Ken E Evans

University of Exeter | UoE · Department of Engineering

Novel mechanical metamaterials inspired by single crystal boron arsenate built at the macroscale using from conventional materials are presented and investigated through finite elements simulations. It was shown that these mechanical metamaterials can mimic, to some extent, the properties of boron arsenate with the more robust rotating rigid‐units...

The mechanical properties of single crystals boron arsenate, BAsO 4 , are simulated through density functional theory simulations over a wide range of pressures from −10 to +90 GPa. It was shown that the elastic constants, as well as the extent of auxeticity, are highly pressure dependent. In particular, the Poisson’s ratio in the (001) plane for s...

The rising popularity of laser sintering (LS) technology has increased by the broadening of available materials for this process. Kepstan 6002 poly (ether ketone ketone) (PEKK) was recently launched as a high-performance polymer grade with a lower processing temperature and unique crystallization kinetics. This study aims to understand the progress...

In a study of unique scale, twenty-four milled Polyaryletherketone (PAEK) polymer powders produced during the development of new grades for Powder Bed Fusion (PBF) were tested using a universal powder rheometer and particle size and shape analyser. Multivariate correlation was used to identify the linear relationships between bulk and dynamic prope...

Boron arsenate, BAsO4, is a β-cristobalite-like crystal which has been reported to exhibit the rather unusual property of negative linear compressibility behaviour at elevated pressures, that is expanding rather than shrinking in a linear dimension when subjected to pressure. This work proposes a ‘geometry—deformation mechanism’-based mathematical...

Auxetic foams counter-intuitively expand (shrink) under stretching (compression). These foams can exhibit superior mechanical properties such as resistance to shear and indentation, improved toughness and energy absorption (EA) under several types of loadings. Their unique deformation mechanism and manufacturing process lead to special multiphysics...

Negative thermal expansion (NTE) materials and structures exhibit the anomalous property of shrinking rather than expanding when heated. This work examines the potential of multi-material planar re-entrant and non-re-entrant honeycombs to exhibit anomalous thermal expansion properties. Expressions for the coefficient of thermal expansion as a funct...

A highly tunable and symmetric system which can exhibit a wide spectrum of Poisson’s ratios, ranging from very negative (auxetic) to large positive values, was investigated by Pierre-Sandre Farrugia and co-workers in article number 2100158. This planar structure can be described as a generalised form of Star-4 system and can be used in a variety of...

Auxetics exhibit the anomalous property of expanding laterally when uniaxially stretched, that is, exhibit a negative Poisson’s ratio, a property which arises from: (i) the presence of specific geometric features within the nano/macro structure of the material and (ii) amenable deformations in response to the applied stimulus. This work explores ho...

An analytical model for a general Star‐4 system, having different ligament lengths, angles and ligament thickness, was derived using Castigliano’s second theorem. The analytical model was validated through finite element analysis and experimental work. The results obtained show that, depending on the parameters used, the general Star‐4 system can s...

Whilst contact loading is known to affect wear, the general stress field is rarely considered. Steel fretting (and low amplitude reciprocating) wear contacts typically develop through a transient regime and wear by multiple mechanisms. It is expected that if these mechanisms were controlled by plasticity or fatigue, the wear rate would be altered b...

Mechanical metamaterials are systems which derive their mechanical properties from their structure rather than their intrinsic material composition. In this work, we investigate a class of highly anisotropic mechanical metamaterials designed by the introduction of diamond and elliptically shaped perforations which possess the ability to show auxeti...

Laser Sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing t...

Due to the intrinsically limited mechanical properties, functionalities and structures of mineral material composites (MMCs) made through traditional fabrication approaches, there is a critical need to develop novel three dimensional (3D) forming techniques that can construct mineral material composites (MMCs) and structures with high performance a...

The effects of debris particles in fretting contacts are substantial and are believed to play a key role in the difference in wear rates observed between fretting and full sliding wear. Studies of debris have shown that the effects can have detrimental or palliative effects on the parent surfaces. This work aims to explore this phenomenon through t...

