G N. Morscher Composites Science and Technology 64 (2004)1311-1319 erformed on 3D architecture composites with double 017;RTp=120° tows in the load-bearing direction and single-tows per pendicular to the loading-direction, the identical narrow 007: RTD =8 stress-range versus ominimatrix was found to exist [25] tress This implies that the narrow stress range for matrix cracking was due to the larger fiber-bridged tow size in he loading-direction because only single-tows were used 0.41stress-rupture. in the 90 direction for these 3D composites. However, the absolute stress-range for matrix cracking would still be dependent on the type of woven architecture and 044RTp=9.50° concentration of composite constituents Finally, it should be noted that the specific results of this study only apply to the Sylramic/BN/MI properties Stress. MPa from this vendor. Changing the fiber, interphase, matrix, Fig. 9. Crack density measured in the load-bearing minicomposites or possibly even the vendor could alter the absolute nature after stress-rupture at 815. C in air compared with the AE activity at of matrix cracking of that composite system compared to room temperature for specimens from the same panel for two different what was found in this study. However, the methodology and approach taken here would be expected to be appli cable for a different composite system as a way to char occurs after initial loading for stress-rupture tests at acterize the stress-dependent crack distribution 815C[24, i.e., little matrix crack formation or growth occurs after attaining the constant stress condition. This confirms that matrix cracks in these composites are 5. Co through-thickness, i.e., progress through the MI matrix, even though not visible after plasma-etch A relationship for matrix cracking versus applied The differences between the double-tow woven and stress was effectively established by assuming that matrix single-tow woven matrix cracking behaviors were strik- cracks originate in the region of the material outside of ing. The double-tow woven composites have shown the load-bearing minicomposite for the 2D woven Syl higher composite strengths for the same volume fraction ramic fiber, BN interphase, melt-infiltrated SiC matrix omposites [17]. However, the relatively lower stress composite system. This was confirmed for a number of range for matrix cracking may be detrimental for use of composite specimens processed with different fiber ar these composites at higher stresses due to strength-de- chitectures and numbers of plies resulting in varied grading oxidation processes. The much narrower matrix concentrations of composite constituents; however, all crack distribution for the double-tow composites ap having the same essential flaw-producer, a standard proaches the more classical assumption of a single ma- sized 90 fiber tow. When an effective larger woven fiber trix crack stress [3]. The narrow matrix cracking stress tow(two standard tows woven together)was used,a range must be due to the larger fiber-count tow. Two different stress-dependent matrix crack distribution possibilities seem apparent: (I)the height of the 90 observed. A simple mathematical relationship for stress bundles is large creating larger effective minimatrix flaws dependent matrix cracking was used to describe the and/or(2) larger but fewer and more spaced out fiber- stress-dependent matrix crack density from which the bridged regions. The dimensions of the 90 bundle, i. e, stress-strain response for a composite component for height and width, were measured for a number of the areas of different thickness, numbers of plies, and/or 2D standard tow composites tested (007, 012, 017, 011, 044) fiber architectures could be modeled. The stress-depen and the 041 double-tow woven composite from polished dent matrix crack distribution also provides the starting wenty-five 90 tows were measured from crack density for modeling the elevated temperature each specimen. The size for all of the standard tow properties of these composites when subjected to me- woven composites fell into a range of h=0. 13+0.01 chanical loads at temperature mm and w=1. 14+0.04 mm. The size for the 041 double-tow woven composite was h=0.12 and w 2. 34. For each individual specimen the scatter was x10%. Acknowledgements Therefore, the height of the tow, presumably the im- portant dimension for flaw size was nearly identical for This work was supported by the Ultra Efficient Engine the standard tow and double-tow woven composites. Technology (UEET) program at NASA Glenn Research However, the load-bearing area of the tow was of course Center. The author also appreciated the suggestion by double for the double-tow woven with twice the number Dr William Curtin of Brown University to assume matrix of fibers per effective load-bearing tow. In recent tests cracking occurs outside the load-bearing minicompositeoccurs after initial loading for stress-rupture tests at 815
