Limits of Linear Elastic Fracture Mechanics

This paper presents a simple procedure for determining the validity limits of linear elastic fracture mechanics (LEFM) calculations for low alloy steel (ferritic) structures. The procedure is limited to structures with flaws whose depths are 50 percent or less of the wall thickness of the component at the flaw location and to components which can be considered to be in a plane strain state. Typical yield and ultimate strengths are assumed to be 60 and 80 ksi (414 and 552 MPa), respectively. The procedure is based upon failure assessment curves derived for typical nuclear vessels and components. Calculations of the ratios of the linear elastic stress intensity factor to the plane strain fracture toughness and the applied load to the plastic collapse load are used in conjunction with these curves to determine the validity limits of LEFM. The procedure is limited to the consideration of primary loading. When the load calculated by LEFM deviates significantly from the load calculated assuming elastic-plastic behavior, LEFM is deemed to be invalid and an elastic-plastic calculation procedure is recommended. Example problems are given which demonstrate the applicability of LEFM analysis in one case and the inapplicability in another case. The paper is an extension of the ideas and work generated from the Electric Power Research Institute research project RP 1237-2.