Cyclic Fatigue in Ceramics: A Balance between Crack Shielding Accumulation and Degradation

Abstract

Cyclic fatigue growth rates in R‐curve ceramics have been observed to depend very strongly on the maximum applied stress intensity, Kmax, and only weakly on the stress intensity range, Δ This behavior is rationalized through measurement of crack wake shielding characteristics as a function of these fatigue parameters in a gas‐pressure‐sintered silicon nitride. In particular, evidence for a mechanical equilibrium between shielding accumulation by crack growth and shielding degradation by frictional wear of sliding interfaces is found for steady‐state cyclic fatigue. This equilibrium gives rise to a rate law for cyclic fatigue. The data suggest that the accumulation process is the origin of the strong Kmax dependence, and that the degradation process is the origin of the weak Δ dependence. These features are shown to be related to the “cyclic”R‐curve and to the cyclic crack opening displacement, respectively.

Model for Fatigue Crack Growth in Grain‐Bridging Ceramics

“…20,21,26,27,29 Specifically, an extremely strong dependence of crack growth rate on K max , unique to ceramics, has been observed in these studies. In contrast, the growth rate dependence on ⌬K is modest and appears to be similar to that observed for metals.…”

In vitro lifetime of dental ceramics under cyclic loading in water

“…In this case, the samples were tested in air and the crack velocity was normalized for comparison with fatigue results obtained for the same material using other measuring techniques. Based on the fatigue empirical equation suggested by previous authors [8,22,25], we used the ratio v/(1ÀR) 2 as normalized velocity for comparison of data measured at different DK I and R values.…”

“…8), it becomes evident that a lower K I,max /K IC ratio is required for crack propagation in the 3Y-TZP and Al 2 O 3 -ZrO 2 -Glass materials, indicating that the zirconia-based ceramics are more prone to subcritical crack growth than the Li 2 O Á 2SiO 2 glass-ceramic. Subcritical crack growth in polycrystalline materials submitted to cyclic stresses may result from the water-assisted breakage of ceramic bonds at the crack tip and/or the degradation of toughening mechanisms involving grain bridging or phase transformations [8,9,25,27]. The higher susceptibility of the 3Y-TZP specimens and the Al 2 O 3 -ZrO 2 -Glass composite to subcritical crack growth is probably related to the degradation of the zirconia transformation toughening mechanism, which is absent in the case of the Li 2 O Á 2SiO 2 glass ceramic.…”

Hertzian contact fatigue on alumina/alumina-zirconia laminated composites

“…In the case of alumina, the main toughening mechanism is grain bridging. 32,33 In the case of Hertzian indentation, the crack is short not only in the perimeter direction of the ring, but specially in the depth direction, as the crack is just some micrometers deep because of the sharp decrease in tensile radial stress with depth, which becomes compressive just after a short depth. 23,34,35 That is, while in the case of stress corrosion cracking the fracture process zone develops without impediment, in the case of cyclic loading this fracture process zone degrades continuously, preventing the development of R-curve.…”