M.L. Antti et al. Journal of the European Ceramic Society 24(2004)565-578 a=0. It is seen that the constants indicate a significant 43. Microstructure and fractography embrittlement with increasing treatment time and tem- perature. The embrittlement is particularly noticeable at Fig. 9 shows representative micrographs of the frac- 100C, 100 h. This is seen, not only in the low values tured, as-received 0/90 material. There were numerous of Ke and co but also as a greater notch sensitivity mea- cracks in the matrix perpendicular to the plies(Fig. 9a) sured as the relative loss in fracture strength of notched one example being shown more closely in Fig. 9b. These samples with respect to the unnotched strength. At a/w never penetrated the fibres. Since the density of cracks 0.25 the loss was 65% for the 1100C/100h material was similar in untested material it is supposed that the compared with 20-30% for the other treatments and the majority are shrinkage cracks formed during production aS-received material rather than multiple matrix cracking generated during The constants can in association with Eq (1)be used loading. The shrinkage cracks widened significantly to predict the effect of notch diameter on the strength of during heat-treatment, probably due to shrinkage of the an infinite plate and these predictions are included as matrix(see later). There were large voids in some of the e curves in Fig 8. The closeness of these curves to the fibre-free areas, but also occasionally inside bundles experimental results provides another justification of the These large voids will in the following be denoted mac- applicability of the model ropores. The infiltration into most bundles was effective, Fig 9. As-received material showing shrinkage cracks, and voids(a and b) and examples of complete(c)and incomplete (d) bundle infiltration. Tw urfaces.(a) Overview, optical microscope (b)Shrinkage crack between two fibres (SEM, SED).(c)Successful infiltration in fibre bundle. (SEM SED. (d)Infiltration not complete. (SEM, BED)a=0. It is seen that the constants indicate a significant embrittlement with increasing treatment time and tem￾perature. The embrittlement is particularly noticeable at 1100
C, 100 h. This is seen, not only in the low values of Kc and c0 but also as a greater notch sensitivity mea￾sured as the relative loss in fracture strength of notched samples with respect to the unnotched strength. At a/w =0.25 the loss was 65% for the 1100
C/100h material compared with 20–30% for the other treatments and the as-received material. The constants can in association with Eq. (1) be used to predict the effect of notch diameter on the strength of an infinite plate and these predictions are included as the curves in Fig. 8. The closeness of these curves to the experimental results provides another justification of the applicability of the model. 4.3. Microstructure and fractography Fig. 9 shows representative micrographs of the frac￾tured, as-received 0/90 material. There were numerous cracks in the matrix perpendicular to the plies (Fig. 9a) one example being shown more closely in Fig. 9b. These never penetrated the fibres. Since the density of cracks was similar in untested material it is supposed that the majority are shrinkage cracks formed during production rather than multiple matrix cracking generated during loading. The shrinkage cracks widened significantly during heat-treatment, probably due to shrinkage of the matrix (see later). There were large voids in some of the fibre-free areas, but also occasionally inside bundles. These large voids will in the following be denoted mac￾ropores. The infiltration into most bundles was effective, Fig. 9. As-received material showing shrinkage cracks, and voids (a and b) and examples of complete (c) and incomplete (d) bundle infiltration. TW surfaces. (a) Overview, optical microscope. (b) Shrinkage crack between two fibres. (SEM, SEI). (c) Successful infiltration in fibre bundle. (SEM, SEI). (d) Infiltration not complete. (SEM, BEI). 572 M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578