KIM et al. CYCLIC FATIGUE OF BRITTLE CERAMICS 4719 (a) (b) n 300um n=103 n n=5×1 Fig. 6. Optical micrographs of Hertzian indentation failure sites in broken inert strength specimens. celain/water. Flexural tension axis horizontal. Surface views, reflected light (Nomarski contrast, after gold coating). Note how fracture origins begin outside the contact circle at low n, and move inside as at equivalent test durations I in extended constant of some rate effect in the radial crack initiation pre load tests revealed no obvious signs of radial crack- cess. ing in any of the materials. Howeve Half-surface and section optical micrographs of ing the fact that the predicted strength function in damage zones in the glass and porcelain in the equations (12a)and (12b) fits the data over the heavy damage region are shown in Fig. 7, for tests entire range of P at n= l in Fig. 3 and the static at load P= 140 N. The sections are viewed in data over the entire range of test time I in Fig. 4, transmitted light, the porcelain after thinning from the failure paths in the glass and porcelain did tend both sides down to the mid-plane. Secondary cone to intersect the contact zone at large P and t(black cracks propagate steeply downward from well symbols in Figs 3 and 4), suggesting the existence within the contact circle. The surface traces of theseat equivalent test durations t in extended constant load tests revealed no obvious signs of radial crack￾ing in any of the materials. However, notwithstand￾ing the fact that the predicted strength function in equations (12a) and (12b) ®ts the data over the entire range of P at n ˆ 1 in Fig. 3 and the static data over the entire range of test time t in Fig. 4, the failure paths in the glass and porcelain did tend to intersect the contact zone at large P and t (black symbols in Figs 3 and 4), suggesting the existence of some rate e€ect in the radial crack initiation pro￾cess. Half-surface and section optical micrographs of damage zones in the glass and porcelain in the heavy damage region are shown in Fig. 7, for tests at load P = 140 N. The sections are viewed in transmitted light, the porcelain after thinning from both sides down to the mid-plane. Secondary cone cracks propagate steeply downward from well within the contact circle. The surface traces of these Fig. 6. Optical micrographs of Hertzian indentation failure sites in broken inert strength specimens, after indentation at P = 500 N, r=3.18 mm, for n values indicated: (a) soda-lime glass/water; (b) por￾celain/water. Flexural tension axis horizontal. Surface views, re¯ected light (Nomarski contrast, after gold coating). Note how fracture origins begin outside the contact circle at low n, and move inside as damage intensi®es at large n. KIM et al.: CYCLIC FATIGUE OF BRITTLE CERAMICS 4719