Modeling fatigue crack nucleation at primary inclusions in carburized and shot-peened martensitic steel

The mechanics of high cycle fatigue crack nucleation (formation of a stable crack that can grow away from the influence of the notch root of the inclusion) at subsurface primary inclusions in carburized and shot-peened martensitic steel subjected to cyclic bending is investigated using three-dimensional (3D) finite element (FE) analysis. FE models are constructed using a voxellation technique to address the shape, size, and distribution of primary inclusions within clusters. The critical depth for fatigue crack nucleation is predicted considering the gradient in material properties arising from carburization, prestrain and compressive residual stress distribution due to shot peening, and the gradient of applied bending stress. The influence of inclusion shape and interface condition (intact or debonded) with the matrix on the driving force for fatigue crack nucleation is examined. It is observed that the inclusion shape has minimal influence on the predicted results while the effect of the interface condition is quite significant. For partially debonded interfaces, the predicted critical depth from surface for fatigue crack nucleation agrees qualitatively with experimental observations.     

基于内部失效机理预测评估渗碳Cr-Ni齿轮钢的超高周疲劳强度

对渗碳Cr-Ni齿轮钢进行应力比为0和0.3的室温超高周疲劳实验,观测试样中诱发裂纹萌生的夹杂和疲劳断口形貌,以全面评估渗碳Cr-Ni齿轮钢疲劳性能。将疲劳失效模式分为有细颗粒区(Fine granular area,FGA)的内部疲劳失效和有表面光滑区(Surface smooth area, SSA)的表面疲劳失效,并阐明了渗碳Cr-Ni齿轮钢的超高周内部疲劳破坏机制。基于累积损伤和位错能量法并结合细颗粒区形成机理和夹杂的最大评估尺寸,分别构建了两种渗碳Cr-Ni齿轮钢内部疲劳强度的预测模型。利用FGA尺寸与夹杂尺寸的比值和夹杂应力强度因子及应力比之间的关系,修正所提出的两种疲劳强度预测模型并给出了最大夹杂尺寸下的l_FGA-S-N曲线。结果表明,基于累积损伤法和位错能量法分别构建的疲劳强度预测模型都可用于预测评估渗碳Cr-Ni齿轮钢在多种应力比下的内部疲劳强度,基于位错能量法的强度预测模型精度较高。     

Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

Simulated extreme value fatigue sensitivity to inclusions and pores in martensitic gear steels

The objective of this work is to model the sensitivity of high cycle fatigue resistance of secondary hardening martensitic gear steels to variability in extrinsic inhomogeneities such as primary inclusions, and pores, coupled with intrinsic microstructure variability. A simplified approach is presented to quantify the variability in the driving force for fatigue crack formation in the matrix at non-metallic inclusions and pores in lath martensitic gear steels using a three-dimensional crystal plasticity constitutive model. The utility of a simulation-based strategy for exploring sensitivity of minimum fatigue lifetime (low probability of failure) to microstructure lies in its inherent capability to consider parametric simulations of hundreds of inclusions and microstructures in contrast to limited numbers of physical experiments. Experiments are used to calibrate the polycrystalline cyclic stress–strain response and mean (50% probability) fatigue crack formation life. Several remote loading conditions are considered in the high cycle fatigue (HCF) regime relevant to typical gear applications. Idealized inhomogenieties (spherical) in the form of hard (Al₂O₃), soft inclusions (La₂O₂S), and pores are systematically investigated in this parametric computational study. Relations between remote loading conditions and local plasticity are discussed as a function of stress amplitude and microstructure. The maximum plastic shear strain range is used in the modified form of Fatemi–Socie parameter evaluated at the grain scale as a measure of the driving force for fatigue crack formation (nucleation and early growth to lengths on the order of several times the average grain size). Multiple realizations of the polycrystal microstructure are considered to obtain a statistical distribution of this fatigue indicator parameter (FIP). The results are used to construct an extreme value Gumbel distribution of the FIPs for the selected microstructures. Subsequently, a computational micromechanics based minimum life estimate that corresponds to 1% fatigue crack formation probability is calculated.     

