89 results found
Nissim L, Butt H, Gao L, et al., 2021, Role of protein concentration on transient film thickness in synovial fluid lubricated joints, Biotribology, ISSN: 2352-5738
A computational model of protein aggregation lubrication has been developed for predicting transient behaviour in lubricated prosthetics. The model uses an advection-diffusion equation to simulate protein transport in order to map concentration changes throughout the contact and inlet zones of an elasto-hydrodynamic contact. Concentration increases lead to exponential increase in fluid viscosity giving rise to lubricating film thicknesses an order of magnitude larger than would be expected using conventional elasto-hydrodynamic theory. The model parameters have been calibrated such that good agreement in transient film thickness is achieved with observed experimental results.KeywordsProtein aggregation lubrication; Elasto-hydrodynamic lubrication; Prostheses
Prado DS, Amigo RCR, Hewson RW, et al., 2021, Functionally graded optimisation of adsorption systems with phase change materials, Structural and Multidisciplinary Optimization, Vol: 64, Pages: 473-503, ISSN: 1615-147X
Adsorption phenomena are encountered in several engineering applications. One of its uses is in the storage and transport of gas in the form of adsorption tanks. The exothermic nature of the adsorption process decreases adsorption capacity presenting an impetus to understand the thermal characteristics of the gas storage process. Studies using mixtures of phase change materials and adsorbents in adsorption tanks demonstrate potential improvements in the adsorption capacity of the tanks. They also show that the distribution of phase change materials and adsorbent are important. Thus, this work presents two approaches for optimising the adsorbent domain. The first is to use a semi-analytic model to determine the best homogeneous material concentration for the adsorbent and phase change material for the vessel composition. The other is to use a 2D axisymmetric model to perform FGM optimisation to distribute material in the tank. Results for both models are presented and discussed for different conditions. The study shows that, for the cylindrical geometry, FGM optimisation is always, at least, marginally better than the homogeneous distribution from the semi-analytic model. However, FGM optimisation demands more computing time increases the complexity of implementation and results assembling. The semi-analytic approach is a possible alternative for optimising adsorption systems with phase change material mixed with adsorbents.
Soltanahmadi S, Raske N, de Boer GN, et al., 2021, Fabrication of cartilage-inspired hydrogel/entangled polymer–elastomer structures possessing poro-elastic properties, ACS Applied Polymer Materials, Vol: 3, Pages: 2694-2708, ISSN: 2637-6105
The ability to replicate the load-bearing properties of articular cartilage is attractive for many engineering applications, particularly bearings where low friction, low wear, and high durability are required. Hydrogels are widely used materials spanning many diverse applications owing to their lubricity and unique mechanical/chemical properties. The poor mechanical characteristics of conventional hydrogels, especially their compressive behavior, limit their application in load-bearing applications despite their favorable properties such as poro/viscoelasticity and lubricity. This paper demonstrates a cartilage-inspired approach to produce a structure that benefits from water-swelling resistant and ultrafast recovery behavior of elastomers as well as the stress-relaxation and energy dissipation properties of hydrogels. A method is presented in this work to fabricate interconnected macro-porous elastomers based on sintering poly(methyl methacrylate) beads. The porous elastomer imparted structural support and resilience to its composite with an infused-grafted hydrogel. At 30% strain and depending upon the strain rate, the composite exhibited a load-bearing behavior that was 14–19 times greater than that of pristine hydrogel and approximately 3 times greater than that of the porous elastomer. The equilibrium elastic modulus of the composite was 452 kPa at a strain range of 10%–30%, which was close to the values reported for the modulus of cartilage tested with similar experimental parameters defined in this study. The dissipated energy for the composite at strain rates of 1 and 10–3 s–1 was enhanced by 25-, 25-, 5-, and 15-fold as compared to that for the pristine hydrogel and the porous elastomer, respectively. The cyclic loading tests at two strain rates showed that the composite immediately recovers its load-bearing properties with the maximum load recovery staying above 95% of its initial values throughout the testing. The permeability of
Murphy R, Imediegwu C, Hewson R, et al., 2021, Multiscale structural optimisation with concurrent coupling between scales, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 63, Pages: 1721-1741, ISSN: 1615-147X
A robust three-dimensional multiscale structural optimization framework with concurrent coupling between scales is presented. Concurrent coupling ensures that only the microscale data required to evaluate the macroscale model during each iteration of optimization is collected and results in considerable computational savings. This represents the principal novelty of this framework and permits a previously intractable number of design variables to be used in the parametrization of the microscale geometry, which in turn enables accessibility to a greater range of extremal point properties during optimization. Additionally, the microscale data collected during optimization is stored in a re-usable database, further reducing the computational expense of optimization. Application of this methodology enables structures with precise functionally-graded mechanical properties over two-scales to be derived, which satisfy one or multiple functional objectives. Two classical compliance minimization problems are solved within this paper and benchmarked against a Solid Isotropic Material with Penalization (SIMP) based topology optimization. Only a small fraction of the microstructure database is required to derive the optimized multiscale solutions, which demonstrates a significant reduction in the computational expense of optimization in comparison to contemporary sequential frameworks. In addition, both cases demonstrate a significant reduction in the compliance functional in comparison to the equivalent SIMP based optimizations.
