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



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 107 cycles and 0% of the beams tested below the predicted endurance strength failed within 107 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.