Fatigue Loading and R-curve Behavior of a Dental Y-TZP

Objective: The objective of this study was to test the following hypotheses: 1. The R-curve behavior of yttria stabilized zirconia polycrystal (Y-TZP) is independent of the aspect ratio of the critical flaw. 2. The R-curve behavior of Y-TZP is dependent on the compressive residual stress acting on surface cracks. 3. Cyclic loading results in reduction in the compressive residual stress acting on surface cracks of Y-TZP.;Materials and methods: The material tested was a dental Y-TZP (IPS e.max ZirCAD). Rectangular beam specimens with dimensions of 25 mm x 4 mm x 3 mm were fabricated such that the tensile surfaces of the specimens were CAD/CAM machined. The specimens were divided into two groups (N=10) according to the treatment given before rapid monotonic loading. The first group was subjected to flexural cyclic loading for 200,000 cycles at a peak stress of 375 MPa and a frequency of 2 Hz. The second group was subjected to slow monotonic loading under a constant loading rate of 0.01 N/s. After initial treatment, the specimens were loaded rapidly at a stress rate of 10 MPa/s until failure. All tests were performed using a fully articulated four-point flexure fixture in deionized water at 37°C. A graph of microbranching stress intensity factor (SIF) against fracture-mirror radius was plotted to determine the residual stress at the crack tip. Fractographic techniques were used to determine the critical flaw to estimate fracture toughness. To determine the presence of R-curve behavior, linear regression was used. The effect of a:b ratio on fracture toughness and the effect of loading method on residual stress was determined using forward stepwise regression. The fracture surfaces and their epoxy replicas were scanned using the atomic force microscope (BioScope Catalyst, Veeco Instruments). The height data was imported into a custom MathCAD script, and FRACTALS software was used to determine the fractal dimensional increment (D*) by the Minkowski cover technique. The Mann-Whitney rank sum test was used to determine the difference in D* for both treatment groups. The relationship between D* and fracture toughness, and the relationship between D* and flaw size were determined using linear regression.;Results: Fractographic examination showed that CAD/CAM machining flaws were the dominant flaws responsible for failure. The linear regression model fit to a data of microbranching SIF against mirror radius. The residual stress was compressive for both treatment groups (p≤.05). Forward stepwise regression showed that fracture toughness was not significantly associated (p=0.72) with a:b ratio. For both groups, there was a significant difference (0.0.05) in the rise of R-curves after subtracting the estimated residual stress from the failure stress. The microbranching SIF did not have significant association (p>0.05) with the loading method. The Mann Whitney rank sum test showed that the D* values for the cyclic loading group were not significantly different (p=0.81) from the D* values for the slow monotonic loading group. The linear regression showed no significant correlation (p=0.64) between fracture toughness and D*. Also, there was no significant correlation (p=0.53) between flaw size and D*. The residual stress values (sigmaR) with 95% confidence intervals and the mean and standard deviation (SD) for D* for both groups are summarized in the table.*.;Conclusion: 1. R-curve behavior is independent of the aspect ratio of the critical flaw for a Y-TZP ceramic IPS e.max ZirCAD. 2. R-curve behavior of this material was dependent on the compressive residual stress acting on surface cracks. 3. Cyclic loading resulted in no difference in the compressive residual stress acting on surface cracks of this Y-TZP.;*Please refer to dissertation for diagrams.