拉-压循环加载下铝合金疲劳裂纹扩展的压载荷效应研究

应用弹塑性有限元方法与增量塑性损伤理论指出疲劳裂纹扩展的压载荷效应是裂纹尖端塑性损伤的结果, 建立了在拉-压循环加载下铝合金疲劳裂纹扩展速率的双参数预报模型, 对LY12-M 高强铝合金MT 试件在应力比R=0、-0.5、-1、-2 进行了疲劳裂纹扩展实验。结果表明:当最大应力强度因子K_max相同时, 恒幅拉压加载(应力比R<0)的疲劳裂纹扩展速率明显高于恒幅拉拉加载(应力比R=0)的情况, 拉-压循环载荷的压载荷部分对疲劳裂纹扩展速率具有促进作用。该文得出的LY12-M 铝合金在拉-压循环加载下的疲劳裂纹扩展速率预报模型与实验结果符合较好。     

缺口应力集中系数对TC4 ELI合金低周疲劳性能的影响

研究了缺口应力集中系数不同的深海潜水器耐压壳用TC4 ELI(Extra-low-interstitial)合金板材在恒总应变幅控制下的低周疲劳行为。结果表明,在应变幅较低(0.7%以下)和应变幅较高(0.8%和0.9%)条件下的光滑试样在循环初期分别发生了循环硬化和循环软化,而缺口试样在0.2%～0.7%应变幅条件下的循环初期均发生了循环硬化。通过循环载荷作用下材料滞回能的变化描述了TC4 ELI合金试样低周疲劳的损伤程度,得到了缺口应力集中系数与低周疲劳性能参数之间的关系,建立了相对裂纹萌生寿命预测模型。利用该模型能较好地预测缺口应力集中系数较低的TC4 ELI合金在高应变幅条件下的相对疲劳裂纹萌生寿命。     

2D-C/SiC缺口试样的拉-拉疲劳损伤

研究了二维正交编织C/SiC双边对称圆弧缺口试样室温和高温真空的拉拉疲劳行为,正弦波疲劳应力比R=0.1,频率60Hz,循环基数106次.循环到规定周次停机,测量试样的共振频率、电阻,并进行SEM观察.结果表明,2D-C/SiC复合材料缺口试样拉-拉疲劳的S-N曲线非常平坦,其疲劳极限是同温度下缺口试样拉伸强度的80%～90%,光滑试样和缺口试样的疲劳极限比值与理论应力集中系数基本相同.缺口试样在疲劳过程中,电阻表征损伤与模量表征损伤的规律基本一致.在疲劳试验初期阶段,缺口附近损伤发展很快,主要表现为产生大量与加载方向垂直的裂纹,随着疲劳次数的增加,损伤发展减缓,但损伤形式逐渐增多,缺口附近与加载方向垂直的裂纹数量明显多于平行加载方向的裂纹数.讨论了电阻表征损伤和模量表征损伤之间的关系.     

TC21合金315℃高周疲劳光滑和缺口试样断口分析

开展了TC21研究合金光滑和缺口试样的315℃高周疲劳实验,并对疲劳断口进行详细观察,研究了缺口对TC21合金疲劳寿命的影响.结果表明,光滑和缺口试样的疲劳强度比值随循环寿命降低而降低;光滑试样的失稳疲劳裂纹长度随循环应力升高而降低;瞬断区所承受的断裂应力随循环应力升高而降低;这说明裂纹失稳决定试样的断裂,缺口试样断口有多个裂纹源,以缺口试样名义应力乘以应力集中因子与光滑试样的应力相等作为比较时,缺口试样主裂纹长度大于光滑试样的裂纹长度,缺口试样的裂纹扩展寿命更长.     

