Ultrasonic autofocus imaging of internal voids in multilayer polymer composite structures

Multilayer polymer composite structures have been playing important roles in various fields, but the voids inside are not allowed in most scenarios. Ultrasonic technology has been widely used to inspect voids in concrete and metal structures. However, the application of ultrasonic imaging in polymer composite structures is severely blocked by the coating and lamination structures and unstable manufacturing induced sound speed variations. In this paper, a method to autofocus imaging of internal voids in multilayer polymer composite structures with ultrasonic phased array is firstly proposed. The method processes the full matrix capture (FMC) and focuses all voids in the multilayer structure automatically without the prior information of the speed of sound (SOS). The method utilizes the focus criterions to evaluate the focusing quality and then estimates the SOS with differential evolution layer by layer from surface to deep, which improves the robustness and computational efficiency. The method was examined with simulation data from three multilayer structures and well-focused all voids with position error less than 0.6 mm and SOS error less than 6 %. Moreover, the method was verified with the experimental data and focused voids with position error less than 1 mm.