Mechanical Fatigue as a Mechanism of Water Tree Propagation in TR-XLPE

The paper reports on an investigation of the fatigue failure of tree retardant cross-linked polyethylene (TR-XLPE) that is relevant to water tree development in underground cable insulation. Finite element calculations were used to estimate the stresses developed in cable insulation by di-electrophoretic forces; these stresses are in the low megaPascal range around inclusions (or water tree branches) that are long and thin. They are insufficient to bring about instantaneous failure of the insulation. However, these stresses might be sufficient to cause cyclic fatigue failure of the insulation, and, accordingly, fatigue measurements were carried out on samples from a commercial cable. The resulting fatigue failures, that occurred at cycle numbers achievable in practical work, suggest that fatigue might be an explanation for slow development of water trees over years of service in the ground. Cycle numbers at failure were found to be lower at higher mechanical stresses, at higher temperatures and in the presence of humic acid or ferric ions; however, the number of cycles to failure was larger in the presence of water.