2602 N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 From these, it is evident that the fracture behaviour of the CFCC in terms of nature of load-displacement curves, values of various fracture toughness parameters(see Table 3)and finally, the nature of crack extension, are grossly different for the two notch orientations. Such differences are more clearly reflected in the variation of total fracture energy release rate o)with the extent of crack extension(8/8), as depicted in Fig. 9. These could be termed as R-curves. These fracture resistance curves show that the material exhibits pronounced R-curve effects( the net increase in fracture resistance with crack extension), indicating sig- nificantly higher fracture resistance(both in terms of initiation(8/8c 1)and propagation(8/8c>1) fracture resistance values) in crack divider orientation as compared to crack arrester orientation. At any value of 8/8c, the Je in the crack divider orientation is more than 4-5 times higher. Further, the R-curve in the crack divider orientation is highly rising in nature, while the same in the crack arrester orientation is relatively flat. These observations clearly point to the fact that crack bridging is all the more effective in the crack divider orientation, in which orientation the fracture resistance of the CFCC is significantly highe 4. Comparison of fracture resistance It is interesting to note that the silica materials, first ever explored for any advanced structural appl cation, are in the form of ultra light weight(96-352 kg/m, highly porous) fibrous ceramic thermal insu- lating structures. The first comprehensive studies on the mechanical properties of these structures were malen ently by Ortiz-Longo and White [9] have comprehensively evaluated the fracture resistance of these in the present study show that the present CFCC material is much superior (though the present CFCC material is also porous, it is much denser (2250 kg/m)in comparison). The conservative estimate of the plane strain fracture toughness, Klc of the present CFCC material (2.03 MPa vm in the crack divider orientation) itself is nearly two orders of magnitude higher than the fracture resistance values reported for these (highly porous) fibrous structural materials. The fracture toughness values(Klc or Kg) of these materials are in the range of 12-160 kPa Vm, depending on the process condition, fibre characteristics, notch orientation and finally, the test temperature [9, 36-40. In another study, Hashida et al. [29] have reported the fracture energy values of several glass-matrix-based CFCCs. These fracture energy values (3.5- 242 Jm; see Table 2 in Ref [29]) vary significantly depending on the combination of the matrix and the reinforcement characteristics. The reported fracture energy values include the crack propagation events and hence, represent the highest possible fracture energy values. The Jc values of the present CFCC material are more than an order of magnitude higher as compared to this class of CFCCs. However, it should be noted here that the Klc values of these glass-matrix CFCCs compare well with the Klc values of the present CFCC material(see data in Tables 2 and 3 of Ref [29]. Hence, this data comparison clearly illustrates again that the crack tip fracture processes(which essentially influence the Klc values)contribute to a small extent to the overall fracture resistance and when the further fracture events of crack extension processes such as fibre bundle failure are included, the total fracture energy release rate values become significantly higher Finally, the Klc of the present CFCC material(2.03 MPa vm in the crack divider orientation and 0.98 MPa Vm in the crack arrester orientation) compare favourably only with monolithic fused silica(0.74 MPa vm)[8]. This again signifies the fact that until all the fracture events other than the matrix micro- cracking are considered, the fracture resistance of the present CFCC material in terms of Klc appears limited. The K values of these silica materials either in the form of fused silica or CFCC are 100-200% lower as compared to the other structural ceramics(Klc N 3-5 MPa vm)and are nearly 10 times lower as compared to the high strength, high performance CFCCs based on the SiC/SiC materials [31, 41-44] (KIc N 20-30 MPa Vm, the best values reported till date for specimens with long cracks). However, theseFrom these, it is evident that the fracture behaviour of the CFCC in terms of nature of load–displacement curves, values of various fracture toughness parameters (see Table 3) and finally, the nature of crack extension, are grossly different for the two notch orientations.Such differences are more clearly reflected in the variation of total fracture energy release rate (Jc) with the extent of crack extension (d=dc), as depicted in Fig.9.These could be termed as R-curves.These fracture resistance curves show that the material exhibits pronounced R-curve effects (the net increase in fracture resistance with crack extension), indicating sig￾nificantly higher fracture resistance (both in terms of initiation (d=dc < 1) and propagation (d=dc > 1) fracture resistance values) in crack divider orientation as compared to crack arrester orientation.At any value of d=dc, the Jc in the crack divider orientation is more than 4–5 times higher.Further, the R-curve in the crack divider orientation is highly rising in nature, while the same in the crack arrester orientation is relatively flat.These observations clearly point to the fact that crack bridging is all the more effective in the crack divider orientation, in which orientation the fracture resistance of the CFCC is significantly higher. 4. Comparison of fracture resistance It is interesting to note that the silica materials, first ever explored for any advanced structural appli￾cation, are in the form of ultra light weight (96–352 kg/m3; highly porous) fibrous ceramic thermal insu￾lating structures.The first comprehensive studies on the mechanical properties of these structures were carried out by Newman in the early 1980s [35].Green et al.[36–38], Komine and Kobayashi [39,40] and more recently by Ortiz-Longo and White [9] have comprehensively evaluated the fracture resistance of these materials.A comparison of the fracture toughness properties of these porous materials with those reported in the present study show that the present CFCC material is much superior (though the present CFCC material is also porous, it is much denser (2250 kg/m3) in comparison).The conservative estimate of the plane strain fracture toughness, KIc of the present CFCC material (2.03 MPa pm in the crack divider orientation) itself is nearly two orders of magnitude higher than the fracture resistance values reported for these (highly porous) fibrous structural materials.The fracture toughness values (KIc or KR) of these materials are in the range of 12–160 kPa pm, depending on the process condition, fibre characteristics, notch orientation and finally, the test temperature [9,36–40].In another study, Hashida et al.[29] have reported the fracture energy values of several glass-matrix-based CFCCs.These fracture energy values (3.5– 242 J/m2; see Table 2 in Ref.[29]) vary significantly depending on the combination of the matrix and the reinforcement characteristics.The reported fracture energy values include the crack propagation events and hence, represent the highest possible fracture energy values.The Jc values of the present CFCC material are more than an order of magnitude higher as compared to this class of CFCCs.However, it should be noted here that the KIc values of these glass-matrix CFCCs compare well with the KIc values of the present CFCC material (see data in Tables 2 and 3 of Ref.[29]).Hence, this data comparison clearly illustrates again that the crack tip fracture processes (which essentially influence the KIc values) contribute to a small extent to the overall fracture resistance and when the further fracture events of crack extension processes such as fibre bundle failure are included, the total fracture energy release rate values become significantly higher. Finally, the KIc of the present CFCC material (2.03 MPa pm in the crack divider orientation and 0.98 MPa pm in the crack arrester orientation) compare favourably only with monolithic fused silica (0.74 MPa pm) [8].This again signifies the fact that until all the fracture events other than the matrix micro￾cracking are considered, the fracture resistance of the present CFCC material in terms of KIc appears limited.The KIc values of these silica materials, either in the form of fused silica or CFCC are 100–200% lower as compared to the other structural ceramics (KIc
3–5 MPa pm) and are nearly 10 times lower as compared to the high strength, high performance CFCCs based on the SiC/SiC materials [31,41–44] (KIc
20–30 MPa pm, the best values reported till date for specimens with long cracks).However, these 2602 N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605