A unified description of micro and macroscopic fatigue crack behaviour

In this paper, the importance of crack front length as a factor controlling growth rate is emphasized. It is shown that fatigue cracks in aluminium alloys do not advance in a coherent manner, but the front is divided into sectors, each of which relates to an individual cracking element. These elements act with some degree of mechanical isolation from their neighbours, and such an arrangement leads to crack front fragmentation and to an increase in the real crack front length.Even on this microscopic scale the crack segments extend as though in a continuum, and, certainly at low stress intensity factors, crack paths are dictated by the local stress direction.     

Successive 3D FE analysis technique for characterization of fatigue crack growth behavior in composite-repaired aluminum plate

An analytical approach using successive finite element analysis technique was conducted to characterize the fatigue crack growth behavior of pre-cracked aluminum plates reinforced with composite patches. For single-sided repairs, due to the asymmetry and the presence of out-of-plane bending, crack front shape would become skewed curvilinear started from a uniform through-crack profile, as observed from previous studies. As the stress intensity factor (SIF) calculated at the crack tip is much influenced by crack front shape, it is necessary to predict the actual crack front shape evolution and take it into account for the accurate analysis of fatigue behavior. Present procedure performed a three-dimensional geometrically nonlinear finite element analysis to determine the SIF distribution at a set of points along the crack front, and then estimated the crack growth increments at these points by invoking a fatigue crack growth rate relationship (power-law relationship). A new crack front was then established for the next step by using a relevant remeshing scheme. Through conducting this procedure successively, the crack path of the patched plate as well as the fatigue life was evaluated with sufficient accuracy. The analytical predictions of both the crack front shape evolution and the fatigue life were in good agreement with the experimental observations.     

3D characterisation of the nucleation of a short fatigue crack at a pore in a cast Al alloy using high resolution synchrotron microtomography

In situ X-ray microtomography is used to visualize the nucleation and growth of a fatigue crack in the bulk of a cast Al alloy. At the end of the fatigue test, the three-dimensional grain structure is revealed by means of a Gallium infiltration technique. The simultaneous visualization of the crack with the surrounding grain structure allows one to correlate irregularities in the crack front shape with the local microstructure of the material.     

On the through-thickness crack with a curve front in center-cracked tension specimens

Three-dimensional finite element analyses are performed on through-thickness cracks with slightly wavy front in center-cracked plates. Considering there is an inherent relationship between the crack shape and the corresponding stress intensity factor (SIF) distribution of a crack, the curved configuration of the crack is determined using a heuristically derived iterative procedure if the SIF distribution function is known. Several simple SIF distribution functions, for instance the constant SIF distribution along the crack front, are assumed to determine the crack shape. Under the assumption that the rate of fatigue crack growth depends on the SIF range or the effective SIF range, possible effects of plate thickness, crack length and crack closure level gradient on the behaviour of crack tunneling are investigated. The stability of the curved shape of a through-thickness crack in fatigue is also discussed, i.e. whether a crack can maintain its shape satisfying the conditions of constant SIF distribution or other distribution along the crack front during fatigue growth. This study will be useful for a better understanding of the behaviour of crack tunneling and help to evaluate the validity of the two-dimensional linear elastic fracture mechanics in cracked plates.     

城轨列车制动盘SiC_(p)/A356复合材料热疲劳裂纹扩展机理EI北大核心CSCD

为研究制动盘服役温度载荷及材料微结构对SiC_(p)/A356复合材料热疲劳裂纹扩展行为的影响,明确其热疲劳裂纹扩展微观机理,开展SiC_(p)/A356复合材料热疲劳裂纹扩展实验。结果表明:裂纹扩展过程包括由SiC颗粒偏转作用和二次裂纹释放扩展驱动力导致的缓慢扩展阶段和主裂纹与裂纹扩展前端微损伤连接的快速扩展阶段;加热温度较低时,裂纹扩展的“台阶状”特征明显,整体扩展速率较低,裂纹宽度较小,裂纹扩展方式为颗粒断裂、轻量基体撕裂和沿界面开裂;加热温度较高时,“斜直线跃升”阶段更为明显,裂纹宽度较大且扩展速率较高,裂纹扩展以颗粒脱落以及大幅度基体撕裂为主;主裂纹总是通过选择沿SiC颗粒群或者直接穿过α-Al基体以阻力较小的方式向前扩展,Si相承载时极易发生断裂,成为裂纹扩展源,同时裂纹扩展前端的微损伤对其扩展具有引导作用。     

