Abstract
Thick composite structures are prone to defects such as fiber wrinkles and voids during manufacturing. It is challenging to accurately evaluate the forming quality of the structures using conventional ultrasonic inspection, due to the coupling effects between time-frequency features of ultrasound and mixed-type defects. Propagation behaviors of through-thickness transmitted ultrasound in thick composites with fiber wrinkles, voids and no defects were comparatively investigated by both simulation and experiment in this study. Influence of out-of-plane fiber wrinkles on the ultrasonic frequency shifting was studied by comparing the time-frequency characteristics of simulated ultrasonic signals after propagating different distances in the intact and wavy composites. Changes of transmission coefficients of converted transverse waves caused by fiber wrinkles were concluded be the reason for the appearance of high-frequency components above the excitation frequency in the transmitted ultrasound signal. Thereby ultrasonic defect inspection indices based on the phenomenon of ultrasonic frequency shifting were defined. Effects of interaction angles between ultrasonic propagation direction and fibers on the defined inspection indices was further investigated quantitatively. The thick composites with diverse defects were successfully diagnosed using the ultrasound frequency shifting and energy dissipation behaviors.
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