J.Am. Ceran.Soc,90o3185-3193(2007 DOL: 10.1111 C 2007 The American No claim to original US government works urna In-Plane Cracking behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber directions gregory n. morscher Ohio Aerospace Institute, Cleveland, Ohio Hee mann yun Matech GSM. Irvine. Califor NASA Glenn Research Center. Cleveland, ohio The tensile mechanical properties of ceramic matrix composites referred to as proportional limit stress), and fiber-pullout mech- CMC in directions off the primary axes of the reinforcing fi- anisms that lead to graceful failure and high ultimate tensile bers are important for the architectural design of CMc com- strength (UTS). This is the case because the high-modulus fi nents that are subjected to multiaxial stress states. In this bers are oriented to carry much of the tensile load applied to the study, two-dimensional (2D)woven melt-infiltrated (MD) sic composite before and after the dfls point, which is caused by Sic composite panels with balanced fiber content in the 0 ai matrix cracks 90 directions were tensile loaded in-plane in the 0 direction and (TTMc)in the CMC. However, when loaded in a direction at a at 45 to this direction. In addition, a 2D triaxially braided MI significant angle to the primary fiber axes, large reductions in Sic/SiC composite panel with a higher fiber content in the key CMC design properties such as low DFLS and UTS can +67 bias directions compared with the axial direction was occur. Whereas on-axis properties are strongly dependent on tensile loaded perpendicular to the axial direction tows (i.e 23 fiber properties, off-axis properties are strongly dependent on from the bias fibers). Stress-strain behavior, acoustic emission, matrix properties, particularly on their stiffness and load-carry nd optical microscopy were used to quantify stress-dependent ing ability, which are typically related to their porosity content matrix cracking and ultimate strength in the panels. It was ob- For example, when CMC with highly porous oxide matrices served that both off-axis-loaded panels displayed higher com- were tested off-axis, highly nonlinear stress-strain behavior and posite onset stresses for through-thickness matrix cracking elatively weak strengths were observed because the porous ma- he 2D-woven 0/90 panels loaded in the primary 0 direction. trix could not carry significant load But when the matrix stiff- d load-c be attributed to higher effective fiber fractions in the loading porous matrix, higher dFl stresses and ultimate strengths were direction, which in turn reduces internal stresses on weak regions obtained, but still not as high as the on-axis value ite tows oriented normal For CMC structural applications, high stresses for TTMC are the loading direction and or critical flaws in the matrix for enerally desired in all directions. This not only allows mainte- posite stress. Both off-axis-oriented panels also nance of composite modulus and thermal conductivity to high showed relatively good ultimate tensile strength when compared stresses but also results in greater composite life for CMC with with other off-axis-oriented composites in the literature, both on nonoxide constituents that can be degraded by environmental an absolute strength basis as well as when normalized by the permeability into TTMC. To achieve these high cracking stress- average fiber strength within the composites. Initial implications es, high-stiffness low-porosity matrices are generally preferred are discussed for constituent and architecture design to improve hich can also help to improve the CMc creep-rupture resis- the directional cracking of SiC/SiC CMC components with MI tance and thermal conductivity. One such CMC system is the matrices melt-infiltrated (MD) SiC matrix system reinforced by the syl- ramic-iBN SiC fiber that is near-stoichiometric in composition and contains a thin in sittt-grown BN layer on its surface. This L. Introducti Sic/SiC composite system is typically processed by taking a woven or braided fiber-preform, coating the fibers with another HE tensile mechanical properties of continuous fiber-rein- thin bn layer by chemical vapor infiltration(CVD, and then forced ceramic matrix composites( CMC) vary according to forming an initial SiC matrix by CV. The remaining matrix the orientation of fibers with respect to the loading direction porosity is then filled with a slurry of SiC particles. After drying CMC mechanical properties are typically measured for tensile the slurry-infiltrated preform, final matrix formation is by mel stresses in a direction parallel to one of the primary fiber axes, infiltration (MD) of liquid Si, which fills nearly all of the large hich generally results in desirable linear stress-st ores in the structure, leaving 5% closed porosity a high stress for deviation from linearity or(DFLS)(also The primary objective of this study was to measure and an- alyze the off-axis in-plane tensile stress-strain behavior of five F. Zok--contributing editor thin-walled panels with the MI Sylramic-iBNSIC system con taining two different basic fiber architectures. A and B. Four panels with architecture A, consisting of a two-dimensional (2D)-woven fabric lay-up with balanced fiber content in the 0 anuscript No. 22879 Received March 5. 