2024铝合金喷丸试件疲劳寿命试验及仿真研究

现有的喷丸材料疲劳性能研究扩展有限元模型没有考虑残余应力对裂纹扩展的影响。对2024铝合金的喷丸与未喷丸试样进行三弯疲劳试验,以明确喷丸工艺对试件疲劳寿命的强化作用。通过ABAQUS建立试件的二维平面应力模型,导入残余应力并利用扩展有限元法模拟循环载荷下裂纹的萌生与扩展,对比试验结果来验证该扩展有限元数值模型的正确性。最后基于该数值模型,改变载荷工况,研究不同载荷工况下残余应力对疲劳寿命的影响,得到喷丸残余应力强化作用与载荷工况的关系。结果表明:喷丸引入的残余应力可以有效地增强试件的疲劳寿命;过大的循环载荷可能造成喷丸残余应力发生松弛;在最大载荷不变的前提下,应力比越小,试件疲劳寿命越短;应力比越大,残余应力对疲劳寿命强化效果越明显。     

加载载荷对ADB610钢疲劳裂纹扩展性能的影响研究

针对ADB610新型低碳贝氏体钢,采取紧凑拉伸试样、恒幅加载方式,在应力比为0.1时进行3种不同加载载荷下多试样的疲劳裂纹扩展试验,并测出裂纹扩展长度a和循环寿命N的a-N试验数据。然后采取七点递增多项式法对a-N试验数据拟合计算疲劳裂纹扩展速率。结果表明,加载载荷较小时,ADB610钢循环寿命较长;在疲劳裂纹扩展的初始阶段,加载载荷小的疲劳裂纹扩展速率相对较慢,但随着疲劳裂纹逐渐扩展,加载载荷小的疲劳裂纹扩展速率反而更大一些。     

循环压缩载荷下缺口残余拉应力场的X射线分析

用细束 x 射线研究了循环压缩载荷下缺口残余拉应力的分布及变化。试验表明,循环压缩加载后形成的残余拉应力大于一次压缩加载后的数值,压缩应力幅对残余拉应力分布的影响大于压缩平均应力的影响,其原因除了加载-降载过程中形成的残余应力直接与应力幅有关外,尚与应力幅加剧材料循环软化程度有关。形成疲劳裂纹后,在完全卸载的裂纹面上,残余拉应力基本松弛,但垂直于裂纹面稍远处的残余拉应力仍保持有相当大的数值,这部分残余应力是否对裂纹扩展起作用,在计算残余应力的应力强度因子时如何予以考虑值得注意。     

拉压加载下纤维增强铝合金层板疲劳裂纹扩展的压载荷效应与预测模型

基于增量塑性损伤理论与纤维增强金属层板疲劳裂纹扩展唯象方法, 推导出在拉-压循环加载下, 纤维增强金属层板疲劳裂纹扩展速率预测模型。并通过玻璃纤维增强铝合金层板在应力比R=-1,-2的疲劳裂纹扩展实验对预测模型进行验证。结果表明, 纤维增强铝合金层板疲劳裂纹扩展的压载荷效应分为两种情况: 在有效循环应力比R_C>0时, 表现为压载荷对铝合金层所承受残余拉应力的抵消作用; 当R_C<0时, 表现为压载荷抵消残余拉应力后, 对纤维增强铝合金层板金属层的塑性损伤, 对疲劳裂纹扩展存在促进作用。纤维铝合金层板疲劳裂纹扩展的压载荷效应不可忽略, 本文中得出的在拉-压循环加载下疲劳裂纹扩展速率预测模型与实验结果符合较好。     

一种多轴向耦合随机激励下缺口试件振动疲劳寿命预测方法EI北大核心CSCD

提出了一种多轴向耦合随机激励下缺口结构振动疲劳寿命预测的频域分析方法。实施了缺口试件的双轴向随机振动疲劳试验,研究了两个振动轴向上载荷谱之间的相干性和相位差对缺口试件疲劳损伤的影响规律;通过随机振动分析计算得到试件缺口根部各节点的应力功率谱密度矩阵,并假设缺口试件裂纹萌生点为历经von Mises应力最大均方根值的节点;缺口试件疲劳临界点可由疲劳裂纹初始点和修正临界距离理论确定;在疲劳临界点处通过Carpinteri-Spagnoli频域准则计算缺口试件的振动疲劳寿命,并与试验结果进行了对比。结果表明:该多轴缺口疲劳预测方法具有较高的预测精度,绝大部分预测结果都在3倍误差带内。     

