An exploration of physical ageing in electrically-stressed polymeric insulating films using the quartz crystal shear wave sensor technique

The mechanical stress induced by a strong electrical field could be a significant contributing factor to the electrical ageing and breakdown of insulating polymeric materials such as polyethylene. An extensive literature exists describing the nature of the failure of polyethylene under mechanical stress. The initial failure is considered to be the generation of sub-microscopic voids, which coalesce to form a crack, and is accompanied by chain scission and electromagnetic radiation. These features bear remarkable resemblance to those, such as trees, appearing in electrical breakdown. If electrically induced mechanical stress is indeed a significant factor in electrical breakdown, then it might be expected that marked changes in the viscoelastic parameters of the polymer, indicative of morphological change, might accompany electrical stressing. Moreover such changes might appear at stresses well below those for breakdown. To test this there are a number of ways in which conventional mechanical testing at low frequency might be adapted to allow simultaneous electrical stressing. We have adopted an unconventional approach, employing a quartz crystal shear wave sensor to determine the shear wave moduli of polyethylene under electrical stress. The technique has been refined. We are now able to demonstrate that the changes in the mechanical viscoelastic moduli of polyethylene in response to electrical stress are quite complex and polarity dependent.