J. Ma et al. Scripta Materialia 54(2006)1967-1971 defects. The second is to investigate the in-plane tensile behavior of the composites and its relation to the micro structures. Also, the differences in tensile behavior between 30L3 warp and weft directions were compared, and suggestions were offered for tailoring the mechanical properties to achieve the desired performance of the components or Fig. 2. Geometry and dimensions of tension specimen(dimensions in structures millimeters) 2. Experimental a chemical vapor deposited Sic coating. The geometry and dimensions of tension specimens are shown in Fig. 2. 2.1. Composite fabrication The tensile stress-strain curves were measured using an Instron 1196 test machine at a crosshead speed of The carbon fiber utilized was T300( Nippon Toray cor- 0.05 mm/min Strains were recorded with an extensometer poration), and each yarn contained 6000 fibers. The fiber with a gauge length of 25 mm. Five specimens were tested preforms fabricated by the 2.5D woven method were sup- for each loading direction in this manner. The fracture sur plied by Nanjing Institute of Glass Fiber, People's Repub- faces of tested specimens were examined by SEM lic of China. A schematic of the fiber preform is shown in ig. 1. Composite panels were processed by isothermal 3 Results and discussion CVI to deposit the pyrolytic carbon(PyC) interphase and the silicon carbide matrix, which has been described previ- 3. 1. Preform characterization ously in detail [1o As shown in Fig. l, the 2.5D preform is a unique kind of 2. Weave architecture and microstructure multilayer fabric. It is composed of layers of straight weft characterization yarns and a set of sinusoidal warp yarns, with the adjacent The as-fabricated composites were cut parallel to the From left to right, the odd column has seven rows of weft warp and weft directions, and then the cross-sections were yarns, while the even column has five rows of weft yarns ground and polished. The microstructural observations For the odd column, the uppermost and lowest weft yarns and the determination of geometrical parameters of the differ significantly from the other weft yarns which are weave architecture were conducted using scanning electron interlocked by the sinusoidal warp yarns. In contrast, in microscopy(SEM). A computer model which was an the case of the even column, all the weft yarns are in the approximate description of the weave architecture was gen- same situation, i.e., the weft yarns are enclosed in the qua- erated using the AutoCAD software drangular outer tubes which are formed by the warp yarns. According to group theory, the preform is a symmetrical 2.3 Tension tests and microstructural observation object. Firstly, the preform is symmetrical in the warp and weft directions with respect to their respective mid Of a total of 15 composite panels processed in the same planes, and secondly, two adjacent layers of warp yarns furnace runs, three composite panels were cut into the are symmetric with respect to the symmetry center dumbbell-shaped tension specimens along the warp and weft directions, and then the specimens were sealed with 3. 2. Weave architecture and microstructure The macro-mechanical performance of the composites is primarily dependent on their final weave architecture or microstructure, so adequate control over their final weave architecture or microstructure is of prime importance. A 3D view of the as-fabricated composites is shown in Fig. 3. It is seen that the warp yarns take an approximately sinusoidal path, and the shapes of lentils and parallelograms alternately. Obviously, the warp yarns undertake dual roles. Firstly, they create the X axis portion which contributes mainly to in-plane strength, and secondly, they form through-the-thickness or Z element which contributes particularly to improved delamination resistance. It is also evident from the trans- Weft yarn verse cross-section(plane y-z)that the warp yarns present Fig. I. Schematic of fiber preform. thought that the different cross-sectional shapes of weftdefects. The second is to investigate the in-plane tensile behavior of the composites and its relation to the micro￾structures. Also, the differences in tensile behavior between warp and weft directions were compared, and suggestions were offered for tailoring the mechanical properties to achieve the desired performance of the components or structures. 2. Experimental 2.1. Composite fabrication The carbon fiber utilized was T300 (Nippon Toray cor￾poration), and each yarn contained 6000 fibers. The fiber preforms fabricated by the 2.5D woven method were sup￾plied by Nanjing Institute of Glass Fiber, People’s Repub￾lic of China. A schematic of the fiber preform is shown in Fig. 1. Composite panels were processed by isothermal CVI to deposit the pyrolytic carbon (PyC) interphase and the silicon carbide matrix, which has been described previ￾ously in detail [10]. 2.2. Weave architecture and microstructure characterization The as-fabricated composites were cut parallel to the warp and weft directions, and then the cross-sections were ground and polished. The microstructural observations and the determination of geometrical parameters of the weave architecture were conducted using scanning electron microscopy (SEM). A computer model which was an approximate description of the weave architecture was gen￾erated using the AutoCAD software. 2.3. Tension tests and microstructural observation Of a total of 15 composite panels processed in the same furnace runs, three composite panels were cut into the dumbbell-shaped tension specimens along the warp and weft directions, and then the specimens were sealed with a chemical vapor deposited SiC coating. The geometry and dimensions of tension specimens are shown in Fig. 2. The tensile stress–strain curves were measured using an Instron 1196 test machine at a crosshead speed of 0.05 mm/min. Strains were recorded with an extensometer with a gauge length of 25 mm. Five specimens were tested for each loading direction in this manner. The fracture sur￾faces of tested specimens were examined by SEM. 3. Results and discussion 3.1. Preform characterization As shown in Fig. 1, the 2.5D preform is a unique kind of multilayer fabric. It is composed of layers of straight weft yarns and a set of sinusoidal warp yarns, with the adjacent layers of weft yarns interlaced together by warp yarns. From left to right, the odd column has seven rows of weft yarns, while the even column has five rows of weft yarns. For the odd column, the uppermost and lowest weft yarns differ significantly from the other weft yarns which are interlocked by the sinusoidal warp yarns. In contrast, in the case of the even column, all the weft yarns are in the same situation, i.e., the weft yarns are enclosed in the qua￾drangular outer tubes which are formed by the warp yarns. According to group theory, the preform is a symmetrical object. Firstly, the preform is symmetrical in the warp and weft directions with respect to their respective mid￾planes, and secondly, two adjacent layers of warp yarns are symmetric with respect to the symmetry center. 3.2. Weave architecture and microstructure The macro-mechanical performance of the composites is primarily dependent on their final weave architecture or microstructure, so adequate control over their final weave architecture or microstructure is of prime importance. A 3D view of the as-fabricated composites is shown in Fig. 3. It is seen that the warp yarns take an approximately sinusoidal path, and the weft yarns present cross-sectional shapes of lentils and parallelograms alternately. Obviously, the warp yarns undertake dual roles. Firstly, they create the X axis portion which contributes mainly to in-plane strength, and secondly, they form through-the-thickness or Z element which contributes particularly to improved delamination resistance. It is also evident from the trans￾verse cross-section (plane y–z) that the warp yarns present the cross-sectional shape of quasi-rectangles. It was Fig. 1. Schematic of fiber preform. thought that the different cross-sectional shapes of weft Fig. 2. Geometry and dimensions of tension specimen (dimensions in millimeters). 1968 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971