1. Davies et al. /Journal of the European Ceramic Sociery 25(2005)599-604 may, for example, shed light on the value of t required for the suppression of crack deflection mechanisms in SiC/SIC composites. Thus, the present work will concern itself with a the investigation of mechanical and physical properties within a single fibre bundle for an orthogonal 3-D woven SiC/SiC composite tensile tested at 1 100oC in air. 2. Experimental procedure The composite under investigation, based on the SiC/SiC system, contained Tyranno LoxM Si-THC-O fibres(800 fibres/bundle)surface-modified so as to achieve a 40 nm arbon-rich layer adjacent to the fibre surface. 29 The aim of 12UA21 5 um surface modification was to promote crack deflection at the carbon-rich layer within the fibre itself, rather than at he fibre surface as is the case for most cmcs. The fibre (b) were woven into an orthogonal 3-d orthogonal configura- tion with fibre volume fractions of.19019 and 0.02 in the x, y, and directions, respectively. Matrix densification of the composite was achieved through the repeated poly mer impregnation and pyrolysis(PIP)of a precursor simi- lar to polytitanocarbosilane(PTCs). The use of similar pre cursors for fibre and matrix components was expected to minimise thermal stresses due to any mismatch in the co- efficients of thermal expansion. This composite system eferred to as"NUSK-CMC'' from the initials of the collab- 3 5 um orating partners and has been the subject of recent work by 321 the authors 29.30 Following machining to a suitable test geometry, 29the Fig. 1. Scanning electron micrographs illustrating the observed fracture composite was heated in air to 1 100C at0.75Cs-, loaded urface types in Tyranno" Si-TiC-O fibres:(a) fracture mirror, and(b) fat and featurele under tension(parallel to the y-axis)to failure, and furnace cooled (initial rate of 3.3Cs); the total time spent at 1 100C being on the order of 600s. Further experimental h, and (iii) whether the fibre exhibited a fracture mirror details are available elsewhere 8,29,31 (Fig. 1(a)) or was flat and featureless(Fig. 1 (b)). A total of In contrast to the complex non-linear stress/strain be- 698 fibres(out of the 800 nominally expected)were mea- aviour noted for similar specimens tested at room temper sured with the difference in number being explained by:(1) ature(RT) and 1200C in vacuum. 8, 29,31 the 1100 C/air shielding of fibres by neighbours, and(ii) the presence of specimens stress/strain curve was approximately linear to holes where fibres had pulled out failure with a low tensile strength(55 MPa)compared to In addition to the above parameters, due to the likely re- the rt(381+41 MPa) and 1200oC/vacuum(405+39 MPa) action of oxygen with the fibre surfaces, So and m were in- cases. In fact, the 1100.C/air specimen tensile strength was vestigated at the centre and edge of the fibre bundle by mea- comparable with that of the stress required for propagation Suring the fracture mirror radius, /'m, of individual fibres and suggesting that the specimen may have failed upon initial fibre strength, 5-2 relationship to determine the individual of matrix cracks within transverse fibre bundles (65 MPa) using the followin matrix microcracking(or shortly thereafter) ollowing failure, the specimen fracture surface was nvestigated using a scanning electron microscope(SEM) (Model JSM-6300F, JEOL, Tokyo, Japan)and the following where Am is known as the "mirror constant and was pre viously determined to be 2.50+0.09MPam /2 for the randomly chosen fibre bundle near the specimen centre: (i SH-Ti-C-O fibres in situ the composite 5.36 and close to position within the fibre bundle, (ii) fibre pullout length the value of 2.51 MPam /2 proposed for nominally similar Nicalon Si-C-O fibres values of S. and m were de duced from cumulative failure probability curves of S after National Aerospace Laboratory of Japan, Ube Industries, Ltd, Shik. applying a suitable correction factor. Fibres wi ibo, Ltd, and Kawasaki Heavy Industries, Ltd are known to be effectively tested at a gauge length, Sc600 I.J. Davies et al. / Journal of the European Ceramic Society 25 (2005) 