Aircraft wing skins contain thousands of fastener holes, which can act as crack initiation sites for structures under severe fatigue conditions. Major cracks have developed around aircraft fastener holes, which can propagate into larger cracks and possible result in catastrophic failure. Compressive residual stresses have been shown to improve fatigue performance for a variety of applications. Current methods of inducing compressive residual stresses in the fastener holes, using cold expansion, do not completely retard the formation and growth of cracks at the entry surface of the holes, since stresses there are lower than at the middle and exit. Gatekey Engineering, Inc., proposes to develop and demonstrate the feasibility of using Electromagnetic Pulse (EM/P) expansion processing to provide deep compressive residual stresses at the entry surface of the hole.
Benefit: This process is non-contact, does not create significant surface roughness, can be used with non-flat surfaces, and can be used on existing aircraft. It is believed that hole dimensions will be minimally impacted and that holes with diameters as small as 0.18 inches may be treated. The process is also cost-effective when compared to other non-contact processes that create deep compressive stresses. If successful, it is felt that EM/P processing could be used to improve fatigue performance of cold expanded holes and could ultimately be used to replace cold expansion of original fastener holes to induce a consistent compressive residual stress along the surface of the hole. It is also possible to treat a variety of repaired or stepped up holes, using modified tooling and process parameters. Cracking around fastener holes as a result of fatigue is an issue for a variety of military and commercial helicopters and fixed wing aircraft. Therefore, if this method is successful it will have broad commercial potential within the aircraft industry. Even though this technology has broad application among manufacturer of military and commercial fixed and rotary wing aircraft, the overall US aircraft industry is relatively small compared to the overall US manufacturing base. However, the technology would be applicable to not only the original airframers, but also to the inspection and repair facilities around the world. All of the commercial airlines and the military have to perform regular inspections of their fleet. Therefore, there is potential to penetrate a high percentage of this market. Outside of the aircraft industry, there are a number of industries that use rivets, including automotive, rail cars, some pressure vessels, etc. However, the impact of fatigue failure around rivets in these industries is not as great as a catastrophic failure of an aircraft. Therefore, application in these markets will need to be further evaluated.
Keywords: expansion, expansion, Electromagnetic Pulse, bolt hole, Crack, Residual, Compressive Stress, Fatigue, peening
