Modeling and description of crack propagation at sharp notched specimens under cyclic loading after overloads

摘要

The lifetime of notched components under cyclic loading consists of 1. a crack initiation stage (initiation and propagation of microcracks) and 2. a crack propagation stage (propagation of small and macrocracks). Essential lifetime portions are spent for crack propagation in low cycle fatigue (LCF) in general and in LCF and high cycle fatigue (HCF) of sharp notched specimens in particular. Initial overloads at the beginning of the fatigue lead to local plasticity and to residual stresses, which influence the crack initiation by affecting the endurance limit due to different mean stresses. These residual stresses also influence considerably the crack propagation by affecting the crack closure. In this research, fatigue of the fine grain steel Metasafe 900 is investigated numeri-caly and experimentaly. Numerical calculations are performed to get the local stress and strain state on sharp notches and ahead of the crack propagating from notches. The simulations show also the different closure of small and macrocracks as experiments. From the local stresses and strains it is possible to calculate straining parameters to describe the lifetime of specimens with initial overloads. The calculated results are compared with the experimental data.