基于主S-N曲线的装载机工作装置疲劳寿命评估

Fatigue-life assessment of loader's working device based on master S-N curve

针对装载机在使用过程中焊接结构的疲劳开裂问题,文中在其设计阶段进行了疲劳寿命评估。首先,建立了工作装置中连杆网格尺寸为10,5,2.5 mm的有限元模型,对其进行静力学求解并提取其焊趾节点的结构应力,结果表明,在这3种网格尺寸下结构应力大小的相对误差不超过1%,验证了主S-N曲线法对网格尺寸不敏感的特性。此外,建立额定载质量9 t的装载机工作装置在台架试验姿态下含有焊缝细节特征的三维有限元模型,采用实测得到的疲劳试验加速加载谱进行加载计算,获得焊线L1~L8的结构应力,根据各焊线所受的载荷模式及所处位置的板厚等参数计算得到了工作装置焊线L1~L8的疲劳寿命,并与采用FE-SAFE软件计算得到的8条焊线疲劳寿命进行对比,二者分析结果均在焊线L2(动臂板与动臂油缸连接处焊缝)处首先发生疲劳破坏,疲劳寿命分别为12376.3和9597.6 h;通过与采用IIW和BS 7608标准计算的寿命进行对比,分析了结果差异的原因,评估结果对改进装载机工作装置结构与焊缝优化提供了依据。

Since the loader's welding structure in operation suffers from fatigue cracking,in this article efforts are made to assess the fatigue life in the design stage.Firstly,the finite-element models with the connecting-rod mesh sizes of 10,5,2.5 mm are set up for the working device.They are statically solved,and the structural stress of their weld-toe nodes is extracted.The results show that as for the three mesh sizes,the relative error of the structural stress does not exceed 1%,which has verified that the main S-N curve method is not sensitive to mesh sizes.Besides,the three-dimensional finite-element model of the 9 t loader's working device with rated load and weld details in a bench-test posture is set up.Then,the fatigue experimental acceleration load spectrum obtained from the actual measurement is used to work out the structural stress of the welding lines LI~L8;the fatigue life of the working device's welding lines L1~L8 is calculated based on the load mode of each welding line and the plate thickness at its location.Then,it is compared with the fatigue life of the 8 welding lines calculated by FE-SAFE.The analytical results show that fatigue failure first occurs in the welding line L2(the weld seam at the connection between the boom plate and the boom oil cylinder),with the fatigue life of 12376.3 and 9597.6 h respectively.By comparing the lifespan with the IIW and BS standards,the reasons for difference in the results are identified.The results obtained from the assessment provide basis for improving the structure of the loader's working device and optimizing the weld seam.