The recent progress in Additive Manufacturing Powder Bed Fusion systems are bound to attract uptake of new high performance materials. Within the Poly(aryl ether ketone) (PAEK) family, Poly(ether ketone ketone) (PEKK) shows promising mechanical properties , comparable with the commercial laser sintering grade PEK HP3, whilst processed at significan...

Shrinkage is extensively mentioned in the literature as one of the main causes for dimensional instability or poor performance in Laser Sintering (LS). This study proposes and examines a methodology to describe shrinkage in cooling from a material perspective. Thermal behaviour and crystallisation effects were measured to determine the influence of...

An extensive investigation is carried out on the missing rib square grid structure using finite element simulations. This showed that, in the cases considered in this study, the type of deformation is primarily dependent on the ratio of the thickness of different ribs with the structure behaving like an anti‐tetrachiral at particular ratios. The an...

Poly(phenylacetylene) sheets that mimic the geometry of a wine‐rack‐like structure have been predicted to exhibit negative Poisson's ratios off‐axis. However, their potential to exhibit negative linear compressibility (NLC) has remained largely unexplored. In this work, the compressibility and other mechanical properties of wine‐rack‐like poly(phen...

An experimental study on the melt and coalescence of different Poly(aryl ether ketones) (PAEK) powders was carried out. The study consisted of evaluating individual particles and neck growth using a hot stage platform. Three different methods of assessing particles dimensions were evaluated, and the coalescence was compared for the grades on three...

This paper develops an auxetic (negative Poisson's ratio) piezoelectric energy harvester (APEH) to increase the power output when harnessing strain energy. The APEH consists of a piezoelectric element bonded to an auxetic substrate. The auxetic substrate concentrates the applied stress and strain into the region of the piezoelectric element and int...

The term “mechanical metamaterials” encompasses a wide range of systems whose anomalous mechanical properties arise primarily from their structure rather than composition. This unique characteristic gives them an edge over many conventional natural or readily available materials and makes them well-suited for a variety of applications where tailor-...

Poly Ether Ether Ketone (PEEK) is a high temperature polymer material known for its excellent chemical resistance, high strength and toughness. As a semi-crystalline polymer, PEEK can become very brittle during long crystallisation times and temperatures helped as well by its high content of rigid benzene rings within its chemical structure. This p...

We simulate, analyse and compare the mechanical properties of a number of molecular sheet-like systems based on fully substituted, penta-substituted, tetra-substituted and tri-substituted poly(phenylacetylene) using static force-field based methods. The networks are modeled in a 3D environment with and without inter-layer interactions in analogy to...

High temperature laser sintering is an Additive Manufacturing (AM) process that allows fabrication layer by layer of complex structures in high performance biomaterial polymers such as Poly Ether Ether Ketone (PEEK) with high design freedom and short lead time. While studies on new material candidates for HT-LS are increasingly appearing, the quali...

The microstructure of wood is highly anisotropic, which suggests that this material could exhibit unusual elastic properties. A few studies have suggested the possibility of auxeticity, or negative Poisson’s ratio (NPR) in some wood samples. In order to conclusively confirm that wood does show NPR, we have used a combination of geometric modelling,...

A novel class of hexagonal nanoscale honeycombs made from penta and tetra substituted polyphenlyacetylenes is proposed and modeled as crystalline systems using force-field based simulations. It is shown that, in-plane, these systems behave rather similarly to crystalline forms of graphyne, graphdiyne, and other fully substituted equivalents but ben...

This paper proposes two methods of preparation of graphene/PEEK powders for Laser Sintering (LS) and investigates their behaviour in relation to their microstructure and their properties. Thin composite films were fabricated in an attempt to replicate the thin layer formation of the powder bed process. Both methods of composite powder preparation (...

