## 3 Experiment
### Acoustoelastic Coefficient Measurement System  
The acoustoelastic coefficient of the epoxy composite is measured using the compressive stress acoustoelastic effect test system and the tensile stress acoustoelastic effect test system, as shown in Figure 3. The compressive stress acoustoelastic effect test system consists of a universal testing machine, an ultrasonic pulser/receiver, an ultrasonic transverse wave probe, a strain instrument, strain gages, a computer, an oscilloscope, and a cuboid sample [2], as shown in Figure 3a. The tensile stress acoustoelastic effect test system is similar to the compressive stress system, but the difference is that the sample is a dumbbell-type tensile sample, and a tensile fixture in a universal testing machine is used, as shown in Figure 3b. The experimental set-up is shown in Figure 4.  
During the test, a CTS-23 ultrasonic pulser/receiver is used to emit ultrasonic waves. The main properties of the CTS-23 ultrasonic pulser/receiver include 0.5-3 MHz bandwidth, A-scan mode, about 30 ns transmitted pulse rising time, and the reflective mode selected to connect one probe. The original unprocessed ultrasonic signal that contains the largest amount of information can be acquired by connecting the oscilloscope to the point in front of the rectification mode in the internal circuit of the ultrasonic pulser/receiver [2]. A transverse wave probe (model 0.5C14S) was used in the experiment, with 0.5 MHz center frequency and 14 mm diameter. Considering the high attenuation of ultrasonic waves in the epoxy composite, composite probes with less clutter and better waveform were used. The piezoelectric wafer of the probe is made of lead zirconate titanate (PZT) and epoxy resin composite material. The probe is connected to the ultrasonic pulser/receiver system with Q9Q6 wires, where the Q9 connector is attached to the ultrasonic pulser/receiver system  
Figure 2: The diagram of specimen A and B: (**a**) Front view and side view of specimen B. The thickness \(a=10\) mm, the width at the middle \(b=25\) mm, the width on both ends \(B=35\) mm, and the clamped portion \(h=50\) mm. The cross-section area in the middle \(S_{0}=2.5\times 10-4\) m\({}^{2}\), \(L_{0}=89.33\) mm, \(L_{C}=139.33\) mm, and \(h_{1}=26.9\) mm. The total length \(L_{t}=293.13\) mm. (**b**) Fabricated specimen A and B.  
and the Q6 connector is attached to the probe. The coupling agent is applied between the probe and the sample [2]. A digital oscilloscope (Tektronix, model DPO3102) is used, with 2.5 GS/s sampling rate (0.4 ns sampling period) and 20 MHz bandwidth. For compressive stress test, the compressing device is a computer-controlled universal testing machine with 100 kN capacity and level 1 accuracy. A level 1 accuracy means that the maximum absolute error is 1 kN within the 100 kN range [30]. Since the epoxy sample would crack under 130 MPa compressive loading, the compressive stress test was carried to 100 MPa. It corresponds to an area of 5 \(\times\) 10\({}^{-4}\) m\({}^{2}\) (a cross-section of 20 mm \(\times\) 25 mm) at a load of 50 kN. For tensile stress tests, the tensile strength of the GIS composite is 70 MPa. The maximum tensile stress load applied is 50 MPa [2]. This corresponds to a load of 12.5 kN on a cross-sectional area of 2.5 \(\times\) 10\({}^{-4}\) m\({}^{2}\) (10 mm \(\times\) 25 mm). For both the compressive stress test and the tensile stress test, the stress load is applied to the samples at an increment speed of 0.5 mm/min and a constant stress gradient of \(\Delta\sigma\) = 5 MPa (stress is the ratio of pressure to cross-sectional area).