inovative fibre - auxetic fibre

8/9/2019 Inovative Fibre - Auxetic Fibre

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A

PaperOn

Innovations In Fibre Auxetic Fibre

- Submitted By -

Mr. Amit M. Saharkar(Final year Textile Engg.)

[email protected]

Department of Textile Engineering

Jawaharlal Darda Institute ofEngineering & Technology, Yavatmal.

(M.S.)


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Content

Sr. no. Title

1 Abstract

2 Introduction

3 What is Auxetic Material?

4 Mechanism and Structure

5 Extrusion of Trimethyleneterephthalate Polyester Fibers

6 Molecular-Based Of Auxetics

7 Applications of Auxetic Material

8 Limitations

9 Conclusion

10 Reference

1. AbstractWhen a material is stretched there is an accompanying reduction in width. A measure of

this dimensional change can be defined by Poissons ratio, = -x/y. For many materials this


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value is positive and reflects a need to conserve volume. Auxetic materials, those with a negative

Poisson ratio (NPR), display the unexpected property of lateral expansion when stretched, withan equal and opposing densification when compressed. Commonly, the tensile strength and

modulus of these polymers is poor. The problem here is one of scale: an open microscopic

structure confers inferior mechanical properties. To impart superior mechanical properties,auxetic structure must exist at the molecular level. We aim to advance scientific understanding

of molecular-level NPR effects. This molecular approach should overcome property limitations

inherent in existing polymers with auxetic microstructures. Initially, we will address the

molecular-level requirements for auxetic fibers that may ultimately furnish a man-made materialsuitable for extrusion and spinning on a commercial scale. These auxetic textiles are used in

many areas like biomedical field, filters, piezoelectric sensors and actuators, medical field, seat

belts and safety harness in automobiles, ballistic protection, reinforcement composites and etc.Auxetic meaning and its various structures and its various applications are explained in detail in

this poster.

2. IntroductionModern technology requires new materials of special properties. One of the reasons for

interest in materials of unusual mechanical properties comes from the fact that they can be used


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as matrices to form composites with other materials of other required properties, e.g. electric,

magnetic, etc. A new field of endeavour is to study materials exhibiting negative Poissons ratio(NPR). Large-scale cellular structures with NPR property were first realized in 1982 in the form

of two-dimensional silicone rubber or aluminium honeycombs deforming by flexure of the ribs.

In 1987, Lakes1 first developed the NPR polyurethane foam with re-entrant structure. Thispolymeric foam had a Poissons ratio of -0.7. These new types of materials were named auxetics

by Evans2, which, in contrast to conventional materials (like rubber, glass, metals, etc.), expand

transversely when pulled longitudinally and contract transversely when pushed longitudinally.

Auxetics comes from the Greek word auxetos, meaning that which may be increased. In thisreport, the term auxetic will be used.

People have known about auxetic materials for over 100 years, but have not given them

much attention. This type of material can be found in some rock and minerals, even animal suchas the skin covering a cows teats. To date, a wide variety of auxetic materials has been

fabricated, including polymeric and metallic foams, micro porous polymers, carbon fibre

laminates and honeycomb structures. A typical example is a well-known synthetic polymer-

polytetrafluorothylene (PTFE), which has been in use for many years. Other materials possessthe NPR property such as micro porous ultra high molecular weight polyethylene (UHMWPE),

polypropylene (PP), several types of rocks. However, their special characteristics are largely

ignored. Only up until recently, Lakes, Evans and other scientists work has attracted moreattention to these auxetic materials.

These auxetic materials are of interest due to the possibility of enhanced mechanical

properties such as shear modulus, plane strain fracture toughness and indentation resistance.Therefore, studying these non-conventional materials is indeed important from the point of view

of fundamental research and possibly practical applications, particularly in medical, aerospace

and defence industries. In fact, some materials with such anomalous (i.e. NPR) properties havebeen used in applications such as pyrolytic graphite for thermal protection in aerospace, large

single crystals of Ni3Al in vanes for aircraft gas turbine engines, and so on.

