L. Davies et al sites Science and Technolog y 59(1999)801-811 earlier that the chemistry of fibres and matrix was attributed to oxygen ingression into the fibre bun approximately similar, only slight differences in coeff dles with only a relatively small number of fibres cients of thermal expansion(CTE) for fibre and matrix having an interface weak enough to allow"weak would be required to produce microcracking at the est link"mechanisms to apply interface between large regions of fibres and matrix, 3. Fibre strength characteristics normalised to a 10-3 particularly when taking into account the low value of t m gauge length indicated fibres tested in air at ele expected for these composites. Further investigation vated temperature to have significantly lower concerning the spatial dependence of in situ fibre prop strengths and average Weibull parameter, m, erties within single fibre bundles for these CMCs is compared to the room temperature, 1200C/ vacuum, and 1300 C/vacuum cases. This was attributed to oxygen damage of the fibre together 3.3. Fibre/matrix interface shear stress with oxidation of the fibre/matrix interface 4. Fibre characteristics for fibres tested at room Values of t shown in Table 3 were calculated from mperature, 1200C/vacuum, and 1300C/vacuum Eq(7)using values of (h and r established elsewhere showed no significant difference between fibre [2, 19]. It can be seen that t was approximately 5 MPa located at the end and centre of the fibre bundle for specimens tested at room temperature with a slight main axis. This was not the case for 1100 and increase to 9 MPa at 1300.C in vacuum. These values 1200 C/air specimens where a significant difference fall within the ran f 2-22 MP was observed and attributed to the difference in average of 9.47 MPa(+3. 50)] suggested by researchers ibre numbers closer to the fibre bundle perimeter for Sic-based fibres with carbon interfaces [6, 8, 27, 30- From this it was suggested that the majority of 33] tested at room temperature, and attributed to the oxygen ingress was from the fibre bundle perimeter low shear strength and friction coefficient of the carbon- 5. The fibre/matrix interface shear strength was low for he room temperature specimens and increased The increase in t with test temperature in vacuum was slightly with temperature when tested in vacuum suggested possibly due to the Cte mismatch being possibly due to changes in the thermal mismatch reduced for specimens tested closer to their processing between fibres and matrix. values for specimens tes temperature. An alternative reason might be slight grain ted at 1100 and 1200'C in air were an order of mag- coarsening at the fibre surface. Even for specimens tes- nitude greater than for room temperature specimens ted at 1300c in vacuum the value of t was still <10 indicating a significant degree of oxidation damage MPa which is normally associated with superior fibre at the fibre/matrix interface to have occurred. pull-out and composite mechanical properties However, expos air at ll00and1200°C increased t by an order of magnitude(55-60 MPa) Acknowledgements compared to that at room temperature. These results are within the range suggested for t after exposure to This work was supported by funding from the Scienc oxidising atmosphere at elevated temperature [8]. It is and Technolog possibly the case for the present materials that the rela- authors(IJ D)was supported as a Science and Technol tively short exposure time allowed production of only a ogy Agency Fellow. The authors gratefully acknowledge partial Sio2 layer at the fibre/matrix interface [30] so Y. Nomura and N. Suzuki for help with mechanical that t might be expected to increase further with addi- testing and Professor Curtin for useful discussion tional exposure time References 4. Conclusions Ishikawa T M, Hirokawa T. Stress/strain ehavior of 3D woven posites. in preparation. 1. In situ fibre fracture characteristics were investi 2] M, Hirokawa T. Optical gated for Si-Ti-C-O fibres after tensile testing up scopy of 3-D woven SiC/SiC-based composites. Comp Sci Tech- to 1380%C in vacuum and in air. The general fibre 3 Davies [J, Ishikawa T, Shibuya M, Hirokawa T. Damage char- morphology for specimens tested in vacuum indi- cterisation of 3-D woven SiC/SiC-based composites. In: Pro- cated fibre decomposition and void formation to be ceedings of the 2lst Symposium on Composite Materials, 31 noticeable at 1300.C with specimens tested above October-I November 1996, Toyama, Japan. Tokyo, Japan: The 1300 C not possessing distinct fracture mirrors 2. Specimens tested in air at 1100 and 1200C gen- 4 Davies IJ, Ishikawa T, Shibuya M. Hirokawa T, Gotoh J. Fibre and interfacial properties measured in situ for a 3-D woven erally had flat fracture surfaces with less than 20% Sic/SiC-based composite with glass sealant Composites Part A of the fibres exhibiting fracture mirrors. This was 199930(4):587-91earlier that the chemistry of ®bres and matrix was approximately similar, only slight di€erences in coe- cients of thermal expansion (CTE) for ®bre and matrix would be required to produce microcracking at the interface between large regions of ®bres and matrix, particularly when taking into account the low value of  expected for these composites. Further investigation concerning the spatial dependence of in situ ®bre prop￾erties within single ®bre bundles for these CMCs is planned. 