Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials

Abstract Objectives: The aim was to determine the effect on apparent shrinkage-strain values of varying specimen aspect ratio in the bonded-disk configuration. Thereby one source of possible inter-laboratory variation might be understood and eliminated. Methods: The bonded-disk procedure was followed and applied to three representative resin-composites. However, specimen sub-groups were examined, each of different diameters (3.5, 5.0, 7.0 and 9.0 mm), corresponding to C -factors in the range 0.45–7.0. Results: For each material, no statistically significant difference was apparent between data obtained with 7 and 9 mm diameter specimens. However, statistically significant reductions ( p d )-dependence of apparent shrinkage-strain ( ϵ ) was given by the curve fit expression: ϵ = A − B / d 2 . Significance: For accurate determination of maximum final equilibrium shrinkage-strain values, a high aspect ratio (7–9:1) of the bonded-disk should be utilised. This is important for accurate comparison of data from proposed low-shrinkage amalgam-replacement materials.