3D modeling of subsurface fatigue crack nucleation potency of primary inclusions in heat treated and shot peened martensitic gear steels

A computational strategy is developed to characterize the driving force for fatigue crack nucleation at subsurface primary inclusions in carburized and shot peened C61^® martensitic gear steels. Experimental investigation revealed minimum fatigue strength to be controlled by subsurface fatigue crack nucleation at inclusion clusters under cyclic bending. An algorithm is presented to simulate residual stress distribution induced through the shot peening process following carburization and tempering. A methodology is developed to analyze potency of fatigue crack nucleation at subsurface inclusions. Rate-independent 3D finite element analyses are performed to evaluate plastic deformation during processing and service. The specimen is subjected to reversed bending stress cycles with R = 0.05, representative of loading on a gear tooth. The matrix is modeled as an elastic–plastic material with pure nonlinear kinematic hardening. The inclusions are modeled as isotropic, linear elastic. Idealized inclusion geometries (ellipsoidal) are considered to study the fatigue crack nucleation potency at various subsurface depths. Three distinct types of second-phase particles (perfectly bonded, partially debonded, and cracked) are analyzed. Parametric studies quantify the effects of inclusion size, orientation and clustering on subsurface crack nucleation in the high cycle fatigue (HCF) or very high cycle fatigue (VHCF) regimes. The nonlocal average values of maximum plastic shear strain amplitude and Fatemi–Socie (FS) parameter calculated in the proximity of the inclusions are considered as the primary driving force parameters for fatigue crack nucleation and microstructurally small crack growth. The simulations indicate a strong propensity for crack nucleation at subsurface depths in agreement with experiments in which fatigue cracks nucleated at inclusion clusters, still in the compressive residual stress field. It is observed that the gradient from the surface of residual stress distribution, bending stress, and carburized material properties play a pivotal role in fatigue crack nucleation and small crack growth at subsurface primary inclusions. The fatigue potency of inclusion clusters is greatly increased by prior interfacial damage during processing.     

Surface Integrity and Fatigue Behavior for High-Speed Milling Ti–10V–2Fe–3Al Titanium Alloy

Influence of cutting parameters on surface integrity when milling Ti–10V–2Fe–3Al is investigated based on high-speed cutting experiments. Surface integrity measurements, fatigue fractography analysis, and fatigue life tests are conducted to reveal the effect of surface integrity on crack initiation and fatigue life. The results show that under given experiment conditions, surface roughness decreases linearly when increasing cutting speed or decreasing feed per tooth. The latter has a greater impact on surface roughness than the former. Compressive stress can be detected on all machined surfaces. With the increase of feed per tooth or cutting speed, respectively, residual stress presents a linear increase. Cutting parameters have no significant impact on micro-hardness. When the surface roughness ranges from 0.5 to 1.0 μm, the effect of surface residual stress on fatigue life is more than that of surface roughness. When the surface residual compressive stress increases, the fatigue life improves significantly. Compared with 60 m/min, when cutting speed is 100 or 140 m/min, the surface roughness decreases, the surface residual compressive stress increases, and the fatigue life improves by 124 and 59%, respectively. Under a tensile load, fatigue crack on machined surface of Ti–10V–2Fe–3Al titanium alloy originates at the cross-edge of the specimen surface. With the increase of surface roughness, the area ratio of fatigue crack propagation zone, and fatigue fracture zone decreases.     

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.     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT

The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K _t?=?1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.     

Influence of surface residual stresses on gigacycle fatigue response of high chromium cold work tool steel

The effect of surface compressive residual stresses (RS) induced by surface grinding and polishing on the gigacycle fatigue behavior of medium‐carbon high‐chromium alloy cold work tool steel was evaluated. Two test series were performed: Specimens of series I revealed high compressive RS of about ‐800 MPa at the surface, resulting from grinding with fine emery paper, which treatment had definitely a beneficial influence on the fatigue endurance strength. The existence of surface RS was also revealed to be responsible for the location of the fatigue crack initiation. High compressive RS favored internal crack origins. In this case crack nucleation sites were primary carbide clusters in the interior of the specimen, forming so‐called fish‐eyes at the fracture surface. In contrast, specimens of test series II had only very low RS, which enabled crack initiation near/at the surface at primary carbides/clusters. Furthermore, it has been shown that the high initial RS are prone to partial relaxation through cyclic loading for which the mechanisms are currently unknown. In this case near‐surface induced failure was obtained. It was possible to confirm the experimentally obtained data by the use of the concept of local fatigue strength as function of effective RS. The relaxation of high initial RS was experimentally confirmed through RS measurements at runout specimens (10¹⁰ cycles without failure).     

A literature survey on the stability and significance of residual stresses during fatigue

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near‐surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.     

