Prediction of fatigue crack nucleation life in polycrystalline AA7075‐T651 using energy approach

A crystal plasticity (CP) simulation and an energy‐based model is presented to predict the fatigue nucleation onset for polycrystalline AA 7075‐T651. Different microstructure morphology and grain sizes are employed in the simulations. Using a simple method, statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) as decoupled with crystal plasticity model are estimated using a double round‐notch specimen test data, and CP simulation. The dislocation density parameter approximated from plastic energy density, stored energy density, elastic energy and accumulated slip validated with double hole experimental data. Sensitivity analysis is performed with respect to different microstructures and dislocation density parameters. Roughly, maximum 30% difference between experimental nucleation life and the simulated one is observed. The simulated predictions are in fair agreement with test data. The proposed strategy is suitable to study the scatter of fatigue nucleation life.     

Determination of lattice level energy efficiency for fatigue crack initiation

Fatigue crack initiation life prediction is a fundamentally challenging problem that is of prime importance as a significant portion of the fatigue life is spent in the initiation phase. In spite of the extensive efforts of research over the past two decades, the concept of crack initiation still remains as an enigma in science. The major challenges in predicting crack initiation life in industry are the evaluation of the crack initiation parameters such as the maximum resolved shear stress range, maximum slip band width and the energy efficiency coefficient. In this paper, we show that the energy efficiency can be successfully estimated with good accuracy by performing lattice level crystal plasticity‐based computational simulations on representative models. The lattice level plasticity‐based finite element computations are reported for the case of single crystal copper in this work, and the results show that this strategy leads to higher accuracy than the existing idea of approximating the efficiency factor. The results show that this strategy could be of great use in improving the reliability in prediction of crack initiation life. The effectiveness of this computational procedure would greatly reduce the financial investments necessary to perform experimental analysis of all structures to determine the crack initiation parameters, as it would require just a single measurement to quantify the measurement of efficiency.     

Physics‐based multiscale damage criterion for fatigue crack prediction in aluminium alloy

In this paper, a physics‐based multiscale approach is introduced to predict the fatigue life of crystalline metallic materials. An energy‐based and slip‐based damage criterion is developed to model two important stages of fatigue crack initiation: the nucleation and the coalescence of microcracks. At the microscale, a damage index is developed on the basis of plastic strain energy to represent the growing rate of a nucleated microcrack. A statistical volume element model with high computational efficiency is developed at the mesoscale to represent the microstructure of the material. Also, the formation of a major crack is captured by a coalescence criterion at mesoscale. At the macroscale, a finite element analysis of selected test articles including lug joint and cruciform is conducted with the statistical volume element model bridging two scale meshes. A comparison between experimental and simulation results shows that the multiscale damage criterion is capable of capturing crack initiation and predicting fatigue life.     

A procedure for estimating the total life in fretting fatigue

ABSTRACT This paper proposes a procedure for estimating the total fatigue life in fretting fatigue. It separately analyses the fatigue crack initiation and propagation lives. The correlation between crack initiation and propagation is made considering a non‐arbitrary crack initiation length provided by the model. The number of cycles to initiate a crack is obtained from the stress distribution beneath the contact zone and a multiaxial fatigue crack initiation criterion. The propagation of the crack is considered using different fatigue crack propagation laws, including some modifications in order to take the short crack growth into account. The results obtained by this method are compared with the fatigue lives obtained in various fretting fatigue tests under spherical contact with 7075‐T6 aluminium alloy.     

Thermodynamic entropy generation in the course of the fatigue crack initiation

Evolution of the thermodynamic entropy generation during fatigue crack initiation life of notched specimens is studied. A set of experimental results of AA7075‐T651 is examined to determine applicability of the thermodynamic entropy generation as an index of fatigue crack initiation. Entropy accumulation is calculated from hysteresis energy and temperature rise. An increasing trend of entropy accumulation with the number of cycle to failure is observed on macroscale measurements. Results also determine that the entropy generations from the samples under the same operating conditions are similar as the crack grows. Scanning electron microscope analysis is performed on the fractured surfaces to observe the fatigue striations, and persistent slip bands are observed employing an optical microscope. A discussion is presented regarding the length scales on which crack initiation occurs and entropy calculation is made.     

