Crack Initiation Depth Research Articles

Crack extension mechanism and scratch stress field model of hard and brittle materials caused by curvature effect

Inhibition of subsurface crack growth under contact loading is key to enhancing the service performance of brittle materials. However, the understanding of crack extension mechanism under complex conditions is still limited. In this paper, scratch experiments and stress field analysis were used to investigate the damage evolution of glass materials induced by the curvature effect of abrasive trajectories. A cycloid or trochoid analytical model was developed to identify curvature-sensitive paths. On this basis, Curvature scratching experiments were conducted on the self-developed scratching experimental platform. The results revealed that surface cracks appeared outside the scratch grooves at the maximum curvature. Scratches with higher curvature peaks demonstrated less depth of crack initiation, and deeper cracks were induced inside the scratch. A novel finite-length, curved stress field model is proposed to evaluate the damage evolution from the inner and outer sides of the scratch. Based on the theory of maximum principal stress and tensile stress, the mechanism of crack extension under the curvature effect is revealed. This study improved the understanding of abrasive scratching and cracks extension under broad physical conditions.

Correlation of residual stress, hardness and surface roughness with crack initiation and fatigue strength of surface treated additive manufactured AlSi10Mg: Experimental and machine learning approaches

Post-processing methods are widely used to address the issues caused by surface imperfections and bulk defects in additive manufactured materials. In our previous studies, we analysed the effects of different peening-based treatments of shot peening (SP), severe vibratory peening (SVP) and laser shock peening (LSP) on fatigue performance of V-notched laser powder bed fusion AlSi1Mg samples. Herein, the fracture surfaces of failed samples were further analyzed and obtained experimental data were further elaborated by machine learning (ML)-based approach to identify the correlation between residual stress, hardness and surface roughness (all affected by the applied post-treatments) with the depth of crack initiation site and fatigue life of the post-treated samples. ML-based model was developed via a six layer deep neural network (DNN) as well as using stacked auto-encoder (SAE) for pre-training of the used data set. Taking the advantages of SAE, the accuracies of more than 0.96 were obtained for the predicted results. Correlations were obtained by performing parametric analyses and the importance of each input factor was assessed through sensitivity analyses. The obtained results revealed that by enhancing surface hardening and inducing higher compressive residual stresses as well as more efficient surface roughness reduction, deeper crack initiation site and superior fatigue life can be obtained. In addition, it was found that the depth of sub-surface crack initiation had direct relation with fatigue life improvement in the samples.

A peridynamic frictional contact model for contact fatigue crack initiation and propagation

Cyclic contact between components can cause cracks in the contact area. Peridynamics can overcome the difficulty of solving cracks and other discontinuities in classical continuum mechanics. A new peridynamic sliding frictional contact model is proposed and verified by a two-dimensional point contact sliding simulation. Then, a two-dimensional plane strain elastoplastic peridynamic model of a single micro-contact asperity is established, and the fatigue crack initiation and propagation in the micro-contact area during fretting wear is analyzed. The results show that in fretting wear, crack initiation originates from the area of maximum shear stress below the surface. The subsurface crack initially propagates in the direction parallel to the surface, and then gradually propagates to the surface. Finally, the crack grows to the free surface and forms wear debris. The contact pressure and the coefficient of friction in the contact surface can affect the crack initiation depth and crack propagation path, thereby affecting the size of wear debris. The crack propagation path is not sensitive to the change of the coefficient of friction of the crack surface. This study helps to understand the mechanism of fretting wear and provides a reference for predicting fretting wear.

Fretting behaviors of a steel up to very high cycle fatigue

In the present work, the fretting fatigue behaviors of GCr15 steel (Chinese GB/T-standard) have been studied up to the very high cycle fatigue (VHCF) regime by using an ultrasonic fatigue system. The main aims of the investigations were the analyses of S–N curves, worn morphology, as well as fracture morphology. As expected, the results of the fretting fatigue tests show a reduction in the lifetime compared to plain fatigue tests without fretting. 2-D and 3-D morphologies of the fretting worn surfaces are combined to study the evolution of the stick zone. With the increasing fatigue life, the stick zone will fully occupy the whole worn zone. Besides, fretting causes severe oxidation and debris in the worn surface. The crack initiation depth increases with the increase in fatigue life, especially in the VHCF regime, the depth increases drastically because of fretting delamination.

Modeling of contact fatigue damage behavior of a wind turbine carburized gear considering its mechanical properties and microstructure gradients

Carburized gears are commonly used in heavy-duty machines such as wind turbines, ships and high-speed rails. The gradient characteristics of mechanical properties and microstructure from the case to the core and the complex subsurface stress state in service make it very challenging to understand the contact fatigue performance of the carburized gears. In this study, a numerical model considering the effects of mechanical properties and microstructure gradients is developed to investigate the contact fatigue damage behavior of a wind turbine carburized gear. The hardness gradient in the case depth is modelled using Thomas empirical equation. The microstructure gradient is considered in terms of the grain size variation adopting the technique of Voronoi tessellation. The damage-coupled elastic-plastic material constitutive relations are developed to capture the intergranular mechanical response and the progressive fatigue damage under repeated gear meshing. The results indicate that the shear stress reversal is not uniformly distributed along the grain boundaries of the same depth. The critical subsurface depth of crack initiation and the fatigue damage evolution obtained from the calculation model compare well to the experimental and numerical results readily available in literature. With the developed framework, the influences of normal load and case carburization on the fatigue damage behavior of the carburized gear are also investigated and discussed in detail.

