Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

Effect of prestress on low-cycle fatigue

An investigation was conducted on HY-80 steel to show how low-cycle fatigue life is influenced by imposition of various stresses prior to fatigue testing. One cycle of static prestress was imposed on each specimen at loads either above or below the yield strength of the material and fatigue testing was done at various levels of stresses. Tests were conducted on notched and tee-fillet welded beam specimens. The magnitude and type of residual stresses at the toe of tee-fillet welds were determined by a hole drilling technique. Experimental data show that residual stresses induced by prestressing can—according to their sign—influence fatigue life. It was observed in the notched beam tests that when stress cycling was performed at stress levels below the yield strength of the material, tensile prestress increased fatigue life and that the higher the prestress, the greater the increase in life. At stress levels above the yield strength, tensile prestress has little or no effect on fatigue life. Compressive prestresses were found to be detrimental to fatigue life regardless of the stress levels at which the fatigue tests were run. The effects of prestress on the tee-weldments were not delinated as clearly. Fatigue life was increased by tensile prestress but no adverse effects were observed for compressive prestress.     

Multiaxial fatigue of welded joints under constant and variable amplitude loadings

Flange-tube joints from fine grained steel StE 460 with unmachined welds were investigated under biaxial constant and variable amplitude loading (bending and torsion) in the range of 10³ to 5 × 10⁶ cycles to crack initiation and break-through, respectively. In order not to interfere with residual stresses they were relieved by a heat treatment. In-phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on the basis of nominal, structural or local strains or stresses. But the influence of out-of-phase loading on fatigue life is severely overestimated if conventional hypotheses are used. However, the hypothesis of the effective equivalent stress that is introduced leads to fairly good predictions for constant as well as for random variable amplitude loads. Therefore, the knowledge of local strains or stresses is necessary. They are determined by boundary element analyses that are dependent on weld geometry. This hypothesis considers the fatigue-life-reducing influence of out-of-phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from weld geometry and loading mode were included. Damage accumulation under a Gaussian spectrum can be assessed for in- and out-of-phase combined bending and torsion using an allowable damage sum of 0.35.     

Residual stresses and fatigue performance

Residual stresses are an inescapable consequence of manufacturing and fabrication processes, with magnitudes that are often a high proportion of the yield or proof strength. Despite this, their incorporation into life prediction is primarily handled through sweeping assumptions or conservative application of statistics. This can lead to highly conservative fatigue design methodologies or unforeseen failures under dynamic loading. The pull from the desire for higher levels of materials performance, coupled with the push of more sophisticated techniques for residual stress measurement, favours a reassessment of the accuracy of assumptions made about residual stresses and their modification during fatigue cycling. A viewpoint is also emerging that the fatigue performance of welded joints might be optimised through careful process control, coupled with understanding of the relative positions of, and interaction between, residual stress peaks, weld defects, hardness and microstructure. This paper will present information regarding the residual stress profiles in aluminium and steel welds, and in shot peened aluminium, obtained via synchrotron and neutron diffraction at the ESRF-ILL in Grenoble. Certain specimens were then subjected to specific cases of fatigue loading and the residual stress field was again measured. Difficulties associated with determining the strain-free lattice spacing will be mentioned, and the potential import of these data for life prediction modelling will be considered.     

A parametric study on fatigue strength of spot‐weld joints

Fastening elements usually lead to high stress concentrations; fatigue failure thus becomes the most critical failure mode for a fastening element itself or the region around it under fluctuating stresses. A designer should seek the ways of increasing fatigue strength of a joint to ensure the safety of the whole structure. Resistance spot welding is the most preferred method to join metal sheets. The design variables for spot‐weld joints affecting their strengths are basically sheet thickness, spot‐weld nugget diameter, number of spot welds and the joint type as exemplified in tensile shear (TS), modified tensile shear (MTS), coach peel (CP) and modified coach peel (MCP) specimens. In this study, the effects of these parameters on the fatigue life of spot‐weld joints have been investigated. For this purpose, one of the most reliable fatigue assessment models, Coffin–Manson approach, was used. In order to accurately determine the stress and strain states, a nonlinear finite element analysis was carried out taking into account plastic deformations, residual stresses developed after unloading and contacting surfaces. The results provide designers with some guidelines to foresee the impact of design changes on fatigue strength of spot‐weld joints.     

