J. Ma et al. Scripta Materialia 54(2006)1967-1971 Warp yarn tion occurring in the composites under increasing tensile stresses. The damage accumulation includes matrix crack- ing and interfacial debonding/sliding, which has been dis- cussed fully in the literature [16] The great tensile anisotropy of the composites is closely related to the differences in yarn density and yarn path between warp and weft yarns As shown in Table 2 the com- posites have 3.33 times as many yarns in the warp direction as compared to the weft direction. However, the tensile Weft varn strength in the warp direction is only 2.2 times that for the weft direction. Such a decrease in strength is expected Fig. 5. Geometric model of 2. 5D weave architecture because the undulation of warp yarns can significantly reduce the in-plane mechanical properties. Obviously, the eduction in the in-plane mechanical properties is influ- enced to a large extent by the waviness of the warp yarns, i.e., the wavier the warp yarns are, the weaker the in-plane mechanical properties of the composites. For the given yarns, the undulation degree of warp yarns can be altered by adjusting the weft density. A lower weft density indicates a lower undulation degree of warp yarns and vice versa. However, a lower weft density will significantly reduce the mechanical properties in the weft direction. That is to say, Weft direction the mechanical properties of the composites can be tailore for specific applications by varying the weave parameter 000.10203040506070809 Strain 3. 4.2. Fracture observation Fig. 6. Tensile stress-strain curves showing mostly nonlinear behavior The fracture surface morphology can well reflect the fracture characteristics of the composites, so extensive frac- Table 2. It is seen that the composites exhibited very differ- tographic studies were conducted by SEM in this work. ent tensile properties between warp and weft directions The top view of the fracture surfaces for tension specimens The mostly nonlinear tensile stress-strain behavior of in both loading directions is shown in Fig. 7. Detailed 2.5D C/SiC composites can be qualitatively understood observations revealed that the yarns fractured at various by the processing-induced damage and damage accumula- elevations. The warp yarns fractured in the yarn crossover areas, while the weft yarns fractured at random positions. The side view of the fracture surfaces indicated that the Table 2 fracture surfaces of the yarns were very Tension tests data for 2.5D C/SiC composites fibers exhibiting multi-step fracture and extensive pullout. Failur nitial The fracture characteristics mentioned above are closel strength(MPa) strain(%) modulus(GPa related to the different yarn path and suitable thickness of Warp direction 326(35) 0682(0.08)153(16) Weft direction 145(16) the Pyc interphase. In the present composites, the warp 0.705(0.04)62(28) and weft yarns take nominally sinusoidal and straight paths Standard deviations are given in brackets. respectively. When the specimens are subjected to tension Fig. 7. Typical SEM images showing the top view of fracture surfaces of tension specimens: (a) specimen cut along warp direction and(b) specimen cutTable 2. It is seen that the composites exhibited very differ￾ent tensile properties between warp and weft directions. The mostly nonlinear tensile stress–strain behavior of 2.5D C/SiC composites can be qualitatively understood by the processing-induced damage and damage accumula￾tion occurring in the composites under increasing tensile stresses. The damage accumulation includes matrix crack￾ing and interfacial debonding/sliding, which has been dis￾cussed fully in the literature [16]. The great tensile anisotropy of the composites is closely related to the differences in yarn density and yarn path between warp and weft yarns. As shown in Table 2 the com￾posites have 3.33 times as many yarns in the warp direction as compared to the weft direction. However, the tensile strength in the warp direction is only 2.2 times that for the weft direction. Such a decrease in strength is expected because the undulation of warp yarns can significantly reduce the in-plane mechanical properties. Obviously, the reduction in the in-plane mechanical properties is influ￾enced to a large extent by the waviness of the warp yarns, i.e., the wavier the warp yarns are, the weaker the in-plane mechanical properties of the composites. For the given yarns, the undulation degree of warp yarns can be altered by adjusting the weft density. A lower weft density indicates a lower undulation degree of warp yarns and vice versa. However, a lower weft density will significantly reduce the mechanical properties in the weft direction. That is to say, the mechanical properties of the composites can be tailored for specific applications by varying the weave parameters. 3.4.2. Fracture observation The fracture surface morphology can well reflect the fracture characteristics of the composites, so extensive frac￾tographic studies were conducted by SEM in this work. The top view of the fracture surfaces for tension specimens in both loading directions is shown in Fig. 7. Detailed observations revealed that the yarns fractured at various elevations. The warp yarns fractured in the yarn crossover areas, while the weft yarns fractured at random positions. The side view of the fracture surfaces indicated that the fracture surfaces of the yarns were very ragged, with the fibers exhibiting multi-step fracture and extensive pullout. The fracture characteristics mentioned above are closely related to the different yarn path and suitable thickness of the PyC interphase. In the present composites, the warp and weft yarns take nominally sinusoidal and straight paths, respectively. When the specimens are subjected to tension Fig. 5. Geometric model of 2.5D weave architecture. Fig. 6. Tensile stress–strain curves showing mostly nonlinear behavior. Table 2 Tension tests data for 2.5D C/SiC composites Specimen Tensile strength (MPa) Failure strain (%) Initial modulus (GPa) Warp direction 326 (35) 0.682 (0.08) 153 (16) Weft direction 145 (16) 0.705 (0.04) 62 (2.8) Standard deviations are given in brackets. Fig. 7. Typical SEM images showing the top view of fracture surfaces of tension specimens: (a) specimen cut along warp direction and (b) specimen cut along weft direction. 1970 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971