Abstract
This paper proposes a new omni-directional electromagnetic acoustic transducer (EMAT) for the ultrasonic Lamb wave (ULW) tomography imaging (TI) of defects in metallic plates. The proposed EMAT is composed of a permanent magnet and a coil with a contra-flexure structure. This new EMAT coil structure is used for omni-directional ULW transmission and reception and ULW TI for the first time. The theoretical background and the working principles of this EMAT are presented and analyzed. The experimental results of its use on a 3 mm thick aluminum plate indicate that the EMAT with a contra-flexure coil (CFC) can transmit and receive a pure single A0 mode ULW with a high signal-to-noise ratio (SNR). Thus, the extraction of the projection data used for ULW TI may be performed accurately. The circumferential consistency of the projection data is only slightly influenced by the distortion of the eddy current field that is induced by the new CFC with an irregular shape. When the new EMAT array is used for ULW TI using the cross-hole method and SIRT arithmetic, a desirable imaging quality can be achieved, and the estimated size of an artificial corrosion defect agreed well with its actual value. The relation between the reconstruction resolution and the number of the new EMATs used is analyzed. More TI experiments are carried out when the aluminum plate defect is in two different locations relative to the EMAT array, for the further investigation of the performances of the new EMATs.
Highlights
Metallic plates are widely used in the aerospace and automobile industries as well as in pressure vessels
Projection data extraction, the circumferential consistency of the projection data and the performances of the ultrasonic Lamb wave (ULW) tomography imaging (TI) for the new electromagnetic acoustic transducer (EMAT), several experiments were conducted using on a 3 mm thick aluminum plate
The non-contacting, omni-directional EMAT is an ideal choice for the ULW TI of the defects in metallic plates
Summary
Metallic plates are widely used in the aerospace and automobile industries as well as in pressure vessels. It utilizes an array of transducers to transmit and receive a group of ULW rays from all directions In this way, the inspected area of the plates can be scanned from different angles to generate the projection data. ULW TI produces quantitative maps of defects in metallic plates with different kinds of projection data, reconstruction algorithms and transducer array geometries. TI, namely reconstruction algorithm for probabilistic inspection of damage (RAPID), has received great attention [16,17,18,19] It generates maps of defects in terms of a presence probability using only a handful of transducers, and uses the feature of signal difference coefficient (SDC) as the projection data. The accuracy of the projection data extraction, the circumferential consistency of the projection data as well as the feasibility and performances of being used for ULW TI of the new EMAT are further investigated experimentally
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