K. E. Evans

University of Exeter, Exeter, England, United Kingdom

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Publications (95)142.52 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.
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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;
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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 01/2013; · 3.12 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.
  • 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. · 4.48 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. · 2.38 Impact Factor
  • W. Miller, C. W. Smith, K. E. Evans
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    ABSTRACT: Honeycombs, as cores in sandwich panels, are in wider use in transport applications where density-specific performance is critical. The ability of honeycombs to withstand through-thickness compression, and in particular resist buckling, is a key performance factor in many applications. However reports of the systematic study of buckling limits in such materials are scant. Honeycomb unit cells with a range of geometries, including conventional hexagonal, re-entrant hexagonal geometries, and modifications of both thereof, were tested mechanically and simulated via finite element modelling. The primary figure of merit was the density-specific peak compressive stress immediately prior to onset of buckling collapse. The conventional hexagonal honeycomb unit cell, as examined here, is not well optimised for density-specific peak stress; in comparison the re-entrant unit cell examined here had approximately 13% higher density-specific peak stress. Modifications to these geometries, such as face stiffeners and fillets, increased absolute values of peak stress but in most cases were deleterious or at best neutral to density-specific peak stress due to the subsequent increase mass of the honeycombs.
    Composite Structures - COMPOS STRUCT. 01/2011; 93(3):1072-1077.
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    ABSTRACT: The coefficient of thermal expansivity (CTE), α, of a 2-D dual-material lattice and the effects of varying the constituent materials and geometry were explored in a parametric study. The lattices had geometries similar to those found in lightweight structures in many transport applications including aerospace and spacecraft. The aim was to determine how to reduce the CTE of such structures to near zero, by using two constituent materials with contrasting CTEs, without incurring penalties in terms of other elastic and failure properties, mass and manufacturability. The results are scale independent and so generic to all such lattices. Lattice CTE was primarily driven by the geometry of the lattice and the mismatch in the constituent’s CTE and elastic moduli, with zero CTE attainable if (i) the relative lengths of internal members a and b were in the range of 1.4–1.6, and (ii) the contrast between αb and αa was at least 4. Large negative CTEs could be obtained easily if in addition the ratio of moduli Eb and Ea was more than 10. It was shown that pairings of commonly used materials, in lattices with commonly used geometries, can give near-zero and negative CTEs. It was shown that this dual-material mechanism effectively exchanges distortion for internal stress. With carefully chosen material pairings there were either small or no penalties for the reduced CTE in terms of other key mechanical performance indices, e.g. premature failure. Two lattices were manufactured, one monolithic and one dual-material (grade 2 titanium and aluminium 6082). Their thermal expansivity was measured and found to match closely the analytical model’s prediction.
    Acta Materialia - ACTA MATER. 01/2011; 59(6):2392-2403.
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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 01/2011; 48(9):1330-1339. · 2.04 Impact Factor
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    ABSTRACT: Models for the nano/micro-structural deformation and mechanical properties of auxetic materials (i.e. materials with a negative Poisson’s ratio) have been previously developed. However, most of these models have been two-dimensional, were usually designed specifically to describe some particular class of auxetic materials, and generally only described the behaviour of one particular plane whilst completely ignoring the out-of-plane behaviour of the material. A three-dimensional model has been developed which can be applied to several classes of auxetic materials, including microporous expanded polymers such as e-PTFE, e-UHMWPE and e-PA, body-centered cubic metals and foams. It is generalised that its underlying structure is not specific to a lengthscale or material as the previous list shows. The new model offers a better insight into the underlying principles behind the observed auxetic behaviour and offers a significant improvement in the agreement of the models with existing experimental data. It is shown that there are geometric limitations to the number of planes that can simultanesously display auxetic behaviour. This has ramifications on the design of ordered auxetic materials.
    Journal of Materials Science 01/2011; 46(2):372-384. · 2.31 Impact Factor
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    Miller W, Smith C W, Scarpa F, Evans K E
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    ABSTRACT: The flatwise compressive behaviour of tetrachiral and hexachiral honeycombs is analysed, using analytical and Finite Element simulations, both with explicit and implicit formulations. The tetrachiral and hexachiral cells are composed by cylinders connected by four and six tangent ligaments respectively. The ligaments act as mixed stiffeners-elastic foundations during flatwise compressive loading, providing different buckling mode shapes during deformation. The models are compared with experimental results obtained using RP-based honeycombs tested according to ASTM C 393-00 and ASTM C365-00.
