## 3 Results and Discussion  
We fabricated GFRP laminate samples with artificial internal defects to test the performance of our THz tomography systems for nondestructive inspection of GFRP composites. Figure 3a,b shows the design of GFRP samples A-1 and A-2, which have dimensions of 130 \(\times\) 130 \(\times\) 5 mm\({}^{3}\) and contain internal defects of various thicknesses. The red rectangles and the blue squares represent delaminations and Teflon inclusions, respectively; these are located at a depth of 1.5 mm. Sample A-1 has defects of 0.025, 0.05, 0.1 and 0.2 mm nominal thickness, while sample A-2 has defects of 0.2, 0.4, 0.7 and 1 mm nominal thickness. Figure 3: Schematic design of GFRP samples containing artificial internal defects with different (**a**,**b**) thicknesses, (**c**) sizes, and (**d**) locations in the depth direction. The numbers indicate millimeters. Figure 4a shows the C-scan images of samples A-1 and A-2 measured with the time-domain THz tomography system. The C-scan data consist of the maximum values in the A-scan data in which the data from the surface were excluded. A-scan data obtained in places with a defect and without any defect using the time-domain THz tomography system are shown in Figure S4a. The 0.025 mm-thick Teflon inclusion is visible, as is the 0.025 mm-thick delamination. We analyzed the amplitudes of the THz signals from defects with various thicknesses in the C-scan images. Figure 4c shows the amplitude of the THz signal according to defect thickness. For both delamination and Teflon inclusion, the amplitude is greatest at a thickness of 0.2 mm and decreases as the thickness either increases or decreases. This is due to Fabry-Perot reflection from the defect layers [15]. Since GFRP and Teflon have refractive indices of about 2.1 and 1.43, respectively, the reflectivity between air and GFRP is higher than that between Teflon and GFRP [20; 21]. Thus, overall, the THz signals from the delaminations are greater than those from the Teflon inclusions, and delaminations are easier to detect than Teflon inclusions. Figure 4e and Video S4e show the 3D images of samples A-1 and A-2 measured with the time-domain THz tomography system. Thinner defects are not visible in the 3D visualization, unlike in the C-scan images. Furthermore, the THz signals from thicker defects are separated into two parts, which indicate the upper and lower interfaces of the defect layers. Figure 4: (**a**) C-scan images, (**c**) THz signal amplitude versus defect thickness, and (**e**) 3D images of samples A-1 and A-2 obtained using the time-domain THz tomography system. (**b**) C-scan images, (**d**) THz signal amplitude versus defect thickness, and (**f**) 3D images of samples A-1 and A-2 obtained using the frequency-domain THz tomography system. Figure 4b,d,f shows the C-scan images, the amplitude of the THz signal versus the defect thickness, and the 3D images of samples A-1 and A-2, respectively, measured with the frequency-domain THz tomography system. In the C-scan images (Figure 4b), delaminations with a thickness of as little as 0.025 mm can be seen, but only Teflon inclusions with a thickness of no less than 0.1 mm are visible. This is because the signal-to-noise ratio of A-scan data is lower in the frequency-domain THz tomography system than in the time-domain THz tomography system. The results for the amplitude of the THz signal versus the defect thickness in Figure 4d are similar to those in Figure 4c. The 3D images in Figure 4f and Video S4f are consistent with the C-scan images in Figure 4b and are similar to the 3D images in Figure 4e and Video S4e. Figure 3c shows the design of GFRP sample B, which has dimensions of 130 \(\times\) 130 \(\times\) 5 mm\({}^{3}\) and contains internal defects of different sizes. Sample B contains square Teflon inclusions that are 1, 2, 3, 4, 5, 6, 7, 8, and 9 mm in size and rectangular delaminations with widths of 1, 2, 3, 4, 5, 6, 7, and 8 mm. All of these defects are 0.2 mm thick and lie at a depth of 1.5 mm.