International Journal of Applied Mechanics and Engineering University of Zielona Góra Department of Mechanics ul. Szafrana 2 65-516 Zielona Góra, POLAND phone: (+48 68) 32 82 496, 32 82 472 fax: (+48 68) 32 47 446 e-mail: E.Walicki@ijame.uz.zgora.pl