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.     

FATIGUE CRACK GROWTH PREDICTION IN SPECIMENS SIMILAR TO SPUR GEAR TEETH

Abstract— The problem of fatigue crack propagation in surface treated specimens similar to gear teeth is analysed. Experimental fatigue tests were carried out on carburized, and carburized and shot peened, specimens. In order to predict crack propagation and to consider the effect of the treatments, different models of the cracked specimens were realized. Two numerical approaches were followed: the finite element method and the weight function technique. Two- and three-dimensional finite element models were constructed, and the stress-intensity factors were evaluated by considering the effect of both the load and the residual stresses due to the treatments. The agreement between these and those obtained through the weight function technique is good. The weight function approach was used directly in the computer software package (which allows crack propagation predictions) and considered the effects of hardness and residual stresses. The comparison between theoretical predictions and experimental results validated the approach followed.     

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10⁷ cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are >99%.     

喷丸强化对材料旋转弯曲疲劳强度影响的定量研究

以往的工作已经提出了金属表面及内部疲劳极限的新概念，成功地分析了喷丸对三点弯曲（应力比Ｒ＝０．０５）条件下材料疲劳强度的影响。本文采用３００Ｍ钢研究了喷丸强化对旋转弯曲疲劳强度的影响。结果表明，经适当表面强化后，疲劳裂纹萌生于试样的次表层，萌生疲劳裂纹的；陆界应力（称内部疲劳极限）为未经喷丸强化试样疲劳极限（称表面疲劳极限）的１．３９倍，表明内部疲劳极限理论在旋转弯曲条件下仍然有效。     

Very high cycle fatigue behavior of a low strength welded joint at moderate temperature

Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370 °C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed.     

Fatigue behaviour of SiCp-reinforced aluminium composites in the very high cycle regime using ultrasonic fatigue

The fatigue behaviour of a 2009/SiC/15p‐T4 DRA composite has been examined in the very high cycle fatigue (VHCF) regime where 10⁷≤N_f≤ 10⁹ cycles. Ultrasonic fatigue was used to achieve the very high cycle counts. Careful processing yielded a composite with a very homogeneous particle distribution with minimal clustering. Fatigue crack initiation was observed almost exclusively at AlCuFe inclusions with no crack initiation observed at SiC particle clusters. Fatigue lives at a given stress level exhibited minimal scatter and subsurface crack initiation was observed in all cases. This behaviour is consistent with the presence of a low number density of critical inclusions that are responsible for crack initiation very early in fatigue life.     

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

In landing gear, an important mechanical component for high responsible applications, wear and corrosion control is currently accomplished by chrome plating or hard anodising. However, some problems are associated with these operations. Experimental results have also shown that chrome‐plated specimens have fatigue strength lower than those of uncoated parts, attributed to high residual tensile stress and microcracks density contained into the coating. Under fatigue conditions these microcracks propagate and will cross the interface coating‐substrate and penetrate base metal without impediment. Shot peening is a surface process used to improve fatigue strength of metal components due to compressive residual stresses induced in the surface layers of the material, making the nucleation and propagation of fatigue cracks difficult. This investigation is concerned with analysis of the shot peening influence on the rotating bending fatigue strength of hard chromium electroplated AISI 4340 steel. Specimens were submitted to shot peening treatment with steel and ceramic shots and, in both cases, experimental results show increase in the fatigue life of AISI 4340 steel hard chromium electroplated, up to level of base metal without chromium. Peening using ceramic shot resulted in lower scatter in rotating bending fatigue data than steel shots.     

