Study of cyclic plastic zone ahead of the fatigue crack tip using Digital Image Correlation system

Abstract

Structural components are susceptible to cyclic loadings during their operation. They are more prone to failure due to localized plastic deformation. The plastically deformed region ahead of the crack tip due to fatigue loading is the important factor responsible for fatigue crack propagation. The plastically deformed region is separated into monotonic and cyclic plastic zones. The cyclic plastic zone (CPZ) is approximately 1/3rd to 1/4th of the monotonic plastic zone (MPZ). The existing analytical expressions to calculate the size of the CPZ are based on elastic perfectly plastic analysis. In this paper, an experimental technique, namely 3D-Digital image correlation (DIC), is used to evaluate the size of the CPZ during the fatigue crack growth rate (FCGR) tests on CT specimens. The DIC system having two cameras is used to capture high-resolution images of crack region of CT specimen surface during the FCGR tests. The FCGR tests are carried out on nuclear piping material steel SA333Gr6 under constant load range and constant load ratio. The post-processing of DIC images revealed an asymmetric butterfly-type shape of the cyclic plastic zone. The DIC evaluated CPZ was compared with the CPZ obtained by a calibrated cyclic plasticity material model-based finite element (FE) analysis. It was observed that the average difference between DIC evaluated experimental CPZ and CPZ calculated by plane stress-based FE analysis is about 5%.