Multiscale investigation on fatigue crack growth and remaining useful life of bogie frame materials under service-induced damage

Abstract To investigate the evolution of fatigue crack growth (FCG) resistance in bogie frame materials after long-term service in high-speed trains, this study systematically evaluates the fatigue fracture behavior of key structural regions before and after aging, using full-scale frame fatigue tests, multiaxial FCG experiments, and numerical simulations. A finite element model was established based on strain measurements from full-scale fatigue tests to determine the equivalent crack loading. FCG experiments were then conducted on the as-welded (AW) and base metal (BM) regions before and after service, and digital image correlation was applied to obtain surface displacement fields for calculating the stress intensity factors. The results indicate that service significantly reduces the crack growth resistance of the frame materials, with a maximum remaining useful life reduction of 70.54% in the AW region and 22.31% in the BM region. Correspondingly, the strain response at the crack tip increases significantly after service, reaching more than twice the original value in the AW region and 1.44 times in the BM region, indicating a reduction in crack growth resistance. Microscopic fracture surface analysis reveals that post-service materials exhibit more secondary cracks, unstable crack paths, and blurred fatigue striations, confirming the detrimental effect of service-induced damage on fatigue performance.