3. What is Auxetic Material?Everyday experience shows us that when a material is stretched there is an expansion in

width. A measure of this dimensional change can be defined by Poissons ratio, = - x / y;


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Materials with a negative Poisson ratio (auxetic) display the unexpected property of lateral

expansion when stretched[1]

Figure 1. Dimensional response in a conventional and auxetic material when a tensile force is

applied

Conventional material Auxetic material

This behavior originates from particular structural characteristics on the cellular level that

give rise to the unfolding of basic structural elements upon stretching and folding back in uponcompression.

Besides the elementary scientific importance of imparting such a fundamental property, a

negative Poisson ratio can give a material many exceptional benefits:

Increased stiffness[2],

Increased indentation resistance[3] and

An ability to form synclastic doubly curved surfaces[4].Although a few auxetics have been discovered in both natural (some minerals, select

skins[5],bone[6], -cristobalite[7]) and man-made materials (foams[3,8] certain micro porous

polymers[9], Gore-Tex[10] and nodular PE[11]) an NPR is still a very rare feature forconventional materials.

4. Mechanism and StructureAs stated above, a material with NPR expands (gets fatter) when stretched, as opposed to

most materials, which tend to get thinner. A typical mechanism is shown in Figure 2. When a

force pulls the structure in one direction (e.g., here vertically), the structure opens up or expands


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in the perpendicular direction (here, horizontally), i.e., the structure gets fatter[12]. Based on this

simple mechanism, a network-like structure can be built up, as shown in Figure 3, where a 2Dstructure of such a material consists of a regular array of rectangular nodules connected by

fibrils. Deformation of the structure is by hinging of the fibrils. For the open geometry, the

cells elongate along the direction of stretch and contract transversely in response to stretchingthe network, giving a positive v (refer to Figure 3 (a)). However, the structure is modified to

adopt a re-entrant4 geometry, Figure 3 (b), and the network now undergoes elongation both

along and transverse to the direction of applied load. In other words, this is an auxetic structure

[12].

Figure2.Schematic of basic deformation mechanism in auxetic material.

Figure 4 shows the variation in width plotted against length variation for two

polypropylene (PP) fibres stretched axially. Fibre 1 is a conventional PP fibre and shows acontraction in width as it is extended, corresponding to a positive Poissons ratio (). Fibre 2 is

processed using extruder temperatures, which lead to the nodule-fibril microstructure. Its width

now increases upon stretching - it displays auxetic behavior [12].

For auxetic honeycombs, which are a special subset of auxetic materials, the NPR effectis due to the geometric layout of the unit cell microstructure, leading to a global stiffening effect

in many mechanical properties such as in-plane indentation resistance, transverse shear modulusand bending stiffness [2]. Figure 5 shows the deformation mechanism of the auxetic

honeycombs along with conventional honeycomb structure. For a conventional hexagonal

geometry (Figure 5 (a)), under the stretch in the y direction the cells elongate along the y-axis

and close up in the x direction, leading to a positive Poissons ratio. However, for an auxeticstructure, the cells undergo elongation both parallel and perpendicular to the direction of the

applied load, shone in figure 5(b).

Figure 3 Comparison of deformation behaviors: (a) non-auxetic and (b)auxetic material


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Figure 4 Width as a function of length variations for polypropylene fibres

Figure 5 Two-dimensional deformation mechanisms, which are subjected to loading in the y

direction:

(a) Conventional honeycomb structure (b) Auxetic honeycomb structure


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For auxetic micro porous polymer, the characteristics of the microstructure can beinterpreted by a simple 2D model, as shown in Figure 6. This basically consists of an

interconnected network of nodules and fibrils. If a tensile load is applied, the fibrils cause lateral

nodule translation, leading to a strain-dependent negative Poissons ratio.

