Fatigue cracks and plastic deformation of parts in jet aircraft engines could be detected, even during engine operation, by use of proposed in situ monitoring devices called "wireless resonant crackwires." Inasmuch as uncontained turbine failures are the leading engine-related hazard for aircraft, early detection of cracks and/or the associated plastic deformation could enable pilots to respond in time to save lives and limit damage. The use of wireless resonant crackwires could also reduce the costs of inspecting engines for fatigue cracks: Often, cracking occurs in engine parts that are accessible only through disassembly of engines. In many cases, the costs of disassembly and reassembly far exceed the costs of inspection, and the disassembly and reassembly processes can cause new damage.
The term "wireless resonant crackwire" is not oxymoronic, though it may appear so at first. "Wireless" as used here represents the absence of wire connections with external equipment and the use of radio signals to interrogate the crackwires.
The principle of operation of wireless resonant crackwires would be an extension of that of radio-frequency (RF) security tags attached to merchandise in some department stores. These tags contain circuits that resonate at frequencies in the vicinity of 9 MHz. Upon leaving a store, a customer must pass by a large loop antenna that emits a signal swept in frequency over a range that includes the resonance frequencies of the security tags. If a security tag has not been deactivated, then it responds to the incident RF signal by ringing and reradiating at its resonance frequency. A second antenna picks up the reradiated signal, and the antenna output triggers an alarm.
To enable the customer to carry the merchandise out of the store without triggering the alarm, a clerk deactivates the tag. Deactivation is accomplished at the store checkout counter by placing the tag on a small antenna that exposes the tag to an overload-level replica of the exit signal in order to burn out a fusible link in the tag circuit. The removal of the link either makes the circuit nonresonant or else moves the resonance to a frequency outside the range of the security equipment.
Resonant crackwires would be bonded to the surfaces of such critical jet-engine components as turbine blades (see Figure 1). Like department-store security tags, resonant crackwires would include links designed to be broken to alter RF resonances. However, these links would not be broken by overload signals; instead, they would be broken by the development of cracks and/or plastic deformation in the engine components to which they were bonded. Thus, if a resonant crackwire monitored by use of a suitable transmitter, antennas, and receiver were to stop resonating at its "installed" resonance frequency, a crack alarm would be triggered.
Of course, because of the unique characteristics of the jet-engine environment (especially high temperature and the metallic nature of the components to be monitored), the design and fabrication of resonant crackwires would differ markedly from those of department-store RF security tags. Resonant crackwire circuits would likely be fabricated by use of techniques developed previously for strain gauges for monitoring engine parts at high temperatures; this would involve the deposition, on the engine parts to be monitored, of thin oxide films as dielectrics and patterned thin films of high-temperature alloys as inductors, electrodes for capacitors, and antennas. The resonance frequencies of these circuits would range upward from several hundred megahertz and possibly into the gigahertz region. In many cases that would involve monitoring of metal components it may prove necessary to resort to stripline circuit designs, utilizing the underlying monitored components as the ground planes of stripline antennas and resonators (see Figure 2).
This work was done by Bruce McKee, Scott Dahl, and Kathy Shkarlet of Innovative Dynamics, Inc., for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 -8
21000 Brookpark Road
Refer to LEW-16758.