To investigate the damage characteristics of XLPE materials in water tree propagation

we investigated the characteristics of the microstructural changes of water trees in different aging periods. Combined with the mechanical fatigue damage characteristics of the materials

the causes for the non-linear increase of the fatigue damage of the material in water tree propagation were revealed. Four groups of XLPE sheet samples A

B

C and D were produced and the samples were subjected to an accelerated water tree aging experiment under 7.5 kV and 400 Hz for 240

480

720 and 1440 hours

respectively. After the water tree aging

the water tree shapes and sizes in the four groups of samples were measured by a microscope

and the microstructural morphologies of the water tree regions were observed by SEM（Scanning electron microscopy）. In addition

one group of dumbbell-shaped XLPE samples（group E） were produced

and the samples were subjected to a mechanical fatigue experiment under an stress ratio of 0.1

stress magnitude of 0.45 MPa and stress frequency of 1 Hz for 4 h. The stress-strain curves of the samples were recorded during the experiments. The water tree aging experiment shows that

with the increase in aging time

the water tree propagation rate first increases and then decreases

while the water tree void density increases non-linearly. Before 480 hours of aging

the water tree void density is low

whereas

after 720 hours of aging

the void density increases significantly. The fatigue experiment shows that

with the increase in the fatigue time

the energy consumption in a fatigue cycle increases. It is analyzed that the non-linear fatigue damage in water tree propagation is mainly caused by the changes of the viscoelastic properties of the material. In the water tree propagation

the material is subjected to an alternating eletro-mechanical fatigue stress and is thus damaged.With the increase in aging time

both the elastic modulus and the viscosity of the material decrease

while the decrement speed of the elastic modulus is much higher than that of the viscosity. As a result

the viscous energy（namely the dissipation energy） of the material in a fatigue cycle increases and the elastic energy decreases

leading to the non-linear increase of the material damage.