The nucleation of fatigue cracks in a low-alloy steel under high-cycle fatigue conditions and uniaxial loading

A basic understanding of the mechanism of fatigue-crack nucleation in a high-strength steel, under true high-cycle conditions, is developed from microstructural observations of rotating-bending fatigue specimens and theoretical results for the stress concentrations at inclusions and holes. The results show that for true high-cycle fatigue (fatigue life > 10⁶ cycles), where the applied stresses are too low to cause localised cyclic-plasticity at the site of inclusions, crack nucleation does not occur while the inclusions remain undamaged and firmly bonded to the matrix. Fatigue-crack nucleation is found to occur only after either the progressive debonding or local fatigue damage of alumina inclusions has led to the formation of holes at the specimen surface: the fatigue cracks being nucleated by a highly localised cyclic plasticity effect at points of maximum stress intensity on the hole boundary. The inclusion/matrix debonding mechanism is therefore not an essential preliminary to the nucleation of fatigue cracks, under true high-cycle conditions.Manganese sulphide inclusions and cementite particles are shown to have virtually no influence on the mechanism of fatigue-crack nucleation under the present high-cycle conditions. For the case of manganese sulphide this is a direct result of the high axial-elongation of the inclusions and the consequent low stress concentration under axial loading: under more general states of fatigue loading manganese sulphide inclusions are unlikely to remain ineffective.     

Study of the Influence of Inclusions on the Behavior of NiTi Shape-Memory Alloys in Thermal Cycling by Means of Finite Element Method

Despite the number of articles on the subject, the influence of inclusions on the behavior of nickel titanium shape memory alloys is still controversial. Numerical simulation can play a fundamental role in providing insight into this subject. As far as superelastic materials are concerned, it has been shown by means of finite element simulations that, in wires loaded in rotary bending conditions, the presence of inclusions greatly increases the stress distribution in the cross section, and that the maximum stress increases as the distance between the inclusion and the neutral axis increases. In this work, a similar approach is used to analyze the effect of inclusions on thermal cycles of wires loaded in tension. By means of a thermomechanical constitutive model implemented in ANSYS, the alternate stress/strain field in the presence of a particle is computed. The particle is either located close to the wire surface or in the center of the wire section. An empirical damage evolution law is applied to the stress strain field showing a region adjacent to the particle where premature mesocrack nucleation is likely to take place.     

32Cr3MoVE轴承钢旋弯疲劳特性及裂纹萌生扩展行为

摘要：采用光滑漏斗状试样对32Cr3MoVE轴承钢进行旋转弯曲疲劳测试，研究了32Cr3MoVE轴承钢旋转弯曲疲劳性能及裂纹萌生扩展行为。采用升降法测得其疲劳极限为860MPa，疲劳断口SEM观察并统计破断试样结果表明：疲劳破坏68.7%是由于非金属夹杂起裂，18.8%由表面加工缺陷起裂，125%为表面粗糙度起裂。当加载应力低于980MPa时，疲劳断裂主要是由于内部非金属夹杂引起的，高于980MPa时，疲劳断裂主要是由于表面粗糙度引起的。表面加工缺陷和表面粗糙度引起的最大应力强度因子分别为3.05和2.97MPa·m1/2，容易引发疲劳裂纹。非金属夹杂物尺寸在5.30～5.90μm范围内，局部应力从859.35MPa升至977.75MPa时，疲劳寿命从1.96×105降低到1.58×105；非金属夹杂物局部应力在840～900MPa范围内，夹杂物尺寸从2.28μm升至5.83μm时，疲劳寿命从1.10×106降低到1.96×105。     

拉伸与低周疲劳载荷作用下夹杂物特征参数对航空用超高强度钢中裂纹萌生与扩展的影响

为了研究非金属夹杂物对航空用超高强度钢性能的影响,采用扫描电镜原位观测的方法,跟踪观察了拉伸和低周疲劳载荷作用下两种航空用超高强度钢中不同种类、形态和尺寸的夹杂物导致裂纹萌生与扩展的微观行为.结果表明,对于单个TiN和AlN夹杂,在拉伸载荷作用下,裂纹均首先在夹杂内部萌生.夹杂面积越大,夹杂内萌生的裂纹条数越多,第1条裂纹萌生所需的应力越小.在疲劳载荷作用下,对于单个TiN夹杂,裂纹也首先萌生于夹杂内部.但对于以点链状形式存在的AlN夹杂,无论是在拉伸还是疲劳载荷作用下,裂纹均首先在点链状夹杂内部两夹杂之间的母材中萌生,然后沿点链状夹杂向两侧扩展.以点链状形式存在的夹杂对材料疲劳性能的危害比单个夹杂严重得多,夹杂物对材料疲劳性能的危害远大于对拉伸性能的危害.     

NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery

Neutron diffraction measurements of internal elastic strains and crystallographic orientation were performed during compressive deformation of martensitic NiTi containing 0 vol pct and 20 vol pct TiC particles. For bulk NiTi, some twinning takes place upon initial loading below the apparent yield stress, resulting in a low apparent Young's modulus; for reinforced NiTi, the elastic mismatch from the stiff particles enhances this effect. However, elastic load transfer between matrix and reinforcement takes place above and below the composite apparent yield stress, in good agreement with continuum mechanics predictions. Macroscopic plastic deformation occurs by matrix twinning, whereby (1 0 0) planes tend to align perpendicular to the stress axis. The elastic TiC particles do not alter the overall twinning behavior, indicating that the mismatch stresses associated with NiTi plastic deformation are fully relaxed by localized twinning at the interface between the matrix and the reinforcement. For both bulk and reinforced NiTi, partial reverse twinning takes place upon unloading, as indicated by a Bauschinger effect followed by rubberlike behavior, resulting in very low residual stresses in the unloaded condition. Shape-memory heat treatment leads to further recovery of the preferred orientation and very low residual stresses, as a result of self-accommodation during the phase transformations. It is concluded that, except for elastic load transfer, the thermal, transformation, and plastic mismatches resulting from the TiC particles are efficiently canceled by matrix twinning, in contrast to metal matrix composites deforming by slip.     

Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue

The structural fatigue of pseudoelastic Ni-Ti wires (50.9 at. pct Ni) was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds. The number of cycles to failure (N _f ) was measured for different bending radii and wire thicknesses (1.0, 1.2, and 1.4 mm). The wires consisted of an alloy with a 50-nm grain size, no precipitates, and some TiC inclusions. In BRF tests, the surface of the wire is subjected to tension-compression cycles, and fatigue lives can be related to the maximum tension and compression strain amplitudes (ɛ _a ) in the wire surface. The resulting ɛ _a -N _f curves can be subdivided into three regimes. At ɛ _a > 1 pct rupture occurs early (low N _f ) and the fatigue-rupture characteristics were strongly dependent on ɛ _a and the rotational speed (regime 1). For 0.75 pct < ɛ _a < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For ɛ _a < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 10⁶ (regime 3). Using scanning electron microscopy (SEM), it was shown that surface cracks formed in regions with local stress raisers (such as inclusions and/or scratches). The growth of surface cracks during fatigue loading produced striations on the rupture surface; during final rupture, ductile voids form. The microstructural details of fatigue-damage accumulation during BRF testing are described and discussed. (on leave from the Centro Atomico Bariloche, Argentina)     

Experimental Study of the Biaxial Cyclic Behavior of Thin-Wall Tubes of NiTi Shape Memory Alloys

Combined torsion-tension cycling experiments were performed on thin-wall tubes (with thickness/radius ratio of 1:20, similar to that found for stents) of nearly equiatomic NiTi shape memory alloys (SMAs). Experiments were controlled by axial displacement and torsional angle with step loading involving torsional loading to a maximum strain, followed by tensile loading, and reverse-order unloading. The superelasticity of the material is confirmed by pure torsion and tension experiments at the test temperature. The evolution of equivalent stress-strain curves as well as the separated tensile and torsional stress-strain curves during cycling is analyzed. Results show that the equivalent stress increases greatly with a small amount of applied axial strain, and the equivalent stress-strain curves have negative slopes in the phase transformation region. The shear stress drops when the torsional strain is maintained at its maximum value and the tensile strain is increased. The shear stress increases with decreasing tensile strain, but it cannot recover to the original value after the complete unloading of the tensile strain. Attention is also paid to dissipated energy density and characteristic stress evolutions during cycling.     

Competing Modes for Crack Initiation from Non-metallic Inclusions and Intrinsic Microstructural Features During Fatigue in a Polycrystalline Nickel-Based Superalloy

Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.     

