ST12汽车钢板拉剪点焊接头的疲劳强度

针对车身使用的ST12低碳钢板,利用生产线上焊接设备制备了电阻点焊接头拉剪疲劳试件.对点焊接头区域进行了金相组织观测,测量了焊点附近的维氏硬度值分布.在MTS材料疲劳试验机上进行了恒幅疲劳加载,得到了ST12低碳钢板的电阻点焊接头的S-N曲线.基于所得的S-N曲线进行了两级疲劳加载试验,对两级加载下的线性累积疲劳损伤进行了分析.累积损伤结果分析表明,ST12低碳钢板电阻点焊件在两级加载下存在加载次序效应,高-低加载次序下加载次序效应更为明显.     

基于累积损伤理论的ST12钢T型点焊件寿命研究

对低碳钢T型撕裂拉伸点焊试件分别进行低-高和高-低两级加载疲劳试验,并利用线性和非线性疲劳累积损伤理论进行寿命预测。对于低-高加载,非线性预测结果较线性稍好一些;对于高-低加载,由于高载下的"过载效应",线性预测结果与非线性预测结果都不太理想。针对高-低加载下的寿命预测误差,本文提出一种基于CHABOCHE修正模型的非线性疲劳累积损伤模型,分别考虑了两级应力幅以及疲劳塑性指数的影响。通过对比后发现,修正后的模型预测点焊寿命能力较原模型有所改善。     

两级低-高加载下拉剪点焊试样的疲劳损伤研究

对拉剪点焊试样进行了两组不同载荷水平的两级低-高循环加载试验,得到随第一级载荷下循环数变化,点焊试样的剩余寿命和总疲劳寿命变化规律。使用Miner线性累积损伤理论计算得到两种不同载荷水平下试样总的疲劳损伤。结果表明:对于低-高加载,随着低载水平下循环数的增多,高载荷水平下的剩余寿命以及损伤值呈逐渐减小趋势,总疲劳寿命逐渐增大。当低载水平载荷幅值较小时,点焊试样破坏时的累计损伤值普遍存在小于1的现象,且这种现象可能对疲劳寿命估算存在一定的影响。     

变幅载荷作用下点焊焊接接头的疲劳损伤

对拉剪点焊试样进行了多级变幅疲劳加载,载荷块分别为低-高、高-低和低-高-低形式,测量了疲劳破坏过程中的固有频率.基于疲劳寿命试验结果和固有频率随疲劳寿命的变化,分析了点焊结构疲劳损伤累积的特点和加载次序对疲劳寿命的影响.结果表明,载荷块形状,即加载次序对疲劳寿命的影响并不明显,使用线性疲劳累积损伤理论预测的疲劳寿命接近实际疲劳寿命.依据固有频率的变化,可以监测疲劳寿命过程中出现的损伤,并确定损伤的程度.按照损伤力学中损伤随寿命的演化规律,可进行疲劳寿命预测,寿命预测结果与试验疲劳寿命吻合较好.     

两级加载下拉剪点焊试件的疲劳累积损伤

对单点点焊试样进行恒幅和两级低-高、高-低加载试验,采用最小二乘法得出恒幅加载条件下的S-N曲线拟合方程。同时由试验数据得出低-高、高-低加载与总寿命有一定的对应关系,可很好地表述低载"锻炼效应"和高-低加载下高应力下裂纹易于形成,致使后继的低应力能使裂纹扩展的效应。对于加载次序对累积损伤的影响,需要把试件的疲劳断裂过程分为裂纹形成和裂纹扩展两个阶段。在裂纹形成阶段,可用疲劳裂纹的超载效应因子予以定量表征。裂纹扩展阶段在考虑载荷次序效应的条件下,必须考虑过载迟滞效应和低载导致裂纹扩展加速的效应。而由曼森和哈尔福德提出的双线性累积损伤准则正好与此对应。     

低周疲劳寿命预测的能量模型探讨

从熵守恒定律和能量守恒定律出发，推导出一个新的低周疲劳寿命预测能量模型，并以该模型为基础，对线性累积损伤法则进行了探讨．通过316L钢420℃环境下应力控制的低周疲劳实验，用该模型及基于该模型的线性累积损伤法则进行了低周疲劳寿命和剩余寿命的预测，预测结果与实测结果符合较好．     