Influence of shot peening on fatigue durability of normalized steel subjected to variable amplitude loading

The influence of shot peening on the fatigue durability of normalized carbon steels subjected to variable amplitude loading has been investigated. The relaxation of residual stresses was recorded during the fatigue life time. Strain amplitude spectra were extracted from real spectra recorded from components in service. The results were compared with data achieved from constant amplitude testing. In both types of tests parallel studies were made on both peened and unpeened specimens. Shot peening leads to pronounced increase in life time, especially for smaller amplitudes. For both variable and constant amplitude loading shot peened specimens exhibit longer life provided the residual stresses during fatigue loading do not relax more than to about 60% of their initial value. To get an improvement in life time of at least a factor two for peened specimens, the stress amplitude in constant amplitude loading or the maximum stress amplitude in variable amplitude history must not be more than 20% larger than the magnitude of the initial residual stresses. This limit corresponds to 1.2 times the yield strength of the unaffected material.     

Analysis of manufacturing effects on fatigue failure of an automotive component using finite element methods

In this study, the fatigue life of an automotive suspension component was analysed using finite element methods with regard to stamping and welding effects. Because automotive suspension components are produced by forming and welding sheet metal, there are various effects on the final product, such as uneven thickness distribution, residual stresses and weld notches. Manufacturing effects may change the mechanical performance of the automotive components; therefore, it is desirable to consider these effects in the early design stage. Residual stresses due to work hardening and thermal deformation were investigated through process simulation. The redistribution and relaxation of residual stresses in a component were investigated in fatigue life analysis under a cyclic loading condition. Various equivalent relaxation curves were investigated and one was selected after comparisons with test results. The fatigue simulation results were compared to the test results; a good correlation between the two was achieved for the residual stress effects in terms of life cycles and failure locations. The simulation results also show that welding produces more detrimental effects than stamping with regard to the fatigue life of a component.     

复杂面内应力状态下平面编织高铝纤维增强氧化铝基复合材料强度及疲劳寿命预测方法

针对平面编织氧化铝基复合材料提出了一种复杂面内应力状态下的强度准则和疲劳寿命预测方法。通过拉伸、压缩及纯剪切试验,分别获得了材料的静强度指标。考虑材料拉、压性能的差异和面内拉-剪联合作用对材料强度的影响机制,提出了修正的Hoffman强度理论。采用该强度理论预测得到的偏轴拉伸强度与试验结果基本一致,偏差不超过10%。开展了偏轴角θ=0°、15°、30°、45°,应力比R=0.1,频率f=10 Hz的拉伸疲劳试验,试验结果表明随着偏轴角的增加,相同轴向拉伸载荷下的疲劳寿命逐渐降低。由于面内剪切应力分量的作用,疲劳失效由纤维主导逐渐过渡到纤维和基体共同主导的模式。基于单轴疲劳寿命曲线,采用Broutman-Sahu剩余强度模型表征剩余强度随疲劳循环次数的变化规律,结合剩余强度演化模型和修正的Hoffman强度理论,提出了一种面内复杂载荷条件下的疲劳寿命预测模型,并引入疲劳剪切损伤影响因子表征拉-剪应力联合作用对材料疲劳行为的影响。采用本文提出的疲劳寿命预测模型,预测不同偏轴角拉伸疲劳寿命,预测结果与试验结果基本一致,偏差在1倍寿命范围内。比较结果表明在给定应力比、温度和疲劳载荷频率条件下,该疲劳寿命预测模型可以用来预测平面编织氧化铝基复合材料拉-剪复杂面内载荷条件下疲劳寿命。      

