The authors present results of experiments in which LDPE (low-density polyethylene) and/or XLPE (cross-linked polyethylene) specimens were subjected to mechanical vibrations in water or to mechanical vibrations and electric stress combined. Water tree growth experiments in stretched films of XLPE cable insulation are also described. Attempts to grow water trees by application of vibrational mechanical stresses to polyethylene immersed in water or containing cavities filled with water were unsuccessful. Vibrational mechanical stresses of moderate amplitude applied to the insulation together with electric stress do not change the water tree growth rates as compared with the insulation subjected to electric stress only. Static tensile stresses enhance the growth of vented water trees but have no effect on bow-tie trees. However, the effect is significant up to 30% strain only.
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