Forming and Crash Induced Damage Evolution and Failure Prediction - Part 2: A Comparison of Damage Models

With increasing requirements on crashworthiness, and light-weight car body structures being a central issue in future automotive development, the use of high strength steel qualities has become widespread in modern cars. Since these materials often show significantly lower ductility than conventional steels, it is of great importance to precisely predict failure under crash loading conditions. It seems more and more evident that the before going treatment of the material through the process chain of rolling to forming significantly influences crash performance of the respective material. An emphasis has been laid on identification of damage parameters and a comparison of relevant damage accumulation models and theories from forming to crash simulation. Special attention was paid to existing differences regarding stress states between forming and crash loading, to clarify differences in failure prediction models respectively. Numerical simulations of Nakazima tests using solid elements were conducted, to get an impression of stress states in these tests used for the determination of Forming Limit Diagrams (FLD). A review of literature and own considerations are shown, to describe a formulation of failure strains dependent on the third invariant of the stress tensor or the Lode angle. Several possibilities to consistently describe damage and failure in both forming and crash simulations are considered.