Nightingale M, Hewson R, Santer M, 2021, Multiscale optimisation of resonant frequencies for lattice-based additive manufactured structures, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 63, Pages: 1187-1201, ISSN: 1615-147X
This paper introduces a novel methodology for the optimisation of resonant frequencies in three-dimensional lattice structures. The method uses a multiscale approach in which the homogenised material properties of the lattice unit cell are defined by the spatially varying lattice parameters. Material properties derived from precomputed simulations of the small scale lattice are projected onto response surfaces, thereby describing the large-scale metamaterial properties as polynomial functions of the small-scale parameters. Resonant frequencies and mode shapes are obtained through the eigenvalue analysis of the large-scale finite element model which provides the basis for deriving the frequency sensitivities. Frequency tailoring is achieved by imposing constraints on the resonant frequency for a compliance minimisation optimisation. A sorting method based on the Modal Assurance Criterion allows for specific mode shapes to be optimised whilst simultaneously reducing the impact of localised modes on the optimisation. Three cases of frequency constraints are investigated and compared with an unconstrained optimisation to demonstrate the algorithms applicability. The results show that the optimisation is capable of handling strict frequency constraints and with the use of the modal tracking can even alter the original ordering of the resonant mode shapes. Frequency tailoring allows for improved functionality of compliance-minimised aerospace components by avoiding resonant frequencies and hence dynamic stresses.
Butt H, Nissim L, Gao L, et al., 2021, Transient mixed lubrication model of the human knee implant, Biosurface and Biotribology
The human knee implant is computationally modelled in the mixed lubrication regime to investigate the tribological performance of the implant. This model includes the complex geometry of the implant components, unlike elliptical contact models that approximate this geometry. Film thickness and pressure results are presented for an ISO gait cycle to determine the lubrication regime present within the implant during its operation. It was found that it was possible for the lubrication regime to span between elastohydrodynamic, mixed and boundary lubrication depending on the operating conditions of the implant. It was observed that the tribological conditions present in one condyle were not necessarily representative of the other. Multiple points of contact were found within the same condyle, which cannot be computed by the elliptical contact solvers. This model can be used to balance forces in all directions, instead of only the normal loads, as often done in elliptical contact models. This work is an initial step towards understanding the role of the complex geometry in the tribological characteristics of the human knee implant when operating in physiological conditions.
De Boer G, Raske N, Soltanahmadi S, et al., 2020, A porohyperelastic lubrication model for articular cartilage in the natural synovial joint, Tribology International, Vol: 149, ISSN: 0301-679X
This work focuses on the proposed mechanisms for the lubrication of synovial joints and applies them to an idealised bearing geometry considering a porohyperelastic material (cartilage) rotating against a stationary rigid impermeable surface. The model captures the behaviour of all lubrication regimes including fluid film formation and boundary contact as the load capacity is increased, representing a major advancement in modelling cartilage mechanics. Transient responses in the fluid phase are shown to be faster than those in the solid phase with the former decaying over time as fluid is exuded from the material. The complex behaviour of fluid migrating to and from the lubricating film is captured which leads to a better understanding of the hydration and friction mechanisms observed.
