## 1 Introduction  
A gas-insulated metal-enclosed switchgear (GIS) fully or partially uses insulating gas instead of atmospheric air as the insulation medium in an electrical substation. With the increasing power demand and the elevation of voltage levels, the GIS is increasingly used in power grids. The basin insulators are significant components in the GIS, and they consist of epoxy insulation, metal inserts, and flange rings. They function as supporting conductors, electrical insulation, and isolation of gas chambers. Thus, their performance is crucial to the safe operation of GIS [1]. The stresses inside basin insulators include residual stress, generated during the manufacturing process, as well as the stress caused by the external loads during installation and operation, as well as during transportation [2]. The GIS basin-type insulators are made through high-temperature curing of integrally poured mixtures of liquid epoxy resin, curing agent, and inorganic powder filler with an aluminum central conductor insert [1]. The origins of residual stress in the pouring and curing process include (1) the curing shrinkage of epoxy resin [3]; (2) different thermal expansion and contraction values between the epoxy composite and metal inters in the case of temperature changing in the curing process [4]; (3) uneven pouring and curing, such as non-uniform mixing of the epoxy resin, inorganic filler particles, and curing agent, or non-uniform sedimentation distribution of the filler material [5]. External loads include jostling and vibration during transportation, mechanical friction, tilted installation of conductor, and an uneven bolt fastening force that can produce stretch or compression in GIS basin-type insulators. In addition, the insulators are also subjected to forces caused by their weight and the weight of conductors during their operation, the pressure difference of SF\({}_{6}\) on two sides [6], mechanical vibration from the switch action [7], and the electrodynamic forces exerted on the conductor by the electromagnetic field of the short circuit alternating current [8], and so on. The stress gradually accumulates under the action of external loads during the operation of the basin insulators. When the stress exceeds the mechanical strength, cracks will easily form and lead to air leakage, partial discharge, insulation ablation, and flashover [9; 10]. The failure rate of GIS due to the insulation problem reached 69%, in a certain region in China from 2010 to 2018 [11]. The internal stress will degrade the insulation mechanical properties. Thus, its measurement is critical for ensuring the mechanical reliability of the insulator [12]. Currently, researchers have used stress-strain sensors such as strain gauges [13] and fiber sensors [14] on insulators to measure the surface stress in hydraulic testing. The Xi'an Jiaotong University group analyzed the failure mode of the basin insulator after the hydraulic testing and simulated the stress field using the finite element method. They found that the tangential stress was the main cause for peeling at the interface [15]. However, there are a limited number of reports on the internal stress measurement in the GIS insulation. Typically, there are two ways of measuring internal stress: destructive testing methods and non-destructive testing methods [16]. Magnetic tests, X-ray imaging, photoelastic methods, and ultrasonic methods are non-destructive testing methods. Only the ultrasonic method, based on the acoustoelastic effect (correlation between wave velocity and stress), is applicable to composite materials [16; 17; 18]. The ultrasonic stress measurement methods, according to the types of ultrasonic waves applied, can be divided into Rayleigh waves, LCR (longitudinal critically refracted) waves, longitudinal waves, combined longitudinal and transverse waves, and transverse waves methods. The Rayleigh waves and LCR waves are typically for subsurface stress testing (within a limited depth of area beneath the surface) [19; 20].