Fatigue life prediction of 3-D problems by damage mechanics with two-block loading

This paper presents the prediction of fatigue lives for 3-D problems in the elasto–plastic range with variable load amplitude. The effect of loading sequence on the fatigue life of a structure member and the validity of Miner's rule are studied. The computational method is derived according to damage mechanics, theory of plasticity and finite element analysis (FEA). A two-block cyclic loading is considered with high–low and low–high load sequence. To consider the effect of the high stress level beyond the yielding point of material in one load block, deformation theory and iteration method are applied to the stress analysis of the first load cycle. The damage evolution under given stress and damage fields for each block loading is determined by a damage evolution equation. Furthermore, an additional loading method is introduced to perform the stress analysis with a given damage field to avoid the reassembling of the stiffness matrix of structure member. In this research, the damage increment at critical element is considered as step length instead of load cycle increment. Comprehensive computer programs are developed to consider both elastic and elasto–plastic cases.