Journal J.Am. Cera. Soc,84S]1043-5102001) Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites Torben K. Jacobsen and Povl brondsted Materials Research Department, Rise National Laboratory, DK-4000 Roskilde, Denmark The elastic and inelastic properties of a chemical vapor but it fails in providing average interfacial properties because of infiltrated (Cvn Sic matrix reinforced with either plain the few interfaces tested woven carbon fibers(C/siC)or SiC fibers (SiC/SiC) have been A further complication occurs for cross-ply or plain-woven investigated. It has been investigated whether the mechanics of composites that are most likely to be used in design of real a plain weave can be described using the theory of a cross-ply components. In cross-ply laminates, cracks in the transverse plies laminate, because it enables a simple mechanics approach to decrease the stiffness of the composite. 6, 12-22 For CMCs, this the nonlinear mechanical behavior. The influences of inter. stiffness decrease is quite severe because the ceramic matrix has a phase, fiber anisotropy, and porosity are included. The ap- similar or higher elastic modulus than the fiber. However, in iber/matrix system with an interface. The tensile behavior is more complex because of bundle waviness, cross-sectional shape fib described by five damage stages. C/SiC can be modeled using of the bundles, porous matrix, and large voids between the one damage stage and a constant damage parameter. The infiltrated bundles. 1, An efficient approach for analyzing the tensile behavior of SiC/SiC undergoes four damage stages. continuum behavior of these materials is continuum damage Stiffness reduction due to transverse cracks in the transverse mechanics, where various damage modes and directions are bundles is very different from cross-ply behavior. Compressive described by phenomenological damage parameters. 1,3 failure is initiated by interlaminar cracks between the fiber In this article, we attempt to treat the plain weave as bundles. The crack path is dictated by the bundle waviness. symmetric cross-ply laminate. Furthermore, the thickness an For siC/SiC, the compressive behavior is mostly linear to elastic properties of the interphase are included and the interphase failure. C/SiC exhibits initial nonlinear behavior because of and fiber are connected to form a modified fiber with properties residual crack openings, Above the point where the cracks dependent on these two constituents. The porosity in the matrix close, the compressive behavior is linear Global compressive counted for, resulting in a matrix with modified properties. The failure is characterized by a major crack oriented at a certain advantage of this approach is the straightforward use of multiple ingle to the axial loading. In shear, the matrix cracks orientate models for inelasticity reported in the literature. 2, 18.122735-39 in the principal tensile stress direction (i.e, 45 to the fiber Models for characteristic damage stages have been collected direction) with very high crack densities before failure, but from the literature and used for setting up a general methodology only Sic/SiC shows significant degradation in shear modulus. for modeling the tensile behavior of cross plies and possibly plain Hysteresis is observed during unloading/reloading sequences weaves. For one of the materials, the tensile behavior can be and increasing permanent strain described by a single, constant damage parameter, TLo. To provide a complete description of the in-plane mechanical properties, shear and compressive tests have been conducted. The nonlinear shear L. Introduction behavior is characterized experimentally with regard to stiffness degradation, permanent strain, and failure mode. Compressive failure appearance is studied, and directions for future modeling or composites( CMCs)is strongly linked to the elastic properties are suggested constant interfacial frictional shear stress t between fiber and matrix depends on volume fractions and elastic properties of the IL. Experimental Procedure constituents -6 variations in t on similar materials observed from Material experimenter to experimenter may reflect variations in the elastic properties and the underlying modeling assumptions Two plain-weave-based CMCs were tested. The materials were Pullout or pushout tests are widely used for estimating interfa- supplied by MAN-Technologie AG, Munich, Germany. The ma- cial properties of single fibers within a composi terials were processed using the CVI method. The fibers were precoated with a thin interphase layer of pyrolytic carbon using pproach particularly appropriate for the chemical vapor infiltra. CVI. Subsequently, the weave was infiltrated with Sic as matrix on(CvI)process is to coat a single fiber with an annulus of interphase( typically carbon or BN) and matrix material(typically material. Two different fibers were used: a carbon fiber(Torayca Sic) and load this microcomposite in tension. Common to all M30, Toray Industries, Ohtsu, Japan) and a Sic fiber(Tyran TY-SIHI6EL, UBE Industries, Yamaguchi, Japan). The Tyran one test specimen to the fiber was different from the Nicalon fiber used in other investig next are observed. The main advantage of the single-fiber test is tions 2, 6 23,24 26737,40 The material with the sic fiber was that it allows for qualitative process optimization of the interphase denoted SiC/SiC, and that with the