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A bi-axial flexure test (piston-on-three-balls), a four-point flexure test, and a diametral tensile test were used to measure the failure stress of four brittle dental materials: zinc phosphate cement, body porcelain, opaque porcelain, and visible light-cured resin composite. Furthermore, the fracture probability of the bi-axial test specimens was predicted from the results of the four-point flexure test, with use of statistical fracture theory. Bi-axial failure stresses calculated from an equation developed by Marshall (1980) exhibited no significant difference for zinc phosphate cement as a function of piston size, specimen thickness, presence or absence of a stress-distributing film, and loading rate. The four-point flexure strength values of zinc phosphate cement and opaque procelain were significantly lower (p less than 0.05) than the corresponding mean bi-axial strength values, while the mean four-point flexure strength values of body porcelain and resin composite were not significantly lower (p greater than 0.05) than the corresponding mean bi-axial strength values. The diametral tensile strength of all materials was significantly lower than the bi-axial flexure strength. The mean bi-axial flexure strengths of zinc phosphate cement and opaque porcelain were much higher than the theoretical values predicted from surface flaw theory, while the strength values for body porcelain and resin composite were comparable with those determined from the four-point flexure test. These results demonstrate that the strength of zinc phosphate cement depends not only upon the geometric factors, but also upon sample preparation conditions.

We hypothesize that the fracture resistance of alumina core/porcelain veneer disks increases and that crack initiation shifts from veneer to core as the core/veneer thickness ratio (t(C)/t(V)) increases from 0.5/1.0 to 1.3/0.2, or as the elastic modulus of the supporting substrate (E(S)) to which it is resin-bonded increases from 5.1 to 226 GPa. When supported by a low-modulus substrate, disks with low t(C)/t(V) ratios exhibited cracks in the veneer and within the core, while those with high t(C)/t(V) ratios demonstrated core cracks, but not veneer cracks. None of the disks supported by Ni-Cr alloy (E = 226 GPa) exhibited core cracks. These results support the hypothesis that the crack initiation site shifts as the t(C)/t(V) ratio increases, but the increase in E(S) did not affect the crack initiation site. This study suggests that the t(C)/t(V) ratio is the dominant factor that controls the failure initiation site in bilayered ceramic disks.

Fluorocanasite (Al2O3-CaO-F-K2O-Na2O-SiO2) glass-ceramics exhibit fracture toughness values of up to 5.0 MPa x m1/2. However, their chemical durability is not adequate for dental applications. The objective of this study was to test the hypothesis that an increased concentration of Al2O3 can increase the chemical durability of fluorocanasite-based glass-ceramics. Glass frits containing 2 wt% (CAN2), 5 wt% (CAN5), and 10 wt% Al2O3 (CAN10) were melted individually, poured into a graphite mold, and cut into 16-mm-diam. x 2-mm-thick disks. Each disk was crystallized at 850 degrees C for 6 hrs. The disks were immersed in a solution of de-ionized-distilled water, 4% acetic acid, or a pH 1 buffer solution, and sealed in 90-mL Teflon containers. Corrosion testing was performed by means of vibrational motion at 60 cycles per min in a shaker-bath at 80 degrees C for 15 days. Solution analyses were performed by means of a pH meter, an atomic absorption spectrophotometer, and an inductively coupled plasma spectrometer. Samples exposed to 4% acetic acid solution exhibited a mean weight loss rate (WLR) for the control group (Dicor) of 0.04+/-0.01 mg/cm2 day, which was significantly lower (p < or = 0.0001) than the mean WLR of the CAN2 (1.08+/-0.02 mg/cm2 x day), CAN5 (1.31+/-0.02 mg/cm2 x day), and CAN10(1.51+/-0.05 mg/cm2 x day) groups. The reduced durability of fluorocanasite-based glass-ceramics with increasing Al2O3 concentration is most likely associated with a more uniform distribution of smaller crystals during heat treatment of the glass.