Analysis of the directivity of longitudinal waves based on double-fold coil phased EMAT

Longitudinal critically refracted (LCR) waves have already been widely applied for residual stress characterization. Such waves are usually generated using mode-conversion at the first critical angle of the incident longitudinal wave, which gives waves that then propagate at a dip-angle, and this places energy close to the surface of the specimen. The dip-angle needs to be minimized to improve both velocity measurement and residual stress characterization sensitivity. This paper reports a novel double-fold coil phased EMAT that can decrease the dip-angle. The performance of this new EMAT was investigated using both a COMSOL model and experiments. Initial model validation was provided through a comparison with experimental data. The EMAT design also enables scanning of samples, and operation in harsh environments where use of a PZT based transducer and couplants can complicate and limit inspection. The use of the EMAT has the potential to give more accurate time of flight (TOF) data and enhances the reliability and accuracy for residual stress measurement.