I.. Davies et al. Composites Science and Technology 59(1999)801-811 the value of m will increase as the range of flaw sizes (i.e 1.2 fia-fm) decreases ■ Centre of fibre bundle Also observed from table 3 is that s and m were 10° Edge of fibre bundle reduced dramatically after exposure to air at 1100 and 1200C, in agreement with previous researchers[8, 27] and in conjunction with microstructural observations 吉06 outlined in section 31. 4. This was believed due to decomposition and oxidation damage to the fibres. More E04 specifically, it was believed that oxygen reacted with the Sic-based fibre with the following mechanisms [28]: 0.0 2SiC+302→2SiO2+2CO 121· Centre of fibre bundle 1.0 0 Edge of fibre bundle SICOy +(/2-y/2+1)O2- SiO2+ XCO (10) 0.8 2C+O2→2CO 9 0.6 The result of these reactions is the evolution of co from 0.4 the fibre and formation by diffusion of a Sio, layer, of 0.2 thickness y, at the fibre surface. It has been shown pre- (b) viously that the sioz layer acts as a defect of size y 0.0 within the fibre, which may significantly reduce the fibre 3456789 strength. Previous work had found the value of y to be 2 um after 1.08×10° s of exposure to650830° C in an Fibre strength(GPa oxidising environment [28] suggesting an average Fig. Il In situ strength(assuming Am=2.5 MPa m'n)of Tyranno" growth rate, y,, of 2x10-12 m s-l. Even taking into Si-Ti-C-O fibres for different regions within a single fibre bundle after account the higher temperatures used for the current tensile testing:(a)1300C in vacuum, and(b)1100C in air materials and that y' would probably have its highes value upon initial exposure, the short time of exposure The difference in behaviour between test conditions at 1100 and 1200@C in air for the current fibres indicates was attributed to ingression of oxygen into specimens y to be in the order of nanometres [28]. Such a small tested at 1100 and 1200oC in air. A suggested mechan value of y would be unlikely to reduce Se by the extent ism to explain the spatial dependence of in situ strength seen in Table 3 suggesting another phenomena to be characteristics for fibres at 1100 and 1200oC in air relies responsible for the observed rapid decrease in Sn such on the fact that fibres in the central portion along the fibre bundle main axis have effectively two sides exposed to the fibre bundle perimeter whereas fibres at the end 3.2. 2. Effect of fibre position within the fibre bundle of the fibre bundle main axis have three sides exposed to Data presented in Section 3. 2. 1 referred to the com- the fibre bundle perimeter. If it were to be assumed that bination of fibres measured at the end and central por- oxygen entered the fibre bundle mainly from the fibre tion of the fibre bundle main axis. However, Fig. ll bundle perimeter then it would be expected that regions illustrates in situ strength data for fibres separated into with the highest fraction of fibres close to the bundle two groups: (i) those located at the end, and (ii) those in perimeter would show a larger degree of oxidation the central portion, of the fibre bundle main axis From damage, at least initially. Such an assumption would Fig. 11(a) it may be observed that fibres tested at explain the lack of a spatial dependence for fibres in 1300 C in vacuum showed similar in situ strength char- specimens tested in vacuum but a significant difference acteristics at the end of the fibre bundle main axis(and in specimens tested in air. Although previous authors also the case for room temperature and 1200oC in have suggested that oxygen ingression for I-D compo- vacuum)whereas Fig. 11(b) shows in situ fibre strength sites occurs mainly along the length of individual fibre characteristics to be different for the two regions within matrix interfaces [22, 23] it was believed in the present he specimen tested at 1100C in air. Thus, fibres in specimens(which have a complex 3-D structure) that specimens tested at room temperature up to 1300C in oxygen entered into fibre bundles mainly via the fibre vacuum show no large-scale correlation between in situ bundle perimeter. The main evidence for such a scenario fibre strength characteristics and fibre bundle position was that fibre pull-out length and probability of fracture whereas fibres in specimens tested at 1100@C in air show mirror occurrence increased with distance away from the a significant spatial dependence fibre bundle perimeter [19, 29]. Although it was statedthe value of m will increase as the range of ¯aw sizes (i.e. f max i ÿ f min  ) decreases. Also observed from Table 3 is that S0 o and m were reduced dramatically after exposure to air at 1100 and 1200C, in agreement with previous researchers [8,27] and in conjunction with microstructural observations outlined in Section 3.1.4. This was believed due to decomposition and oxidation damage to the ®bres. More speci®cally, it was believed that oxygen reacted with the SiC-based ®bre with the following mechanisms [28]: 2SiC ‡ 3O2 ! 