Constitutive parameters identification based on DIC assisted thermo-mechanical tensile test for hot stamping of boron steel

Abstract

Hot stamping of Boron steel is a thermal-mechanical and phase transformation coupled process. The reliable testing and parameter identification method of the constitutive behavior is crucial for accurate simulation on hot stamping. The imprecise strain and strain rate calibration due to the inevitable temperature gradient along the gauge length in the Gleeble tensile test has been a great challenge to the accurate description on constitutive behavior at elevated temperatures. In the present work, a novel design of grips is developed to flexibly regulate the length of Uniform Temperature Zone (UTZ) along the specimen gauge length. A system with new speckle preparation method and great optical anti-interference ability is developed for the accurate strain calibration at elevated temperature and high-speed deformation based on digital image correlation (DIC). The influence of the length of UTZ and the strain measurement methods on the accuracy of parameters identification is further investigated. The DIC based calibration method on full-field strain and strain rate makes the parameter identification more robust on the length variation of UTZ, that is, the identified parameters are nearly the same using different UTZ length of specimen. The appropriate UTZ length should be satisfied to prevent the ends of gauge length from blue brittleness and the undesirable end failure. The identified constitutive parameters are verified by comparison between simulation and experiments of the hot stamping hat-type parts. It shows that the newly developed DIC-based method is the most precise within the context of the tested approaches to determine constitutive parameters of hot stamping process.