Mixed mode I/II failure prediction of thin U-notched ductile steel plates with significant strain-hardening and large strain-to-failure: The Fictitious Material Concept

Abstract

In wide range of ductile materials, the strain-to-failure and the strain-hardening are simultaneously significant, for which, there is no valid K-based fracture toughness (KIc or Kc) based on the standard test methods. So, prediction of the load-carrying capacity (LCC) of notched specimens made of these materials by means of the Equivalent Material Concept (EMC) is impossible. On the other hand, because of large value of the predicted failure load resulted from the large strain-to-failure, the modified EMC (MEMC) seems inefficient in LCC prediction of notched members. Consequently, for such ductile materials, both EMC and MEMC are null. The Fictitious Material Concept (FMC) is proposed in the present study to overcome this major limitation, by which fictitious values of Young's modulus and fracture toughness are defined and calculated for the real ductile material to predict LCC of notched ductile specimens. For this aim, first, U-notched specimens made of SUS 316 stainless steel with good ductility, large strain-to-failure, high elastic modulus, and significant strain-hardening, are evaluated for fracture under pure mode I and mixed mode I/II loadings both experimentally and theoretically. The U-notched specimens with different notch tip radii and notch inclination angles are produced and tested, and their failure loads are experimentally recorded. Then, the test results are theoretically predicted by linking FMC to the maximum tangential stress (MTS) and mean stress (MS) brittle fracture criteria with no need for time-consuming and complex elastic-plastic analyses. A good agreement is observed between the predictions of both FMC-MTS and FMC-MS combined criteria and the experimental results. Meanwhile, the experimental observations as well as the results of elastic-plastic finite element analysis confirm that all of the specimens tested fail by the gross-yielding (GY) regime.