*2.2. Ultrasonic Test Setup*

To find out the optimal material with appropriate mechanical properties and dimensions, four different metallic rods (namely steel rods with a diameter of 3.17 mm and 6.35 mm, and aluminum rods with a diameter of 3.17 mm and 6.35 mm) were embedded into the samples during the tests (see Figure 1a). The length of each rod was 304.8 mm, and the embedded length was 152.4 mm. A Plexiglas plate with four holes was used to keep the rods in place and to reduce the moisture evaporation as well. Low cost small piezo discs (SMD07T02S412, manufactured by STEINER & MARTINS, Inc., Miami, FL. USA) (see Figure 1a,c) bonded on top of the rods were used to excite the rods in the axial direction and to receive the reflected signals. In order to get the signals from the four rods alternately, an Agilent multiplexer 34970A with the plug-in 34903A card (Agilent, Santa Clara, CA, USA) (see Figure 1b) was used to connect these four piezo discs to a pulser-receiver (Olympus 5077PR, Tokyo, Japan) and a digitizer (NI-PXI 5133, Austin, TX, USA). The 34903A card is simply a set of 20 independent single-pole, double-throw reed relays, and each of the 20 relays could be independently controlled. In this study, only four relays were used and closed alternately to get the signals from the four rods. In order to reduce the noise ratio, a total of 200 signals obtained from each rod were averaged separately and then saved each time.

Meanwhile, shear wave testing was carried out using two shear wave transducers (Olympus V151, Olympus, Tokyo, Japan) in the same batch of samples independently. The setup for shear wave testing was the same as the one used in the author's previous work [5], as shown in Figure 1d, the mold was formed by two plastic plates on the sides and a rubber container in between. A soft foam layer was installed between the internal container and external plastic plate, aiming to reduce the direct transmitted ultrasonic waves. Two shear wave transducers were put on each side of the mold. In this setup, the sample thickness was about 27 mm, and the actual thicknesses will be tested after shear wave testing. During the shear wave testing, one transducer served as an actuator, and the other was used as a receiver. Both transducers were connected to an Olympus 5077PR pulser-receiver (Olympus, Tokyo, Japan), with a gain of 40 dB. A 200 V square wave pulse sent from the pulser-receiver was used to drive the actuating transducer. The receiving signals were then sampled at a 10 MHz sampling rate by a NI-USB 5133 digitizer (National Instruments, Austin, TX, USA), which was also used to transfer digitized signals to the connected computer. Furthermore, 200 signals were first obtained and then averaged, as used in the guided wave testing setup, to reduce the noise ratio. Both shear wave velocity tests and guided wave tests were carried out with a 5 min interval until the final setting of the samples.

**Figure 1.** Ultrasonic Test setups: (**a**) guided wave testing; (**b**) 34903A switching card; (**c**) 7 mm diameter piezo disc and (**d**) shear wave measurement.
