Inspection of Defect Under Thick Insulation Based on Magnetic Imaging With TMR Array Sensors

A lot of industrial structures are covered by insulation. Defects under insulation (DUI) frequently distress the structure maintenance and may lead to serious safety concerns. In this article, a probe with tunneling magnetoresistance (TMR) array sensors is proposed to inspect defects under thick insulation. The probe contains 64 TMR sensors with the size of each sensor length
$\times $
width = 0.45 mm
$\times0.45$
mm, which can generate a magnetic field image with high spatial resolution. Excitation current with extremely low frequency (in the order of a few hundred Hertz) is utilized for inspection of defects with large liftoff distances. In this low-frequency range, the TMR sensors have much better sensitivity than conventional induction coils. A data processing method based on discrete wavelet transform (DWT) is utilized to reduce noise and highlight defect indication. Experimental results demonstrate that the probe is capable of detecting a machined rectangular defect with dimensions of 6 mm (length), 3 mm (width), and 1 mm (depth) under 10 mm thick insulation. The minimum dimensions of the machined defect under 20 mm insulation that can be detected by the probe are 9 mm (length), 3 mm (width), and 1 mm (depth). As a comparison, for a commercial pulsed eddy current probe (PECA-HR-SM), the length of minimum detectable detect is 19 mm under 18 mm thick insulation. If the thickness of the insulation is 30 mm, a machined rectangular defect with dimensions 21 mm (length), 3 mm (width), and 7 mm (depth) can be detected by the proposed probe. The experimental results demonstrate that the proposed probe is a promising alternation with excellent sensitivity for inspection of defects with large liftoff distances.