Taper Junctions in modular total hip arthroplasty

Morse tapers are widely used in modular total hip joint replacements (THRs) to attach the femoral head onto the femoral stem. Despite the success of THR procedures, the taper junction has been associated with failures due to damage at the interface generated by fretting and corrosion. Taper design parameters including taper clearance, length, diameter, and assembly conditions have been previously shown to influence the generation of micromotions associated with taper damage. However, the role of taper geometry and surface topography on the mechanisms that influence damage is not well understood. 3D FE models of CoCr alloy femoral heads assembled onto Ti alloy trunnions were developed to investigate the contact environment and the relative motions generated in the taper interface during loading. The maximum accumulated micromotions (49 µm) over a walking cycle were found in tapers with an oval trunion and high clearance. The components of the micromotions (pistoning, normal and tangential) varied with the taper geometry and loading activity. The largest contribution of micromotion in the oval taper arise from tangential motion whilst for a round taper the relative motions were dominated by normal and pistoning. The effect of surface topography (form, waviness and roughness) on the micromotions generated were studied using 2D and 3D models. Contact conditions in the taper interface as a result of the interaction of different surface topographies at different assembly forces were studied. Idealised taper surfaces were found to be different from real, measured surfaces when assembled and generated different contact conditions. Power Spectrum Density (PSD) analysis showed that idealised surfaces comprised of only one spatial wave frequency while measured data contained 3 - 7 spatial wave frequencies. When surfaces where assembled, the measured surfaces showed flattening of the roughness peaks and large plastic strains which reached values that indicated material failure (>0.6). In measured bore surfaces, intermittent contact of deformed trunnion peaks was identified as the assembly force increased. The interaction of the surfaces in the 2D models led to the estimation of a global coefficient of friction (COF); surface roughness and adhesion had a significant effect on this estimation, for example when using a local COF of 0.21 to simulated adhesion, the estimated global COF ranged from 0.21 to 0.46. Using a value of friction from this estimated global COF in the 3D taper model under walking conditions influenced the results obtained; a high COF (0.46) decreased the magnitude of resultant micromotions but increased the magnitude of normal and tangential relative motions magnitudes by 15% and 115% respectively and decreased pistoning by 145% compared to motions generated with a global COF of 0.21. Findings from these studies suggest that the surface topography variations comprising roughness, waviness and form determine the taper performance. These findings help to understand the role of surface design in tapers and highlight the importance of manufacturing processes which will significantly affect a taper’s performance

An improved discrete bat algorithm for symmetric and asymmetric traveling salesman problems

Bat algorithm is a population metaheuristic proposed in 2010 which is based on the echolocation or bio-sonar characteristics of microbats. Since its first implementation, the bat algorithm has been used in a wide range of fields. In this paper, we present a discrete version of the bat algorithm to solve the well-known symmetric and asymmetric traveling salesman problems. In addition, we propose an improvement in the basic structure of the classic bat algorithm. To prove that our proposal is a promising approximation method, we have compared its performance in 37 instances with the results obtained by five different techniques: evolutionary simulated annealing, genetic algorithm, an island based distributed genetic algorithm, a discrete firefly algorithm and an imperialist competitive algorithm. In order to obtain fair and rigorous comparisons, we have conducted three different statistical tests along the paper: the Student's $t$-test, the Holm's test, and the Friedman test. We have also compared the convergence behaviour shown by our proposal with the ones shown by the evolutionary simulated annealing, and the discrete firefly algorithm. The experimentation carried out in this study has shown that the presented improved bat algorithm outperforms significantly all the other alternatives in most of the cases

Combined frequency-amplitude nonlinear modulation: theory and applications

In this work we formulate a generalized theoretical model to describe the nonlinear dynamics observed in combined frequency-amplitude modulators whose characteristic parameters exhibit a nonlinear dependence on the input modulating signal. The derived analytical solution may give a satisfactory explanation of recent laboratory observations on magnetic spin-transfer oscillators and fully agrees with results of micromagnetic calculations. Since the theory has been developed independently of the mechanism causing the nonlinearities, it may encompass the description of modulation processes of any physical nature, a promising feature for potential applications in the field of communication systems.Comment: 8 pages, 4 figures, to be published on IEEE Transactions on Magnetic

The role of gas infall in the evolution of disc galaxies

Spiral galaxies are thought to acquire their gas through a protracted infall phase resulting in the inside-out growth of their associated discs. For field spirals, this infall occurs in the lower density environments of the cosmic web. The overall infall rate, as well as the galactocentric radius at which this infall is incorporated into the star-forming disc, plays a pivotal role in shaping the characteristics observed today. Indeed, characterising the functional form of this spatio-temporal infall in-situ is exceedingly difficult, and one is forced to constrain these forms using the present day state of galaxies with model or simulation predictions. We present the infall rates used as input to a grid of chemical evolution models spanning the mass spectrum of discs observed today. We provide a systematic comparison with alternate analytical infall schemes in the literature, including a first comparison with cosmological simulations. Identifying the degeneracies associated with the adopted infall rate prescriptions in galaxy models is an important step in the development of a consistent picture of disc galaxy formation and evolution

High spatial resolution and high contrast optical speckle imaging with FASTCAM at the ORM

In this paper, we present an original observational approach, which combines, for the first time, traditional speckle imaging with image post-processing to obtain in the optical domain diffraction-limited images with high contrast (1e-5) within 0.5 to 2 arcseconds around a bright star. The post-processing step is based on wavelet filtering an has analogy with edge enhancement and high-pass filtering. Our I-band on-sky results with the 2.5-m Nordic Telescope (NOT) and the lucky imaging instrument FASTCAM show that we are able to detect L-type brown dwarf companions around a solar-type star with a contrast DI~12 at 2" and with no use of any coronographic capability, which greatly simplifies the instrumental and hardware approach. This object has been detected from the ground in J and H bands so far only with AO-assisted 8-10 m class telescopes (Gemini, Keck), although more recently detected with small-class telescopes in the K band. Discussing the advantage and disadvantage of the optical regime for the detection of faint intrinsic fluxes close to bright stars, we develop some perspectives for other fields, including the study of dense cores in globular clusters. To the best of our knowledge this is the first time that high contrast considerations are included in optical speckle imaging approach.Comment: Proceedings of SPIE conference - Ground-based and Airborne Instrumentation for Astronomy III (Conference 7735), San Diego 201