N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 increases with increase in the displacement as higher displacements include more fracture processes and events that enhance the fracture resistance of these materials. microstructural observations of the fractured pecimens have shown that the cracks in all the three cases of varied crack length extend in the shear mode; not, in the intended tensile mode, once a displacement of value of about 600-800 um(depending on the initial crack length, termed here as the maximum stable displacement, Smax)is reached. Hence no dis placements are considered for the e evaluation in the present study, which are in excess of this Smax value Further, in the displacement region where shear fracture is dominant, any further crack extension(increase in the displacement, d) does not significantly increase the total fracture energy release rate ( Je). It is also observed that soon after the event of first fibre bundle fracture. the crack front becomes unstable, but the specimen regains stability(shown as the increase in the load with crack extension) after crack extension of a few tens of microns. We have chosen the displacement value at which such an unstable/stable crack extension occurs as the critical displacement value 8e. This Sc is taken as the one corresponding to the average 8 at peak load of the three or four specimens that are tested with different initial crack lengths. A 8 value of 260 um for crack divider orientation and 400 um for crack arrester orientation are found to be appropriate for the present case. The derived values of total fracture energy release rate at varied dis placements(given as displacements normalised by the 8c)are shown in Fig. 9 for the CFCC material in the crack arrester orientation. The data in Fig. 9 clearly show that the CFCC material exhibits maximum fracture energy, asymptotic values of Je of 3.6 and 0.84 kJ/m- for crack divider and crack arrester orien tations, respectively It should be noted here that the standard Jic evaluation procedures for the metallic materials [21-24]do not recommend the practice of using the calculated fracture energies for such large displacements as 83 for two Firstly, the sudden load drop corresponding to the breakage of a minimum of one fibre bundle is actually an event of unstable crack extension. (Though, this could be contested on the basis of the Crack Divider Fig. 9. Variation of total fracture energy release rate with normalised displacement in case of the CFCC material in the two notch orientations. Note the asymptotic values of Jc of 3.6 and 0.84 kJ/m2 respectively, represent the acture energies for the terial in two notch orientations of crack divider and crack arrester orientationsincreases with increase in the displacement as higher displacements include more fracture processes and events that enhance the fracture resistance of these materials.Microstructural observations of the fractured specimens have shown that the cracks in all the three cases of varied crack length extend in the shear mode; not, in the intended tensile mode, once a displacement of value of about 600–800 lm (depending on the initial crack length, termed here as the maximum stable displacement, dmax) is reached.Hence, no dis￾placements are considered for the Jc evaluation in the present study, which are in excess of this dmax value. Further, in the displacement region where shear fracture is dominant, any further crack extension (increase in the displacement, d) does not significantly increase the total fracture energy release rate (Jc).It is also observed that soon after the event of first fibre bundle fracture, the crack front becomes unstable; but, the specimen regains stability (shown as the increase in the load with crack extension) after crack extension of a few tens of microns.We have chosen the displacement value at which such an unstable/stable crack extension occurs as the critical displacement value dc.This dc is taken as the one corresponding to the average d at peak load of the three or four specimens that are tested with different initial crack lengths.A dc value of 260 lm for crack divider orientation and 400 lm for crack arrester orientation are found to be appropriate for the present case.The derived values of total fracture energy release rate at varied dis￾placements (given as displacements normalised by the dc) are shown in Fig.9 for the CFCC material in the crack arrester orientation.The data in Fig.9 clearly show that the CFCC material exhibits maximum fracture energy, asymptotic values of Jc of 3.6 and 0.84 kJ/m2 for crack divider and crack arrester orien￾tations, respectively. It should be noted here that the standard JIc evaluation procedures for the metallic materials [21–24] do not recommend the practice of using the calculated fracture energies for such large displacements as d3 for two reasons.Firstly, the sudden load drop corresponding to the breakage of a minimum of one fibre bundle is actually an event of unstable crack extension.(Though, this could be contested on the basis of the Fig.9.Variation of total fracture energy release rate with normalised displacement in case of the CFCC material in the two notch orientations.Note the asymptotic values of Jc of 3.6 and 0.84 kJ/m2 respectively, represent the maximum fracture energies for the material in two notch orientations of crack divider and crack arrester orientations. 2600 N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605