1044 Journal of the American Ceramic Sociery-Jacobsen and Brondsted Vol 84. No 5 Compressive specimens(Fig. 2(b)were initially cast into one cylinder, then two cylinders were put into an alignment fixture, and the specimen was cast into the second cylinder. The testing entity was then placed and tested between two plane-parallel disks mounted in the testing machine. The in-plane C-z)shear tests Pointy were conducted using the losipescu test specimen, with ar optimized V-notch angle,43(Fig. 2(c)) Interbundle mental results Bundle Figs. 3(a) and 4(a) for SiC/SiC and C/SiC, respectively. Shear stress-strain curves are shown in Fig. 5. Stress-strain curves for compressive behavior are shown in Fig. 6 for both materials. The elastic composite properties from the initial part of the respective stress-strain curves are shown in Table I. where E is the initial 1mm stiffness. y the Poisson ratio. g the shear stiffness. and g and T the stress at the first measurable deviation from linearity of the ig. 1. Plain-woven structure of C/SiC. Sample tensile and the shear stress-strain curve, respectively. Calculated point-wise connectivity between the bundles and interbundle pores is properties are the coefficient of thermal expansion (a), the residual stress on the 90 ply(), and the radial residual stress acting at the fiber/matrix interface(o ) The tensile/compressive loading is applied in the y-direction, and the in-plane properties are in the between the bundles. The fabric design of SiC/SiC was 1600 J-z-plane(ig. 2). In tension, C/SiC has no initial elastic region filaments/bundle and 0.59 rovings/mm. The C/SiC fabric was 1000 gme=0). The instantaneous elastic stiffnesses at a given peak filaments/bundle and 0.90 rovings/mm. As-received SiC/SiC stress(Ey) and G of the damaged composite are the slope of the showed no matrix cracks, whereas the C/Sic was significantly unloading curve at the peak stress 36.44 The literature reports that precracked in the 90 bundles Ey is approximated by an average modulus defined as the stress range divided by the strain range, i.e., neglecting the anelastic (2) Experimental Procedure hysteresis behavior. Tension, compression, and shear testing of the two materials The strength properties of the materials are shown in Table Il, were performed in air and at room temperature using servo- where S is the tensile strength, Es the tensile strain to failure, Tthe ontrolled test machines with 100 kN load cells (Instron Corp shear strength, yr the engineering shear strain to failure, Scom the anvers, MA). The tension tests were performed at constant load compressive strength, and Ecom the compressive strain to failure rate(5 MPa/s). Constant displacement rates were applied for the compression(0.001 mm/s)and shear tests(0. 1 mm/min). Periodic (4 Damage Mechanisms unloadings at peak loads were performed to measure changes in (A) SiC/SiC: The matrix-cracking sequence obtained from stiffness and hysteresis. Tensile and compressive specimens were the replicas of SiC/SiC revealed that, above the elastic limit unloaded to 70% of the previous peak load, and replicas were continuous matrix cracking took place until fracture. First, matrix taken of a polished edge. The replicas were then examined using cracks emanated from large porosities at stresses above - 0.1IS. optical light microscopy. Fracture surfaces were examined usin Second tunneling matrix cracking in the 90 bundles occurred at anning electron microscopy(SEM; Model JSM-6300F, JEOL tresses from 0.45S to -0 82S, above which the matrix cracking abody, MA). Test specimens are shown in Fig. 2. Strain gauge in the 90o bundles saturated. Tunneling cracks in SiC/SiC are measured the in-plane and out-of-plane strains and were mounted shown in Fig. 7. At stresses above 0.55S tunneling matrix cracks on all test specimens. A 10 mm gauge length extensometer started to grow into the 0 bundles. A few cracks penetrated the 0 (Instron)measured the strain in the loading direction for the tensile bundles below 0. 