4718 KIM et al: CYCLIC FATIGUE OF BRITTLE CERAMICS Table 1. Fracture parameters for each material Soda-lime glass Silicon nitride 22 1900 650 factor in the crack driving force. Examination in tact stresses has been well documented in earlier ossed polars revealed the presence of considerable Vickers indentation studies [54-56] residual stress around the more heavily damaged Analogous surface observations of damage sites tes. Such a stabilizing influence from residual con- at high loads P in fast single-cycle contact tests 104 300um n=1 n=5×103 300um n三 n=10 300um Fig. 5. Optical micrographs of Hertzian indentation sites, comparing damage after single-cycle (n=D) ycle(n>1)loading: (a) soda-lime glass/water 500N, r=3. 18 mm);(b)porcelainw (P=500 N. r=3.18 mm);(c) silicon nitride/air(P= 2200 N, r=1.98 mm). Surface views: glass ar lain viewed in transmitted light (porcelain after back-surface thinning ) silicon nitride in reflected (Nomarski contrast, after gold coating). Note appearance of radial cracks at contact peripheryfactor in the crack driving force. Examination in crossed polars revealed the presence of considerable residual stress around the more heavily damaged sites. Such a stabilizing in¯uence from residual con￾tact stresses has been well documented in earlier Vickers indentation studies [54±56]. Analogous surface observations of damage sites at high loads P in fast single-cycle contact tests or Table 1. Fracture parameters for each material Material N P1 (N) s1 (MPa) Soda-lime glass 17.9 228 90 Porcelain 30 205 120 Silicon nitride 22 1900 650 Fig. 5. Optical micrographs of Hertzian indentation sites, comparing damage after single-cycle (n ˆ 1) and multi-cycle (nw1) loading: (a) soda-lime glass/water (P = 500 N, r=3.18 mm); (b) porcelain/water (P = 500 N, r=3.18 mm); (c) silicon nitride/air (P = 2200 N, r=1.98 mm). Surface views: glass and porcelain viewed in transmitted light (porcelain after back-surface thinning), silicon nitride in re¯ected light (Nomarski contrast, after gold coating). Note appearance of radial cracks at contact periphery after cycling. 4718 KIM et al.: CYCLIC FATIGUE OF BRITTLE CERAMICS