A Dalmas et al Crack density inside the transverse yarns(i. e. yarns elevated temperature. Cracks produced during the perpendicular to the loading direction). The estima- cooling phase of material manufacture have less effect tion has been conducted for cracks that cross the on material behavior as the temperature is increased. At whole thickness of the yarns. This value increases the same time the tendency to produce new cracking is from the initial crack density (2.4+0.7)by a factor reduced of 1. 3 for unfailed specimens(600 and 1000 C) An increase in the modulus of the material is noted and by a factor of 1-7 for those failed by fatigue during cyclic loading at 600C, accompanied by an (room temperature and 1500oC). It should be increase in the residual deformation within the material noted that after the test at 1500 C. these cracks This results from fact that fatigue produces a were well open, up to 50 um(Fig. 10) reduction in the stress transfer responsible for crack Debonding between the entities of the architecture. propagation, most notably where the interfacial shear We observe the first debonding cracks at the stress is sufficiently high. However, the complex archi- nterfaces between weft and warp yarns. mainly at tecture of the matcrial mcans that the crack surfaces are 600 and 1000"C(Fig. 9). The same damage not fat( because of cracking across the transverse yarns has been observed by Lee after failure testing of following fiber contours). Also, their displacement is a 2D C/SiC at 1000C. This kind of damage is multi-directional, producing both crack opening and more likely to occur as the weave is more pro- shearing If this lateral displacement is too large(greater nounced. Some debonding appears between the than the period of the surface roughness of the crack longitudinal fiber bundles and the matrix in the faces). contact is made between the crack faces. stop specimen tested and failed at 1500oC, as well as ping load redistribution racks around transverse yarns at the yarn/ matrix interface Influence of the macroporosity Macropores have 5 CONCLUSIONS an effect on the initiation of cracks parallel to the longitudinal yarns, this is observed at all tempera- The cyclic fatigue behavior of a 2. 5D C/SiC composite tures(Figs 8 10). This cracking tends to separate has been studied over a range of temperatures; 20, 600, the adjacent woven plies and propagates along the 1000 and 1500oC. The analysis of the cyclic fatigue interface between longitudinal and transverse behavior and the damage accumulated has allowed us to yarns(Fig. 9) or through the transverse yarn describe the fatigue mechanism operating within the Figs 8 and 9) material. The effect of cyclic fatigue is, in part, depen- dent on the shear strength at the fibre/ matrix interface (in this work considered on a mesoscopic scale, for the 4 DISCUSSION yarn/yarn interaction). For temperatures below 1000oC cyclic fatigue behavior is governed by both the friction First, it should be noted that, for this material, fatigue between the yarn/yarn or the yarn/ matrix and the resi behavior is independent of temperature below 1000oC. dual stress produced when cooling during manufacture and is linked to the number of cycles and not to the At temperaturcs above 1000C the effect of thermal period of the test. This behavior is in turn, dependent stresses appears to be masked by other effects: these on mechanical effects at the fibre/matrix interface. At merit further investigation temperatures above 1000C, the test period appears to The hypothesis proposed to explain the stiffening influence the fatigue behavior. but this effect requires observed with an increasing number of cycles is to be urther observation further investigated, notably by high-temperature ten Residual stresses in the material are in tension in the sile tests in situ in the sem nteriors of the fiber bundles transverse to the fibers which produces the observed cracking. It is also in ten- s and their coat. ACKNOWLEDGEMENTS ing, promoting debonding at the interface. The stress redistribution that is produced controls thc macroscopi The authors are grateful to the SEp and CNRS for their mechanical behavior nterest and the support in this work. Thanks are parti Cyclic loading produces a cyclic sliding that degrades cularly due to Dr M. Bourgeon, Dr F. Abbe and Dr J the interface. It is this process that leads to a reduction Ph. Richard from SEP, for numerous fruitful discus in fatigue damage since it is more difficult for the matrix sions and for providing the material cracks to propagate into the fiber ncreasing test temperature augments both the life and the elastic modulus of the material. Up to 1000C REfEReNces he fatigue mechanisms operating are attenuated by an increase in temperature. This effect is due, in most part Arends. F.J. The Mail. K. and Kuhnle. 