A generalised hybrid damage mechanics model for steel sheets and heavy plates

The damage onset and evolution are of significant importance in the forming processes of high strength steels. These two features and their influence on fracture have challenged the predictive capability of the conventional damage mechanics models. The present thesis contributes to the accurate fracture prediction of both high strength steel sheets and heavy plates by proposing a new generalised hybrid damage mechanics model. A dual-phase steel sheet (DP600) and a high strength low alloy steel plate (S355J2+N), which show very different relation patterns between damage and fracture, are investigated. For both steels, an easy and systematic material parameter calibration procedure with different experiments is designed. Good prediction applying the model to Nakajima tests for the steel sheet and to bending tests for the heavy plate is achieved. This validates the generalised transferability and flexibility of the proposed model for high strength steels with complex damage-fracture relation under various stress states. As the model is formulated in a phenomenological sense, it also suffers from two drawbacks: having a large number of material parameters and a weak link to the material microstructure. Therefore, two approaches are provided to overcome these shortcomings: justified simplification of the model formulation for specific applications and linking the microstructure to the phenomenological material parameters by multiscale modelling.