1566 Journal of the American Ceramic Sociery-0 ol.84.No.7 Japan)between 100 and 1000C. The UD composite was esti- mated to contain 36.3 vol% fiber and 25.2 vol% porosity Nano-indentation tests(Model ENT-1 100, Elionix, Japan)were conducted in order to estimate the matrix elastic modulus with the basic theory for this method being explained elsewhere. Essen- tially, the elastic modulus of a material may be estimated using the load/displacement curve obtained from the nano-indentation test In this study, a relative com of elastic moduli between the matrix and fiber component 2n Il. Experimental Results (1 Stress/Strain Behavior and Multiple Microcracking Stress/strain curves and hysteresis loops obtained during load- ing/unloading tensile cycles are summarized in Fig. 3, while optical micrographs of the replica films, illustrating matrix crack ing within the longitudinal (0%)and transverse(90%)fiber bundles, are shown in Fig. 4. The micrographs labelled(a) to(e)in Fig. 4 correspond to the loading stages(a) to(e)in Fig 3 The following damage processes for each load stage are under a) The stress/strain curve is linear with an initial elastic modulus, E 141 GPa, and no microscopic damage is observed 2mm up to a tensile stress of 65 MPa.(b) Propagation of matrix cracks within transverse fiber bundles is observed above 65 MPa. In this paper, the term transverse crack is used to indicate matrix crack Fig. 1. Optical microphotographs of Si-Ti-C-O fiber/Si-Ti-C-O matrix composite (NUSK-CMC) illustrating the orthogonal 3-D woven fiber within transverse(90%) fiber bundles The onset and evolu- tion of matrix cracks in longitudinal fiber bundles is observed above 180 MPa Matrix cracks that originate in the transverse fiber bundles only partially penetrate the longitudinal fiber bundles. In this paper, the term matrix crack is used to describe matrix cracks to a flat finish such that the interlacing loops shown in Fig. I were in the longitudinal(0%)fiber bundles.(d) The matrix crack not present in the final specimens. density in longitudinal fiber bundles increases with the applied load up to 300 MPa. A small amount of transverse crack propa (2) Tensile Tests gation is also observed. (e) Matrix crack densities in both Tensile testing was conducted on a servo-hydraulic testing rig transverse and longitudinal fiber bundles are saturated above a tress level of 300 MPa Model 8501, Instron, USA)at room temperature in air using a onded to the specimen end regions with the load being applied (2 Transverse and Matrit Crack Densities using hydraulic wedge grips. A clip gauge-type extensometer Crack ity measurements for the composite are shown in (gauge length 25 mm; Model 632.11C-20, MTS, USA)was used Fig. 5, and indicate that matrix cracking initiates at Ume =180 to measure the longitudinal strain. Matrix cracking characteristics MPa, and is saturated by o= 300 MPa with a crack saturation were investigated using the replica film method with surface pacing of s= 45.4 um. In contrast to this, the onset of transverse replicas being taken under load at various stages of the loading crack propagation is 65 MPa with the crack density first cycle. Tensile tests were conducted with a number of loading/ rapidly up to 120 MPa, then more slowly above 200 MPa, and is unloading cycles applied to each specimen. saturated beyond 300 MPa. In this final stage, oblique transverse () Thermal Expansion and Nano-indentation Tests The thermal expansion behavior of the UD and 3-D woven 500 composites was investigated in order to estimate the coefficient of qs=423.8[MPa] thermal expansion(CTE)of the matrix and fiber components using a thermal mechanical analyzer (TMA-6300, Seiko Instruments, 9/8 0002040.60.81.01.21.4 Strain [% Units: mm Fig. 3. Stress/strain curve and hyster agonal 3-D Fig. 2. Specimen configuration and dimensions used for unidirectionalto a flat finish such that the interlacing loops shown in Fig. 1 were not present in the final specimens. (2) Tensile Tests Tensile