High-temperature-resistant epoxy composites play a crucial role in enhancing the operational reliability and service life of devices such as DC bushings, which is of great significance for the long-term stable operation of ultra-high voltage and flexible power transmission and distribution systems. In this study, the epoxy composite was prepared, and long-term thermal aging tests were conducted at 250 °C and 270 °C. The changes in physical properties, electrical characteristics, and bending strength of epoxy composite were systematically investigated, and the thermal aging mechanism of these materials was elucidated. The experimental results revealed that with the progression of thermal aging, the epoxy composites exhibited volume shrinkage due to the breaking of chemical bonds. After 10 thermal aging cycles at 270 °C, the mass loss rate of the epoxy composite reached 20.52%. At 250 °C, the breakdown strength decreased by 9.9% compared to the unaged state. After aging at 250 °C and 270 °C, the volume resistivity decreased by a maximum of 53.75% and 76.94%, respectively, while the dielectric constant decreased by a maximum of 50.34% and 41.94%, respectively. After 10 aging cycles at 250 °C and 270 °C, the bending strength of the cured epoxy composite decreased by 39.79% and 53.91%, respectively. These findings provide valuable insights into the thermal aging characteristics of epoxy composites used in DC bushings and other electrical devices, offering a scientific basis for material selection and reliability assessment in high-voltage insulation applications.

Epoxy resins serve as critical materials for encapsulating high-end electrical equipment and advanced electronic devices, yet their applications are constrained by the intrinsic thermal resistance-toughness trade-off. This study proposes a molecular design strategy employing binary imide curing agents with tunable rigid-flexible architectures to overcome the incompatibility between thermal stability and toughening in epoxy composites. Systematic modulation of rigid polycyclic dimensions and flexible chain lengths achieves the design of 25 novel imide curing agents, enabling concurrent enhancement and controllable regulation of thermal resistance and toughness. Molecular dynamics simulations reveal that expanding dianhydride polycyclic dimensions reduces hydrogen bond-dominated non-bonding interactions, decreasing cohesive energy density by 18% while increasing free volume fraction by 27%, thereby facilitating epoxy toughening. Concurrently, long-chain structures from diamine induce molecular entanglement, where localized stresses at entanglement nodes dissipate through chain slippage and disentanglement mechanisms. This energy dissipation paradigm preserves 94% of the tensile modulus in modified epoxy composites. Such molecular-level coordination establishes an wide range operational temperature window of 199 K, coupled with tunable toughness (K/G) ranging from 17.17 to 27.83. This work provides new molecular design strategy for developing epoxy formulations with superior thermal resistance and toughening, and establishes an effective design framework for imide-modified epoxy materials.

Porcelain insulators are critical to ultra-high-voltage transmission systems, yet the mesoscopic failure mechanisms of internal defects under electro-thermal-mechanical coupling remain unclear. Here, a phase-field cohesive model is developed to investigate the coupled damage behavior induced by bubbles, iron inclusions, and pre-existing cracks. The thermal effect of partial discharge is represented by equivalent partial discharge (PD) temperature. Results indicate that partial-discharge heating significantly redistributes the crack tip stress field, altering the propagation mode. Under bubble-surface heating, the crack path shifts from near-vertical growth under pure mechanical loading to interfacial propagation along iron inclusions. Thermal stress reduces the maximum load-bearing capacity by up to 66.7%, with 300 °C identified as a critical temperature for damage-mechanism transition. Larger iron inclusions and longer initial cracks further degrade load-bearing capacity and accelerate fracture. This work provides new insight into the coupled damage evolution in defective porcelain insulators, offering a theoretical basis for degradation assessment and life prediction of UHV equipment.

During the manufacturing of gas-insulated switchgear (GIS), basin insulators are susceptible to void defects that may escape conventional non-destructive testing. These defects can deteriorate mechanical integrity and promote local electric-field distortion during long-term operation. In this paper, an electro-thermo-mechanical coupled finite-element model incorporating the fracture phase-field method is developed to investigate the failure behavior of GIS basin insulators containing void defects. In the model, the cumulative thermal effect associated with repeated partial discharge activity is represented by an equivalent localized thermal load at the void wall, enabling a parametric evaluation of the influence of local heating on crack initiation and propagation. The results show that void defects significantly intensify local stress concentration. With increasing defect size, the maximum first principal stress increases, whereas the critical failure load decreases continuously. The localized thermal load reconstructs the internal thermal stress field and induces a distinct spatial shift in the crack-initiation site. Simplified three-point bending tests on epoxy specimens with artificial void-like defects further support the predicted defect-size effect and crack-initiation shift. Moreover, a comprehensive parametric sensitivity analysis reveals that crack propagation exhibits pronounced heterogeneity governed jointly by defect size, temperature-rise intensity, and spatial location, identifying critical thresholds that trigger failure-mode transitions. The proposed model and findings provide theoretical support for defect assessment, structural optimization, and reliability improvement of GIS basin insulators.