1. State Key Laboratory for Mechanical Behavior of Materials Xi'an Jiaotong University Xi'an 710049 China
2. High Voltage Switchgear Insulation Materials Laboratory of State Grid Pinggao Group Co., Ltd Pingdingshan 467001 China
3. State Key Laboratory of Electrical Insulation and Power Equipment Xi'an Jiaotong University Xi'an 710049 China
4. Department of Mechanical Engineering Nagaoka University of Technology Nagaoka 940−2188 Japan