1. Program of Materials Science and Engineering, Univ. of California, San Diego, La Jolla, CA 92093.
2. Dept. of Structural Engineering, Univ. of California, San Diego, La Jolla, CA 92093-0085.
3. Dept. of Structural Engineering and Program of Materials Science and Engineering, Univ. of California, San Diego, La Jolla, CA 92093-0085 (corresponding author).