2007; approved June 2.2007. uthor to whom correspondence should be addressed. e-mail: gmorscher(a and 90 directions, were tensile loaded in the 0 direction(panels grc.nasa.go Al-3)and at 45. to this direction(panel A4). Another panel 3185In-Plane Cracking Behavior and Ultimate Strength for 2D Woven and Braided Melt-Infiltrated SiC/SiC Composites Tensile Loaded in Off-Axis Fiber Directions Gregory N. Morscherw Ohio Aerospace Institute, Cleveland, Ohio Hee Mann Yun Matech GSM, Irvine, California James A. DiCarlo NASA Glenn Research Center, Cleveland, Ohio The tensile mechanical properties of ceramic matrix composites (CMC) in directions off the primary axes of the reinforcing fi- bers are important for the architectural design of CMC com￾ponents that are subjected to multiaxial stress states. In this study, two-dimensional (2D)-woven melt-infiltrated (MI) SiC/ SiC composite panels with balanced fiber content in the 01 and 901 directions were tensile loaded in-plane in the 01 direction and at 451 to this direction. In addition, a 2D triaxially braided MI SiC/SiC composite panel with a higher fiber content in the 7671 bias directions compared with the axial direction was tensile loaded perpendicular to the axial direction tows (i.e., 231 from the bias fibers). Stress–strain behavior, acoustic emission, and optical microscopy were used to quantify stress-dependent matrix cracking and ultimate strength in the panels. It was ob￾served that both off-axis-loaded panels displayed higher com￾posite onset stresses for through-thickness matrix cracking than the 2D-woven 0/90 panels loaded in the primary 01 direction. These improvements for off-axis cracking strength can in part be attributed to higher effective fiber fractions in the loading direction, which in turn reduces internal stresses on weak regions in the architecture, e.g., minicomposite tows oriented normal to the loading direction and/or critical flaws in the matrix for a given composite stress. Both off-axis-oriented panels also showed relatively good ultimate tensile strength when compared with other off-axis-oriented composites in the literature, both on an absolute strength basis as well as when normalized by the average fiber strength within the composites. Initial implications are discussed for constituent and architecture design to improve the directional cracking of SiC/SiC CMC components with MI matrices. I. Introduction THE tensile mechanical properties of continuous fiber-rein￾forced ceramic matrix composites (CMC) vary according to the orientation of fibers with respect to the loading direction. CMC mechanical properties are typically measured for tensile stresses in a direction parallel to one of the primary fiber axes, which generally results in desirable linear stress–strain behavior, a high stress for deviation from linearity or (DFLS) (also referred to as proportional limit stress), and fiber-pullout mech￾anisms that lead to graceful failure and high ultimate tensile strength (UTS).1 This is the case because the high-modulus fi- bers are oriented to carry much of the tensile load applied to the composite before and after the DFLS point, which is caused by the development of transverse through-thickness matrix cracks (TTMC) in the CMC. However, when loaded in a direction at a significant angle to the primary fiber axes, large reductions in key CMC design properties such as low DFLS and UTS can occur.2 Whereas on-axis properties are strongly dependent on fiber properties, off-axis properties are strongly dependent on matrix properties, particularly on their stiffness and load-carry￾ing ability, which are typically related to their porosity content. For example, when CMC with highly porous oxide matrices were tested off-axis, highly nonlinear stress–strain behavior and relatively weak strengths were observed because the porous ma￾trix could not carry significant load.3 But when the matrix stiff￾ness and load-carrying ability were increased via sintering the porous matrix, higher DFL stresses and ultimate strengths were obtained, but still not as high as the on-axis value. For CMC structural applications, high stresses for TTMC are generally desired in all directions. This not only allows mainte￾nance of composite modulus and thermal conductivity to high stresses but also results in greater composite life for CMC with nonoxide constituents that can be degraded by environmental permeability into TTMC. To achieve these high cracking stress￾es, high-stiffness low-porosity matrices are generally preferred, which can also help to improve the CMC creep-rupture resis￾tance and thermal conductivity. One such CMC system is the melt-infiltrated (MI) SiC matrix system reinforced by the Syl￾ramic-iBN SiC fiber that is near-stoichiometric in composition and contains a thin in situ-grown BN layer on its surface.4 This SiC/SiC composite system is typically processed by taking a woven or braided fiber-preform, coating the fibers with another thin BN layer by chemical vapor infiltration (CVI), and then forming an initial SiC matrix by CVI. The remaining matrix porosity is then filled with a slurry of SiC particles. After drying the slurry-infiltrated preform, final matrix formation is by melt infiltration (MI) of liquid Si, which fills nearly all of the large pores in the structure, leaving B5% closed porosity. The primary objective of this study was to measure and an￾alyze the off-axis in-plane tensile stress–strain behavior of five thin-walled panels with the MI Sylramic–iBN/SiC system con￾taining two different basic fiber architectures, A and B. Four panels with architecture A, consisting of a two-dimensional (2D)-woven fabric lay-up with balanced fiber content in the 01 and 901 directions, were tensile loaded in the 01 direction (panels A1–3) and at 451 to this direction (panel A4). Another panel F. Zok—contributing editor w Author to whom correspondence should be addressed. e-mail: gmorscher@ grc.nasa.gov Manuscript No. 22879. Received March 5, 2007; approved June 2, 2007. Journal J. Am. Ceram. Soc., 90 [10] 3185–3193 (2007) DOI: 10.1111/j.1551-2916.2007.01887.x r 2007 The American Ceramic Society No claim to original US government works 3185