Generation and Detection of Higher-Order Mode Clusters of Guided Waves (HOMC-GW) Using Meander-Coil EMATs

This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-GW) using a meander-coil (MC) electromagnetic acoustic transducer (EMAT) in plates at frequencies significantly higher than the lower-order plate modes. These wave modes are considerably less dispersive and they occur at much higher frequency-thickness (f x d) products. Our studies cover the f x d range of 13 to 20 MHz.mm. Experimental measurements were carried out on Al plate samples of different thicknesses using three different EMAT coil periods. To understand the generation and propagation characteristics of HOMC-GW with EMATs, several simulations were carried out using 2-D finite element models at different f x d products. These simulations captured all features observed in the experiments. The time-frequency smoothed pseudo Wigner-Ville distribution (SPWVD) was used to analyze the HOMC-GW modes. Defect detection measurements using HOMC-GW generated using EMATs were made on Al plates with machined defects.