Damage tolerant ceramic matrix composites Table 1. Properties of fibres, matrices and composites Sic/CAS JI SiC/LAS I Sic/MAS SiC/1723 Fibre a (un) Matrix Em(Gpa) 85 86 Gm(J/m2) 035 0.34050 0.3504 Ts(MPa) 2 1·5 004 004 004 400 1·2 om(MPa) 175 69 The data for SiC/MAS are from ref. 34, "supplemented by data for SiC/MAS-L36 Data for SiC/CAS II are from ref. 19, except for (ref. 17) The data for SiC/LAS III are from ref. 32: from ref. 33 "Value calculated from the initial composite modulus, E Lowest value from ref. 31, highest value from ref. 14 /Data from ref. 28 FThe data for Sic/1723 are from ref. 12 4 Results can be calculated from Ec to bef=03. For the SiC/MAS composite a value of am= 5 X 10-6 4.1 Available material data oc-I was used in the calculation of the residual Table 1 shows the available material data for four stresses. 5 However, Martin et al.3b reported am continuous fibre-reinforced ceramic composites. 3x 10-6oC-I for a MAS-L matrix The values of Ga were taken from Marshall and Oliver, who measured this property on SiC/LAS 4.2 Comparison of predictions to experiments II with a single fibre push-in method. Since the Using the material data given in Table I and eqns interphases of most composites consist mainly of (12)(15), (17)(19)and(21), the individual contri- carbon, , u an identical value of Gah was assigned bution of each mechanism can be calculated, and for all composites. In the literature there is some using eqn(7)the total energy uptake, the value of variation on the values of the interfacial sliding U can be predicted(Table 2). The experimental friction for SiC/CAS IL, ranging from Ts= 5 MPa results lie close to the theoretical predictions (estimated from the energy dissipation calculated Some deviations, however, are found, particularly by the fatigue hysteresis loop method )to 15 MPa for the SiC/MAS composite. For this material the (found by a single fibrc indentation mcthod 4). predicted value is lower than the measured. How For the SiC/LAS III composite Cao et al.3 ever, this may be attributed to poor estimates of eported the measured composite modulus Ec to the residual stresses, since there appears to be be 120 GPa and a fibre volume fraction off=0-5. some disagreement on the reported values of the However, using the rule of mixtures,(eqn 29), f thermal expansion mismatch. Neglecting the residual Table 2. Toughness, predictions and experiments SicAS I SiCAS III SIC/MAS SC/723 Ume(MJ/m) 0.04005 0260 28 Ucb(MJ/m) 00030-005 0005 0·003-0.004 Φ°(MJm3) 0-007-0-010 0-018019 φ?(MJ/m3 0012-0-014 φm(MJ/m3) 0036033 0003-0011 0000007 033_0.48 9(MJ/m) 44-2.45 90-316 3.95-461 Ws(MJ/m) 017-042 10016 002010 084089 U (MJ/m) Predicted 2.7-3.0 20-3-4 0.37046 5462 Experimental data for U/ are from ref. 19 for SiC/CAS IL, ref. 33 for SiC/LAS III, ref. 34 for Sic/MAS and from ref. 12 for SiC/1723Damage tolerant ceramic matrix composites Table 1. Properties of fibres, matrices and composites 1053 SiCKAS II Sic/LAS ZZZ SiC/MAS SIC/l 723 Fibre Er @Pa) 200 200 200 200 a (pm) 7.5 7.5 7.5 7.5 Matrix -%, (GPa) 98 85 75” 86 G, (J/m*) 25’ 30’ 40” 40 Composite f 0.35 0.3%.50 0.45 0.3550.4 rE (MPa) 5-15’ 2 1.5” 86 Gdb (J/m21 0.04 0.04 0.04 0.04 s (pm) 160 400 100 100 E” (W 0.9 0.77’ 0.36 1.2 a2 (MPa) 70 -50’ 175 69 The data for SiC/MAS are from ref. 34. “sunnlemented bv data for SiC/MAS-L36. Data for SiCKAS II are from ref. 19, except-for b(ref. 17j. The data for SIC/LAS III are from ref. 32; ‘from ref. 33. devalue calculated from the initial composite modulus, E,. ‘Lowest value from ref. 31, highest value from ref. 14. fData from ref. 28. gThe data for Sic/1723 are from ref. 12. 4 Results 4.1 Available material data Table 1 shows the available material data for four continuous fibre-reinforced ceramic composites. The values of G,, were taken from Marshall and Oliver,” who measured this property on Sic/LAS III with a single fibre push-in method. Since the interphases of most composites consist mainly of carbon,29,30 an identical value of Gdb was assigned for all composites. In the literature there is some variation on the values of the interfacial sliding friction for SiC/CAS II, ranging from TV = 5 MPa (estimated from the energy dissipation calculated by the fatigue hysteresis loop method3*) to 15 MPa (found by a single fibre indentation method14). For the Sic/LAS III composite32 Cao et ~1.~~ reported the measured composite modulus E, to be 120 GPa and a fibre volume fraction off = 0.5. However, using the rule of mixtures, (eqn 29), f can be calculated from E, to be f = 0.3. For the SiC/MAS composite34 a value of (Y, = 5 X 10m6 “Cm’ was used in the calculation of the residual stresses.35 However, Martin et ~1.~~ reported a;, = 3 X 10m6 ‘C-l for a MAS-L matrix. 4.2 Comparison of predictions to experiments Using the material data given in Table 1 and eqns (12)-( 15), (17)-( 19) and (21), the individual contri￾bution of each mechanism can be calculated, and using eqn (7) the total energy uptake, the value of U can be predicted (Table 2). The experimental results lie close to the theoretical predictions. Some deviations, however, are found, particularly for the SiC/MAS composite. For this material the predicted value is lower than the measured. How￾ever, this may be attributed to poor estimates of the residual stresses, since there appears to be some disagreement on the reported values of the thermal expansion mismatch. Neglecting the residual Table 2. Toughness, predictions and experiments SiCKAS ZZ Sic/LAS ZZZ SiC/MAS Sic/I 723 U,, (MJ/m3) U,, (MJ/m’) @‘,” (MJ/m’) @p (MJ/m’) ad” (MJ/m’) @ lf’ (MJ/m’) W,, (MJ/m’) U (MJ/m’) Predicted Experiment 0.10 0.040.05 0.22 0.260.28 0.004 0~003-0~005 0.005 0~003-0~004 0.016 0~007-0~010 0.11 0.018+19 0.015 0~002-0~003 0.05 0.0124.014 0.036-0.33 0~003-0011 0~00-0~007 0.33-0.48 2442.45 1.90-3.16 0.29 3.95-4.61 0.17-0.42 0~10-0~16 0~02-0~10 0.84-0.89 2.7-3.0 2.0-3.4 0.37-0.46 5.46.2 3.1 2.7 0.57 5.4 Experimental data for U are from ref. 19 for SiCKAS II, ref. 33 for Sic/LAS III, ref. 34 for SiC/MAS and from ref. 12 for SiC/1723