Process Optimization and Non-Destructive Evaluation of Micro-Voids in Submarine Composite Structures for Enhanced Mechanical Performance

This study presents a systematic approach to enhancing the mechanical performance of composite materials for submarine applications by quantitatively evaluating and control-ling internal micro-voids generated during the manufacturing process. Three non-destructive evaluation techniques—ultrasonic testing, optical microscopy, and mi-cro-computed tomography (Micro-CT)—were employed to assess void content in fi-ber-reinforced composite specimens fabricated under various processing conditions. Ten-sile and flexural strength tests were conducted to investigate the correlation between void content and mechanical properties. Among the methods, ultrasonic testing exhibited the strongest negative correlation (correlation coefficient = –0.703), confirming its effectiveness as a representative non-destructive evaluation technique. Furthermore, statistical design of experiments, including factorial design, steepest ascent method, and response surface methodology (RSM), identified defoamer concentration and mixing time as the most in-fluential process parameters in void reduction. The optimal processing conditions were determined to be 0.049% defoamer and 232 minutes of mixing. Under these conditions, void content was minimized, and mechanical properties were significantly improved. These findings offer practical guidance for void control and non-destructive evaluation in large-scale composite structures, contributing to improved reliability in underwater structural applications.