Lightweight design of vehicle components based on strengthening effects of low‐amplitude loads below fatigue limit

A new lightweight design method for vehicle components is proposed based on the strengthening effects of low‐amplitude loads below the fatigue limit. The new method is technically based on the strength feature of strengthening and damaging of vehicle components under loading spectrum, while combining dynamic strength equations with the residual strength of vehicle components. It ensures the maximum exploitation of the material's strength potential and fully realises the lightweight design of vehicle components at a low cost. As an application of the new lightweight design method, a light truck's front axle was redesigned. The lightweight potential of the front axle was first estimated by fatigue and static strength experiments of four‐point bending. Then the lightweight design was realised by finite element analysis and experimental results. The weight of this front axle was reduced by 5.5 kg.     

加载频率与应变比对10CrNi5Mo高强钢低周疲劳寿命的影响

通过对圆形横截面光滑试样进行轴向应变控制的低周疲劳试验,研究了加载频率与应变比对10CrNi5Mo高强钢疲劳寿命的影响。结果表明:在较低频率(0.05～0.1 Hz)范围内,频率对疲劳寿命的影响不明显,随着加载频率的增加(0.1～0.8 Hz),疲劳寿命显著提高,当加载频率增加到0.8 Hz时,疲劳寿命达到最大值,随着加载频率的继续增加(0.8～1.0 Hz),疲劳寿命反而有所下降;在最大应变不变条件下,随着应变比的增加(-1～-0.3),疲劳寿命平稳上升,当应变比增加到-0.30时,随着应变比的继续增加,材料的疲劳寿命急剧升高。研究结果为10CrNi5Mo高强钢的工程应用提供了依据。     

基于循环载荷的单搭胶接接头残余强度分析

为研究循环载荷下单搭胶接接头的残余强度及失效机理,以5052铝合金单搭胶接接头为研究对象,先后对其进行静强度测试、疲劳强度测试和残余强度测试,引入威布尔分布对试验数据进行分析,检验其有效性,并采用超声扫描显微镜和扫描电子显微镜对失效胶层进行失效机理分析。结果表明,在疲劳循环载荷作用下,接头刚度基本稳定,而残余强度随着疲劳循环载荷周次的增加,呈现出先增大后减小的变化趋势;疲劳裂纹从接头搭接端部的界面端点处开始萌生,并快速向中间扩展,当疲劳循环达到一定次数时,胶层瞬间断裂,裂纹萌生阶段几乎占据了其全部疲劳寿命,失效后的胶层会出现"凹台"状微观结构。     

Procedure for reliable durability validation of train axles

The structural durability of wheelset axles depends on the operational loading as well as on the fatigue strength of axles. To validate an axle design the time‐varying stresses in individual critical areas of the axle, represented by their stress spectra, must be taken into account. The allowable stresses in individual axle areas depend both on the fatigue strength and on the spectrum of operational stresses. The procedure for the axle structural durability validation, presented in this paper, includes the numerical fatigue life estimation as well as the experimental durability approval. Also the influences of axle design, material properties and manufacturing technology on structural durability are discussed.     

Lebensdauer von Schweißverbindungen unter mehrachsigen Belastungen mit variablen Amplituden – Schadensakkumulation und Hypothese der wirksamen Vergleichsspannung

