## 3 Results and Discussion  
The proposed algorithm was experimentally validated. A double-sided inspection of a polymer composite was carried out. During the measurements a commercially-available material which consists of 30 plies of glass fiber fabric with regularly-oriented fibers subjected to stress forces below 140 MPa was utilized. The photography of the exemplary specimen is presented in Figure 9. After applying mechanical excitation, several delaminations were created on various depths of the layered structure and both surfaces were perforated. Material in this condition has significantly worse mechanical properties.  
The exemplary A-scan signals \(g_{\mathrm{mCHI}}(t)\) and \(-g_{\mathrm{mCHI2}}(-t)\) obtained using the proposed double-sided inspection and signal processing are presented in Figure 10. One can observe constructive interference of \(\mathrm{FSR}_{\mathrm{CHI}}\)-\(\mathrm{BS}_{\mathrm{CHI2}}\), \(\mathrm{FSR}_{\mathrm{CHI2}}\)-\(\mathrm{BS}_{\mathrm{CHI1}}\) and defect/layer reflection pulses, respectively. Similarly, B-scan signals obtained during a single line scan along the sample are presented in Figure 11 (deconvolved) and Figure 12 (channel 2 signal time-reversed). In each case BSR pulse has smaller amplitude and frequency content in comparison with FSR pulse. This is caused by attenuation of the pulse energy during propagation through multilayered structures and defocusing. As mentioned before, layers' reflection pulses (in the case of layers close to back surfaces) are blurred, as well. Based on Figure 12, one can see that \(g_{\mathrm{mCHI}}(t)\) and \(g_{\mathrm{mCHI2}}(-t)\) signals contain complementary information about the layers.  
Figure 8: Analytical model: (**a**) cascade connection of multiple layers; (**b**) block scheme of reflected signal calculation; (**c**) time domain excitation signal (and its spectrum) in the form of a wideband Gaussian pulse; and (**d**) the signal obtained using the analytical model for reflection arrangement (and its spectrum).  
Figure 11: Exemplary B-scan signals obtained using double-sided inspection of a composite sample subjected to stress: (**a**) deconvolved channel 1 signal \(g_{\text{mCH1}}(t,x)\); and (**b**) deconvolved channel 2 signal \(g_{\text{mCH2}}(t,x)\).  
Figure 10: Exemplary results of double-sided pulsed THz measurements for given (\(x\),\(y\)) position (signal from channel 2 was multiplied by \(-1\) and time-reversed according to the proposed algorithm).  
Figure 9: Photo of the multilayered polymer composite structure subjected to stress resulting in multiple delaminations.  
The B-scan signal combined using the proposed algorithm is presented in Figure 13. In comparison to the results of standard, single-side inspection (shown in Figure 11), here both material-air interfaces have \"sharp\" indications and the layers' response is noticeable over the whole cross-section of the examined structure (with maximum values in the vicinity of material boundaries). Moreover, the pulses caused by reflection from defects (mainly delaminations) are detectable even on the background of the layers' responses.  
Figure 12: B-scan signals prepared for weighted summing: (**a**) deconvolved channel 1 signal \(g_{\text{mCH1}}(t,x)\); and (**b**) time reversed channel 2 signal \(g_{\text{mCH2}}(-t,x)\).  
Figure 13: Combined B-scan signal \(g_{\text{cmb}}(t,x)\) using the proposed algorithm.