Instrumented indentation characterisation of mouth-guard materials

The aim of the study was to investigate the mechanical properties and energy absorption behavior of mouth-guard materials. Interpretation of indentation force-displacement data has been used to determine these properties.

An ultra micro-indentation system (UMIS) was used to determine near surface properties of mouth-guard materials with nanometer and micro-Newton displacement and force resolution. The measurement procedure was conducted with a small spherical steel indenter (R=500 microm), with impressions that were equally spaced (250 microm). Measurements of force-displacement response of surfaces at maximum forces of 10, 40 or 50 mN, 10 indentations were made at each of these forces. Tests were undertaken in two different modes to determine the mechanical properties, namely: (a) continuous load to the maximum force and then unload, (b) multiple loading and partially unloading sequence to the maximum load.

The force-displacement results were analysed to determine the elastic modulus and contact pressure versus depth of penetration as well as the energy loss. Energy absorption for each material was determined from the ratio of the hysteresis energy loss to the total energy at maximum load and ranged from 10 to 24% among different mouth-guard materials. Energy absorption ratio, elastic modulus and contact pressure were significantly different between different materials with same thickness (FC, GC and C3) (p<0.01) and significantly different between materials with different thickness (C1 and C3) (p<0.01).

The present approach provides a simple and efficient method to readily measure the elastic-plastic (hysteretic) response of mouth-guard materials. The indentation technique lends itself to investigate the influence of ageing, heat treatment, sterilisation, moisture etc in a simple systematic manner.