边界载荷反求法分析推耙机的焊接结构疲劳

Fatigue analysis of weld structure of rake dozer based on reverse determination of structural boundary loads

针对工程装备焊接结构模型复杂、计算数据庞大导致的疲劳寿命难以预测问题,该文将带有缺陷的局部结构体分离出来单独进行分析,采用有限元和应力解析计算方程获得测试应力(应变)和结构体边界载荷之间的关系,以便反求得到局部结构体的外载荷,进而应用降维多轴疲劳损伤模型开展局部结构体裂纹扩展寿命的研究。以存在多条焊缝的推耙机H形架横梁为对象设置初始裂纹来模拟焊接缺陷,基于测试数据确定横梁的等效边界载荷并进行疲劳寿命计算,预测得到的H形架横梁使用寿命与工程实际寿命误差在5%以内,说明该方法是切实可行的。该研究可为工程结构设计及制造精度控制提供参考。

Due to the bad working conditions and the weld defects in the structure which is subject to random multiaxial loadings, the fatigue failure occurs frequently in the rake dozer. When the defects exist in the working structure, if the whole engineering structure is taken as the fatigue analysis object, not only the computation model is complex, but also the data are enormous, which will make it difficult to predict the fatigue life of structure. Therefore, it is necessary to separate the local structure where defects are located from the whole one in order to reduce the complexity of the computation model. In this way, the determination of the boundary loads of the local structure is a key to realize successful analysis of structure fatigue. For irregular and regular structures, the finite element method （FEM） and the stress analytical computation equation are respectively used to establish the relationship between the test stress （strain） and the boundary load, so that the loads can be reversely determined. For the FEM method, the cross section of structure is firstly discretized, the center of the strain gage is regarded as the location of the corresponding FEM node, and all the force components of the boundary loads are applied to the boundary nodes of the cross section, and thus the relationships between the boundary loads and the test stresses or strains can be established through the FEM equations. According to the number of unknown boundary loads, the number of three-element rectangular rosettes can be determined according to the equation. For the structure with regular cross section, the number of rosettes can be determined in the same way based on the analytical equations. Next, the strain or stress data are collected and transformed to obtain the maximum and minimum normal stresses of the test points, and the load case of the structure is determined according to the stress states derived from the maximum and minimum normal stresses. In the paper, the cross section of the beam of the machine H-shaped frame is studied since there are potential structure defects in the multiple weld joints. Firstly, the object＆#39;s approximately equivalent boundary conditions are determined based on the test data; secondly, an XFEM model is set up, in which the weld defects in the corner of the beam are taken as the initial cracks, and the material is Q235; then stress components on 6 directions of all elements such as the stresses of the crack tip are obtained; thirdly, the multiaxial fatigue life is predicted based on the multiaxial fatigue damage model proposed by Liu and Mahadevan. The fatigue life predicted by this method is 3 559 h, i.e. the structure will be subjected to fatigue failure after 445 d if the rake dozer runs 8 h a day. The fatigue life is close to the actual situation, which has an error of less than 5%. The method proposed in this paper is feasible for predicting the fatigue life of big and defective engineering structures which are subject to complex working loads, and the prediction result provides an important reference for the structure design and the measurement of manufacture quality. Future work will be directed to the analysis of the errors caused by the area of the resistance strain gages and the distribution of forces exerted on the local structure, so that the method can be applied to other engineering structures.