Fatigue Threshold R‐curves Predict Fatigue Endurance Strength for Self‐Reinforced Silicon Nitride

Abstract

There is a need for methods that can help predict and avoid fatigue failures of silicon nitride ceramic components. The fatigue threshold R-curve has been proposed as potential solution to this problem. In this study, the fatigue threshold R-curve for small, semielliptical surface cracks was calculated for a silicon nitride ceramic using the published bridging stress distribution developed from fatigue threshold tests on macroscopic crack specimens. To test the accuracy of the endurance strengths predicted using the fatigue threshold R-curve, fatigue tests were conducted using four-point bend beams of silicon nitride containing semielliptical surface cracks introduced by Knoop indentation. The effectiveness of the methodology was verified; indeed, 77% of the beams tested at stress levels above the predicted endurance strength failed within 10 7 cycles and 0% of the beams tested below the predicted endurance strength failed within 10 7 cycles. Furthermore, using the bridging stress distribution, which is thought to be a material property, the need for prohibitively difficult fatigue threshold experiments on small surface cracks is avoided. Accordingly, this methodology is potentially quite practical for use in the engineering design of ceramic mechanical components. D. Marshall-contributing editorManuscript No. 33525.

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:…”

Fatigue Threshold R-Curves for Dental Lithium Disilicate Glass–Ceramics

“…However apparent, based solely on the extent of R-curve degradation observed here, ceramics with larger crystal sizes are expected to present lower fracture rates at earlier lifetimes due to their higher strength and steeper R-curve ( Kruzic et al 2005 ; Greene et al 2013 , 2014 ), with fracture rates converging at later stages with those of nanostructured materials. This has been known for structural ceramics for some time now.…”

“…The reduction of tractions at bridging elements is therefore cycle dependent, and thus K I,sh (N) = K I,sh exp(−N/N 0 ), which also depends on K I,appl,max and the R-ratio. Yet, this process is still concomitant to new bridges being created at the crack front whenever the crack advances subcritically (Kruzic et al 2005;Fett 2012;Hartelt et al 2013;Greene et al 2014).…”