Varas Casado JM, Hewson R, 2020, Multicomplex number class for Matlab, with a focus on the accurate calculation of small imaginary terms for multicomplex step sensitivity calculations, ACM Transactions on Mathematical Software, Vol: 46, Pages: 1-26, ISSN: 0098-3500
A Matlab class for multicomplex numbers was developed with particular attention paid to the robust and accurate handling of smallimaginary components. This is primarily to allow the class to be used to obtainn-order derivative information using the multicomplexstep method for, amongst other applications, gradient-based optimization and optimum control problems. The algebra of multicomplexnumbers is described as is its accurate computational implementation, considering small term approximations and the identification ofprinciple values. The implementation of the method in Matlab is studied, and a class definition is constructed. This new class definitionenables Matlab to handlen-order multicomplex numbers, and perform arithmetic functions. It was found that with this method, thestep size could be arbitrarily decreased toward machine precision. Use of the method to obtain up to the 7th derivative of functions ispresented, as is timing data to demonstrate the efficiency of the class implementation.
de Boer GN, Raske N, Soltanahmadi S, et al., 2020, Compliant-poroelastic lubrication in cartilage-on-cartilage line contacts, Tribology: Materials, Surfaces and Interfaces, Vol: 14, Pages: 151-165, ISSN: 1751-5831
The mechanisms of friction in natural joints are still relatively unknown and attempts at modelling cartilage-cartilage interfaces are rare despite the huge promise they offer in understanding bio-friction. This article derives a model combining finite strain, porous and thin-film flow theories to describe the lubrication of cartilage-on-cartilage line contacts. The material is modelled as compliant and poroelastic in which the micro-scale fibrous structure is interstitially filled with synovial fluid. This fluid flows over the interface between the bodies and is coupled to pressure generated by relative motion in the thin-film region formed under load. A Stribeck analysis demonstrated that this type of contact is determinable to conventional elastic lubrication but that the friction performance is improved by this interfacial flow. Moreover, the inclusion of periodic flow conditions when contact is onset is a specific novelty which elucidates new observations in the lubrication mechanisms pertaining to natural joints.
Murphy R, Imediegwu C, Hewson R, et al., 2020, Multiscale concurrent multi-objective structural optimization of a goose neck hinge
A robust multiscale concurrent optimization framework, which enables the precise functional-grading of mechanical properties within structures over two-scales, is presented within this paper and applied to a practical aerospace application — the mass minimization of a Goose Neck Hinge. The novelty of this framework lies in the concurrent nature of the response surface which enables the efficient calculation of small-scale mechanical properties during large-scale optimization. The efficacy of this approach permits a large number of design variables to be used in the parameterization of the small-scale without incurring a significant computational expense. The mass minimization of the Goose Neck Hinge constitutes a multi-objective optimization problem, constrained by a single maximum displacement constraint. Optimization of the Goose Neck Hinge was undertaken using both the framework presented within this paper and a density based topology optimization, to understand the relative performance of the multiscale framework to an industry standard method for structural optimization. The optimized multiscale geometry was able to satisfy the maximum displacement constraint using 20% less material than the density based topology optimization. This indicates that this framework has the potential to deliver a new generation of optimized aerospace structures.
Imediegwu C, Murphy R, Hewson R, et al., 2019, Multiscale structural optimization towards three-dimensional printable structures, Structural and Multidisciplinary Optimization, Vol: 60, Pages: 513-525, ISSN: 1615-147X
Murphy RD, Imediegwu C, Hewson R, et al., 2019, Multiscale concurrent optimization towards additively manufactured structures, AIAA Scitech 2019 Forum, Publisher: American Institute of Aeronautics and Astronautics
This work establishes a robust concurrent multiscale optimization framework which facilitates the precise functional-grading of mechanical properties within structures, over two scales.The novelty lies in the concurrent nature of the response surface which connects the small-scalegeometry to the large-scale domain. A concurrent implementation enables an efficient application of computational resources, such that a large number of design variables can be usedwithout a significant computational penalty. This framework also takes advantage of the process flexibility and precision of additive manufacturing techniques to ensure that all optimizedstructures are manufacturable and suitable for an aerospace based application. A complianceminimization case is compared against a standard topology optimization algorithm, resultingin superior functional values and demonstrates the efficacy of the presented approach. A further application of this framework is highlighted through a target deformation case, where acomplex deformation field is obtained through simple loading conditions. Results from both ofthe example problems indicate that this framework has potential within the field of adaptivestructures, to inspire a new generation of multifunctional designs.
