Nonlinear modelling and fracture analysis of implantable prostheses

Abstract

Implantable prostheses exist at the nexus of advanced material properties, biocompatibility requirements, significant variability in surrounding tissues and loading conditions, as well as tangible benefit to patients. In such devices, nonlinear behaviour (including plasticity and fracture) must be considered if the design space is to be fully explored, and in many cases microstructural characteristics need to be considered.

Coronary stents must balance the requirements of flexibility and service life under challenging loading with minimal biological impact and shear load on the artery wall. A major failure mode of stents is fracture, and a distinctive challenge of such designs is the influence of the individual grains of the unknown microstructure on the fracture behaviour. A common approach to such unknowns in the literature is to use stochastic methods to generate the grain boundaries. This study applies the stochastic approach to the material properties of the grains, however, the grain properties are applied to an experimentally determined microstructure. XFEM was implemented to simulate the distribution of cracks across the specimen in a nondeterministic context, and both linear elastic and crystal plastic approaches were considered in the grains.

Dental implantable prostheses require longevity, stability, stiffness and toughness. A successful material for this is Zirconia, which exhibits a distinctive effect - regularly spaced transformation bands. The development of these bands arises from multi-scale, bi-directional phenomena whereby the global stress state influences the development of the local transformation, and the dilation at the local level relaxes the global stress state. This thesis includes the first instance of this phenomena being captured in a numerical model.