Monitoring of rotating bending fatigue damage by axial-shear resonance using magnetostrictively-coupled EMAT

Abstract

In this paper, we present a technique to generate axial shear acoustic resonance in a cylindrical rod or pipe by a magnetostrictively-coupled electromagnetic acoustic transducer (EMAT). We apply this ultrasonic technique to monitor rotating bending fatigue damage. The technique is based on the electromagnetic acoustic resonance (EMAR), which is a combination of the resonant technique and a noncontacting EMAT. The EMAT for the axial shear wave consists of a solenoidal coil to apply a static field along the sample rod (or pipe) and a meanderline coil to induce the dynamic field, relying on the magnetostrictive effect in ferromagnetic metals. The axial shear wave propagates along the circumference of the cylindrical rod or pipe with axial polarization. The resonant mode is determined by the sample dimensions and the wavelength. The major advantage is the capability of evaluating the inhomogeneous materials character in the radial direction using the different resonant modes. The attenuations and the resonant frequencies of the first three resonant modes are measured during the rotating fatigue test. The attenuation shows a remarkable sensitivity to damage accumulation, mainly due to the evolution of the dislocations and the grain size. The ultrasonic evaluation is interpreted in terms of the photomicrograph and transmission electron micrograph observations.