    Composites Science and Technology 07/2010; http://dx.doi.org/10.1016/j.compscitech.2009.10.022(70):1049-1056. · 4.48 Impact Factor
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    ABSTRACT: This work describes the out-of-plane linear elastic mechanical properties of trichiral, tetrachiral and hexachiral honeycomb configurations. Analytical models are developed to calculate the transverse Young’s modulus and the Voigt and Reuss bounds for the transverse shear stiffness. Finite Element models are developed to validate the analytical results, and to identify the dependence of the transverse shear stiffness vs. the gauge thickness of the honeycombs. The models are then validated with experimental results from flatwise compressive and simple shear tests on samples produced with rapid prototype (RP)-based techniques.
    Composites Science and Technology 07/2010; http://dx.doi.org/10.1016/j.compscitech.2009.07.008(70):1057-1063. · 4.48 Impact Factor
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    ABSTRACT: Finite Element models are developed for the in-plane linear elastic constants of a family of honeycombs comprising arrays of cylinders connected by ligaments. Honeycombs having cylinders with 3, 4 and 6 ligaments attached to them are considered, with two possible configurations explored for each of the 3- (trichiral and anti-trichiral) and 4- (tetrachiral and anti-tetrachiral) connected systems. Honeycombs for each configuration have been manufactured using rapid prototyping and subsequently characterised for mechanical properties through in-plane uniaxial loading to verify the models. An interesting consequence of the family of 'chiral' honeycombs presented here is the ability to produce negative Poisson's ratio (auxetic) response. The deformation mechanisms responsible for auxetic functionality in such honeycombs are discussed.
    IMRI: Journal Articles (Peer-Reviewed). 01/2010;
  • W. Miller, C W Smith, F.I. Scarp, K. E. Evans
    EPJ Web of Conferences. 01/2010;
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    ABSTRACT: The use of negative thermal expansivity (NTE) particles as composite fillers is relatively new and the particle–matrix interface is not well studied. This lack of understanding of the particle–matrix interface is further complicated as in many engineering applications, such as microchip packaging, the composite is constrained by its surroundings and is not free to expand upon heating; an important consideration that is often not taken into account. This paper presents a systematic theoretical study of the behaviour at the particle–matrix interface under varying particle coefficient of thermal expansivity (CTE), Poisson’s ratios (including negative Poisson’s ratios), Young’s moduli, boundary conditions and particle separation distances via finite element modelling, and describes how to optimise composite formulation for problems of thermal mismatch through tailoring of particle–matrix interaction. The effects of reduced CTE are explored via models of electronic chip package assembly.
    Composites Science and Technology - COMPOSITES SCI TECHNOL. 01/2010; 70(2):318-327.
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    ABSTRACT: In this work we present concepts and prototypes of a novel class of chiral honeycomb core with embedded sensing characteristics for potential structural health monitoring (SHM) and other multifunctional applications. The cellular structure concepts, all having negative Poisson’s ratio behaviour, have piezoelectric sensors and their hardware support embedded on the surface, or within the unit cell plates. Both the sensors and the infrastructure provide not only the capability of detecting signals proportional to the external mechanical loading, but act also as load-bearing units. The honeycombs have been produced using vacuum-casting techniques and resin transfer moulding methods, with micro fibre composites (MFCs) embedded in their cell walls. The sensing and mechanical performance of the prototypes are evaluated using finite element simulations, static tests, broadband vibration excitation and impact at low kinetic energy levels using an airgun.
    Composites Science and Technology 01/2010; · 4.48 Impact Factor
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    ABSTRACT: In this work, the deformation of material under localised tangential loading has been investigated. An analytical expression to predict deformation and strain patterns under tangential loading over a rectangular patch on a surface is validated against experiential tests using surface displacement measurement and finite element modelling. The predicted force- displacement data and displacement/strain patterns show close agreement with experimental results a rubber test material and FE results. The ranges of specimen geometries that minimise the boundary effects have been determined. KeywordsSurface displacement analysis-Deformation-Analytical-FE modelling
    Experimental Mechanics 01/2010; 50(5):651-659. · 1.55 Impact Factor

Publication Stats

1k Citations
142.52 Total Impact Points

Institutions

  • 1994–2013
    • University of Exeter
      • • Department of Engineering
      • • Department of Computer Science
      Exeter, England, United Kingdom
  • 2006–2009
    • University of Bolton
      • Institute for Materials Research and Innovation
      Bolton, ENG, United Kingdom
  • 2001
    • University of Malta
      • Department of Chemistry
      Msida, Malta
  • 1991–1994
    • University of Liverpool
      Liverpool, England, United Kingdom