Ultrasonic autofocus imaging of internal voids in multilayer polymer composite structures

•Achieve ultrasonic imaging of voids in multilayer polymer composite structures.•Process the full matrix data and improve the imaging ability of multilayer structures.•Determine speed of sound (SOS) and focus void location layer by layer automatically.•The Brenner criterion is better than the Tenenbaum and normalized variance criterion.•The void position error is within 0.6 mm, and SOS estimation error is within 6%.