1. Tribology Research Institute; Southwest Jiaotong University; Chengdu 610031 China
2. Biomechanics Section; TUHH Hamburg University of Technology; Hamburg 21073 Germany
3. Institute of Mechanics and Ocean Engineering; TUHH Hamburg University of Technology; 21073 Hamburg Germany
4. Politecnico di Bari. V.le Gentile 182; 70125 Bari Italy