Fatigue Damage Evaluation of Discontinuous Carbon Fiber-Reinforced Polymer Composites Using Thermoelastic Temperature Variations

Carbon fiber-reinforced polymer (CFRP) composites are expected to be increasingly adopted in automotive structures to achieve car weight reductions that yield an effective reduction of carbon dioxide emissions. Fatigue damage evaluation of CFRP composites is indispensable to guarantee their long-term use. In a thermography-based approach, a suitable temperature component to evaluate the fatigue damage of discontinuous CFRP composites—CFRP composites of discontinuous fibers—should be elucidated. In this study, the non-dimensional thermoelastic temperature amplitude, obtained through thermoelastic temperature variation measurements, is employed. This amplitude can be used to evaluate the stress state of a material subjected to cyclic loading. Moreover, the relationship between the non-dimensional thermoelastic temperature amplitude and fatigue damage is investigated under tension–tension cyclic loading for carbon fiber sheet molding compound (C-SMC), which is a discontinuous CFRP composite produced via sheet molding compound methods. Experimental results reveal that a decrease in the non-dimensional thermoelastic temperature amplitude is associated with the fatigue damage. This decrease is attributed to two factors. One is a change in the stress state applied in the longitudinal and transverse directions of the fiber caused by a shift in the dominant fiber orientation as fatigue damage progresses. The other is the difference in the thermal expansion coefficients in the longitudinal and transverse directions of the fiber. Therefore, the possibility of monitoring the fatigue damage evolution using the non-dimensional thermoelastic temperature amplitude is confirmed. Future studies should assess the remaining fatigue life of CFRP composites using thermoelastic temperature variations.