1. Associate Professor, Engineering Materials Research Group, Dept. of Production Engineering, EAFIT Univ., Cra 49 Nr 7 sur 50, Medellin, Colombia.
2. Reader in Mechanics of Materials, Dept. of Engineering, Cambridge Univ., Trumpington St., Cambridge CB2 1PZ, U.K.
3. Professor of Mechanical Engineering, Dept. of Engineering, Cambridge Univ., Trumpington St., Cambridge CB2 1PZ, U.K. (corresponding author).