Dependence of electrical tree inception and growth on mechanical properties

The mechanical properties of an unsaturated polyester resin have been varied by the addition of a plasticising polyester resin. Specifically, the tensile strength, modulus of elasticity and fracture toughness have been determined over a range of plasticisation. The initiation and growth of electrical trees in a needle-plane geometry in this range of materials have been investigated. Two conflicting criteria determine the initiation and growth: the presence of internal mechanical strain, and the intrinsic mechanical properties of the material. The initial internal mechanical strain, resulting from the manufacturing process, greatly enhances tree inception and propagation. This effect is dominant and masks the intrinsic dependence of tree initiation and growth on mechanical properties. In strain-free samples, however, the reduction in inception time and the increase in growth rate resulting from the plasticisation of the material, and the ensuing changes in mechanical properties, are clearly shown.