Stress and Fracture Analyses of Semi-Elliptical Surface Cracks

The present paper concentrates on both elastic and elastic-plastic finite-element stress analyses of the surface crack in a plate subjected to tension and bending loads. Stress-intensity factor (K) equations that cover a wider range of crack-length-to-width ratios, than those previously developed by Newman and Raju, for various crack-depth-to-crack-length ratios and crack-depth-to-plate-thickness ratios have been developed and are presented. These equations are used in the subsequent fracture analyses of surface crack specimens subjected to tension and bending loads. From elastic-plastic finite-element analyses, the variations of a hyper-local constraint parameter (αh) along the surface-crack front were studied to identify the region of maximum constraint and the critical fracture location. (The hyper-local constraint parameter is based on the average normal stresses acting over the plastic-zone region on a line in the crack plane perpendicular to the crack front.) The application of linear-elastic fracture mechanics to fracture of surface-crack specimens made of a high-strength D6AC steel are presented for both tension and bending loads. Two methods were used to characterize fracture: the K2-integral around the crack front and K at a critical fracture location (ϕc). The critical fracture location was the location of the maximum of the product of K times αh. These two methods were used to evaluate the fracture toughness for both the crack-initiation loads and at the maximum failure load conditions. For tension and bending loads, the K2-integral method correlated 90% of the fracture data within ±25% in terms of load, whereas K at the critical fracture location correlated the data within ±20%.