TC17钛合金电子束焊接接头的疲劳裂纹扩展规律及疲劳剩余寿命

首先通过试验取得母材及焊接接头的疲劳裂纹扩展速率,然后结合TC17钛合金电子束焊接接头CTOD试验结果及裂纹容限计算值,以估算其疲劳剩余寿命.结果表明:在低应力水平或低△K下,TC17电子束焊缝的da/dN数据与母材的基本相当;然而随着应力水平的增加,焊缝的da/dN值越来越大.在初始裂纹尺寸相同的情况下,TC17合金电子束焊缝与母材疲劳裂纹扩展寿命曲线存在交叉点.当应力幅大于交叉点应力幅时,TC17母材疲劳裂纹扩展到临界裂纹尺寸的剩余寿命要高于相应焊缝的剩余寿命;当应力幅小于交叉点应力幅时,TC17母材扩展到临界裂纹尺寸的剩余寿命要低于相应焊缝的剩余寿命.     

蠕变疲劳交互作用下裂纹萌生的有限元模拟

通过结合初始应力应变场与连续损伤力学理论以及单元失效和裂纹萌生准则,构建了蠕变-疲劳交互作用下裂纹萌生的预测模型,将模型编写为UMAT耦合到ABAQUS有限元分析软件中,实现了初始无缺陷结构蠕变-疲劳交互作用下裂纹萌生的有限元模拟,并分析了影响裂纹萌生寿命的因素.通过与线性累计损伤理论对比发现,裂纹萌生位置蠕变损伤和疲劳损伤具有相互促进作用,蠕变疲劳的交互作用使裂纹萌生寿命减小;蠕变和疲劳载荷加载顺序对损伤的累积具有很大影响,模拟发现承受蠕变载荷的结构在承受后续循环载荷下总损伤值更大,裂纹萌生寿命更短.     

自然环境-疲劳交替作用对2024典型连接结构累积损伤的影响研究

目的 揭示自然环境和疲劳交替作用下，2024典型连接结构环境损伤规律与损伤机制。方法 采用静态暴露试验和静态暴露-疲劳交互试验，对比研究2024典型连接结构的累积损伤特性和疲劳寿命退化规律。通过电液伺服材料试验机测试不同试验周期的试样疲劳寿命，利用环境扫描电镜（SEM）和金相显微镜（OM）观察和表征试样疲劳断口形貌及微观腐蚀特征，利用X射线应力分析仪测量了周期加载引起的孔边残余应力变化。结果 金相分析结果表明，周期疲劳加载对2024连接结构的腐蚀损伤进程有一定加速性，交互试验2年的腐蚀深度（51.9 μm）大于静态暴露试样（47.0 μm）。中值疲劳寿命变化曲线显示，相同环境腐蚀条件下，静态暴露的试样疲劳寿命较“疲劳+静态暴露”交互试验方式有所提高，裸材试样的中值疲劳寿命约为交互试验的1.5～2.3倍，带涂层试样的中值疲劳寿命约为交互试验的1.1～1.4倍。紧固孔周边残余应力检测数据表明，受外加交变应力和环境腐蚀的叠加作用，孔边残余压应力逐渐减小，产生局部塑性变形，这会对疲劳寿命造成不利影响。结论 “疲劳+静态暴露”交互试验条件下的试样疲劳寿命退化与低载锻炼效应和电化学效应相互竞争、孔边残余压应力变化及微动磨损密切相关，从而呈现出先升高、后小幅振荡下降的趋势。     

基于模糊理论的随机振动条件下叠层PBGA焊点可靠性分析

对叠层塑料球栅阵列(plastic ball grid array package, PBGA)焊点在随机振动条件下的可靠性进行了研究. 通过模态分析,提取固有频率和振型,接着进行随机振动分析,得到叠层PBGA焊点的应力分布特性;引入模糊理论修正钎料的应力—寿命曲线(S-N curve),并结合三带技术对叠层PBGA焊点随机振动疲劳寿命进行了计算. 结果表明,组件在一阶固有频率下振动时,芯片处的振幅最大;组件边角叠层焊点受到的应力最大,且芯片侧应力大于PCB侧;模糊理论的引入使得1 σ与2 σ应力水平对叠层PBGA焊点产生的损伤得以考虑,从而实现对叠层PBGA焊点振动疲劳寿命更准确预测.     

