ISSI 2021: Terahertz Spectroscopic Characterization and Thickness Evaluation of Internal Delamination Defects in GFRP Composites

Abstract In recent years, terahertz time-domain spectroscopy (THz-TDS) has attracted significant attention due to its superior time and spatial resolution as well as its capabilities of localizing and characterizing internal defects in glass-fiber-reinforced polymer-matrix (GFRP) composite materials. In this study, the THz-TDS system is used to detect and evaluate hidden multi-delamination in plastic multilayered GFRP composite laminates. To remove the noise from the time-of-flight (TOF) signals, a wavelet shrinkage de-noising algorithm was used, leading to enhanced THz signals and further enabling an easy localization and evaluation of overlapping multi-delamination defects in GFRP composite laminates. In particular, both the thicknesses and locations of these delamination defects in the z-direction (i.e., through-the-thickness direction) were analyzed and measured considering the interaction between pulsed THz waves and the GFRP composite laminate. THz wave power and absorbance levels at every interface with different refractive indices in the GFRP composite samples were discussed based on analytical and experimental results. The present study shows the capabilities of the THz-TDS for the spectroscopic characterization of composite materials and helps users to know how they can theoretically estimate the thickness of the composite material that can be nondestructively evaluated by the THz-TDS.