On the use of a linear array EMAT for remote thickness gauging using the reflected modes on a steel pipe

Guided wave nondestructive testing refers to a set of fast and reliable techniques used to scan structures such as plates or pipes over medium to long range distances. The structural filtering method uses high-order modes and their cutoff frequency-thickness products to estimate the thickness along an inspection line. A corroded region, which can be approximated by a local loss of thickness, will act as a filter on the propagating modes. Because of their cutoff frequency-thickness product, some modes will not be able to propagate and will be either reflected or converted. Shear horizontal guided waves are the main candidate for structural filtering due to the simplicity of interpretation of their dispersion curves and their low sensitivity to non-viscous surface fluid loading. Electromagnetic acoustic transducers (EMAT) provide fast and efficient SH wave transduction but the most common designs, namely, periodic and permanent magnet EMAT is a mono element probe allowing modes to be excited only around a fixed wavelength. Recent work has shown that a linear array EMAT can be combined with a phase velocity excitation to allow almost complete control of the emitted waves in the frequency wavenumber space. In this paper, the linear array EMAT developed is used to estimate the minimum thickness of a defect machined on a 10.2 mm thick steel pipe with an external diameter of 323.8 mm. The measurements and finite element simulations carried out enabled us to estimate the thickness in several positions, with a maximum error of 0.4 mm and locate the defect with an error of 10 mm over a propagation distance of 600 mm.