Nondestructive Measurement of Residual Stress on Epoxy Insulators Using Thermoelastic Method

Abstract

Residual stress is inevitable in epoxy insulators, which easily leads to small cracks and even insulation breakdown during the operation of gas-insulated transmission line (GIL)/gas-insulated switchgear (GIS). This study proposes a nondestructive method to measure the residual stress on epoxy insulators using thermoelastic effects. First, the laser-induced temperature rise of the epoxy/Al₂O₃ composite was measured under different mechanical stresses to establish a relationship between the relative temperature rise and mechanical stress. Then, the residual stress distributions on full-sized insulators were reconstructed based on the stress–temperature relationship by scanning and measuring the relative temperature rise values at distributed points. The results show that the temperature rise of the epoxy/Al₂O₃ composite is promoted by tensile stress but inhibited by compressive stress, reflecting the impacts of mechanical stress on the thermal properties of epoxy insulators. Compressive stress is present on the outer side, whereas tensile stress is concentrated on the inner side of both basin-type and tri-post insulators, with maximum values around 30 MPa. During curing, a higher temperature on the outer side of the mould leads to a faster curing rate of the insulator than on the inner side. This, combined with the mismatch of thermal expansion coefficients between epoxy and aluminium, contributes to the generation of residual stress. The measurement results of residual stress are consistent with the theoretical analysis results, verifying the effectiveness of the proposed method and offering new insights into the measurement of residual stress.