JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 209(2009)572-576 573 Carbon fibers transfered Layer n+1 when needling layer n+ Laver n-1 m,点 120 Fig 2- Schematic of tension specimens(dimensions in Fig. 1-Generation of 3D architecture preform with needling process. low cost processing. The needled preforms have good dimen- sional stability and good drapeability, making them suitable for fabricating composites with complicated shape(for exam- shaped parts In order to utilize these novel composites most effciently, thorough understanding of their mechanical properties is essential. It is well known that the fber architecture determines the composite microstructures and properties Therefore, in the present work, the microstructure and tensile behavior of the multiply needled C/Sic composite fabricated Strain/% Fig 3- Tensile stress-strain curves of the multiply needled C/Sic composite at room temperature 2. Experimental procedure 2.1. Preform preparation open porosity of the composite were 2.0-2.1g/cm and 14-17% respectively, determined by the Archimedes' method. The fibrous preforms fabricated by the through-the-thickness needling technique were supplied by Jiangsu Tianniao Insti- 2.3. Tension tests and microstructure observation tute of Carbon Fiber, China. The preforms were composed of unidirectional fibrous plies(also named as nonwoven web) Five specimens for tensile tests were prepared in the same fur- and short-chopped-fiber fabrics. In the present work, HTA nace runs. The shape and dimensions of the specimens are carbon fibers from Toho apan) were used for unidirectional shown in Fig. 2. To avoid local fracture at loading points, alu fibrous plies and T-700 12K carbon fibers from Toray (apan) minum end tabs were bonded to the specimens using an epoxy were used for short-chopped-flber fabrics. The ratio of unidi- resin adhesive. The tensile tests were conducted on an Instron rectional fibrous plies to short-chopped-fiber fabrics was 3: 1. 1196 test machine with a crosshead speed of 0.05 mm/min One unidirectional fibrous ply and one short-chopped-fiber Strains were recorded using an extensometer with a gauge fabric were named as one unit layer After each unit layer lami- length of 25 mm. Microstructure of the composite and the nated in desired orientations(0 /90)and sequences, needling fracture surfaces of the tested specimens were observed by process was carried out to keep adjacent units together with scanning electron microscopy(SEM, $4700 carbon fibers carried, as shown in Fig. 1. Hooks were designed on needle so that fibers stayed where they have been car- ried when needles left preform. As a result, each part of the 3. Results and discussion preform has received the same amount of transferred fibers 3.1. Tensile behavion through the thickness, and this provides the preform with good through-the-thickness homogeneity. The fiber volume The tensile stress-strain curves are shown in Fig.3.From fraction of the preform is about 30-32% Fig 3, the tensile stress-strain curves can be divided into three regions: a very small initial linear region with a low 2. Densification processing limitation stress followed by a large nonlinear region and finally a quasi-linear region. The large nonlinear region was To protect the carbon fibers from damage in the CVI process accompanied by a significant decrease in the modulus In fibers and the SiC matrix(Naslain, 1998, 2004), a pyrolytic gradually tended to be stable up to the failure of the compas and to weaken the interfacial bonding between the carbon the quasi-linear region, the modulus recovered very little ar carbon( Pyc) layer was deposited on the surface of carbon ite. As listed in Table 1, the average tensile strength and failure fibers as fiber/matrix interphase prior to the densification of strain for the composite are 158.9 MPa and 0.71%, respectively. Sic matrix CVI was employed to deposit PyC interphase and The average initiation modulus obtained by linear fitting of the Sic matrix. The conditions for CVi process were the same as stress-strain curves from o to 40 MPa is 75 GPa for the needled that described in reference(Xu et al., 1998). The density and C/Sic compositejournal of materials processing technology 209 (2009) 572–576 573 Fig. 1 – Generation of 3D architecture preform with needling process. low cost processing. The needled preforms have good dimen￾sional stability and good drapeability, making them suitable for fabricating composites with complicated shape (for exam￾ple, T-shaped and bell-shaped parts). In order to utilize these novel composites most efficiently, thorough understanding of their mechanical properties is essential. It is well known that the fiber architecture determines the composite microstructures and properties. Therefore, in the present work, the microstructure and tensile behavior of the multiply needled C/SiC composite fabricated by CVI were investigated. 2. Experimental procedure 2.1. Preform preparation The fibrous preforms fabricated by the through-the-thickness needling technique were supplied by Jiangsu Tianniao Insti￾tute of Carbon Fiber, China. The preforms were composed of unidirectional fibrous plies (also named as nonwoven web) and short-chopped-fiber fabrics. In the present work, HTA carbon fibers from Toho (Japan) were used for unidirectional fibrous plies and T-700 12 K carbon fibers from Toray (Japan) were used for short-chopped-fiber fabrics. The ratio of unidi￾rectional fibrous plies to short-chopped-fiber fabrics was 3:1. One unidirectional fibrous ply and one short-chopped-fiber fabric were named as one unit layer. After each unit layer lami￾nated in desired orientations (0◦/90◦) and sequences, needling process was carried out to keep adjacent units together with carbon fibers carried, as shown in Fig. 1. Hooks were designed on needle so that fibers stayed where they have been car￾ried when needles left preform. As a result, each part of the preform has received the same amount of transferred fibers through the thickness, and this provides the preform with good through-the-thickness homogeneity. The fiber volume fraction of the preform is about 30–32%. 2.2. Densification processing To protect the carbon fibers from damage in the CVI process and to weaken the interfacial bonding between the carbon fibers and the SiC matrix (Naslain, 1998, 2004), a pyrolytic carbon (PyC) layer was deposited on the surface of carbon fibers as fiber/matrix interphase prior to the densification of SiC matrix. CVI was employed to deposit PyC interphase and SiC matrix. The conditions for CVI process were the same as that described in reference (Xu et al., 1998). The density and Fig. 2 – Schematic of tension specimens (dimensions in millimeter). Fig. 3 – Tensile stress–strain curves of the multiply needled C/SiC composite at room temperature. open porosity of the composite were 2.0–2.1 g/cm3 and 14–17%, respectively, determined by the Archimedes’ method. 2.3. Tension tests and microstructure observation Five specimens for tensile tests were prepared in the same fur￾nace runs. The shape and dimensions of the specimens are shown in Fig. 2. To avoid local fracture at loading points, alu￾minum end tabs were bonded to the specimens using an epoxy resin adhesive. The tensile tests were conducted on an Instron 1196 test machine with a crosshead speed of 0.05mm/min. Strains were recorded using an extensometer with a gauge length of 25mm. Microstructure of the composite and the fracture surfaces of the tested specimens were observed by scanning electron microscopy (SEM, S4700). 3. Results and discussion 3.1. Tensile behavior The tensile stress–strain curves are shown in Fig. 3. From Fig. 3, the tensile stress–strain curves can be divided into three regions: a very small initial linear region with a low limitation stress followed by a large nonlinear region and finally a quasi-linear region. The large nonlinear region was accompanied by a significant decrease in the modulus. In the quasi-linear region, the modulus recovered very little and gradually tended to be stable up to the failure of the compos￾ite. As listed in Table 1, the average tensile strength and failure strain for the composite are 158.9 MPa and 0.71%, respectively. The average initiation modulus obtained by linear fitting of the stress–strain curves from 0 to 40 MPa is 75 GPa for the needled C/SiC composite