Doctor of Philosophy / The commercial introduction of glass infiltrated slip cast ceramic technology as a core dental ceramic for crowns and bridges began in 1990. The system known as Inceram® provided a metal free ceramic core with a flexural strength that was reported to be between 400 and 600 MPa. The first aim of this study was to investigate in vitro fractures for this type of crown that occurred from high localized occlusal loading. The second aim was to propose suitable guidelines for the construction of crowns that are more crack resistant when loaded by a spherical object such as an opposinc cusp. The review of the literature identified Young’s modulus (E) as an important aspect for the behaviour of a brittle solid when it is exposed to load. Young’s modulus is involved in the total energy of the solid, its free surface energy, its toughness, parameters involved in contact areas, stresses from indenters on those contact areas and at the interface between two materials with different moduli. This mismatch was to become more important when Finite Element Analysis was applied to the test results. A second physical property that has importance in bilaminar minerals is Poisson’s ratio. Less important than Young’s modulus, Poisson’s ratio still provides some of the answers to crack propagation at an interface, highlighted by the description of Dundurs parameters (Mencik 1996). The use of Weibull statistics provided information concerning reliability for different core to veneer thickness ratios in the experimental models and crown designs. The first experimental part was to establish a crown model suitable for investigation. The porcelain jacked crown restoration for an upper incisor was chosen because it provided the best opportunity to vary the construction parameters of the crown without sacrificing the aesthetics. The results from this initial in vitro study established thickness ranges for the standard design of these crowns, the loads that could be expected and the two types of fracture patterns that might occur when the bilaminar system is loaded via a hard steel ball indenter in the middle of the palatal surface. The data from chapter three were then compared with two basic bilaminar and monolithic disc designs to establish the relevance of the dimensions of the ISO standard test specimen design for flexural strength. The results of this chapter confirmed that the loads were in the same range for the bilaminate discs, and that the flexural strengths for the two component parts were in agreement with other known results. The use of Finite Element Analysis (FEA) was employed to provide an additional method for testing the model to establish areas when principal stresses might lie and how they were distributed. It was found that the ISO flexural strength test was not ideal for testing bilaminates because it did not account for the large mismatch in Young’s modulus between the two component materials. The FEA revealed an unexpected increase in tensile stress on the bottom surface of the In-Ceram when it was the bottom layer in the bilaminate. The FEA did confirm the difference between a flat punch and a ball indenter for stress intensities around the contact area with the ball indenter producing higher stresses for a given load…
| Identifer | oai:union.ndltd.org:ADTP/225527 |
| Date | January 2001 |
| Creators | Ironside, James G |
| Publisher | University of Sydney., Faculty of Dentistry |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | The author retains copyright of this thesis., http://www.library.usyd.edu.au/copyright.html |
Page generated in 0.002 seconds