Affiliation: | 1Professor, Institute of Mechanics, Acad. G. Bonchev bl. 4, 1113 Sofia, Bulgaria. E-mail: ivanova@imbm.bas.bg 2Research Associate, Institute of Mechanics, Acad. G. Bonchev bl. 4, 1113 Sofia, Bulgaria (corresponding author). E-mail: gery@imbm.bas.bg 3Associate Professor, Institute of Mechanics, Acad. G. Bonchev bl. 4, 1113 Sofia, Bulgaria. E-mail: petia@imbm.bas.bg 4Professor, Technical Univ. Darmstadt, 64289 Darmstadt, Germany. E-mail: becker@mechanik.tu-darmstadt.de
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