C [24], i.e., little matrix crack formation or growth occurs after attaining the constant stress condition. This confirms that matrix cracks in these composites are through-thickness, i.e., progress through the MI matrix, even though not visible after plasma-etch. The differences between the double-tow woven and single-tow woven matrix cracking behaviors were strik￾ing. The double-tow woven composites have shown higher composite strengths for the same volume fraction composites [17]. However, the relatively lower stress range for matrix cracking may be detrimental for use of these composites at higher stresses due to strength-de￾grading oxidation processes. The much narrower matrix crack distribution for the double-tow composites ap￾proaches the more classical assumption of a single ma￾trix crack stress [3]. The narrow matrix cracking stress range must be due to the larger fiber-count tow. Two possibilities seem apparent: (1) the height of the 90
bundles is large creating larger effective minimatrix flaws and/or (2) larger but fewer and more spaced out fiber￾bridged regions. The dimensions of the 90
bundle, i.e., height and width, were measured for a number of the standard tow composites tested (007, 012, 017, 011, 044) and the 041 double-tow woven composite from polished micrographs. Twenty-five 90
tows were measured from each specimen. The size for all of the standard tow woven composites fell into a range of h ¼ 0:13 0:01 mm and w ¼ 1:14 0:04 mm. The size for the 041 double-tow woven composite was h ¼ 0:12 and w ¼ 2:34. For each individual specimen the scatter was
10%. Therefore, the height of the tow, presumably the im￾portant dimension for flaw size was nearly identical for the standard tow and double-tow woven composites. However, the load-bearing area of the tow was of course double for the double-tow woven with twice the number of fibers per effective load-bearing tow. In recent tests performed on 3D architecture composites with double￾tows in the load-bearing direction and single-tows per￾pendicular to the loading-direction, the identical narrow stress-range versus rminimatrix was found to exist [25]. This implies that the narrow stress range for matrix cracking was due to the larger fiber-bridged tow size in the loading-direction because only single-tows were used in the 90
direction for these 3D composites. However, the absolute stress-range for matrix cracking would still be dependent on the type of woven architecture and concentration of composite constituents. Finally, it should be noted that the specific results of this study only apply to the Sylramic/BN/MI properties from this vendor. Changing the fiber, interphase, matrix, or possibly even the vendor could alter the absolute nature of matrix cracking of that composite system compared to what was found in this study. However, the methodology and approach taken here would be expected to be appli￾cable for a different composite system as a way to char￾acterize the stress-dependent crack distribution. 5. Conclusions A relationship for matrix cracking versus applied stress was effectively established by assuming that matrix cracks originate in the region of the material outside of the load-bearing minicomposite for the 2D woven Syl￾ramic fiber, BN interphase, melt-infiltrated SiC matrix composite system. This was confirmed for a number of composite specimens processed with different fiber ar￾chitectures and numbers of plies resulting in varied concentrations of composite constituents; however, all having the same essential flaw-producer, a standard sized 90
fiber tow. When an effective larger woven fiber tow (two standard tows woven together) was used, a different stress-dependent matrix crack distribution was observed. A simple mathematical relationship for stress￾dependent matrix cracking was used to describe the stress-dependent matrix crack density from which the stress–strain response for a composite component for areas of different thickness, numbers of plies, and/or 2D fiber architectures could be modeled. The stress-depen￾dent matrix crack distribution also provides the starting crack density for modeling the elevated temperature properties of these composites when subjected to me￾chanical loads at temperature. Acknowledgements This work was supported by the Ultra Efficient Engine Technology (UEET) program at NASA Glenn Research Center. The author also appreciated the suggestion by Dr. William Curtin of Brown University to assume matrix cracking occurs outside the load-bearing minicomposite. Fig. 9. Crack density measured in the load-bearing minicomposites after stress-rupture at 815
C in air compared with the AE activity at room temperature for specimens from the same panel for two different panels. 1318 G.N. Morscher / Composites Science and Technology 64 (2004) 1311–1319