Experimental and numerical investigation of localized thinning in hydroforming of micro-tubes

An experimental program has been carried out for hydroforming of stainless steel micro-tubes. Under careful control, it was found that failure takes place randomly, which is significantly different from observations of failure in hydroforming of macro-tubes, where failure loads and locations are predictable. This occurs because wall thinning of micro-tubes in forming processes is non-uniform, i.e. localized necking takes place randomly. To investigate the localized thinning mechanism, an integrated crystal plasticity finite element (CPFE) modeling system has been developed. In this paper, a simplified plane strain CPFE model is presented and used to investigate the localized thinning and failure features in hydroforming of micro-tubes. The crystal plasticity equations were implemented in the ABAQUS/Explicit FE code through a user-defined material subroutine, VUMAT. Single crystal, three-grain and polycrystal FE models were generated to study the localized thinning/necking mechanism and the effect of differing adjacent grain orientations, as well as the number of grains across the smallest specimen dimension, on the necking features. It has been confirmed from the analyses that the localized thinning observed in hydroforming of micro-tubes is significantly affected by the microstructure and grain orientations of the material.