Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

• The low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel to validate the linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN).
• The plastic deformation at each Gauss Point accumulates progressively as the crack tip approaches it. Progressive ratcheting behavior has been observed in the cyclic plastic zone and that accumulative plastic strain increases with the increase of R-ratio.

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.