October 2007 Tensile Mechanical Properties of Ceramic Matrix Composites 500 7.9 epcm [Orgo From Table l, it can be seen that in-plane E values are similar 8.7 epcm [o/90 for all panels with the exception of the double-tow 5HS-woven composite(panel A3). This suggests that for the Sic/SiC system 350[0/467] double studied, the in-plane elastic modulus is not strongly dependent 3 95 epem [090 on 2D fiber architectures or on tensile-loading direction. For in- 300 f=0.39 plane UTS, the 2D-woven 0/90 panels aligned in the primary fiber axes are of course the strongest because they were test 8.7 epem [+45] parallel to the fiber direction. Nevertheless, the braided panel with over three-quarters of the fibers oriented 23 from the loading-axis displayed a high in-plane UTS; however, the [+45 panel displayed a relatively low UTS. But perhaps the most 50 striking mechanical results were the high in-plane DFLs values a3 for both the braided and the [+] panel 040.506 These high DFLS results correlate(Fig. 4 and Table In) with higher stress ranges over which high-energy ae events were re- corded in the off-axis panels. In Fig. 4. the aE activity versus Fig. 2. Stress-strain curves of off-axis and orthogonally aligned com- applied CMC stress is plotted as normalized cumulative AE en- psites with the hysteresis loop removed for clarity. An example of the ergy, which is the cumulative AE energy of each AE event up ysteresis loops for 0/+67 braid (I)is shown in the inset. a given event divided by the total AE energy of all the eve o For the MI siC/SiC system, it has been shown that this type of plot represents a good relative distribution of transverse or direction in order to ns were cut and polished along the TTMC. In addition, multiplying the final matrix crack density measured from polished sections after failure, by the normalized The polished specimens were plasma etched(CFa at 500 cumulative AE energy is a very good estimate of the actual for 30 min) in order to enhance the matrix cracks in the CvI sic stress-dependent matrix crack density versus applied stress. The of the matrix. matrix cracks were counted over lengths of neasured matrix crack densities at failure are shown in Table ll approximately 10 mm in order to obtain a matrix crack density. and the estimated matrix crack density with stress in Fig. 5. The difference in the shape of the matrix crack distribution between e double-tow and single-tow 2D-woven composites is hypoth esized to be due to the greater concentration and longer lengt II Results of back-to-back 90 minicomposites (see Section IV). Two The room-temperature tensile stress-strain curves for the off- axis-oriented SiC/SiC panels A4 and Bl and for the orthogonal ifferent off-axis-loaded specimens. The [+45] specimen has a oriented 2D-woven SiC/SiC panels Al-3 are shown in Fig. 2 ery steep curve, whereas the braided specimens were similar in Hysteresis loops used for residual stress determination have shape to the single-tow 2D composites. Neither of the off-axis been eliminated from these curves except for a braided panel composites appears to have reached matrix crack saturation be BI specimen, which is given as an example in Fig. 2. The values ause a decrease in the rate of ae activity was never achieved at of initial elastic modulus e. UTs. and residual stress on matrix her stresses. It is also interesting to note that at a composit stress of 200 MPa, the off-axis-tested braided specimens had are listed in Table I and are consistent with other composite little or no cracks: however, the double-tow 2D-woven compo panels fabricated with the same fiber types and fiber architec tures , As shown in Fig. 3, the offset strain construct method ites had -5 TTMC/mm. was also used to determine DFLs. It consists of drawing a line For this study, several aE criteria were used to evaluate stress levels near initiation of matrix cracking as these values ith the same slope as the initial elastic modulus, but offset by some amount of positive strain, where the dFls would be de- t upper design limits beyond which CMC mod- termined by the intersection of that curve with the stress-strain and axial thermal conductivity irreversibly decrease and the curve. Typical offset-strain values used are 0.005% and bility for adverse environmental effects for Sic/SiC com- 0.002%. 0 As indicated in Table II. both values were dete ites exists. These stress levels. which are indicated in Fig. 6 n this study and Table Il, are associated with(I)the first AE event, (2) the first loud ae event. which is defined as an ae event with an energy of at least one-tenth the highest energy event not corresponding to final failure of the composite, and ( 3)the effective AE onset stress, which is determined by extrapolation of the steep slope of the normalized cumulative AE energy with increasing stress back down to the zero energy axis(see Fig. 6 For the braided specimens, there was an initial increase in slope (AE activity) followed by a further increase in slope, which con- tinued at the higher rate until failure. For AE onset stress, the Stress-strain curve initial moderate AE slope(Fig. 6) was used because it was con- 0. 005% offset firmed that high-energy AE events were occurring in this regime It is evident in Table II that except for the first AE event, the 0. 