2SiO2 ‡ 2CO …9† SiCxOy ‡ …x=2 ÿ y=2 ‡ 1†O2 ! SiO2 ‡ xCO …10† 2C ‡ O2 ! 2CO …11† The result of these reactions is the evolution of CO from the ®bre and formation by di€usion of a SiO2 layer, of thickness , at the ®bre surface. It has been shown pre￾viously that the SiO2 layer acts as a defect of size within the ®bre, which may signi®cantly reduce the ®bre strength. Previous work had found the value of to be 2 mm after 1.08106 s of exposure to 650±830C in an oxidising environment [28] suggesting an average growth rate, 0 , of 210ÿ12 m sÿ1 . Even taking into account the higher temperatures used for the current materials and that 0 would probably have its highest value upon initial exposure, the short time of exposure at 1100 and 1200C in air for the current ®bres indicates to be in the order of nanometres [28]. Such a small value of would be unlikely to reduce S 0 o by the extent seen in Table 3 suggesting another phenomena to be responsible for the observed rapid decrease in S 0 o such as thermal decomposition. 3.2.2. E€ect of ®bre position within the ®bre bundle Data presented in Section 3.2.1 referred to the com￾bination of ®bres measured at the end and central por￾tion of the ®bre bundle main axis. However, Fig. 11 illustrates in situ strength data for ®bres separated into two groups: (i) those located at the end, and (ii) those in the central portion, of the ®bre bundle main axis. From Fig. 11(a) it may be observed that ®bres tested at 1300C in vacuum showed similar in situ strength char￾acteristics at the end of the ®bre bundle main axis (and also the case for room temperature and 1200C in vacuum) whereas Fig. 11(b) shows in situ ®bre strength characteristics to be di€erent for the two regions within the specimen tested at 1100C in air. Thus, ®bres in specimens tested at room temperature up to 1300C in vacuum show no large-scale correlation between in situ ®bre strength characteristics and ®bre bundle position whereas ®bres in specimens tested at 1100C in air show a signi®cant spatial dependence. The di€erence in behaviour between test conditions was attributed to ingression of oxygen into specimens tested at 1100 and 1200C in air. A suggested mechan￾ism to explain the spatial dependence of in situ strength characteristics for ®bres at 1100 and 1200C in air relies on the fact that ®bres in the central portion along the ®bre bundle main axis have e€ectively two sides exposed to the ®bre bundle perimeter whereas ®bres at the ends of the ®bre bundle main axis have three sides exposed to the ®bre bundle perimeter. If it were to be assumed that oxygen entered the ®bre bundle mainly from the ®bre bundle perimeter then it would be expected that regions with the highest fraction of ®bres close to the bundle perimeter would show a larger degree of oxidation damage, at least initially. Such an assumption would explain the lack of a spatial dependence for ®bres in specimens tested in vacuum but a signi®cant di€erence in specimens tested in air. Although previous authors have suggested that oxygen ingression for l-D compo￾sites occurs mainly along the length of individual ®bre/ matrix interfaces [22,23] it was believed in the present specimens (which have a complex 3-D structure) that oxygen entered into ®bre bundles mainly via the ®bre bundle perimeter. The main evidence for such a scenario was that ®bre pull-out length and probability of fracture mirror occurrence increased with distance away from the ®bre bundle perimeter [19,29]. Although it was stated Fig. 11. In situ strength (assuming Am ˆ 2:5 MPa m1/2) of Tyranno1 Si±Ti±C±O ®bres for di€erent regions within a single ®bre bundle after tensile testing: (a) 1300C in vacuum, and (b) 1100C in air. I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811 809