Numerical modeling of TRIP steel in axial crashworthiness

With their high strain rate hardening capacity and good combination of strength and ductility, transformation induced plasticity (TRIP) steels offer lightweight solutions for reinforcement components undergoing crash events. The TRIP effect correlates with stress-stimulated martensitic transformation from austenite, which depends on a myriad of kinematic ingredients, including three-dimensional plastic deformation, temperature change, strain-rate, and stress state. The complex interdependency of these kinematic ingredients requires stepwise and time iterative numerical analyses, especially when boundary-value problems are imposed. Typically engineering desired boundary value problems involve simulations designed to control energy absorption under complex crash scenarios. In this paper, we demonstrate a new phenomenological framework for thermo-mechanical modeling of multi-phase TRIP steel that captures the evolution of phase transformation. The model was implemented into the user-defined subroutine of the explicit dynamic version of the commercial finite element software LS-DYNA. Simulations were performed on a top-hat crush tube made of an industrial multiphase TRIP 800 steel. The results were utilized within an optimization simulation framework to pinpoint composition windows, which maximize the energy absorption capacity. When not concerned about peak crush force, larger compositions of austenite (typically ∼70%) will outperform all other compositions. However, lowering the austenite volume fraction with higher volume fractions of bainite (∼80%) with balanced ferrite could produce a top-hat crush rail with a superior crush force and efficiency compared to TRIP 800 steel without increasing the peak crush force.