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

Abstract

The design of an electromagnetic acoustic transducer (EMAT) array device for the inspection of large areas of metallic plate-like structures using the S0 guided wave mode is described. The reasons for using the S0 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 2 can be inspected from a single location. Spurious signals in the results are caused both by the unwanted A0 mode and by S0 sidelobes, the latter occurring at the same radial distance from the array as the genuine S0 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 S0 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 S1 guided wave mode to inspect a 20 mm thick plate is also demonstrated.

Optimization of guided wave array for inspection of large plate structures

“…This relationship agrees with one-third of the wavelength criterion empirically obtained in [5]. Using relationships (9) and (11) the number of elements required in the ring array can be estimated as …”

“…Based on this principle several guided wave array prototypes [5,6] and different signal processing methods have been developed [3,4]. However, the problem of optimal inter-element spacing in a sparse array hasn't been fully investigated.…”

“…However, the problem of optimal inter-element spacing in a sparse array hasn't been fully investigated. Numerical calculations performed in [5] showed that there is no precise relationship between maximum inter-element spacing and the appearance of grating lobes in the image, but in order to avoid the appearance of grating lobes the maximum element spacing must be approximately equal to 3. Note, that in this case the specific array geometry (circular array) and imaging technique (phased addition method) were used.…”

“…For example, in [5] electro-magnetic acoustic transducers (EMAT) were used to generate and detect S0 Lamb wave mode. However, the optimal EMAT size needs to be of the order of two-thirds of a wavelength in order to maximize the sensitivity to the S0 mode [5]. These two conflicting requirements mean that the transducers must be placed with overlapping, which makes the whole array construction significantly more complicated.…”

Improving Synthetic Aperture Image by Image Compounding in Beamforming Process

“…The increasing use of arrays in non-destructive evaluation (NDE) is also present in ultrasonic guided wave inspection, and various array geometries and signal processing techniques have been employed and evaluated to obtain good defect detection and sizing [2,3,4]. As the changes in the inspected structure are relatively slow, high imaging rates are not necessary, and synthetic aperture techniques (SA) can be employed.…”