Prediction of shape change of corner crack by fatigue crack growth circles

As a fatigue crack grows, its shape changes. Conventional method for predicting the shape change typically requires the computation of several hundred increments to get accurate results. In this paper, a new finite element simulation technique that uses fatigue crack growth circles is developed. Since the circles are perpendicular to the new crack front as well as to the current crack front, they can represent the real path of the fatigue crack well and yield more accurate results. The new technique is validated by applying it to the case of a quarter-elliptical corner crack in a plate with an open hole subjected to tension loading. The effect of the Paris–Erdogan exponent is also investigated.     

Numerical estimation of the fatigue crack front shape for a specimen with finite thickness

An iterative process for the estimation of a fatigue crack front based on linear elastic fracture mechanics using values of the stress singularity exponent is presented. Based on the assumption of a constant stress singularity exponent along the crack front, a numerical approach leading to crack shape determination is suggested and applied. The crack front was approximated by a spline curve. In each node defining the crack front the stress singularity exponent was estimated and a complete crack front shape was found. The difference between thin and thick specimens is then described and discussed. The approach presented leads to better estimation of the crack front shape for structures with different thicknesses and a more accurate determination of fatigue crack fracture parameters. The results presented can be helpful for a better understanding of fatigue failure and more reliable prediction of residual lifetime.     

Non‐planar mixed‐mode growth of initially straight‐fronted surface cracks,in cylindrical bars under tension,torsion and bending,using the symmetric Galerkin boundary element method‐finite element method alternating method

In this paper, the stress intensity factor (SIF) variations along an arbitrarily developing crack front, the non‐planar fatigue‐crack growth patterns, and the fatigue life of a round bar with an initially straight‐fronted surface crack, are studied by employing the 3D symmetric Galerkin boundary element method‐finite element method (SGBEM‐FEM) alternating method. Different loading cases, involving tension, bending and torsion of the bar, with different initial crack depths and different stress ratios in fatigue, are considered. By using the SGBEM‐FEM alternating method, the SIF variations along the evolving crack front are computed; the fatigue growth rates and directions of the non‐planar growths of the crack surface are predicted; the evolving fatigue‐crack growth patterns are simulated, and thus, the fatigue life estimations of the cracked round bar are made. The accuracy and reliability of the SGBEM‐FEM alternating method are verified by comparing the presently computed results to the empirical solutions of SIFs, as well as experimental data of fatigue crack growth, available in the open literature. It is shown that the current approach gives very accurate solutions of SIFs and simulations of fatigue crack growth during the entire crack propagation, with very little computational burden and human–labour cost. The characteristics of fatigue growth patterns of initially simple‐shaped cracks in the cylindrical bar under different Modes I, III and mixed‐mode types of loads are also discussed in detail.     

A simplified method for the evaluation of fatigue crack front shapes under mode I loading

Two-dimensional elastic or elasto-plastic models dominate the current fatigue crack growth assessment and life prediction procedures for plate components with through-the-thickness cracks. However, as demonstrated in many theoretical and experimental papers, the stress field near the crack tip is always three-dimensional and, as a result, the fatigue crack front is not straight. It is normally curved towards the plate faces. Over the past few years there were a number of very careful numerical studies focusing on the evaluation of fatigue crack front shapes. However, the application of the direct numerical techniques to fatigue phenomena is a very tedious and time consuming process and, sometimes, it is quite ambiguous. In the current paper we develop a simplified method for the evaluation of the front shapes of through-the-thickness fatigue cracks. Further, we validate the developed method against experimental results, investigate the influence of various parameters on the crack front shapes at stable (steady-state) propagation and analyse the differences in the results of fatigue crack growth evaluation obtained with two- and three-dimensional approaches.     

