## 4 Results and Discussion  
A comparison of the sensitivity and resolution of the PuC-based system with those of a standard PA ultrasonic testing [39] was carried out by doing measurements on a thick, quasi-isotropic CFRP sample. The sample had 59 layers of carbon fiber fabric with a fiber orientation of \(0^{\circ}/90^{\circ}\) and \(-45^{\circ}/45^{\circ}\) degrees in the successive plies. The matrix material was a standard Epoxy resin. The volumetric density of the material was 1.14 g/cm\({}^{3}\), while its compression and tensile modulus were 3.2 GPa and 3.0 GPa, respectively, measured at a temperature of 23 \({}^{\circ}\)C. The total thickness of the sample was 60 mm, with known standard defects, which were basically a series of side drilled holes. There were seven side drilled holes, 2 mm diameter each, drilled 20 mm deep to the side surface and 10 mm apart from each other while changing in depth from the surface of inspection, as depicted in the picture and sketch of the sample in Figure 3.  
Figure 2: Illustration of the total focusing method (TFM) image reconstruction procedure when the full matrix capture (FMC) data are acquired using a pulse-compression-based linear phased-array (PuC-PA) system. Two separated array transducers are used for the transmission and reception of ultrasonic testing (UT) signals, and the plane in the middle of the two transducers is visualized. The pulse-echo measurements were performed using a commercial system from Advanced OEM Solutions in combination with a linear array transducer \"2.25L64-A2\" from Olympus. The transducer had the central frequency of 2.25 MHz and a total number of 64 elements. The pitch and length of a single element were 0.75 mm and 12 mm, respectively. To maintain an equivalent aperture in the transducer system used for the PuC-based method, only 43 out of 64 active elements of the array transducer were used. For the PuC measurements, a pair of 16-element linear array transducers manufactured by IMG Ultrasounds with a central frequency of 2.25 MHz were used. The pitch and length of an element were 2 mm and 16 mm, respectively. The transmission and reception transducers were placed in a pitch-catch configuration and aligned side-by-side along the array direction to obtain a configuration as close as possible to that used for pulse-echo measurements. For the excitation, a linear chirp signal ranging from 150 kHz to 2.85 MHz was used, corresponding to a central frequency of 1.5 MHz and a relative bandwidth equal to 180% of the central frequency. No window function, i.e., envelope amplitude modulation, was applied to the excitation signal \(s(t)\). On the other hand, the acquired signal at each channel of the array was convolved with a windowed matched filter to retrieve the impulse response, i.e., an A-scan echo signal, while reducing the sidelobe levels. An example of the typical signals in a PuC measurement is presented in Figure 4, where the input signal is a linear chirp with approximately 0.54 ms of duration and the same frequency range as indicated above. The impulse response is then retrieved from the convolution of the acquired signal with the matched filter, where a reactance-Tukey window function \(w(t)\) was used, as explained in Section 2. In the case of the sector scan, due to limitations in the on-board memory and data throughput, the excitation chirp was approximately five times shorter, i.e. corresponding to a 1 ms duration. The shown envelope of the echo signal is obtained by exploiting the Hilbert transform of the impulse response. Figure 3: (a) Sketch and (b) picture of the (60 mm thick carbon fiber-reinforced polymers (CFRP)) sample under test, indicating the side drilled holes located at different depths from the surface of inspection (all the dimensions are in mm).