Fatigue Damage Evaluation of Short Carbon Fiber Reinforced Plastics Based on Thermoelastic Temperature Change and Second Harmonic Components of Thermal Signal

Short fiber reinforced plastics (SFRPs) have excellent moldability and productivity compared to continuous fiber composites. In this study, thermoelastic stress analysis (TSA) was applied to detect delamination defects in short carbon fiber reinforced plastics (SCFRPs). The thermoelastic temperature change ΔT(E), phase of thermal signal θ(E), and second harmonic temperature component ΔT(D) were measured. In the fatigue test of SCFRP, it was confirmed that changes in ΔT(E), θ(E), and ΔT(D) appeared in the damaged regions. A staircase-like stress level test for a SCFRP specimen was conducted to investigate the generation mechanism of the ΔT(D). The distortion of the temperature change appeared at the maximum tension stress of the sinusoidal load—and when the stress level decreased, the temperature change returned to the original sinusoidal waveform. ΔT(D) changed according to the change in the maximum stress during the staircase-like stress level test, and a large value of ΔT(D) was observed in the final ruptured region. A distortion of the temperature change and ΔT(D) was considered to be caused by the change in stress sharing condition between the fiber and resin due to delamination damage. Therefore, ΔT(D) can be applied to the detection of delamination defects and the evaluation of damage propagation.