This paper introduces a novel extrusion process for manufacturing helical auxetic yarn (HAY). A range of semi-coextruded HAYs have been manufactured in a cost-effective, consistent and readily usable form. The semi-coextruded HAYs were compared to the conventional spun yarns in terms of tensile properties and auxetic behavior. The results show the...

In this work, we investigate the deformation mechanism of auxetic hierarchical rotating square systems through a dynamics approach. We show how their deformation behaviour, hence their mechanical properties and final configuration for a given applied load, can be manipulated solely by altering the resistance to rotational motion of the hinges withi...

New materials for laser sintering (LS) are usually developed using a trial and-error approach that consists of a series of builds within LS systems. This strategy is time consuming, costly and focuses only on the optimisation of the processing parameters, ignoring the powder properties of the materials under examination. Being able to predict proce...

This paper presents an experimental investigation of the dynamic thermo-mechanical and impact properties of helical auxetic yarns (HAYs). A series of thermoplastic polyurethane (TPU) core fibres fabricated using an extrusion process have been wrapped with either ultra-high-molecular-weight polyethylene (UHMWPE) wrap or stainless steel wire wrap to...

New materials for Laser Sintering (LS) are usually developed using a trial and-error approach that consists of a series of builds within LS systems. This strategy is time consuming, costly and focuses only on the optimisation of the processing parameters, ignoring the powder properties of the materials under examination. Being able to predict proce...

This paper presents a systematic study of the helical auxetic yarn (HAY) via careful in-house fabrication and characterisation of a wide range of polymeric fibres and yarns. It provides a better understanding of the auxetic behavior of the HAY in order to tailor their properties for specific applications. The study focused on three parameters: comp...

The crystal structure of Poly Aryl Ether Ketones (PAEKs) fabricated using a range of manufacturing processes has been investigated by high resolution electron microscopy. The study has revealed a hierarchical spherulitic crystal structure, different from the common spherulite morphology with lamellae grown in a radial direction. The results show th...

We investigate the ability of counting rules drafted from engineering to predict the flexibility or rigidity of bar-and-joint or body-and-joint assemblies representing metal organic frameworks. We show that while scalar counting rules are not reliable, group-theoretical approaches are able to disentangle mechanisms from states of self-stress and to...

This paper proposes a new technique for fabrication of vertically aligned carbon nanotube (VACNT) structures, controlled in shape, height and functionality, through continuous successive growth of VACNT layers by chemical vapour deposition (CVD) combined with patterning strategies. This was achieved by vacuum deposition of additional catalyst mater...

The Double-Shear Lap Joint (DSLJ) is a novel damping insert sited internally within a structure which is particularly well suited for lightweight sandwich structures with internal voids, such as honeycomb core sandwich panels. In high performance lightweight structures, the insertion of relatively more dense dampers of any type may increase the tot...

Almost all matter including viscoelastic cellulose respond to energy changes which almost invariably influence its atomic and or molecular bond characteristics (length, orientation, and geometry). Changes in bonding characteristics of conformationally isomeric cellulose is characterised by variations in the relative amounts of rotamers which has im...

Materials with a negative Poisson's ratio known also as auxetic materials [1] exhibit unusual property of getting thicker when stretched and thinner when compressed. The helical auxetic yarn (HAY) is a recently invented auxetic reinforcing structure for composites [2]. A helical auxetic yarn (HAY) consists of two fibres: a low modulus elastomeric c...

Functional auxetic composite materials can be fabricated from conventional or from auxetic components. The helical auxetic yarn (HAY) is a very recently invented auxetic reinforcing component for composite materials. This paper investigates the Poisson’s ratio behaviour of a further development of the HAY, needed for many practical applications. Th...

Almost all matter including viscoelastic cellulose respond to energy changes which almost invariably influence its atomic and or molecular bond characteristics (length, orientation, and geometry). Changes in bonding characteristics of conformationally isomeric cellulose is characterised by variations in the relative amounts of rotamers which has im...

