K. E. Evans

University of Exeter, Exeter, England, United Kingdom

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Publications (139)292.11 Total impact

  • P. Aumjaud, C.W. Smith, K.E. Evans
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    ABSTRACT: Constrained layer dampers (CLD) are in widespread use for passive vibration damping, in applications including aerospace structures which are often lightweight. The location and dimensions of CLD devices on structures has been the target of several optimisation studies using a variety of techniques such as genetic algorithms, cellular automata, and gradient techniques. The recently developed double shear lap-joint (DSLJ) damper is an alternative method for vibration damping, and can be placed internally within structures. The performance of the DSLJ damper is compared in a parametric study with that of CLD dampers on beam and plate structures under both cantilever and simply supported boundary conditions, using finite element analysis. The objective was to determine which damper and in which configuration produced the highest modal loss factor and amplitude reduction for least added mass, as would be important for lightweight applications. The DSLJ tend to be more mass efficient in terms of loss factor and amplitude reduction for cantilevered beam and plate structure, and are competitive with CLD dampers in simply supported beam and plate structures. The DSLJ works well because it has the potential to magnify global flexural deformation into shear deformation in the viscoelastic more effectively than traditional CLD dampers.
    Composite Structures 01/2015; 119:322–332. DOI:10.1016/j.compstruct.2014.09.005 · 3.12 Impact Factor
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    ABSTRACT: Materials with a negative Poisson’s ratio are referred to as auxetic. One recently invented example of this is the helical auxetic yarn (HAY). This has been proved to successfully exhibit auxetic behaviour both as a yarn and when incorporated into fabric. The HAY is based on a double-helix geometry where a relatively stiffer ‘wrap’ is helically wound around a compliant core fibre. This paper studies the effect of the interaction between the core and the wrap fibre on the auxetic behaviour of the HAY, including the effect of their relative moduli. Assessment of the Poisson’s ratio of the HAYs has revealed that an elevated difference in component moduli causes the wrap fibre embedding itself into the core fibre, thus decreasing the auxetic effect. Careful determination of an optimum core–wrap moduli ratio where the ratio is high enough to yield an auxetic effect and low enough to prevent the core-indentation effect can lead to the fabrication of a yarn with largest negative Poisson’s ratio.
    Composites Science and Technology 10/2014; 102:87–93. DOI:10.1016/j.compscitech.2014.07.023 · 3.63 Impact Factor
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    ABSTRACT: This paper presents an investigation into the properties of Poly Ether Ketone (PEK) components using the commercial high temperature laser sintering system, EOSINT P800. The shrinkage and the mechanical performance of components across the entire build chamber have been tested and a non-linear shrinkage profile has been obtained. The middle of the build chamber recorded the highest degree of shrinkage and the shrinkage in Z direction had the largest variation. The laser sintered components built in X and Y directions recorded a 10% lower tensile strength than the injection moulded samples of the same material where those built in the Z direction showed an approximately 50% decrease in strength in comparison with the injection moulded test specimens. The crystallinity between the skin and the core of the sintered samples was different; varied with the position within the build chamber and coincided with noticeable sample colour changes.
    Materials and Design 09/2014; 61:124–132. DOI:10.1016/j.matdes.2014.04.035 · 3.17 Impact Factor
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    ABSTRACT: In an attempt to expand the range of engineering polymers used for laser sintering, this paper examines the morphology, flowability and interparticle interactions of two commercially available poly (ether ether) ketone (PEEK) powders, not yet optimised for the LS process, by comparison with the LS optimised Polyamide (PA) and Polyetherketone (PEK) powdered polymers. The effect of incorporating fillers and additives on the flow behaviour is also analysed. The Particle Size Distribution (PSD) results alone do not allow ranking the powder materials in relation to the flow behaviour. The particle morphology has a stronger influence on the flow characteristics for materials with similar PSDs. The work also provides additional characterization parameters to be considered when analysing LS powders.
    European Polymer Journal 08/2014; 59. DOI:10.1016/j.eurpolymj.2014.08.004 · 3.24 Impact Factor
