Some observations on failure of austenitic stainless steel: effects of in- and out of plane constraint

Abstract

Modern engineering design for safety-critical structures demands meticulous failure assessments. While conventional fracture toughness parameters (e.g. JIC or KIC) are perceived as a reliable tool for characterizing the fracture behaviour of many engineering components, excessive plasticity in low-strength materials with high strain hardening capability, such as austenitic stainless steels, can change the predominant failure mode to plastic collapse. In this case, assessing the component integrity assuming fracture can introduce significant conservatism in its estimated load bearing capacity. Low constraint structures such as thin sections or sections with short cracks can suffer from plastic collapse rather than fracture but there is not a measure that can predict the change of failure mechanism. In this study, the effect of crack tip constraint in terms of the thickness and initial crack size on failure behaviour of AISI Type 316L austenitic stainless steel is studied. To this end, several single edge notched bending (SENB) specimen were manufactured with different initial crack lengths and thicknesses to account for in- (i.e. crack length) and out of (i.e. thickness) plane constraint effect. Fracture tests performed according to ASTM E1820-18 except for the sample thickness and crack lengths. The challenges for measuring the crack extension for resistance curve method as a result of deviating from the standard is discussed. The failure of the samples was also assessed by Failure Assessment Diagram (FAD). The results showed that unloading compliance extremely underestimate the crack extension in the presence of significant plastic deformation of the samples. The results also showed that the plastic collapse occurs in all samples before crack starts to grow, in the blunting regime.