81 / 2018-08-24 01:03:04

Ultrasonic Velocity Measurement of Epoxy Composite for GIS Based on Through-transmission Method

Acoustoelastic principle,Ultrasonic longitudinal wave,Epoxy composite,GIS,Velocity measurement

Epoxy resin is largely employed as a structural material in the insulator of Gas-Insulation Substation (GIS) and Gas-Insulated Transmission Line (GIL). Residual stress in epoxy resin insulator can lead to cracks, and even cause more serious problems, such as partial discharge, flashover, inner breakdown and explosion. Therefore, residual stress measurement in epoxy resin is highly important for the stable operation of GIS and GIL. The ultrasonic method, as a nondestructive testing method, can be applied in measuring residual stress. The residual stress detection technology by using ultrasonic method is based on the acoustoelastic principle, and the principle is that the velocity of acoustic wave has linear relationship with variation in stress in the material. By far, the ultrasonic method is largely applied in metal, such as alloy and stainless steel, only a few on composite material, such as rubber, ceramics and epoxy resin. The residual stress detection technology using ultrasonic method include ultrasonic longitudinal method, ultrasonic transverse method, lamb wave method, and etc. The ultrasonic longitudinal wave, parallel to the stress or strain, is most sensitive to the stress or strain. Therefore, this kind of ultrasonic wave, such as ultrasonic critical refracted longitudinal wave, is widely applied in residual stress detection among existing ultrasonic methods. However, little research has been made on the ultrasonic method with longitudinal wave perpendicular to stress or strain direction. This article mainly introduces the residual stress detection technology in residual stress by applying the ultrasonic longitudinal wave, which is perpendicular to the stress direction. Firstly, based on theory of acoustic elasticity, the formula between the change of residual stress and the change of longitudinal wave with direction perpendicular to stress was derived. Then, the ultrasonic reflection method and the ultrasonic penetration method was briefly explained, from which the ultrasonic velocity could be calculated from the transmitting time and distance Next, the composition of residual stress detection system for GIS epoxy resin was introduced in detail, including ultrasonic defect detector, ultrasonic probe, oscilloscope, compression testing machine and computer. Moreover, the procedure of residual stress detection experiment was clearly demonstrated: we used compression testing machine to put stress on epoxy resin specimen to create residual stress by setting equal stress increment, and then we performed test on epoxy resin specimen with employing the longitudinal wave, perpendicular to the stress direction. The longitudinal wave speed was acquired from accurately measuring ultrasonic wave transmitting time and distance. Based on the experimental results, we explored the relation between wave speed and applied stress, otherwise known as the acoustoelastic coefficient. The results showed that the velocity of ultrasonic longitudinal wave is linearly related to residual stress, by using ultrasonic reflection method or ultrasonic penetration method. From comparing these two methods, it had been found that the ultrasonic penetration method was more accurate than the ultrasonic reflection method, due to the surface reflection wave involved in the ultrasonic reflection method. The experimental results provided evidence that acoustoelastic principle can also be applied in epoxy resin of GIS, and the longitudinal wave speed was sensitive to the perpendicular stress. These findings support the notion that the method holds promise as a method to measure stress in composite materials, and acoustic elasticity effect should not be ignored in the case of longitudinal wave perpendicular to stress direction.