Omnidirectional guided wave inspection of large metallic plate structures using an EMAT array

The design of an electromagnetic acoustic transducer (EMAT) array device for the inspection of large areas of metallic plate-like structures using the S/sub 0/ guided wave mode is described. The reasons for using the S/sub 0/ mode are discussed and it is shown how the choice of mode determines the nature of the EMAT array elements. A novel array construction technique is shown to be necessary whereby the EMAT coils for adjacent elements are overlapped in order to achieve the required element density. Results are presented that illustrate the operation of the device on steel and aluminum plate specimens in the thickness range from 5 to 10 mm. An area of at least 10 m/sup 2/ can be inspected from a single location. Spurious signals in the results are caused both by the unwanted A/sub 0/ mode and by S/sub 0/ sidelobes, the latter occurring at the same radial distance from the array as the genuine S/sub 0/ signal from a reflector, but in the wrong direction. The signal-to-coherent noise performance of the complete system is determined by the amplitude ratio of the largest genuine S/sub 0/ signal to the largest spurious signal. This is typically around 30 dB. The sensitivity of the device to artificial defects and genuine corrosion patches is demonstrated and the limitations of its operation are discussed. The feasibility of using the device with the S/sub 1/ guided wave mode to inspect a 20 mm thick plate is also demonstrated.