Fastener Tilt Assessment Based on Magnetic Field Image Obtained With Array Tunnel Magnetoresistance Sensors

Abstract

Assessing the tilt of fasteners in lap joints is crucial in the aviation industry to maintain the reliability of structures. However, low-cost, portable, and fast inspection tools are still lacking. This work investigates a novel probe with array tunneling magnetoresistance (TMR) sensors for tilt assessment. The operating principle of the probe is based on electromagnetic induction, where eddy currents are induced in the conductive sample, and then, the magnetic field images are recorded by the TMR sensors. The magnetic field images are symmetry for nontilt fasteners and asymmetry for tilt ones. This image symmetric is exploited for tilt measurement. A finite-element method model is employed to investigate the feasibility of the method, based on which an image-processing algorithm has been developed and a feature named asymmetry parameter D is extracted. It is found that D is monotonicity related to the altitude of tilt. A prototype probe, including 64 integrated TMR sensors and rectangular excitation coils, has been developed and tested. The sensors are microfabricated and then wire-bonded on a printed circuit board. The spatial resolution of the image obtained by the array sensors is as fine as 0.5 mm. Experiment results show that the probe can discriminate fasteners that tilt in any orientations from well-mounted ones. The probability of detection and the probability of false alarm are 90.9% and 13.1%, respectively, for a fastener with tilt altitude difference 0.27 mm along the scan direction. Due to its low-cost and fast inspection speed, this method is an economical and efficient alternative tool in aircraft manufacture and maintenance for fastener tilt assessment.