High strength steel fracture: fracture initiation analysis by the essential work of fracture concept

Abstract

Advanced High Strength Steels (AHSS) are used in the automotive industry in form of thin sheets, to reduce weight and increase crashworthiness. However their moderate ductility can be a disadvantage for their conformation, these type of steels are prone to crack after forming. These cracks normally nucleate from notches or edge irregularities. Therefore knowing their behaviour at fracture is necessary to predict their performance in service. But traditional approaches to measure fracture toughness are not valid for thin sheets. Essential Work of Fracture (EWF) is an experimental methodology that can measure fracture toughness in thin sheets using an energetic approach. In this work two Dual Phase (DP) AHSS steels, DP780 and DP1000, that are vastly used in the automotive industry, are studied. Double Edge Notched Tests (DENT) are simulated by Finite Element Modelling (FEM), aimed at introducing a method to identify fracture initiation using the same EWF energetic approach. This method is experimentally validated by testing it on a notched hole specimen specially designed for this purpose. It will allow to extrapolate known fracture initiation data from DENT specimens to other specimens with the same thickness, notch size and stress triaxiality. Finally several fracture properties are related with the state of stress in the specimens by using two parameters, stress triaxiality and lode parameter.