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.     

变幅载荷下316N不锈钢的力学性能

为了分析一批承受50万周次拉伸变幅载荷Type 316N不锈钢螺栓材料的力学性能变化,对该批中未失效螺栓进行室温拉伸、冲击和疲劳寿命测试,对已断裂失效螺栓进行组织和位错观察。结果表明:在循环载荷作用下,材料内部产生高密度的位错,并形成驻留滑移带,从而使服役后的螺栓材料屈强比上升,塑性下降。对未失效螺栓进行疲劳寿命试验,发现该批螺栓服役寿命仅为恒幅载荷条件下设计寿命的1/2左右,这是由于变幅载荷与恒幅载荷下材料的力学行为存在较大差异,从而导致其与按恒幅载荷设计的疲劳寿命出现偏差。     

结构疲劳裂纹扩展寿命的区间预测北大核心

针对结构疲劳裂纹扩展寿命预测中参数不确定性会导致结构裂纹扩展寿命偏差较大的问题,在分析过程中考虑结构材料特性、载荷和裂纹尺寸等的不确定性,将影响结构疲劳裂纹扩展寿命的不确定性参数视为区间变量,给出了疲劳裂纹开始扩展的区间条件;以Paris公式恒幅载荷疲劳裂纹扩展寿命计算公式为基础,提出了恒幅载荷和变幅载荷作用下结构疲劳裂纹扩展寿命预测的区间模型,并且给出了该区间模型求解的优化方法。通过工程实例计算,表明该方法是可行、有效和简便的,它可以作为结构疲劳裂纹扩展寿命预测的一种新方法。     

AZ31B/TA15异种材料电阻点焊接头的疲劳寿命预测

对镁合金AZ31B/钛合金TA15异种材料电阻点焊拉剪接头进行了恒幅疲劳测试,并使用扫描电镜观察了疲劳断口特征.在实验中观察到,在不同的载荷水平下,试件有两种失效模式.在实验结果的基础上,对点焊试件建立有限元模型,分析了焊点附近的弹塑性应力应变.进而分别应用缺口应力法和局部应力应变法预测点焊接头的疲劳寿命,并与实验寿命...     

电阻法检测金属构件损伤及预测疲劳寿命

基于非线性连续疲劳损伤理论,考虑应力幅带来的非线性累积效应,以电阻值定义金属构件损伤参量,提出基于电阻变化的金属高周疲劳损伤累积模型及疲劳载荷应力幅对损伤参量的影响函数,给出通过测量电阻来检测金属构件疲劳损伤状态的方法及预测高周疲劳剩余寿命的理论公式,并通过45号钢高周疲劳试验进行验证。     

恒变幅载荷下Al-Si-Mg合金的疲劳行为及微观结构演变(英文)

研究了恒变幅疲劳载荷作用下Al-Si-Mg合金的疲劳行为及微结构对其疲劳性能的影响。对恒变疲劳载荷作用下Al-Si-Mg合金的单轴疲劳寿命以及疲劳行为进行了研究。利用透射透射电子显微镜观察在恒变幅载荷条件下Al-Si-Mg合金中位错以及位错带的演变情况。结果表明,Al-Si-Mg合金的疲劳寿命随变幅载荷的变化而变化;位错结构在Al-Si-Mg合金中的运动具有一定的规律性。变幅加载条件会引起材料微结构发生变化,使得材料的硬化效果和变形行为表现出明显的差异,从而影响了损伤的演化进程,材料的疲劳寿命强烈地依赖于加载历史所导致的材料微观结构变化。     

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。     

应力应变、疲劳与脆断、断裂力学

局部加热法影响转轮焊接残余应力的试验分析；工艺参数对板管间接点焊过程的影响；MoSi2／不锈钢连接梯度过渡层的残余应力；基于固有频率变化的两焊点接头的疲劳损伤参量；大型焊接构件疲劳寿命评估系统[编者按]     

基于BS7608标准的点焊试样疲劳寿命预测

基于BS7608标准对点焊接头进行疲劳寿命预测。运用壳单元、CWELD/ELPAT点焊连接单元建立点焊拉剪试样有限元模型,利用Nastran软件做线弹性分析,将线弹性分析结果导入疲劳分析软件,以BS7606标准提供的焊接接头S-N曲线为依据,对点焊试样进行疲劳寿命评估。疲劳寿命预测结果与疲劳试验结果对比表明,不同载荷下的点焊疲劳寿命预测结果与试验寿命有良好的一致性。     

基于Pavlou方法的焊接结构疲劳寿命预测

变幅载荷作用下的疲劳寿命预测一直是焊接结构完整性评估的重要内容.基于Pavlou提出的疲劳损伤区概念,采用BS7608标准中推荐的S-N曲线,运用有限元热传导分析技术,提出了一种焊接结构疲劳寿命的预测方法.针对承载型十字焊接接头、非承载型十字焊接接头和对接焊接接头,开展了二级变幅载荷谱块的拉伸疲劳试验,分别采用Miner模型、M-H模型和Pavlou方法估算了试件的疲劳寿命.结果表明,Pavlou方法的预测精度明显高于其它2种模型,预测寿命与试验寿命误差散射图中数据点的分布形态更加合理,验证了Pavlou方法的精确性和有效性.进一步讨论了焊接接头S-N曲线存活率对疲劳寿命预测精度的影响,提出2.3%存活率可以获得较为满意的预测结果.     