A level set model for simulating fatigue-driven delamination in composites

This paper proposes a level set model for simulating delamination propagation in composites under high-cycle fatigue loading. For quasi-static loading conditions, interface elements with a cohesive law are widely used for the simulation of delamination. However, basic concepts from fatigue analysis such as the notion that the crack growth rate is a function of energy release rate cannot be embedded in existing cohesive laws. Therefore, we propose a model in which the cohesive zone is eliminated from the computation while maintaining the flexibility that the crack shape is not bound to element edges. The model is able to predict the delamination growth rate and its front shape accurately. To demonstrate the validity of the model, several tests under different fracture modes are conducted and the results are compared with experimental data, analytical solutions and results from cohesive zone analysis.     

SURFACE CRACK GROWTH IN A PLATE UNDER THE REMOTE HIGH STRAIN-CONTROLLED CYCLIC LOADING

Abstract— —Low cycle fatigue surface crack growth in a plate of finite width under remote strain-controlled cyclic loading has been studied for a mild steel. Both crack growth in the depth direction and along the surface plane were measured. It is found that the cyclic J -integral, estimated by Dowling and Begley's approach, can be used as the mechanical driving force to characterize crack growth rates in both directions. The maximum strain, strain ratio and strain rate have no influence on the relation between the crack growth rates and the corresponding cyclic J -integral. The shape of the crack, idealized as a semi-ellipse, gradually changes from shallow to deep at the beginning of a test and remains unchanged after the ratio of crack depth to thickness of the plate attains a value of 0.7. The change of crack shape is not affected by maximum strain, strain rate and strain ratio, but is dependent on the precracked shape.     

A fracture mechanics analysis of fatigue crack growth in a complex cross section

This paper describes research conducted to determine the fatigue crack growth behavior of corner cracks which occur in a beam with a complex “double-T” shaped cross section. Particular objectives were to determine the changes in crack shape as they grow from one leg of the double-T cross section into the wider portion of the specimen, and to determine whether crack growth could be delayed or arrested in this area. Since it is difficult, if not impossible, to obtain details of crack growth inside metal specimens, it was decided to test transparent polymer models which allow in situ photographs of the crack plane. Fatigue crack growth rates measured at various positions along the crack front were used to compute cyclic stress intensity factors from the baseline fatigue crack growth behavior of the test material. The results of these experiments do indicate that cracks may be delayed once they sever one leg of the double-T cross section, although crack arrest may be compromised if a second crack develops in the opposing leg.     

An optimized predictor–corrector scheme for fast 3D crack growth simulations

An optimized predictor–corrector scheme for the accelerated simulation of 3D fatigue crack growth is presented. Based on experimental evidence, it is assumed that the crack front shape ensures a constant energy release rate. Starting from a crack front satisfying this requirement a predictor step is performed. Usually, the new crack front does not fulfill the requirement of a constant energy release rate. Therefore, several corrector steps are needed. Within the new predictor–corrector scheme the history of crack growth is taken into account to reduce the number of corrector steps. The efficiency of the new scheme is shown on two numerical examples providing a speed up of a factor above three.     

Crack propagation along polymer/non-polymer interfaces

Mechanisms of the propagation of crack fronts along interfaces between a glassy polymer and metal or glass are discussed. Specifically, the systems studied are Poly-Ethylene Terephthalate (PETG) spin-coated on Al, PETG-glass and PETG hot-pressed on Cr-sputtered glass. Cracks studied propagate in an Assymetric Double Cantilever Beam (ADCB) geometry. Dependence of microscopic crack front geometry on propagation speed is found. The local stress state is found to have an impact on macroscopic as well as microscopic crack front geometry. Simple lattice model calculations of propagating crack fronts illustrate the impact of disorder and residual stress state on propagation mechanisms and macroscopic crack front shape respectively.     

