BackgroundThe management and prognosis of aortic dissection (AD) is often challenging and the use of personalised computational models is being explored as a tool to improve clinical outcome. Including vessel wall motion in such simulations can provide more realistic and potentially accurate results, but requires significant additional computational resources, as well as expertise. With clinical translation as the final aim, trade-offs between complexity, speed and accuracy are inevitable. The present study explores whether modelling wall motion is worth the additional expense in the case of AD, by carrying out fluid-structure interaction (FSI) simulations based on a sample patient case.MethodsPatient-specific anatomical details were extracted from computed tomography images to provide the fluid domain, from which the vessel wall was extrapolated. Two-way fluid-structure interaction simulations were performed, with coupled Windkessel boundary conditions and hyperelastic wall properties. The blood was modelled using the Carreau-Yasuda viscosity model and turbulence was accounted for via a shear stress transport model. A simulation without wall motion (rigid wall) was carried out for comparison purposes.ResultsThe displacement of the vessel wall was comparable to reports from imaging studies in terms of intimal flap motion and contraction of the true lumen. Analysis of the haemodynamics around the proximal and distal false lumen in the FSI model showed complex flow structures caused by the expansion and contraction of the vessel wall. These flow patterns led to significantly different predictions of wall shear stress, particularly its oscillatory component, which were not captured by the rigid wall model.ConclusionsThrough comparison with imaging data, the results of the present study indicate that the fluid-structure interaction methodology employed herein is appropriate for simulations of aortic dissection. Regions of high wall shear stress were not significantly altered by the wall motion, however, certain collocated regions of low and oscillatory wall shear stress which may be critical for disease progression were only identified in the FSI simulation. We conclude that, if patient-tailored simulations of aortic dissection are to be used as an interventional planning tool, then the additional complexity, expertise and computational expense required to model wall motion is indeed justified.

This article describes an investigation into the contact behaviour of polymeric gear transmissions using numerical finite element (FE) and analytical techniques. A polymer gear pair was modelled and analysed using the ABAQUS software suite and the analytical results were calculated using the BS ISO 6336 rating standard. Before describing the results, the principles of the strategies and methods employed in the building of the FE model have been discussed. The FE model dynamically simulated a range of operating conditions. The simulations showed that the kinematic behaviour of polymeric gears is substantially different from those predicted by the classical metal gear theory. Extensions to the path of contact occur at the beginning and end of the meshing cycle. These are caused by large tooth deflections experienced by polymer gear teeth, as a result of much lower values of stiffness compared to metallic gears. The premature contact (occurring at the beginning of the meshing cycle) is hypothesized to be a factor in pitch line tooth fractures, whereas the extended contact is thought to be a factor in the extreme wear as seen in experiments. Furthermore, the increase in the path of contact also affects the induced bending and contact stresses. Simulated values are compared against those predicted by the international gear standard BS ISO 6336 and are shown to be substantially different. This is particularly for the case for bending stresses, where analytically derived values are independent of contact stiffness. The extreme tooth bending and the differences between analytical and numerical stresses observed in all the simulations suggest that any future polymeric gear-rating standard must account for the effects of load sharing (as a result of tooth deflection) and friction (particularly in dry-running applications).

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The onset of fretting fatigue is characterized by material microstructural changes in which the extent of the damage is comparable to grain size, and hence, the microstructure characteristics could have a significant effect on fatigue crack initiation. In this paper, a three‐dimensional finite element crystal plasticity framework is presented for simulation of the fretting fatigue. Controlled Poisson Voronoi tessellation (CPVT) method is employed to generate the polycrystalline region. In the CPVT method, regularity parameter controls the shape of grains. In this study, the impact of grain size and regularity parameter on crack initiation life and initiation site has been investigated. Cumulative plastic slip was used as a parameter of microstructure‐sensitive fatigue indicator. This parameter could effectively predict the location of crack initiation and its life. The results show that regularity parameter has a significant effect on the location of crack initiation. Furthermore, the effect of grain size on the fretting fatigue life of 316L stainless steel was investigated experimentally through testing different specimens with different grain sizes, to validate the simulation results.

People suffering from neuromuscular diseases may also face certain abnormalities in their walking pattern. Patients with quadriceps muscle weakness suffer from flexion contracture as well as flexion instability during the gait cycle. In this article, a knee-ankle-foot orthosis design is proposed with two different mechanisms for the stance and swing phases, addressing the needs of patients with quadriceps muscle weakness. The stance phase mechanism locks the knee joint movement from the initial contact until the end of mid-swing and after mid-stance phase, the knee joint can flex freely. OpenSim was utilized to simulate patients with muscle weakness as well as calculating the required moment to mimic the stiffness of a normal knee joint. The super-elasticity of shape memory alloys was then used to reproduce the calculated moment for different levels of muscle weakness. It is shown that by designing patient-specific orthosis, the stiffness profile of normal joint for each patient with distinct level of muscle weakness can be reproduced.

Carbon fibers significantly improve thermal and mechanical properties of nanocomposites, and many researchers have focused their studies on determining the effective thermal and mechanical properties of these composites. Much effort has gone into determining how mechanical loading changes the effective properties of the nanocomposite, and studying its behavior under further mechanical loading. In the present study, a computer simulation of three different volume fractions of carbon fibers in natural rubber was subjected to eight loading scenarios, each, to study the effect of loading conditions on the effective thermomechanical properties of the nanocomposites. Results suggest that mechanical loading can improve the effective thermal conductivity and increase the elastic modulus of the nanocomposite.

We report a coupled in-situ micro-Laue diffraction and nano-indentation experiment, with spatial and time resolution, to investigate the deformation mechanisms in [001]-oriented single crystal MgO. Crystal plasticity finite element modelling was applied to aid interpretation of the experimental observations of plasticity. The Laue spots showed both rotation and streaking upon indentation that is typically indicative of both elastic lattice rotation and plastic strain gradients respectively in the material. Multiple facets of streaking of the Laue peaks suggested plastic slip occurring on almost all the {101}-type slip planes oriented 45° to the sample surface with no indication of slip on the 90° {110} planes. Crystal plasticity modelling also supported these experimental observations. Owing to asymmetric slip beneath the indenter, as predicted by modelling results and observed through Laue analysis, sub-grains were found to nucleate with distinct misorientation. With cyclic loading, the mechanical hysteresis behaviour in MgO is revealed through the changing profiles of the Laue reflections, driven by reversal of plastic strain by the stored elastic energy. Crystal plasticity simulations have also shown explicitly that in subsequent loading cycles after first, the secondary slip system unloads completely elastically while some plastic strain of the primary slip reverses. Tracking the Laue peak movement, a higher degree of lattice rotation was seen to occur in the material under the indent, which gradually decreased moving laterally away. With the progress of deformation, the full field elastic strain and rotation gradients were also constructed which showed opposite lattice rotations on either sides of the indent

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