N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 without the contributions from matrix fracture events. have led to unified models for the fracture resistance in materials that exhibit crack bridging [2-6]. The toughening in these cases of crack bridging is essentially due to ductile or brittle reinforcements. In the case of present CFCCs, it is the later that makes contri butions to the toughening. In the present paper, the fracture behaviour of a two-dimensional (2D) silica fibre-reinforced, silica matrix composite is presented and discussed. Various parameters of fracture resistance have been used to quantify the fracture resistance of the material. These include, the plane strain fracture toughness(Kl) elastic-plastic fracture toughness (ic) and total fracture energy release rate (e). Also reported and dis- cussed are the effects of notch orientation and notch depth on the fracture resistance in these composites. 2. Experimental details he two-dimensionally weaved silica fibre preforms are vacuum impregnated using colloidal silica Ition precursor to provide the matrix for the silica-silica continuous fibre-reinforced, ceramic-matrix composites (referred to as"silica-silica CFCC"or simply"CFCC"). The interconnected network of capillaries in the preforms facilitates solution impregnation, thus providing uniform matrix for the CFCC After infiltration, the CFCC is dried and during this drying process, water content of the matrix gel solution is gradually removed. These dried CFCCs are then sintered to impart interparticle bonding and in turn, this Facilitates load transfer from the matrix to the fibre and vice-versa There are no standard test procedures for the evaluation of fracture toughness/energy of ceramic materials, especially for the advanced ceramic composites such as CFCCs. However, several studies have been reported in the recent past which describe in detail the procedures adopted for and the fracture behaviour observed of the monolithic ceramics and ceramic-matrix composites, including CFCCs(see Refs. [11-18] for details and a summary of these details in Ref [19D. Since ceramic materials exhibit brittle fracture, the AsTM Standard E-399, describing the standard practice for the evaluation of plane strain fracture toughness of metallic materials [20], can conveniently be adopted to determine the fracture oughness of these materials. However, since the CfCCs also exhibit limited extent of non-linear fracture, the J-integral technique once again developed for metallic materials(fundamentals and standard practices described in Refs. [21-23] and [24], respectively) also applies equally Single edge notch beam(SENB)specimens of 8 mm thickness, 10 mm width and a span length of 40 mm were used. The fracture toughness/energy was evaluated in two notch orientations, namely (i)crack divider orientation, in which the notch is along the orientation of the plies in the thickness direction and (ii)crack arrester orientation, in which the notch is perpendicular to the orientation of the plies in the thickness direction(the third orientation of crack delamination could not be studied because of specimen size limi tations). In both cases, notch is perpendicular to the longitudinal plies Notches of varied length were introduced using 0.3 mm thick diamond wafer blades, mounted on a standard Isomet cutting machine. A specially designed jig was used to obtain straight notches by moving the job across the cutting plane. The notches thus introduced were found to have a finite root radius, p, typically of the order of 160 um. The p values were determined by Delta TM 35 x-y profile projector. The notch root radii. either in the crack divider or crack arrester orientation were found to be similar. The crack lengths were maintained in the range of 0.35 to 0. 7 times the specimen width. Among these, specimens with crack lengths in the range specified by the ASTM standard E-399[20](0.45-0.55 times the specimen width) were only considered for the determination of Klc values. The other specimens with large lengths were employed essentially to determine the work of fracture [25], which results will be reported separately. The fracture energy determined from the load-displacement data were used to determine the elastic-plastic fracture toughness, JIe and the total fracture energy release rate, Jc. The later two fracture resistance parameters are based on J-integral [21]without the contributions from matrix fracture events, have led to unified models for the fracture resistance in materials that exhibit crack bridging [2–6].The toughening in these cases of crack bridging is essentially due to ductile or brittle reinforcements.In the case of present CFCCs, it is the later that makes contri￾butions to the toughening. In the present paper, the fracture behaviour of a two-dimensional (2D) silica fibre-reinforced, silica– matrix composite is presented and discussed.Various parameters of fracture resistance have been used to quantify the fracture resistance of the material.These include, the plane strain fracture toughness (KIc), elastic–plastic fracture toughness (JIc) and total fracture energy release rate (Jc).Also reported and dis￾cussed are the effects of notch orientation and notch depth on the fracture resistance in these composites. 2. Experimental details The two-dimensionally weaved silica fibre preforms are vacuum impregnated using colloidal silica solution precursor to provide the matrix for the silica–silica continuous fibre-reinforced, ceramic–matrix composites (referred to as ‘‘silica–silica CFCC’’ or simply ‘‘CFCC’’).The interconnected network of capillaries in the preforms facilitates solution impregnation, thus providing uniform matrix for the CFCC. After infiltration, the CFCC is dried and during this drying process, water content of the matrix gel solution is gradually removed.These dried CFCCs are then sintered to impart interparticle bonding and in turn, this facilitates load transfer from the matrix to the fibre and vice-versa. There are no standard test procedures for the evaluation of fracture toughness/energy of ceramic materials, especially for the advanced ceramic composites such as CFCCs.However, several studies have been reported in the recent past which describe in detail the procedures adopted for and the fracture behaviour observed of the monolithic ceramics and ceramic–matrix composites, including CFCCs (see Refs.[11–18] for details and a summary of these details in Ref.[19]).Since ceramic materials exhibit brittle fracture, the ASTM Standard E-399, describing the standard practice for the evaluation of plane strain fracture toughness of metallic materials [20], can conveniently be adopted to determine the fracture toughness of these materials.However, since the CFCCs also exhibit limited extent of non-linear fracture, the J-integral technique once again developed for metallic materials (fundamentals and standard practices described in Refs.[21–23] and [24], respectively) also applies equally. Single edge notch beam (SENB) specimens of 8 mm thickness, 10 mm width and a span length of 40 mm were used.The fracture toughness/energy was evaluated in two notch orientations, namely (i) crack divider orientation, in which the notch is along the orientation of the plies in the thickness direction and (ii) crack arrester orientation, in which the notch is perpendicular to the orientation of the plies in the thickness direction (the third orientation of crack delamination could not be studied because of specimen size limi￾tations).In both cases, notch is perpendicular to the longitudinal plies. Notches of varied length were introduced using 0.3 mm thick diamond wafer blades, mounted on a standard Isomet cutting machine.A specially designed jig was used to obtain straight notches by moving the job across the cutting plane.The notches thus introduced were found to have a finite root radius, q, typically of the order of 160 lm.The q values were determined by Delta TM 35 x–y profile projector.The notch root radii, either in the crack divider or crack arrester orientation, were found to be similar.The crack lengths were maintained in the range of 0.35 to 0.7 times the specimen width. Among these, specimens with crack lengths in the range specified by the ASTM standard E-399 [20] (0.45–0.55 times the specimen width) were only considered for the determination of KIc values.The other specimens with larger crack lengths were employed essentially to determine the work of fracture [25], which results will be reported separately.The fracture energy determined from the load–displacement data were used to determine the elastic–plastic fracture toughness, JIc and the total fracture energy release rate, Jc.The later two fracture resistance parameters are based on J-integral [21]. N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605 2591