Miniaturised SH EMATs for Fast Robotic Screening of Wall Thinning in Steel Plates

Electromagnetic acoustic transducers (EMATs) are well suited to generating and detecting a variety of different ultrasonic wavemodes, without the need for couplant, and they can be operated through some coatings. EMATs can be used to generate shear horizontal (SH) waves, which show promise for fast screening of wall thinning and other defects. However, commercial SH-wave EMATs are not suitable for robotic implementation on ferritic steel due to the large magnetic drag force from the magnets. This article describes the design and characterisation of miniaturised SH guided wave EMATs, which significantly reduce the magnetic drag and enable mounting onto a small crawler robot for sample scanning. The performance of the miniaturised EMATs is characterised and compared to a commercial EMAT. It is shown that signal to noise ratio is reduced, but remains within an acceptable range to use on steel. The bandwidth and directivity are increased, depending on the exact design used. Their ability to detect flat bottomed holes mimicking wall thinning is also tested.

I. Introduction

There is a growing need for faster, more reliable, automated inspection of a variety of structures and materials. Robotic inspection is a potential solution for automated inspection; however, current inspection techniques use point-by-point scanning, accumulating large amounts of data and requiring operator presence and input [1]. Point-by-point scanning typically uses ultrasound (longitudinal or shear wavemodes at normal incidence to sample surface) to measure the thickness of the material at each point and detect defects. This is normally done using a piezoelectric transducer, which requires couplant and consistent contact with the material under test. Non-contact methods of inspection are preferable as they can simplify the process [2].