Figure 6 Schematic of the microstructure of a typical auxetic polymer. (a) The polymer at rest

and (b) The polymer at the tensile load

5. Extrusion Of Trimethyleneterephthalate Polyester FibersAn initial extrusion experiment was carried out with trimethyleneterephthalate granules

to observe the behavior of the polymer during extrusion. The melt spinning process wasperformed using an Emerson and Renwick Ltd Labline extruder which consists of a 25.4 mm


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screw diameter, 3 : 1 compression ratio, length/diameter 24 : 1, five temperature zones each

having individual thermostatic controls, and a die slot with 40-filament die having each hole sizeof 550 m (Figure 7). The processing temperatures investigated were based on thermal analysis

studies and were carried out at intervals of 5C, leading to flat temperature profiles in the range

of 230 to 210C with screw speeds of 0.525 rad/s and take-up speed 0.075 m/s. The extrusionwas carried out at higher temperature (230C) profile and gradually decreased the temperature to

210C until the viscosity of the powder bulk was too high to allow free flow through the die-

zone. The polyester powder was fed through a hopper into the barrel and transferred through the

barrel zones to the die zone, thereby undergoing sintering due to the maintained temperaturesalong the zones of the extruder. The fibers were extruded from the die head and cooled before

winding.[13]

Figure7.Schematic diagram of melt extruder used to produce auxetic polyester fiber

6. Molecular-Based Of AuxeticsRecent attention has focused on the theoretical design and synthesis of auxetic structures

on a molecular scale. We suggest that a liquid crystalline (LC) polymer might exhibit an auxeticresponse if transverse rigid rods are incorporated in the main chain. A unique feature of liquid


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crystalline polymers is the tendency for the transverse rods in the un-stretched state to orient

themselves roughly parallel to each other and to the terminally attached LC rods [1]. Uponstretching, the rigid rods should reorient themselves while retaining site-connectivity to give a

significant increase in the transverse dimension of the stretched polymer chain (figure 8) [1].

The accompanying increase in inter-chain spacing upon stretching should lead to anauxetic response. It is apparent that oligo-paraphenyls should be suitable for this purpose, and

ter-, quater- and penta-phenyls are good candidates [1]. Their initial approach is to create a main-

chain LC polymer consisting of both terminally attached liquid crystalline rods with ter-phenyl

and ultimately even longer pent phenyl rods capable of a full 90 transverse re-orientation uponstretching [1].

Figure 8.Terminally Attached Transverse Rods Pre-aligned with surrounding liquid crystal

field (top) Re-oriented with a lateral expansion upon stretching (bottom)

We have reported liquid crystalline polymers containing laterally attached ter phenyls

[14]. Due to the specific chemistry and the attachment sites on the central phenyl ring, these ter

phenyls make a 60 angle with the polymer main-chain when the polymer is fully stretched - thisis the maximum limit for rotation of the laterally attached rods. We have also seen a class of

polymer consisting of longer laterally attached p-quarter phenyl rods that could rotate to a

maximum of 75 with respect to polymer main-chain when the polymer chain is fully stretchedand could push the neighboring chains further apart[15]. Figure 9 shows the orientation between

unique attachment sites of the terminally attached rods to the flexible spacer. In both approaches,

we successfully produced simple fibres from a co-polymer designed by this molecular approach.

These showed that laterally attached ter phenyl rods incorporated into their main chain gave anincrease in inter-chain distances when their fibres were stretched a prerequisite for our auxetic

design.


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Figure 9.Progressive improvements in transverse-rod site connectivity from a lateral 1, 4-

phenylene and 2,2-biphenyl connectivity to a terminal 1,3-resorcinol (or isophthalate)

connection

The maximum extension capable from these laterally attached rods was 60 and 75,

respectively, when stretched; the majority likely giving statistically smaller re-orientationsanywhere within this distribution span [1]. We now know from these results that much largerreorientations are now necessary. The improved design we now propose, will exploit terminally

attached p-terphenyl and even longer p-pentaphenyl rods capable of a 90 re-orientation when

stretched (Figure 10)[1].

