October 2007 Tensile Mechanical Properties of Ceramic Matrix Composites 3189 orthogonal this study D[o+67] double tow braid (hoop direction)-this study Fiber Fraction in Loading Direction emission(AE) matrix cracking(onset)stress for two- osites with fibers tows oriented perpendicular to the loading direction. panel displays the lowest onset stress and the smallest stress dis- tribution for TTMC in the on-axis panels In an effort to model architectural effects on the onset of TTMC in 2D-woven 0/90 SiC/SiC composites tested in the 0o direction, a previous study has examined the important micro- tructural factors affecting the internal stresses on the 90 mini composites since these appear to be the most critical flaws within the matrix that eventually cause TTMC. Two important facto 1 mm were identified: (1)a built-in residual stress on the matrix and (2) the applied composite stress as modified by the load shared by the 0%minicomposites. As such, one can estimate the stress on the matrix region containing the 90 minicomposites(=mini- matrix) by the following simple rule of mixtures relationship (oc +Oth)Ec-fmini em ominimatrix 1) Fig 8. Micrograph of a two-dimensional Ec posite after room temperature tensile failure urce.Most notably, there was also a very na Here,σ is the composite stress, Oth is the residual tribution for TTMC(Fig. 4), which would corres ess in the matrix Ec is the measured composite ela population(high Weibull modulus)of flaws and tic modulus from the oe curve (Table I): mini is the volume that propagate through-thickness at and slightly above the ma ically -2 times the fiber content in the 0 direction for the com-(panel A4) displayed a higher onset stress for TTMC than its can be reduced by increasing a compressive residual stress on the cracking stress and the narrow stress-distribution for matrix matrix and/or by increasing mini. As shown in Table l, one ben acking for this system is primarily due to a lack of minicom- efit of the mi sic/SiC system is a compressive residual stress on posites perpendicular to the direction of stress so that higher the as-fabricated matrix, but this stress did not vary much with tresses are required for tunnel crack propagation at an ang the 2D architectures of this study. The exact source of this stress 45 to the loading axis. There is also the possibility that the is not completely understood, but probably can urce of matrix cracking are surface flaws or pores in the ma silicon content in the matrix and to the composite fabrication trix and not the +45-oriented minicomposites Matrix cracks conditions. On the other hand, fiber content in the or primary were observed to be perpendicular to the loading direction as MI SiC/SiC system, which in turn should increase the composite stress for the onset of ttmc. that this latter mechanism can indeed be utilized is seen in Fig. 7, which plots as open circles the Ae onset cracking strength as a function of primary for the (3) Matrix Cracking in 01+67 2D-Braided Composite Tested 0o-loaded 0/90 MI SiC/SiC composites of this study and for a in the Hoop Direction For the triaxially braided specimen, approximately 77% of the fibers or a total fiber fraction of 0. 26 were oriented 23 from the loading axis. This fraction is effectively higher than that of the (2) Matrix Cracking in 0/90 2D-Woren Composites Tested 2D-woven composites loaded in the primary fiber direction, a rhe45° Direction thus appears to be one cause for the higher matrix cracking The stress-distribution for matrix cracking in the [+45]off-axis stress for the braided composite. As described above for the 0/90 panel A4 differs considerably from those of the [o/90] compos- composites, matrix cracks emanate from the region outside the ites tested in a 0 or fiber direction first ae events show that load-bearing minicomposites, e.g., from the minicomposites ori some small microcracks were formed at low stresses in the [t45 ented 90 to the loading axis. The same type of analysis can be composite (Table ID), the source of which has not been deter- applied here to the braided composites to determine whether the mined. There was considerable porosity in the matrix region stress ranges in the TTMC flaw-source regions(axial tow mini- tween the outer two plies ( Fig 8), which could have been one composites, see Fig. 1)of the matrix are similar to standardpanel displays the lowest onset stress and the smallest stress dis￾tribution for TTMC in the on-axis panels. In an effort to model architectural effects on the onset of TTMC in 2D-woven 0/90 SiC/SiC composites tested in the 01 direction, a previous study7 has examined the important micro￾structural factors affecting the internal stresses on the 901 mini￾composites since these appear to be the most critical flaws within the matrix that eventually cause TTMC. Two important factors were identified: (1) a built-in residual stress on the matrix and (2) the applied composite stress as modified by the load shared by the 01 minicomposites. As such, one can estimate the stress on the matrix region containing the 90 minicomposites ( 5 mini￾matrix) by the following simple rule of mixtures relationship7 : sminimatrix ¼ ðsc þ sthÞ Ec Ec fminiEmini 1 fmini