Fatigue Life of Welded Joints under Multiaxial Variable Amplitude Loading – Damage Accumulation and the Effective Equivalent Stress Hypothesis Flange‐tube joints from fine grained steel FeE 460 with different notch factors were investigated under biaxial constant and Gaussian variable amplitude loading (bending and torsion) in the range of 10⁴ to 5 · 10⁶ cycles to crack initiation and break‐through, respectively. In order not to interfere with residual stresses they were stress relieved by a heat treatment. Damage accumulation under a Gaussian spectrum can be assessed for in‐ and out of phase combined bending and torsion using an allowable damage sum of 0.35 for weldments in the as‐welded state and 0.16 for joints with machined weldments with a lower notch factor of the critical area. For the evaluation of multiaxial in‐ and out‐of‐phase stresses the introduced hypothesis of the effective equivalent stress leads to good results. For this, the knowledge of local strains or stresses is necessary. This hypothesis considers the fatigue‐life reducing influence of out‐of‐phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from the weld geometry and loading mode were included.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Thermisch‐mechanisches Ermüdungsverhalten der partikelverstärkten Aluminiumknetlegierung EN AW‐6061‐T6 und die Entwicklung von Eigenspannungen im Matrixwerkstoff unter thermisch‐mechanischer Beanspruchung

Thermal mechanical fatigue behaviour of particle reinforced EN AW‐6061‐T6 and development of residual stresses in the matrix material by thermal mechanical loading The behaviour of non reinforced and 15 Vol.‐% α‐alumina particle reinforced wrought aluminium alloy EN AW‐6061‐T6 in thermal mechanical fatigue loading was investigated at different maximum temperatures. The tests were performed in strain controlled mode by means of an electro‐mechanical testing machine. Alternating load deformation and life cycle behaviour either materials were compared. It came out, that the reinforcement leads to an decreasing thermal mechanical fatigue life cycle while keeping constant the maximum temperature and mechanical loading. The two materials showed softening behaviour due to high maximum temperatures of 573 K to 673 K. However, there is an intense scatter of the number of cycles to failure of the non reinforced alloy aggravating the interpretation of the results. On the other hand the thermal mechanical life cycle increases in combination with increasing maximum temperatures. Simultaneously the part of plastic deformation in mechanical loading increases for both materials, while for a constant total strain range the effective maximum and minimum stresses are decreasing. Furthermore, the development of residual stresses in the matrix of the reinforced alloy by thermal mechanical fatigue loading was analysed. It was observed that only small absolute values of residual stresses will be obtained for these loads. Nevertheless, tendencies of mounting tensile residual stresses can be identified in the direction of thermal mechanical fatigue loading and subsequently reduction of the residual stresses.     

Effect of the amount of adhesive on the bending fatigue strength of adhesively bonded aluminum honeycomb sandwich beams

The effect of the amount of adhesive for bonding face sheets and cores on the bending fatigue strength of aluminum honeycomb sandwich beams was analyzed. It was experimentally proved that the fatigue strength increases as increasing the amount of adhesive. Furthermore, the applied loading parameter is not correlated with the fatigue life data of all studied specimens with various amounts of adhesive because the global parameter has no clear physical meanings with respect to the failure mechanism. From the observations made during fatigue testing, debonding at the interface between the honeycomb core and face sheet is the main cause of fatigue failure. Finite element analyses were conducted to obtain the local stress states at the interface, and these simulated stresses were employed in fatigue life prediction parameters. Three local interfacial parameters were adopted and correlated with the experimental data for the studied specimens. The predicted failure locations using the three interfacial parameters were also examined by comparing the observation results in fatigue tests. Among the three studied interfacial parameters, the combined interfacial peeling and shear stress parameter is recommended for use in fatigue design as it provides good fatigue life correlations and predicts the correct locations of failure initiation simultaneously.     

Festigkeitsverhalten von Schweißverbindungen unter kombinierten phasengleichen und phasenverschobenen mehrachsigen Beanspruchungen

Fatigue Behaviour of Welded Joints under Combined In-and Out-of-Phase Multiaxial Loading Welded crucifix-type specimens from fine grained steel StE 290 and flange-tube as well as tube-tube joints from fine grained steel StE 460 with unmachined and machined welds were investigated under biaxial constant amplitude loading in the range of 10³ to 5 · 10⁶ cycles to crack initiation and break-through, respectively. In order not to interfere with residual stresses they were relieved by a heat treatment. Inphase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on basis of nominal, structural or local strains or stresses. But the influence of out-of-phase loading on fatigue life is severely overestimated if conventional hypotheses are used. However, the introduced hypothesis of the effective equivalent stress leads to fairly well predictions. Herefore, the knowledge of local strains or stresses is necessary. They are determined by boundary-elemente analyses in dependency of weld geometry. This hypothesis considers the influence of out-of-phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from weld geometry and loading mode were included.     

