214 v. Kostopoulos et al Fatigue test for a /o.-075 coefficient in our case or through the hysterisis Steady-state loops elsewhere and could be a very reliable damage indicator for the material under investigation 4 Conclusions The present work deals with the characterisation of damage development in 3-D SiC/SiC composites under fatigue loading using as indicators of the Number of Fatigue keycles fatigue damage the effective dynamic modulus of elasticity and the corresponding damping coeffi Fig.6.Correlation between AE activity related to fibre frac- cient. The main conclusions of the present stud ure and normalised effective dynamic modulus for fatigue loading of 3-D Sic/SiC up to 0-75 of UTS. are the following: The fatigue the effective dynamic modulus damage produces a progressive and increase of internal damping. Due to the redistribution of the stress field around the fibre during the first 30 kcycles of fatigue life(for during this period, the crack saturation phase maximum applied stress of 0.7 of UTS) while approaches and an unstable damage propagation the damping coefficient increases drastically initiates. This period involves an increase rate of Damping coefficient is more sensitive to fati- fibre fracture and corresponds to the burn-out gue damage than the storage modulus phase. It leads to the materials final failure. The The damage stage associated to the interfacial burn out phase is absent from Fig. 5 since th rear and internal friction is very well corre material is loaded lower to its endurance fatigue lated to the monitored damping coefficient limit. In contrast to this, in the case of Fig. 6 a. The damping coefficient is in a direct relation more massive destruction of the material appears to the elsewhere used area of the hysterisis from the beginning of loading, since the applied loop which is also a fatigue damage indicator load exceeds the endurance fatigue limit, the num in the case of CMcs ber of fibre breakages is higher and close to the end of the life of the material the burn out phase The combination of dynamic response monitor appears. These results are in good correlation with ing with other damage evaluation methods such as that provided by the monitoring of the dynamic Ae could be a powerful tool in the study of the response of the material during fatigue fatigue damage of CMCs and could be used for The monitoring of the dynamic response of the establishing safe criteria for the stability of fatigu material could be accounted as a macroscopic damage and the remaining life of the material approach, while the AE monitored stands for a under investigation microscopic approach and based on this, the cri tical role of fibre fracture on the 3-D Sic/Sic stiff- ness degradation could be confirmed. The Acknowledgements degradation of the effective dynamic storage mod ulus is directly related to the number of fibre frac- The authors want gratefully to acknowledge the tures and this is confirmed comparing the results support of the Greek Ministry of Development, presented in Figs 5 and 6. On the other hand, General Secretariat of Research and Technology to additionaly to the matrix cracking that occurred the present work within the frame of pened pro- during fatigue loading, the failure of a fibre bundle ject( Contract no 615) leads to a stress redistribution in the vicinity of the broken fibre. This produces a stress concentration site, and as a result to fibre-matrix debonding. References Then the friction is the only load transfer mechan ism between matrix and fibres, but is the presence 1. Evans, A G Zok, F.w. and MeMeeking, R. M. Fati- of this friction that is recognised as internal damp- ing increase. Thus, the fibre fracture promotes Mater.,1995,43,859-875 2. Reynaud, P, Cyclic fatigue of ceramic-matrix composites fibre-matrix debonding and then friction plays a very critical role for the mechanical behaviour of Tech,1996,56,809814 CMCs during fatigue. The presence of the internal 3. Evans, A. G, Design and life prediction issues for high perature engineering ceramics and their composites. friction is monitored either through damping Acta metall. Mater.1997.45. 23-40.and increase of internal damping. Due to the redistribution of the stress ®eld around the ®bre during this period, the crack saturation phase approaches and an unstable damage propagation initiates. This period involves an increase rate of ®bre fracture and corresponds to the burn-out phase. It leads to the materials ®nal failure. The burn out phase is absent from Fig. 5 since the material is loaded lower to its endurance fatigue limit. In contrast to this, in the case of Fig. 6 a more massive destruction of the material appears from the beginning of loading, since the applied load exceeds the endurance fatigue limit, the num￾ber of ®bre breakages is higher and close to the end of the life of the material the burn out phase appears. These results are in good correlation with that provided by the monitoring of the dynamic response of the material during fatigue. The monitoring of the dynamic response of the material could be accounted as a macroscopic approach, while the AE monitored stands for a microscopic approach and based on this, the cri￾tical role of ®bre fracture on the 3-D SiC/SiC sti€- ness degradation could be con®rmed. The degradation of the e€ective dynamic storage mod￾ulus is directly related to the number of ®bre frac￾tures and this is con®rmed comparing the results presented in Figs 5 and 6. On the other hand, additionaly to the matrix cracking that occurred during fatigue loading, the failure of a ®bre bundle leads to a stress redistribution in the vicinity of the broken ®bre. This produces a stress concentration site, and as a result to ®bre±matrix debonding. Then the friction is the only load transfer mechan￾ism between matrix and ®bres, but is the presence of this friction that is recognised as internal damp￾ing increase. Thus, the ®bre fracture promotes ®bre-matrix debonding and then friction plays a very critical role for the mechanical behaviour of CMCs during fatigue. The presence of the internal friction is monitored either through damping coecient in our case or through the hysterisis loops elsewhere and could be a very reliable damage indicator for the material under investigation. 4 Conclusions The present work deals with the characterisation of damage development in 3-D SiC/SiC composites under fatigue loading using as indicators of the fatigue damage the e€ective dynamic modulus of elasticity and the corresponding damping coe- cient. The main conclusions of the present study are the following: . The fatigue damage produces a progressive reduction of the e€ective dynamic modulus during the ®rst 30 kcycles of fatigue life (for maximum applied stress of 0.7 of UTS) while the damping coecient increases drastically. . Damping coecient is more sensitive to fati￾gue damage than the storage modulus. . The damage stage associated to the interfacial wear and internal friction is very well corre￾lated to the monitored damping coecient . The damping coecient is in a direct relation to the elsewhere used area of the hysterisis loop1 which is also a fatigue damage indicator in the case of CMCs. The combination of dynamic response monitor￾ing with other damage evaluation methods such as AE could be a powerful tool in the study of the fatigue damage of CMCs and could be used for establishing safe criteria for the stability of fatigue damage and the remaining life of the material under investigation. Acknowledgements The authors want gratefully to acknowledge the support of the Greek Ministry of Development, General Secretariat of Research and Technology to the present work within the frame of PENED pro￾ject (Contract no. 615). References 1. Evans, A. G., Zok, F. W. and McMeeking, R. M., Fati￾gue of ceramic matrix compositesÐoverview. Acta metall. Mater., 1995, 43, 859±875. 2. Reynaud, P., Cyclic fatigue of ceramic-matrix composites at ambient and elevated temperatures. Comp. Sci. and Tech., 1996, 56, 809±814. 3. Evans, A. G., Design and life prediction issues for high￾temperature engineering ceramics and their composites. Acta Metall. Mater., 1997, 45, 23±40. Fig. 6. Correlation between AE activity related to ®bre frac￾ture and normalised e€ective dynamic modulus for fatigue loading of 3-D SiC/SiC up to 0.75 of UTS. 214 V. Kostopoulos et al