An improved damage evolution model to predict fracture of steel sheet at elevated temperature

Author(s): 
B.T. Tang, S. Brusch,i A. Ghiotti, P.F. Bariani

CHALLENGE - Sheet forming processes of High Strength Steel (HSS) at elevated temperature are being increasingly applied to produce car parts body-in-white, which were previously produced at room temperature. In this paper, a damage model based on the Continuum Damage Mechanics was modified to account for the elevated temperature influence, enabling a comprehensive description of the coupled thermo-mechanical-microstructural events that interact during the hot stamping process. The objective is to propose a novel damage evolution law, which is a modification of the damage model by Lemaitre.

SOLUTION - The material parameters of the improved Lemaitre damage model were identified through the joint use of numerical and experimental techniques. The modified damage model was then implemented in the numerical model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550 ºC and 850 ºC, at different strain rates, to identify the material parameters necessary for the calibration of the proposed damage model. Nakajima-type experiments were conducted and numerically simulated to validate the proposed damage criteria. 

BENEFITS - The optimization software modeFRONTIER was used to conduct a stochastic search to generate solutions, by way of the genetic algorithm based method NSGA-II to minimize the objective function based on the obtained response surface. The comparison between numerical and experimental results in terms of critical load at crack growth initiation and fracture location shows that the proposed criterion is able to predict the ductile fracture onset in hot stamping.​