## 1 Introduction
The longitudinal wave, combined longitudinal and transverse wave, and transverse wave methods have been investigated for measuring the internal stress in materials, such as bolts and concrete [21; 22]. The longitudinal wave method and the combined longitudinal and transverse wave method work effectively in the condition that the stress direction is along, with wave propagation due to the high sensitivity of the wave velocity to the stress in this case [2; 23]. The combined longitudinal and transverse wave method can eliminate the influence of thickness and deformation on test results, but the required probe must be specifically manufactured [22]. As for the transverse wave method, some researchers have investigated the acoustoelastic effect of transverse waves and measured the internal stress in wood, patch-welded disks, concrete prisms, and so on. [PERSON] found that the transverse wave velocity linearly changed with stress in mild steel using the transverse waves with polarization parallel or vertical to stress and thus verified the acoustoelastic effect. Then, they measured the residual stress in a patch-welded plate and found that the results obtained using the ultrasonic stress method agreed well with the conventional destructive method [21]. [PERSON] applied the transverse-wave electromagnetic acoustic transducer to measure the axial stress in bolts. They applied the pulse-echo method, in which two reflection echoes from the bottom are selected to determine the propagation time in bolts. The accuracy in this research is high (approximately 5% at 5 tons) [24]. [PERSON] verified the acoustoelastic effect in railway wheel by using the transverse waves with polarization parallel or normal to stress, based on the pulsed-echo method. Then, they obtained the corresponding acoustoelastic coefficients and measured the biaxial residual stress in a railway wheel [25]. Currently, most ultrasonic stress measurements are focused on metals, like bolts [20], patch-welded plates [21], and non-metallic materials, like concrete [22] and wood [26]. Research on stress measurements in epoxy composite materials is still needed. [PERSON] et al. investigated the acoustoelastic effect of carbon fiber composites with an epoxy matrix using the LCR wave. They found that the stress varies linearly with the ultrasonic transmitting time and calculated the acoustoelastic coefficients [16]. Based on the acoustoelastic effect, Tianjin University applied the longitudinal wave pulse-echo method to measure the residual stress, induced in the curing process, in aramid reinforced epoxy composites of GIS pull rods [27]. So far to date, the ultrasonic testing method had been applied to investigate the density uniformity or stress condition of a GIS basin insulator. The density uniformity was detected using the longitudinal wave pulse-echo method [11]. The internal stress, caused by a temperature gradient in the working conditions, was calculated based on the relationship between the transmitting time and temperature. The maximum relative error was 28.7%, compared with simulation results [11]. In the previous work, 0-50 MPa subsurface stress of GIS composite was calculated based on the acoustoelastic effect by applying the LCR waves. However, the LCR waves are applied to measure the sub-surface stress, not the internal stress [28]. The 0-70 MPa compressive internal stress in GIS composites was measured with the longitudinal waves, based on the acoustoelastic effect. In this research, due to the applied through-transmission method, the effect of coupling agent thickness had to be eliminated. The relative errors are relatively high in a low-stress state, reaching 55-85% below 25 MPa [2]. The relative high error was due to the fact that the ultrasonic longitudinal wave is insensitive to the stress, if the longitudinal waves propagate normally to the stress, i.e., the tangential stress. Thus, it is necessary to seek a more accurate method to measure the tangential internal stress inside a GIS composite.