Journal of the American Ceramic Society-Morscher and Martines-Fernandes Vol. 82. Ne stress. For single-fiber microcomposites, a direct relationship 12E+00 existed between the AE energy and number of cracks pre luced. Figure al shows the data from a single-fiber SCS-6 CVD-SiC fiber with a diameter of 143 um, Textron Specialt 1E+006 Materials, Lowell, MA)microcomposite tensile test.29The same tensile-test setup as that described by Morscher et al 800000 was used. The load was increased to a set load and held at that Nic-3MBN load. The cracks were opened and could be observed via ste Hi-Ni reomicroscopy. The position of each crack and the cumulative PBN AE energy at the load were recorded. The load was then in- creased and held, and the crack position and AE energy were again recorded. This procedure was continued up to a load of 45N. which did not cause failure. The number of cracks and AE energy was normalized by the normalize ea cracl 20000 mulative ae energy at 45 N and plotted in Fig. Al. The not malized number of cracks and aE energy, as a function of load Syl-PBN are almost identical, indicating a linear relationship between the ae energy and the number of cracks that are formed. the 050100150200250300350 epoxy mounting and gripping technique for this microcompos of Cracks In Gauge ite test was exactly the same as used for the estimated from crack spacing cause matrix cracks were not observable on the surface Fig. A2, nship between crack formation and AE energy fo of minicomposites, the relationship between AE energy and minicomposites. Each data point corresponds to a differe the number of cracks must be determined indirectly, i. e, after the tensile test. The crack spacing at failure was determined for each minicomposite from polished longitudinal sections of a major portion of the gauge section. The number of and interface sliding events to the cumulative AE energy is cracks formed during the tensile test in the gauge section was very small. On work with macrocomposite where it was then approximated from the crack spacing of the polished por possible to convincingly separate fiber-failure events from ma- trix-crack events, the fiber-failure events had amplitudes and failure. It was assumed that the energy of an ae source event energies that were orders of magnitude lower than the higher- from a matrix crack was proportional to the area of the mini- omposite. Therefore, absolute cumulative AE energy normal ergy approach was taken in the macrocomposite work that was ed by the minicomposite area was plotted versus the number ed in this study fiber failure would only account for-2.5% of cracks in the gauge, as shown in Fig. A2. Each data point in the total cumulative energy of the tensile test after failure Fig. A2 represents a different minicomposite that has been Matrix cracking would have accounted for at least 85% of the tested to failure. There does seem to be a linear relationsh total cumulative energy of the tensile test. This result is rea- (the dashed line in Fig. A2)between the AE energy and the sonable because the surface energy that is created from a ma- umber of cracks formed for minicomposites. However, there cantly larger than that produced by an in- significant scatter in this data. the greatest source of this dividual fiber failure. For the minicomposites, AE events that scatter is probably the inconsistency in the ae signal intensi- occur near failure( which would most likely be individual fiber es, as measured by the transducers through the epoxy for each failures)for all the minicomposites that have been tested in this mple. Because the epoxy surface is not flat, the measured study are much-lower-energy events than the events that occur signal intensity will be dependent on the transducer contact earlier in the stress-strain curve. Therefore, this assumption is area and the epoxy thickness, which varied from sample to considered to be reasonable The assumption that the ae energy is directly related to the Acknowledgments: The authors wish to thank Drs. Stanley Levine and number of cracks implies that the contribution of fiber failure and Drs. Ti Force Base(Wright-Patterson AFB, OH) for their input on the patterson Air Parthasarathy and Ron Kerans of roughness on interfacial shear stress Normalized References A. DiCarlo, ""Creep Limitations of Current Polycrystalline Ceramic Fi 0.8 Fof Cracks 比mmA小 of the l ww kasai, T. Seguchi, and K. Stoichiometric B-SiC Com- Ceram. Eng. Sci. Proc., 18 3 147-57(1997) Kumagawa, H. Yamaoka, M. Shibuya, and T. Yamamura,""Thermal and Chemical Corrosion Resistance of Newly Developed normalized -O Tyranno Fiber, Ceram. Eng. Sci. Proc., 18 3) 113-18 SN. Lissart and J. Lamon, ""Analysis of Damage Failure in Model Temperature Ceramic-Matrix Composites I. Edited by A G. Evans and bs. T. Gonczy, R C. Sprandel, and K. T. Faber, ""Tensile Tests of Miniature Fiber Reinforced Ceramic Composites 0152025303540 Tensile Load, N with Carbon and Boron Nitride Interphases at Elevated Temperatures in Air J Am Ceram Soc., 80 18 N. Morscher. *The Effect of Static and Cvelic Tensile Stress and Te Fig. Al. Relationship between crack formation and AE energy for a perature on Failure for Precracked Hi-Nicalon/BN/CVD SiC Minicomposites in Air, Ceram. Eng. Sci. Proc., 183]737-45(1997stress. For single-fiber microcomposites, a direct relationship existed between the AE energy and number of cracks pro￾duced. Figure A1 shows the data from a single-fiber SCS-6 (CVD-SiC fiber with a diameter of 143 mm, Textron Specialty Materials, Lowell, MA) microcomposite tensile test.29 The same tensile-test setup as that described by Morscher et al.11 was used. The load was increased to a set load and held at that load. The cracks were opened and could be observed via ste￾reomicroscopy. The