The effect of the environment on the crack initiation toughness of dental poly(methyl methacrylate).

The development of linear elastic fracture mechanics has been traced briefly, with emphasis on the approach based on stress intensity. Factors affecting fracture toughness include the effects of the environment, but little about these is known at present. Fracture behavior in two-phase acrylic systems has also been reviewed. In the light of this, SEN specimens of heat-cured denture-base PMMA were saturated in a range of liquids and extended to failure at chewing speed. Crack initiation toughnesses and critical flaw sizes were thus determined. The effects of the chemical environment on the relationship between these two parameters showed that vegetable oil had increased the notch sensitivity of the resin, while 4% ethanol and soap solution increased the plasticization of the crack tip beyond that in water alone. Although the behavior at the crack tip was similar for all the other aqueous media, the craze resistance of the resin could be altered by the chemical nature of the liquid; this might explain in part the behavioral strengths of the un-notched material.