testing was conducted on a servo-hydraulic testing rig (Model 8501, Instron, USA) at room temperature in air using a specimen geometry as shown in Fig. 2. Cardboard tabs were bonded to the specimen end regions with the load being applied using hydraulic wedge grips. A clip gauge-type extensometer (gauge length 25 mm; Model 632.11C-20, MTS, USA) was used to measure the longitudinal strain. Matrix cracking characteristics were investigated using the replica film method with surface replicas being taken under load at various stages of the loading cycle. Tensile tests were conducted with a number of loading/ unloading cycles applied to each specimen. (3) Thermal Expansion and Nano-indentation Tests The thermal expansion behavior of the UD and 3-D woven composites was investigated in order to estimate the coefficient of thermal expansion (CTE) of the matrix and fiber components using a thermal mechanical analyzer (TMA-6300, Seiko Instruments, Japan) between 100° and 1000°C. The UD composite was esti￾mated to contain 36.3 vol% fiber and 25.2 vol% porosity. Nano-indentation tests (Model ENT-1100, Elionix, Japan) were conducted in order to estimate the matrix elastic modulus with the basic theory for this method being explained elsewhere.19 Essen￾tially, the elastic modulus of a material may be estimated using the load/displacement curve obtained from the nano-indentation test. In this study, a relative comparison of elastic moduli between the matrix and fiber components was conducted. III. Experimental Results (1) Stress/Strain Behavior and Multiple Microcracking Stress/strain curves and hysteresis loops obtained during load￾ing/unloading tensile cycles are summarized in Fig. 3, while optical micrographs of the replica films, illustrating matrix crack￾ing within the longitudinal (0°) and transverse (90°) fiber bundles, are shown in Fig. 4. The micrographs labelled (a) to (e) in Fig. 4 correspond to the loading stages (a) to (e) in Fig. 3, respectively. The following damage processes for each load stage are under￾stood: (a) The stress/strain curve is linear with an initial elastic modulus, E ' 141 GPa, and no microscopic damage is observed up to a tensile stress of 65 MPa. (b) Propagation of matrix cracks within transverse fiber bundles is observed above 65 MPa. In this paper, the term transverse crack is used to indicate matrix crack within transverse (90°) fiber bundles. (c) The onset and evolu￾tion of matrix cracks in longitudinal fiber bundles is observed above 180 MPa. Matrix cracks that originate in the transverse fiber bundles only partially penetrate the longitudinal fiber bundles.12 In this paper, the term matrix crack is used to describe matrix cracks in the longitudinal (0°) fiber bundles. (d) The matrix crack density in longitudinal fiber bundles increases with the applied load up to 300 MPa. A small amount of transverse crack propa￾gation is also observed. (e) Matrix crack densities in both the transverse and longitudinal fiber bundles are saturated above a stress level of 300 MPa. (2) Transverse and Matrix Crack Densities Crack density measurements for the composite are shown in Fig. 5, and indicate that matrix cracking initiates at s# mc 5 180 MPa, and is saturated by s# s 5 300 MPa with a crack saturation spacing of l # s 5 45.4 mm. In contrast to this, the onset of transverse crack propagation is 65 MPa with the crack density first increasing rapidly up to 120 MPa, then more slowly above 200 MPa, and is saturated beyond 300 MPa. In this final stage, oblique transverse Fig. 1. Optical microphotographs of Si-Ti-C-O fiber/Si-Ti-C-O matrix composite (NUSK-CMC) illustrating the orthogonal 3-D woven fiber architecture. Fig. 2. Specimen configuration and dimensions used for unidirectional tensile testing. Fig. 3. Stress/strain curve and hysteresis loops for the orthogonal 3-D woven Si-Ti-C-O fiber/Si-Ti-C-O matrix composite (NUSK-CMC) under loading/unloading testing. 1566 Journal of the American Ceramic Society—Ogasawara et al. Vol. 84, No. 7