Journal J.Am. Cera. Soc.84D]1565-7(2001) Multiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite Toshio Ogasawara and Takashi Ishikawa National Aerospace Laboratory of Japan, Mitaka, Tokyo, 181-0015, Japan Hiroshi Ito and naoyuki Watanabe Aerospace Systems Department, Tokyo Metropolitan Institute of Technology, Hino, Tokyo, 191-0065, Japan an. Davies Advanced Fibro Science, Kyoto Institute of Technology, Sakyo-ku, Kyoto, 606-8585, Japan This paper presents experimental results for the multiple It is now well understood that unidirectional (UD)CMCs microcracking and tensile behavior of an orthogonal 3-d exhibit nonlinear stress-strain behavior under unidirectional ten- woven Si-Ti-C-O fiber (Tyranno M Lox-M)/Si-Ti-C-O matrix sile loading as a result of multiple microcracking and fiber pullout composite with a nanoscale carbon fiber/matrix interphase within the longitudinal(0%)fiber bundles. An overview of CMC Ind processed using a polymer impregnation and pyrolysis mechanical properties has been proviede by Evans and Zok'and oute. Based on microscopie observations and unidirectional Evans.The change in stiffness due to multiple matrix cracking tensile tests. it is revealed that the inelastic tensile stress/strain has been estimated by two different approaches: (1)elastic ehavior is governed by matrix cracking in transverse (90o) analysis based on the Lame problem"and(2)shear-lag analysis o fiber bundles between 65 and 180 MPa, matrix cracking in In cross-ply composites, this change also involves initial cracking longitudinal (0%)fiber bundles between 180 and 300 MPa, and in the transverse(90%) plies as tunneling cracks. Subse- fiber fragmentation above 300 MPa. A methodology for esti- quently, transverse cracks penetrate the longitudinal plies as the mation of unidirectional tensile behavior in orthogonal 3-D load is increased. Based on the energy criterion and finite element composites has been established by the use and modification of analysis, transverse crack propagation in cross-ply brittle-matrix existing theory. A good correlation was obtained between the composites has been analyzed. Shear-lag analysis is often used to predicted and measured composite strain using this procedure. estimate transverse crack propagation within polymer-matrix com- and this method has also been applied to cross-ply CMCs. 0, I3 Following matrix crack saturation, estimates for the L. Introduction fiber pullout length, pullout work, and the ultimate tensile strength of unidirectional CMCs have been derived by Curtin, ,based on T IS well known that monolithic ceramics do not possess a level the statistical analysis of fiber strength and stress redistribution due of damage tolerance that is sufficient for aerospace applications to fiber fragmentation processes For this reason, a great deal of effort has been devoted to the This paper presents experimental results for the multiple micro development of continuous-ceramic-fiber-reinforced ceramic- cracking and tensile behavior of an orthogonal 3-D woven Si-Ti matrix composites(CMCs or CFCCs) for jet engine comp C-O fiber/Si-Ti-C-O matrix composite(NUSK-CMC)fabricated nents, rocket engine nozzles, and the thermal protection systems (TPS)of future space transportation vehicles. -The National were estimated from hysteresis loop analysis of loading/unloading Aerospace Laboratory of Japan, Ube Industries Ltd, Shikib cycles. A methodology for estimating the unidirectional tensile Ltd, and Kawasaki Heavy Industries Ltd. have conducted a behavior of orthogonal 3-D composites has been provided by using joint program in order to develop and evaluate a continuous- and modifying the above-mentioned theories fiber-reinforced CMC. The composite contains Tyranno M Lox M fiber (Si 54%, Ti 2%, C 32%, 0 12%(mass%))with an additional surface modification process. The fibers are woven Il. Experimental Procedure into an orthogonal 3-D structure that has advantages from the (0 Material and Specimens rocessing,and improved delamination resistance and tensile The composite under investigation contained Tyranno Lox-M rength. Using these technologies. the composite exhibits volume fractions of 19%, 19%, and 2% in the duration with fiber fibers woven into an orthogonal 3-D confi excellent tensile stre at room temperature", and creep and- directions trength at elevated temperature. The composite is referred to respectively. Optical micrographs in Fig. I illustrate the fiber as"NUSK-CMC" from the initials of the collaborating partners architecture of the present composites with each fiber bundle atmosphere, resulting in the formation of a 10 nm Sio- rich layer surrounding an inner 40 nm carbon-rich layer at the fiber surface B. N. Cox--contributing editor The nanoscale carbon- rich layer is believed to result in an interphase with desirable properties between the fiber and the matrix.Polytitanocarbosilane was used as the matrix precursor with eight impregnation and pyrolysis cycles, the average com- ipt No. 188832. Received January 3, 2000: approved March 6, 2001 posite bulk density was 2.20 g/cm". Tensile specimens were Currently with Nagoya Aerospace Systems, Mitsubishi Heavy Industries Ltd machined from the composite plates such that the loading direction Nagoya, Japan. was parallel to the y-axis. The specimen surfaces were also groundMultiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite Toshio Ogasawara* and Takashi Ishikawa National Aerospace Laboratory of Japan, Mitaka, Tokyo, 181-0015, Japan Hiroshi Ito† and