Multiaxial fatigue model describing crack growth behavior and its application in welded structures of railway frames

Welded joints are typically key regions for crack initiation and propagation, making it especially important to study their fatigue behavior under different loading conditions. This study extends the classic mode I Christopher-James-Patterson (CJP) model and proposes a multiaxial CJP model tailored for welded structures. The study incorporates parameters that drive and resist crack propagation, defining the equivalent stress intensity factor K CJP-eq for mixed-mode cracks. I + II mixed-mode fatigue crack growth experiments were conducted on as-welded and post-weld heat treatment specimens under different loading angles. Digital image correlation techniques were used to obtain displacement field data at the crack tip, and the improved multiaxial CJP model was applied to accurately calculate the crack propagation driving parameters. The results indicate that the improved multiaxial CJP model effectively accounts for residual stress effects, with the largest deviation in K CJP-eq due to residual stresses in the early stages of crack propagation being 10.89 %. Furthermore, for the as-welded specimens, the maximum error with the theoretical value occurs at a loading angle of 30°, reaching 2.89°.