Research On Determination Of Structural Boundary Loads And Its Application

For the structural design and multiaxial fatigue life analysis of complex engineering machinery, on the one hand, structural strength and reliability should be guaranteed to meet the demands, on the other hand, the fatigue failure problem should be as a key research content.Due to the bad working conditions and the weld defects in the structure which is subject to random multiaxial loadings, fatigue failure occurs frequently in the push rake machine. When 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 is enormous, which will make the research work difficult to carry out. Therefore, it is necessary to pick out the local structure where defects are located in as the single analysis object, 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 structure fatigue analysis.For irregular and regular structures, FEM and stress computation equation based methods are respectively taken to establish the relationship between the test strain and the boundary loads, so that the loads can be reversely determined. For the FEM method, the cross section of structure is firstly discretized, and the center of the strain gage is regarded as the location of the corresponding node, thus the equations between the boundary loads and the test stress or strain can be established. 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 equation. Next, the strain data are collected and transformed to obtain the maximum and minimum normal stresses of the test points, 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-frame is studied since there are potential structure defects in the multiple weld joints. Firstly, the object’s approximate equivalent boundary conditions are determined based on the test data; Secondly, an XFEM mode is set up, in which the weld defects are taken as the initial cracks; then stress components on six directions of all element, including the stresses of the crack tip, are obtained. Thirdly, the fatigue life predicted based on the multiaxial fatigue damage model by using this method is close to the actual situation, and thus the method provides an important reference for the structure design and measurement of manufacturing quality.In the paper, the error is analyzed for three-node triangular element, four- node rectangular element and six-node triangular element, at the same time, advantages and disadvantages of every element are compared. Some reasonable suggestions also be proposed about equivalent nodal location, the choice of element types and the relationship between grid density and computational expense.