The effect of specimen thickness on the experimental and finite element characterization of CTOD in extra deep drawn steel sheets

Abstract

Recent experimental results by us have indicated that the load-drop technique can serve as a valid fracture criterion for predicting elastic-plastic fracture in extra deep drawn (EDD) steel sheets or in predominantly plane stress conditions. The purpose of this investigation is to examine the validity of a J-integral as a fracture parameter and the J-CTOD relation for the determination of critical CTOD in predominantly plane stress fracture (CTOD-crack tip opening displacement). Fracture tests were performed and experimental results were generated on fracture behaviour of EDD (0.06%C) steel sheets with CT specimens and using 'load-drop' as a fracture criterion. Critical CTOD was determined by using the J-CTOD relation in addition to several existing techniques. A full 3-D finite element model was formulated to verify the critical load, critical CTOD and plastic-zone size. The critical CTOD was shown to increase with increasing specimen thickness and appeared to be approaching a higher limiting value. The characteristic features of predominantly plane stress fracture or general yielding fracture mechanics are summarized in conclusion.