Luna Innovations Inc. has devised a photonic solution to monitoring structural fatigue in aircraft. By replacing the traditional electrode sensors with a strand of optical fiber, the Blacksburg, Va., company has eliminated weight and space issues and problems with electromagnetic interference typically associated with strain tests.Fiber optic strain sensors for detecting aircraft fatigue weigh far less and take up much less space in the vehicle than traditional electronic sensors. The images illustrate the cabling necessary to support foil strain gauge sensors (bottom) and the fiber system (top). As aircraft age, decompressions of the cabin can weaken the structure, deicing agents can corrode the metal, and flexion and vibration can compromise the joints and control surfaces. To avoid catastrophic failure, vehicles periodically are pulled off-line and tested for evidence of fatigue using foil strain gauge sensors in the aircraft. Each of these sensors requires multiple lead wires, and the cabling takes up space and may weigh as much as several thousand pounds.The photonic technique employs fiber optic sensing with optical frequency domain reflectometry. This permits spatially distributed measurements from hundreds or thousands of fiber Bragg grating sensors along a single fiber.A fiber Bragg grating is fabricated by exposing the core of an optical fiber to a pattern of UV radiation, forming a periodic change in the refractive index that acts as a diffraction grating. In the company's application, the fiber is glued to the aircraft surface, and as it is stretched or compressed by temperature or strain, the refractive index changes the spectral response.Each sensor acts as an independent interferometer with a beat frequency proportional to its length along the fiber. A swept-wavelength tunable laser interrogates the sensors over a 7-nm range at 60 nm/s, which corresponds to a strain range of approximately ±3500 microstrain and a scan time of 0.12 s. The reflected light is then detected, demodulated and analyzed.The farther the sensor is along the fiber, the lower its beat frequency. Because each sensor has a unique spatial position, individual spectra can be "windowed" using filtering. The resulting data are displayed as strain field maps that indicate the location of faults.This system has been evaluated on a US Navy P-3C Orion submarine hunter aircraft at Lockheed Martin Aeronautics Co. in Marietta, Ga. The P-3C is being considered for extended service and for additional orders. Although the trials were restricted to ground tests, the results have encouraged Luna Innovations to pursue the development of a system for in-flight, continuous monitoring that could be mounted on military and civilian aircraft.