1. Davies et al./Journal of the European Ceramic Society 25 (2005)599-604 Table I Values of fibre strength Weibull parameters, So and m, measured in situ an orthogonal 3-D woven SiC/SiC composite tested in air Room temperature 100°C/air So(GPa) S0(GPa)(L=10-3m) So(GPa) So (GPa)(Lo 10-m)m 3.04±002 3.78±0. 4.18±0.14 4.13±0.03 225±0.25 2.96±0.09 3.15±0.06 3.93±0. 4.10±0 2.45±0.0 1.39±0.15 8.89±047 a significant difference was noted for the 1 100C/air case populations. As a first approximation, the authors assumed a (Fig. 4(b)) with the normalised So for edge fibres being 62% linear variation in So and m between the centre and furthest that of the centre value. However, even So for the centre fibres with non-zero pullout lengths when applying Eq. (4) fibres(2.25+ 0. 25 GPa)was still considerably below that This assumption was justified on the ground that, as shown of the rt specimen( 3.86 GPa), which itself was approx- later, changes in t from Eq (4) would be dominated by (h) imately 30% below that of the"as received"fibres. The rather than So or n reduction in fibre strength for the 1100 C/air specimen was attributed to the formation of a surface oxide layer on the 3.3. Mean fibre pullout length tributed to increased oxidation and hence a thicker oxide It was next decided to investigate the variation of proper layer. Whereas m for fibres in the RT specimen was approx- ties along the fibre bundle minor axis between perimeter and 8.89+0.47 for edge centre with the data in Fig. 2 bei fibres in the 1100C/air specimen with the increase being at- rical) horizontal strips. Whilst this procedure neglects the tributed to the relatively even thickness of the surface oxide effects of oxidation from either end of the major axis, the layer. The lower m value for centre fibres in the 1100 C/air large major/minor axis length ratio would make oxidation specimen(2.96+0.09)was tentatively attributed to a frac- along the minor axis by far the dominant component Re- tion of the fibres having their strength determined by surface sults presented in Fig. 5 confirm, as previously mentioned flaws(i.e. similar to the rt case)with the remainder being the existence of a strong correlation between fibres exhibit- determined by the surface oxide layer, i.e. two distinct fibre ing pullout and those exhibiting fracture mirror behaviour n fact, the correlation in Fig. 5 is more specific as it clearly Center Edge (a)0.0 0 0. 086 Distance from fibre bundle perimeter(odm) Fig.4. Fibre strength distributions(normalised to a scale length of 10-3m) measured in situ an orthogonal 3-D woven SiC/SiC composite: (a)room Fig. 5. Distribution of fibre properties along the minor axis within an temperature, and (b) 1100C in air. "Centre" and "Edge" refer to the individual fibre bundle in an orthogonal 3-D woven SiC/SiC composite general position within the fibre bundle where the measurements were tested at 1100C in air: (a)mean fibre pullout length, (h), and(b)fraction of fibres exhibiting fracture mirrors.602 I.J. Davies et al. / Journal of the European Ceramic Society 25 (2005) 599–604 Table 1 Values of fibre strength Weibull parameters, So and m, measured in situ an orthogonal 3-D woven SiC/SiC composite tested in air Room temperature 1100 ◦C/air So (GPa) So (GPa) (Lo = 10−3 m) m So (GPa) So (GPa) (Lo = 10−3 m) m Centre 3.04 ± 0.02 3.78 ± 0.13 4.18 ± 0.14 4.13 ± 0.03 2.25 ± 0.25 2.96 ± 0.09 Edge 3.15 ± 0.06 3.93 ± 0.13 4.10 ± 0.05 2.45 ± 0.01 1.39 ± 0.15 8.89 ± 0.47 a significant difference was noted for the 1100 ◦C/air case (Fig. 4(b)) with the normalised So for edge fibres being 62% that of the centre value. However, even So for the centre fibres (2.25 ± 0.25 GPa) was still considerably below that of the RT specimen (∼3.86 GPa), which itself was approx￾imately 30% below that of the “as received” fibres.2 The reduction in fibre strength for the 1100 ◦C/air specimen was attributed to the formation of a surface oxide layer on the fibres;10–13 the lower strength of the edge fibres being at￾tributed to increased oxidation and hence a thicker oxide layer. Whereas m for fibres in the RT specimen was approx￾imately 4.1, the respective value was 8.89 ± 0.47 for edge fibres in the 1100 ◦C/air specimen with the increase being at￾tributed to the relatively even thickness of the surface oxide layer. The lower m value for centre fibres in the 1100 ◦C/air specimen (2.96 ± 0.09) was tentatively attributed to a frac￾tion of the fibres having their strength determined by surface flaws (i.e. similar to the RT case2) with the remainder being determined by the surface oxide layer, i.e. two distinct fibre Fibre strength (GPa) 1 2 3 4 5 6 78 Cumulative failure 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative failure 0.0 0.2 0.4 0.6 0.8 1.0 Center Edge (a) (b) Fig. 4. Fibre strength distributions (normalised to a scale length of 10−3 m) measured in situ an orthogonal 3-D woven SiC/SiC composite: (a) room temperature, and (b) 1100 ◦C in air. “Centre” and “Edge” refer to the general position within the fibre bundle where the measurements were taken. populations. As a first approximation, the authors assumed a linear variation in So and m between the centre and furthest fibres with non-zero pullout lengths when applying Eq. (4). This assumption was justified on the ground that, as shown later, changes in τ from Eq. (4) would be dominated by h rather than So or m. 3.3. Mean fibre pullout length It was next decided to investigate the variation of proper￾ties along the fibre bundle minor axis between perimeter and centre with the data in Fig. 2 being divided into (symmet￾rical) horizontal strips. Whilst this procedure neglects the effects of oxidation from either end of the major axis, the large major/minor axis length ratio would make oxidation along the minor axis by far the dominant component. Re￾sults presented in Fig. 5 confirm, as previously mentioned, the existence of a strong correlation between fibres exhibit￾ing pullout and those exhibiting fracture mirror behaviour. In fact, the correlation in Fig. 5 is more specific as it clearly Mean pullout length ( ∝m) 0 20 40 60 80 Distance from fibre bundle perimeter (∝m) 0 10 20 30 40 50 60 Fraction of fibres exhibiting 0.0 0.2 0.4 0.6 0.8 1.0 fracture mirrors (a) (b) Fig. 5. Distribution of fibre properties along the minor axis within an individual fibre bundle in an orthogonal 3-D woven SiC/SiC composite tested at 1100 ◦C in air: (a) mean fibre pullout length, h, and (b) fraction of fibres exhibiting fracture mirrors