TECHNOLOGY AREAS: Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA 261, Heavy Lift Helicopter

OBJECTIVE: Develop non-contact technologies and processes to enhance the residual stresses around aluminum cold worked fastener holes on wing assemblies.

DESCRIPTION: Fastener holes on aircraft wing skins can develop extensive cracking after extended periods of service time. The cracking can initiate around the fastener holes and propagate into large cracks, resulting in catastrophic failure. Fleet crews have to inspect thousands of fastener holes and conduct repair to the cracks. Existing approaches rely on compressive residuals through a cold expansion approach, which does not completely retard the fatigue initiation/growth on the surface of the rivet holes because the residual stresses on the entry surface are lower than those of middle and hole exits.

Innovative, non-contact methods for enhancing the residual stresses on the entry surface, which will be equivalent of the cold work section, are sought. The proposed process should be cost-effective and without risk of damage to the aluminum surface and hole dimensions. The process should be able to induce deep residual stresses onto the surface around the holes with little changing of material surface roughness or dimensions of fastener holes. The process must be used on existing aircraft without any foreign object damage (FOD). Ideally, the process would be suitable for non-flat geometry, such as wing curvatures, and also suitable for an inside hole diameter as small as 0.18 inch.

PHASE I: Develop innovative concepts for imparting residual stresses in cold worked fastener holes by non-contact means. Demonstrate the feasibility of the developed approach through limited testing of flat dog bone coupons with a 0.195 inch cold worked hole.

PHASE II: Fully develop the concept conceived in Phase I into a prototype process. Provide validation and verification that the process meets the requirements.

PHASE III: Transition the developed technology to industry and aircraft maintainers.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Successful development of this technology could be transitioned to commercial airlines for the improvement of the cold worked fastener holes.

REFERENCES:
1. Poussard, C., Pavier, M.J. & Smith, D.J. (1995). Analytical and Finite Element Predictions of Residual Stresses in Cold Worked Fastener Holes. Journal of Strain Analysis, Vol. 30, No. 4, pp. 291-304.

2. Fujimoto, W.T. (2001). Analytic Testing of Joints with Cold Worked Holes. Fatigue & Damage Advanced Structural Technology, Inc. (203) 878-8327, pp 1-23.

KEYWORDS: Cold Work; Fastener Holes; Fatigue Life; Residual Stress; Aluminum; Low Cost Process

** TOPIC AUTHOR (TPOC) **

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