Deterioration Law of Mechanical Properties of Insulation Pressboard Under Multifield Coupled Ageing

Converter transformers can experience progressive deterioration of their oil–pressboard insulation under elevated temperature, combined AC–DC electrical stress and persistent mechanical vibration. This study investigates how mechanical vibration affects the mechanical properties of insulation pressboard and clarifies the associated microscopic degradation mechanisms under thermal–electrical–mechanical (TEM) multifield coupled ageing. A thermal–electrical–mechanical multifield coupled ageing platform was used to conduct accelerated ageing tests on pressboard specimens, and complementary molecular dynamics simulations were performed to model material responses under oil–pressboard interfacial conditions. Tensile stress–strain curves of aged pressboard were measured at a constant strain rate to quantify the variation in mechanical degradation. In parallel, the elastic deformation and fracture behaviours under quasistatic tension were analysed through combined molecular dynamics simulations and experiments. The results show that mechanical stress reduces yield stress by enhancing cellulose‐chain mobility and promoting the coarsening of cellulose crystallites. The elastic modulus first decreases due to enhanced segmental mobility and chain flexibility and then partially recovers at the late stage, likely owing to fatigue‐induced structural rearrangement and preferential chain alignment under sustained vibration. Tensile strength decreases due to weakened intermolecular interactions and an intensified stress concentration effect.