Towards energy-based fracture modelling for crashworthiness applications

Sammanfattning: The automotive industry is currently adapting to progessively more stringent emission and safety regulations imposed by governmental agencies. This introduces significant design difficulties due to the conflicting nature of passenger safety in automotive manufacturing, namely that increased crashworthiness generally leads to heavier vehicles, which in turn leads to more severe crashes. Significant industry effort to introduce lightweight materials into automotive Body-in-White (BIW) design has thus been introduced in recent years to reduce curb weight while improving crashworthiness. Third generation Advanced High Strength Steels (3rd-gen AHSS) and new generations of press hardening steels (PHS) has emerged as cost-effective and natural substitutes in the safety critical crush zones of the vehicle. The limited ductility of these higher strength materials can however make them more prone to cracking, which in turn make reliable deformation behaviour difficult in a crash event. Thus, predicting cracks in the material and its resistance to further propagate them are essential in evaluating crash performance of a design. Fracture toughness measured within the frame of fracture mechanics using the Essential Work of Fracture (EWF) has shown to correlate well with AHSS crashworthiness for steel sheets, making it an interesting parameter for further study in this area. EWF is however strain rate dependent, and most available EWF testing for AHSS is still performed using quasi-static loading rates, conditions completely different from common high-speed crash scenarios. Furthermore, since full-scale testing is a costly endeavor, numerical modelling is used in Computer Aided Engineering (CAE) to test designs before proceeding with a physical prototype. To promote the use of new high strength steel grades in the industry, reliable and properly characterised material models are thus necessary. These models then need to be validated with component experiments to ensure that the models are accurate enough. This is usually done using crash box components in an axial compression or three-point bending setup because of their similarity to real structural components used in crash zones. In this work, EWF at the higher loading rates common in crash scenarios is further investigated to contribute additional data regarding strain rate dependence of fracture toughness measured within the frame of fracture mechanics for AHSS sheets. Furthermore, the crashworthiness of dynamically loaded axially compressed AHSS and PHS crash boxes are evaluated both experimentally using full-field measurements and numerically using a commercially available damage model. The high-speed photography allow for a more efficient component crashworthiness evaluation with fewer components due to the possibility to track crack initiations and their propagation during the deformation. The results from the commercial damage model show that although the prediction of the first cracks is decent, the damage evolution is not captured accurately. These results show the need for further development of economically feasible (shell) damage models that take propagation energy into account in crash simulations. This would also help promote the use of fracture toughness in the automotive industry.