Very high frequency EMAT for resonant measurement

Very high frequency resonant EMAT technique is described at frequencies over 200 MHz which is a very high frequency room-temperature measurement of this type. This high frequency EMAT is very small, 25 mm×25 mm×40 mm, and is made from a permanent magnet with 20 mm diameter and a flat circular coil with 3 mm diameter. Longitudinal waves, slow and fast shear waves are generated and detected on thin metal sheets down to 20 μm. Single crystals of steel alloy with 3% Si are measured (thickness 0.253 mm) and distinctive thickness resonances are obtained. From the measured resonance frequencies, crystallographic direction is obtained. In addition, the results on a thin sheet bonded to a bulk sample are given. The nature of the bonding is shown to strongly influence the resonant peak heights. The wall thickness of an aluminum beer can was probed using this technique at various positions along its length. Through these examples it should be clear this high frequency resonant EMAT is a promising technique for the nondestructive measurement of material characteristics of plates and thin sheets