Defect Sensitivity in Ultrasonic Testing of Al ₂ O ₃ /Epoxy Composites: Insights from Multiscale Mechanics and Simulation

Epoxy composites are widely used in electrical insulation due to their high specific strength and excellent dielectric properties. However, adding Al ₂ O ₃ particles changes the material modulus and ultrasonic propagation, making internal defect detection more difficult. This study develops an ultrasonic testing model for Al ₂ O ₃ /epoxy composites. COMSOL simulations are used to evaluate the maximum particle size affecting ultrasonic transmission. The effects of different Al ₂ O ₃ concentrations on composite modulus are assessed through nanomechanical measurements and compression tests. Due to the high attenuation in epoxy systems, a 0.5 MHz longitudinal ultrasonic wave is employed. Experimental and simulated results are combined to analyze how wave velocity, defect contrast, attenuation, and detection sensitivity vary with modulus. The results show that micron‐sized Al ₂ O ₃ particles have negligible influence on low‐frequency ultrasonic wave paths. Furthermore, composites with higher modulus exhibit lower attenuation and stronger defect signal amplitudes, improving detection performance. A mechanism of ultrasonic propagation in high‐modulus composites is proposed, providing a theoretical and experimental basis for designing materials with enhanced mechanical strength and detectability.