Figure 10. Representation of proposed auxetic target polymer structure showing resorcinol-

based terphenyl auxogens (R = H, n-Pr, CN, CF3, SiMe3, Ph) with spacer chemistry that may

be methylene (X = CH2), ethylene oxide (X = O), or siloxane

(X = Si(CH3)2OSi) based.

This synthesis will hinge on the preparation of resorcinol- or isophthalate-based p-ter-

and p-pentaphenyl rods with flexible arms suitable for polymerization into hydrocarbon,ethylene oxide or siloxane-based linear and cross linked chains depending on glass transition

temperature (Tg) requirements (Figure 10)[1]. The terminus of each rod can then have

substitutions (H, n-Pr, CN, SiMe3, and CF3) suitable for tailoring liquid crystallinity, melting,solubility, or to enhance pre-alignment prior to any stretching. In the non-liquid crystalline cases

where we expect to look at polymers containing only auxetic rod, the rods should pack


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somewhat randomly in the polymer, most oriented in the direction of the chain axis for packing

efficiency [1]. Although this statistical packing of terminally attached rods along each chain axismay reduce the number of overall acute angle geometries capable of an auxetic expansion in the

non-liquid crystalline polymers, it is not expected to eliminate auxetic behavior: most isotropic

crystals have random orientations yet display remarkably large negative Poisson ratios oversmall strains. In any case, the likelihood of auxetic rods pre-aligning with their neighbors

increases with length and may eventually have to be synthesized to be calamitic themselves[1].

7. Applications Of Auxetic MaterialFollowing are the applications of auxetic material.

7.1 Auxetic Fibres

The breakthrough development of a continuous process to produce auxetic materials infibrous form has created the opportunity to apply their unique characteristics in a wide range of

applications previously not possible. Fibres can be used in single or multiple filament structures

and can be used to produce a woven structure. Typical performance characteristics expected ofauxetic fibres and structures. Current research on auxetic composites is concentrated on the useof non-auxetic constituents and so benefits due to the auxetic effect occur at a macro structural

level. Employing auxetic fibres as the reinforcement will enable benefits, such as impact energy

and acoustic energy absorption, to be achieved at the micro structural level [12,16].

7.2 Auxetic Yarns and Textiles

We envision single or multi-filament auxetic fibers woven or knitted into unique fabrics

with a wide range of applications presently unattainable from fibers with conventional elastic

properties. It also follows that any future for advanced fibers may include multi-filament yarns

where the auxetic fibers are wrapped with one or more conductive or dye able yarns. Thus,auxetic benefits can be incorporated with other beneficial properties for smart textiles.

Employing auxetic fibers as reinforcements should confer improved impact resistance,

indentation resistance and energy absorption properties in textiles. These fabric properties areparticularly advantageous and potentially attractive for commercialization in protective clothing

uses for military and Homeland Security, such as superior performance outfits, combat jackets

and body armor (i.e. bullet-proof vests, helmets). At present, protective materials of this typeneed to be about 1cm thick making them stiff, heavy and inflexible. Auxetic body armor could

give the same safeguard but be thinner, lighter and conform better to the synclastic double

curvatures of the human body.

7.3 Piezoelectric Sensors and ActuatorsAnother area relates to the use of auxetic materials in piezoelectric sensors and actuators.

Auxetic metals could be used as electrodes sandwiching a piezoelectric polymer, or piezoelectricceramic rods could be embedded within an auxetic polymer matrix. These are expected to

increase piezoelectric device sensitivity by at least a factor of two, and possibly by ten or a

hundred times. The development of auxetic materials for micro and nano mechanical andelectromechanical devices is also being investigated.


