In product design environments where safety is critical, industries can often be reluctant to adopt emerging technologies. This is the case for many endovascular device manufacturers, where advanced non-linear Finite Element Analysis methods are still to be incorporated into their product design functions. To encourage adoption this thesis explores the merits and limitations of newly developed constitutive models as implemented into leading numerical analysis software suites. Specific attention is given to the Holzapfel soft tissue and Auricchio Nitinol constitutive models. Limitations and merits are explored through the development of an integrated modelling framework. This framework was constructed around a case study for which Vascutek's AnacondaTM stent graft system was chosen. In its final iteration the framework took the form of a series of python scripts which could be imported into Abaqus 6.11 to generate a variety of FE studies. For the development of the python scripts, a generalised artery material model was required for which a vascular characterisation programme was initiated. This programme mechanically characterised three human abdominal aortas using uniaxial and biaxial characterisation methods. By deploying Anaconda proximal ring devices within cadaver specimens and recording deformations, the programme produced detailed out-of-plane deformation data for vascular tissue. Where possible, results obtained were combined with those found in the academic literature. From this, comprehensive data sets were created from which a generalised mechanical description for the abdominal aorta was proposed. In this study, the accuracy of such a generalised arterial description was also explored for the first time. The work herein present a range of advanced Finite Element modelling techniques. These techniques range from a bespoke three layered abdominal aorta arterial model to novel wire bundle modelling methods. A comprehensive study into Nitinol's load path dependency effects is also presented. The study correlated the effect as captured within experimental results against those predicted by Finite Element Analysis. Findings of this study highlight limitations within the Auricchio constitutive model as implemented into Abaqus 6.11 and emphasise the need to model the entire loading regime when conducting Finite Element studies. The extensive validation procedures used to determine the accuracy of the integrated modelling framework are presented. It is shown for the case study chosen, that the framework is capable of capturing a range of complex device-artery interactions. It is also shown that the model is capable of predicting deformations within 35% of that measured during device deployment and cadaver specimen pressure-inflation experiments. It is a conclusion of this thesis, that from an understanding of the limitations involved, current FE technologies could be feasibly integrated into product design methodologies through the development of robust, computationally efficient design tools.
|Date of Award||24 Sep 2015|
- University Of Strathclyde
|Supervisor||William Dempster (Supervisor) & David Nash (Supervisor)|