(1) Here, sc is the applied composite stress, sth is the residual stress in the matrix (Table I); Ec is the measured composite elas￾tic modulus from the se curve (Table I); fmini is the volume fraction of the 01 minicomposites in the loading direction (typ￾ically B2 times the fiber content in the 01 direction for the com￾posites of this study); and Emini is the effective modulus of these 01 minicomposites. Thus, the stress on the 90 minicomposites can be reduced by increasing a compressive residual stress on the matrix and/or by increasing fmini. As shown in Table I, one ben￾efit of the MI SiC/SiC system is a compressive residual stress on the as-fabricated matrix, but this stress did not vary much with the 2D architectures of this study. The exact source of this stress is not completely understood, but probably can be related to the silicon content in the matrix and to the composite fabrication conditions. On the other hand, fiber content in the 01 or primary fiber direction, fprimary, can be increased to a large degree for the MI SiC/SiC system, which in turn should increase the composite stress for the onset of TTMC. That this latter mechanism can indeed be utilized is seen in Fig. 7, which plots as open circles the AE onset cracking strength as a function of fprimary for the 01-loaded 0/90 MI SiC/SiC composites of this study and for a previous study.7 (2) Matrix Cracking in 0/90 2D-Woven Composites Tested in the 451 Direction The stress-distribution for matrix cracking in the [745] off-axis panel A4 differs considerably from those of the [0/90] compos￾ites tested in a 01 or fiber direction. First AE events show that some small microcracks were formed at low stresses in the [745] composite (Table II), the source of which has not been deter￾mined. There was considerable porosity in the matrix region between the outer two plies (Fig. 8), which could have been one source. Most notably, there was also a very narrow stress-dis￾tribution for TTMC (Fig. 4), which would correspond to a large population (high Weibull modulus) of flaws and/or microcracks that propagate through-thickness at and slightly above the ma￾trix cracking stress. The important fact is that this composite (panel A4) displayed a higher onset stress for TTMC than its theoretically equivalent 0/90 composite (panel A1) loaded along its primary fiber axis. It is suggested that the higher matrix cracking stress and the narrow stress-distribution for matrix cracking for this system is primarily due to a lack of minicom￾posites perpendicular to the direction of stress so that higher stresses are required for tunnel crack propagation at an angle 451 to the loading axis. There is also the possibility that the source of matrix cracking are surface flaws or pores in the ma￾trix and not the 7451-oriented minicomposites. Matrix cracks were observed to be perpendicular to the loading direction as has been reported before.2 (3) Matrix Cracking in 0/767 2D-Braided Composite Tested in the Hoop Direction For the triaxially braided specimen, approximately 77% of the fibers, or a total fiber fraction of 0.26, were oriented 231 from the loading axis. This fraction is effectively higher than that of the 2D-woven composites loaded in the primary fiber direction, and thus appears to be one cause for the higher matrix cracking stress for the braided composite. As described above for the 0/90 composites, matrix cracks emanate from the region outside the load-bearing minicomposites, e.g., from the minicomposites ori￾ented 901 to the loading axis. The same type of analysis can be applied here to the braided composites to determine whether the stress ranges in the TTMC flaw-source regions (axial tow mini￾composites, see Fig. 1) of the matrix are similar to standard 100 120 140 160 180 200 220 240 0.1 0.15 0.2 0.25 0.3 Fiber Fraction in Loading Direction AE Matrix Cracking Stress, MPa 2D orthogonal [7] 2D orthogonal - this study 2D double tow orthogonal [7] 2D double tow orthogonal this study [0/+67] double tow braid (hoop direction) - this study Fig. 7. Effect of fiber fraction in the loading direction on acoustic emission (AE) matrix cracking (onset) stress for two-dimensional com￾posites with fibers tows oriented perpendicular to the loading direction. Fig. 8. Micrograph of a two-dimensional woven [745]-oriented com￾posite after room temperature tensile failure. October 2007 Tensile Mechanical Properties of Ceramic Matrix Composites 3189