1. Doctoral Student, Institute of Engineering Mechanics, Univ. of Innsbruck, Technikerstr. 13, 6020 Innsbruck, Austria.
2. Professor of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125.
3. Professor of Engineering Mechanics, Univ. of Innsbruck, Technikerstr. 13, 6020 Innsbruck, Austria (corresponding author).