S.B. Leen

A corrosion model for bioabsorbable metallic stents

In this study a numerical model is developed to predict the effects of corrosion on the mechanical integrity of bioabsorbable metallic stents. To calibrate the model, the effects of corrosion on the integrity of biodegradable metallic foils are assessed experimentally. In addition, the effects of mechanical loading on the corrosion behaviour of the foil samples are determined. A phenomenological corrosion model is developed and applied within a finite element framework, allowing for the analysis of complex three-dimensional structures. The model is used to predict the performance of a bioabsorbable stent in an idealized arterial geometry as it is subject to corrosion over time. The effects of homogeneous and heterogeneous corrosion processes on long-term stent scaffolding ability are contrasted based on model predictions.

Comparing coronary stent material performance on a common geometric platform through simulated bench testing

Absorbable metallic stents (AMSs) are a newly emerging cardiovascular technology which has the potential to eliminate long-term patient health risks associated with conventional permanent stents. AMSs developed to date have consisted of magnesium alloys or iron, materials with inferior mechanical properties to those used in permanent stents, such as stainless steel and cobalt-chromium alloys. However, for AMSs to be feasible for widespread clinical use it is important that their performance is comparable to modern permanent stents. To date, the performances of magnesium, iron, and permanent stent materials have not been compared on a common stent platform for a range of stent performance metrics, such as flexibility, radial strength, and recoil. In this study, this comparison is made through simulated bench testing, based on finite-element modelling. The significance of this study is that it allows potential limitations in current AMS performance to be identified, which will aid in focusing future AMS design. This study also allows the identification of limitations in current AMS materials, thereby informing the on-going development of candidate biodegradable alloys. The results indicate that the AMSs studied here can match the recoil characteristics and radial strength of modern permanent stents; however, to achieve this, larger strut dimensions are required. It is also predicted that the AMSs studied are inferior to permanent stents in terms of maximum absolute curvature and longitudinal stiffness.

An investigation of the fatigue and fretting performance of a representative aero-engine spline coupling

The fatigue behaviour of a representative high-performance aero-engine spline coupling is investigated under test conditions designed to simulate in-service conditions. The test load cycles consist of major cycle torque and axial load, simulating maximum thrust, combined with minor cycle rotating bending moment and fluctuating torque, simulating life-limiting conditions at take-off. The objective of the study is to develop understanding of the fatigue behaviour of the coupling over a range of loading conditions, including interaction between low-cycle fatigue, fretting fatigue and fretting wear. This information is necessary for the development of fatigue and fretting-fatigue life prediction techniques. The test results are interpreted with the help of three-dimensional finite element models, which include the frictional contact between the spline teeth.

Macroscopic fretting variables in a splined coupling under combined torque and axial load

A three-dimensional study of frictional contact in a helical splined coupling for the assessment of macroscopic fretting variables is presented. The study is based on an experimentally validated finite element model of the coupling under combined torque and axial loads. The effect of axial profile modification for reduced contact stresses in spline teeth and the effect of friction coefficient are considered. The motivation for the work is the need for representative information about fretting variable distributions in splined couplings for the development and application of simplified fretting test configurations.

Effect of particle impact on residual stress development in HVOF sprayed coatings

The application of thick high-velocity oxyfuel (HVOF) coatings on metallic parts has been widely accepted as a solution to improve their wear properties. The adherence of these coatings to the substrate is strongly influenced by the residual stresses generated during the coating deposition process. In an HVOF spraying process, due to the relatively low processing temperature, significant peening stresses are generated during impact of molten and semimolten particles on the substrate. At present, finite-element (FE) models of residual stress generation for the HVOF process are not available due to the increased complexities in modeling the stresses generated due to the particle impact. In this work, an explicit FE analysis is carried out to study the effect of molten particle impingement using deposition of an HVOF sprayed copper coating on a copper substrate as an example system. The results from the analysis are subsequently used in a thermomechanical FE model to allow the development of the residual stresses in these coatings to be modeled.

