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Submitted on: 04 May 2004.
Revised on: 24 August 2004.
Accepted on: 06 September 2004.
___________________Ain Shams Engineering Journal---ELSEVIER
Submitted on: 18 January 2011.
Revised on: 13 March 2011.
Accepted on: 09 April 2011.
___________________Ain Shams Engineering Journal---ELSEVIER
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