1. Laboratory of Mechanics, Materials and Structures, UFR Sciences, University of Reims Champagne Ardenne, Rue des Craye`res, BP1035, 51100 Reims Cedex 2, France
2. Laboratoire d’ Analyse des Contraintes Meécaniques IFTS, University of Reims Champagne Ardenne, Laboratory of Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France