Zur Steigerung der Schwingfestigkeit durch mechanisch erzeugte Druckeigenspannungen

About the Fatigue Strength Improvement due to Compressive Residual Stresses Induced by Coldworking The fatigue strength improvement due to compressive residual stresses induced by coldworking depends essentially on the material, the type of load sequence, and the load level. The estimation of the fatigue life improvement is very difficult, because the load sequence can also induce residual stresses due to plastification in the notch root. In this case it gives complicate interaction effects between the two types of residual stresses. By tests the fatigue strength improvement due to stress coining was determined for two types of materials (an Al-alloy AlCuMg₂, and a fine grained structural steel FeE 47) under three different load sequences, at two load levels and on notched specimens of four different sizes. Besides the residual stresses induced by stress coining also those caused by the load sequences were evaluated. The results show, that the fatigue strength improvement of higher strength materials is superior to that of lower strength materials. In the case of higher strength materials the main effect results from the reduction of the mean stress by the compressive residual stresses while for the lower strength materials also the strain hardening by coldworking is important. Possibilities for the estimation of fatigue strength improvement will be presented.     

On the interaction of normal and shear stresses in multiaxial fatigue damage

One of the important issues in assessing multiaxial fatigue damage is interactions between different components of stress such as normal and shear stresses. The present study investigated this interaction effect on the fatigue behavior of materials with shear failure mode when subjected to multiaxial loading conditions. A method is introduced to model this interaction based on the idea that two types of influence are caused by the normal stress acting on the critical plane orientation. These two types of influence are affecting roughness induced closure, as well as fluctuating normal stress which affects the growth of small cracks in mode II. Shear‐based critical plane damage models which use normal stress as a secondary input, such as FS damage model, could then use the summation of these terms. In order to investigate the effect of the method, constant amplitude load paths with different levels of interaction between the normal and shear stresses, as well as variable amplitude tests with histories both taken from service loading conditions and generated using random numbers were designed for an experimental program. The proposed method was observed to result in improved fatigue life estimations where significant interactions between normal and shear stresses exist.     

Festigkeit und Spannungsabbau bei der dynamischen Belastung von Stahl- und Aluminiumproben mit bekannten Eigenspannungen

Strength and Relaxation of Stress During Dynamic Loading of Steel and Aluminium Specimens with known Residual Stress Distributions Residual stresses of the first kind can be introduced in multiply – connected bodies without accompanying changes in the material. The circular ring represents the simplest example in this group. For the circular ring the residual and applied stresses in a fatigue test can be calculated accurately with relations from the theory of elasticity. Circular rings with and without residual stresses were subjected to fatigue testing. It was established that tensile residual stresses reduce the fatigue life and compressive residual stresses have the opposite effect, with the stresses referred to the point of crack initiation. The fatigue behaviour is altered appreciably even by small magnitudes of residual stress. The effect on fatigue life is the same, whether at the point of crack initiation a residual stress or a mean stress of the same magnitude and direction acts. The stress relaxation depends primarily on the difference between the maximum superposed stress and the yield strength or the 0.2% proof stress. The degree of relaxation is particularly high when the yield strength is reached. The first few cycles in a fatigue test are important for the fading of stress. The state of residual stress changes only slightly thereafter. The stress relaxation increases again somewhat only after a very large number of cycles. The changes observed when the sum of residual and loading stresses is below the yield strength may be attributed to the small plastic deformations in favourably oriented crystallites.