Influence of Tapered Brittle Coatings on Stresses in Layered Structures: Relevance to Failure of Dental Crowns

Abstract

This paper uses Finite Element Analysis to examine stresses in loaded curved bi-layer structures. The model system consists of glass shells, both constant thickness and tapered, filled with dental composite. These systems, simulating brittle crowns on tooth dentine, are loaded with ultra-compliant disk indenters, and hard spherical indenters for comparison, along the (convex) axis of symmetry. The resulting maximum principal stress patterns are analysed. Previous studies have generally utilised hard spherical indenters of various radii indenting constant thickness coatings, and examined stresses leading to crack initiation. However, the peak stresses observed in this traditional contact problem – inducing surface cone cracking or flexureinduced radial cracking - occurred close to or inside the (small) contact area, and do not explain the margin failures in dental crowns commonly observed by dentists. Furthermore, the effect of varying coating thickness, especially tapering towards thinner margins, has not previously been examined. The use of an ultra-compliant indenter distributes the indentation force over a large contact area, generating a compressive zone underneath the contact, and consequently, previously insignificant stresses at the support margin become dominant, and the focus shifts to the support margin, rather than the area close to the contact. In this study, balsa wood is used as the disk indenter model material, with a modulus several orders of magnitude lower than the indented materials. Stress patterns from the same systems indented by hard spherical indenters are included for comparison. The specific focus is the effect of tapered coatings, examining stress patterns from several geometries. Results confirm not only a shift in the peak maximum principal stress from the near-contact area (under hard spherical indenters) to the margin area (under ultra-compliant indenters), but also show that coating taper can have a significant influence on the margin stress under a soft indenter. In the same systems indented by a hard indenter, coating taper has very little effect on the more localised stresses induced.