Machine Design

Sensors help gauge if jets are fit to fly

As aircraft reach or exceed their planned life spans, the U.S. Air Force is turning to advanced nondestructive evaluation methods to decide whether the planes are fit for continued service.

A low-profile magnetostrictive sensor detected a crack and monitored its growth on an A-10 aircraft test article. With additional development, the technology will monitor structural components in high-cycle aircraft, such as T-37 and T-38 trainers, as well as A-10, F-16, F-15, and other military combat aircraft. It also has potential for commercial fleets.

In this regard, Southwest Research Institute (SwRI), San Antonio, has developed a flexible thin-film deposition process to make lowprofile magnetostrictive sensors (MsS). The devices detect and monitor defects in aircraft without costly teardowns or unnecessary inspections.

SwRI pioneered the use of MsS technology as a nondestructive evaluation tool for pipelines. The method uses guided waves to rapidly detect corrosion and defects for assessing overall pipeline integrity. A magnetic field around the pipe generates the guided waves with a ferromagnetic strip and coil. External magnets, however, are impractical for the small components and confined spaces aboard aircraft.

"What we needed was a lightweight, low-profile sensor that could maintain a residual magnetic field without an external magnet," says Senior Research Engineer Clint J. Thwing of SwRI's Sensor Systems and Nondestructive Evaluation Technology Dept.

Thin layers of iron cobalt and iron terbium are deposited on low-oxidation aluminum foils under ultrahigh vacuum, then heat treated. This makes them pliable and easier to apply. The deposition process is highly reproducible, and the resulting MsS have effectively monitored crack growth and determined the approximate defect size (cross section) on a simulated A-10 aircraft structure.

MsS used by the pipeline industry are limited by a "dead zone" — the distance associated with the time required for the excitation pulses to saturate the receiver. At 32 kHz, the dead zone extends 2 to 3 ft. This distance is inconsequential when testing a miles-long pipeline. But it is a problem in aircraft bulkheads and fuselage components where the region being inspected is usually only a few inches from the monitoring probe. The new method shrinks the dead zone by using a much shorter wavelength or higher frequency signal.

MsS technology is also ideal as a health and usage-monitoring system, or HUMS, says Thwing. "HUMS can reduce costs, improve readiness, and increase safety by identifying mechanical problems or maintenance issues while the aircraft keeps flying."

Southwest Research Institute,
(210) 684-5111,

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