Detection of incipient thermal damage in carbon fiber-epoxy composites using fluorescent thermal sensor films

Carbon fiber epoxy composites have become prevalent in a variety of industries, especially in aerospace. The significant non-destructive evaluation challenges of composites require new solutions, especially in detecting the onset of thermal damage (incipient thermal damage). The initial stages of thermal damage are chemical rather than physical, and can cause significant reduction in properties such as interlaminar shear strength (ILSS) well before physical damage becomes detectable in ultrasonic testing. This work demonstrates the use of thermochromic fluorescent molecules dispersed in films as sensors for such detection. A molecule has been developed which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature-time combinations that can cause damage in composites. This molecule is dispersed in a removable polymer film and attached to composite parts to form a sensor. This work presents an evaluation of the sensor film's performance in comparison to ultrasonic C-scan as a method to detect incipient thermal damage. Carbon fiber-epoxy samples exposed to varying thermal exposures are evaluated using the fluorescent thermal sensor films and with ultrasonic imaging, and the results are compared to the results of short beam shear tests for ILSS.