A mechanical origin for electrical ageing and breakdown in polymeric insulation

The failure of polyolifinic solids, such as polyethylene and polypropylene, by fracture under mechanical stress has been studied extensively. The initial stages of such failure are considered to involve the scission of the main polymeric bonds, the generation of free radicals which induce bond-breaking chain reactions and the consequent growth of a population of sub-microscopic voids which ultimately coalesce into a propagating crack. Accompanying these processes is so-called fracto-emission, the generation of charged particles and electromagnetic radiation by fracture. All these features bear such remarkable resemblance to those occurring in the early stages of electrical breakdown of these same polymers that it becomes imperative to consider whether electrical breakdown has a mechanical origin. It has been suggested already that an electrical field in a dielectric will set up a mechanical stress which can become significant when the field approaches breakdown values (>10/sup 8/ V/m). The presence of this mechanical stress, which must be taken into account when considering the internal equilibrium of a dielectric, is not intuitively obvious but a number of results support its existence. In this paper, we outline the main features of the theoretical model for this stress and then describe new experiments which strikingly demonstrate its presence in polyethylene and polypropylene films under electrical stress. The implications of this demonstration for electrical ageing and breakdown are clear.