Micro air gaps or micro cracks may be induced by the stress concentration of GIL three-post insulators

which will reduce the mechanical and insulation properties. In this paper

mechanical characteristics of three-post insulators in a 550 kV GIL under radial and axial loads are simulated and tested. New measurement methods of acoustic elastic coefficient under the parallel stress in standard specimens and ultrasonic longitudinal wave detecting mechanical stress in GIL three-post insulators are proposed

respectively. The acoustic elastic coefficients of parallel stress and vertical stress are measured with standard specimens. Based on the coefficients and the equation of acoustic elasticity

the stress of three post insulators is obtained by measuring the ultrasonic velocity. The results show that the acoustic elastic coefficients of epoxy composite material under the parallel stress and vertical stress are 4.477×10-4 MPa-1 and 8.908×10-5 MPa-1

respectively. The stress of stressed post in the insulator can be detected by an ultrasonic penetration method under radial loads. The stress of epoxy outside the insert can be detected by an ultrasonic reflection method under axial loads. The testing values are consistent with the simulation values

and the errors are analyzed. The methods can be taken as a non-destructive detection technology for mechanical stress of three-post insulators in GIL. Meanwhile

it is also found that the insulator always fractures at the epoxy-insert interface

which belongs to the brittle fracture. The failure of the insulator under radial loads depends on the tensile strength of the epoxy at the maximum curvature of the epoxy-insert interface

and the failure under axial loads depends on the shear strength of the interface.