Simultaneous in-situ monitoring of temperature and degree of cure in CFRP using a microfiber based acousto-ultrasonic system

Abstract Degree of cure (DoC) that significantly affects the quality of composites is difficult to be in-situ monitored as lack of sensing system with high sensitivity and minimal impact on composites. In this study, an acousto-ultrasonic system using encapsulated cantilever microfiber Bragg grating was proposed to simultaneously track temperature and ultrasonic signals during curing process of carbon fiber reinforced plastic (CFRP). Temperature acquired by the system was consistent with thermocouple measurements in either CFRP lamina or laminates. The peak of envelope and peak of spectrum extracted from the ultrasonic signals propagating in the CFRP lamina showed a strong correlation with the DoC calculated from off-line differential scanning calorimeter test. The phase transition of CFRP lamina was also identified and validated by dynamic mechanical analysis. Furthermore, in-situ monitoring of CFRP laminates confirmed that DoC evaluation remained effective regardless of the angle between optical fiber and carbon fiber. The relationship between the ultrasonic characteristic parameters and DoC was established using a modified logistic function and substantiated in a curing experiment of quasi-isotropic CFRP laminate. The results indicated that the newly proposed microfiber based acousto-ultrasonic system enables multi-parameter monitoring in the curing process of composites.