Fracture toughness of automotive steel at high strain rate

Abstract

The aim of this thesis is to study and push the development of methodology surrounding fracture toughness at high strain rates. This project primarily focuses and adapts the Essential Work of Fracture (EWF) method for application at high strain rates. The EWF method is an extremely powerful tool to observe the fracture resistance of thin ductile sheets using an energy approach, calculating the work done by the fracture process and the plastic process. For safety applications, research and testing of the fracture toughness in automotive steel at high strain rate is very valuable. However, one of the main limitations of testing a materials fracture toughness at high strain rate conditions is that current common test procedures that determine material fracture toughness rely on apparatus that cannot be utilized at high speeds or methodology that can record at static or quasi-static conditions. The EWF method can obtain a value of fracture toughness that not only doesn’t require complex apparatus but also can be performed and obtained at high speeds, making the primary focus of this study, the utilization of the EWF methodology in high strain rate applications and determining its practicality and reliability. This research has attempted to find a successful and usable methodology based around the EWF in the advanced high strength steels: dual phase and TWIP steels, determining how they each respond to low and high strain rate conditions, and validation of the methodology through numerical solutions using finite element analysis. Success in this study will allow the opportunity to define more accurate fracture toughness properties, where the application of automotive steels can be chosen depending on its use cases. A material’s strain rate dependent fracture toughness could be the deciding factor whether it is safer in certain applications.