Amigo RCR, Prado DS, Paiva JL, et al., 2018, Topology optimisation of biphasic adsorbent beds for gas storage, Structural and Multidisciplinary Optimization, Vol: 58, Pages: 2431-2454, ISSN: 1615-147X
Adsorption is a retention mechanism of fluid molecules on solid surfaces and presents a wide range of applications, such as fuel storage, refrigeration and separation processes. This work describes the modelling of gas adsorption on porous media and presents an optimisation approach for the design of adsorption systems based on biphasic adsorbent beds by topology optimisation. A comprehensive formulation for the adsorption phenomenon is presented, detailing the derivation of governing equations and respective weak forms and discretisation for the implementation of the finite element method (FEM). A new topology optimisation material model based on offset hyperbolic tangents is introduced. The derivation of sensitivities is presented in detail, based on a transient adjoint problem. A diverse set of optimised adsorbed natural gas (ANG) tanks, considering real material properties of activated carbon and steel, is presented. Results indicate the suitability of the method in optimising the distribution of phases across adsorbent beds and show that biphasic ANG tanks can perform significantly better than traditional tanks.
Gao L, Hua Z, Hewson RW, 2018, Can a “pre-worn” bearing surface geometry reduce the wear of metal-on-metal hip replacements? – A numerical wear simulation study, Wear, Vol: 406-407, Pages: 13-21, ISSN: 0043-1648
Total Hip Replacement (THR) is generally a highly successful treatment for late stage hip joint diseases and wear, however, wear continues to be one of the major causes of metal-on-metal THR's failure. Hip replacements typically experience a two-stage wear; a higher initial wear rate in the beginning followed by a lower steady-state one with the surface profile changed. This alludes to the potential use of a cup with a non-spherical interior cavity with an initial geometry similar to a worn surface which may benefit from lower wear rate. In this paper wear is numerically simulated with a cup having a non-spherical geometry inspired by the initial stage of wear.A wear model was recently developed by the authors for the THR, which considered the lubricated contact in both elastohydrodynamic lubrication (EHL) and mixed lubrication regime, rather than a dry contact used in most of other studies of wear modelling in the academic literature. In this study the wear model has been updated by introducing the ‘λ ratio’ (the ratio of film thickness to surface roughness) and addressing the non-Newtonian shear-thinning properties of the synovial fluid. This wear model was able to describe the non-linear wear evolution process due to the change of worn profiles. Firstly the wear of a spherical hip joint was simulated until a steady-state wear rate is achieved. Then a non-spherical joint was proposed in which the cup bearing geometry was generated by the previously predicted worn profile from the spherical joint. At last the wear of this “pre-worn” hip bearing was simulated and compared to the spherical one. Approximately 40% reduction in the steady-state wear rate and 50% in the total accumulated wear has been observed in the non-spherical hip joint. This study presented a full numerical analysis of the relationship between lubrication, wear reduction and the geometry change, and quantitatively suggested the optimal geometry to reduce running-in wea
Imediegwu C, Murphy R, Hewson RW, et al., 2018, Multiscale structural and thermal optimization towards 3D printable structures, The 9th International Conference on Computational Methods
Gao L, Hua Z, Hewson R, et al., 2018, Elastohydrodynamic lubrication and wear modelling of the knee joint replacements with surface topography, Biosurface and Biotribology, Vol: 4, Pages: 18-23, ISSN: 2405-4518
This numerical study predicted wear of lubricated total knee replacements with the existing of textured surface and the possibility of surface designs to reduce wear. In the first part, a wear model of metal-on-polyethylene total knee replacement was developed. The medial and lateral knee compartments was accounted for separately, with the contact force and motion during walking cycles applied. An adapted Archard wear formula was employed where the wear factor was an exponential function of the `Lambda ratio' (film thickness to the average roughness). Wear of the soft bearing surface (polyethylene insert) was simulated with regularly geometry update until a steady-state wear observed. In the second part, the effect of surface topography of the knee replacements was investigated. The surface texturing techniques have shown promising benefit to machine components in many areas of engineering practice. The texture parameters were designed using the Taguchi method for the geometry, size, and distribution of the micro dimples. It was observed that the lateral compartment may benefit from surface texturing if dimples were properly designed, while the texturing showed hardly advantageous effect on the medial surface in terms of lubrication enhancement and wear reduction. Some results were presented in the 6th World Tribology Conference.
