S. Baste/Composites Science and Technology 61(2001)2285-2297 Under cyclic tensile loading, a 2D carbon SiC com- active again. The compliances reach the value they had posite exhibits quite immediately a non-linear behaviour before unloading [15] ( Fig. 21). The cycles do not show any hysteresis but Fig. 23 shows the relative part of the elastic and nevertheless the residual strains are far from negligible inelastic strains on the total strain. As an example, the The lack of hysteresis let think that the sliding occurs cycle at 280 MPa has been chosen. The linear variation with little or no friction at the fibre matrix interface [15]. of the total strain is the sum of both the non-linear S33, S44 and Sss modified by the presence of a trans- variations of the elastic and inelastic strains. The elastic verse cracks system are the only compliances to exhibit strain naturally comes back to zero with the stress a variation during the cycles(Fig. 22). Their progressive whereas the inelastic strain is still present as a function decrease during the unloading cycles is relevant of of the transverse cracks which is still active [15] progressive closure of the cracks. As some cracks close, The successive process of prediction experimental they are active no more and they lose the effect they had data confrontation allows us the optimal determination on the compliances. When the sample is reloaded, all of the evolution laws of the cracks density and the le cracks that have been created re open and become opening closure variable (Fig. 24). Fig. 22 plots the experimental changes of the three most influenced com pliances under cyclic loading and the prediction plotted in straight lines. According to Eqs.(17)and(19), the behaviour during the cycles is fully described by the variables: B,8 and f that predict the compliances var- iations(Fig. 22). During the loading/unloading cycles, only the transverse crack system has an influence on the behaviour due to the opening-closure of the cracks [15] During unloading. it is obvious that the effect of closure of the cracks must be taken into account. The proportion Total Strain (% of transverse cracks which is still active under cyclic 040.5060.708 loading is described with the opening-closure function Fig. 21. Stress-strain curve of a 2D C-SiC under cyclic loading F. Eq(4)becomes ein=8.FB Furthermore, some cracks will probably not com- pletely close and have a residual opening. This opening can be due to the roughness of the cracks edges coming from both wear debris at the sliding interface [29] and inelastic Fig. 23. Under load strain partition during cyclic loading at 280 MPa of a 2D C-Sic 删 Fig 22. The most influenced compliances of a 2D C-SiC under cyclic Fig. 24. Variation of the crack density parameter and of the opening- loadin closure function of a 2D C-sicUnder cyclic tensile loading, a 2D carbon SiC com￾posite exhibits quite immediately a non-linear behaviour (Fig. 21). The cycles do not show any hysteresis but nevertheless the residual strains are far from negligible. The lack of hysteresis let think that the sliding occurs with little or no friction at the fibre matrix interface [15]. S33, S44 and S55 modified by the presence of a trans￾verse cracks system are the only compliances to exhibit a variation during the cycles (Fig. 22). Their progressive decrease during the unloading cycles is relevant of a progressive closure of the cracks. As some cracks close, they are active no more and they lose the effect they had on the compliances. When the sample is reloaded, all the cracks that have been created re open and become active again. The compliances reach the value they had before unloading [15]. Fig. 23 shows the relative part of the elastic and inelastic strains on the total strain. As an example, the cycle at 280 MPa has been chosen. The linear variation of the total strain is the sum of both the non-linear variations of the elastic and inelastic strains. The elastic strain naturally comes back to zero with the stress whereas the inelastic strain is still present as a function of the transverse cracks which is still active [15]. The successive process of prediction experimental data confrontation allows us the optimal determination of the evolution laws of the cracks density and the opening closure variable (Fig. 24). Fig. 22 plots the experimental changes of the three most influenced com￾pliances under cyclic loading and the prediction plotted in straight lines. According to Eqs. (17) and (19), the behaviour during the cycles is fully described by the variables: ,  and F that predict the compliances var￾iations (Fig. 22). During the loading/unloading cycles, only the transverse crack system has an influence on the behaviour due to the opening-closure of the cracks [15]. During unloading, it is obvious that the effect of closure of the cracks must be taken into account. The proportion of transverse cracks which is still active under cyclic loading is described with the opening-closure function F. Eq. (4) becomes: "in ¼ F ð20Þ Furthermore, some cracks will probably not com￾pletely close and have a residual opening. This opening can be due to the roughness of the cracks edges coming from both wear debris at the sliding interface [29] and Fig. 22. The most influenced compliances of a 2D C–SiC under cyclic loading. Fig. 23. Under load strain partition during cyclic loading at 280 MPa of a 2D C–SiC. Fig. 24. Variation of the crack density parameter and of the opening￾closure function of a 2D C–SiC. Fig. 21. Stress-strain curve of a 2D C–SiC under cyclic loading. 2292 S. Baste / Composites Science and Technology 61 (2001) 2285–2297