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I.. Davies et al. Composites Science and Technology 59(1999)801-811 1200.C). This would also tend to increase the uncer- 1.0 Room temperature 1200°/ actin tainty for values of S"' and m 0.8 △1300°/ acuum One important point to note is the large difference in 1100°c/air 1200 C/air appearance between Fig. 10(a)and(bthis emphasises that comparison between in situ S, and m data should only be made after normalising to a standard gauge ngth. For example, data taken at different gauge lengths [Fig. 10(a)] indicates fibres tested in air to have 0.2 similar in situ fibre strength characteristics at 1 100 and 1200oC, both of which are approximately 80% that of room temperature and vacuum(1200, 1300oC)data 1.0 Legend:as above However, following normalisation to L=10-3m [ Fig. 10(b)] it is observed that fibres tested in air actually possess different strength characteristics(1200.C data is 9% lower compared to the 1100oC data)and further more. that values are about 35% that of room tem perature and vacuum (1200, 1300C)data. These substantial differences can be attributed to variations in m between specimens (Table 3)when applied to Eq(4) Table 3 indicates that s decreases with increased (b) vacuum. from 3.85 GPa at perature to 3. 18 GPa at 1300C. Conversely, m increa 0.50.7 234568 ses with increased temperature in vacuum, from 4.19 at Fibre strength(GPa) room temperature to 6.56 at 1300.C. One suggest mechanism for these changes in S and m would be Fig. 10. In situ fibre strength of Tyranno"Si-Ti-C-O fibres after interaction between intrinsic (surface)flaws within the tensile testing in vacuum and air up to 1380C: (a) after correction of fibre and voids introduced through heating the fibre to fracture mirror parameters according to Fig. 2 5]. and (b) normalised to a 10-3m gauge length(assuming Am= 2.5 MPa m 2). elevated temperature in vacuum. It is known that for- mation of voids within SiC-based fibres at the tempera ture tested in this report would tend to produce and vacuum fibres and 1.3% for fibres tested in air additional flaws distributed homogeneously within the whereas the uncertainty for m and m was typically 1.5 fibre bulk, and initially with a small average flaw size, and 4.3% for fibres tested at room temperature(and fw, and narrow distribution with largest size fm. with vacuum) and air respectively. This indicates differences increasing temperature and or time above a1000oC the between Eqs. (1)and (3)(S=S, m=m,)to be sig- value of fa will start to increase(Section 3. 1.3)until at nificant for fibres tested at room temperature and in some point the flaw distribution will approach and vacuum(1200, 1300oC)but within experimental error overlap the intrinsic flaw distribution, which has aver for fibres tested in air(1100, 1200 C). However, these age size fa(with fav >> fav) and a minimum value of uncertainty in S" and m depends to a large extent on failed due to flaws of size fmn (i.e. the strongest fibres) the quality of the data, in particular the number of data will start to instead fail due to flaws of size fmx. The points. Furthermore, Eqs. (1)and (3)assume a single result of this will be that the average strength of the flaw distribution which is suspected not to be the case fibres at a given gauge length, So, will decrease (as the for fibres tested in vacuum(1200, 1300C)and air(1100, average flaw size within the fibres will increase) whilst Table 3 In situ strength parameters and fibre/matrix interface shear stress for Tyranno*SH-Ti-C-O fibres after testing in vacuum and air up to 1380.C Test condition S(GPa) So(GPa) )(10-3m)2S(L=10-3m)r(MPa) Room temperature3.11(±0.01)4.18(±0.05)3.09(±0.01)4.19(±005)0.81(±0.02)1.29(±0.02)3.85(±0.13)494(±0.16) 200C/ actin3.12(±001)5.59(±0.10)307(±0.01)5.72(±0.10)0.59(±0.03)1.19(±0.02)3.45(±0.20)6.27(±0.35) 1300°C/ vacuum3.10(±0.01)6.36(±0.11)3.04(±0.01)6.56(±0.11)0.39(±0.04)1.16(±0.02)3.18(±0.34)9.13(±0.96 1100°C/air 240(±0.03)299(±0.14)247(±0.03)291(±0.13)007(±0.01)1.43(±0. 