Prediction of thermomechanical fatigue lives in metal matrix composites

To identify the role of silicon carbide participate reinforcement on high-temperature thermomechanical fatigue behavior of Al 2xxx-T4, experiments have been conducted under thermomechanical out-of-phase and in-phase loading conditions. A general constitutive representation, based on Eshelby’s inclusion theory, is used for the determination of volumetric average stresses and strains under cyclic loading of the metal matrix composite. This constitutive representation is used with a life prediction model, based on the matrix stress-strain behavior, which predicts contributions of fatigue, creep, and environmental damages to failure under both isothermal and thermomechanical fatigue loading. In isothermal fatigue experiments at 200 °C and 300 °C, pure fatigue damage and creep damage are the dominant damage mechanisms in the short-life regime. In the long-life regime, however, the stress levels are too low to induce considerable creep damage; so, oxidation damage becomes dominant. When fatigue damage is dominant, the model predicts a decrease in life, based on strain range, with increasing volume fraction of reinforcement. Based on stress range, improved fatigue lives are predicted with increasing volume fraction of reinforcement. The reinforced alloy exhibits longer lives when compressive hydrostatic stresses in the matrix at the high-temperature end of the cycle reduce the creep damage. Temporarily Director, Mechanics and Materials Program, National Science Foundation, Washington, DC 20550     

Evaluation of the fatigue behavior of ductile irons with various matrix microstructures

The stress-failure (S-N) curves for ferritic irons, pearlitic irons, and austempered ductile irons (ADIs) have been determined under tension-tension loading with a stress ratio of 0.1. The effects of Ti contents of up to 0.10 wt pct (resulting from the deliberate use of Ti-containing steel scrap) on fatigue behavior were investigated. It was found that ferritic and pearlitic ductile irons can contain up to 0.10 wt pct Ti without any adverse effect on fatigue behavior. In ADIs, fatigue properties deteriorate at such high Ti contents. Tests were also conducted to investigate the effects of microstructural features on fatigue properties. It was found that the effect of the graphite nodule count (the number of graphite particles on a unit area of a polished surface) on the fatigue limit is significant only in ADIs. Scanning electron microscope (SEM) analysis has shown that cracks usually initiate from surface dross-type defects. However, in ADIs, fatigue cracks can also initiate at shrinkage cavities and at surface or subsurface locations. An offset bilinear S-N curve behavior (the linear S-N curve at higher stress levels is separated from the linear S-N curve at lower stress levels) has been observed in ADIs. This is attributed to surface residual compressive stresses, which prohibit fatigue crack initiation from surface positions at lower applied stress levels. In ferritic and pearlitic ductile irons, the offset bilinear S-N curve behavior is not observed because of the rapid relaxation of the residual compressive stresses.     

Kinetics of small fatigue cracks in steel during cyclic loading

The kinetics of development of fatigue microcracks in a high-strength 08Kh14AN4MD steel is studied when a cantilever specimen with several notches is subjected to rotational bending tests. The specific feature of the tests consists in the fact that, when the specimen is loaded at a constant load, different stress amplitudes are realized in different notches. As a result, after the sample has failed across the section with the maximum stress, a longitudinal polished section of the specimen contains fatigue cracks having nucleated in the mouths of the other notches at lower stress amplitudes. A relation between inflection points in the fatigue curve and the conditions of local and developed yield in a notch has been established. Fatigue fracture mechanisms at a superhigh number of loading cycles are studied, and their relation to the structure near a crack is found. A rapid method for estimating the fatigue limit at a superhigh number of loading cycles is proposed.     

Cyclic-Tension Fatigue Behavior in a Rolled AZ31B Magnesium Alloy Studied Using Ultrasonic Shear Waves

Nondestructive evaluation of cyclic-tension fatigue in a rolled magnesium alloy, Mg-3Al-1Zn, was performed using vertically polarized shear wave (SV) reflection and shear horizontal wave (SH) transmission methods. Internal friction measured by SV reflection increased rapidly in the early stages of the fatigue and finally saturated, showing dominating interactions of movable dislocations and twinning boundaries with the waves as acoustic nonlinearities. The propagation time and logarithmic damping ratio in the SH transmission method followed a repeated increase and subsequent sudden decrease pattern, and finally converged toward fatigue failure due to acoustoelasticity, which represents the interaction with residual stresses. The wave and phase data were determined using an optical microscope, a scanning electron microscope, a surface roughness tester, and X-ray diffraction. The results demonstrated that during the fatigue process, residual stress accumulated on the compressive side of the specimen, despite the applied cyclic-tension loading. Brittle cracks that originated in inclusions provided sudden relief from the residual stress.     