An Overview of Fatigue Strength of Case-Hardening TRIP-Aided Martensitic Steels

Surface-hardened layer characteristics and fatigue strength properties of transformation-induced plasticity-aided martensitic steels subjected to heat-treatment or vacuum carburization followed by fine-particle peening are revealed for automotive applications specially for powertrain parts. The as-heat-treated steels without the case-hardening process possess excellent impact toughness and fatigue strength. When the steels are subjected to fine-particle peening after heat-treatment, the fatigue limits of smooth and notched specimens increase considerably, accompanied with low notch sensitivity. Vacuum carburization and subsequent fine-particle peening increases further the fatigue strength of the steels, except notch fatigue limit. The increased fatigue limits are principally associated with high Vickers hardness and compressive residual stress just below the surface, resulting from the severe plastic deformation and the strain-induced martensitic transformation of metastable retained austenite, as well as low surface roughness and fatigue crack initiation depth.

Effect of ultrasonic nanocrystal surface modification on the characteristics of AISI 310 stainless steel up to very high cycle fatigue

AbstractEffects of ultrasonic nanocrystal surface modification (UNSM) on the very high cycle fatigue response of AISI 310 stainless steel have been investigated. The higher impact force used in UNSM treatment showed a higher fatigue life improvement. The fatigue life improvement was higher in crack initiation from the surface of specimens. The subsurface crack initiation depth in the alloy increased with increase in the fatigue failure cycles. It was concluded that UNSM treatment can increase the life of the alloy significantly up to very high cycle fatigue.

Effect of Size and Depth of Small Defect on the Rolling Contact Fatigue Strength of Bearing Steel JIS-SUJ2

To quantitatively estimate rolling contact fatigue strength as a crack problem, rolling contact fatigue tests were performed on JIS-SUJ2 bearing steel using specimen plates into which small holes of various diameters and depths had been drilled. In all the tests, fatigue cracks initiated at the edge near the bottom of the hole, and then propagated by the shear-mode. Even in the specimens that did not fail after being tested up to N = 2×108 cycles, short fatigue cracks were found at the edge. The fatigue life Nf plotted against the maximum contact pressure qmax varied greatly on the basis of the diameter and depth of the hole. The effect of the depth of crack initiation on Nf was uniquely characterized by using the nominal shear stress amplitude τa, instead of qmax Further, by considering the rolling contact fatigue strength as a small shear-mode crack problem, the fatigue life data was plotted using the novel parameter, τa/(√area)−1/6, where the area is a projected area of the hole. Consequently, all the fatigue life data was successfully fitted to a unified line irrespective of the diameter and depth of the hole, i.e., a defect size dependence on the rolling contact fatigue strength was manifested in a small crack regime.

Influence of a white layer on the performance of hard machined surfaces in rolling contact

This study investigates the influence of a machining-induced white layer on the performance of hard machined surfaces in rolling contact. Based on the residual stress and microhardness distribution of a specimen without a white layer and one with a white layer, fatigue parameters are computed to investigate its effect on the fatigue performance. Fatigue tests are then performed to substantiate the computations. It is noted that the white layer moves the crack initiation depth to a deeper location at low maximum Hertzian stresses, whereas it moves the crack initiation depth to a shallower location at the highest maximum Hertzian stress. The white layer can increase the maximum shear stress at crack initiation depth by 54 per cent. The white layer can reduce the crack initiation life by 75 per cent, while reducing the crack propagation life by 89 per cent. Accordingly, it can reduce the fatigue life by 85 per cent. The effect of the white layer on crack initiation life, crack propagation life, and fatigue life decreases significantly as the loading increases. The presented results demonstrate that the white layer induces a highly variable change in fatigue parameters, and its effect on fatigue performance can be significantly lessened if the loading is increased.

A Comparative Study of Residual Stress Distribution Induced by Hard Machining Versus Grinding

This study investigates the residual stress distribution induced by hard machining and grinding and compares its impact on fatigue parameters. The residual stress distribution below hard turned and ground surfaces is investigated after a thermally damaged layer is removed. Fatigue parameters are computed based on the residual stress distribution to compare the impact of the residual stress distribution on the fatigue performance. Rolling contact fatigue tests are then performed to substantiate the computations. The effect of residual stresses on crack initiation depth is shown to be significant for the ground specimen. The maximum shear stress at crack initiation depth of the hard turned specimen is smaller than that of the ground specimen. Due to a significant increase in crack initiation life, the predicted rolling contact fatigue life of the hard turned specimen is longer than that of the ground specimen. The overall average in the ratios of predicted life to experimental life for the hard turned specimen is closer to 1 than that for the ground specimen. The results demonstrate that the hard turned specimen shows better rolling contact fatigue performance and better accuracy in the fatigue life prediction.

