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 anal; yses of surface crack specimens subjected to tension and bending loads. From elastic-plastic finite-element analyses, the variations of a hyper-local constraint parameter (alpha(h)) 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 K-2-integral around the crack front and K at a critical fracture location (phi(c)). The critical fracture location was the location of the maximum of the product of K times alpha(h), 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 K-2-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%.