Naoyuki Watanabe Aerospace Systems Department, Tokyo Metropolitan Institute of Technology, Hino, Tokyo, 191-0065, Japan Ian J. Davies* Advanced Fibro Science, Kyoto Institute of Technology, Sakyo-ku, Kyoto, 606-8585, Japan This paper presents experimental results for the multiple microcracking and tensile behavior of an orthogonal 3-D woven Si-Ti-C-O fiber (Tyranno™ Lox-M)/Si-Ti-C-O matrix composite with a nanoscale carbon fiber/matrix interphase and processed using a polymer impregnation and pyrolysis route. Based on microscopic observations and unidirectional tensile tests, it is revealed that the inelastic tensile stress/strain behavior is governed by matrix cracking in transverse (90°) fiber bundles between 65 and 180 MPa, matrix cracking in longitudinal (0°) fiber bundles between 180 and 300 MPa, and fiber fragmentation above 300 MPa. A methodology for esti￾mation of unidirectional tensile behavior in orthogonal 3-D composites has been established by the use and modification of existing theory. A good correlation was obtained between the predicted and measured composite strain using this procedure. I. Introduction I T IS well known that monolithic ceramics do not possess a level of damage tolerance that is sufficient for aerospace applications. For this reason, a great deal of effort has been devoted to the development of continuous-ceramic-fiber-reinforced ceramic￾matrix composites (CMCs or CFCCs) for jet engine compo￾nents, rocket engine nozzles, and the thermal protection systems (TPS) of future space transportation vehicles.1–3 The National Aerospace Laboratory of Japan, Ube Industries Ltd., Shikibo Ltd., and Kawasaki Heavy Industries Ltd. have conducted a joint program in order to develop and evaluate a continuous￾fiber-reinforced CMC. The composite contains Tyranno™ Lox-M fiber (Si 54%, Ti 2%, C 32%, O 12% (mass%)) with an additional surface modification process. The fibers are woven into an orthogonal 3-D structure that has advantages from the points of view of the polymer impregnation and pyrolysis (PIP) processing, and improved delamination resistance and tensile strength. Using these technologies, the composite exhibits excellent tensile strength at room temperature4,5 and creep strength at elevated temperature.6 The composite is referred to as “NUSK-CMC” from the initials of the collaborating partners. It is now well understood that unidirectional (UD) CMCs exhibit nonlinear stress–strain behavior under unidirectional ten￾sile loading as a result of multiple microcracking and fiber pullout within the longitudinal (0°) fiber bundles. An overview of CMC mechanical properties has been proviede by Evans and Zok7 and Evans.8 The change in stiffness due to multiple matrix cracking has been estimated by two different approaches: (1) elastic analysis based on the Lame problem9 and (2) shear-lag analysis.10 In cross-ply composites, this change also involves initial cracking in the transverse (90°) plies as tunneling cracks.10–13 Subse￾quently, transverse cracks penetrate the longitudinal plies as the load is increased. Based on the energy criterion and finite element analysis, transverse crack propagation in cross-ply brittle-matrix composites has been analyzed.14 Shear-lag analysis is often used to estimate transverse crack propagation within polymer-matrix com￾posites,15,16 and this method has also been applied to cross-ply CMCs.10,13 Following matrix crack saturation, estimates for the fiber pullout length, pullout work, and the ultimate tensile strength of unidirectional CMCs have been derived by Curtin,17,18 based on the statistical analysis of fiber strength and stress redistribution due to fiber fragmentation processes. This paper presents experimental results for the multiple micro￾cracking and tensile behavior of an orthogonal 3-D woven Si-Ti￾C-O fiber/Si-Ti-C-O matrix composite (NUSK-CMC) fabricated using the PIP method. Constituent properties of the composite were estimated from hysteresis loop analysis of loading/unloading cycles. A methodology for estimating the unidirectional tensile behavior of orthogonal 3-D composites has been provided by using and modifying the above-mentioned theories. II. Experimental Procedure (1) Material and Specimens The composite under investigation contained Tyranno Lox-M fibers woven into an orthogonal 3-D configuration with fiber volume fractions of 19%, 19%, and 2% in the x, y, and z directions, respectively. Optical micrographs in Fig. 1 illustrate the fiber architecture of the present composites with each fiber bundle containing 1600 fibers. The composite preform plate (240 mm 3 120 mm 3 6 mm) was treated at elevated temperature in a CO atmosphere, resulting in the formation of a 10 nm SiOx-rich layer surrounding an inner 40 nm carbon-rich layer at the fiber surface.4 The nanoscale carbon-rich layer is believed to result in an interphase with desirable properties between the fiber and the matrix.5 Polytitanocarbosilane was used as the matrix precursor with eight impregnation and pyrolysis cycles; the average com￾posite bulk density was 2.20 g/cm3 . Tensile specimens were machined from the composite plates such that the loading direction was parallel to the y-axis. The specimen surfaces were also ground B. N. Cox—contributing editor Manuscript No. 188832. Received January 3, 2000; approved March 6, 2001. *Member, American Ceramic Society. † Currently with Nagoya Aerospace Systems, Mitsubishi Heavy Industries Ltd., Nagoya, Japan. J. Am. Ceram. Soc., 84 [7] 1565–74 (2001) 1565 journal