Identification of invisible fatigue damage of thermosetting epoxy resin by non-destructive thermal measurement using entropy generation

To quantify carbon fiber-reinforced plastic (CFRP) fatigue, herein, we investigate the relationship between fatigue and an epoxy resin used in CFRPs. Generally, fatigue is related to the entropy, which comprises the mechanical entropy, calculated from the dissipated energy and temperature, and thermal entropy, calculated from the relationship between specific heat capacity and temperature. According to previous studies, mechanical entropy generation and thermal entropy generation are equal. Herein, 100 cyclic loading tests are conducted on epoxy resin specimens consisting of 4,4’-DDS and bisphenol a diglycidyl ether. The dissipated energy is determined based on stress–strain curves, and mechanical entropy generation is quantified. An equation for the relationship between the specific heat capacity and temperature is developed based on the Debye model, and the increase in specific heat capacity is calculated for equal mechanical and thermal entropy generations. Generally, differential scanning calorimetry is used for specific heat capacity measurements; however, because these measurements are performed by cutting the specimen, a nondestructive measurement method is required. In this study, the specific heat capacity is measured using lock-in thermography (LIT), and the measured and estimated values are comparable. Thus, fatigue can be estimated by quantifying the thermophysical properties, and the lock-in thermography method is a suitable thermophysical property measurement method for this application.