Dynamic thermo-mechanical and impact properties of helical auxetic yarns

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 form helical auxetic yarns. Dynamic mechanical analysis (DMA) measurements indicated that the core/wrap diameter ratio and the initial wrap angle influenced significantly the dynamic thermo-mechanical behaviour of HAYs. The impact test results have shown that the fibre property, impact velocity and the initial wrap angle had great effect on the impact response of a HAY. Importantly, in this work it is shown that an optimal wrap angle can be found to give the best combination of stiffness, energy absorption and auxetic performance of HAYs.This work is supported by the UK Engineering and Physical Science Research Council (EPSRC grant No. EP/J004553/1). The authors would like also to acknowledge their colleagues Yat-Tarng Shyng and the late Dave Baker for technical support

Glass bead filled Polyetherketone (PEK) composite by High Temperature Laser Sintering (HT-LS)

Copyright © 2015 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Materials and Design. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Materials and Design, Vol. 83 (2015). DOI: 10.1016/j.matdes.2015.06.005Thermal expansion behaviour and mechanical performance of high temperature laser sintered PEK (LS-PEK) and glass bead filled PEK composite (LS-GB/PEK) are reported and discussed for the first time. The laser sintered PEK and its composite show anisotropic thermal expansion properties related with their multilayer structure created by the laser sintering method. Compared with the PEK manufactured by conventional injection moulding (IM-PEK), LS-PEK has a 10% lower tensile strength but a higher hardness due to its greater crystallinity. Addition of glass bead to PEK increases the hardness of PEK without affecting the ultimate tensile strength and also improves the thermal stability of laser sintered parts. The rule of mixtures is applied to simulate the coefficient of thermal expansion (CTE) of the LS-GB/PEK composite structures and compare with experimental results. The measured CTE values match the calculated results below Tg and deviate slightly from the simulated trend line above Tg

Morphology of polymeric powders in Laser Sintering (LS): from polyamide to new PEEK powders

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 characterisation parameters to be considered when analysing LS powders

The manufacture and mechanical properties of a novel negative Poisson’s ratio 3-component composite

This paper was presented at the ICCM 20 Conference - 20th International Conference on Composite Materials in Copenhagen, 19-24 July 2015. Full conference proceedings are available via the link in this recordMaterials 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 core and a high modulus wrap fibre in a double helix structure. When a tensile load is applied the core of the HAY becomes wider as the wrap straightens out, resulting in a lateral expansion of the core, and therefore a large negative Poisson’ ratio behaviour. The auxetic behaviour of the HAY can be tailored by altering fibre properties, the initial geometry and also the applied strain to comply with specific applications, such as composites [3, 4], blast mitigation, and filtration [5]. This paper introduces a further development to the current HAY by addition of a third component (a sheath). The presence of the sheath is expected to overcome problems such as slippage of the wrap and inconsistency in the initial wrap angle previously encountered during the manufacture of the HAY. The auxetic performance of conventional and novel systems is investigated and Poisson’s ratio data are presented.Engineering and Physical Sciences Research Council (EPSRC

The fabrication and mechanical properties of a novel 3-component auxetic structure for composites

Copyright © 2015 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Composites Science and Technology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Composites Science and Technology (2015), DOI: 10.1016/j.compscitech.2015.06.012Functional 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. The 3-component auxetic yarn is based on a stiff wrap fibre (the first component) helically wound around an elastomeric core fibre (the second component) coated by a sheath (the third component). The resultant structure can overcome problems such as slippage of the wrap and changes in wrapping angles previously encountered during the manufacture and utilisation of the two-component HAY. The mechanical performance of conventional and novel systems is investigated; with emphasis on the differences between the engineering and true Poisson’s ratio. The importance of the utilisation of a true tensile modulus and a true Poisson’s ratio is demonstrated. This is the first time reported in the literature that an experimental auxetic effect analysis of HAYs was carried out by comparing true and engineering Poisson’s ratio. We show that depending on the coating thickness of the third component, the 3-component auxetic system can demonstrate auxetic behaviour, and the coating thickness can be employed as a new design parameter to tailor both the Poisson’s ratio and modulus of this novel composite reinforcement for a wide range of applications.Engineering and Physical Science Research Council (EPSRC

Processability of PEEK, a New Polymer for High Temperature Laser Sintering (HT-LS)

