Resonance EMAT system for acoustoelastic stress measurement in sheet metals

Abstract

A practical method of acoustoelastic stress measurement based on electromagnetic acoustic resonance is presented. This overcomes fundamental limitations of conventional procedures and exhibits the stress resolution to 0.1 MPa for thin aluminum plates and the spatial resolution to several millimeter square in a noncontacting operation. The proposed method successfully combines an electromagnetic acoustic transducer (EMAT) and a superheterodyne phase- sensitive detector. An EMAT is excited by a high-power rf burst in the 0.5–20-MHz range and generates ultrasonic oscillations in a plate through a Lorentz force mechanism. The signals in the plate are then received by this same EMAT and analyzed for the amplitude spectrum using swept-frequency phase-sensitive detection circuitry. The resonance frequencies can be determined for longitudinal and shear modes simultaneously. To illustrate the resonance spectrum technique, experimental results are shown for the measurement of a two-dimensional stress field in a thin aluminum plate. Liftoff sensitivity is also determined.