Grain size characterization by laser-based ultrasonics based on the centroid frequency shift method

To solve such problems as the sensitivity of ultrasonic attenuation method for grain size evaluation to geometric spreading, reflection and transmission effects, and the accuracy of ultrasonic echo amplitude measurements, this paper addresses the application of the centroid frequency shift method for characterization of mean grain size in polycrystalline materials. A general model was presented for the correlation of the mean grain size to the downshift in the spectral centroid frequency of a Gaussian pulse during propagation in a medium with a nonlinear frequency dependence of attenuation. The model was further extended to a particular case of laser-ultrasonic inspection, and an explicit formula for the mean grain size was derived in terms of the centroid frequency ratio of two successive echoes under a quadratic frequency assumption. Experimental inspection for mean grain size was then conducted in α-titanium alloys by a laser-based ultrasonic method. Microstructures with different mean grain sizes of 26–49 μm were obtained by annealing at 800°C for different holding times. The centroid frequency shift reveals a linear dependence on the mean grain size. Referring to the metallography analysis, the nondestructive evaluation errors over all the test samples are smaller than 5.4%. The provided results help support the application of the centroid frequency shift method for the grain size evaluation in polycrystalline materials, and this method is more insensitive to effects of wave diffraction, reflection, and transmission in comparison with some other nondestructive methods.