N161-009 TITLE: Innovative Sensing Fasteners for Aircraft Fatigue Monitoring

TECHNOLOGY AREA(S): Air Platform, Sensors

ACQUISITION PROGRAM: PMA 274 Executive Transport Helicopter Program

OBJECTIVE: Develop a sensor capability that can be incorporated onto a common aerospace fastener to monitor for in-hole fatigue crack initiation in multi-layered joints.

DESCRIPTION: One of the primary damage modes in layered joints on aircraft is fatigue cracking that originates at fastener through-holes. If fatigue cracks are undetected, they have the potential to cause a joint failure that could result in catastrophic consequences. However, in-hole inspections of joints, particularly the inner layers of joints, are problematic, costly, and time consuming. With current methods, joints need to be disassembled to be inspected. Following fastener removal, each hole needs to be reamed clean, inspected by eddy-current probe, and then have a new fastener installed. Low profile ultrasonic and eddy current sensors are currently being used for Structural Health Monitoring (SHM) applications, typically incorporated onto a structure via flexible films. This technology has shortcomings since it is fragile, sensitive to orientation, and potentially difficult to install on small curved surface areas.

To minimize the number of required fastener hole inspections, an innovative sensor capability, which can be incorporated onto an existing fastener, is sought. The resulting self-sensing fastener should be capable of detecting crack initiation inside of a borehole without requiring any disassembly of the aircraft structure. The self-sensing fastener should also be able to be integrated into the assembly of an aircraft with minimal impact to weight, structural strength, and durability of the parent joints. Lastly, the self-sensing fastener should be capable of interfacing with an existing Health and Usage Monitoring System (HUMS), such as the B.F. Goodrich systems currently being utilized aboard H-53E/K, H-60R/S, and H-1 (Ref 7). The ultimate goal is to perform a full system airworthiness qualification onboard a Navy or Marine aircraft, which will include test and evaluation of structural strength, fatigue, environmental, vibration, and shock per MIL-STD-810, electromagnetic environmental effects (E3) per MIL-STD-464, and electromagnetic interference (EMI) per MIL-STD-461.

PHASE I: Determine feasibility for the development of a sensor capability that can be incorporated onto common aerospace fasteners, such as AN or Hi-Lok series. Demonstrate feasibility of the self-sensing fastener concept by bench testing in a lab environment. Develop appropriate models required to design a sensor network with self-sensing fasteners.

PHASE II: Build a prototype self-sensing fastener based upon Phase I approach. Evaluate data quality and accuracy by fatigue testing a simple lap joint with a self-sensing fastener installed. This data should be collected with using an existing aircraft HUMS recorder or with a standalone data aggregator that has the ability to interface with an existing aircraft HUMS unit. Develop a plan for full system airworthiness qualification onboard a Navy or Marine aircraft.

PHASE III DUAL USE APPLICATIONS: Perform a full system airworthiness qualification onboard a Navy or Marine aircraft, specific target platforms to be provided during Phase II as appropriate. Evaluate qualification test results and provide procurement specification for transition to an actual production platform. Consider a qualification plan for commercial applications. The development of these sensors will have broad commercial applications for structural health monitoring of mechanical joints on commercial aircraft, ships, and civil structures such as bridges.

REFERENCES:

1. Rakow, R., Chang, F.K. (11-15 November 2007). "Design and Experimental Validation of a Structural Health Monitoring Fastener", ASME 2007 International Mechanical Engineering Congress and Exposition, Volume 10: Mechanics of Solids and Structures, Parts

2. Ibrahim, M.E., Ditchburn, R.J. (2009). "Monitoring of Fatigue Cracks Using Permanently-Mounted Conformable Eddy-Current Sensors," Institute of Materials Engineering Australasia Ltd, Materials Forum Volume 33.

3. Goldfine, N., Schlicker, D., Sheiretov, Y., Washabaugh, A., Zilberstein, V., Grundy, D. (2002). "Surface Mounted and Scanning Periodic Field Eddy-Current Sensors for Structural Health Monitoring," IEEE, Aerospace Conference Proceedings Volume 6.

4. MIL-STD-461F: Department of Defense Interface Standard: Requirements For the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment (10 Dec 2007).

5. MIL-STD-810G CHG-1� Department of Defense Test Method Standard: Environmental Engineering Considerations Laboratory Tests (15April 2014).

6. MIL-STD-464C, Department of Defense Interface Standard: Electromagnetic Environmental Effects Requirements for Systems (01 Dec 2010).

7. B.F Goodrich (UTC Aerospace) HUMS. <http://utcaerospacesystems.com/cap/systems/sisdocuments/Health%20and%20Usage%20Management%20Systems%20(HUMS)/Health%20and%20Usage%20Management%20Systems%20(HUMS).pdf>

KEYWORDS: Sensor; Maintenance Reduction; Structural Health Monitoring; Fastener; Damage Detection; Condition Based Maintenance

TPOC-1: 301-757-1314

TPOC-2: 301-342-8396

TPOC-3: 301-342-9359

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