599–604 may, for example, shed light on the value of τ required for the suppression of crack deflection mechanisms in SiC/SiC composites. Thus, the present work will concern itself with the investigation of mechanical and physical properties within a single fibre bundle for an orthogonal 3-D woven SiC/SiC composite tensile tested at 1100 ◦C in air. 2. Experimental procedure The composite under investigation, based on the SiC/SiC system, contained Tyranno® LoxM Si–Ti–C–O fibres (800 fibres/bundle) surface-modified so as to achieve a 40 nm carbon-rich layer adjacent to the fibre surface.29 The aim of the surface modification was to promote crack deflection at the carbon-rich layer within the fibre itself, rather than at the fibre surface as is the case for most CMCs. The fibres were woven into an orthogonal 3-D orthogonal configura￾tion with fibre volume fractions of 0.19, 0.19, and 0.02 in the x, y, and z directions, respectively. Matrix densification of the composite was achieved through the repeated poly￾mer impregnation and pyrolysis (PIP) of a precursor simi￾lar to polytitanocarbosilane (PTCS). The use of similar pre￾cursors for fibre and matrix components was expected to minimise thermal stresses due to any mismatch in the co￾efficients of thermal expansion. This composite system is referred to as “NUSK-CMC” from the initials of the collab￾orating partners† and has been the subject of recent work by the authors.29,30 Following machining to a suitable test geometry,29 the composite was heated in air to 1100 ◦C at 0.75 ◦C s−1, loaded under tension (parallel to the y-axis) to failure, and furnace cooled (initial rate of 3.3 ◦C s−1); the total time spent at 1100 ◦C being on the order of 600 s. Further experimental details are available elsewhere.8,29,31 In contrast to the complex non-linear stress/strain be￾haviour noted for similar specimens tested at room temper￾ature (RT) and 1200 ◦C in vacuum,8,29,31 the 1100 ◦C/air specimen’s stress/strain curve was approximately linear to failure with a low tensile strength (∼55 MPa) compared to the RT (381±41 MPa) and 1200 ◦C/vacuum (405±39 MPa) cases. In fact, the 1100 ◦C/air specimen tensile strength was comparable with that of the stress required for propagation of matrix cracks within transverse fibre bundles (∼65 MPa),8 suggesting that the specimen may have failed upon initial matrix microcracking (or shortly thereafter). Following failure, the specimen fracture surface was investigated using a scanning electron microscope (SEM) (Model JSM-6300F, JEOL, Tokyo, Japan) and the following parameters measured for each fibre visible within a single randomly chosen fibre bundle near the specimen centre: (i) position within the fibre bundle, (ii) fibre pullout length, † National Aerospace Laboratory of Japan, Ube Industries, Ltd., Shik￾ibo, Ltd., and Kawasaki Heavy Industries, Ltd. Fig. 1. Scanning electron micrographs illustrating the observed fracture surface types in Tyranno® Si–Ti–C–O fibres: (a) fracture mirror, and (b) flat and featureless. h, and (iii) whether the fibre exhibited a fracture mirror (Fig. 1(a)) or was flat and featureless (Fig. 1(b)). A total of 698 fibres (out of the 800 nominally expected) were mea￾sured with the difference in number being explained by: (i) shielding of fibres by neighbours, and (ii) the presence of holes where fibres had pulled out. In addition to the above parameters, due to the likely re￾action of oxygen with the fibre surfaces, So and m were in￾vestigated at the centre and edge of the fibre bundle by mea￾suring the fracture mirror radius, rm, of individual fibres and using the following relationship to determine the individual fibre strength, S: 32–34 S = Am √rm (1) where Am is known as the “mirror constant” and was pre￾viously determined to be 2.50 ± 0.09 MPa m1/2 for the Si–Ti–C–O fibres in situ the composite35,36 and close to the value of 2.51 MPa m1/2 proposed for nominally similar Nicalon® Si–C–O fibres37 Values of So and m were de￾duced from cumulative failure probability curves of S after applying a suitable correction factor.38 Fibres within CMCs are known to be effectively tested at a gauge length, δc