1. McCormick Institute Professor and W.P. Murphy Professor, Dept. of Civil and Mechanical Engineering, and Materials Science, Northwestern Univ., 2145 Sheridan Rd., CEE/A127, Evanston, IL 60208 (corresponding author).
2. Graduate Research Assistant, Northwestern Univ., 2145 Sheridan Rd., CEE/A127, Evanston, IL 60208.