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7.4 Filters

Auxetic foam and honeycomb filters offer enhanced potential for cleaning fouled filters,for tuning the filter effective pore size and shape, and for compensating for the effects of

pressure build-up due to fouling [16]. These benefits rely on the pores opening up both along and

transverse to the direction of a tensile load applied to an auxetic filter. The pores of a non-auxetic filter open up in the stretching direction but close up in the lateral direction, leading to

poorer filter performance; however, stretching an auxetic filter improves performance by

opening pores in both directions. The effect of stretching on the de-fouling of an auxetic

polymeric honeycomb fouled with glass beads has been investigated. For the particular

honeycomb studied the value of is dependent on the stretching direction. The studies clearly

demonstrate that defueling is enhanced when the filter is loaded in the direction with the largest

negative Poisson ratio [16].

Figure 11.A macro model of the auxetic filtration mesh principle can be seen in the

unstrained (above) and strained (below) states.

7.5 Medical Sutures / Controlled Release of Drugs

Auxetic dental floss offers several key benefits,

including the ability to expand to fit the widely differing gaps

between human teeth and the ability to deliver

chemotherapeutics, fluorides or flavors directly to the gum line[16]. The porous nature of auxetic floss would also assist in

debris removal, making the flossing process more efficient.

The market is currently held back because the procedure offlossing is dull, and the benefits to the user are not

immediately apparent. This has lead to the situation where a

large proportion of users only floss on an occasional basis [16].

It is thought that if the flossing experience could be made moresatisfying, then the market would grow appreciably.

We have recently made the first prototypes of a 'smart-suture'. Basically, it is a part-

braided, part wound system that has a core which can be soaked in an agent, such as achemotherapeutic. When you stretch the yarn, the outer cover expands - as it does so, it opens up

a large number of pores - simultaneously, it squeezes the core, forcing the agent out [16]. This

would appear to be very useful for in-situ drug delivery. For instance, if it was soaked in vitaminE, it would help reduce the formation of scar-tissue, or if antibiotics were used, it may be


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appropriate for use in third-world countries, or on battlefields. A further enhancement is that the

suture's circumferential expansion under strain would help to reduce cheese wire traumas causedby the suture cutting into the flesh as the result of patient movement [16].

Figure12 Medical sutures can be made that are able to deliver agents such as

chemotherapeutics or flavors by using the system shown here. a) A semi-braided, semi-wrapped auxetic yarn containing an absorptive core. b) The same material treated with

walnut oil. c) When this is placed

under a tensile load, a series of pores

open up and the oil is squeezed out.

7.6 Seat Belts & Safety Harnesses

Auxetic materials offer solutions to

many everyday problems. As an example,consider how a passenger seat belt behaves in a

vehicle collision[16]. In an accident, the

passenger is usually thrown forwards the forcesinvolved can be enormous. In attempting to

restrain this movement, the seat belt gets

stretched and, much like an elastic band beingpulled becomes narrower. This is exactly the

opposite of the behavior that you want at such a

critical time, for, in getting narrower; it concentrates all the forces into a much smaller area [16].In a healthy adult, this can cause significant abrasion traumas. For someone who is elderly,

pregnant, or very young, such injuries can be much more serious. An auxetic seat belt, however,

would get wider this would spread the loads over a much larger area, potentially reduce any

injuries experienced [16].

7.7 Fibre Reinforced Composites

It is expected that auxetic reinforcing fibres should enhance fracture resistance ofcomposites. It is well known that the interface between matrix and fibres is the weakest part of a

composite material (Figure 13 (a)). Fibre pull-out is a major failure mechanism in fibre

reinforced composites [12]. For example, a unidirectional composite loaded in tension willundergo lateral contraction of both the matrix and fibres in conventional composite materials,

leading to failure at the fibre/matrix interface as shown in Figure 13 (b).However, replacing of

conventional fibre by auxetic fibres could delay the pull-out of fibres, potentially helping to

resist crack growth, because the possibility of maintaining the interface by careful matchingof the Poisson's ratios of the matrix and fibre leading to fibreexpansion during pull-out, as shown in Figure 13 (c)[12].