基于有限元方法的硬质合金TiCN梯度涂层的热应力分析

采用有限元方法分析硬质合金TiCN梯度涂层的热应力缓和机制,比较了单层TiC、TiN涂层和TiC/TiCN/TiN梯度复合涂层对热应力场的影响。计算表明:单层的TiN涂层表面应力最大,为1728MPa;单层的TiC涂层应力最小,为1067MPa;在TiN和TiC之间增加TiCN过渡层时,应力减小为1 674 MPa;只有TiC/TiCN时,应力减小幅度最大,只有1352MPa。TiCN层内应力梯度非常明显,其中等效应力最大值出现在边角上,为3148MPa。因此,热裂纹应从刀片边缘开始,与实验结果吻合较好。     

Contact of crack surfaces during fatigue: Part 1. formulation of the model

A model has been developed to predict crack opening and closing behavior for propagating fatigue cracks which undergo significant sliding displacements at crack flanks. Crack surfaces were described statistically by assuming a random distribution of asperity heights and a mean density of asperities and asperity radii. The propagating crack was subdivided into strips, and each strip was treated as a contact problem between two randomly rough surfaces. The remote tensile stresses were varied in a cyclical manner. The contact stresses at minimal load were determined by analyzing the local crushing of asperities via a sliding mechanism. Then, upon loading, the crack opening stress levels were computed when the contact stresses were overcome. Part 1 of this article includes a discussion of the previous models, then introduces statistical contact mechanics concepts which are utilized in the fatigue crack growth simulations. In addition, the numerical algorithms for the modeling work and the sensitivity of results to model parameters are described. The role of stress ratio, maximum stress level, crack length, and the geometry of crack surfaces on the crack growth behavior will be discussed in Part 2 of this article.     

Residual Stress Measurement and Fatigue Crack Growth Prediction after Cold Expansion of Cracked Fastener Holes

The cold expansion technique is often used to introduce beneficial compressive stress at fastener holes, and can be used for remedial work where cracks already exist. In this paper, results are presented showing the effect of preexisting cracks on the residual stress field produced by cold expanding a fastener hole, and on subsequent fatigue crack growth. The effect on the residual stresses was experimentally evaluated in two ways: indirectly, in terms of retained expansion and directly, by measurement of the stresses using the X-ray and neutron diffraction techniques. The retained expansion ratio showed that cold expansion is more sensitive to the existence of precracks at lower levels of applied interference, and the inlet and outlet faces have different behavior. The stress measurements showed that preexisting cracks reduce the compressive residual stresses more on the mandrel inlet face than on the outlet face and in the middle of the specimen. The effect on fatigue crack growth rates was modeled using a linear-elastic fracture mechanics approach. It was found that cold expansion of a hole containing a preexisting crack longer than 1 mm introduces little benefit for subsequent fatigue crack growth behavior.     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.     

Progress in the understanding of NiTi shape memory alloys

Shape memory alloys (SMAs) are a special class of metallic materials which respond with a considerable change in their properties to small changes in temperature or stress. The SMAs offer two interesting characteristics, viz., shape memory effect (SME) and superelasticity (SE), also called pseudoelasticity which make them attractive for applications in engineering and biomedical fields. Among the various SMAs, NiTi base alloys have been the most commercially exploited ones because of their superior SME and SE, mechanical properties, corrosion resistance and biocompatibility. Since the pioneering discovery of NiTi SMA in early 1960s, significant progress has been made in the processing and understanding of the behaviour of these alloys. In spite of these efforts, the NiTi SMAs continue to offer challenges to the scientists and engineers, and new findings are being made continuously. This paper provides an overview of the developments in NiTi SMAs.     

Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.     

Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

Air fatigue of notched 1018 steel in the endurance limit range

Reverse bend, constant deflection fatigue tests were performed in ambient temperature air on notched 1018 steel specimens. Endurance limit for these exhibited scatter in excess of 25 pct, and this was correlated with concentration of manganese sulfide inclusions at the notch root vicinity. For specimens designated as “high inclusion” the endurance limit was 95 MN/m² (14 ksi); and for “low inclusion” ones it was 130 MN/m² (19 ksi). By monitoring crack length as a function of number of cycles it was observed that unbroken specimens tested above approximately 85 MN/m² (12.5 ksi) contained nonpropagating cracks. Also, high inclusion specimens fatigued at stresses above their endurance limit, but less than about 140 MN/m² (20 ksi) exhibited fatigue crack hesitation; that is, cracks initiated and grew to microscopic size but then arrested for a finite number of subsequent cycles. The endurance limit for high inclusion specimens was then a stress above which arrested cracks repropagate and below which they do not. Repropagation of hesitant fatigue cracks is projected to result from inclusion related cyclic stress damage near the crack tip, which was possibly stimulated by atmospheric interactions.