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

This paper presents the results of experimental investigation on fatigue behaviors of friction stir welded joints in AA7075-T6 with ultrasonic fatigue test system (20 kHz). Two kinds of particles, Fe-rich intermetallic compounds and Mg₂Si-based particles, governed the fatigue crack initiation. The plastic deformation and recrystallization during welding process led to the changes in particle size and micro crack occurrence between thermo-mechanically affected zone (TMAZ) and nugget zone (NZ). Therefore, the fatigue crack initiation sites leaned to be located at the TMAZ in short fatigue life, or at the NZ in very high cycle fatigue regime.     

Improvement of very high cycle fatigue properties in an AA7075 friction stir welded joint by ultrasonic peening treatment

The present paper aims to investigate the effect of ultrasonic peening treatment on the very high cycle fatigue resistance of an AA7075 friction stir welded joint. Microscopy observation, microhardness and X‐ray diffraction measurements were carried out to characterize the treated surface of peened specimens. Fatigue crack initiation sites were investigated through scanning electron microscope, and the role of enhanced surface on fatigue resistance was analyzed. The results indicate that a sensible fatigue strength improvement can be obtained through application of ultrasonic peening treatment and that fatigue cracks can initiate from the interior of the specimen. To clarify the fatigue failure mechanism, we analyzed the microstructure characteristics, compressive residual stress profile and intermetallic inclusion distribution in the surface layers, and we discussed the capability of ultrasonic peening treatment to hinder the surface crack initiation.     

Verhalten gekerbter Biegeproben aus der Aluminiumlegierung AA7075 im Kurzzeitermüdungsbereich

Prediction on Fatigue Life of Notched Specimens under Cyclic Bending Loading Pulsating 3P‐bending fatigue tests are conducted on edge‐notched specimens of AA7075. Measurements of electrical potential drop across notches were used to determine the number of cycles up to crack initiation. Cyclic material data determined from strain–controlled constant amplitude loading are use in FE‐analyses to the determination time functions of the local stresses and strains at the notch root using non‐linear material model according to Chaboche and Lemaitre. Using these FE computations, the fatigue life is predicted by the equivalent strain approach of the “ASME Boiler and Pressure Vessel Code” and compared with the results of the plastic strain energy approach. It is found that both approaches lead to relatively good predictions.     

Fatigue life prediction based on an equivalent initial flaw size approach and a new normalized fatigue crack growth model

A general procedure for fatigue life prediction of structural details based on Fracture Mechanics approach is presented in this paper, taking advantage of the new normalized fatigue crack growth model proposed by Castillo et al., here denoted as CCS model. An extension to the CCS model is proposed by adopting the cyclic J-integral range instead of the stress intensity factor range as reference parameter. This enables the generalized elastoplastic conditions resulting for the cracked geometry of the structural detail to be considered by means of the cyclic J-integral values obtained from a finite element analysis, for different loading levels and crack lengths. As a practical application, the proposed approach is applied to a notched plate made of P355NL1 steel, using the equivalent initial flaw size (EIFS) concept. Fatigue crack growth data for CT specimens from the literature is evaluated to estimate the modified CCS crack growth model parameters. The predicted fatigue propagation lifetime prediction is compared with the results and, finally, the goodness of the predictions is analysed and deviations discussed.     

Life prediction by ferrite–pearlite microstructural simulation of short fatigue cracks at high temperature

This paper dealt with fatigue behavior simulation based on ferrite–pearlite microstructure modeled by correctional Voronoi-polygons. The model took grain size, grain orientation and the percentage of pearlite and ferrite into consideration. The basal energy was proposed to represent the inherent energy for slip-band and grain boundary to cracking. The driving force for crack initiation and propagation caused by load condition was considered as the energy increment of slip-band and grain boundary. The fatigue behavior including crack initiation, propagation, coalescence and interference were simulated based on Monte Carlo method. The simulation results show a satisfying agreement with the experimental ones.     

