I.. Davies et al. Composites Science and Technology 59(1999)801-811 (a) 1.0 Ex situ standard Ex situ surface-modified 40 um 0.0 345678 Fibre strength(GPa) (b) Fig 9. Ex situ and in situ fibre dtrength of Tyranno Si-TiC-0 fibres normalised to a 10-3m gauge length(assuming Am=2.5 Pa m2) after tensile testing at room temperature an estimate and may be subject to revision at a later date. The smaller value of m for the in situ fibres indi- cated a wide distribution of faw sizes to have been introduced during composite manufacture. In situ fibre strength data for SiC-based fibres has been presented in Fig. 10 1. five sets of in data with different specimen test conditions were inves- tigated: (i) room temperature, (ii)1200oC in vacuum, Fig8.Scanning electron micrographs illustrating TyrannoSH-Ti-C-o (iii)1300oC in vacuum,(iv) 1100C in air, and(v) fibres tested in situ at 1100 and 1200'C in air: (a) general view of the 1200 C in air. Fig. 10(a)illustrates in situ strength, s, outer edge of a fibre bundle, and (b)detailed view of a fibre fracture after correcting the fracture mirror parameters using Fig. 2 [5 whilst data in Fig. 10(b) was normalised to a gauge length of Lo=10-3m. For comparison, Weibull (8) strength parameters have been presented in Table 3 for (i) uncorrected(S, m),(ii) corrected fracture mirror where G; and Gr are the critical strain energy release (So). Although Lo =10- m was confirmed to be a sui- rates for interface and fibre, respectively. A low value table gauge length for fibres tested in vacuum(Table 3) for t would also imply G; to be small whereas oxidation as 0.35x10-3<(h)<0.81x10-3m [19], the significantly of the fibre/matrix interface tends to increase t(due to lower (h) for specimens tested in air (0.06 and increased u)and hence increase Gi to the point where 0.07x10-3m) indicates a more appropriate value of La the inequality in Eq(8)is no longer satisfied. At this to have been 10-4 m for these latter specimens. How point, crack deflection at the fibre/matrix interface will ever, in order to accurately compare in situ strength for no longer be energetically favourable, allowing the crack different test conditions there is little option but to nor to continue through the fibre with resultant featureless malise the gauge length to a single value although the fibre fracture surfaces(Fig. &)and flat composite fracture comments above should be kept in mind when later surfaces comparing data between specimens tested in vacuum and that tested in air 3. 2. In situ fibre strength parameters The first point to note from Table 3 is that, to S, the value of So was approximately 1. 5% lower for 3.2.1. Effect of test temper fibres tested at room temperature and vacuum (1200, Fig 9 illustrates fibre strength for standard and sur- 1300C)and 3.5% higher for fibres tested in air(1100, face-modified fibres tested ex situ at room temperature 1200oC)due to differences in m. Likewise, compared [14]. The value of Am in Eq (2) for Tyranno" Si-Ti-C-o to m,, values of m were approximately 2% higher and fibres was estimated to be 2.5 MPa m/2 as this provided 3% lower for fibres tested at room temperature(and a 30% decrease for in situ strength compared to ex situ vacuum) and in air, respectively. The second point to strength as noted in previous ceramic fibre systems. note is that values of s and So generally had associated However, it is accepted that Am=2.5 MPa m/ is only uncertainties in the range 0.3% for room temperatureGi Gf 4 1 4 …8† where Gi and Gf are the critical strain energy release rates for interface and ®bre, respectively. A low value for  would also imply Gi to be small whereas oxidation of the ®bre/matrix interface tends to increase  (due to increased ) and hence increase Gi to the point where the inequality in Eq. (8) is no longer satis®ed. At this point, crack de¯ection at the ®bre/matrix interface will no longer be energetically favourable, allowing the crack to continue through the ®bre with resultant featureless ®bre fracture surfaces (Fig. 8) and ¯at composite fracture surfaces. 3.2. In situ ®bre strength parameters 3.2.1. E€ect of test temperature and atmosphere Fig. 9 illustrates ®bre strength for standard and sur￾face-modi®ed ®bres tested ex situ at room temperature [14]. The value of Am in Eq. (2) for Tyranno1 Si±Ti±C±O ®bres was estimated to be 2.5 MPa m1/2 as this provided a 30% decrease for in situ strength compared to ex situ strength as noted in previous ceramic ®bre systems. However, it is accepted that Am ˆ 2:5 MPa m1/2 is only an estimate and may be subject to revision at a later date. The smaller value of m for the in situ ®bres indi￾cated a wide distribution of ¯aw sizes to have been introduced during composite manufacture. In situ ®bre strength data for SiC-based ®bres has been presented in Fig. 10. In total, ®ve sets of in situ data with di€erent specimen test conditions were inves￾tigated: (i) room temperature, (ii) 1200C in vacuum, (iii) 1300C in vacuum, (iv) 1100C in air, and (v) 1200C in air. Fig. 10(a) illustrates in situ strength, S, after correcting the fracture mirror parameters using Fig. 2 [5] whilst data in Fig. 10(b) was normalised to a gauge length of L0 o ˆ 10ÿ3 m. For comparison, Weibull strength parameters have been presented in Table 3 for: (i) uncorrected …S; m†, (ii) corrected fracture mirror parameters …So; m†, and (iii) normalised to L0 o ˆ 10ÿ3 m (S0 o). Although L0 o ˆ 10ÿ3 m was con®rmed to be a sui￾table gauge length for ®bres tested in vacuum (Table 3) as 0.3510ÿ3 4hhi40.8110ÿ3 m [19], the signi®cantly lower hhi for specimens tested in air (0.06 and 0.0710ÿ3 m) indicates a more appropriate value of L0 o to have been 10ÿ4 m for these latter specimens. How￾ever, in order to accurately compare in situ strength for di€erent test conditions there is little option but to nor￾malise the gauge length to a single value although the comments above should be kept in mind when later comparing data between specimens tested in vacuum and that tested in air. The ®rst point to note from Table 3 is that, compared to S, the value of So was approximately 1.5% lower for ®bres tested at room temperature and vacuum (1200, 1300C) and 3.5% higher for ®bres tested in air (1100, 1200C)Ðdue to di€erences in m. Likewise, compared to m, values of m were approximately 2% higher and 3% lower for ®bres tested at room temperature (and vacuum) and in air, respectively. The second point to note is that values of S and So generally had associated uncertainties in the range 0.3% for room temperature Fig. 9. Ex situ and in situ ®bre dtrength of Tyranno1 Si±Ti±C±O ®bres normalised to a 10ÿ3 m gauge length (assuming Am=2.5 Pa m1/2) after tensile testing at room temperature. Fig. 8. Scanning electron micrographs illustrating Tyranno1 Si±Ti±C±O ®bres tested in situ at 1100 and 1200C in air: (a) general view of the outer edge of a ®bre bundle, and (b) detailed view of a ®bre fracture surface. I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811 807