Surface characteristics and fatigue performance of warm shot peened wrought magnesium alloy Mg–9Gd–2Y

Abstract

The surface characteristics and fatigue performance of the warm shot peened Mg–9Gd–2Y alloys were investigated. Compared to conventional shot peening (SP) at room temperature, warm shot peening (WSP) at 240°C induces higher subsurface hardening and larger maximum compressive residual stress in the subsurface of the specimens. The optimum Almen intensity of WSP is 0·15 mm N, whereas it is 0·10 mm N for SP. The main reason is that the surface of warm shot peened specimen is more plastically deformed but less damaged at the optimum Almen intensity due to the increase in plastic deformation ability of the tested alloys at elevated temperature. The fatigue strength of the tested alloy at 10⁷ cycles is increased from 125 to 175 MPa by optimum SP and to 185 MPa by optimum WSP.     

Fatigue strength prediction of carburized 12Cr steel alloy: Effects of evaluation of maximum crack sizes and residual stress distribution

Axial loading fatigue tests of carburized 12Cr steel alloy in long‐life regime were performed under stress ratios of ?1 and 0. Fatigue fracture can be divided into surface failure, interior failure without fine granular area (FGA), and interior failure with FGA. By considering the effects of the tensile limit, the evaluation of maximum crack sizes (inclusion sizes and FGA sizes), and residual stress distribution, the fatigue strength prediction model of carburized 12Cr steel alloy for inclusion‐FGA‐fisheye induced failure in low stress level region can be established. By comparing the predicted results evaluated by generalized extreme values (GEV) with generalized Pareto (GP), the GP distribution is more suitable to predict the fatigue limit of the carburized 12Cr steel alloy. Furthermore, by using the relationship between inclusion sizes and FGA sizes, the fatigue limit prediction model for the design of components can be established, and the result is extremely accurate for the carburized 12Cr steel alloy.     

Fatigue properties of two case hardening steels after carburization

Fatigue properties of two case hardening steels after carburization have been investigated by means of rotating bending fatigue tests and rolling contact fatigue tests. Results show that the steel with higher Al and N contents has higher rotating bending fatigue limit and rolling contact fatigue limit, increasing from 865 to 950 MPa and from 3575 to 3725 MPa, respectively. It is also shown that the steel with higher Al and N contents has finer prior austenite grain sizes and higher hardness in the carburized case. Scanning electron micrographic observations on the fractured surface of specimens for rotating bending fatigue tests show that fatigue crack usually initiated from oxide inclusions and propagated along prior austenite grain boundaries, indicating that the finer grain size and higher hardness in the carburized case of the steel with higher Al and N contents can contribute to its higher fatigue properties.     

Investigation of rolling contact fatigue cracks in ball bearings

Rolling contact fatigue in a ball bearing is studied using the experimental method. Fatigue pits and spallings on the rolling surface are investigated, and the strain-hardening beneath the contact surface is studied using the microhardness profile. Moreover, surface and subsurface crack layouts and the effect of inclusions on crack nucleation are studied by optical and electron microscopy. Additionally, a simulated model is used to study the influences of the crack inclined angle and the inclusion’s hardness on fatigue damage in bearings.     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...     

Fatigue crack growth through a residual stress field introduced by plastic beam bending

We present predictions and measurements of fatigue crack growth rates in plastically bent aluminium 2024‐T351 beams. Beam bending and fatigue were carefully controlled to minimize factors other than residual stress that could affect the fatigue crack growth rate, such as large plastic strains or residual stress relaxation. The residual stress introduced by bending was characterized by a bending method and by the slitting method, with excellent agreement between the two methods. Crack growth rates were predicted by three linear elastic fracture mechanics (LEFM) superposition based methods and compared to experimental measurements. The prediction that included the effects of partial crack closure correlated with experimental data to within the variability normally observed in fatigue crack growth rate testing of nominally residual stress free material. Therefore, we conclude that crack growth through residual stress fields may be predicted using the concept of superposition as accurately as crack growth through residual stress free material, provided that the residual stress is accurately known, the residual stress remains stable during fatigue, the material properties are not changed by the introduction of residual stress, and that the effect, if any, of partial crack closure is taken into account.     

Rotating bending fatigue properties of two case hardening steels after nitriding treatment

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 10⁷ cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000 Hv and compressive stress of 400 MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.