1. Research Fellow, Manchester Materials Science Centre, Univ. of Manchester and UMIST, Grosvenor St., Manchester M1 7HS, UK.
2. Reaserch Fellow, Dept. of Materials Engineering, The Open Univ., Walton Hall, Milton Keynes MK7 6AA, UK.
3. Professor of Structural Integrity, Dept. of Materials Engineering, The Open Univ., Walton Hall, Milton Keynes MK7 6AA, UK.
4. Senior Lecturer, Dept. of Materials Engineering, The Open Univ., Walton Hall, Milton Keynes MK7 6AA, UK (corresponding author).