1. 1Presenter and corresponding author. Institute of Dynamics and Vibration Research, Leibniz University of Hannover, Appelstr. 11, D-30167 Hannover, Germany. Telephone: (+49) 0511-762 4176. Fax: (+49) 0511-762 4164. E-mail: ripka@ids.uni-hannover.de
2. 2Institute of Dynamics and Vibration Research, Leibniz University of Hannover, Appelstr. 11, D-30167 Hannover, Germany.