Investigation of microstructure effect on fretting fatigue crack initiation using crystal plasticity

The onset of fretting fatigue is characterized by material microstructural changes in which the extent of the damage is comparable to grain size, and hence, the microstructure characteristics could have a significant effect on fatigue crack initiation. In this paper, a three‐dimensional finite element crystal plasticity framework is presented for simulation of the fretting fatigue. Controlled Poisson Voronoi tessellation (CPVT) method is employed to generate the polycrystalline region. In the CPVT method, regularity parameter controls the shape of grains. In this study, the impact of grain size and regularity parameter on crack initiation life and initiation site has been investigated. Cumulative plastic slip was used as a parameter of microstructure‐sensitive fatigue indicator. This parameter could effectively predict the location of crack initiation and its life. The results show that regularity parameter has a significant effect on the location of crack initiation. Furthermore, the effect of grain size on the fretting fatigue life of 316L stainless steel was investigated experimentally through testing different specimens with different grain sizes, to validate the simulation results.     

Observations and modeling of the small fatigue crack behavior of an extruded AZ61 magnesium alloy

The objective of this paper is to quantify the microstructurally small fatigue crack growth of an extruded AZ61 magnesium alloy. Fully reversed and interrupted load-controlled tests were conducted on notched specimens that were taken from the material in the longitudinal and transverse orientations with respect to the extrusion direction. In order to measure crack growth, replicas of the notch surface were made using a dual-step silicon-rubber compound at periodic cyclic intervals. By using microscopic analysis of the replica surfaces, crack initiation sites from numerous locations and crack growth rates were determined. A marked acceleration/deceleration was observed to occur in cracks of smaller length scales due to local microheterogeneities consistent with prior observations of small fatigue crack interaction with the native microstructure and texture. Finally, a microstructure-sensitive multistage fatigue model was employed to estimate the observed crack growth behavior and fatigue life with respect to the microstructure with the most notable item being the grain orientation. The crack growth rate and fatigue life estimates are shown to compare well to published findings for pure magnesium single crystal atomistic simulations.     

Mixed-mode fatigue crack growth under biaxial loading

Studies on crack growth in a panel with an inclined crack subjected to biaxial tensile fatigue loading are presented. The strain energy density factor approach is used to characterize the fatigue crack growth. The crack growth trajectory as a function of the initial crack angle and the biaxiality ratio is also predicted. The analysis is applied to 7075-T6 aluminium alloy to predict the dependence of crack growth rate on the crack angle. The effect of crack angle on the cyclic life of the component and on the cyclic life ratio is presented and discussed.     

On the expression of fatigue crack initiation life considering the factor of overloading effect

In the present study attempts are made to give an expression of the fatigue crack initiation life of notched elements with the consideration of overloading effects. This expression may be used to predict the fatigue crack initiation life of notched element under variable amplitude loading. Experimental work on LY12CZ alloy show that the test results of fatigue crack initiation life after tension overloading can be well fitted by the formula developed before for fatigue crack initiation life. Tension overloading increases the fatigue crack initiation threshold but has no effect on the coefficient of the resistance to fatigue crack initiation. The overloading ratio has no markable effect on crack initiation life. The increase of the crack initiation threshold results in the increase of crack initiation life, in particular, in long life range. The same results are also obtained by reanalysing some existing test results of overloading effect on crack initiation life given in literature. Consequently, the expression of the fatigue crack initiation life can be obtained by the method given in this paper. However, the overloading stress should be determined from the theoretical stress concentration factor of notched element and the maximum nominal stress in the load spectrum of elements.     

Fatigue behaviour prediction of steel reinforcement bars using an adapted Navarro and De Los Rios model

Fatigue cracks tend to initiate on the rebar surface and therefore, the surface conditions may control their fatigue behaviour. This study investigates the influence of surface microstructure and roughness dispersion on the scatter and fatigue life of hot rolled (HR)–cold worked (CW) and quenched and self-tempered (QST) rebars. The stochastic nature of the fatigue life is mainly affected by the scatter of short cracks in the crack initiation phase. A model adapted from Navarro and De Los Rios (N–R) was developed to predict the crack initiation, including short crack growth, and long crack propagation phases. The crack initiation phase includes the dispersion inherent to the grain size, grain orientation ratio and multiple phases i.e., ferrite–pearlite and martensite as well as the roughness dispersion determined on the rebar surface and the influence of the rib geometry. The stress concentration factor due to the rib geometry was considered as a constant parameter. In the long crack propagation phase, all microstructural features are considered as constants. The model results were compared to experimental data from the literature.     