3.3. Fibre/matrix interface shear stress Values of  shown in Table 3 were calculated from Eq. (7) using values of hhi and r established elsewhere [2,19]. It can be seen that  was approximately 5 MPa for specimens tested at room temperature with a slight increase to 9 MPa at 1300C in vacuum. These values fall within the range of 2±22 MPa [and close to the average of 9.47 MPa (‹3.50)] suggested by researchers for SiC-based ®bres with carbon interfaces [6,8,27,30± 33] tested at room temperature, and attributed to the low shear strength and friction coecient of the carbon￾rich interface. The increase in  with test temperature in vacuum was suggested possibly due to the CTE mismatch being reduced for specimens tested closer to their processing temperature. An alternative reason might be slight grain coarsening at the ®bre surface. Even for specimens tes￾ted at 1300C in vacuum the value of  was still <10 MPa which is normally associated with superior ®bre pull-out and composite mechanical properties. However, exposure to air at 1100 and 1200C increased  by an order of magnitude (55±60 MPa) compared to that at room temperature. These results are within the range suggested for  after exposure to oxidising atmosphere at elevated temperature [8]. It is possibly the case for the present materials that the rela￾tively short exposure time allowed production of only a partial SiO2 layer at the ®bre/matrix interface [30] so that  might be expected to increase further with addi￾tional exposure time. 4. Conclusions 1. In situ ®bre fracture characteristics were investi￾gated for Si±Ti±C±O ®bres after tensile testing up to 1380C in vacuum and in air. The general ®bre morphology for specimens tested in vacuum indi￾cated ®bre decomposition and void formation to be noticeable at 1300C with specimens tested above 1300C not possessing distinct fracture mirrors. 2. Specimens tested in air at 1100 and 1200C gen￾erally had ¯at fracture surfaces with less than 20% of the ®bres exhibiting fracture mirrors. This was attributed to oxygen ingression into the ®bre bun￾dles with only a relatively small number of ®bres having an interface weak enough to allow ``weak￾est link'' mechanisms to apply. 3. Fibre strength characteristics normalised to a 10ÿ3 m gauge length indicated ®bres tested in air at ele￾vated temperature to have signi®cantly lower strengths and average Weibull parameter, m, compared to the room temperature, 1200C/ vacuum, and 1300C/vacuum cases. This was attributed to oxygen damage of the ®bre together with oxidation of the ®bre/matrix interface. 4. Fibre characteristics for ®bres tested at room temperature, 1200C/vacuum, and 1300C/vacuum showed no signi®cant di€erence between ®bres located at the end and centre of the ®bre bundle main axis. This was not the case for 1100 and 1200C/air specimens where a signi®cant di€erence was observed and attributed to the di€erence in ®bre numbers closer to the ®bre bundle perimeter. From this it was suggested that the majority of oxygen ingress was from the ®bre bundle perimeter. 5. The ®bre/matrix interface shear strength was low for the room temperature specimens and increased slightly with temperature when tested in vacuum, possibly due to changes in the thermal mismatch between ®bres and matrix. Values for specimens tes￾ted at 1100 and 1200C in air were an order of mag￾nitude greater than for room temperature specimens indicating a signi®cant degree of oxidation damage at the ®bre/matrix interface to have occurred. Acknowledgements This work was supported by funding from the Science and Technology Agency of Japan whilst one of the authors (I.J.D) was supported as a Science and Technol￾ogy Agency Fellow. The authors gratefully acknowledge Y. Nomura and N. Suzuki for help with mechanical testing and Professor Curtin for useful discussion. References [1] Davies IJ, Ishikawa T, Shibuya M, Hirokawa T. Stress/strain behavior of 3-D woven SiC/SiC-based composites, in preparation. [2] Davies IJ, Ishikawa T, Shibuya M, Hirokawa T. Optical micro￾scopy of 3-D woven SiC/SiC-based composites. Comp Sci Tech￾nol 1999;59(3):429±37. [3] Davies IJ, Ishikawa T, Shibuya M, Hirokawa T. Damage char￾acterisation of 3-D woven SiC/SiC-based composites. In: Pro￾ceedings of the 21st Symposium on Composite Materials, 31 October±1 November 1996, Toyama, Japan. Tokyo, Japan: The Japan Composite Society. pp. 103±4. [4] Davies IJ, Ishikawa T, Shibuya M, Hirokawa T, Gotoh J. Fibre and interfacial properties measured in situ for a 3-D woven SiC/SiC-based composite with glass sealant. Composites Part A 1999;30(4):587±91. 810 I.J. Davies et al. / Composites Science and Technology 59 (1999) 801±811