Investigation of crack propagation and mechanical field evolution at the tip of a growing crack under variable loading in dissimilar metal welded joints

Abstract

This study investigates crack propagation paths at different locations in a dissimilar metal welded joint (DMWJ) within a nuclear power plant (NPP) and examines the distribution of mechanical fields at the tip of the growing crack under various loading conditions. First, the microstructure and mechanical properties of the DMWJ are thoroughly analyzed. Next, the field subroutine “USDFLD” is used to establish a correlation between the non-uniformly distributed material mechanical properties and their spatial positions, thus characterizing the mechanical heterogeneity within the welded joint. Building upon this, a crack growth criterion based on the crack tip opening stress (CTOS) is developed using XFEM in conjunction with the UDMGINI subroutine. The influence of mechanical heterogeneity on stress corrosion cracking (SCC) propagation paths are then investigated. Finally, utilizing the identified crack propagation paths, the mechanical fields at the tip of the growing crack are analyzed under different loading conditions. The results indicate significant differences in the microstructure of local regions of the DMWJ, leading to an uneven distribution of local mechanical properties. Chemical composition gradients exist in the interface region, and significant fluctuations in mechanical properties are often observed near the interface. The material on the higher yield strength side has lower plastic constraints and crack resistance, which tends to result in a higher crack tip driving force. SCC cracks usually propagate towards the higher yield strength side, and the higher the yield strength, the longer the crack propagation length. Crack propagation induces the unloading process at the crack tip. As the crack extends, residual stresses or residual plastic strains are released and redistributed at the crack tip. A single tensile overload can relax the stress near the crack tip within a certain distance, reducing the strain rate.