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Figure 13 Fibre pull-out in composites

7.8Defence Auxetic textile

Auxetic currently has five main areas of interest in the defence sector. These are: Window-protection

Ballistic-fragment protection for military tents

Ballistic camouflage nets

Enhancements for body armor

Window-protection

Recent terrorist threats have renewed concerns about the protection offered to our

buildings both domestically and internationally [16]. One issue that has received a lot of

attention in recent years is the hazard posed by flying window glass. Typically more than 80% of

deaths and serious injuries are caused by flying debris. Blast curtains can be used to significantlyreduce the number of casualties caused by this during a terrorist strike. Auxetic has developed a

range of blast-fabrics that can be used for window-protection [16].

Ballistic-fragment protection for military tents

Conventional military tents are extremely vulnerable to attack by enemy forces, andprovide little to no ballistic-protection. Auxetic is developing variants of the blast-mitigation

fabrics which can be used as liners for the fly sheets [12.16]. This could significantly reduce the

ingress of shrapnel (from mortars, RPGs and grenades), thus reducing the number of deaths andserious injuries sustained in action [16].

Ballistic camouflage netsConventional camouflage nets, as used by the world's military forces, do not provide

anything more than a degree of cover from observation. Ballistic-protection fabrics which have

been printed with camouflage patterns can, however, also provide significant protection against

primary and secondary fragmentation [12,16].Auxetic is also developing thinner forms of the ballistic-protection fabrics for use as

military mosquito nets. Whilst still working well as insect screens, these would also act as


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secondary fragmentation shields, providing extra protection in the event of a local mortar or

grenade burst[16].

Enhancements for body armor

Spalling is a process that occurs when a hard structure is hit by a projectile or explosionand the opposite face bursts and scabs of material fly off, often at ballistic speeds. The terms'Scabbing', 'Ablation' and 'Spalling' all refer to the same thing. It can be a serious problem with

vehicles, aircraft, boats and buildings [12,16]. The principle here is that the blast-fabrics are

fixed to the inside of a threatened structure, and in the event of a projectile strike, reduce any potentially lethal back-face bursting. This role is especially significant in weight-sensitive

applications, as backings made from ballistic-protection fabrics make it possible to use a thinner

armor, with consequent savings in both cost and weight [16].

7.9 Structure

The counterintuitive property of auxetic materials, namely, lateral expansion

(compression) under longitudinal tensile (compression) loads, is essential from the point of viewof modern technology. Many applications for auxetic materials have been designed in various

fields of human activity, from vascular implants, strain sensors, shock and sound absorbers,

"press-fit" fasteners, gaskets and air filters, to fillings for highway joints. Materials containinginclusions of negative stiffness constitute another class of systems with unusual mechanical

properties [16]. The recent interest in such systems has its origin in their very high damping

properties. The mechanical properties of auxetic honeycombs are highly sensitive to themicrostructure unit cell geometric parameters. This is a feature that could be used to design

optimized sandwich structures for various applications. As an example, regular hexagonal

honeycombs do not excel in sound absorption applications. Selected combinations ofmicrostructure properties of auxetic honeycombs have been proven to increase the transmission

loss factors inside cylindrical shells. Wave dispersion properties can also be custom tuned byvarying the auxetic microstructure layout [16].

7.10 Other Application

Apart for the above applications, materials with NPR should have more applications in

aerospace industry [16]. For example, from the mechanical point of view, the Poissons ratiodoes not depend on scale, so auxetic materials should not be difficult to be extended to structures

built from conventional components that would open up when stretched. These expandable

structures can be particularly useful for the manufacture of space structures such as largeantennas and sun shields that could be launched into space in a closed compact form and then

open up at a later stage in space [16].

Auxetic materials have higher resistance against shearing (tearing force) and twisting

than conventional materials. These properties are particularly important for structuralcomponents, which may fail under shear strain (such as beams in buildings, sheets used in

aircraft or cars etc) [16].

