Comparison of Stress Intensity Factors for Cracks at Countersunk Holes

Abstract

This paper focuses on the study and comparison of stress intensity factor solutions for cracks emanating from countersunk holes in a plate subjected to remote tension and bending moments. To explore the difference in the crack behavior at a straight-shank and countersunk rivet hole, models of holes with different shapes at various ratios of countersunk depth to plate thickness were generated, in which there were two extreme cases of countersunk holes: knife edge hole and straight-shank hole. The stress intensity factor solutions for cracks at the countersunk holes were predicted using the FEM-BEM alternating technique. The FEM-BEM alternating method, which is described in this paper, was verified by carrying out a study of the stress intensity factor for a crack at a straight- shank hole. A range of crack shapes and sizes under remote tension and bending moments were analyzed. The results from the crack analyses revealed that the crack behavior is considerably different at the countersunk holes with different countersunk depths. UNTERSUNK-RIVETED lap joints are commonly used in the aircraft industry to achieve aerodynamically ooth surfaces. However, due to the existence of high stress concentrations, corrosion pillowing stresses and fretting damage around the rivet holes, cracks could easily nucleate and grow from the edge of these holes, which could eventually lead to the catastrophic failure of a transport aircraft. Although a great deal of effort has been made to investigate the fracture mechanism around the rivet holes, accurate estimations of the stress intensity factors for cracks at countersunk holes are still needed to predict joint strength and fatigue life. Currently, no solutions for such structural configurations have been reported in stress intensity factors handbooks and they are still lacking in fatigue life prediction software. C Research is being carried out at the National Research Council Canada on crack growth simulation with an objective to develop computational modeling techniques to simulate the fatigue crack propagation in complex aircraft structures. In the present work, fracture analysis was carried out to predict stress intensity factors for cracks emanating from countersunk holes in a large plate subjected to remote tension and bending moments. To explore the difference in the crack behavior at countersunk rivet holes, models of holes with different shapes at various ratios of countersunk depth to plate thickness (h/t = 0, 0.05, 0.25, 0.5, 0.75, 1.0) were generated and analyzed. The parametric studies were carried out by varying the crack shapes, sizes and load conditions to represent typical experimental observations. The finite element and boundary element (FEM-BEM) alternating technique was employed to obtain the stress intensity factor solutions for cracks at the countersunk holes. The aim of this work is to compare and reveal the differences in the crack characteristics in the different countersunk holes.