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Propagation of ultrasonic waves has been studied in random particulate composites at low, intermediate, and high frequencies. The principal objective of this experimental work is to demonstrate the existence of two separate branches of wave propagation: a low-frequency acoustical branch where the inclusions move essentially in phase with the matrix and a high-frequency optical branch where inclusions move essentially out of phase with the matrix. The two are separated by a well-defined cutoff frequency which corresponds to the rigid-body-translation resonance of the relatively heavy inclusions. The measured values of the cutoff frequency were found to be in excellent agreement with an elementary model of Moon and Mow. To the best of our knowledge this is the first experimental observation of the optical branch and of the cutoff frequency.
composite materials, wave propagation, ultrasonics, dispersion, attenuation, acoustical branch, optical branch, cutoff frequency
Associate professor, Texas A&M University, College Station, TX