1. Duke University, P.O. Box 90300, Durham, NC 27708-0300, USA,
2. North Carolina State University, Campus Box 7910, Raleigh, NC 27695-7910, USA
3. Structures and Mechanics Branch, Engineering Sciences Directorate, U.S. Army Research Office, Box 12211, Research Triangle Park, NC 27709-2211, USA
4. Mechanical Behavior of Materials Branch, Physical Sciences Directorate, U.S. Army Research Office, Research Triangle Park, NC27709-2211, USA