基于循环孔洞扩张模型的Q355钢超低周疲劳断裂数值模拟

基于微观断裂力学的循环孔洞扩张模型是进行钢材超低周疲劳断裂分析的有效手段。通过光滑圆棒循环加载试验,确定了Q355钢混合强化模型材料参数;进行了不同加载方式下Q355钢单边缺口试件的超低周疲劳试验,确定了试件的超低周疲劳寿命及其断裂发展过程;建立了单边缺口试件有限元模型,基于有限元分析结果,采用循环孔洞扩张模型对试件的超低周疲劳寿命进行了预测,并通过编写用户子程序,删除断裂单元以模拟试件的断裂扩展过程,对试件的超低周疲劳断裂全过程进行了数值模拟。数值模拟结果与试验结果基本吻合,验证了循环孔洞扩张模型对钢材超低周疲劳断裂全过程数值模拟的适用性。     

钠钙玻璃的缺口弯曲强度

钠钙玻璃的缺日弯曲强度与试样的应力集中系数K_t成反比;而缺口根部断裂应力与K_t无关．如弯曲强度服从Weibull分布,其模量m是材料常数,而标度因子σ₀与K_t成反比．如弯曲强度服从正态分布,其平均值及标准差均与K_t成反比,而变异系数为常数．     

Notched fatigue behavior including load sequence effects under axial and torsional loadings

Notch effects on axial and torsion fatigue behaviors of low carbon steel were investigated. Fully-reversed tests were conducted on thin-walled tubular specimens with or without a transverse circular hole. A shear failure mechanism was observed for both smooth and notched specimens and under both axial and torsion loadings. The notch effect was more pronounced under axial loading, in spite of higher stress concentration factor in torsion. The commonly used nominal S–N approach with fatigue notch factor in conjunction with von Mises effective stress resulted in overly conservative life predictions of both smooth and notched torsion fatigue lives. Neuber’s rule yielded notch root stress and strain amplitudes close to the FEA results for both axial and torsion loadings. The local strain approach based on effective strain obtained from Neuber’s rule or FEA resulted in poor correlation of the fatigue life data of smooth and notched specimens. The Fatemi–Socie critical plane parameter represented the observed failure mechanism and resulted in very good correlations of smooth and notched specimens fatigue data under both axial and torsion loadings. In block loading tests with equal number of alternating axial and torsion cycles at the same stress level, beneficial effect of axial loading was observed. Possible potential reasons for this unexpected behavior are discussed.     

Notch fatigue life prediction considering nonproportionality of local loading path under multiaxial cyclic loading

An innovative numerical methodology is presented for fatigue lifetime estimation of notched bodies experiencing multiaxial cyclic loadings. In the presented methodology, an evaluation approach of the local nonproportionality factor F for notched specimens, which defines F as the ratio of the pseudoshear strain range at 45° to the maximum shear plane and the maximum shear strain range, is proposed and discussed deeply. The proposed evaluation method is incorporated into the material cyclic stress‐strain equation for purpose of describing the nonproportional hardening behavior for some material. The comparison between multiaxial elastic‐plastic finite element analysis (FEA) and experimentally measured strains for S460N steel notched specimens shows that the proposed nonproportionality factor estimation method is effective. Subsequently, the notch stresses and strains calculated utilizing multiaxial elastic‐plastic FEA are used as input data to the critical plane‐based fatigue life prediction methodology. The prediction results are satisfactory for the 7050‐T7451 aluminum alloy and GH4169 superalloy notched specimens under multiaxial cyclic loading.     

