the recrystallized nugget. It was also observed that other closure mechanisms, such as oxide-induced closure may affect the fatigue behavior of the 2050 FSW’s. Two methods to predict fatigue crack growth rates in the weld zones were attempted: using the residual stress profiles determined by cut compliance and using the effective stress intensity factor, Δ K eff, obtained during fatigue crack growth testing.

A new method for predicting fatigue crack propagation rates

A method has been developed for predicting fatigue crack growth rates in Ti 6246 specimens based on a damage accumulation driving force for crack propagation. Cracks were assumed to propagate via a process of quasi-static incremental steps through the crack tip damage zone. The number of cycles to reach a critical level of damage in the process zone ahead of a crack of any given length was determined based on finite element analysis and through correlation with the measured fatigue response of smooth specimens tested under strain-control. By summing up the individual micro-increments in crack growth, predictions were made for the relationship between fatigue crack growth rate and Δ K, the stress intensity range. Model predictions were assessed against experimental crack growth rate data obtained on beta forged Ti 6246 alloy. An approach based on a crack step micro-increment size equivalent to the cyclic plastic zone size was found to predict the general trend and magnitude for stage II crack growth rate in this alloy. The ability of the method to account for load ratio effects is assessed and discussed with reference to K max and Δ K dependency of the crack growth increment.

Prediction of fatigue crack growth rate in welded tubular joints using neural network

In the past, several methods have been proposed to predict fatigue crack growth rate in tubular joints of offshore structures, however reasonably accurate solution for this problem is still lacking. Dramatic increase in the use of neural neural networks (NN) in material science, specially fatigue area, inspired an investigation on the application of NN in estimating fatigue crack growth rates and stress intensity factors in tubular joints. In this research, four MLP networks are developed to predict weld magnification factor for weld toe cracks in T-butt joints under membrane and bending loading. The training data for these networks are obtained from results of finite element modeling. In addition, two types of neural networks, i.e. MLP and RBF are developed to predict stress intensity modification factors for deepest point of fatigue cracks in tubular T-joint, under axial loading. Experimental data are used to train these networks. The results of above mentioned networks are used to predict fatigue lives of tubular T-joints. The comparison between network results and fatigue lives reported in experiments shows that NN is a successful prediction technique if properly used in this area.

Application of Probabilistic Fracture Mechanics in Structural Design of Magnet Component Parts Operating Under Cyclic Loads at Cryogenic Temperatures

Abstract This paper presents a methodology for the use of probabilistic fracture mechanics concepts to estimate the design fatigue life and reliability of structural materials used in superconducting coils and magnet intercoil components of the International Thermonuclear Experimental Reactor (ITER). The orbitally welded conductor jackets of superconducting coils are subjected to fluctuating tensile stresses. The paper uses fatigue crack growth (FCGR) data at 7K from type 316LN stainless steel, in parent and weld material conditions, to predict fatigue crack growth rates using Monte Carlo analysis. Two stages are considered in which scatter could be quantified in the analysis. Initially, the unknown in crack length is taken as a stochastic variable. Secondly, the model assumes that the scatter observed in the correlation of the FCGR data versus ΔK is directly due to factors such as testing methods, measurement, material, and geometric variability and can therefore be quantified statistically. Either a normal or lognormal distribution of the scatter is assumed depending on the parameter. The purpose of this approach is to illustrate some of the advantages over a deterministic approach that most design codes use. An example is presented that compares the design life of a conductor jacket section containing a single crack and multiple cracks. A comparison of a sensitivity analysis for multiple cracks, for failure times at probabilities of 1 % and 0.001 %, with times calculated from a deterministic analysis using appropriate safety factors, suggests that the deterministic analysis give less conservative failure times.

Prediction of elevated temperature fatigue crack growth rates in TI-6AL-4V alloy – neural network approach

Abstract The results obtained from two experimental test programs (TP-1 and TP-2) were used to train neural networks to predict elevated temperature, fatigue crack growth rates in Ti-6Al-4V alloy. Two programs, TP-1 and TP-2, were conducted at room and elevated temperatures under high humidity and laboratory air environments, respectively. While elevated temperature effects were investigated in TP-2, stress ratio effects were studied in TP-1 using several stress ratios. Networks were trained using the elevated temperature data to predict the crack growth rates at a given stress intensity under different temperatures. The experimental and predicted fatigue crack growth rates showed a least squared error of 0.03. Thus, this approach was found to predict fatigue crack growth rates in Ti-6Al-4V alloy at elevated temperatures.

A Study on Fatigue Crack Growth and Life Modelling using Backpropagation Neural Networks.

Fatigue crack growth and life is estimated by various fracture mechanical parameters but affected by load, material and environment. Fatigue character of component without surface notch cannot be e valuated by above-mentioned parameters due to microstructure of in-service material. Single fracture mechanical parameter or nondestructive parameter cannot predict fatigue damage in arbitrary boundary condition but multiple fracture mechanical parameters or nondestructive parameters can Fatigue crack growth modelling with three point representation scheme uses this merit but has limit on real-time monitoring. Therefore, this study shows fatigue damage model using backpropagatior. neural networks on the basis of X-ray half breadth ratio B/ fractal dimension and fracture mechanical parameters can predict fatigue crack growth rate da/dN and cycle ratioN/ at the same time within engineering estimated mean error(5%).

Closure‐free biaxial fatigue crack growth rate and life prediction under various biaxiality ratios in SAE 1045 steel

Biaxial fatigue tests were performed on thin‐walled tubular 1045 steel specimens in a test fixture that applied internal and external pressure and axial load. There were two test series, one in which constant amplitude fully reversed strains (CAS) were applied and another in which large periodic compressive overstrain (PCO) cycles causing strains normal to the crack plane were inserted in a constant amplitude history of smaller strain cycles. Ratios of hoop strain to axial strain of λ = −1, −0.625, −ν and +1 were used in each test series. Fatigue crack growth behaviours under CAS and PCO histories were compared, and revealed that the morphology of the fracture surface near the crack tip and the crack growth rate changed dramatically with the application of the compressive overstrains. When the magnitude of the compressive overstrains was increased, the height of the fracture surface irregularities was reduced as the increasing overstrain progressively flattened the fracture surface asperities near the crack tip. The reduced asperity height was accompanied by drastic increases in crack growth rate and decreases in fatigue life.Using a pressurizing device attached to the confocal scanning laser microscope (CSLM), crack opening measurements were obtained. Crack opening measurements showed that the biaxial cracks were fully open at zero internal pressure for block strain histories containing in‐phase PCO cycles of yield stress magnitude. Therefore, for the shear‐strained samples, there was no crack face interference and the strain intensity range was fully effective. For PCO tests (with biaxial strain ratios of −0.625 and +1), effective strain intensity data were obtained from tests with positive stress ratios for which cracks did not close. A number of strain intensity parameters derived from well‐known fatigue life parameters were used to correlate fatigue crack growth rates for the various strain ratios investigated. Predicted fatigue lifetimes based on a fatigue crack growth rate prediction program using critical shear plane parameters showed good agreement with the experimental fatigue life data.