Damage Mechanics-Finite Element Method To Predict The Fatigue Life Of Plate Specimen

The studies carried out to predict the crack initiation fatigue lives for plane problems in linearly elastic and elasto-plastic ranges are presented. The damage mechanics based upon finite element formulation and the method of additional loads are applied for this computational investigation .By means of this damage mechanics-finite element-additional loads method, the structural stiffness matrix is kept unchanged once the material is deteriorated during cyclic loading, but additional loading is adjusted accordingly. The damage evolution under given stress field and damage field is determined by damage evolution equation. In this approach, the damage increment at critical element is taken as the step length instead of the increment of load cycles. To consider the scatter of test data quantitatively by computational mechanics, the initial damage caused by manufacturing or other causes is introduced.We introduce the equal stress into the damage epuation. For the coupling question between stress and damage, we use the iteration method to analysis the damage field and stess field. The prediction of fatigue lives for cycle fatigue problems, where the maximum equivalent stress produced by the initial loading is greater than the initial yielding stress and the range of variation of equivalent stress caused by the subsequent loading is still within the subsequent elastic range, are performed in the use of damage mechanics, theory of plasticity and finite element method. The idealization of materials for nonlinearly hardening cases is carried out. The reduction of maximum and minimum loads is considered during the subsequent loading because of the existence of plastic deformation. The initial damage caused by plastic deformation is also introduced, which can be expressed as a power function of plastic work ratio. We should ensure the maximum equal stess of each element, the get the damage degree of each element and the tatal damage field. We chose constant strain triangle element to program on computer, this program can predict every 2D fatigue life problem. In finite element analysis, the convergence test is performed for the optimization of mesh generation and damage step length. To verify the availability of the damage mechanics-finite element-additional loads method presented, the numerical results of crack initiation fatigue lives for plate specimens are obtained. It is reveal that the computational results given by developed programs show a good agreement with the experimental data. The damage mechanics-finite element-additional loads method can not only predict the fatigue life of specimen, but also predict the crack extention direction, and optimize the structure of the component.