1.37(±0.15)54.75(±5 1120°C/air 2.38(±0.03)277(±0.12)248(±0.03)268(±0.11)0.06(±0.01)1.47(±0061.26(±0.18)60.50(±8.43) Note: Values of S, So and S, were calculated assuming Am=2.5 MPa m 2, whilst values of (h) have been taken from Ref. 2and vacuum ®bres and 1.3% for ®bres tested in air whereas the uncertainty for m and m was typically 1.5 and 4.3% for ®bres tested at room temperature (and vacuum) and air respectively. This indicates dierences between Eqs. (1) and (3) (S S, m m) to be signi®cant for ®bres tested at room temperature and in vacuum (1200, 1300C) but within experimental error for ®bres tested in air (1100, 1200C). However, these conclusions cannot be applied generally to CMCs as the uncertainty in S and m depends to a large extent on the quality of the data, in particular the number of data points. Furthermore, Eqs. (1) and (3) assume a single ¯aw distribution which is suspected not to be the case for ®bres tested in vacuum (1200, 1300C) and air (1100, 1200C). This would also tend to increase the uncertainty for values of S and m. One important point to note is the large dierence in appearance between Fig. 10(a) and (b)Ðthis emphasises that comparison between in situ So and m data should only be made after normalising to a standard gauge length. For example, data taken at dierent gauge lengths [Fig. 10(a)] indicates ®bres tested in air to have similar in situ ®bre strength characteristics at 1100 and 1200C, both of which are approximately 80% that of room temperature and vacuum (1200, 1300C) data. However, following normalisation to L0 o 10ÿ3 m [Fig. 10(b)] it is observed that ®bres tested in air actually possess dierent strength characteristics (1200C data is 9% lower compared to the 1100C data) and furthermore, that values are about 35% that of room temperature and vacuum (1200, 1300C) data. These substantial dierences can be attributed to variations in m between specimens (Table 3) when applied to Eq. (4). Table 3 indicates that S0 o decreases with increased temperature in vacuum, from 3.85 GPa at room temperature to 3.18 GPa at 1300C. Conversely, m increases with increased temperature in vacuum, from 4.19 at room temperature to 6.56 at 1300C. One suggested mechanism for these changes in S 0 o and m would be interaction between intrinsic (surface) ¯aws within the ®bre and voids introduced through heating the ®bre to elevated temperature in vacuum. It is known that formation of voids within SiC-based ®bres at the temperature tested in this report would tend to produce additional ¯aws distributed homogeneously within the ®bre bulk, and initially with a small average ¯aw size, f a , and narrow distribution with largest size f max . With increasing temperature and/or time above 1000C the value of f a will start to increase (Section 3.1.3) until at some point the ¯aw distribution will approach and overlap the intrinsic ¯aw distribution, which has average size f a i (with f a i f a ) and a minimum value of f min i , i.e. f max 5f min i . At this point, ®bres that originally failed due to ¯aws of size f min i (i.e. the strongest ®bres) will start to instead fail due to ¯aws of size f max . The result of this will be that the average strength of the ®bres at a given gauge length, S 0 o, will decrease (as the average ¯aw size within the ®bres will increase) whilst Table 3 In situ strength parameters and ®bre/matrix interface shear stress for Tyranno1 Si±Ti±C±O ®bres after testing in vacuum and air up to 1380C Test condition S (GPa) m So (GPa) m hhi (10ÿ3 m) l S 0 o
L0 o 10ÿ3 m (GPa) (MPa) Room temperature 3.11 (0.01) 4.18 (0.05) 3.09 (0.01) 4.19 (0.05) 0.81 (0.02) 1.29(0.02) 3.85 (0.13) 4.94 (0.16) 1200C/vacuum 3.12 (0.01) 5.59 (0.10) 3.07 (0.01) 5.72 (0.10) 0.59 (0.03) 1.19 (0.02) 3.45 (0.20) 6.27 (0.35) 1300C/vacuum 3.10 (0.01) 6.36 (0.11) 3.04 (0.01) 6.56 (0.11) 0.39 (0.04) 1.16 (0.02) 3.18 (0.34) 9.13 (0.96) 1100C/air 2.40 (0.03) 2.99 (0.14) 2.47 (0.03) 2.91 (0.13) 0.07 (0.01) 1.43 (0.07) 1.37 (0.15) 54.75 (5.40) 1120C/air 2.38 (0.03) 2.77 (0.12) 2.48 (0.03) 2.68 (0.11) 0.06 (0.01) 1.47 (0.06) 1.26 (0.18) 60.50 (8.43) Note: Values of S, So and S0 o were calculated assuming Am 2:5 MPa m1/2, whilst values of hhi have been taken from Ref. 2. Fig. 10. In situ ®bre strength of Tyranno1 Si±Ti±C±O ®bres after tensile testing in vacuum and air up to 1380C: (a) after correction of fracture mirror parameters according to Fig. 2 [5], and (b) normalised to a 10ÿ3 m gauge length (assuming Am 2:5 MPa m1/2). 808 I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811