Investigation of crack nucleation in the vicinity of cold-worked fastener holes

During one of the scheduled inspections conducted within an aircraft full scale testing, a single crack was inspected at the side windshield retainer, between the window cutout and the nearby fastener hole. The fastener hole was cold-worked during the aircraft assembly process, to enhance its expected fatigue lifetime. The crack was detected and verified by means of two different NDI methods, namely, High Frequency Eddy Current (HFEC) and detailed visual inspection. As part of a root cause analysis conducted, a detailed finite element model was constructed. This model simulates the cold-working procedure that was applied to the retainer during assembly, as well as the applied loads during typical flight. The finite element analysis implied that the cold-working process at the fastener holes induced residual tensile stress in the nearby cutout radius. Combining this effect with the typical operating loads acting on the windshield retainer leads to crack nucleation at the window cutout. These preliminary conclusions were validated by means of fractographic analysis. However, the cold-working process has some beneficial effects. The crack propagation was further monitored during test, and it was concluded that the cold-working process slowed the crack growth rate of the continuing damage from the fastener hole up to complete failure of the retainer, as compared to predictions.