Spherical micro-indentation process of polymer-based materials: a finite element study

This paper describes finite element (FE) elastic and elastic /plastic analysis of the spherical indentation process of polymers. Efforts are targeted towards polymers used in dental restorative materials which behave in a brittle manner. A key component of the wear modelling of such materials is an understanding of the stresses encountered by restorative materials during service, when two-body or three-body interactions cause failure on the surface of dental restorations. The primary objective is to study the distribution of the stresses and the corresponding strain behaviour around the contact area in order to edify crack phenomena. The large-deformation analysis is repeated for various indentor penetration depths. The commercial FE program initially selected for the analyses proved to be incapable of handling the complex large displacement elasto-plastic problem and eventually a second program was implemented to accomplish the tasks. It was shown how, at a certain load, the tensile stresses are sufficient to form a ring crack in the sample surface around the indenter. The same analysis was then used to show the formation of other median cracks in the sample. The results were closely matched and verified with experimental investigations. This comparison of theoretical and experimental investigations resulted in further study of the unloading process of the micro-indentation. It showed that upon unloading, the resulting residual stresses produced in the material are large enough to cause the Hertzian-like ring cracks.