Viscoelastic braided stent: finite element analysis and validation of crimping behaviour

Abstract

In this thesis, validated Finite Element (FE) models for a viscoelastic braided stent are created, to investigate the mechanical behaviour of the stent subjected to crimping and to predict the stent’s radial forces. The research is focused on developing and validating two linear viscoelastic material models: one for Polydioxanone, a biodegradable polymer and one for Nitinol, a Nickel Titanium shape memory alloy. Both models are developed using Abaqus, a commercial FE software package. The existing analytical model is corrected and adapted to include the braiding angle as opposed to the pitch angle. The model is also extended by introducing additional design information such as the mandrel design to enable correct fabrication, the braid cover factor and the condition in which the braid is in jammed state. A tensile test and tensile creep experiment are performed on the polymer monofilament in order to characterise its viscoelastic response. Data acquired are used to calibrate the polymer material model. Two experiments are performed on the Nitinol material: a uniaxial tensile test to capture its superelasticplastic behaviour and a Discrete Scanning Calorimeter analysis to investigate its transformation temperatures. Data acquired alongside creep data available in the literature are used to calibrate the Nitinol viscoelastic material model. Finally, both viscoelastic configurations of the FE models for the braided stent used in a crimping simulation are validated experimentally on in-house built prototypes. The outcomes of the study demonstrate the great benefits in using FE analysis to assess the mechanical behaviour of a viscoelastic braided stent during crimping. The viscoelastic material model developed for Polydioxanone shows an improvement in the accuracy of the stent’s mechanical behaviour in comparison to a purely elastic material model. The viscoelastic model developed for Nitinol predicts the radial resistive forces more accurately; however, overall, the superelastic-plastic model gives more accurate estimations. The significance of the work is in confirming the effectiveness of using FE method in the braided stent analysis and in helping design engineers to better understand the mechanical behaviour of the viscoelastic braided stent and therefore to better optimise its design for particular clinical applications.