Phase-field simulation of fracture-mode transition induced by external surface defects in ductile metallic expanding shells

The fracture behavior of expanding metallic shells is a complex dynamic process, involving multiple coupled factors, and related research holds significant scientific and engineering value. While the influence of loading factors on failure behavior is well understood, understanding of how material defects influence fracture behavior is severely inadequate. In this study, a novel phase-field damage fracture model was successfully developed to effectively capture the competitive interaction between tensile- and shear-fracture modes. The model is quantitatively validated against benchmark tests covering both tension‑ and shear‑dominated fracture in ductile metals (304 stainless steel and AA6061). Through phase-field fracture modeling, we investigated the effect of pre-defined external surface defect depth on the transition between tensile- and shear-fracture modes in cylindrical shells. The pre-defined defect is represented by the phase field order parameter. The results showed that the depth of an external surface defect significantly influenced the fracture mode during the expansion of cylindrical shells. Particularly, as the pre-defined defect depth increased, the fracture mode transitioned from shear to mixed tensile-shear mode, and finally to tensile fracture. This fracture mode transition yielded a three-stage fracture timing vs. pre-defined defect depth characteristic (“first decline – then rise – finally decline ”). Further analysis demonstrated that the fracture mode transition resulted from altered stress states induced by pre-defined defect-depth variations and the evolving crack-opening angles, and that plastic strain localization acts as a precursor for shear cracks. Thus, the study established a defect depth-fracture timing relationship, thereby providing theoretical support for failure prediction in shell structures with pre-defined external surface defects.