Testing of a hot stamped axial crush member with tailored properties - Experiments and models

The effect of tailoring the properties of a hot stamped axial crush rail on its axial crush response is investigated. Four configurations of rails of thickness 1.2 and 1.8 mm were formed: a non-tailored (fully martensitic) configuration and three tailored configurations in which one-half of the rail was quenched while the other half was formed in tooling that was heated at different temperatures (in the range 400-700°C). Impact experiments showed that the non-tailored, fully hardened components did absorb the highest energy (15.4-24.1 kJ at 165 mm displacement), but exhibited extensive tearing and fracture. The tailored configurations with a single soft zone were less susceptible to fracture, but the thinner rails were more likely to buckle and absorbed less energy (9.7-20.5 kJ at 165 mm) as a result. Graded tailored configurations with two soft zones and one hard zone did not buckle and absorbed slightly higher energy. The results show that tailoring can introduce graded properties to promote a progressive folding deformation mode, thereby improving energy absorption without fracture. Numerical models of the forming and impact response were developed in which strain rate-sensitive constitutive properties and fracture limit strain versus triaxiality loci were prescribed to be a function of the as-formed hardness and microstructural phase fractions. The models were able to predict the energy absorption of the various axial crush rails to within 10% accuracy, as well as the large difference in extent of tearing occurring in the fully martensitic versus tailored configurations.