002% offset other two stress measures for matrix cracking are significantl higher for the off-axis specimens than for the on-axis specimens. Underlying mechanisms and technical significance for these stress levels will be discussed in the following sections Strain. The effect of architecture and orientation on matrix cracking(or Fig3. Deviation from linearity-stress construction for a braided spec- DFLS) and UTs are key properties that need to be understood imen. Note, only the low strain portion of the stress-strain curve is for applications using MI SiC/SiC composites under multiaxial plotted ress states. In the following sections, the results shown in Figs.The tested specimens were cut and polished along the loading direction in order to measure transverse matrix cracks optically. The polished specimens were plasma etched (CF4 at 500 Watts for 30 min) in order to enhance the matrix cracks in the CVI SiC of the matrix. Matrix cracks were counted over lengths of approximately 10 mm in order to obtain a matrix crack density.7 III. Results The room-temperature tensile stress–strain curves for the off￾axis-oriented SiC/SiC panels A4 and B1 and for the orthogonal￾oriented 2D-woven SiC/SiC panels A1–3 are shown in Fig. 2. Hysteresis loops used for residual stress determination have been eliminated from these curves except for a braided panel B1 specimen, which is given as an example in Fig. 2. The values of initial elastic modulus E, UTS, and residual stress on matrix are listed in Table I and are consistent with other composite panels fabricated with the same fiber types and fiber architec￾tures.4,7 As shown in Fig. 3, the offset strain construct method8 was also used to determine DFLS. It consists of drawing a line with the same slope as the initial elastic modulus, but offset by some amount of positive strain, where the DFLS would be de￾termined by the intersection of that curve with the stress–strain curve. Typical offset-strain values used are 0.005%9 and 0.002%.10 As indicated in Table II, both values were determined in this study. From Table I, it can be seen that in-plane E values are similar for all panels with the exception of the double-tow 5HS-woven composite (panel A3). This suggests that for the SiC/SiC system studied, the in-plane elastic modulus is not strongly dependent on 2D fiber architectures or on tensile-loading direction. For in￾plane UTS, the 2D-woven 0/90 panels aligned in the primary fiber axes are of course the strongest because they were tested parallel to the fiber direction. Nevertheless, the braided panel with over three-quarters of the fibers oriented 231 from the loading-axis displayed a high in-plane UTS; however, the [745] panel displayed a relatively low UTS. But perhaps the most striking mechanical results were the high in-plane DFLS values for both the braided and the [745] panels. These high DFLS results correlate (Fig. 4 and Table II) with higher stress ranges over which high-energy AE events were re￾corded in the off-axis panels. In Fig. 4, the AE activity versus applied CMC stress is plotted as normalized cumulative AE en￾ergy, which is the cumulative AE energy of each AE event up to a given event divided by the total AE energy of all the events. For the MI SiC/SiC system, it has been shown7 that this type of plot represents a good relative distribution of transverse or TTMC. In addition, multiplying the final matrix crack density, measured from polished sections after failure, by the normalized cumulative AE energy is a very good estimate of the actual stress-dependent matrix crack density versus applied stress.7 The measured matrix crack densities at failure are shown in Table II and the estimated matrix crack density with stress in Fig. 5. The difference in the shape of the matrix crack distribution between the double-tow and single-tow 2D-woven composites is hypoth￾esized to be due to the greater concentration and longer lengths of back-to-back 901 minicomposites (see Section IV). Two different matrix crack distributions are evident for the two different off-axis-loaded specimens. The [745] specimen has a very steep curve, whereas the braided specimens were similar in shape to the single-tow 2D composites. Neither of the off-axis composites appears to have reached matrix crack saturation be￾cause a decrease in the rate of AE activity was never achieved at higher stresses. It is also interesting to note that at a composite stress of 200 MPa, the off-axis-tested braided specimens had little or no cracks; however, the double-tow 2D-woven compos￾ites had B5 TTMC/mm. For this study, several AE criteria were used to evaluate key stress levels near initiation of matrix cracking as these values typically represent upper design limits beyond which CMC mod￾uli and axial thermal conductivity irreversibly decrease and the possibility for adverse environmental effects for SiC/SiC com￾posites exists.11 These stress levels, which are indicated in Fig. 6 and Table II, are associated with (1) the first AE event, (2) the first loud AE event, which is defined as an AE event with an energy of at least one-tenth the highest energy event not corresponding to final failure of the composite, and (3) the effective AE onset stress,7 which is determined by extrapolation of the steep slope of the normalized cumulative AE energy with increasing stress back down to the zero energy axis (see Fig. 6). For the braided specimens, there was an initial increase in slope (AE activity) followed by a further increase in slope, which con￾tinued at the higher rate until failure. For AE onset stress, the initial moderate AE slope (Fig. 6) was used because it was con- firmed that high-energy AE events were occurring in this regime. It is evident in Table II that except for the first AE event, the other two stress measures for matrix cracking are significantly higher for the off-axis specimens than for the on-axis specimens. Underlying mechanisms and technical significance for these stress levels will be discussed in the following sections. IV. Analysis and Discussion The effect of architecture and orientation on matrix cracking (or DFLS) and UTS are key properties that need to be understood for applications using MI SiC/SiC composites under multiaxial stress states. In the following sections, the results shown in Figs. Fig. 2. Stress–strain curves of off-axis and orthogonally aligned com￾posites with the hysteresis loop removed for clarity. An example of the hysteresis loops for 0/767 braid (1) is shown in the inset. 0 50 100 150 200 250 0 0.02 0.04 0.06 0.08 0.1 Strain, % Stress, MPa Stress-strain curve 0.002% offset 0.005% offset Fig. 3. Deviation from linearity-stress construction for a braided spec￾imen. Note, only the low strain portion of the stress–strain curve is plotted. October 2007 Tensile Mechanical Properties of Ceramic Matrix Composites 3187