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    ABSTRACT: Laser sintering (LS) of polymers has high potential for growth as a manufacturing technique into a wide range of applications provided the range of engineering polymers available for LS expands and machines and LS process conditions are optimised for such materials.This study is the first investigation into laser sintering of both virgin and used polyether ketone (PEK) powder using a bespoke high temperature (HT) polymer laser sintering machine (known commercially as EOSINT P800). The physico-chemical results reveal that, in spite of polymer degradation, used PEK has a viable processing window for LS manufacturing which, combined with optimisation of specific parameters can successfully lead to manufacture of good quality parts. The proposed sintering mechanism of both, virgin and used powders is supported by the experimental data. The incorporation of 30% used HP3 PEK powder led to an approximately 17% drop in tensile strength.
    Journal of Materials Processing Technology 04/2014; 214(4):969–978. DOI:10.1016/j.jmatprotec.2013.11.007 · 2.04 Impact Factor
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    ABSTRACT: The introduction of material into the void of honeycomb-like structures, such as foam, viscoelastic or particulate filling, has been credited with improving the damping properties of the honeycombs. Optimisation of such damping inserts has been investigated, and indicates that partial occupation of the void could be more efficient, on a density basis, than full filling. The main goal of this study is to explore fully damping in honeycomb cells with inserts from the point of view of minimal increase in density and location of inserts. In this paper, damping of vibrations in the plane is investigated using analytical, finite element and topological optimisation methods to find the best locations of a damping insert within the cell.
    Composite Structures 12/2013; 106. DOI:10.1016/j.compstruct.2013.05.036, · 3.12 Impact Factor
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    ABSTRACT: Two-dimensional regular theoretical units that give a negative Poisson’s ratio (NPR) are well documented and well understood. Predicted mechanical properties resulting from these models are reasonably accurate in two dimensions but fall down when used for heterogeneous real-world materials. Manufacturing processes are seldom perfect and some measure of heterogeneity is therefore required to account for the deviations from the regular unit cells in this real-life situation. Analysis of heterogeneous materials in three dimensions is a formidable problem; we must first understand heterogeneity in two dimensions. This paper approaches the problem of finding a link between heterogeneous networks and its material properties from a new angle. Existing optimisation tools are used to create random two-dimensional topologies that display NPR, and the disorder in the structure and its relationship with NPR is investigated.
    Mechanics of Materials 04/2013; DOI:10.1016/j.mechmat.2009.04.008 · 2.23 Impact Factor
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    ABSTRACT: Recent advances have shown effective reinforcement of a polymer matrix using vertically aligned carbon nanotube (VACNT) forests grown using chemical vapour deposition (CVD). Such structures are known to wet readily through capillary interactions to form fully wetted composites that retain the dispersion and alignment of the CNT within the matrix thus overcoming two major problems in CNT composite production. So far VACNT composite fabrication has been limited by available forest size and as a result mechanical characterisation of such materials has been restricted and has generally been conducted using nanoindentation techniques. In this work VACNT composite samples are produced that are of a sufficiently large size to conduct Dynamic Mechanical Thermal Analysis (DMTA) in single cantilever mode allowing macroscale testing of the complete composite sample. Results from experiments are compared to current VACNT composite modelling techniques that consider CNT waviness which is known to exist within such composite samples. Previously the effects of as grown VACNT waviness have been considered detrimental to the overall mechanical properties of these novel composites as analysis has typically been conducted in the axial direction where applied load and VACNT axis are parallel. In this work a positive effect on sample modulus resulting from VACNT waviness has been found in the transverse plane, perpendicular to the CNT axis. Specifically an increase in modulus of over 20% is observed with only a 2 vol.% CNT loading and is in agreement with wavy CNT composite modelling.
    Composites Science and Technology 03/2013; 77:1–7. DOI:10.1016/j.compscitech.2013.01.001 · 3.63 Impact Factor
  • Z. Xi, O. R. Ghita, K. E. Evans
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    ABSTRACT: This study examines the thermal expansion behaviour of virgin and recyclate polytetrafluoroethylene (PTFE)/glass fibre (GF) composites using two new sintering approaches – with and without pressure. In the case of composites sintered without pressure, the thermal expansion behaviour showed an unusual trace for both virgin and recycled samples. The introduction of a small amount glass fibre (<15 vol%) will increases the overall thermal expansion in the compression direction. This unusual behaviour could relate to pores entrapped and the alignment of the glass fibre in the structure.
    Composites Part A Applied Science and Manufacturing 11/2012; 43(11):1999–2006. DOI:10.1016/j.compositesa.2012.07.013 · 3.01 Impact Factor