Imediegwu C, Murphy R, Hewson RW, et al., 2018, The design of a lattice-based periodic microstructure model towards 3D printable optimized structures, 10th European Solid Mechanics Conference
Navadeh N, Hewson RW, Fallah AS, 2018, Dynamics of transversally vibrating non-prismatic Timoshenko cantilever beams, Engineering Structures, ISSN: 0141-0296
Wang H, Kow J, Raske N, et al., 2017, Robust and high-performance soft inductive tactile sensors based on the Eddy-current effect, Sensors and Actuators A: Physical, Vol: 271, Pages: 44-52, ISSN: 0924-4247
Tactile sensors are essential for robotic systems to interact safely and effectively with the external world, they also play a vital role in some smart healthcare systems. Despite advances in areas including materials/composites, electronics and fabrication techniques, it remains challenging to develop low cost, high performance, durable, robust, soft tactile sensors for real-world applications. This paper presents the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. SITSs measure the inductance variation caused by changes in AC magnetic field coupling between coils and conductive films. Design methodologies for SITSs are discussed by drawing on the underlying physics and computational models, which are used to develop a range of SITS prototypes. An exemplar prototype achieves a state-of-the-art resolution of 0.82 mN with a measurement range over 15 N. Further tests demonstrate that SITSs have low hysteresis, good repeatability, wide bandwidth, and an ability to operate in harsh environments. Moreover, they can be readily fabricated in a durable form and their design is inherently extensible as highlighted by a 4 × 4 SITS array prototype. These outcomes show the potential of SITS systems to further advance tactile sensing solutions for integration into demanding real-world applications.
Taherkhani AR, Gilkeson CA, Gaskell PH, et al., 2017, Aerodynamic CFD based optimization of police car using Bezier curves, SAE International Journal of Materials and Manufacturing, Vol: 10, Pages: 85-93, ISSN: 1946-3987
This paper investigates the optimization of the aerodynamic design of a police car, BMW 5-series which is popular police force across the UK. A Bezier curve fitting approach is proposed as a tool to improve the existing design of the warning light cluster in order to reduce drag. A formal optimization technique based on Computational Fluid Dynamics (CFD) and moving least squares (MLS) is used to determine the control points for the approximated curve to cover the light-bar and streamline the shape of the roof. The results clearly show that improving the aerodynamic design of the roofs will offer an important opportunity for reducing the fuel consumption and emissions for police vehicles. The optimized police car has 30% less drag than the non-optimized counter-part.
de Boer G, Raske N, Wang H, et al., 2017, Design optimisation of a magnetic field based soft tactile sensor, Sensors, Vol: 17, Pages: 1-20, ISSN: 1424-8220
This paper investigates the design optimisation of a magnetic field based soft tactile sensor, comprised of a magnet and Hall effect module separated by an elastomer. The aim was to minimise sensitivity of the output force with respect to the input magnetic field; this was achieved by varying the geometry and material properties. Finite element simulations determined the magnetic field and structural behaviour under load. Genetic programming produced phenomenological expressions describing these responses. Optimisation studies constrained by a measurable force and stable loading conditions were conducted; these produced Pareto sets of designs from which the optimal sensor characteristics were selected. The optimisation demonstrated a compromise between sensitivity and the measurable force, a fabricated version of the optimised sensor validated the improvements made using this methodology. The approach presented can be applied in general for optimising soft tactile sensor designs over a range of applications and sensing modes.
de Boer G, Hewson R, Bryant M, et al., 2017, An investigation into the contact between soft elastic and poroelastic bodies rotating under load, Tribology - Materials, Surfaces & Interfaces, Pages: 1-9, ISSN: 1751-5831
Raske N, Hewson RW, Kapur N, et al., 2017, A predictive model for discrete cell gravure roll coating, Physics of Fluids, Vol: 29, Pages: 062101-1-062101-13, ISSN: 1070-6631
A heterogeneous multiscale model for discrete cell gravure roll coating is presented along with experimental results for the purpose of model validation. The cell volume, generalized cell shape, and the gravure patterning are considered in the model which is based on a multiscale description of the flow in the coating bead. The inclusion of a web-to-roll contact term accounts for the special gravure case when the web-roll separation tends to zero. The results show how the coating bead responds to changes in operating conditions. These are presented as profile plots of the fluid properties and coating bead shape.