Potential fatigue strength improvement of AA 5083-H111 notched parts by wire brush hammering: Experimental analysis and numerical simulation

The effects of milling as machining process and a post-machining treatment by wire-brush hammering, on the near surface layer characteristics of AA 5083-H111 were investigated. Surface texture, work-hardening and residual stress profiles were determined by roughness measurement, scanning electron microscope (SEM) examinations, microhardness and X-ray diffraction (XRD) measurements. The effects of surface preparation on the fatigue strength were assessed by bending fatigue tests performed on notched samples for two loading stress ratios R_0.1 and R_0.5. It is found that the bending fatigue limit at R_0.1 and 10⁷ cycles is 20% increased, with respect to the machined surface, by wire-brush hammering. This improvement was discussed on the basis of the role of surface topography, stabilized residual stress and work-hardening on the fatigue-crack network nucleation and growth. The effects biaxial residual stress field and surface work-hardening were taken into account in the finite element model. A multi-axial fatigue criterion was proposed to predict the fatigue strength of aluminum alloy notched parts for both machined and treated states.     

Barkhauseneigenspannungsmessungen an einem schadhaften Hydraulikzylinder mit tribotechnischer Funktionsschicht

Barkhausen Noise Analysis of a Tribofunctional Coated Hydraulic Cylinder with a Defect An airplane hydraulic cylinder showed a leak caused by a fatigue crack with its origin on the inside of the cylinder. The main reason for the development of the fatigue crack were regions with tiny cracks and residual tensile stresses produced during final machining of the cylinder base material. Residual stress differences on the inner surface were measured by Barkhausen noise analysis without removing the tribofunctional coating. The Barkhausen noise results were confirmed by some x-ray stress measurements.     

High- and very high-cycle plain fatigue resistance of shot peened high-strength aluminum alloys: The role of surface morphology

The present paper is aimed at investigating the effect of shot peening on the high and very-high cycle plain fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 10⁵ and 10⁸ cycles. Three peening treatments were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests. Fatigue crack initiation sites were investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment. The capability of shot peening to hinder the initiation and to retard the subsequent propagation of surface cracks is discussed on the basis of a model combining a multiaxial fatigue criterion and a fracture mechanics approach.     

Fretting fatigue crack nucleation in Ti–6Al–4V

Fretting fatigue crack nucleation in Ti?6Al?4V when fretted against itself is investigated to determine the influence of contact pressure, stress amplitude, stress ratio, and contact geometry on the degradation process. For the test parameters considered in this investigation, a partial slip condition generally prevails. The resulting fatigue modifying factors are 0.53 or less. Cycles to crack nucleation, frictional force evolution, crack orientations and their relationship to the microstructure are reported. The crack nucleation process volume is of the same order as the microstructural length scales with several non‐dominant cracks penetrating 50 μm or less. The effective coefficient of friction increases during early part of fretting. Observations suggest that cyclic plastic deformation is extensive in the surface layers and that cyclic ratchetting of plastic strain may play a key role in nucleation of the fretting cracks. A Kitagawa–Takahashi diagram is used to relate the depth of fretting damage to the modifying factor on fatigue life.     

Einfluß der Oberflächenbeschaffenheit auf die Schwingfestigkeit von Aluminium-Druckguß

Effect of Surface Condition on Fatigue Strength of Die-Cast Aluminium Surface processing like milling or shot peening involves modifications of the surface and subsurface condition of a material that can be described by changes of the residual stresses, the hardness, and the surface roughness. Moreover, there is the possibility of introducing additional surface defects. In this paper the influence of these modifications on the fatigue behaviour of the aluminium die-casting alloy GD-AlSi8Cu3 is presented. S-N curves are determined for four surface conditions produced by milling, shot peening, and a combined treatment. The improvement of fatigue limit by shot peening is confirmed for this material. The additional effect of mean stresses is determined. Investigations of the fatigue fracture surfaces show that pores and pipes as well as surface defects induced by shot peening act as fatigue crack initiation sites.     

Effect of cutting speed and tool rake angle on the fatigue life of 2024-T351 aluminium alloy

Ring-shaped specimens of 2054-T351 aluminium alloy were machined orthogonally on a lathe equipped with a quick-stop device at cutting speeds of 0.5–1.5 m s^?1 with tools having positive rake angles in the range 10–30°. The machined specimens were then fatigued at a selected stress and the resulting fatigue lives were compared with that of the virgin material. The surfaces of the specimens were examined using optical and scanning electron microscopy.The fatigue life of the machined specimens was found to increase with increasing cutting speed or tool rake angle. The fatigue life of the specimens machined at higher cutting speeds was higher than that of the virgin material, due to the presence of compressive residual stresses in the surface layers. At lower cutting speeds the surface damage was so severe that, in spite of the presence of compressive residual stresses in the surface layers, the fatigue life of the machined specimens was lower than that of the virgin material.     