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    ABSTRACT: The introduction of hierarchy into structures has been credited with improving their elastic and other properties. Similarly, functional grading has been demonstrated to increase the damage tolerance of honeycomb structures, although with the penalty of reduced Young’s modulus or increased density. The combination of both hierarchy and functional grading has not been reported for honeycomb structures, although it is known in natural materials. A parametric numerical modelling study has been made of the in-plane elastic properties of honeycombs and how they are affected by functional grading and hierarchy, and importantly to establish whether it is possible to avoid reductions in Young’s modulus. A set of analytical models has been developed to describe functional grading and hierarchy in honeycombs, based upon beam mechanics and the transform section method. The conditions for transition of a hierarchical honeycomb in behaviour from that of a discrete structure to that of a continuum are established. Furthermore, conditions are established for which hierarchical honeycombs, uniform or functionally graded, can surpass in-plane Young’s moduli of conventional honeycombs a by factor of up to 2, on an equal density basis.
    Composite Structures 07/2012; 94(8):2296–2305. DOI:10.1016/j.compstruct.2012.01.021 · 3.12 Impact Factor
  • W. Miller, Z. Ren, C. W. Smith, K. E. Evans
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    ABSTRACT: The manufacture of negative Poisson’s ratio (auxetic) composites, containing inherently auxetic phases is rare and has been confined to relatively low modulus composite systems with stiffnesses several orders of magnitude below those of structural composites. This paper presents the use of an auxetic double helix yarn that is used to produce a unidirectional fibre composite with both relatively high stiffness (4 GPa) and negative Poisson’s ratio (−6.8), at 30% fibre volume fraction, compared to other auxetic composites. This is the first structural auxetic composite to be produced using carbon fibre and importantly it was produced using standard manufacturing techniques and therefore is potentially applicable in a variety of engineering applications.
    Composites Science and Technology 04/2012; 72(7):761–766. DOI:10.1016/j.compscitech.2012.01.025 · 3.63 Impact Factor
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    ABSTRACT: Most materials compress axially in all directions when loaded hydrostatically. Contrary to this, some materials have been discovered that exhibit negative linear compressibility and, as such, expand along a specific axis or plane. This paper analyses a fundamental mechanism by using a combination of finite element simulations and analytical derivations to show that negative linear compressibility can be found in a body-centred or face-centred tetragonal network of nodes connected by a network of beams. The magnitude and direction of this behaviour depends on the cross geometry in the network.
    Mechanics of Materials 03/2012; 46:123–128. DOI:10.1016/j.mechmat.2011.12.007 · 2.23 Impact Factor
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    ABSTRACT: The study reports mechanical performance of the recycled glass fibres produced from a water-based solvolysis technology, known as the hydrolysis process. The chemical reaction was carried out using sub-critical water to dissolve polyester resin and recover the glass fibres from composites. The effect of temperatures, times, catalyst and water amount on mechanical properties of the recovered glass fibres were investigated. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and time-of flight secondary ion mass spectroscopy (ToF-SIMS) analyses were also employed to examine the fibre surface associated with the polyester resin eliminated level after the hydrolysis reaction. The results revealed that by carefully adjusting the hydrolysis parameters the tensile strength and failure strain of the recycled fibres decrease by approximately 40–70% in comparison with virgin fibres while Young’s moduli remain similar. The relationship between the hydrolysis conditions, recovered fibres and mechanical performance was discussed in this study.
    Composites Part A Applied Science and Manufacturing 03/2012; 43(3):398–406. DOI:10.1016/j.compositesa.2011.11.011 · 3.01 Impact Factor
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    ABSTRACT: Materials having a negative Poisson's ratio (auxetic) get fatter rather than thinner when uniaxially stretched. This phenomeno has been often explained through models that describe how particular geometric features in the micro or nanostructure of th material deform when subjected to uniaxial loads. Here, a new model based on scalene rigid triangles rotate relative to eac other will be presented and analysed. It is shown that this model can afford a very wide range of Poisson's ratio values the sign and magnitude of which depends on the shape of the triangles and the angles between them. This new model has th advantage that it is very generic and may be potentially used to describe the properties in various types of materials, includin auxetic foams and their relative surface density. Specific applications of this model, such as a blueprint for a system tha can exhibit temperature-dependent Poisson's ratios, are also discussed.
    Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences 03/2012; 468(2139):810-830. DOI:10.1098/rspa.2011.0273 · 2.00 Impact Factor
  • Zhoutian Xi, Oana R. Ghita, Ken E. Evans