3 lo the reduction of residual stresses at the interface at Mechanical behavior nt sized C/C-SiC tubes under698 A. Dulmuz et al. l Crack density inside the transverse yarns (i.e. yarns perpendicular to the loading direction). The estima￾tion has been conducted for cracks that cross the whole thickness of the yarns. This value increases from the initial crack density (2.4 f 0.7) by a factor of I.3 for unfailed specimens (600 and IOOO’C) and by a factor of I .7 for those failed by fatigue (room temperature and I 500°C). It should be noted that after the test at 1_5OO”C, these cracks were well open, up to 50 pm (Fig. 10). l Debonding between the entities of the architecture. We observe the first debonding cracks at the interfaces between weft and warp yarns, mainly at 600 and 1000°C (Fig. 9). The same damage has been observed by Lee4 after failure testing of a 2D CjSiC at 1000°C. This kind of damage is more likely to occur as the weave is more pro￾nounced. Some debonding appears between the longitudinal fiber bundles and the matrix in the specimen tested and failed at 15OO”C, as well as cracks around transverse yarns at the yarn/matrix interface. l Influence of the macroporosity. Macropores have an effect on the initiation of cracks parallel to the longitudinal yarns, this is observed at all tempera￾tures (Figs 8%10). This cracking tends to separate the adjacent woven plies and propagates along the interface between longitudinal and transverse yarns (Fig. 9) or through the transverse yarn (Figs 8 and 9). 4 DISCUSSION First, it should be noted that, for this material, fatigue behavior is independent of temperature below lOOo”C, and is linked to the number of cycles and not to the period of the test. This behavior is. in turn, dependent on mechanical effects at the fibrelmatrix interface. At temperatures above lOOO”C, the test period appears to influence the fatigue behavior. but this effect requires further observation. Residual stresses in the material are in tension in the interiors of the fiber bundles, transverse to the fibers. which produces the observed cracking. It is also in ten￾sion at the interface between the yarns and their coat￾ing, promoting debonding at the interface. The stress redistribution that is produced controls the macroscopic mechanical behavior. Cyclic loading produces a cyclic sliding that degrades the interface. It is this process that leads to a reduction in fatigue damage since it is more difficult for the matrix cracks to propagate into the fibers. Increasing test temperature augments both the life and the elastic modulus of the material. Up to 1000°C the fatigue mechanisms operating are attenuated by an increase in temperature. This effect is due, in most part, to the reduction of residual stresses at the interface at elevated temperature. Cracks produced during the cooling phase of material manufacture have less effect on material behavior as the temperature is increased. At the same time the tendency to produce new cracking is reduced. An increase in the modulus of the material is noted during cyclic loading at 6OO”C, accompanied by an increase in the residual deformation within the material. This results from the fact that fatigue produces a reduction in the stress transfer responsible for crack propagation, most notably where the interfacial shear stress is sufficiently high. However, the complex archi￾tecture of the material means that the crack surfaces are not flat (because of cracking across the transverse yarns, following fiber contours). Also, their displacement is multi-directional. producing both crack opening and shearing. If this lateral displacement is too large (grcnter than the period of the surface roughness of the crack faces). contact is made between the crack faces, stop￾ping load redistribution. 5 CONCLUSIONS The cyclic fatigue behavior of a 2.5D C/Sic composite has been studied over a range of temperatures; 20, 600, 1000 and 1500°C. The analysis of the cyclic fatigue behavior and the damage accumulated has allowed us to describe the fatigue mechanism operating within the material. The effect of cyclic fatigue is, in part, depen￾dent on the shear strength at the fibre/matrix interface (in this work considered on a mesoscopic scale, for the yarn/yarn interaction). For temperatures below 1000°C cyclic fatigue behavior is governed by both the friction between the yarn/yarn or the yarn/matrix and the resi￾dual stress produced when cooling during manufacture. At temperatures above 1000°C the effect of thermal stresses appears to be masked by other effects; these merit further investigation. The hypothesis proposed to explain the stiffening observed with an increasing number of cycles is to be further investigated, notably by high-temperature ten￾sile tests in situ in the SEM. ACKNOWLEDGEMENTS The authors are grateful to the SEP and CNRS for their interest and the support in this work. Thanks are parti￾cularly due to Dr M. Bourgeon, Dr F. Abbt and Dr J. Ph. Richard from SEP, for numerous fruitful discus￾sions and for providing the materials. REFERENCES I Arendts, F. J.. Theuer, A., Mail, K. and Kuhnle. J., Mechanical behavior of different sized C/C-SC tubes under