Abstract
The periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is a sensor that can generate and receive shear horizontal (SH) waves without direct contact with the inspected medium using the Lorentz mechanism. However, the PPM EMAT experiences high signal variance on ferromagnetic steel under specific conditions, such as a change in signal amplitude when the sensor is moved in the direction of SH wave propagation. Magnetostriction effects are hypothesized to be the cause of these anomalous behaviors; the objective of this paper is to determine the relative strengths of the magnetostriction and Lorentz wave generation mechanisms for this type of EMAT on steel. This goal is accomplished through the use of a second EMAT, which induces only magnetostriction (MS-EMAT), to calibrate a novel semi-empirical magnetostriction model. It is found that magnetostriction effects reduce the amplitude of the SH wave generated by this particular PPM EMAT transmitter by an average of 29% over a range of input currents. It is also determined that magnetostriction is significant only in the investigated PPM EMAT transmitter, not the receiver. In terms of practical application, it is shown that the MS-EMAT is less sensitive to changes in the static and dynamic fields than PPM EMATs at specific operating points; this makes the MS-EMAT a viable alternative for nondestructive evaluation despite lower amplitudes.
Highlights
Aboveground storage tanks and pipes are large steel structures that experience corrosion and can be inspected with guided waves using electromagnetic acoustic transducers (EMATs) [1]
This paper aims primarily to determine the relative magnitudes of Lorentz and magnetostriction contributions in a particular periodic permanent magnet (PPM) EMAT transmitter operating on mild steel plates
An MS-to-MS experiment is compared against an MS-to-PPM experiment with the same MS-EMAT magnet liftoff (8.92 mm) for both transmitter and receiver and input current (35.1 A) in the transmitter (Figure 11)
Summary
Aboveground storage tanks and pipes are large steel structures that experience corrosion and can be inspected with guided waves using electromagnetic acoustic transducers (EMATs) [1]. For inspecting plate structures, using a periodic permanent magnet (PPM) EMAT has been a popular choice [2,3,4,5,6] due to their high signalto-noise ratio and ability to generate shear horizontal (SH) ultrasonic waves. Andruschak et al, used PPM EMATs to generate SH waves near the inflection point of the dispersion curve to optimize the detection of defects in the presence of support contacts [2]. Shi et al, proposed a method for detecting circumferential cracks on a pipe with SH waves produced by a PPM EMAT by analyzing the reflections from the edges of the defect [6]
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