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    ABSTRACT: This study examines the recycling of polytetrafluoroethylene (PTFE)/glass fiber (GF) printed circuit boards (PCB) laminates through mechanical grinding and the re-manufacture of new composites incorporating PTFE/GF recyclate using sintering processes with and without pressure (SWOP and SWP). The PTFE/GF recyclate was tested for mechanical and dielectric performance. Experimental data were fitted to modified Lichtenecker and Effective-Medium Theory (EMT) theoretical equations to estimate the dielectric constant of PTFE/GF recyclate subsheets for further use in new PTFE/GF laminates. It was found that the experimental values were inside of the Wiener bounds and fitted well the two theories for both manufacturing methods proposed. Overall, the results showed that PTFE/GF recyclate could be used as replacement for virgin PTFE/GF when incorporated in specific concentrations. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.
    Journal of Applied Polymer Science 11/2011; 122(4). DOI:10.1002/app.34350 · 1.40 Impact Factor
  • M. A. Beard, O. R. Ghita, K. E. Evans
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    ABSTRACT: Near infrared spectroscopy has been used to monitor the effects of changing build parameters on the sintering process of selective laser sintering components. The surface roughness of the parts produced has been studied whilst modifying laser scan speed and laser power build parameters. Near infrared spectroscopy is shown to be a powerful tool in detecting subtle variations in the coalescence of particles that form the surface topology of the component. Principal component analysis (PCA) performed on the diffuse reflectance spectra obtained from the surface of the components shows a strong correlation between near infrared (NIR) spectra and build parameters. Using the chemometric model produced from the PCA analysis it is possible to calculate build parameters for unknown components, making NIR a useful aid for quality control of additive manufacturing technologies. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
    Journal of Applied Polymer Science 09/2011; 121(6). DOI:10.1002/app.33898 · 1.40 Impact Factor
  • M. R. Sloan, J. R. Wright, K. E. Evans
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    ABSTRACT: This paper introduces a novel fibre structure known as the helical auxetic yarn (HAY). The geometry of the yarn is defined and the manufacturing process described. A range of HAYs have been manufactured that vary the geometric properties of the structure. A systematic study of the yarns has been completed to evaluate the effect on the auxetic behaviour of the geometry. We also characterise the component fibres and yarns and discuss the influence of geometric and material effects on the observed Poisson’s ratio of the yarns.It can be shown that the starting wrap angle of the yarn has the greatest effect on auxetic behaviour both in terms of magnitude and the strain range over which it may be observed.The maximum negative Poisson’s ratio observed for a yarn manufactured from conventionally available monofilaments with positive Poisson’s ratio is −2.7.
    Mechanics of Materials 09/2011; 43(9):476–486. DOI:10.1016/j.mechmat.2011.05.003 · 2.23 Impact Factor
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    ABSTRACT: Introducing hierarchy into structures has been credited with improving elastic properties and damage tolerance. Specifically, adding hierarchical sub-structures to honeycombs, which themselves have good-density specific elastic and energy-absorbing properties, has been proposed in the literature. An investigation of the elastic properties and structural hierarchy in honeycombs was undertaken, exploring the effects of adding hierarchy into a range of honeycombs, with hexagonal, triangular or square geometry super and sub-structure cells, via simulation using finite elements. Key parameters describing these geometries included the relative lengths of the sub- and super-structures, the fraction of mass shared between the sub- and super-structures, the co-ordination number of the honeycomb cells, the form and extent of functional grading, and the Poisson’s ratio of the sub-structure. The introduction of a hierarchical sub-structure into a honeycomb, in most cases, has a deleterious effect upon the in-plane density specific elastic modulus, typically a reduction of 40 to 50% vs a conventional non-hierarchical version. More complex sub-structures, e.g. graded density, can recover values of density specific elastic modulus. With careful design of functionally graded unit cells it is possible to exceed, by up to 75%, the density specific modulus of conventional versions. A negative Poisson’s ratio sub-structure also engenders substantial increases to the density modulus versus conventional honeycombs.
    International Journal of Solids and Structures 05/2011; 48(9):1330-1339. DOI:10.1016/j.ijsolstr.2011.01.017 · 2.04 Impact Factor

Publication Stats

3k Citations
292.11 Total Impact Points

Institutions

  • 1994–2014
    • University of Exeter
      • • College of Engineering, Mathematics and Physical Sciences
      • • Department of Engineering
      • • Department of Computer Science
      Exeter, England, United Kingdom
  • 2001
    • University of Malta
      • Department of Chemistry
      Msida, Malta
  • 1987–1998
    • University of Liverpool
      • School of Engineering
      Liverpool, England, United Kingdom