MULTIAXIAL CREEP-FATIGUE LIFE EVALUATION UNDER PROPORTIONAL LOADING

A new method was proposed for the multiaxial creep-fatigue life evaluation under proportional loadings. Because this method was derived from the strain range partitioning method with a multiaxiality factor, it was possible to consider the influence of both creep-fatigue interaction and multiaxial stress state on fatigue life. In order to predict the combined axial-torsional fatigue life the damage under combined loading was defined as linear summation of the damages under axial loading and torsional loading. Axial-torsional creep-fatigue tests were carried out using tubular specimens of 316LC austenitic stainless steel and the ferritic rotor steel. This rotor steel was developed for the permanent magnet type eddy current retarder in heavy trucks. Experimentally obtained lives of both steels were well corresponded with the lives predicted by the proposed method. It was found that the proposed method was effective in multiaxial fatigue life evaluation under proportional creep-fatigue Ioadings.     

基于红外热像法的AZ31B镁合金疲劳寿命预测

采用红外热像法测量AZ31B镁合金板材在疲劳过程中表面温度场的变化,结果表明,当疲劳载荷大于疲劳强度时,镁合金试样表面的温度变化经历5个阶段:温度上升阶段、温度下降阶段、稳定阶段、断裂前的温升阶段和断裂后温度下降阶段。通过疲劳过程中镁合金试样表面的温度变化规律,基于不同的理论提出了2种预测疲劳寿命的方法:由试验过程中试样表面温升(?TM)特征确定其疲劳寿命,?TM–Nf曲线拟合结果表明,?TM=3.89℃为温升极限值,即?TM>3.89℃时,试样会发生断裂,与实测疲劳试样的温升值(3.68℃)相比,误差为5.7%;利用能量法提出了镁合金疲劳寿命的计算公式,用能量法和传统试验方法分别绘制S-N曲线,结果具有很好的一致性,用2种方法分别计算了循环次数为1×107时的疲劳强度ΔσeΦ=99.3 MPa、ΔσeSN=99.8 MPa,误差为0.5%。采用红外热像法估算疲劳寿命具有简单、省时等优势。     

Influence of weld geometry and process on fatigue crack growth characteristics of AISI 304L cruciform joints containing lack of penetration defects

Abstract

The influence of weld geometry in conjunction with the gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) processes on fatigue properties of AISI 304L load carrying transverse fillet welded cruciform joints, containing lack of penetration (LOP) defects, has been studied. The fatigue lives of the joints were evaluated using conventional S -N (stress - number of cycles to failure) and fracture mechanics methods. The fatigue lives were calculated according to the two stage approach in which both the fatigue crack initiation and the crack propagation phases were considered. Constant amplitude fatigue experiments with stress ratio R=0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. An automatic crack monitoring system based on crack propagation gauges was used to obtain the crack initiation and propagation data during the fatigue process. The predicted life was compared with the experimental values. It was found that the fatigue lives of the joints fabricated via GTAW were longer than those of the corresponding GMAW joints. It was also observed that the fillet geometry plays a major role in determining the failure mode and life. Test results have been compared with the BS 5400 design curve.     

圆形电源连接器随机振动分析与结构优化

对电连接器进行模态分析和随机振动分析,运用正交试验的方法,研究接触件不同的结构参数对电连接器固有频率、最大应力和接触件间隙的影响,选出最优的水平组合,并对优化前后电连接器的固有频率、最大应力和接触件间隙值进行比较;最后根据Miner累积损伤理论对电连接器的疲劳寿命进行预测。结果表明:径向加载时的电连接器最大应力值与接触件间隙值比轴向加载时大;冠簧宽度对电连接器一阶固有频率影响最大,冠簧长度对电连接器最大应力影响最大,冠簧厚度对接触件间隙影响最大;通过优化冠簧结构尺寸,可以减小电连接器受振动载荷时的接触件间隙值,最大应力值也会大幅下降,一阶固有频率值有明显上升;优化前后电连接器都未发生疲劳失效,优化后的总体损伤值与优化前相比下降较大,优化后的结构尺寸可以大幅增强电连接器的疲劳寿命。     

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.     

Fatigue life prediction using two stage model for AISI 304L cruciform joints,with different fillet geometry,failing at toe

Abstract

Reports in the literature indicate that the fillet geometry affects the fatigue properties of cruciform welded joints in structural steels. In an attempt to study the above effect with respect to stainless steel sheet metal welded joints, load carrying transverse fillet welded cruciform joints having a two fillet geometry were fabricated from AISI 304L stainless steel using gas tungsten arc welding and gas metal arc welding with 308L electrodes. The objective of the present work is to predict the fatigue life of AISI 304L cruciform joints failing at the weld toe using a two stage model. The local stress life method was applied to calculate the fatigue crack initiation life, whereas the fatigue crack propagation life was estimated using fracture mechanics concepts. Constant amplitude fatigue tests with stress ratio R ~ 0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. An automatic crack monitoring system based on crack propagation gauges was used to obtain the propagation data during the fatigue process. The predicted lives were compared with the experimental values.     

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.     

基于雨流计数法的齿轮程序载荷谱研究

载荷谱是齿轮剩余疲劳寿命预测的关键,为了加速疲劳试验以提高剩余疲劳寿命的预测效率,提出一种基于雨流计数的齿轮程序载荷谱编制方法。通过分析一个啮合周期齿轮轮齿受载情况,抽取相应动载荷谱,基于雨流计数递归算法实现该载荷谱的雨流计数。通过假设检验确定计数结果的数学模型,应用最大似然估计确定概率密度函数,在此基础上确定极大值载荷,建立了程序载荷谱。根据所得程序载荷谱应用Miner疲劳累计假说以及局部应力应变法预测齿轮寿命,并与疲劳试验结果进行比对,结果基本一致,证明了该程序载荷谱的有效性。