ELECTROMAGNETICALLY EXCITED ACOUSTIC-RESONANCE FOR EVALUATING ATTENUATION COEFFICIENT AND GRAIN-SIZE IN POLYCRYSTALLINE METALS

An experiment on an acoustic resonance spectrometer for evaluating attenuation characteristics and grain size of polycrystalline metals is described. A noncontacting electromagnetic acoustic transducer is excited to establish a shear-wave ringing in a plate sample and the amplitude spectrum is obtained by sweeping the driving rf frequency through the resonances. At each resonance frequency, the sample rings down exponentially with time. The wave form is acquired to determine the time constant, or the attenuation coefficient, whose frequency dependence is finally related to the average grain size through the Rayleigh scattering theory. This nondestructive evaluation agreed with the photomicrographic examination to an accuracy of 6 mum for a relatively narrow distribution of grain size in low carbon steels.