1050 Journal of the American Ceramic Sociery-Jacobsen and Brondsted Vol 84. No 5 shear loading 0094 sKU 0.1m Delamination cracks Vv ig. 15. Optic dure in C/SiC Fig. 13. Shear fracture of SiC/Sic amination in the 90 bundles and fiber breaks in the o bundles No fiber kinking is observe volume-dependent strength, because the gauge volume of the losipescu test is much smaller than the gauge volume of the tensile specimens. C Optical microscopy and SEM examinations of two plain-woven MCs show that fiber packing is very irregular, that large interbundle porosity is present, and that C/SiC is precracked in the Delamination 90 bundles. Therefore, a transition scheme for converting a plain weave to a cross-ply laminate has been applied, because it enables a simpler mechanics approach to the mechanical performance. In 10 mm the first step, the porosity is included in the matrix, and the SiC/SiC interphase is attached to the fiber, resulting in a reduction of the composite system to a fiber and matrix system with an interface Bundles decoupled by interlaminar cracks The second step consists of calculating the elastic properties of ult in unidirectional ply. The third step consists of calculating the elastic of failure bundle properties of the cross ply. Compared with the measured elastic properties, the agreement is good for both materials, and it concluded that the elastic properties of plain weaves can be calculated using the theory for a cross ply Fig. 14. Failure appearance of compressive specimens and failure mech- n tension, four characteristic damage stages are identified anism by interlaminar shear failur SiC/SiC undergoes all four stages, whereas C/SiC in its as received condition is in stage Ill, where the inelastic deformation is controlled by the 0 plies. Furthermore, we can simulate the tensile stress-strain behavior of C/Sic using a constant interfacial (26) damage parameter and a residual stress term. The tensile behavior of SiC/SiC is more complex, and the damage in the 90 plies has where gm. is the maximum interlaminar shear stress. For SiC/ e energy SiC, omax= 70 MPa,2 resulting in 0=6.3. A value of om Therefore, the mechanical analysis of laminated cross plies cannot 50 MPa has been measured for C/Sic by the manufacturer be applied to plain weaves in stage Il, because it is believed that in 0=5.5. Knock-down factors of 2. 44 for SiC/SiC and he fracture energy for tunneling cracking does not vary much C/SiC are needed to obtain agreement between the model because of small crack openings In the other stages, the agreement (26) and experiments, i.e., a reasonable compressive sive between theory and simulation is good prediction for plain-woven CVI-SiC composites would be e In compression, the stress-strain behavior is linear until very se to failure. Failure occurs by interlaminar matrix cracks, followed by bundle buckling. A simple criteria based on the maximum bundle misalignment angle, interlaminar shear strength and knock-down factor of "2.5 predicts the compressive strength The angle of the failure plane is coincident with the maximum The compressive strength of the materials is more than twice as angle of bundle misalignment. Therefore, it is proposed that high as the tensile strength, but strain-to-failure is of the same compressive failure is initiated by 0-oriented interlaminar crack resulting in"pinned-end" wavy bundle columns. Subsequently, the The shear behavior of SiC/Sic shows almost no hysteresis unt cracks link up and form the major crack plane, as shown in Fig 14. very close to the failure strain, indicating that the shear deforma- (C) Shear. The expected tensile strength at +45 to the fiber tion is controlled by matrix cracking and no delamination between directions is 2T, which, for both materials, exceeds the tensile the plies. C/SiC exhibits only a slightly decreasing shear stiffness, trength in the fiber directions S(Table If). This indicates a but large permanent deformations and hysteresis, indicating thatScom 5 sxy max u (26) where sxy max is the maximum interlaminar shear stress. For SiC/ SiC, sxy max 5 70 MPa,25 resulting in u 5 6.3°. A value of sxy max 5 50 MPa has been measured for C/SiC by the manufacturer, resulting in u 5 5.5°. Knock-down factors of 2.44 for SiC/SiC and 2.60 for C/SiC are needed to obtain agreement between the model in Eq. (26) and experiments, i.e., a reasonable compressive strength prediction for plain-woven CVI-SiC composites would be Scom 5 sxy max 2.5u (27) The angle of the failure plane is coincident with the maximum angle of bundle misalignment. Therefore, it is proposed that compressive failure is initiated by u-oriented interlaminar cracks, resulting in “pinned-end” wavy bundle columns. Subsequently, the cracks link up and form the major crack plane, as shown in Fig. 14. (C) Shear: The expected tensile strength at 645° to the fiber directions is ;2T, which, for both materials, exceeds the tensile strength in the fiber directions S (Table II). This indicates a volume-dependent strength, because the gauge volume of the Iosipescu test is much smaller than the gauge volume of the tensile specimens. VI. Summary Optical microscopy and SEM examinations of two plain-woven CMCs show that fiber packing is very irregular, that large interbundle porosity is present, and that C/SiC is precracked in the 90° bundles. Therefore, a transition scheme for converting a plain weave to a cross-ply laminate has been applied, because it enables a simpler mechanics approach to the mechanical performance. In the first step, the porosity is included in the matrix, and the interphase is attached to the fiber, resulting in a reduction of the composite system to a fiber and matrix system with an interface. The second step consists of calculating the elastic properties of a unidirectional ply. The third step consists of calculating the elastic properties of the cross ply. Compared with the measured elastic properties, the agreement is good for both materials, and it is concluded that the elastic properties of plain weaves can be calculated using the theory for a cross ply. In tension, four characteristic damage stages are identified. SiC/SiC undergoes all four stages, whereas C/SiC in its as￾received condition is in stage III, where the inelastic deformation is controlled by the 0° plies. Furthermore, we can simulate the tensile stress–strain behavior of C/SiC using a constant interfacial damage parameter and a residual stress term. The tensile behavior of SiC/SiC is more complex, and the damage in the 90° plies has to be fitted to an increasing tunnel-cracking-mode fracture energy. Therefore, the mechanical analysis of laminated cross plies cannot be applied to plain weaves in stage II, because it is believed that the fracture energy for tunneling cracking does not vary much because of small crack openings. In the other stages, the agreement between theory and simulation is good. In compression, the stress–strain behavior is linear until very close to failure. Failure occurs by interlaminar matrix cracks, followed by bundle buckling. A simple criteria based on the maximum bundle misalignment angle, interlaminar shear strength, and knock-down factor of ;2.5 predicts the compressive strength. The compressive strength of the materials is more than twice as high as the tensile strength, but strain-to-failure is of the same order. The shear behavior of SiC/SiC shows almost no hysteresis until very close to the failure strain, indicating that the shear deforma￾tion is controlled by matrix cracking and no delamination between the plies. C/SiC exhibits only a slightly decreasing shear stiffness, but large permanent deformations and hysteresis, indicating that Fig. 13. Shear fracture of SiC/SiC. Fig. 14. Failure appearance of compressive specimens and failure mech￾anism by interlaminar shear failure. Fig. 15. Optical microscopy photograph of compressive failure in C/SiC due to delamination in the 90° bundles and fiber breaks in the 0° bundles. No fiber kinking is observed. 1050 Journal of the American Ceramic Society—Jacobsen and Brøndsted Vol. 84, No. 5