This paper investigates the detection of internal defects in composite samples composed of Onyx, a material made with a nylon matrix reinforced by chopped carbon fibers. Artificial defects, in the form of internal air voids, were intentionally introduced during the additive manufacturing process. The primary objective was to determine the depth detection capabilities of ultrasound by varying the excitation frequency and determining whether internal defects remained identifiable at different subsurface levels. Ultrasonic lock-in thermography was utilized to excite the samples. Experimental measurements were conducted using the Edevis UTvis system, while the thermal response to ultrasonic excitation was captured with a FLIR SC7500 infrared camera equipped with an actively cooled detector. As the frequency is modified, the depth of wave propagation also changes, a phenomenon well established in homogeneous materials. However, the heterogeneous nature of Onyx, with conductive carbon fibers distributed anisotropically throughout the matrix, introduces complexities into wave propagation. The recorded thermographic data were processed in MATLAB, where advanced post-processing and image analysis techniques were applied to enhance defect visibility and assess signal response.

Acknowledgement: This work was supported by the VEGA 1/0753/24 and the KEGA 020ŽU-4/2025.