High-Density Large-Scale TMR Sensor Array for Magnetic Field Imaging

Magnetic field imaging (MFI) is widely used in electromagnetic nondestructive evaluation (ENDE). This paper presents a high-density, large-scale tunneling magnetoresistance (TMR) sensor array for MFI applied in ENDE. The TMR array consists of 180 sensing elements with the distance of 0.5 mm between any pair of TMR sensors adjacent to each other, which enables to acquire an image with high spatial resolution. With a total length of 90 mm, the TMR array is capable of imaging a rectangular area as wide as 90 mm in a single scan resulting in a fast inspection speed for the evaluation of structure with a large surface area. The proposed TMR sensor array has high sensitivity to the magnetic field in a wide frequency range. Therefore, it can be used for the detection of deeply embedded defects in multilayer riveted structures with excitation currents at ultralow frequencies. Experimental results show that machined notches under fasteners head at a distance of 8 mm from the top surface of a multilayer riveted aluminum structure can be detected with a proper fusion of the magnetic field images at 100 and 1000 Hz. Furthermore, surface notch with dimension
$300\,\,\mu \text {m} \times 300\,\,\mu \text {m} \times 400\,\,\mu \text{m}$
on an aluminum sample can be detected from the obtained magnetic field image with excitation current frequency 20 kHz.