694 1. Alma- et al fatigue loading, and to define the changes in behavior stresses in the material during cooling after manu lused by test temperature. The mechanical load facture. These stresses are probably partially relaxed applied as a sinusoidal signal from 0 to a given max- generating a network of cracks in the matrix(Fig. 7) imum amplitude. The frequencies of the load We can note three types of pre-existent cracks unloading cycle were I and 10 Hz. Two types of test have been conducted at room temperature at 35 and I. Matrix cracks perpendicular to the longitudina 75% of the fracture stress. The effect of the temperature yarns: the intercrack distance evolves from has also been studied for a load of 75% of the fracture 240±117mto506±32m, for a matrix thick stress at 600. 1000 and 1500%( ess of 40 um and 100 um, respectively. No evi- a quantitative analysis of these phenomena was con lence of crack propagation parallel to the fiber ducted on the basis of the different parameters(Youngs matrix interface was noted modulus, residual deformation. hysteresis loop)repre- 2. Cracks within the transversal yarns: all the yarns Senting the macroscopic mechanical behavior of the are cracked in a direction parallel to the short axis of the ellipse and exhibit, on average, 2. 5 crack mposite Tests were conducted on tensile specimens with a per yarn, across the whole thickness of the yarns parallel section of 16 x 50 mm. The thickness of 2 a tew cracks parallel to the long axis were also corresponds to that of the standard composite. Tests present (0.2 per yarn) with a mean length of were conducted in an Instron(8502servo-hydraulic test 400 um. Macroporosity does not seem to initiate rig and aA E.T. furnace. Force was measured by an the crack network Instron load cell ( 50 kN)and the deformation by an Instron high-temperature extensometer (+%). The 3.1.2 Thermal stress field specimen was placed in cooled grips and positioned in Pre-existent cracks in the matrix and within the yarns the center of the furnace. The latter was heated by an are due to the partial or total relaxation of the thermal induction healing element with thermal insulation. A stresses induced during fabrication special type of insulation allowed tests to be conducted A finite-element analysis of the composite was con up to 2000C under an inert atmosphere or 1600C ducted in order to estimate the thermal stresses in each component and to study the crack propagation mechanisms after fabrication or during tensile loading The mechanical and dilatometric properties of each 3 RESULTS AND DISCUSSION constituent are presented in table First, a study was conducted at the microscopic fiber 3. 1 Characterization of the as-received composite matrix level in order to determine the stress in a cylin- The transverse and longitudinal yarns each consist of drical system made up of carbon fiber and matrix 1000 carbon fibers of diameter 7 um (Fig. 7). These according to the constitution of the composite. The yarns form cylinders of elliptical cross-section of results by finite-element analysis, after cooling fromm the 1200 um long axis by 200 um short axis. The archi- elaboration to room temperature, are tecture. the specific infiltration of pyrocarbon and the atrix of the composite produce macroporosities alongitudinal in matrix =350MPa between the longitudinal yarns. The fibers are highl Longitudinal in fiber =-600MPa, compact (hexagonal or cubic arrangements) and just Ordial at the interface 36MPa allow the infiltration of pyrocarbon. A small amount of microporosity within the yarns is also noted. The matrix The matrix is in tension in the longitudinal direction therefore exclusively deposited around the yarn and and the interface in the radial direction. Since the ther constitutes a coating. It should be noted that. in the mal stress is higher than the Sic fracture strength composite. the longitudinal yarns are parallel to the (or: 294 MPa), matrix cracks are genet loading direction. analytical calculations, taking into account radial and longitudinal effects, are in accordance with these results 3.1.1 Crack network Since the pyrocarbon interlayer ry im t to C he difference in the thermal expansion coefficient the mechanical behavior of the composite. a cylindrical system with three elements was considered. The thermal Table 1. Constituent properties Longitudinal GPa) longitudinal(C) (C1) Carbon fiber T300 430 48 48×106694 .4. Dultnu: fatigue loading, and to define the changes in behavior caused by test temperature. The mechanical load was applied as a sinusoidal signal from 0 to a given max￾imum amplitude. The frequencies of the loading; unloading cycle were 1 and IO Hz. Two types of test have been conducted at room temperature at 35 and 750/o of the fracture stress. The effect of the temperature has also been studied for a load of 75% of the fracture stress at 600. 