Experimental Investigation of Dispersion Characteristics of Ultrasound in Fine Weave Pierced C/C Composites

Reliable nondestructive evaluation of fine weave pierced carbon/carbon (C/C) composites is essential because these materials are increasingly used in critical components, yet ultrasonic inspection is often compromised by dispersion and frequency-selective filtering that distort waveforms and complicate imaging. This study aimed to experimentally characterize the anisotropic acoustic dispersion and frequency-filtering behavior of up-to-date fine weave pierced C/C composites with a pitch-based matrix. Phase velocities along the three principal directions (x, y, z) were measured over a frequency range of 0.5–5.0 MHz. Along the z-direction, phase velocity increases from 7250 m/s to 13,500 m/s with rising frequency, revealing four selective passbands. This indicates pronounced geometric dispersion and a wave-filtering effect due to the larger-scale fibers aligned in this direction. In contrast, the x- and y-directions exhibit only a single low-frequency passband dominated by the strong viscoelasticity of the matrix, with phase velocities of 8100 m/s at 0.5 MHz and 7100 m/s at 0.3 MHz, respectively. Furthermore, temperature-dependent measurements in the z-direction demonstrate a transition from viscoelastic-dominated to geometric-dominated dispersion as temperature increases. These results provide frequency-selection guidance for reliable ultrasonic nondestructive evaluation of advanced C/C composite components.