EFFECTS OF 2-SCALE TRANSVERSE CRACK SYSTEMS ON THE NONLINEAR BEHAVIOROF A 2D SIC-SIC COMPOSITE

By using both an ultrasonic device and an extensometer, it is possible to know which stiffness coefficients change during the damage process of a material and which part of the global strain is either elastic o r inelastic. The influence of the two damage mechanisms is described f or a woven 2D SiC-SiC composite. It appears that the two scales of thi s composite have a great influence on its behaviour. Two elementary me chanisms occur at both scales of the material: at the mesostructure le vel consisting of the bundles as well as of the inter-bundle matrix an d at the microstructure level made from both the fibres and the intra- bundle matrix. The inelastic strains are sensitive to this two-scale e ffect: an increment of strain at constant stress that comes to saturat ion corresponding to the inter-bundle damage process and a strain whic h needs an increase in stress as cracking occurs at the fibres scale. With the help of a model that predicts the compliance changes caused b y a crack system in a solid, it is possible to predict the crack densi ty variation at both scales as well as the geometry of the various cra ck systems during monotonous loading. Furthermore, when the crack open ing is taken into account, it appears that the inelastic strain is gov erned by the transverse crack density. (C) 1998 Elsevier Science S.A. All rights reserved.