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Objectives-The aim of this study was to test the hypothesis that the flexural strengths and critical flaw sizes of dental ceramic specimens will be affected by the testing environment and stressing rate even though their fracture toughness values will remain the same.Methods-Ceramic specimens were prepared from an aluminous porcelain (Vitadur Alpha; VITA Zahnfabrik, Bad Säckingen, Germany) and an alumina-zirconia-glass composite (In-Ceram ® Zirconia; VITA Zahnfabrik). Three hundred uniaxial flexure specimens (150 of each material) were fabricated to dimensions of 25 mm × 4 mm × 1.2 mm according to the ISO 6872 standard. Each group of 30 specimens was fractured in water using one of four different target stressing rates ranging on a logarithmic scale from 0.1 to 100 MPa/s for Vitadur Alpha and from 0.01 to 10 MPa/s for InCeram ® Zirconia. The fifth group was tested in inert environment (oil) with a target stressing rate of 100 MPa/s for Vitadur Alpha and 1000 MPa/s for In-Ceram ® Zirconia. The effects of stressing rate and environment on flexural strength, critical flaw size, and fracture toughness were analyzed statistically by Kruskal-Wallis one-way ANOVA on ranks followed by post-hoc comparisons using Dunn's test (α=0.05). In addition, 20 Vitadur Alpha specimens were fabricated with controlled flaws to simplify fractography. Half of these specimens were fracture tested in water and half in oil at a target stressing rate of 100 MPa/s, and the results were compared using Mann-Whitney rank sum tests (α=0.05). A logarithmic regression model was used to determine the fatigue parameters for each material.Results-For each ceramic composition, specimens tested in oil had significantly higher strength (P≤0.05) and smaller critical flaw size (significant for Vitadur Alpha, P≤0.05) than those tested in water but did not have significantly different fracture toughness (P>0.05). Specimens tested at faster stressing rates had significantly higher strength (P≤0.05) but did not have significantly different fracture toughness (P>0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram ® Zirconia specimens (P≤0.05) but not for Vitadur Alpha specimens (P>0.05). Fatigue parameters, n and ln B, were 38.4 and −12.7 for Vitadur Alpha and were 13.1 and 10.4 for InCeram ® Zirconia.Significance-Moisture assisted subcritical crack growth had a more deleterious effect on InCeram ® Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified

Fracture load of the three ceramic systems was found to be influenced by ceramic composition. Moreover, cement and fatigue condition influenced the fracture loads of the crown specimens evaluated in this study.

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