Toughness evaluation of high strength steels sheets by means of the essential work of fracture

Advanced high strength steels (AHSS) are increasingly being applied in safety related automotive components. Thus, a detailed knowledge of their fracture properties, as fracture toughness, is needed to properly design high performance components. Fracture toughness (KIC) cannot be readily measured in metal sheet. It is because the small thickness (1-3 mm) is not enough to develop a plane strain state, requirement that must be met to properly calculate KIC, as states the ASTM E399 procedure. In this sense the essential work of fracture (EWF) has been successfully applied to determine fracture toughness in polymers films and some metal sheets as aluminum alloys, low Carbon steels, stainless steels, brass, etc. However, there is no enough information about the applicability of this methodology to AHSS, due to the assumption that the ligament area must be fully yielded before the onset of crack propagation. In these terms, optical 3D deformation analysis allows to measure the strain on the ligament area during sample loading and to calculate the yielded area. In the present work the fracture toughness of some advanced high strength steel sheets has been measured by means of the EWF. The results show that AHSS sheets can meet the requirements of the method, and therefore the values obtained for the EWF are valid. Thus, the EWF is postulated as methodology to evaluate the fracture toughness and can be used as a mechanical parameter to characterize the crash behavior of AHSS steel sheets. Introduction Advanced high strength steels (AHSS) are increasingly being applied in safety related automotive components taking advantage of their high mechanical properties. AHSS allows lightweight construction by reducing the component cross section in comparison to conventional steels. Moreover, AHSS present high energy absorption during crash test, which allows producing components with high crashworthiness. Structural components such as B-pillars, longitudinal beams, bumper reinforcements among others are currently being manufactured with different grades of AHSS. Automotive industry is continuously looking for strategies for further weight reduction. The thickness of structural components can still be reduced if the fracture behavior during crash is optimized. In this sense fracture toughness would be a useful property to take into account when evaluating crashworthiness of metal sheets for structural parts. However, fracture toughness cannot be readily measured in metal sheets because its limited thickness (1-3 mm) does not allow developing a plain strain state in front of the crack tip. The essential work of fracture (EWF) can be used to overcome such problem. This method has been successfully applied to determine fracture toughness in polymers films and many metal sheets. The EWF is experimentally evaluated by following the methodology developed by Cotterell and Reddel [1]. They proposed that the total work of fracture (Wf) during the ductile fracture of a double edge notch specimen (DENT, Fig. 1) can be separated into two components: i) The essential work of fracture (we) spent in the fracture process zone (FPZ), and ii) non-essential plastic work (wp) dissipated in an outer region.