Influence of test method on failure stress of brittle dental materials

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.