C. A. Rideout, D. W. Akers, Positron Systems, Inc., Boise, ID

Summary:

The ability to detect cracks within Taper-Lok fastener holes associated with thick, multilayer wing structures without removing the fasteners presents significant challenges to existing nondestructive inspection technologies. Reliable inspection of critical fastener holes for sections of the B-1B Bomber wing structures is costly, requiring removal of the Taper-Lok fasteners followed by eddy current inspection. Because of the complexity of the thick, multilayer wing configuration, the potential for wing damage during inspection, and the required aircraft downtime/man-hours expended, a nondestructive inspection technique without disassembly/fastener removal is required. The work scope for this project was to demonstrate the effectiveness of Photon Induced Positron Annihilation (PIPA) as a material characterization tool to assess the buildup of fatigue damage or microcracking in Taper-Lok fastener holes that would be suitable for the rapid, low-cost inspection of wing structures without removal of the fasteners. Multiple PIPA measurements were performed on Taper-Lok fastener hole wing configuration specimens to characterize damage in the thick aluminum center section around the Taper-Lok fasteners. The primary focus of the PIPA measurements was the outboard Taper-Lok fastener Hole #1 where the titanium and aluminum layers are 0.5 inches and 1.8 inches thick, respectively, and the aluminum layer is expected to have corner cracks at the 0� or 180� locations. Preliminary assessment of the PIPA measurement results for the larger Taper-Lok configuration indicates that PIPA may be highly sensitive to the loss of interference fit and transfer of the Taper-Lok compressive loading to other locations in the fastener hole caused by fatigue damage, resulting in a viable inspection means to detect damage in Taper-Lok fastener hole configurations. These results indicate that the PIPA process has the potential to provide the basis for the development of both current damage estimates and remaining life assessment for thick, multi-layer wing configurations