Gao L, Hua Z, Hewson RW, et al., 2017, EHL and Wear Modelling of the Knee Joint Replacements with Surface Topography, The 6th World Tribology Conference
Wang H, de Boer G, Kow J, et al., 2017, A low-cost soft tactile sensing array using 3D hall sensors, Procedia Engineering, Vol: 168, Pages: 650-653, ISSN: 1877-7058
Tactile sensors are essential for robotic systems to safely interact with the external world and to precisely manipulate objects. Existing tactile sensors are typically either expensive or limited by poor performance, and most are not mechanically compliant. This work presents MagTrix, a soft tactile sensor array based on four 3D Hall sensors with corresponding permanent magnets. MagTrix has the capability to precisely measure triaxis force (1 mN resolution) and to determine contact area. In summary, the presented tactile sensor is robust, low-cost, high-performance and easily customizable to be integrated into a range of robotic and healthcare applications.
de Boer GN, Gao L, Hewson RW, et al., 2016, Heterogeneous multiscale methods for modelling surface topography in EHL line contacts, Tribology International, Vol: 113, Pages: 262-278, ISSN: 1879-2464
A multiscale method for the Elastohydrodynamic Lubrication (EHL) of line contacts is derived based on the Heterogeneous Multiscale Methods.Periodicity applies to the topographical features and lubricant flow, data is homogenised over a range of variables at amicro-scale and coupled into amacro-scalemodel. This is achieved using flow factorsas calculated from metamodels, which themselves evolve with the solution procedure. Results are givenfor an idealised topography and illustrate significant deviationsfrom smooth surface assumptions as quantified by the flow factors. Improvements in the accuracy andefficiency with previous work and large fluctuations due to micro-EHL are also presented.Validation of the multiscale method with a deterministic topography is provided demonstrating good accuracy and efficiency.
Cea Esteban A, 2016, Multiscale Topology Optimization towards 3D PrintedStructures
Wang H, de Boer G, Kow J, et al., 2016, Design methodology for magnetic field-based soft tri-axis tactile sensors, Sensors, Vol: 16, Pages: 1-20, ISSN: 1424-8220
Tactile sensors are essential if robots are to safely interact with the external world and to dexterously manipulate objects. Current tactile sensors have limitations restricting their use, notably being too fragile or having limited performance. Magnetic field-based soft tactile sensors offer a potential improvement, being durable, low cost, accurate and high bandwidth, but they are relatively undeveloped because of the complexities involved in design and calibration. This paper presents a general design methodology for magnetic field-based three-axis soft tactile sensors, enabling researchers to easily develop specific tactile sensors for a variety of applications. All aspects (design, fabrication, calibration and evaluation) of the development of tri-axis soft tactile sensors are presented and discussed. A moving least square approach is used to decouple and convert the magnetic field signal to force output to eliminate non-linearity and cross-talk effects. A case study of a tactile sensor prototype, MagOne, was developed. This achieved a resolution of 1.42 mN in normal force measurement (0.71 mN in shear force), good output repeatability and has a maximum hysteresis error of 3.4%. These results outperform comparable sensors reported previously, highlighting the efficacy of our methodology for sensor design.
Gao L, Hewson RW, 2016, Development of A Wear Model for LubricatedMetal-On-Metal Hip Joints, 5th International Conference on Biomedical Engineering and Biotechnology 2016
de Boer GN, Wang H, Ghajari M, et al., 2016, Force and Topography Reconstruction Using GP and MOR for the TACTIP Soft Sensor System, 17th Annual Conference, TAROS 2016, Publisher: Springer International Publishing, Pages: 65-74, ISSN: 0302-9743
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