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Investigation of High-Frequency Magnetostrictive Sensor Technology as a Health and Usage Monitoring System for Aging Aircraft, 14-R9550

Principal Investigator
Clinton J. Thwing

Inclusive Dates:  06/20/05 – 11/04/05

Background - A number of aging United States Air Force (USAF) aircraft will be reaching the end of their design life in the next few years. With no new aircraft in line to take over the operations of the aging aircraft, the USAF is turning to advanced nondestructive evaluation (NDE) methods monitoring to determine the fitness for duty of these aircraft. This process is referred to as health and usage monitoring systems (HUMS). Southwest Research Institute has been involved in similar work for the pipeline industry using magnetostrictive sensor (MsS^®) technology. Magnetostrictive sensor technology has demonstrated propagation at 1 MHz over a distance of approximately 10 to 15 inches in plate-like aluminum structures with a defect detection sensitivity on the order of 0.3 inch by 0.1 inch in size. SwRI has recently completed the development of a USB-interfaced magnetostrictive sensor instrument (called the MSSR^® 3030A) that operates in the frequency range of 10 to 250 kHz. The MsSR 3030 was modified for higher frequencies, such as 500 kHz to 1 MHz, for use on aircraft structures.

Approach - To accomplish this project, the modification of the low-frequency MsSR 3030 was necessary. This included removing and replacing the output power transmitter, oscillator, power supply module, and a variety of electrical components on the main board to accommodate high-frequency operation. Firmware on the system board was upgraded, as was the computer software needed to acquire and analyze data. Testing of the system was conducted on test specimens developed to simulate a variety of aircraft structure. Baseline data were then acquired from each specimen before simulated defects were introduced.

Accomplishments - The first step in the verification and validation process of the upgraded MsSR 3030 instrument (see Figure 1) was to acquire data on several specimens simulating aircraft structure. The MsS probe consists of the excitation coil, shown in Figure 2, placed over a nickel or iron-cobalt film approximately 0.005 inch thick × 1.5 inch × 0.5 inch. This signal, transformed through the magnetostrictive property of the nickel or iron cobalt into an ultrasonic guided wave, propagating outward from the sensor in both directions, was evaluated for velocity and mode. Using the formula for velocity, ν = f /λ, where f = 1 MHz and λ = 2.99 mm, it was determined that the sensor produced a shear wave with a velocity of 1.17 × 10⁵ inch/sec. This was necessary to determine the wave characteristics for future data analysis with the computer analysis software. A simulated test specimen with several holes and known defects was tested (see Figure 3), and the results from these tests can be seen in Figure 4.

Figure 1. MsSR 3030A Instrument	Figure 2. MsS Probe Coil

Figure 3. MsSR Probe and Coil on a Test Specimen

Figure 4. Results from Test on Test Specimen

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