1. Centerfor Nondestructive Evaluation, The Johns Hopkins University, 3400 North Charles Street, Maryland Hall Room 102, Baltimore, MD 21218
2. Dipartimento di Meccanica ed Aeronautica, Universita' degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy
3. Department of Materials Science and Mechanics, Michigan State University, 3536 Engineering Building, East Lansing, MI 48824-1226