Durability of countersunk fastener holes
The objective of this research was to investigate the fatigue crack growth behavior around the countersunk fastener hole. Both experimental and computational studies were carried out and the results were compared when possible.
In the early stage of this research, the fatigue failures at the mechanical joint specimens were investigated and characterized. The three-dimensional finite element analyses revealed that the degree of load transfer through the countersink fastener can influence the cracking behavior at the countersunk fastener hole. This finding was verified by investigating the fracture surfaces of the mechanical joint specimens. The cracking location's dependence on the load transfer can be attributed to the non-uniform bearing load distribution along the straight bore section caused by the load transfer.
Fatigue crack growth from the countersunk fastener hole under remote tensile loading was studied using the PMMA transparent material. Single edge corner crack at the bottom of the plate and single internal surface crack at the sharp intersection between the bore and the countersunk were pre-cracked in the PMMA specimen and fatigue tested subsequently. By applying the ‘back-calculation’ method, the stress intensity factors at several crack fronts were experimentally derived. For the derivation, both apparent and effective stress intensity factor ranges were used and the results were compared. The ‘back-calculation’, without the crack closure consideration near the free surface, underestimated the stress intensity factors. The experimental stress intensity factors, corrected by the crack opening load variation, agreed well with the computational results at selected crack fronts. The experimental stress intensity factors are presented in tabular forms.
0548: Mechanical engineering