Nondestructive Detection of Depth-Dependent Defects in Carbon-Fiber-Reinforced Polymer Composites by Terahertz Time-Domain Spectroscopy

Terahertz (THz) time-domain spectroscopy was used to detect the artificially pre-embedded circular polytetrafluoroethylene (PTFE) slice defects at different depth (0.225–0.9 mm) in carbon-fiber-reinforced polymer composites. The THz reflectance peak-to-peak value, maximum value, and delay time in the time-domain waveform, and spectral characteristics in frequency domain of defects were analyzed. It was found that the defects in the specimen can be effectively distinguished in the frequency range of 0.16–2.0 THz by THz reflection imaging technology. With the increase of depth of the defects in the specimen, the peak-to-peak value and maximum value of THz signals show a linear attenuation, and the delay time increases accordingly. The analysis results of frequency-domain signals of the defects in specimens also confirm the accuracy of the former results. This technology exhibits potential applications in distinguishing and sorting the defects at different depths in a carbon-fiber composite, and can provide an important reference for nondestructive composite failure detection in this area.