Objectives : The objective of this study was to compare the Weibull parameters (characteristic strength, σ 0 , and Weibull modulus, m ) and slow cracking growth (SCG) parameters (stress corrosion coefficient, n , and scalar parameter, σ f0 ) obtained for a dental porcelain using two different specimen configurations.
Materials and methods : Seventy porcelain specimens (VM7, Vita) were produced for each of the following specimen configurations: a) bars (4 mm × 2 mm × 16 mm) and b) discs (12 mm × 1 mm). Before mechanical testing, all specimens were polished up to 1 μm. For determination of the Weibull parameters, 30 specimens of each type were tested in either a three-point flexure test (bars) or biaxial piston-on-three-balls design (discs), using a universal testing machine at a constant stress rate of 1.0 MPa/s in artificial saliva at 37 °C. The SCG parameters, were determined using the same flexural test designs described above, but using 5 constant stress rates ( n = 10): 10 2 , 10 1 , 10 0 , 10 −1 , 10 −2 MPa/s. The Weibull statistics was used to determine σ 0 and m , and the SCG parameters were statistically analyzed according to the guidelines presented in ASTM C 1368-00.
Results : The results are presented in the Table (in parenthesis are shown standard deviations for SCG parameters, and 95% confidence intervals for Weibull parameters). As for SCG parameters, bar-shaped specimens showed a significantly higher stress corrosion coefficient ( n ) compared to disc-shaped specimens, however the latter showed a slightly higher early strength ( σ f0 ). With respect to the Weibull parameters, no effect of specimen configuration was observed, as both σ 0 and m were similar for bars and discs (confidence intervals overlapped).
| n | σ f0 (MPa) | σ 0 (MPa) | m | |
|---|---|---|---|---|
| Bars | 36 (11) | 65.08 (0.02) | 74.6 (69.4–80.0) | 5.8 (4.2–7.8) |
| Discs | 26 (7) | 70.55 (0.03) | 76.7 (70.8–82.9) | 5.2 (3.8–7.0) |
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