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

Abstract

Sheet forming processes of High Strength Steel (HSS) at elevated temperature are being increasingly applied to produce parts of the car body-in-white, which were previously produced at room temperature. The proper selection of the temperature-related parameters is one of the most critical aspects in the process design with the numerical simulation. In particular, the description of the material formability by Forming Limit Diagrams (FLDs), which are usually implemented in dedicated FEM codes, appears strongly limited when applied to the hot stamping process, as the material behavior is dependent on both the temperature and strain rate. In this paper, a damage model based on the Continuum Damage Mechanics (CDM) 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 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°C and 850°C, at different strain rates, to identify the material parameters necessary for the calibration of the proposed damage model. Nakajima-type experiments at elevated temperature were conducted and numerically simulated to validate the proposed approach. The comparison between numerical and experimental results in terms of critical load at crack growth initiation and fracture location shows that the proposed model is able to predict the ductile fracture onset in hot stamping process.