Dependence of electrical tree inception and growth on mechanical properties

In order to study systematically the relationship between mechanical properties and tree initiation and growth, a range of mechanical properties was obtained by the plasticization of the original polyester resin by the addition of controlled amounts of a plasticizing polyester resin (Crystic 586). It was found that an initial mechanical stress is necessary for the relatively easy initiation and growth of trees, casting light on the problem reported in earlier work of explaining the development of permanent mechanical stress in polyester specimens under the application of the electrostatic cyclical forces produced by an alternative electric field. Progressive plasticization of the polyester resin reduces the initial stress produced in the specimens during manufacture, and the initiation time is progressively increased as a consequence. Two conflicting mechanisms act in determining the dependence of tree growth on degree of plasticization. On one hand, increased plasticization reduces the internal stress and will reduce tree growth. On the other, plasticization decreases the fracture toughness, and propagation of trees by mechanical fracture will be enhanced