Prediction of crack formation in the progressive folding of square tubes during dynamic axial crushing

Experimental and numerical investigations at both lab and structure scales on the plasticity and failure behavior of an automotive dual-phase steel sheet (DP1000) are performed in this study. On the lab level, an extensive experimental program considering the temperature and strain rate effects on the plastic deformation behavior and the stress state dependency of the ductile fracture behavior is designed. On the structure level, dynamic square tube crushing tests are performed on a drop tower. The extended hybrid damage mechanics model is formulated in the study to describe the deformation and damage/fracture behavior of DP1000 considering the influence of temperatures, strain rates, and loading histroy. The plasticity and fracture description at the lab scale is used to calibrate the material parameters of the model, which is further applied to predict the crashworthiness of the square tube crushing tests. The proposed model has been proven to show very good predictive capability at both lab and structure scales. It is further found that for the modern high-strength steels short cracks could be developed in the tube crushing tests and the crack formation mechanics is shearing and local second-level bending caused by the self-contact between folds.