Fatigue life evaluation of tension‐compression asymmetric material using local stress–strain method

In this paper, the low‐cycle fatigue characteristics of cold‐drawn steel were investigated under strain‐controlled uniaxial fatigue load. Cyclic softening was observed throughout fatigue life except for the initial relatively short period which exhibited cyclic hardening. Positive mean stress was found under fully reversed strain loading, indicating that there was a significant cyclic asymmetry. A modified local stress–strain method was proposed to estimate fatigue life of notched tension‐compression asymmetric material. In order to verify this method, fatigue experiments on two kinds of notched specimens with different notch radius were carried out under constant and block load spectrum. It was found that the modified local stress–strain method was more accurate than the traditional ones, the maximum relative error between predicted and experimental fatigue life was less than 6%.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Prediction of Fatigue Life of a Notched Body by the Local Stress-Strain State. Part 3. Allowing for Stress and Strain Gradients

We propose a method for calculating the fatigue life of bodies with stress concentrators prior to the fatigue-crack initiation. It is based on the energy criterion of fracture. Energy dissipated in a notch is taken as a determining parameter. The method allows the fatigue life of notched bodies under regular symmetrical loading to be predicted from the cyclic strain and fatigue characteristics of the material, as obtained in tests of smooth specimens. We analyze the influence of absolute dimensions of a stress concentrator on the fatigue life and propose relationships for calculating the gradients of dissipated energy and the residual strain amplitude. The data we obtained are compared with the test results for notched specimens of 30KhGSA steel and D16AT and AMtsM aluminum alloys.     

NOTCHED FATIGUE BEHAVIOUR OF TWO COLD ROLLED STEELS

Abstract— A SAE1010 plain carbon steel and a SAE945X HSLA steel were cold rolled to various thickness reductions. Centre notched specimens were tested under stress control at a stress ratio of—1. The effect of loading direction on the fatigue strength was examined. The notched specimen fatigue strength was only slightly increased by cold rolling, since two opposing factors: the smooth specimen fatigue strength and the notch sensitivity, were increased by cold rolling. The notched specimen fatigue strength in the transverse direction was approximately the same as that in the longitudinal direction. An empirical equation and equations derived from fracture mechanics and Neuber's rule were applied to predict the fatigue notch factor for the sharp and blunt notch geometries examined. A reasonable agreement between the predictions and the experimental results was observed for the sharp notches. For the blunt notches, the predicted fatigue notch factors were conservative.     

Local stress–strain field intensity approach to fatigue life prediction under random cyclic loading

According to the characteristic of the local behavior of fatigue damage, on the basis of stress field intensity approach, a theory of local stress–strain field intensity for fatigue damage at the notch is developed in this paper, which can take account of the effects of the local stress–strain gradient on fatigue damage at the notch. In order to calculate the local stress–strain field intensity parameters, an incremental elastic-plastic finite element analysis under random cyclic loading is used to determine the local stress–strain response. A local stress–strain field intensity approach to fatigue life prediction is proposed by means of elastic-plastic finite element method for notched specimens. This approach is used to predict fatigue crack initiation life, and good correlation was observed with U-shape notched specimens for normalized 45 steel.     

A notch critical plane approach of multiaxial fatigue life prediction for metallic notched specimens

An approach based on the local stress response is proposed to locate the fatigue critical point for metallic blunt notched specimens under multiaxial fatigue loading. According to the stress analysis, both stress gradient and gradient of loading nonproportionality exist at notch root. The plane in the vicinity of the notch that passes through the fatigue critical point and experiences the maximum shear stress amplitude is defined as the critical plane for notch specimens (CPN). Furthermore, the Susmel's fatigue damage parameter is modified to assess fatigue life of notched components by combining CPN and the theory of critical distance (TCD). The multiaxial fatigue test of the thin‐walled round tube specimens made of Ni‐base alloy GH4169 is carried out to verify the above approaches. In addition, test data of two kinds of materials are collected. The results show that the maximum absolute error of the fatigue critical point is 9.6° and the majority of the predicted life falls within the three‐time scatter band.     

残余应力对金属疲劳强度的影响

残余应力对光滑试样高周疲劳极限的影响可以用Goodman关系来描述，但必须要得到残余应力作用系数m、合理地提取残余应力的表征值和区分开其它因素的影响。残余应力对缺口疲劳极限的作用大于对光滑试样的作用，是由于残余应力也存在应力集中现象，而且不易衰减。残余应力的应力集中系数不仅与缺口几何因素有关，还与材料特性有关。试验研究还表明，表层残余压应力对于承受轴向载荷且疲劳残纹萌生于表面的零件也十分有益。