carbon fiber C/SiC. The plates were 5 mm thick, which is twice the thickness of previous studies of SiC/SiC. Figure I shows the interior plain-woven structure of C/SiC. The bundles are point-wise connected from sheet to B. N. Cox--contributing editor sheet(similar appearance for SiC/SiC). This was also observed in Ref. 29 The fiber packing was nonuniform, with the highest fiber volume fraction in the center of the bundles. Small porosities Manuscript No 190608 Received October 28, 1997; approved March 15, 2000. existed within the bundles, and large interbundle pores existedMechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites Torben K. Jacobsen and Povl Brøndsted Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain￾woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of inter￾phase, fiber anisotropy, and porosity are included. The ap￾proach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain. I. Introduction THE modeling of the mechanical performance of ceramic-matrix composites (CMCs) is strongly linked to the elastic properties of the nominally damage-free material. The magnitude of the constant interfacial frictional shear stress t between fiber and matrix depends on volume fractions and elastic properties of the constituents.1–6 Variations in t on similar materials observed from experimenter to experimenter may reflect variations in the elastic properties and the underlying modeling assumptions.7 Pullout or pushout tests are widely used for estimating interfa￾cial properties of single fibers within a composite.7–10 Another approach particularly appropriate for the chemical vapor infiltra￾tion (CVI) process is to coat a single fiber with an annulus of interphase (typically carbon or BN) and matrix material (typically SiC) and load this microcomposite in tension.11 Common to all these methods is that large variations from one test specimen to the next are observed. The main advantage of the single-fiber test is that it allows for qualitative process optimization of the interphase, but it fails in providing average interfacial properties because of the few interfaces tested. A further complication occurs for cross-ply or plain-woven composites that are most likely to be used in design of real components. In cross-ply laminates, cracks in the transverse plies decrease the stiffness of the composite.6,12–22 For CMCs, this stiffness decrease is quite severe because the ceramic matrix has a similar or higher elastic modulus than the fiber. However, in CVI-manufactured plain-woven CMCs, the cracking behavior is more complex because of bundle waviness, cross-sectional shape of the bundles, porous matrix, and large voids between the infiltrated bundles.21,23–33 An efficient approach for analyzing the continuum behavior of these materials is continuum damage mechanics, where various damage modes and directions are described by phenomenological damage parameters.31,34 In this article, we attempt to treat the plain weave as a symmetric cross-ply laminate. Furthermore, the thickness and elastic properties of the interphase are included and the interphase and fiber are connected to form a modified fiber with properties dependent on these two constituents. The porosity in the matrix is accounted for, resulting in a matrix with modified properties. The advantage of this approach is the straightforward use of multiple models for inelasticity reported in the literature.12,18,22,35–39 Models for characteristic damage stages have been collected from the literature and used for setting up a general methodology for modeling the tensile behavior of cross plies and possibly plain weaves. For one of the materials, the tensile behavior can be described by a single, constant damage parameter, tL0. To provide a complete description of the in-plane mechanical properties, shear and compressive tests have been conducted. The nonlinear shear behavior is characterized experimentally with regard to stiffness degradation, permanent strain, and failure mode. Compressive failure appearance is studied, and directions for future modeling are suggested. II. Experimental Procedure (1) Materials Two plain-weave-based CMCs were tested. The materials were supplied by MAN-Technologie AG, Munich, Germany. The ma￾terials were processed using the CVI method. The fibers were precoated with a thin interphase layer of pyrolytic carbon using CVI. Subsequently, the weave was infiltrated with SiC as matrix material. Two different fibers were used: a carbon fiber (Torayca M30, Toray Industries, Ohtsu, Japan) and a SiC fiber (Tyranno TY-S1H16EL, UBE Industries, Yamaguchi, Japan). The Tyranno fiber was different from the Nicalon fiber used in other investiga￾tions.12,16,21,23,24,26,37,40 The material with the SiC fiber was denoted SiC/SiC, and that with the carbon fiber C/SiC. The plates were 5 mm thick, which is twice the thickness of previous studies of SiC/SiC. Figure 1 shows the interior plain-woven structure of C/SiC. The bundles are point-wise connected from sheet to sheet (similar appearance for SiC/SiC). This was also observed in Ref. 29. The fiber packing was nonuniform, with the highest fiber volume fraction in the center of the bundles. Small porosities existed within the bundles, and large interbundle pores existed B. N. Cox—contributing editor Manuscript No. 190608. Received October 28, 1997; approved March 15, 2000. J. Am. Ceram. Soc., 84 [5] 1043–51 (2001) 1043 journal