[1] Institute of Solid Mechanics of Romanian Academy, C-tin Mille 15, Bucharest 1, Romania
[2] Politehnica University of Bucharest, Splaiul Independentei 125, Bucharest 5, Romania
[3] Industrial Engineering and Technology VTC, Burla Vasile 24, Bucharest 6, Romania
[4] The National University Research Council, Schitu Magureanu 1, Bucharest 5, Romania