Effects of CTE-induced residual stresses around hard alpha particles on fatigue crack growth in Ti–6Al–4V

The presence of hard alpha (HA) anomalies in titanium alloys represents a significant potential degradation to gas turbine component performance. Although HA defects in titanium alloys are rare, when they are present, they can crack and ultimately result in failure. In static fracture and fatigue test specimens, embedded HA defects had significantly higher fracture strengths than anticipated. The objective of this work was to determine if residual stresses caused by thermal expansion mismatch during material fabrication were the cause of the observed behaviour. The residual stress fields in and around surface and embedded HA particles in Ti–6Al–4V (Ti–6–4) were determined using an elasticity solution and measured coefficient of thermal expansion (CTE) data. The calculated stress distributions serve as the foundation for comparisons of the local stress and the fracture strength, the stress intensity factor K and the crack growth threshold ΔK_th, with the experimentally determined fatigue lives. The analytical results indicated that CTE‐induced residual stress around HA particles can contribute to the fatigue strength of Ti–6–4 by delaying microcracking of HA anomalies and reducing the driving force (effective ΔK) of the fatigue crack. Based on the analysis results, the differences between the surface and subsurface results as well as the difference between predicted and measured fatigue lives could be largely attributed to the residual stress effects caused by the mismatch of the particle and matrix properties.     

Determination of fatigue related discontinuity state of 7000 series of aerospace aluminum alloys

The current methods of fatigue life estimation do not model the interaction of stress with microstructural discontinuities. An experimental study of fatigue crack nucleation in 7075-T6 and 7079-T6 aluminum alloy sheet was performed to obtain more information on the role of discontinuities in the microstructure and to determine the relative contributions of each type of discontinuity. Some specimens were machined from recently manufactured material without any coating (cladding or anodizing). Some were fabricated from old clad-anodized material from an aircraft fuselage and old anodized material from a stock of unused wing panels. In the uncoated materials, large constituent particles and the surface roughness were found to be important factors in fatigue crack nucleation. In the coated materials, discontinuities related to the cladding and anodizing appeared to be the controlling factors in crack nucleation and particles did not play any role. The coated specimens showed a low scatter and a relatively low fatigue life. Multiple nucleation sites were observed in the coated materials.     

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.     

HXD1机车牵引电机转轴组件断裂失效分析

HXD1型电力机车的牵引电机转轴和小齿轮轴采用圆锥过盈配合传动结构(下称转轴组件),使用中该组件出现了早期断裂失效.本文通过理化检测、断口和配合面宏/微观形貌观察等失效分析技术对失效组件进行了分析.结果表明,材料成分、组织和显微硬度正常,小齿轮轴和电机转轴的失效形式分别为高周疲劳断裂和微动疲劳断裂.造成组件失效的原因和过程是,小齿轮轴近齿端油槽-油孔交界线处有较大的结构应力集中,油槽底部周向加工刀痕造成附加应力集中,在应力集中和旋转弯曲疲劳载荷作用下油孔边两个应力集中点萌生了疲劳裂纹并扩展;随小齿轮轴裂纹的不断扩展转轴组件结构刚度减小,继而诱发了与小齿轮轴匹配的电机轴配合面的微动疲劳,电机轴疲劳裂纹萌生于微动区的边缘处;电机转轴先于小齿轮轴完全断裂.基于本文的分析结果提出了提高组件抗疲劳断裂的技术措施.     

Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement

Shot peening is a dynamic cold‐working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly used as a surface treatment technique within the aerospace industry during manufacturing to improve fatigue performance of structural components. The compressive residual stress induced during shot peening results in fatigue crack growth retardation, improving the performance of shot‐peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Because of the relatively soft nature of AA7050‐T7451, shot peening can result in cracking of the constituent precipitate particles, creating an initial damage state. The aim of this paper is to understand the balance and fundamentals of these competing phenomena through a comparative study throughout the fatigue lifecycle of baseline versus shot‐peened AA7050‐T7451. Microstructure and surface topology characterization and comparison of the baseline and shot‐peened AA7050‐T7451 has been performed using scanning electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and optical profilometry techniques. A residual stress analysis through interrupted fatigue of the baseline and shot‐peened AA7050‐T7451 was completed using a combination of X‐ray diffraction and nanoindentation. The fatigue life performance of the baseline versus shot‐peened material has been evaluated, including crack initiation and propagation. Subsurface particles crack upon shot peening but did not incubate into the matrix during fatigue loading, presumably due to the compressive residual stress field. In the baseline samples, the particles were initially intact, but upon fatigue loading, crack nucleation was observed in the particles, and these cracks incubated into the matrix. In damage tolerant analysis, an initial defect size is needed for lifetime assessment, which is often difficult to determine, leading to overly conservative evaluations. This work provides a critical assessment of the mechanism for shot peening enhancement for fatigue performance and quantifies how incubation of a short crack is inhibited from an initially cracked particle into the matrix within a residual stress field.     

An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58–60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 µm and severe plastic deformations beneath the machined surface.