N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 observation that the major load drops observed in the present study are followed by an increase in the load with the subsequent crack extension, which indicates that the subsequent crack growth occurs in a stable manner. ) Secondly, the mode of fracture is not the same for the various combinations of crack length(a) and displacement(O). For lower crack lengths and small displacements(as in the case of specimens with ay and a2 crack lengths and 8, displacement), the mode of failure is purely mode I or tensile. On the other hand, for large crack length(a3)specimens and large displacement(8, and 83)values, the mode of fracture involves significant mode Il or in-plane shear component of fracture 3.5. Comparison of various fracture toughness parameters The present CFCC material is isotropic in the in-plane directions. Hence, we assume that the following equation to derive the fracture energy (ko) from the Klc values [33] is valid (Jkg)=k2(1-y2) where v is the poissons ratio, whose value was assumed to be 0.3 for both the orientations The values of elastic modulus(E) were experimentally measured as 18.8 GPa in the two in-plane directions [34]. The ko values derived from Klc are up to an order of magnitude lower(333 J/m in the crack divider orientation and 47 J/m- in the crack arrester orientation) than the Jic and Jc values(the maximum value of Jc in the crack divider orientation is 3.6 kJ/m"). This is because the Klc or the ko includes the fracture events at the most up to first fibre bundle failure(mostly confined to the events of matrix microcracking) and does not include any further events of crack-reinforcement interactions. Finally, all the three fracture resistance parameters, namely the kQ(fracture energy value corresponding to the Klc), JIc and the Je values, determined in the present study for the two notch orientations are com- pared in Table 3. This data comparison clearly reveals that the Klc values grossly underestimate the fracture resistance of the material as compared to Jc or J values. The Jc values, themselves are 2 to 3 times lower as compared to the Je values, a finding that demonstrates the effectiveness of the fibre bundle failure and the related increase in the fracture resistance by crack bridging. This is reflected in the values of b, the energy consumed in the development of the wake zone (3.27 kJ/m- in crack divider orientation and 0. 79 kJ/m" in the crack arrester orientation), whose values compare very well with the Je values and are considerably higher than the Jle values. This indicates that the present CFCC material exhibit large degree of nonlinear fracture region with pronounced R-curve effects, with large fracture energies that are involved in the cre- ation of a highly effective fracture zone at the wake of the crack tip, a behaviour that nearly matches with the recent results reported in the case of 2D SiC/SiC CFCCs [31]. In view of the above, it is appropriate consider the Jc values of the material and refer to these values as the most effective design parameter. 3.6. Anisotropy in fracture resistance Major differences in the fracture behaviour of the present CFCC material in the two orthogonal notch orientations of crack divider and crack arrester orientations are described in the preceding paragraphs Table 3 Fracture toughness parameters of the CFCC material, values evaluated in the present study Crack divider orientation Crack arrester orientation Plane strain oughness(Klc), MPavm 2.03 And equivalent Jgo or Jup, kJ/m2 2. Mode I, elastic-plastic fracture toughness, le or Jo), kJ/m- 1.30 Total fracture energy release rate (c), kJ/m-observation that the major load drops observed in the present study are followed by an increase in the load with the subsequent crack extension, which indicates that the subsequent crack growth occurs in a stable manner.) Secondly, the mode of fracture is not the same for the various combinations of crack length (a) and displacement (d).For lower crack lengths and small displacements (as in the case of specimens with a1 and a2 crack lengths and d1 displacement), the mode of failure is purely mode I or tensile.On the other hand, for large crack length (a3) specimens and large displacement (d2 and d3) values, the mode of fracture involves significant mode II or in-plane shear component of fracture. 3.5. Comparison of various fracture toughness parameters The present CFCC material is isotropic in the in-plane directions.Hence, we assume that the following equation to derive the fracture energy (JKQ) from the KIc values [33] is valid: ðJKQÞ ¼ K2 Icð1  m2 Þ=E; ð5Þ where m is the Poisson’s ratio, whose value was assumed to be 0.3 for both the orientations. The values of elastic modulus (E) were experimentally measured as 18.8 GPa in the two in-plane directions [34]. The JKQ values derived from KIc are up to an order of magnitude lower (333 J/m2 in the crack divider orientation and 47 J/m2 in the crack arrester orientation) than the JIc and Jc values (the maximum value of Jc in the crack divider orientation is 3.6 kJ/m2).This is because the KIc or the JKQ includes the fracture events at the most up to first fibre bundle failure (mostly confined to the events of matrix microcracking) and does not include any further events of crack–reinforcement interactions. Finally, all the three fracture resistance parameters, namely the JKQ (fracture energy value corresponding to the KIc), JIc and the Jc values, determined in the present study for the two notch orientations are com￾pared in Table 3.This data comparison clearly reveals that the KIc values grossly underestimate the fracture resistance of the material as compared to JIc or Jc values.The JIc values, themselves are 2 to 3 times lower as compared to the Jc values, a finding that demonstrates the effectiveness of the fibre bundle failure and the related increase in the fracture resistance by crack bridging.This is reflected in the values of Jb, the energy consumed in the development of the wake zone (3.27 kJ/m2 in crack divider orientation and 0.79 kJ/m2 in the crack arrester orientation), whose values compare very well with the Jc values and are considerably higher than the JIc values.This indicates that the present CFCC material exhibit large degree of nonlinear fracture region with pronounced R-curve effects, with large fracture energies that are involved in the cre￾ation of a highly effective fracture zone at the wake of the crack tip, a behaviour that nearly matches with the recent results reported in the case of 2D SiC/SiC CFCCs [31].In view of the above, it is appropriate to consider the Jc values of the material and refer to these values as the most effective design parameter. 3.6. Anisotropy in fracture resistance Major differences in the fracture behaviour of the present CFCC material in the two orthogonal notch orientations of crack divider and crack arrester orientations are described in the preceding paragraphs. Table 3 Fracture toughness parameters of the CFCC material, values evaluated in the present study S.no. Description Crack divider orientation Crack arrester orientation 1.Plane strain fracture toughness (KIc), MPa pm 2.03 0.98 And equivalent JKQ or Jtip, kJ/m2 0.333 0.047 2.Mode I, elastic–plastic fracture toughness, (JIc or JQ), kJ/m2 1.36 0.66 3.Total fracture energy release rate (Jc), kJ/m2 3.6 0.84 N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605 2601