574 JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 209(2009)572-576 Table 1- Tensile properties of the multiply needled C/SiC composites Specimens UTS(MPa) Strain to UTS (% Initiation modulus(GPa) 15 1564 73 T4 75 1574 Avera 158.9 75.0 Fig 4- Microstructure of the multiply needled C/SiC composite (a) Top view and(b Side view The nonlinear tensile stress-strain behavior of the multiply 3.2. Microstructure observation needled C/Sic composite can be understood by the process induced damage and damage accumulation occurred in the Typically nonlinear tensile stress-strain curves indicated a composite under the increasing tensile stresses. The dam- typical non-brittle fracture behavior of the multiply nee- age accumulation included micro-cracks propagating, matrix dled C/Sic composite. To further understand the fracture cracking, interfacial debonding/sliding, and fibers breaking characteristics of the multiply needled C/Sic composite, the (Wang and Laird, 1995). Due to the mismatch of the ther- microstructures of the composite and the fracture surfaces mal expansion coeffcients between the carbon fibers and tested specimens were observed by SEM in this work. Fig 4 C matrix, there are some unavoidable microcracks existed shows the microstructure of the multiply needled C/ Sic com- within the Sic matrix. These microcracks propagated when posite before mechanical loading, and Fig. 5 shows the typical the tensile loading increased, accompanying with new micro- fracture surface of tested specimens. As shown in Fig. Sa, the racks initiated in the matrix. After the local stress exceeded fracture surface was very ragged, and the fracture of clus the load bearing capability of the Sic matrix, the microcracks ters mainly occurred at the crossover of needling fibers and joined together to form macrocracks, leading to the crack- unidirectional fibers Detailed observations revealed that the ing of the matrix. Then the tensile loading is mainly bone clusters fractured at various elevations(Fig 5b) by the carbon fibers From the mixture law for the compos The fracture characteristics mentioned above are closely ites, Ec=VAEf+VmEm, the modulus of the composites is mainly related to the microstructure of the composite and suitabl depended on ViEt, so the modulus of the composite decreased. thickness of the Pyc interphase. As shown in Fig. 4a, the Fig 5- Typical fracture morphologies of the multiply needled C/SiC composite (a) Fracture surface and (b)Pullout of carbon574 journal of materials processing technology 209 (2009) 572–576 Table 1 – Tensile properties of the multiply needled C/SiC composites Specimens UTS (MPa) Strain to UTS (%) Initiation modulus (GPa) T1 161.5 0.66 76.7 T2 166.6 0.71 78.2 T3 156.4 0.73 75.6 T4 152.7 0.75 70.6 T5 157.4 0.72 73.7 Average 158.9 0.71 75.0 Deviation 5.3 0.03 2.9 Fig. 4 – Microstructure of the multiply needled C/SiC composite. (a) Top view and (b) Side view. The nonlinear tensile stress–strain behavior of the multiply needled C/SiC composite can be understood by the process￾induced damage and damage accumulation occurred in the composite under the increasing tensile stresses. The dam￾age accumulation included micro-cracks propagating, matrix cracking, interfacial debonding/sliding, and fibers breaking (Wang and Laird, 1995). Due to the mismatch of the ther￾mal expansion coefficients between the carbon fibers and SiC matrix, there are some unavoidable microcracks existed within the SiC matrix. These microcracks propagated when the tensile loading increased, accompanying with new micro￾cracks initiated in the matrix. After the local stress exceeded the load bearing capability of the SiC matrix, the microcracks joined together to form macrocracks, leading to the crack￾ing of the matrix. Then the tensile loading is mainly borne by the carbon fibers. From the mixture law for the compos￾ites, Ec = VfEf + VmEm, the modulus of the composites is mainly depended on VfEf, so the modulus of the composite decreased. 3.2. Microstructure observation Typically nonlinear tensile stress–strain curves indicated a typical non-brittle fracture behavior of the multiply nee￾dled C/SiC composite. To further understand the fracture characteristics of the multiply needled C/SiC composite, the microstructures of the composite and the fracture surfaces of tested specimens were observed by SEM in this work. Fig. 4 shows the microstructure of the multiply needled C/SiC com￾posite before mechanical loading, and Fig. 5 shows the typical fracture surface of tested specimens. As shown in Fig. 5a, the fracture surface was very ragged, and the fracture of clus￾ters mainly occurred at the crossover of needling fibers and unidirectional fibers. Detailed observations revealed that the clusters fractured at various elevations (Fig. 5b). The fracture characteristics mentioned above are closely related to the microstructure of the composite and suitable thickness of the PyC interphase. As shown in Fig. 4a, the Fig. 5 – Typical fracture morphologies of the multiply needled C/SiC composite. (a) Fracture surface and (b) Pullout of carbon fibers