B G. Nair et al. Materials Science and Engineering 4300(2001)68- men and more than three times that for = 90. Fur- At 1300%C. 30% of the inelastic strain was recovered thermore, a misorientation of just 200 from the loading for a =0 specimen on dropping the level of stress direction causes an increase in Ess by a factor of 30 over down to x3 MPa at the conclusion of a stress-stepped the on-axis case test. For off-axis geometries, =20 composites showed the highest strain-recovery(- 14% of inelastic 5.5}T=1300°c strain)after such a stress drop. For all other orienta tions. the recovered strain was minimal and could not be estimated accurately due to noise in displacement Fig. 10 shows data for the creep of the unreinforced CAS-II matrix material. The data indicates a clear transition from Newtonian (n= 1)rheology at low stress(-0,15 MPa) to a non-Newtonian (n>2) rheology at higher stresses. Q for matrix creep increased from x 900 kJ mol-l at 15 mPa to about 1080 kJ ol- at 35 MPa 3.3. Optical microscopy Log F-o, MPa) Optical micrographs for undeformed and deformed 2D composite specimens are presented in Fig. Ila-d 55}T=1300°c Fig. lla shows the microstructure of the undeformed (b) 2D material. Some intrinsic damage (i.e. as created in the composite fabrication process) in the form of a 1D:φ=20° network of randomly oriented matrix microcracks most of which are under 100 um in length, is present Such a network of cracks was also seen in the deformed 2D composite specimens; tthe 0/-90(Fig 1lb)and the 40/-50(Fig llc)composites had microstructures very similar to that of the undeformed composite. However, visual study of these micrographs suggested that the length of cracks(in the 90 and 50 plies)in the direction deformed composites as compared to the undeformed 13 1.6 composite- a few cracks of length as high as 400 um Log F-o (MPa)l in the direction of the applied stress. There was no significant increase in the number of cracks, however, (c) suggesting that the existing pre-cracks from the unde 50T=1300°c formed composite must have elongated in the direction of the compressive stress during creep. There was very little creep-induced cavitation at the fiber-matrix inter- face in the 50 or 90 plies as compared to ID com- posites with =40-90. The 20/-70% com however, showed a very different microstructure(Fig I ld)as compared to the other orientations. The mi- crostructure was characterized by very long crack (600-800 um long in some cases)that originate in the 70 plies and eventually get deflected toward the 20o l 1.6 4. Discussion Fig. 5. Comparison of creep data at 1300C of 2D composite creep data to ID data for specimens with o=y and(90-y):(a)0/-90 The ometry in (b)20/-70°,(c)40/-50° experiments enforces an isostrain condition on theB.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 73 men and more than three times that for 8=90°. Fur￾thermore, a misorientation of just 20° from the loading direction causes an increase in o; ss by a factor of 30 over the on-axis case. At 1300°C, 30% of the inelastic strain was recovered for a 8=0° specimen on dropping the level of stress down to 3 MPa at the conclusion of a stress-stepped test. For off-axis geometries, 8=20° composites showed the highest strain-recovery (14% of inelastic strain) after such a stress drop. For all other orienta￾tions, the recovered strain was minimal and could not be estimated accurately due to noise in displacement data. Fig. 10 shows data for the creep of the unreinforced CAS-II matrix material. The data indicates a clear transition from Newtonian (n=1) rheology at low stress (−s115 MPa) to a non-Newtonian (n\2) rheology at higher stresses. Q for matrix creep increased from 900 kJ mol−1 at 15 MPa to about 1080 kJ mol−1 at 35 MPa. 3.3. Optical microscopy Optical micrographs for undeformed and deformed 2D composite specimens are presented in Fig. 11a–d. Fig. 11a shows the microstructure of the undeformed 2D material. Some intrinsic damage (i.e. as created in the composite fabrication process) in the form of a network of randomly oriented matrix microcracks, most of which are under 100 mm in length, is present. Such a network of cracks was also seen in the deformed 2D composite specimens; tthe 0/–90° (Fig. 11b) and the 40/–50° (Fig. 11c) composites had microstructures very similar to that of the undeformed composite. However, visual study of these micrographs suggested that the length of cracks (in the 90 and 50° plies) in the direction of the applied stress was somewhat higher in these deformed composites as compared to the undeformed composite — a few cracks of length as high as 400 mm in the direction of the applied stress. There was no significant increase in the number of cracks, however, suggesting that the existing pre-cracks from the unde￾formed composite must have elongated in the direction of the compressive stress during creep. There was very little creep-induced cavitation at the fiber-matrix inter￾face in the 50 or 90° plies as compared to 1D com￾posites with 8=40–90°. The 20/–70° composites however, showed a very different microstructure (Fig. 11d) as compared to the other orientations. The mi￾crostructure was characterized by very long cracks (600–800 mm long in some cases) that originate in the 70° plies and eventually get deflected toward the 20° plies. 4. Discussion The specimen geometry in the 2D composite creep experiments enforces an isostrain condition on the plies. Fig. 5. Comparison of creep data at 1300°C of 2D composite creep data to 1D data for specimens with 8=c and (90−c); (a) 0/–90°; (b) 20/–70°, (c) 40/–50°.