Fatigue lives of friction stir spot welds in aluminum 6061-T6 sheets

The fatigue lives of friction stir spot welds in aluminum 6061-T6 lap-shear specimens under cyclic loading conditions are investigated in this paper. The paths of fatigue cracks near friction stir spot welds in lap-shear specimens are first examined. The experimental observations suggest that under cyclic loading conditions, the fatigue crack is initiated near the possible original notch tip in the stir zone and propagates along the circumference of the nugget, then through the sheet thickness and finally grows in the width direction to cause final fracture. A fatigue crack growth model based on the Paris law for crack propagation and the local stress intensity factors for kinked cracks is then adopted to predict the fatigue lives of friction stir spot welds. The global and local stress intensity factors are used to estimate the local stress intensity factors of kinked cracks with experimentally determined kink angles. The results indicate that the fatigue life predictions based on the Paris law and the local stress intensity factors as functions of the kink length agree well with the experimental results.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

A CUMULATIVE FATIGUE DAMAGE RULE UNDER THE ALTERNATIVE OF CORROSION OR CYCLIC LOADING

Fatigue damage increases with the applied loading cycles in a cumulative manner and the material deteriorates with the corrosion time. A cumulative fatigue damage rule under the alternative of corrosion or cyclic loading was proposed. The specimens of aluminum alloy LY12-CZ soaked in corrosive liquid for different times were tested under the constant amplitude cyclic loading to obtain S-N curves. The test was carried out to verify the proposed cumulative fatigue damage rule under the different combinations among corrosion time, loading level, and the cycle numbers. It was shown that the predicted residual fatigue lives showed a good agreement with the experimental results and the proposed rule was simple and can be easily adopted.     

Joint strength and failure mechanism of laser spot weld of mild steel sheets under lap shear loading

Abstract

Ultimate strength and failure mechanism of laser spot welds under lap shear loading were investigated. Optical micrographs of cross-section of spot welds before and after failure were examined to understand the failure behaviour. The experimental results indicate that laser spot welds can fail in two distinct modes, namely interfacial and pullout failure. A failure mechanism which was confirmed by SEM investigations was proposed to describe these two failure modes. According to the experimental observations, a simple stress solution related to the far field load was conducted and the critical weld nugget diameter to ensure pullout failure mode was estimated. The results were compared with the experimental data and also with the test data of resistance spot welds. It was observed that the critical nugget diameter of laser spot welding was larger than that of resistance spot welding due to the different failure location in pullout mode. Furthermore, the effect of welding parameters on joint strength and failure mode was studied.     

十字接头恒幅和变载荷历程疲劳试验结果及预测

对十字接头分别进行恒辐轴向载荷和变载荷历程疲劳试验,接头疲劳裂纹通常开始发生在焊缝根部,但如果接头变形引起的弯曲应力较大,疲劳裂纹也可能发生在焊缝趾部。本研究试图找出影响焊接接头疲劳寿命各参数之间的相互关系,并利用作者引伸的焊接接头疲劳裂纹起始－扩展模型较精确地预测复杂焊件的疲劳寿命。该模型把疲劳裂纹起始寿命看成是疲劳裂纹萌生,早期生成并聚合成主疲劳裂纹的循环次数,利用应变控制疲劳数据和Ｐａｌｍｇ     

ACM建模下焊点疲劳计算的网格敏感性分析

焊点通常是车身疲劳开裂的位置，用有限元方法准确计算焊点疲劳损伤在汽车设计阶段非常重要，本文研究网格前处理对焊点疲劳损伤计算精度的影响。焊点的建模采用主流的区域接触模型（Area Contact Model，ACM），网格敏感性分析主要考虑载荷类型、网格尺寸和偏心距3个因素。结果表明:在纯剪载荷下，网格尺寸对焊点损伤计算没有影响；在拉伸和弯矩载荷下，网格尺寸越大，焊点损伤越大；纯剪载荷时，偏心距越大，焊点损伤越大，而拉弯载荷下正好相反。最后统计了网格细化对229个车身结构焊点疲劳损伤的影响，并对焊点ACM建模提出指导性建议。     

基于连续损伤力学的高低周复合疲劳损伤

基于连续损伤力学(CDM)的经典损伤理论和不可逆热力学原理，分别对高周和低周疲劳载荷下的损伤演化模型进行了研究，进而推导出一个新的高低周复合疲劳损伤模型；将该模型编写为UMAT耦合到ABAQUS有限元分析软件中，实现了对缺口材料高低周复合疲劳损伤的模拟及裂纹萌生位置和萌生寿命的预测；同时研究了不同的高低周循环比对裂纹萌生寿命的影响.结果表明,裂纹容易在缺口根部应力集中处萌生；该模型考虑了高低周循环的交互作用，模拟计算结果更符合实际情况；同时通过研究发现高的循环比会使裂纹的萌生加速.