Cyclic elastoplasticity-based life assessment of fatigue crack initiation and subsequent propagation in rib-to-deck welded joints

Abstract

Conventional S-N curve methods for fatigue life assessment fail to consider the different cracking mechanisms of crack initiation and propagation, resulting in less accurate predictions of fatigue life. The present study aims to assess the fatigue crack initiation and subsequent propagation life of rib-to-deck welded joints separately by accurate prediction of local cyclic elastoplasticity. The welding residual stress was simulated with high precision by thermal-mechanical analysis based on the real bead geometry and welding parameters, and defined as initial stress condition. The performance of an unconventional Fatigue Subloading Surface constitutive model (FSS model, hereafter) to predict the cyclic elastoplasticity in the sub-yield stress state was proved by comparing to the experiment results and conventional plasticity theory. The FSS model was then embedded to distinct zones of welded joints for cyclic structural analyses. Considering the relaxation of residual stress, the fatigue crack initiation by the local strain approach based on the local cumulative plasticity damage, and propagation life was assessed by the analytical approach based on the LEFM parameters of materials. As a connection between the crack initiation and propagation, the initial crack featured by 0.5 mm depth and semi-circular shape was proposed. The current work is verified to assess the fatigue life of rib-to-deck welded joints with high accuracy under the pure bending load against experiment results.