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

Journal articleThis 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.Irish Research Council (RS/2010/2392) [EMBARK Initiative Scheme]; Irish Centre for High-End Computing [ICHEC

Mechanistic modelling of cyclic voltage-capacity response for lithium-ion batteries

One of the challenging tasks related to lithium-ion batteries (LIBs) remains a comprehensive approach for battery behaviour modelling. An approach is presented that enables modelling the voltage-capacity response of LIBs that are subjected to variable temperature and current load histories. A detailed presentation of the developed macro-scale phenomenological model embedding the mechanistic properties of the Prandtl type hysteresis operator and the concept of the force-voltage analogy is made. The necessary input data preparation for the model calibration is also presented. Accuracy of the model is confirmed with experimental observations for both nested current load history at two different temperatures and for arbitrary current load history. The same measured data is used to calibrate and to simulate response of the first order Thevenin equivalent circuit topology in order to amply compare the obtained results

Continuous modelling of cyclic ageing for lithium-ion batteries

The energy industry, transportation and even the smallest consumer electronics benefit from the practical applications of rechargeable batteries. Expectations of battery performance are greatly related to capacity, power output and available lifetime. However, the lifetime is affected by gradual chemical and mechanical degradation of the internal battery structure that cannot easily be predicted prior to installation. The reduction in performance is closely related to a particular usage pattern which is unique to the user and application, and is thus difficult to predict. Reliable real-time prediction of the remaining battery life therefore remains an important research topic. In this paper we show that fading battery performance under cyclic loading can be effectively and continuously followed by introducing the concept of the damage parameter derived from mechanical durability modelling approaches. The damage parameter is calculated continuously by the novel macro-scale hysteresis damage operator model. The hysteresis model is formed by a system of constitutive spring-slider modelling elements, here bridging the complex relation between the battery load and the durability data. The spring and the slider properties are individually calibrated for lithium nickel manganese cobalt oxide (NMC) batteries, however other battery structures can also be used. The durability data is obtained experimentally under controlled steady thermal and cyclic loading (constant charge/discharge current) conditions. The approach is validated on a standardised driving pattern with a complex current history. The predicted battery life is in good agreement with observed repetitions of a simulated load block until 90% of the initial battery capacitywith 589, 590 and 698 repetitions for the combined test and simulation prediction, full simulation prediction and experiment, respectively. When compared to established equivalent circuit or analytical approaches, the proposed approach requires only a small number of cyclic durability tests with constant current and temperature. In addition, the approach supports the battery design process by allowing simulations for different usage patterns, material and durability data

Comparison of Covered Laser-cut and Braided Respiratory Stents: From Bench to Pre-Clinical Testing

Lung cancer patients often suffer from severe airway stenosis, the symptoms of which can be relieved by the implantation of stents. Different respiratory stents are commercially available, but the impact of their mechanical performance on tissue responses is not well understood. Two novel laser-cut and hand-braided nitinol stents, partially covered with polycarbonate urethane, were bench tested and implanted in Rhön sheep for 6 weeks. Bench testing highlighted differences in mechanical behavior: the laser-cut stent showed little foreshortening when crimped to a target diameter of 7.5 mm, whereas the braided stent elongated by more than 50%. Testing also revealed that the laser-cut stent generally exerted higher radial resistive and chronic outward forces than the braided stent, but the latter produced significantly higher radial resistive forces at diameters below 9 mm. No migration was observed for either stent type in vivo. In terms of granulation, most stents exerted a low to medium tissue response with only minimal formation of granulation tissue. We have developed a mechanical and in vivo framework to compare the behavior of different stent designs in a large animal model, providing data, which may be employed to improve current stent designs and to achieve better treatment options for lung cancer patients

PulmoStent: In Vitro to In Vivo Evaluation of a Tissue Engineered Endobronchial Stent

Currently, there is no optimal treatment available for end stage tumour patients with airway stenosis. The PulmoStent concept aims on overcoming current hurdles in airway stenting by combining a nitinol stent with a nutrient-permeable membrane, which prevents tumour ingrowth. Respiratory epithelial cells can be seeded onto the cover to restore mucociliary clearance. In this study, a novel hand-braided dog bone stent was developed, covered with a polycarbonate urethane nonwoven and mechanically tested. Design and manufacturing of stent and cover were improved in an iterative process according to predefined requirements for permeability and mechanical properties and finally tested in a proof of concept animal study in sheep for up to 24 weeks. In each animal two stents were implanted, one of which was cell-seeded by endoscopic spraying in situ. We demonstrated the suitability of this membrane for our concept by glucose transport testing and in vitro culture of respiratory epithelial cells. In the animal study, no migration occurred in any of the twelve stents. There was only mild granulation tissue formation and tissue reaction; no severe mucus plugging was observed. Thus, the PulmoStent concept might be a step forward for palliative treatment of airway stenosis with a biohybrid stent device