Papers by Author: Mark Bush

Abstract: This paper summarizes some of the recent advances that have been made as a result of contact damage tests on bi-layer structures containing one brittle layer on a polymeric support base. The effects of indenter modulus (hard/soft indenters) and of the sate of loading (load location reference to the specimen axis of symmetry) on contact damage in bi-layer systems were investigated. Convex specimens having curvature of 12 mm inner coating diameter and 1mm thick were produced, and loaded along the axis of symmetry and off axis at 45o using flat indenters of six different moduli. The influence of indenter modulus on radial crack initiation and damage evolution was examined in respect to the load location, with particular attention paid to the relevance of such damage to lifetime-limiting failures of biomechanical layered systems. The results of this study illustrates that the fracture behaviour of brittle layered structures is not dominated by certain parameters. Critical loads for initiation of radial cracks are sensitive to indenter modulus (hardness) but not sensitive to load location. Load location plays an important role in crack propagation and subsequent damage patterns, especially at specimen margins.

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.

Abstract: This paper explores the so-called “margin failures” observed in loaded curved bi-layer structures. Hemispherical bi-layer model test specimens consisting of glass shells with varying margin geometry filled with epoxy resin, simulating brittle crowns on tooth dentine, are loaded with compliant indenters along the (convex) axis of symmetry. Using this unique setup, the influence of margin geometry on margin failure is examined. Nearly all previous studies have utilised hard spherical indenters of various radii, and examined crack initiation and evolution at the contact point. However, the modes of fracture observed in this traditional contact problem, surface cone cracking or flexure-induced radial cracking initiate close to or inside the (small) contact area, and thus not explain the margin failures commonly observed by dentists. Crack growth at the margins distant from the contact zone cannot be generated under indentation using hard spherical indenters. The use of a compliant (soft) indenter distributes the indentation force over a large contact area, generating a compressive zone underneath the contact, and effectively inhibiting the modes of fracture typically observed using hard indenters (radial and cone cracking). Consequently, significant tensile stresses at the support margin become dominant, and the focus shifts to fracture initiating at the support margins. In this study, cylindrical indenters composed of PTFE Teflon, with a modulus several orders of magnitude lower than the indented materials, are used to examine margin fracture in brittle crown like structures. The specific focus is the effect of margin geometry – Chamfered; Round; Shoulder margins are examined, and their influences on crack initiation and damage evolution are reported.

Abstract: The effects of indenter/coating modulus mismatch on contact damage in bilayer systems composed of brittle coating layers on compliant polymeric underlayers is investigated. Convex specimens having curvature of 12 mm inner coating diameter are produced, and loaded using flat indenters of four different moduli. Glass plates of d = 1 mm thick are used as representative of the brittle coating, and soft epoxy filler is used as an underlayer. Specimens are loaded along the convex axis of symmetry. In this fashion the influence of indenter modulus on radial crack initiation and damage evolution is examined. Previous studies utilised hard spherical indenters of various sizes, and did not consider the effect of a compliant indenter on radial crack evolution and subsequent damage patterns. It is demonstrated that critical loads for initiation of radial cracks and subsequent damage patterns especially at specimen margins are sensitive to indenter modulus (indenter/coating modulus mismatch).

Abstract: This paper aims to assess the effect of shape irregularity on contact damage in a brittle coating on a stiff metal substrate. Hertzian contact damage in a dental porcelain layer over a Ni-Cr alloy substrate in both curved and flat geometries was studied using finite element analysis and experimental investigation. Three failure modes were examined with varying porcelain layer thickness: cone cracking at the top surface of the porcelain, interface cracking at the layer/substrate interface and plastic deformation below the contact area in the substrate. It is shown that curvature has very little effect on the initiation of surface cone cracks in this system, but substantial effect on the initiation of interface radial cracks. In particular, curvature reduces the critical load for the onset of interface cracks