Life of polythene at very high stresses

Polythene samples, 0.09¿0.18 cm thick, with artificial voids have been subjected to pulses lasting for 10¿6¿10¿2s with a wide range of waveforms, at electric stresses of 0.2¿0.9MV/cm. It was found that, for a given peak stress, the life obtained with unidirectional pulses was independent of waveshape; it was independent of the pulse-repetition frequency from 1 to 4 pulses/s, but longer lives were obtained at 50 pulses/s. Life was shorter on oscillatory than on unidirectional pulses of the same peak stress and was unaffected by sample thickness. The results are consistent with life being controlled by the intrinsic electric strength of the polythene, and indicate that a space charge may be set up and decay with a time constant of the order of 10¿3s.