Evaluation of Finite-Element Models and Stress-Intensity Factors for Surface Cracks Emanating from Stress Concentrations

Abstract

This paper presents an evaluation of the three-dimensional finite-element models and methods used to analyze surface cracks at stress concentrations. Previous finite-element models used by Raju and Newman for surface and corner cracks at holes were shown to have “ill-shaped” elements at the intersection of the hole and crack boundaries. These ill-shaped elements tended to make the model too stiff and, hence, gave lower stress-intensity factors near the hole-crack intersection than models without these elements. Improved models, without these ill-shaped elements, were developed for a surface crack at a circular hole and at a semicircular edge notch. Stress-intensity factors were calculated by both the nodal-force and virtual-crack-closure methods. Both methods and different models gave essentially the same results. Comparisons made between the previously developed stress-intensity factor equations and the results from the improved models agreed well except for configurations with large notch-radii-to-plate-thickness ratios. Stress-intensity factors for a semi-elliptical surface crack located at the center of a semicircular edge notch in a plate subjected to remote tensile loadings were calculated using the improved models. A wide range in configuration parameters was considered. The ratio of crack depth to crack length ranged from 0.4 to 2; of crack depth to plate thickness from 0.2 to 0.8; and of notch radius to plate thickness from 1 to 3. The finite-element or nonsingular elements models employed in the parametric study had singularity elements all along the crack front and linear-strain (eight-noded) elements elsewhere. The models had about 15 000 degrees of freedom. Stress-intensity factors were calculated by using the nodal-force or virtual-crack-closure method.