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A major cause of fracture in complete lower dentures is due to the denture falling onto a hard floor. Impact failure has never been investigated in complete dentures and in a material which is strain rate dependent, a knowledge of the maximum strain and strain rate at failure is essential if laboratory tests are to evaluate materials in a realistic manner. The work described includes brittle lacquer studies to establish the strain pattern which occurs when a denture is dropped from a height of 1 metre. This was used to orientate strain gauges and experimental dentures were then hit by a specially constructed ram driven at known velocities by a spring, whilst the strain was recorded on a storage oscilloscope. Photographs of the stored wave form were then used to establish the results at ram velocities equal to that of a denture falling 1 metre. Maximum strains were achieved of 11 x 10(3) microstrain and strain rates of 1.04 x 10(3) microstrain/ms. Laboratory impact machines for testing denture materials should therefore be designed to achieve these figures.

This paper describes the application of two fracture toughness tests, the compact tension (CT) and the double torsion (DT) to homogenous poly(methyl methacrylate) and two-phase acrylics. Materials were tested in air as processed and after saturation for one month in water. Most materials gave stable crack propagation with these tests, allowing accurate measurement of crack velocity. Modulus and un-notched fracture strength (sigma f) were determined at an identical strain rate in bending to that used in the fracture toughness tests, allowing accurate calculation of the flaw size (a) for a material and an assessment of how applicable Linear Elastic Fracture Mechanics are to these materials.

A rapid-cure polymerization of cure, including a terminal boil, with respect to transparency, residual monomer, mechanical properties, and generation of gaseous porosity. It was found that in order for porosity-free resin to be produced during rapid curing, a low concentration (approximately 0.26%) of benzoyl peroxide initiator in the powder component is vital, and a very small concentration (approximately 0.025%) of the chemical activator dimethyl-p-toluidine in the monomer component is an advantage. Mechanical properties were only influenced where substantial porosity existed, but it was recognized that a material "tailor-made" for rapid curing would be most welcome.