J. Ma et al Scripta Materialia 54(2006)1967-1971 The yarns are the fundamental structural elements of the Warp yarn composites. The geometrical structures of the composites are mainly determined by the cross-section and the center- line configurations of their constituent yarns In the present study, the basic idea of the computer modeling is to create each yarn by sweeping a closed curve along a centerline path. The closed curve and centerline path represent the ctional shape and Weft yarn the constituent yarns, respectively. Assuming the cross-sec- tional shapes of the warp and weft yarns to be rectangular and elliptical/parallelogrammic, and the spatial path of Fig 3. Three-dimensional view of as-fabricated composites warp and weft yarns to be sinusoidal and straight, a geo. metric model(shown in Fig. 5) was generated. Although yarns are attributable to the combination of compaction in it is only an approximate description of the spatial path the graphite fixture and the symmetrical characteristics of and geometry of the yarns in the composites, it does conve- fiber preform. niently visualize the weave architecture and quantify Shown in Fig. 4 are five kinds of cracks in the compos- geometric characteristics that can be correlated with the ites: transverse cracks in the warp and weft yarns, longitu- global composite properties dinal cracks in the warp and weft yarns and splitting cracks between two adjacent warp yarns. Transverse cracks corre- 3.4. Monotonic tension test results spond to the pre-existing cracks due to processing-related thermal residual stresses, while longitudinal cracks are 3.4.1. Tension tests formed by the propagation of transverse cracks into adja- The tensile stress-strain curves for monotonic loading cent yarns. Since all these cracks are observed after com- parallel to the warp and weft directions are shown in posite processing and before mechanical loading, they are Fig. 6. All the curves exhibited mostly nonlinear behavior referred to as"processing-induced cracks The curves can be generally divided into three regions: a very small initial linear region followed by a large nonlin- 3.3. Geometric model ear region and finally a quasi-linear region. The large non linear region was accompanied by a significant decrease in The cross-sections parallel to the warp and weft yarns the elastic modulus. In the quasi-linear region, the elastic were examined by SEM, and some of the major geometri- modulus recovered a little and gradually tended to be sta cal parameters (listed in Table 1)were evaluated approxi- ble up to the failure of the composites. For the convenience mately to generate the geometric model. of comparison, the tension test data are summarized in Fig 4. SEM images showing five kinds of processing-induced cracks. The geometrical parameters of weave architecture in the final 2. 5D C/SiC composites Rectangular warp Crimp angle of Weft density Elliptical weft yarn Parallelogrammic warp yarns(deg) (Picks/cm) Width Height Length of side 0.39 0.26 0.71yarns are attributable to the combination of compaction in the graphite fixture and the symmetrical characteristics of fiber preform. Shown in Fig. 4 are five kinds of cracks in the compos￾ites: transverse cracks in the warp and weft yarns, longitu￾dinal cracks in the warp and weft yarns and splitting cracks between two adjacent warp yarns. Transverse cracks corre￾spond to the pre-existing cracks due to processing-related thermal residual stresses, while longitudinal cracks are formed by the propagation of transverse cracks into adja￾cent yarns. Since all these cracks are observed after com￾posite processing and before mechanical loading, they are referred to as ‘‘processing-induced cracks’’. 3.3. Geometric model The cross-sections parallel to the warp and weft yarns were examined by SEM, and some of the major geometri￾cal parameters (listed in Table 1) were evaluated approxi￾mately to generate the geometric model. The yarns are the fundamental structural elements of the composites. The geometrical structures of the composites are mainly determined by the cross-section and the center￾line configurations of their constituent yarns. In the present study, the basic idea of the computer modeling is to create each yarn by sweeping a closed curve along a centerline path. The closed curve and centerline path represent the cross-sectional shape and the centerline configurations of the constituent yarns, respectively. Assuming the cross-sec￾tional shapes of the warp and weft yarns to be rectangular and elliptical/parallelogrammic, and the spatial path of warp and weft yarns to be sinusoidal and straight, a geo￾metric model (shown in Fig. 5) was generated. Although it is only an approximate description of the spatial path and geometry of the yarns in the composites, it does conve￾niently visualize the weave architecture and quantify geometric characteristics that can be correlated with the global composite properties. 3.4. Monotonic tension test results 3.4.1. Tension tests The tensile stress–strain curves for monotonic loading parallel to the warp and weft directions are shown in Fig. 6. All the curves exhibited mostly nonlinear behavior. The curves can be generally divided into three regions: a very small initial linear region followed by a large nonlin￾ear region and finally a quasi-linear region. The large non￾linear region was accompanied by a significant decrease in the elastic modulus. In the quasi-linear region, the elastic modulus recovered a little and gradually tended to be sta￾ble up to the failure of the composites. For the convenience of comparison, the tension test data are summarized in Fig. 3. Three-dimensional view of as-fabricated composites. Fig. 4. SEM images showing five kinds of processing-induced cracks. Table 1 The geometrical parameters of weave architecture in the final 2.5D C/SiC composites Warp density (Ends/cm) Rectangular warp yarn Crimp angle of warp yarns (deg) Weft density (Picks/cm) Elliptical weft yarn Parallelogrammic weft yarn Width (mm) Height (mm) Major axis (mm) Minor axis (mm) Length of side (mm) 10 1 0.39 14 3 1.8 0.26 0.71 J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971 1969