1. Department of Mechanical Engineering, University of Delaware, Newark, DE 19716
2. U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21001
3. Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716
4. Department of Mechanical Engineering, University of Delaware, Newark, DE 19716; Fenn College of Engineering, Cleveland State University, Cleveland, OH 44115-2214 e-mail: