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Submitted on: 05 August 2013.
Revised on: 16 October 2013.
Accepted on: 29 October 2013.
___________________Journal of Theoretical and Applied Mechanics-Bulgaria
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Submitted on: 02 November 2009.
Revised on: 20 December 2009.
Accepted on: 02 February 2010.
___________________Journal of Theoretical and Applied Mechanics-Bulgaria
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Submitted on: 23 August 2000.
Revised on: 25 November 2000.
Accepted on: 29 December 2000.
___________________Journal of Theoretical and Applied Mechanics-Bulgaria