Photo courtesy of the U.S. Navy / Mass Communication Specialist 1st Class Peter Lewis
The U.S. Navy wants proposals by mid-February from industry partners interested in investigating how fasteners might be turned into “smart” devices for detecting cracking in airframe structural joints.
Fatigue cracking in aircraft structures often originates in holes drilled for fasteners to hold multiple layers of skin or to attach skin to frames, bulkheads, longerons and stringers. Undetected cracks can lead to structural failure with catastrophic consequences.
But finding cracks is “problematic, costly and time consuming,” said the Navy in a solicitation under its fiscal 2016 Small Business Innovation Research program. “Innovative Sensing Fasteners for Aircraft Fatigue Monitoring” is one of 72 topics included in that solicitation, which is part of a larger Defense Dept. effort targeting high-tech companies with fewer than 500 employees.
Detecting fatigue cracking can require disassembling joints, reaming out individual fastener holes and performing eddy-current inspections on them. After inspection and necessary repairs, those holes must be rebored for reassembly of the joints with new fasteners.
Although low-profile ultrasonic and eddy-current sensors are used for structural health monitoring applications (typically incorporated via flexible films), the Navy noted, this approach has shortcomings. “The sensors are fragile, sensitive to orientation and potentially difficult to install on small curved surface areas.”
The service is asking for proposals by Feb. 17 on the feasibility of minimizing fastener hole inspections by developing a sensor capability that could be incorporated onto common aerospace fasteners (such as AN or Hi-Lok series ones) to monitor for the start of in-hole fatigue crack in multi-layered joints.
The “smart” fastener “should be capable of detecting crack initiation inside of a borehole without requiring any disassembly of the aircraft structure,” said the Navy. It also should be able “to be integrated into the assembly of an aircraft with minimal impact to weight, structural strength and durability of the parent joints.” It also should be capable of interfacing with an existing health and usage monitoring system, such as the B.F. Goodrich systems used on Sikorsky Aircraft H-53Es and H-60Rs and S and Bell Helicopter H-1s.
You can find the information in the Navy’s Small Business Innovation Research solicitation here