The Effect of Surface Roughness on Contact Fatigue Behavior Using Mesoscopic Approach

The effect of surface roughness on the contact fatigue was investigated in this study. To accomplish this goal, contact analysis based on the influence functions and the rectangular patch solutions was performed to obtain the subsurface stress. Mesoscopic multiaxial fatigue criterion is then applied to predict fatigue damage. Rainbow counting method and damage rule were used to evaluate the fatigue life under random loadings caused by the rough surface contact. As a result of the fatigue analysis, the relationship among the fatigue life, the crack initiation depth, and the surface roughness were revealed. It is observed that the fatigue life has hardly changed and a crack is initiated at subsurface for the surface with the surface roughness below a critical value. However, if the surface roughness exceeds the critical value, fatigue life is remarkably reduced and the site of crack initiation moves to the near surface.

Influence of tool flank wear on performance of finish hard machined surfaces in rolling contact

This study investigates the influence of tool flank wear on the performance of finish hard machined surfaces in rolling contact. The residual stress and micro-hardness distribution below the machined surface and the surface roughness of the machined surface were measured to investigate the rolling contact fatigue performance. The fatigue life predictions and the fatigue test results indicate that the change in fatigue life by tool flank wear is highly variable depending on the machining conditions. It is conceivable that tool flank wear reduces the fatigue life, since less compressive residual stresses and more softened layer typically decrease the rolling contact fatigue life. However, tool flank wear does not reduce the fatigue life for some specimens. It is noted that tool flank wear can even increase the fatigue life by lower stress at crack initiation depth or lower driving force for crack propagation although tool flank wear induces less compressive residual stresses and more softened layer.

浸炭したＳＣＭ４１５鋼の転がり疲労特性とき裂発生深さ

Fatigue strength and crack initiation depth were investigated in carburized SCM415 steel under rolling contact fatigue (RCF). RCF tests were carried out until the number of cycles reached 1.2×108. The fatigue strength of carburized SCM415 was found to be more than 4000MPa. Observation of a flaking surface showed that the crack initiated around an inclusion or a defect, the depth of which was about 50μm. Based on the observations, we concluded that the subsurface crack was propagated when its shape remained circular. We evaluated the crack growth behavior in terms of contact stresses and stress intensity factors. The contact stresses and the stress intensity factors were calculated by using Hanson and Fabrikant's analytical method. It is found that the absolute values of KIImax and KIImin were larger than other stress intensity factors and the subsurface crack was dominantly propagated in mode II. The flaking depths vary depending on the KII peak curbing slightly. The friction between crack faces causes the flaking failure which is shallower than predicted by the maximum shear stress.

Influence of Sliding Conditions on Pitting Fatigue Strength of Carburized Steel.

The influence of sliding conditions on pitting fatigue strength has been investigated. In order to elucidate the influence of sliding conditions, fatigue tests were carried out on various conditions of specific sliding (0∼-82%) and rolling velocity (500∼5000 rpm) with carburized SCr420 Steel rollers. As the results, there were two pitting forms, internal initiation and surface initiation. In the case of 0% specific sliding, internal initiate pitting occurred. The crack initiation depth and the fatigue strength of internal initiation coincided with the values estimated from shear stress and shear strength related Vickers hardness. In the case of -8∼-82% specific sliding, surface initiate pitting occurred. The fatigue strength was related to specific sliding rather than rolling velocity, surface friction coefficient and surface temperature, and the fatigue strength decreased as specific sliding increased.

Optimum carburization in external spur gears

In order to determine the surface durability of case-hardened gears and the optimum design conditions required to prevent spalling fatigue, an analytical model was developed which correlates the subsurface shear stress produced by the transmitted load with the material strength of the gear. Consequently, it was made clear that there is an optimum case depth for the required surface durability and that the crack initiation depth of spalling coincides quite well with the depth where the subsurface shear stress exceeds the material shear strength. The implications of the model for optimizing the carburizing process and preventing gear spalling fatigue are discussed.

The Surface Durability of the Case-Hardened Nickel Chromium Steel and Its Optimum Case Depth

In order to clarify the surface durability of the surface hardened steels and the optimum design condition of the surface hardened gears, the carburized and hardened steel rollers were tested with a rolling contact fatigue testing machine. Consequently, it was made clear that there is an optimum case depth for the surface durability and that the crack initiation depth of spalling coincides quite well with the depth where the amplitude of the ratio of orthogonal shear stress to Vickers hardness becomes maximum. By using these test results, the authors introduced a tentative method for calculating the optimum case depth for surface durability of roller pairs which had various relative radii of curvature. And this calculation method was successfully discussed with the practical case-hardened gears.