To promote the use of newhigh-strengthmaterials in the automotive industry, the evaluation of crashworthiness is essential, both in terms of finite element (FE) analysis aswell as validation experiments. Thiswork proposes an approach to address the crash performance through high-speed imaging combined with 3D digital image correlation (3D-DIC). By tracking the deformation of the component continuously, cracks can be identified and coupled to the load and intrusion history of the experiment. The so-called crash index (CI) and its decreasing rate (CIDR) can then be estimated using only one single (or a few) component, instead of a set of components with different levels of intrusion and crushing. Crash boxes were axially and dynamically compressed to evaluate the crashworthiness of TRIP-aided bainite ferrite steel and press-hardenable steel. Acalibrated rate-dependent constitutivemodel, and a phenomenological damage model were used to simulate the crash box testing. The absorbed energy, the plastic deformation, and the CIDR were evaluated and compared to the experimentally counterparts. When applying the proposed method to evaluate the CIDR, a good agreement was found when using CI:s reported by other authors using large sets of crash boxes. The FE analyses showed a fairly good agreement with some underestimation in terms of energy absorptions. The crack formation was overestimated resulting in too high a predicted CIDR. It is concluded that the proposed method to evaluate the crashworthiness is promising. To improve the modelling accuracy, better prediction of the crack formation is needed and the introduction of the intrinsic material property, fracture toughness, is suggested for future investigations and model improvements.

The gradually more stringent environmental and safety regulations in the transport sector have made third generation Advanced High Strength Steel (3rd-gen AHSS) grades excellent alternatives to lower strength steel grades and have continuously been adopted by the automotive industry for body-in-white parts and energy absorbing safety components. Recently, essential work of fracture (EWF) has emerged as a viable material characterisation method to rationalise edge crack resistance and crashworthiness. However, much of the published data is still based on quasi-static conditions, which do not reflect the conditions during crash situations typically involving high deformation rates. This paper presents an experimental study on the deformation rate-dependence of fracture characteristics of three 3rd-gen AHSS grades. The results show that the fracture toughness, measured using the EWF method, increases significantly with the loading rate, although the differences in conventional tensile properties are modest. The increase is due to a combination of rate-dependent hardening combined with a much more ductile failure at a higher loading rate.

The automotive industry is currently in a paradigm shift transferring the fleet over from internal combustion vehicles to battery electric vehicles (BEV). This introduces new challenges when designing the Body-In-White (BIW) due to the sensitive and energy-dense battery that needs to be protected in a crash scenario. Press hardening steels (PHS) have emerged as an excellent choice when designing crash safety parts due to their ability to be manufactured to complex parts with ultra-high strength. It is however crucial to evaluate the crash performance of the selected materials before producing parts. Component testing is cumbersome and expensive, often geometry dependent, and it is difficult to separate the bulk material behaviour from other influences such as spot welds. Fracture toughness measured using the essential work of fracture method is a material property which has shown to be able to rationalise crash resistance of Advanced High Strength Steel (AHSS) grades and is thereby an interesting parameter in classifying steel grades for automotive applications. However, most of the published studies have been performed at quasi-static loading rates, which are vastly different from the strain rates involved in a crash. These higher strain rates may also lead to adiabatic self-heating which might influence the fracture toughness of the material. In this work, two PHS grades, high strength and very high strength, intended for automotive applications were investigated at lower and higher strain rates to determine the rate-dependence on the conventional tensile properties as well as the fracture toughness. Both PHS grades showed a small increase in conventional mechanical properties with increasing strain rate, while only the high-strength PHS grade showed a significant increase in fracture toughness with increasing loading rate. The adiabatic heating in the fracture process zone was estimated with a high-speed thermal camera showing a significant temperature increase up to 300 degrees Celsius.