## 2 Materials and Methods
### Measurement Method  
Before the ultrasonic test using the noncontact ultrasonic technique, the boards were visually inspected for possible irregularities that may occur in them. Then, the velocities of longitudinal and transverse waves in the boards were measured using the classical ultrasonic method (the probes are in contact with the material).In this study, we used a pair of shear transducers (V151, PANAMETRICS(r), GE Measurement & Control, Billerica, MA, USA) and longitudinal transducers (V318-SU, PANAMETRICS(r)), excited by a pulse signal generator (5058 PR, PANAMETRICS(r)).The signals were recorded using a LeCroy Wavejet 324A (Teledyne LeCroy, Chestnut Ridge, NY, USA) oscilloscope. A gel designed for ultrasonic testing was used as a coupling material between the transducers for longitudinal waves while honey was used for acoustic coupling to generate and record transverse waves. Honey is characterized by high viscosity, allowing transverse vibrations to transfer from the transducer to the plate. The values of these velocities are required to estimate the angle of incidence (critical angle) to generate Lamb waves in boards using the noncontact ultrasonic method. The pilot noncontact ultrasonic test was performed for both materials to ensure the boards were free from internal defects. When artificial defects were introduced, care was taken to maintain a constant temperature in the boards by cooling the area close to the generated defect with compressed air and controlling the temperature with the thermal imaging camera (FIR-Flir E5, Teledyne Technologies, Wilsonville, OR, USA). The pilot studies have shown the temperature change at the point of introducing defects in polymeric materials affects the measured amplitude of the Lamb wave. Following the preliminary test, the noncontact ultrasonic studies of the boards without defects (reference data) were performed first. Then, three kinds of artificial defects were applied and the ultrasonic tests were repeated for such modified materials. Figure 2 shows the photographs and placement of these three kinds of defects: (i) delamination at the board edge (see Figure 2a), (ii) cracking at the board edge (see Figure 2b), and (iii) cracking (discontinuity) in the middle part of the board (see Figure 2c). Figure 3 shows a photograph of the noncontact ultrasonic system used in the measurements. During the experiments, the board was mounted on a movable table (2). The linear motion range was approximately 1500 mm (scanning step 3 mm) perpendicular to the axis of the emitter (8) and receiver (9). The stand (1) provided adequate stiffness to mount the transducers and motion system (2). The experimental setup is also equipped with elements that isolate the waves propagating in air directly from the transmitter to the receiver acoustically (10). During the measurements, damping barriers (polyurethane foams) were used. These barriers are characterized by strong absorption of acoustic and ultrasonic waves in the frequency range from 2 kHz to 40 kHz. The barrier thickness was about 20 cm and was chosen experimentally based on the measured signals of the Lamb wave. Their presence is essential from the point of view of effective attenuation of unwanted acoustic waves reflected from the board (or the other components of the experimental setup). The test stand is positioned using a three-phase hybrid Easy Servo Motor type ES-M32320 (Leadshine Technology Co. Ltd., Shenzen, China) (3) with a holding torque of 2 Nm, a speed range of up to 3000 rpm with an integrated incremental encoder of 1000 pulses/rev in the NEMA23 (Leadshine Technology Co. Ltd., Shenzen, China) standard. The motor works with a dedicated Easy Servo ES-508 controller (Leadshine Technology Co. Ltd., Shenzen, China) (7) with a power supply ranging from 20 V to 45 V and a maximum current per phase from 0.5 A to 8 A. The engine and the controller ensure the precise operation of the transport system. Figure 4 presents a diagram of the measuring system.