Copyright © 2015 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in European Polymer Journal. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in European Polymer Journal (2015), DOI: 10.1016/j.eurpolymj.2015.04.003Currently, 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 and characterisation of Poly Ether Ether Ketone (PEEK), a polymer which is part of the PAEK family with a 30 ˚C lower melting temperature than PEK. The systematic characterisation of laser sintered samples of PEEK revealed a very good overall performance in comparison with the HP3 PEK material, with no change in storage modulus and only 25 % drop in tensile strength. The possibility of variable building configurations available within the HT-LS system, i.e. reduced, half and full chamber building modes, is examined in relation to the mechanical performances of the components. The effect of the post sintering time, an additional heating phase supplied to the powder bed at every layer, found only in the HT-LS system EOSINT P 800, is also examined

A study of interface adhesion between polyamide 6 (PA6) and nitrile rubber (NBR)

This is the peer reviewed version of the following article: Wang Y., Ghita O., Kavanagh D., and Chandler D. (2014), A study of interface adhesion between polyamide 6 (PA6) and nitrile rubber (NBR), Surf. Interface Anal., 10-11, pages 1000–1004, which has been published in final form at 10.1002/sia.5488. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving: http://olabout.wiley.com/WileyCDA/Section/id-820227.html#termsThe mechanism of interface bonding of a selected nitrile rubber compound vulcanized on polyamide 6 was studied using transmission electron microscopy. The two types of interfaces identified were formed through mechanical clamping and enhanced by hydrogen bonding between polyamide 6 and silane coupling agent in nitrile rubber compoun

Powder characteristics, microstructure and properties of graphite platelet reinforced Poly Ether Ether Ketone composites in High Temperature Laser Sintering (HT-LS)

This is the author's accepted manuscript. The final published article is avilable from the publisher via DOI: 10.1016/j.matdes.2015.09.094Copyright © 2015 Elsevier Ltd. All rights reserved.The properties of graphite platelet reinforced Poly Ether Ether Ketone (PEEK/GP) composites from powder to laser sintered parts were investigated in this study. The flowability, particle size and laser absorption characteristics of PEEK/GP powders with various graphite loadings were studied. It was found that the addition of graphite improved laser absorption; however, the flowability of powder was reduced. Micro-CT scanning was used to study the distribution, dispersion and the orientation of graphite platelets as well as the porosity and maximum pore size of laser sintered PEEK/GP composites. The graphite platelets were observed to be distributed evenly in the structure without significant agglomeration. Most of the graphite had their in-plane surface orientated in the X-Y plane of fabrication, which increased the tensile strength of the composites incorporating 5. wt.% graphite. The investigation also demonstrated that the porosity and maximum pore size increased with increasing amounts of graphite. A significant increase in porosity and pore size was found in PEEK/GP composites with 7.5. wt.% graphite, and it is believed to be responsible for the drop in tensile strength. DMA analysis showed no reduction of the damping properties in the composites incorporating up to 5. wt.% graphite, whereas the composites with 7.5. wt.% graphite showed increased stiffness

Varying the performance of helical auxetic yarns by altering component properties and geometry

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.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 behaviour of the HAY in order to tailor their properties for specific applications. The study focused on three parameters: component moduli, the core/wrap diameter ratio and the initial wrap angle. The results show that a larger difference in component moduli, a higher core/wrap diameter ratio and a lower initial wrap angle can produce a larger maximum negative Poisson's ratio value and thereby a better auxetic performance for HAYs. All three parameters could be carefully utilised when in combination to achieve the required auxetic behaviour of HAYs. Moreover, the instantaneous true Poisson's ratio analysis accurately presents the instantaneous behaviour of highly strain dependent HAYs.This work is supported by the UK Engineering and Physical Science Research Council (EPSRC Grant No. EP/J004553/1). The authors would like also to acknowledge their colleagues Dave Baker and Yat-Tarng Shyng for technical support

Enhanced Ductility of PEEK thin film with self-assembled fibre-like crystals

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.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 paper presents a simple quench crystallization method for preparation of PEEK thin films with the formation of a novel fibre-like crystal structure on the surface of the films. These quenched crystallised films show higher elongation at break when compared with conventional melt crystallised thin films incorporating spherulitic crystals, while the tensile strength of both types of films (quenched crystallised and conventional melt) remained the same. The fracture analysis carried out using microscopy revealed an interesting microstructure which evolves as a function of annealing time. Based on these results, a crystal growth mechanism describing the development of the fibre-like crystals on the surface of the quenched crystallised films is proposed.This work is supported by the UK Engineering and Physical Science Research Council (EPSRC Grant No EP/L017318/1-Particle Shape and Flow behaviour in Laser Sintering: from modelling to experimental validation)