Measurement of Elastic Constants Using HalbachArray Enhanced EMAT

Abstract

Elastic constants of metal can be extracted by measuring the ultrasound longitudinal and shear wave velocities. The Electromagnetic Acoustic Transducer (EMAT) technique provides a non-coupling approach to generate ultrasound. Aiming at the rapid determination of elastic constants, we propose an EMAT that can simultaneously excite and receive longitudinal and shear waves. In one aspect, the pole directions of permanent magnets in this array are arranged so that it obtains both horizontal and vertical magnetic fields. These two magnetic fields are required to generate longitudinal wave and shear wave. In another aspect, this magnetic array enhances the magnetic flux density. A planer array is fabricated with permanent magnets with an initial magnetic flux density of 444mT, the resulting magnetic flux density of the magnetic array is as high as 918mT. A square spiral coil is placed under the enhanced magnetic field to excite both shear and longitudinal waves. By measuring the time of flight (TOF) of longitudinal and shear echoes within a single receiving signal, ultrasonic velocities of shear wave and longitudinal wave are calculated with the knowledge of plate thickness. Elastic constants of the 7075 aluminum alloy and TA2 titanium plates are extracted subsequently. The comparison of the measured elastic constants and the previous researches’ result implies the feasibility of the approach.