Deducing the Fatigue Crack Growth Rates of Natural Flaws in Silicon Nitride Ceramics: Role of R‐Curves

Abstract

Fatigue failure is a concern when high-strength, high-toughness silicon nitride ceramics are used in mechanical components and the growth of natural flaws will determine the usable upper bound strength. In this study a fracture resistance curve (R-curve) model is incorporated into an established method for deducing natural flaw growth rates from a combination of strength and fatigue life data for smooth specimens. Experimental data for a commercial silicon nitride, SL200, were examined. When compared with results deduced using a constant fracture toughness model, the new method gives more physically realistic growth rate results. Specifically, by incorporating the R-curve the deduced fatigue threshold is equal to the reported intrinsic toughness for crack propagation of 2.2 MPa√m, whereas the constant fracture toughness model gives a physically unrealistic threshold value. Furthermore, much better agreement is achieved with the growth rates measured using macroscopic compacttension specimens. Overall, it is concluded that the R-curve effect should not be ignored when deducing the fatigue crack growth rates of natural flaws in high-toughness silicon nitride ceramics.

Ceramics and ceramic coatings in orthopaedics

“…It uniquely incorporates and demonstrates the importance of R-curves in determining fatigue resistance. First elucidated by Kruzic et al [388] using Al 2 O 3 and Si 3 N 4 as model systems, and later expanded and verified by Gallops et al [389], Hartelt et al [390], and Greene et al [391,392], it deconvolutes K TH into two terms:…”

Long‐term ceramic reliability analysis including the crack‐velocity threshold and the “bathtub” curve

“…Analytical lifetime solutions including contact effects exist for assumed power‐law crack velocities . Additionally important are microstructural effects, typically varying as g ~ −(1 − c − X ) (with X > 1), written to reflect a resistance to crack propagation that depends on microstructure and increases with crack length to a steady‐state value (an R ‐curve) . R ‐curve effects are usually absent in fine‐grain ceramics and glasses and mostly evident in large‐grain polycrystals and phase transforming ceramics, greatly affecting lifetimes and measured or deconvoluted crack velocities .…”

“…16 Additionally important are microstructural effects, typically varying as g~À(1 À c ÀX ) (with X > 1), written to reflect a resistance to crack propagation that depends on microstructure and increases with crack length to a steady-state value (an R-curve). [3][4][5]17 R-curve effects are usually absent in fine-grain ceramics and glasses and mostly evident in large-grain polycrystals and phase transforming ceramics, greatly affecting lifetimes and measured or deconvoluted crack velocities. [3][4][5]17 As many R-curves derive from microstructural-scale stress distributions coupled to crystallographic CTE inhomogeneity, temperature changes could affect lifetimes through variable crack resistance effects.…”

“…[3][4][5]17 R-curve effects are usually absent in fine-grain ceramics and glasses and mostly evident in large-grain polycrystals and phase transforming ceramics, greatly affecting lifetimes and measured or deconvoluted crack velocities. [3][4][5]17 As many R-curves derive from microstructural-scale stress distributions coupled to crystallographic CTE inhomogeneity, temperature changes could affect lifetimes through variable crack resistance effects. Prior crack propagation or damage, such as occurs in cyclic fatigue, can also modify fracture resistance, especially the threshold 18 and thereby also affect lifetime.…”

A Theoretical Model with the Effect of Cracks in the Local Spalling of Full Ceramic Ball Bearings

“…However, the bending stress exerted by the rolling elements on the outer ring during the process of passing through the spalling position of the outer ring was not taken into account in the study, and the bending stress may also be the reason for the initiation and propagation of the cracks. Researchers [23][24][25][26][27][28] have conducted deep analyses on the crack formation mechanism of the cracks under contact stress. On the basis of the principle of fracture mechanics, the influence of cracks caused by bending moment on stiffness can be analyzed by calculating the strain energy caused by cracks [29][30][31].…”