This study conducts multi-scale numerical simulations spanning atomic to macroscopic scales (i.e., from nanometer to millimeter scale) to investigate the fatigue crack propagation behavior in the welded heat-affected zone (HAZ) of AH36 shipbuilding steel. A coupled molecular dynamics–finite element method (MD-FEM) was employed to establish a multi-scale model. Through the transfer of boundary displacements, equivalent mapping of crack morphology, and crack-tip tracking, an iterative multi-scale simulation of 600 tension–tension fatigue cycles was achieved. The results indicate that the crack propagation rate is significantly influenced by crack tip morphology (blunting/sharpening) and growth direction. Notably, the peak strain at the boundary is not the sole determining factor. Periodic blunting of the crack tip occurs during cyclic loading, accompanied by a decrease in the propagation rate. Additionally, the stress field near the crack tip induces microscopic defects such as voids in the nearby area, affecting the crack propagation. This study, based on multi-scale analysis, reveals the microscopic mechanism and evolution law of fatigue crack propagation in the heat-affected zone of AH36 steel welds.

This study presents a multiscale numerical simulation of the behavior of crack growth in the heat-affected zone (HAZ) of AH36 marine steel welded joints under fatigue loading from the micro-scale to the macro-scale. The MD-FEM method and the multiscale coupling-optimized XFEM method were used to simulate fatigue crack propagation from micro-scale to meso-scale and from meso-scale to macro-scale. A total of 10,900,788 tension–tension fatigue cycles was realized. Information across different scales was transferred via boundary displacement transfer, crack morphology equivalence, and tip tracking. Building upon our previous investigation into fatigue crack growth behavior at the micro-scale, in which crack extension was limited to 469 Å, the present study encompasses the complete process of fatigue cracking from micro-scale initiation to macro-scale instability. Furthermore, the crack tip morphology and propagation pathways obtained from micro-scale molecular dynamics simulations are employed to optimize and calibrate the corresponding XFEM simulations at both the meso- and macro-scales. Results demonstrate that the phenomenon of interconnection between voids and the main crack near the crack tip has a significant influence on the crack propagation rate and path. During cycling, the propagation rate of the main crack increases significantly during its interconnection with voids, whereas crack propagation is significantly hindered when the interconnection is completed or when voids undergo self-closure. Furthermore, both theoretical simulations and experiments revealed the occurrence of crack propagation instability at the meso-scale. The present examination of the entire fatigue crack propagation process indicates that the MD-FEM method and the multiscale coupling optimized XFEM method in this study are fundamentally accurate in representing both the crack propagation process and the crack tip morphology. The results obtained in this paper can serve as a reasonable prediction of fatigue damage mechanisms in the HAZ of AH36 marine steel welded joints.