Ultrasonic Resonance Method with EMAT for Stress Measurement in Thin Plates

Abstract

Metallic thin plate structures such as airplanes are often severely loaded due to high load/weight requirement. Measurement of stress in the loading test are normally done with wire strain gauges. Since such a stress measurement technique is rather inefficient, a new technique of stress measurement has been requested. Birefringence acoustoelasticity is one of the possible alternatives [1, 2, 3]. Stresses in the material make it acoustically anisotropic and the ultrasonic shear wave shows acoustic birefringence. The principal stress difference is proportional to the difference in the propagation velocities of two shear waves polarized in the principal stress directions [4]. For the precise time-of-flight measurement, the sing-around and pulse-echo-overlap methods have been used. However, these methods are inadequate for thin plates, because the propagation distance is short and the accuracy becomes insufficient. Overlapping of echoes sometimes makes the measurement totally impossible.