Optimizing the design of a bioabsorbable metal stent using computer simulation methods.

Computer simulation is used extensively in the design of permanent stents. In order to address new challenges that arise in the design of absorbable metal stents (AMSs), such as corrosion and the limited mechanical properties of bioabsorbable alloys, new simulation and design techniques are needed. In this study a new method for simulating AMS corrosion is developed to study the effects of corrosion on the mechanical performance of a range of stent designs. The corrosion model is combined with an optimization strategy to identify AMS features that give optimal corrosion performance in the body. It is found that strut width is the predominant geometrical factor in determining long-term AMS scaffolding performance. An AMS with superior scaffolding performance to a commercial design is identified, based on deployment and corrosion simulations in stenosed vessels. These simulation and design techniques give new insights into in-vivo AMS performance and the role of device geometry in determining long-term scaffolding performance.

Development of a representative test specimen for frictional contact in spline joint couplings

The frictional contact conditions in a helical spline joint coupling under torsional and axial loads are studied using finite element analyses. Comparisons of spline root torsional stresses are made with photoelastic measurements. Surface contact tractions, relative slip distributions and subsurface localized stress component and equivalent stress distributions are presented for both the external and the internal spline contact surfaces. The work is important as a basis for understanding and predicting the fatigue, contact fatigue, fretting fatigue, wear and fretting wear characteristics of the coupling. A representative test specimen concept is presented which seeks to capture the local contact variable distributions in the spline coupling, with a view to predicting the wear and the contact and fretting fatigue performance of the coupling.

Experimental characterisation for micromechanical modelling of CoCr stent fatigue.

Fatigue of CoCr alloy stents has become a major concern in recent times, owing to cases of premature fracture, often driven by microstructural phenomena. This work presents the development of a micromechanical framework for fatigue design, based on experimental characterisation of a biomedical grade CoCr alloy, including both microscopy and mechanical testing. Fatigue indicator parameters (FIPs) within the micromechanical framework are calibrated for the prediction of microstructure-sensitive fatigue crack initiation (FCI). A multi-scale CoCr stent model is developed, including a 3D global J2 continuum stent-artery model and a 2D micromechanical sub-model. Several microstructure realizations for the stent sub-model allow assessment of the effect of crystallographic orientations on stent fatigue crack initiation predictions. Predictions of FCI are compared with traditional Basquin-Goodman total life predictions, revealing more realistic scatter of data for the microstructure-based FIP approach. Comparison of stent predictions with performance of a 316L stent for the same generic design exposes the design as over-conservative for the CoCr alloy. In response, the micromechanical framework is used to modify the stent design for the CoCr alloy, improving design efficiency.

Influence of statistical size effects on the plastic deformation of coronary stents.

The dimensions of coronary stent struts are similar to those of the metallic grains of their constituent alloys. This means that statistical size effects (SSEs), which are evident in polycrystals with few grains through their dimensions, can have detrimental effects on the mechanical performance of stent struts undergoing large plastic deformation. Current trends in coronary stent design are towards thinner struts, potentially increasing the influence of SSEs. In order to maintain adequate device performance with decreasing strut thickness, it is therefore important to assess the role of SSEs in the plastic deformation of stents. In this study, finite element modelling and crystal plasticity theory are used to investigate SSEs in the deformation of struts in tension and bending. The relationships between SSEs and microstructure morphology, alloy strain hardening behaviour and secondary phases are also investigated. It is predicted that reducing the number of grains through the strut cross section and increasing the number of grains along the strut length have detrimental effects on mechanical performance. The magnitudes of these effects are predicted to be independent of the uniformity of the studied microstructures, but dependent on alloy strain hardening behaviour. It is believed that model predictions will aid in identifying a lower bound on suitable strut thicknesses in coronary stents for a range of alloys and microstructures.

Micro-scale testing and micromechanical modelling for high cycle fatigue of CoCr stent material

This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material. Micro-scale specimens, representative of stent struts, are manufactured via laser micro-machining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterisation of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments.