Characterization of superelastic nitinol wire for application to aortic stent graft design

Student thesis: Doctoral Thesis

Abstract

Aortic stent graft devices are required to treat patients with life-threatening vascular disease. These devices depend on superelastic Nitinol material for their ability to be delivered through a minimally invasive endovascular approach and then to self-expand for long-term implantation at the target site. In this safety-critical application, Nitinol stent components are subjected to challenging in-service conditions in terms of thermo-mechanical loading. Characterization of the material’s response to these loading conditions is therefore essential for its safe and effective implementation in the design of medical devices. This thesis reports material characterization work performed on superelastic Nitinol stent wire under conditions relevant to its in-service application. The product life cycle of Nitinol stent graft components is investigated, highlighting the importance of the material’s bending behaviour and the associated tensile and compressive mechanical responses. With tensile behaviour and test methods already well established for Nitinol, the work first focuses on developing a method for compressive testing of representative Nitinol wire material, building on a previous method to enable testing to higher strains. This allows characterization of the material in compression for thermo-mechanical loading representative of large compaction deformations, in-vivo cycling and different temperatures seen during production, sterilization and implantation. The results also allow a clear understanding of the material’s tension-compression stress-strain asymmetry, which is essential to understanding its bending behaviour. This investigation concludes with a feasibility study into a novel compression test method developed by the author, using short wire samples with high-resolution ‘microtester’ equipment to obtain improved results.The work then focuses on development of a test method for bend testing of thin Nitinol wires to investigate the load response to in-service deformations at relevant test temperatures. This allows characterization of the wire’s load-history dependent bending response, whereby the force exerted at a given deflection during unloading depends on the maximum deflection during loading, with interesting application possibilities for stent components. The testing also allows the material’s temperature dependence, cyclic behaviour and large deformation response in bending to be studied. Following this, full-field strain measurement of thin Nitinol wires in bending is presented, achieved through application of 3-D microscopic Digital Image Correlation(DIC) technology. The development of a novel test method, together with extensive data analysis, provides results for characterization of the material’s complex bending behaviour, allowing new insight to its tension–compression asymmetry, localised deformation, load-unload strain hysteresis and load-history dependence of strain state inbending. This testing provides useful quantitative characterisation data including neutral axis eccentricity at high bend deformations. Finally, Abaqus FEA software is used to investigate the effectiveness of its in-built superelastic constitutive model for representing the Nitinol stent wire’s bending behaviour, and ultimately its suitability for use in stent design and analysis. The uniaxial stress-strain test results are used for input to the model, and then bending simulation results are compared against the experimental results, both in terms of force and strain outputs.Key findings include the model’s inability to represent the strain localisation seen in bending experiments, leading to under-representation of the maximum strains for ‘intermediate’ bend deflections, and also the model’s under-representation of unloading forces at these deflections (unless input parameters are adapted to compensate). Despite these limitations of th
Date of Award1 Oct 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsVascutek Limited
SupervisorWilliam Dempster (Supervisor) & David Nash (Supervisor)

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