position of each crack and the cumulative AE energy at the load were recorded. The load was then in￾creased and held, and the crack position and AE energy were again recorded. This procedure was continued up to a load of 45 N, which did not cause failure. The number of cracks and AE energy was normalized by the normalized cracks and cu￾mulative AE energy at 45 N and plotted in Fig. A1. The nor￾malized number of cracks and AE energy, as a function of load, are almost identical, indicating a linear relationship between the AE energy and the number of cracks that are formed. The epoxy mounting and gripping technique for this microcompos￾ite test was exactly the same as used for the minicomposites of this study. Because matrix cracks were not observable on the surface of minicomposites, the relationship between AE energy and the number of cracks must be determined indirectly, i.e., after the tensile test. The crack spacing at failure was determined for each minicomposite from polished longitudinal sections of a major portion of the gauge section. The number of cracks formed during the tensile test in the gauge section was then approximated from the crack spacing of the polished por￾tion of the gauge length for the entire tested gauge length at failure. It was assumed that the energy of an AE source event from a matrix crack was proportional to the area of the mini￾composite. Therefore, absolute cumulative AE energy normal￾ized by the minicomposite area was plotted versus the number of cracks in the gauge, as shown in Fig. A2. Each data point in Fig. A2 represents a different minicomposite that has been tested to failure. There does seem to be a linear relationship (the dashed line in Fig. A2) between the AE energy and the number of cracks formed for minicomposites. However, there is significant scatter in this data. The greatest source of this scatter is probably the inconsistency in the AE signal intensi￾ties, as measured by the transducers through the epoxy for each sample. Because the epoxy surface is not flat, the measured signal intensity will be dependent on the transducer contact area and the epoxy thickness, which varied from sample to sample. The assumption that the AE energy is directly related to the number of cracks implies that the contribution of fiber failure and interface sliding events to the cumulative AE energy is very small. On work with macrocomposites where it was possible to convincingly separate fiber-failure events from ma￾trix-crack events, the fiber-failure events had amplitudes and energies that were orders of magnitude lower than the higher￾energy matrix-cracking events.30 If the same cumulative en￾ergy approach was taken in the macrocomposite work that was used in this study, fiber failure would only account for ∼2.5% of the total cumulative energy of the tensile test after failure. Matrix cracking would have accounted for at least 85% of the total cumulative energy of the tensile test. This result is rea￾sonable because the surface energy that is created from a ma￾trix crack is significantly larger than that produced by an in￾dividual fiber failure. For the minicomposites, AE events that occur near failure (which would most likely be individual fiber failures) for all the minicomposites that have been tested in this study are much-lower-energy events than the events that occur earlier in the stress–strain curve. Therefore, this assumption is considered to be reasonable. Acknowledgments: The authors wish to thank Drs. Stanley Levine and James DiCarlo of NASA–Lewis Research Center for review of the manuscript and Drs. Triplicane Parthasarathy and Ron Kerans of Wright-Patterson Air Force Base (Wright-Patterson AFB, OH) for their input on the effects of fiber roughness on interfacial shear stress. References 1 J. A. DiCarlo, ‘‘Creep Limitations of Current Polycrystalline Ceramic Fi￾bers,’’ Compos. Sci. Technol., 51, 213–22 (1994). 2 M. Takeda, Y. Imai, H. Ichikawa, T. Ishikawa, N. Kasai, T. Seguchi, and K. Okamura, ‘‘Thermomechanical Analysis of the Low Oxygen Silicon Carbide Fibers Derived from Polycarbosilane,’’ Ceram. Eng. Sci. Proc., 14 [7–8] 540– 47 (1993). 3 J. Lipowitz, J. A. Rabe, A. Zangvil, and Y. Xu, ‘‘Structure and Properties of Sylramic Silicon Carbide Fiber—A Polycrystalline, Stoichiometric b-SiC Com￾position,’’ Ceram. Eng. Sci. Proc., 18 [3] 147–57 (1997). 4 K. Kumagawa, H. Yamaoka, M. Shibuya, and T. Yamamura, ‘‘Thermal Stability and Chemical Corrosion Resistance of Newly Developed Continuous Si-Zr-C-O Tyranno Fiber,’’ Ceram. Eng. Sci. Proc., 18 [3] 113–18 (1997). 5 N. Lissart and J. Lamon, ‘‘Analysis of Damage Failure in Model Unidirec￾tional CVI Composites’’; pp. 241–46 in Ceramic Transactions, Vol. 57, High Temperature Ceramic-Matrix Composites I. Edited by A. G. Evans and R. Naslain. American Ceramic Society, Westerville, OH, 1995. 6 S. T. Gonczy, R. C. Sprandel, and K. T. Faber, ‘‘Tensile Tests of Miniature Fiber Reinforced Ceramic Composites for Screening Process-Property Rela￾tions,’’ Ceram. Eng. Sci. Proc., 18 [3] 729–36 (1997). 7 G. N. Morscher, ‘‘Tensile Stress-Rupture of SiCf /SiCm Minicomposites with Carbon and Boron Nitride Interphases at Elevated Temperatures in Air,’’ J. Am. Ceram. Soc., 80 [8] 2029–42 (1997). 8 G. N. Morscher, ‘‘The Effect of Static and Cyclic Tensile Stress and Tem￾perature on Failure for Precracked Hi-Nicalon/BN/CVD SiC Minicomposites in Air,’’ Ceram. Eng. Sci. Proc., 18 [3] 737–45 (1997). Fig. A1. Relationship between crack formation and AE energy for a single-fiber SCS-6 minicomposite. Fig. A2. Relationship between crack formation and AE energy for several single-tow minicomposites. Each data point corresponds to a different minicomposite. 154 Journal of the American Ceramic Society—Morscher and Martinez-Fernandez Vol. 82, No. 1