Evaluation of Crashworthiness and Fracture Toughness at High Deformation Rates for Advanced High Strength Steel sheets

Gradually more stringent environmental and safety regulations in the transport sector have made third generation Advanced High Strength Steel (3rd-gen AHSS) grades and new generations of press hardening steels (PHS) cost-effective and natural substitutes in the automotive industry. Increasing the strength of steel allows for potentially downgauging the sheet thickness while maintaining or improving structural performance, and thus reducing the weight of the vehicle. 3rd-gen AHSS and PHS grades have been continuously adapted by the automotive industry for body-in-white parts and energy-absorbing safety components. However, the limited ductility of these higher-strength materials can make them more prone to cracking, which in turn has a negative impact on the folding behaviour of safety structures in a crash. For further introduction of new high-strength steel grades in the design and production of safety parts, proper calibrated material models are needed, and their crash behaviour must be investigated and quantified. Plane stress fracture toughness measured with the Essential Work of Fracture (EWF) method has recently emerged as a viable material parameter to rationalise edge crack resistance and crashworthiness. EWF offers a small-scale laboratory methodology capable of characterising important fracture characteristics of modern automotive steel grades. Hence, EWF together with well-instrumented crash tests in the laboratory are powerful tools for estimating the crashworthiness and quantifying energy absorption. However, much of the published fracture toughness data is based on quasi-static conditions, which do not reflect the conditions in a crash typically involving high deformation rates. To characterise the material for crash scenarios and validate simulation models, further investigation is necessary at higher deformation rates. In this PhD thesis, the crashworthiness and fracture characteristics of 3rd-gen AHSS and PHS grades at higher deformation rates were investigated. The crashworthiness and energy absorbing capacity were evaluated by studying dynamically loaded axially crushed crash boxes both experimentally using full-field deformation measurements and numerically by finite element analysis using a commercially available damage model. Stereo high-speed imaging allowed for more efficient evaluation of crash performance with fewer components and aided in model validation. Furthermore, the rate dependence of fracture toughness and the underlying mechanisms were explored, revealing that crack propagation resistance after crack initiation significantly influences fracture toughness at higher loading rates. It was also experimentally shown that there is significant adiabatic heating in the fracture process zone using the EWF methodology at higher loading rates, which can influence the value of fracture toughness.