Micro-scale damage tolerance of AHSS steels as function of microstructure and stress/strain state (Microdamage) : final report

This project investigates the effect of prestrain on the susceptibility of AHSS microstructures to void formation with the aim of developing a micro-scale damage tolerance model for use in development of new AHS steels. The work has included studies of industrially produced steels and a vast matrix of laboratory microstructures and their properties as well as development of a FEM model of micro damage tolerance. The results have shown that the strength difference in the microstructure, together with the strength level and work hardening controls the microdamage behaviour in the steels investigated here. If the strength difference is high, void nucleation starts at low strains. The volume increase of voids with increasing strain is controlled by nucleation of new voids rather than growth of already nucleated ones. Growth of voids as an important feature for damage evolution is only relevant if nucleation of new voids is limited, as in single phase material. The strength difference and the fineness of the microstructure, i.e. the number of nucleation sites, controls the volume increase of voids and hence, the damage evolution. The findings can be used to optimise microstructures in terms of work hardening and damage tolerance.