Analysis of subcritical crack growth in dental ceramics using fracture mechanics and fractography.

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 mmx4 mmx1.2 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 In-Ceram 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 (alpha=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 (alpha=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 (P0.05). Specimens tested at faster stressing rates had significantly higher strength (P0.05). Regarding critical flaw size, stressing rate had a significant effect for In-Ceram Zirconia specimens (P0.05). Fatigue parameters, n and lnB, were 38.4 and -12.7 for Vitadur Alpha and were 13.1 and 10.4 for In-Ceram Zirconia.

Significance: Moisture assisted subcritical crack growth had a more deleterious effect on In-Ceram Zirconia core ceramic than on Vitadur Alpha porcelain. Fracture surface analysis identified fracture surface features that can potentially mislead investigators into misidentifying the critical flaw.