55S. Third, at stresses above 0.82S and until tests fractured delamination, cracks started to grow between the 0 and Lamina Tabs # se Strain gauges in +45°and-45° directions SiC/SIC C/SIC 200 b) Fig. 2. Test geometries, coordinate system, and specimen dimensions for(a) tensile specimens, (b) compressive specimen, and (c) shear specimenbetween the bundles. The fabric design of SiC/SiC was 1600 filaments/bundle and 0.59 rovings/mm. The C/SiC fabric was 1000 filaments/bundle and 0.90 rovings/mm. As-received SiC/SiC showed no matrix cracks, whereas the C/SiC was significantly precracked in the 90° bundles. (2) Experimental Procedure Tension, compression, and shear testing of the two materials were performed in air and at room temperature using servo￾controlled test machines with 100 kN load cells (Instron Corp., Danvers, MA). The tension tests were performed at constant load rate (5 MPa/s). Constant displacement rates were applied for the compression (0.001 mm/s) and shear tests (0.1 mm/min). Periodic unloadings at peak loads were performed to measure changes in stiffness and hysteresis. Tensile and compressive specimens were unloaded to 70% of the previous peak load, and replicas were taken of a polished edge. The replicas were then examined using optical light microscopy. Fracture surfaces were examined using scanning electron microscopy (SEM; Model JSM-6300F, JEOL, Peabody, MA). Test specimens are shown in Fig. 2. Strain gauges measured the in-plane and out-of-plane strains and were mounted on all test specimens. A 10 mm gauge length extensometer (Instron) measured the strain in the loading direction for the tensile tests. Compressive specimens (Fig. 2(b)) were initially cast into one cylinder, then two cylinders were put into an alignment fixture, and the specimen was cast into the second cylinder. The testing entity was then placed and tested between two plane-parallel disks mounted in the testing machine. The in-plane (y– z) shear tests were conducted using the Iosipescu test specimen,41 with an optimized V-notch angle42,43 (Fig. 2(c)). (3) Experimental Results Stress–strain curves for the tension experiments are shown in Figs. 3(a) and 4(a) for SiC/SiC and C/SiC, respectively. Shear stress–strain curves are shown in Fig. 5. Stress–strain curves for compressive behavior are shown in Fig. 6 for both materials. The elastic composite properties from the initial part of the respective stress–strain curves are shown in Table I, where E is the initial stiffness, n the Poisson ratio, G the shear stiffness, and smc and tmc the stress at the first measurable deviation from linearity of the tensile and the shear stress–strain curve, respectively. Calculated properties are the coefficient of thermal expansion (a), the residual stress on the 90° ply (sR ), and the radial residual stress acting at the fiber/matrix interface (sr R ). The tensile/compressive loading is applied in the y-direction, and the in-plane properties are in the y-z-plane (Fig. 2). In tension, C/SiC has no initial elastic region (smc 5 0). The instantaneous elastic stiffnesses at a given peak stress (Ey) and Gyz of the damaged composite are the slope of the unloading curve at the peak stress.36,44 The literature reports that Ey is approximated by an average modulus defined as the stress range divided by the strain range, i.e., neglecting the anelastic hysteresis behavior. The strength properties of the materials are shown in Table II, where S is the tensile strength, εS the tensile strain to failure, T the shear strength, gT the engineering shear strain to failure, Scom the compressive strength, and εcom the compressive strain to failure. (4) Damage Mechanisms (A) SiC/SiC: The matrix-cracking sequence obtained from the replicas of SiC/SiC revealed that, above the elastic limit, continuous matrix cracking took place until fracture. First, matrix cracks emanated from large porosities at stresses above ;0.11S. Second, tunneling matrix cracking in the 90° bundles occurred at stresses from ;0.45S to ;0.82S, above which the matrix cracking in the 90° bundles saturated. Tunneling cracks in SiC/SiC are shown in Fig. 7. At stresses above ;0.55S tunneling matrix cracks started to grow into the 0° bundles. A few cracks penetrated the 0° bundles below 0.55S. Third, at stresses above ;0.82S and until fractured delamination, cracks started to grow between the 0° and Fig. 1. Plain-woven structure of C/SiC. Sample was split and the point-wise connectivity between the bundles and interbundle pores is shown. Fig. 2. Test geometries, coordinate system, and specimen dimensions for (a) tensile specimens, (b) compressive specimen, and (c) shear specimen (Iosipescu). 1044 Journal of the American Ceramic Society—Jacobsen and Brøndsted Vol. 84, No. 5