基于结构应力法的点焊S-N曲线研究

为获取列车不锈钢车体点焊结构的疲劳特性,对点焊试件进行拉剪疲劳试验,并绘制载荷-寿命(F-N)散点图。为将载荷-寿命散点图归一化到应力-寿命(S-N)曲线,首先,建立点焊实体仿真模型,利用有限元分析,将应变结果与静载试验结果对比,验证点焊试件有限元约束加载的准确性。然后,建立梁单元的点焊仿真模型,计算并比较梁单元节点力与体单元焊核截面力,验证梁单元模拟焊核的可行性。最后,将与疲劳试验相同的载荷施加到梁单元模拟焊核的仿真模型中,提取梁单元的力和力矩,根据结构应力公式计算得到点焊试件的结构应力,在双对数坐标系中,以试验所得寿命为横坐标,计算所得结构应力为纵坐标,利用最小二乘法拟合出点焊的S-N曲线。结果显示,点焊的结构应力能较好的归一化为载荷-寿命曲线,载荷比对S-N曲线有较大影响。

Research on S-N Curve of Spot Welding Based on Structural Stress Formula

In order to obtain the fatigue characteristics of the spot welding structure of stainless steel carbody, the shear fatigue test was performed and the load-life(F-N) scatter diagram was drawn. In order to normalize the load-life curve to the stress-life(S-N) curve, firstly, an entity simulation model was built and a finite element analysis was performed. The result was compared with fatigue test to prove the accuracy of constraints and loading. Then another simulation model was created with shell and beam elements. The joint force of beam element was compared with cross section force of the weld nugget to prove the accuracy of using beam element to simulate the spot welding. Finally, the same load as fatigue test was applied to the finite element model. The force and moment were read to calculate the structural stress by the structural stress formula. In the double logarithmic coordinate system, the S-N curve of spot welding is fitted by least square method with the life as the abscissa and the structural stress as the ordinate. The results show that the structural stress of spot welding structure can normalize the load-life curve well, and the load ratio has a great influence on the S-N curve.