A modification of UniGrow 2‐parameter driving force model for short fatigue crack growth

In this paper, a modification of the UniGrow model is proposed to predict total fatigue life with the presence of a short fatigue crack by incorporating short crack propagation into the UniGrow crack growth model. The UniGrow model is modified by 2 different methods, namely the “short crack stress intensity correction method” and the “short crack data‐fitting method” to estimate the total fatigue life including both short and long fatigue crack propagations. Predicted fatigue lives obtained from these 2 methods were compared with experimental data sets of 2024‐T3, 7075‐T56 aluminium alloys, and Ti‐6Al‐4V titanium alloy. Two proposed methods have shown good fatigue life predictions at relatively high maximum stresses; however, they provide conservative fatigue life predictions at lower stresses corresponding high cycle fatigue lives where short crack behaviour dominates total fatigue life at lower stress levels.     

A multiparameter approach to the prediction of fatigue crack growth in metallic materials

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, d a /d N , as functions of multiple variables, including stress intensity factor range, Δ K , stress ratio, R , crack closure stress intensity factor, K _cl , the maximum stress intensity factor K _max , nominal specimen thickness, t , frequency, Ω , and temperature, T . A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY-80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.     

A low-cycle fatigue life prediction model of ultrafine-grained metals

A (high strain) low‐cycle fatigue (LCF) life prediction model of ultrafine‐grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two‐phase ‘composite’ consisting of the ‘soft’ matrix (all the grain interiors) and the ‘hard’ reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue‐damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high‐strain LCF conditions, the fatigue‐damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding‐off process within it. The fatigue crack growth rate is directly correlated to the range of the crack‐tip opening displacement. The empirical Coffin–Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high‐cycle fatigue (HCF) regime.     

Experimental and numerical investigation of fatigue of thin tensile specimen

This paper presents the results of experimental and numerical investigation on fatigue of thin 304 stainless steel tensile specimens. In order to achieve the experimental aspects of this investigation a Micro Fatigue Test Rig (MFTR) was designed and developed to evaluate fatigue life and failure mechanism of tensile specimen. A 3D finite element model was also developed to investigate the fatigue damage of thin tensile specimen and to account for the effects of topological randomness of material microstructure on fatigue lives. The topology of the material grain structure was modeled using randomly generated 3D Voronoi tessellations corresponding to the measured grain size. Continuum damage mechanics was used to model the progressive material degradation. The damage parameters were obtained from the experimentally obtained S–N curve. A 3D mesh partitioning procedure was developed to consider both crack initiation and propagation stages considering the predominant transgranular, non-planar crack growth observed in the experiments. The stress–life results obtained from the fatigue damage model are in good agreement with the experimental data. The progression of damage and the proportion of life spent in crack initiation obtained from the model are consistent with empirical observations. The fatigue damage model was used to assess the influence of microstructure randomness accompanied by material inhomogeneity and internal voids on fatigue life dispersion.     

Three-dimensional finite element analysis using crystal plasticity for a parameter study of microstructurally small fatigue crack growth in a AA7075 aluminum alloy

Three-dimensional finite element analysis using a crystal plasticity constitutive theory was performed to understand and quantify various parametric effects on microstructurally small fatigue crack growth in a AA7075 aluminum alloy. Plasticity-induced crack opening stresses (S_o/S_max) were computed, and from these results the crack propagation life N was obtained. A design of experiments (DOE) technique was used to study the influences of seven parameters (maximum load, load ratio, particle modulus, the number of initially active slip systems, misorientation angle, particle aspect ratio, and the normalized particle size) on fatigue crack growth. The simulations clearly showed that the load ratio is the most influential parameter on crack growth. The next most influential parameters are maximum load and the number of initially active slip systems. The particle modulus, misorientation angle, particle aspect ratio, and the normalized particle size showed less influence on crack growth. Another important discovery in this study revealed that the particles were more important than the grain boundaries for inducing resistance for microstructurally small fatigue crack growth.