Stress intensity factors in shafts subjected to torsion and axial loading

The finite element method has been used to determine stress intensity factors (SIFs) of surface cracks in plain and fillet notched shafts subjected to torsion and axial loadings. The SIFs were obtained as part of a simulation of surface crack growth under fatigue conditions. The shape of the crack front was also predicted and compared to the crack front shapes observed on several test specimens.     

Finite element modelling and analysis of crack shape evolution in mode-I fatigue Middle Cracked Tension specimens

A numerical procedure was employed to study the shape evolution of fatigue cracks in Middle Cracked Tension specimens. This iterative procedure consists of a 3D finite element analysis to obtain the displacement field in the cracked body, calculation of stress intensity factors along crack front and definition of local crack advances considering the Paris law. Numerical predictions were compared with experimental crack shapes with a good agreement. The evolution of crack shape was analysed for different propagation conditions considering robust dependent parameters. Two main propagation stages were identified: an initial transient stage highly dependent on initial crack shape and a stable stage where the crack follows preferred paths. Mathematical models were proposed for transient and stable stages consisting of exponential and polynomial functions, respectively. The transition between both stages was defined considering two criteria: the rate of shape variation and the distance to stable shape. Finally, the crack shape change was linked with the distribution of stress intensity factor along crack front.     

Stress intensity factors for surface fatigue crack in a round bar under cyclic axial loading

In this paper, the surface fatigue crack growth shape for an initial straight-fronted edge crack in an elastic bar of circular cross-section is determined through experiments under pure fatigue axial loading. Three different initial notch depths are discussed. The relations of the aspect ratio (b/c) and relative crack depth (b/D) are obtained, and it is shown that there is a great difference in the growth of cracks with different initial front shapes and crack depths. Further, using the three-dimensional finite element method, the stress intensity factors (SIFs) are determined under remote uniform tension loading. Since the relationship of b/c and b/D changes during the fatigue crack growth, the SIFs are determined for different surface crack configurations.     

Prediction of stable crack growth in surface cracked plate of Aluminum 7050 alloy

To simulate stable crack growth, three-dimensional finite element analysis using the constant Crack Tip Opening Angle (CTOA) fracture criterion was performed for a thin plate made of Aluminum 7050 alloy. The critical CTOA value was experimentally obtained by the Rubber Impression Method, which directly measures the three-dimensional crack profiles by inserting the gel-state silicon rubber into the crack, and taking out the solid-state rubber later. From the microscopic observation from the broken specimen, it was found that, as the crack extends, the amount of crack growth near the free surface is more than that in the depth/thickness direction, which creates the special tunneling, e.g., canoe-shaped crack extension. For the numerical simulation, the surface-cracked plate was analyzed by quasi-static elastic-plastic finite strain analysis with the node release and the reloading technique. Consistent with experimental observations, numerical simulation with constant CTOA fracture criteria produced tunneling of a surface crack, but the shape of the crack front deviated from the experimental crack front as the free surface was approached. To address the local crack tip constraint effect on the stable crack growth, various fracture parameters - crack tip triaxiality, equivalent plastic strain, and void growth ratio as the crack extends – were also investigated.     

A New Method of Retarding Fatigue Crack Growth on Pressure Vessels

An artificial wedge to retard;fatigue crack growth in tension has been investigated. The results show that an artificial wedge can reduce the growth rate of fatigue crack on surface fatigue crack and, the fatigue crack growth behavior is essentially similar in-depth and width directions. Based on a theoretical analysis, a model for the effective crack growth parameter DeltaK(eff) is presented. It is shown that the relationship between the calculated DeltaK(eff) value and crack speed are almost the same as those of cracked specimens without a wedge. Therefore this model can be applied to estimate retardation behavior.