Methodology improvement of bulk compressive creep test: Deformation measurement and loading rate

(1) To identify improvements when bulk compressive creep testing of dental resin composite materials to reduce the sensitivity to the surface morphology and parallelism of specimens, to generate more accurate strain (displacement) measurement values. (2) To investigate the effect of loading rate on the creep and recovery behavior under bulk compressive creep test.

Cylindrical composite resin specimens were subjected to bulk compressive creep test with conventional and modified methodology (with/without introduction of stainless steel hemisphere and preload process). Furthermore, specimens undertook different loading rates ranging from 1 N/s to 50 N/s. Maximum deformation, creep deformation, permanent set as well as percentage of recovery during the creep and recovery procedure were compared, and surface topography changes before and after preload process was evaluated by laser scanning confocal. Burgers model was used to investigate the effect of improvements to each part of viscoelastic deformation of resin specimens.

(1) The influence of surface evenness of resin specimens could be reduced by addition of preload process before the bulk compressive creep test resulting in significantly decreased permanent set (p = 0.002), and increased recovery to 91.7 % (p < 0.001). While the standard deviation of maximum deformation, permanent set and percentage of recovery had the smallest values when hemisphere was introduced to loading chain. (2) With increasing loading rate of bulk compressive creep tests, creep deformation increased and this trend became statistically significant when the loading rate reached 50 N/s.

The accuracy of deformation measurement during bulk compressive creep test could be improved by means of introducing stainless steel hemisphere to the loading chain, and adding preload process to loading protocol.