Mechanical aspects of water treeing

Water treeing is a long-term degradation phenomenon in polymeric materials which lowers the breakdown strength of insulation, for example in cables, leading to failure. The water-filled tree-like microstructures which characterize this phenomenon originate at microvoids and impurities in the bulk of the materials or at defects and protrusions at the insulation surface. Water tree initiation and growth require the presence of moisture at a point of electrical stress concentration although the fields required for water trees to grow are much lower than for electrical trees and there appears to be an absence of discharge activity. The authors have attempted to establish experimentally a correlation between the initiation and growth of water trees in a material and its mechanical properties. The material chosen for investigation was a clear casting polyester resin (Resin C) whose mechanical properties could be systematically modified by the progressive plasticisation of the material through the addition of controlled amounts of a plasticising polyester resin