The sensitivity of a sonar receiver was increased by an order of magnitude by replacing anon-auxetic matrix with an isotropic auxetic matrix. A further advantage of using auxetic

materials is their behavior when bent[16]. The double curvature is one of the important


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deformation mechanisms in auxetic materials. This feature can be used in moulding and shaping

sandwich panels for aircraft components such as nose cones or car body parts [16].

8. Limitations

As stated above, auxetic materials potentially have many applications, because of theirwonderful properties compared to conventional (i.e. non-auxetic) materials. However, they also

have their own limitations like other materials.The special micro structural features for auxetic materials need space to allow the

hinges to flex, or the nodules to spread out. The materials often need substantial porosity.

Therefore, materials with negative Poissons ratio are substantially less stiff than the solids fromwhich they are made and this causes limitations on the structural applications of the materials

with negative Poissons ratio. For example, they are normally not stiff enough or not dense

enough for load-bearing applications.

9. Conclusion

Auxetics are having specialty in their nature, even though these are the natures wealth,today research & development is going on for mimicking these structures. The advantages in this

negative poisons ratio material are showing the several applications in various fields. Today somany researches are going on around the world. Hope for the success in those projects and that

will show the new views to the conventional world.

10. Reference


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1. Anselm C. Griffin, leader; Satish Kumar,Philip J. McMullan (Georgia Tech) Textile FibersEngineered from Molecular Auxetic Polymers, NTC Project: M04-GT21, National Textile Center

Research Briefs Materials Competency: June 2005.

2. K.E. Evans, Chem. Ind. 1990, 20, 654-657.3. K.L. Anderson, A.P. Pickles, P.J. Neale, K.E. Evans, Acta Metall. Mater. 1994, 42, 2261-2266.4. R.S. Lakes, Science, 1987, 235, 1038-1040.5. (a) L.M. Frohlich, M. Labarbera, W.P. Stevens, J. Zool. Lond. 1994, 232, 231-252; (b) C.Lees, J.F.V. Vincent, J.E. Hillerton, Biomed. Mater. Eng. 1991,1,19; (c) D.R. Veronda, R.A.

Westmann, J. Biomech. 1970, 3, 111-124.

6. J.L. Williams, J.L. Lewis, Trans. ASME, J. Biomech. Eng. 1982, 104, 5-56.7. (a) Y. Yeganeh-Haeri, D.J. Weidner, J.B. Parise, Science, 1992, 257, 650-652; (b) N.R.Keskar, J.R. Chelikowski, Nature 1992, 358, 222-224.

8. E.A. Friis, R.S. Lakes, J.B. Park, J. Mater. Sci. 1988, 23, 4406-4414.9. (a) K.L. Alderson, K.E. Evans, Polymer 1992, 33, 4435-4438; (b) K.E. Evans, Endeavour1991, 15, 170-174.

10.(a) B.D. Caddock, K.E. Evans, Biomaterials 1995, 16, 1109-1115; (b) B.D. Caddock, K.E.Evans, J. Phys. D.: Appl. Phys. 1989, 22, 1877-1882.11.(a) K.L. Alderson, V.R. Simkins, Patent WO 00/53830, 2000; (b) K.L. Anderson, K.E.Evans,Polymer 1992, 33, 4435-4438.

12.Q. Liu, Literature Review: Materials with Negative Poisson's Ratios and Potential Applicationsto Aerospace and Defence (DSTO-GD-0472), Australian Govt. Dept. of Defence Science and

Technology Organisation.13. Naveen Ravirala, Kim L. Alderson, Philip J. Davies, Virginia R. Simkins and Andrew Alderson1,

Negative Poissons Ratio Polyester Fibers, Textile Research Journal, Centre for Materials Research

& Innovation, University of Bolton, Deane Road, Bolton, BL3 5AB, UK.

14. C. He, P. Liu, A.C. Griffin, Macromolecules 1998, 31, 3145-3147.

15. C. He, P. Liu, P.J. McMullan, A.C. Griffin, Phys. Stat. Sol. B. 2005, 242, 576-584.

16.http://www.auxetic.com

inovative fibre - auxetic fibre

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