Single crystals can commonly have negative Poisson’s ratio in a few directions; however more generalised auxeticity is rarer. We propose a typology to distinguish auxetic materials. We characterise numerous single crystals and demonstrate that partial auxeticity occurs for around 37%. We find average auxeticity to be limited to α-cristobalite and n...

Negative linear compressibility (NLC) is still considered an exotic property, only observed in a few obscure crystals. The vast majority of materials compress axially in all directions when loaded in hydrostatic compression. However, a few materials have been observed which expand in one or two directions under hydrostatic compression. At present,...

The 1D bundles of cellulose microfibrils (lignified flax fibre) and 2D networks of cellulose microfibrils form tunicate, bacterial and microfibrillated celluloses were strained in tension, and their molecular deformation followed by Raman spectroscopy in order to fully understand the origins and magnitudes of in-plane auxetics for the information o...

Currently, the HT-LS sector is predominantly based around one commercial poly ether ketone (PEK) polymer. Although the combination of polymer and process works well, a lower melting temperature polymeric material, part of the same Poly Aryl Ether Ketone (PAEK) family would be preferable in certain applications. This study presents the optimisation...

The use of discontinuous carbon fibre plastics (DCFP) in automotive applications is increasing. The material offers a light weight and stiff alternative to conventional short-fibre glass-reinforced composites such as sheet moulding compounds (SMC). Prohibiting factors to wider utilisation of this material are cost and formability. Carbon moulding c...

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...

The axial and lateral straining of naturally composited straight 1D lignified flax fibre, and straight and bent 2D cellulose networks, namely tunicate cellulose, bacterial cellulose and microfibrillated cellulose, were investigated by straining in tension using a Deben micro-tester, and following molecular deformation with Raman spectroscopy with t...

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 wou...

Unlike conventional (positive Poisson's ratio) materials, when auxetic (negative Poisson's ratio) materials are stretched, they expand in at least one direction at 90° to the direction of stretch. Cellulose is believed to be auxetic. To fully appreciate the auxetics of non-native (processed) cellulose, a 1D bundle of cellulose microfibrils and 2D n...

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...

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)...

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 p...

This paper considers effective strain tensors within the context of linear elastic equilibrium theory. The elastic properties of structured materials are often averaged over subvolumes of various scales inside the material. For subvolumes smaller than a representative volume element, simple volume-averaging of the stress and strain may not preserve...

This paper investigates the effects of particles size and morphology on flowability of a range of polymeric powders (SLS and non-SLS grades). The effect of additives incorporation, as well as drying or sieving, on the flowability characteristics of the powders is also analyzed. The results show that the particle morphology has a stronger influence...

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 bas...

Negative compressibility is the ability to expand in at least one dimension rather than shrinking upon the application of an externally applied hydrostatic pressure. It is shown that, contrary to current perception, negative linear compressibility may be obtained from re-entrant hexagonal truss systems of specific geometric features which deform th...

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 measur...

Selective Laser Melting (SLM) is an increasingly employed additive manufacturing process for the production of medical, aerospace, and automotive parts. Despite progresses in material flexibility and mechanical performances, relatively poor surface finish still presents a major limitation in the SLM process.In this study an investigation of surface...

An auxetic material comprises an outer component of a relatively compliant material, and an inner component of a relatively stiff material, wherein the inner component is located within a cavity provided within the outer component, the cavity being of non-linear form in the longitudinal direction of the material when the material is at rest.

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 th...

Micromachined polymeric honeycomb membranes having conventional and re-entrant cell geometries have been fabricated using femtosecond laser ablation. Mechanical properties characterization confirms that the re-entrant membrane is auxetic (possesses negative Poisson's ratios: nu(xy) = -1.82 +/- 0.05 and nu(yx) = -0.51 +/- 0.01) whereas the conventio...

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...

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 g...

Support structures are required in several additive manufacturing (AM) processes to sustain overhanging parts, in particular for the production of metal components. Supports are typically hollow or cellular structures to be removed after metallic AM, thus they represent a considerable waste in terms of material, energy and time employed for their c...