Damage Investigation in Dual-Phase 1000 Steel and Behaviour Prediction Using Microstructure Based Modelling

Dual phase (DP) steels are advanced high strength steels that are being progressively used in the automotive industry in order to reduce weight while enhancing safety. The development of the next generation DP steels requires better understanding of the deformation and damage development at the scale of their microstructure in order to predict their mechanical response, especially work-hardening up to the Ultimate Tensile Strength and elongation to fracture. A combined methodology of experimental measurements and modelling at micro-scale was used in this project to investigate deformation and damage in DP1000 steels. A digital image correlation (DIC) experimental technique was used to measure deformation of the microstructure. The displacement results of DIC were extracted and used as boundary conditions for microstructure simulation. The uniqueness of this method is to ensure deformation of modelling matches the actual deformation, which thus allows further investigation for strain and stress values at the damage locations. The method was then used to investigate a crack initiation criterion in the martensite phase and predict crack propagation. A critical maximum principal stress value of 1700 MPa is suggested to initiate damage in the martensite phase of DP1000 steel. Additionally, a continuum-based damage model, namely Gurson, was used for the ferrite phase in order to predict the stress/strain curve of the material. A new approach based on microstructure simulations was used to adjust the Gurson damage parameters. The transferability of these parameter values was examined using different specimen geometries with different stress triaxiality, including a notch bar of DP1000 steel. The results showed a reasonable agreement between stress predictions from microstructure simulations and experimentally measured true stress values. The method of using combined DIC results and microstructure simulation for damage initiation criterion in the martensite and for calibrating damage model parameters for the ferrite phase is then discussed, and the significance of the results obtained in this work for the prediction of overall stress/strain curves of advanced high strength steels is then drawn.