J. Ma et al. Materials Letters 61(2007)312-315 (a) Warp yarn Warp yarn Weft yarn Fig. 1. Weave patten of the carbon fiber preform and the three-dimensional view of the composites (a)Weave pattem of the carbon fiber preform (b)Three- imensional view of the composite weft directions, and then the specimens were sealed with CVd further qualitative analysis. It is seen that the warp yarns presented the Sic to cover all exposed fiber ends. All the mechanical testing shape of a quasi rectangle and contacted tightly with each other, while the was performed on Instron 1196 machine at room temperature. weft yams presented the shape of a lentil and parallelogram alternately Tension tests were performed according to the ASTM C1275 along the warp direction and distributed separately with each other. All test method. Three-point bending tests with a span-to-depth these phenomena are attributed to the combination of compaction in the ratio of 16 were conducted according to the ASTM C1341 test the carbon fiber preforms method. In-plane shear tests were carried out based on the guidelines of the ASTM C1292 test method. Short-beam shear 3.2. Tension tests tests were conducted to measure interlaminar shear strength according to the ASTM D2344 test method; the span-to-depth The typical tensile stress-strain curves of the composites are shown ratio was approximately 5 in Fig. 2. It is seen that all the stress-strain curves exhibited largely nonlinear behavior, which can be qualitatively understood by the 3. Results and discussion processing-induced damage and damage accumulation occurring in the composites under increasing tensile stresses. The damage accumulation 3. 1. Carbon fiber preform architecture includes the matrix cracking and interfacial debonding/sliding which had been discussed in full detail in literature [7]. The tension test data It is a well-known fact that the macro-mechanical properties of fiber are summarized in Table 2, and it is seen that the composites had much reinforced composite materials are primarily dependent on their fiber higher tensile properties in the direction than those in the weft preform architecture, so a thorough understanding of their fiber preform direction, which are attributed to the large differences in yarn density architecture can facilitate the qualitative analysis of their mechanical and yarn path between warp and weft yarns. properties. Closer observations of fracture surfaces revealed that the fracture It can be seen from Fig. 1(a) that the carbon fiber preforms are some warp yarns occurred mostly in the yarn crossover areas, while the variations of layer-to-layer angle interlock fabric and consist of two sets of yams. The weft yams, which take a nominally straight path, create the Y axis portion; while the warp yarns, which take a nominally sinusoidal path, travel from one weft layer to the adjacent weft layer, and back, thus holding together all the weft layers. Obviously, the warp yams undertake dual roles. Firstly, they create the X axis portion which contributes mainly warp direction to in-plane strength, and secondly, they form through-the-thickness reinforcements which contribute to improved delamination resistance, thereby the superior in-service impact resistance. Considering that the mechanical properties of the composites depend upon their final fiber preform architecture, the three-dimensional view of weft direction the real composites was illustrated in Fig. 1(b) for visualization and parameters of the fiber preforms 000.1020.30.4050.60.70.8 layers Ends/cm/layer Picks/cm/layer fraction Warp( K) 6 10 27.7 Weft(6 K) Fig. 2. Tensile stress-strain curves for two loading directions showing mostlyweft directions, and then the specimens were sealed with CVD SiC to cover all exposed fiber ends. All the mechanical testing was performed on Instron 1196 machine at room temperature. Tension tests were performed according to the ASTM C1275 test method. Three-point bending tests with a span-to-depth ratio of 16 were conducted according to the ASTM C1341 test method. In-plane shear tests were carried out based on the guidelines of the ASTM C1292 test method. Short-beam shear tests were conducted to measure interlaminar shear strength according to the ASTM D2344 test method; the span-to-depth ratio was approximately 5. 3. Results and discussion 3.1. Carbon fiber preform architecture It is a well-known fact that the macro-mechanical properties of fiber reinforced composite materials are primarily dependent on their fiber preform architecture, so a thorough understanding of their fiber preform architecture can facilitate the qualitative analysis of their mechanical properties. It can be seen from Fig. 1(a) that the carbon fiber preforms are some variations of layer-to-layer angle interlock fabric and consist of two sets of yarns. The weft yarns, which take a nominally straight path, create the Y axis portion; while the warp yarns, which take a nominally sinusoidal path, travel from one weft layer to the adjacent weft layer, and back, thus holding together all the weft layers. 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 reinforcements which contribute to improved delamination resistance, thereby the superior in-service impact resistance. Considering that the mechanical properties of the composites depend upon their final fiber preform architecture, the three-dimensional view of the real composites was illustrated in Fig. 1(b) for visualization and further qualitative analysis. It is seen that the warp yarns presented the shape of a quasi rectangle and contacted tightly with each other, while the weft yarns presented the shape of a lentil and parallelogram alternately along the warp direction and distributed separately with each other. All these phenomena are attributed to the combination of compaction in the graphite fixture during the CVI process and symmetrical characteristics of the carbon fiber preforms. 3.2. Tension tests The typical tensile stress–strain curves of the composites are shown in Fig. 2. It is seen that all the stress–strain curves exhibited largely nonlinear behavior, which can be qualitatively understood by the processing-induced damage and damage accumulation occurring in the composites under increasing tensile stresses. The damage accumulation includes the matrix cracking and interfacial debonding/sliding, which had been discussed in full detail in literature [7]. The tension test data are summarized in Table 2, and it is seen that the composites had much higher tensile properties in the warp direction than those in the weft direction, which are attributed to the large differences in yarn density and yarn path between warp and weft yarns. Closer observations of fracture surfaces revealed that the fracture of warp yarns occurred mostly in the yarn crossover areas, while the Fig. 1. Weave pattern of the carbon fiber preform and the three-dimensional view of the composites. (a) Weave pattern of the carbon fiber preform. (b) Three￾dimensional view of the composites. Table 1 Weave parameters of the fiber preforms Weave direction Number of yarn layers Yarn density Fiber volume fraction Ends/cm/layer Picks/cm/layer Warp (6 K) 6 10 27.7 Weft (6 K) 7/5 3 8.3 Fig. 2. Tensile stress–strain curves for two loading directions showing mostly nonlinear behavior. J. Ma et al. / Materials Letters 61 (2007) 312–315 313