Critical parameters for ductile fracture of surface flaws

Abstract

A surface flaw in a section of finite thickness is a problem of practical importance. A number of linear elastic fracture mechanics (LEFM) analyses of this problem have been conducted and have provided descriptive parameters for predicting a fracture stress for LEFM conditions. However, these parameters have not been shown to be effective for predicting the fracture stress under ductile conditions, even though a strong demand exists for such capabilities. This paper describes an experimental approach to defining the critical parameters for surface flaws in ductile materials. A test program was conducted using plate specimens of annealed Type 304 stainless steel containing surface flaws of ellipsoidal, triangular, or rectangular shapes. An empirical approach, based on gross strain, is used to develop equations for predicting initiation of crack growth, conditions for the crack to penetrate the 6.35 mm (0.25 in.) wall thickness, and for plastic instability. These equations provide a means to identify the parameters that quantify the severity of the surface flaw in a ductile material.