1000 and 1500°C. A quantitative analysis of these phenomena was con￾ducted on the basis of the different parameters (Young’s modulus, residual deformation. hysteresis loop) repre￾senting the macroscopic mechanical behavior of the composite. Tests were conducted on tensile specimens with a parallel section of 16 x 50 mm. The thickness of 2 mm corresponds to that of the standard composite. Tests were conducted in an lnstron (8502) servo-hydraulic test rig and a A.E.T. furnace. Force was measured by an Instron load cell (+ 50 kN) and the deformation by an lnstron high-temperature extensometer ( + 5%). The specimen was placed in cooled grips and positioned in the center of the furnace. The latter was heated by an induction heating element with thermal insulation. A special type of insulation allowed tests to be conducted up to 2000°C under an inert atmosphere or 1600°C in air. 3 RESULTS AND DISCUSSION 3.1 Characterization of the as-received composite The transverse and longitudinal yarns each consist of 1000 carbon fibers of diameter 7Llrn (Fig. 7). These yarns form cylinders of elliptical cross-section of 1200~m long axis by 200pm short axis. The archi￾tecture, the specific infiltration of pyrocarbon and the matrix of the composite produce macroporosities between the longitudinal yarns. The fibers are highly compact (hexagonal or cubic arrangements) and .just allow the infiltration of pyrocarbon. A small amount of microporosity within the yarns is also noted. The matrix is therefore exclusively deposited around the yarn and constitutes a coating. It should be noted that. in the composite, the longitudinal yarns are parallel to the loading direction. 3.1.1 Crack nrtrtwrk The difference in the thermal expansion coefficient between the constituents produces thermal residual et al. stresses in the material during cooling after manu￾facture. These stresses are probably partially relaxed. generating a network of cracks in the matrix (Fig. 7). We can note three types of pre-existent cracks: Matrix cracks perpendicular to the longitudinal yarns: the intercrack distance evolves from 24Oi 117 I_cm to 506~t 321_~m, for a matrix thick￾ness of 40pm and 1001_~m, respectively. No evi￾dence of crack propagation parallel to the fiber:’ matrix interface was noted. Cracks within the transversal yarns: all the yarns are cracked in a direction parallel to the short axis of the ellipse and exhibit, on average, 2.5 cracks per yarn, across the whole thickness of the yarns. A few cracks parallel to the long axis were also present (0.2 per yarn) with a mean length of 4001_~m. Macroporosity does not seem to initiate the crack network. 3. I .2 Thrrnzal .stre.s.s firid Pre-existent cracks in the matrix and within the yarns are due to the partial or total relaxation of the thermal stresses induced during fabrication. A finite-element analysis of the composite was con￾ducted in order to estimate the thermal stresses in each component and to study the crack propagation mechanisms after fabrication or during tensile loading. The mechanical and dilatometric properties of each constituent are presented in Table 1. First, a study was conducted at the microscopic fiber: matrix level in order to determine the stress in a cylin￾drical system made up of carbon fiber and matrix, according to the constitution of the composite. The results by finite-element analysis, after cooling from the elaboration to room temperature, are: %ngltudlnal I” matnx - - - ISOMPa, ~longltudinal I” fiber = -600MFk ~radlal at the mterl’ace = - 36MPa. The matrix is in tension in the longitudinal direction, and the interface in the radial direction. Since the ther￾mal stress is higher than the SIC fracture strength ~~~~~~~~~~ = 294 MPa), matrix cracks are generated. The analytical calculations,5 taking into account radial and longitudinal effects, are in accordance with these results. Since the pyrocarbon interlayer is very important to the mechanical behavior of the composite,’ a cylindrical system with three elements was considered. The thermal Table 1. Constituent properties Carbon fiber T300 Pyrocarbon SIC %>“gltudlnal K ‘) %,dl;il (“C ’ ) 1x10 h 7x10 h 3x10 h 28x10 h7 4.8x 10 h 4.8x10 h