Plastic-yielding analysis in the pipe-bending process of AZ31 and AA7050 using the normalized Cockcroft and Latham criterion

Compared with other sheet-metal-forming processes, the deformation conditions of pipe bending are affected by the material, equipment, die, and multiple other factors. Therefore, it is urgent to explore methods that avoid forming defects and improve the bend-forming quality. Wall-thinning induced ductile fracture is one of the dominant defects in the pipe-bending process, especially for materials with lower ductility. In this study, the normalized Cockcroft and Latham criterion is embedded into the user subroutine USDFLD within the Abaqus finite element (FE) software to predict the fracture tendency of a bent pipe. Through a comparison of theoretical calculations and simulation results, a simplified FE model of rotary draw bending was established, and the damage evolution and distribution of bent pipes composed of AZ31 and AA7050 materials were investigated. The results show that the two materials have similar regularity, and damage mainly generates and accumulates in the outer surface of the deformation zone due to wall thinning. The damage value remains at a certain level after the bending deformation enters the stable stage. This study provides a concept and strategy for more efficiently predicting the fracture tendency of a bent pipe.