Analysis of Decay-like Fracture Failure in Core Rods of On-Site Composite Interphase Spacers of 500 kV Overhead Power Transmission Lines

Composite interphase spacers are essential components in ultra-high-voltage (UHV) transmission lines to suppress conductor galloping. This study investigates the first reported case of a core-rod fracture in a 500 kV composite spacer and elucidates its degradation mechanism through multi-scale characterization, electrical testing combined and electric field and mechanical simulation. Macroscopic inspection and industrial computed tomography (CT) show that degradation initiated at the unsheltered high-voltage sheath–core interface and propagated axially, accompanied by continuous interfacial cracks and void networks whose volume ratio gradually decreased along the spacer. Material characterizations indicate moisture-driven glass-fiber hydrolysis, epoxy oxidation, and progressive interfacial debonding. Leakage current test further indicates humidity-sensitive conductive paths in the degraded region, confirming the presence of moisture-activated interfacial channels. Electric-field simulations under two shed configurations demonstrated that local field intensification was concentrated within 20–30 cm of the HV terminal, where the sheath and core surface fields increased by approximately 9.3% and 5.5%. Mechanical modeling demonstrates a pronounced bending-induced stress concentration at the same end region. The combined effects of moisture ingress, electrical stress, mechanical loading, and chemical degradation lead to the decay-like fracture. Improving sheath hydrophobicity, enhancing interfacial bonding, and optimizing end-fitting geometry are recommended to mitigate such failures and ensure the long-term reliability of UHV composite interphase spacers.