Defence Research and Development Canada Recherche et de ´ veloppement pour la de ´ fense Canada Manufacture and characterization of auxetic foams Royale S. Underhill DRDC – Atlantic Research Centre Defence Research and Development Canada Scientific Report DRDC-RDDC-2017-R099 September 2017

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Page 1: Manufacture and characterization of auxetic foams · Manufacture and characterization of auxetic foams Royale S. Underhill DRDC – Atlantic Research Centre ... September 2017. Manufacture

Defence Research andDevelopment Canada

Recherche et developpementpour la defense Canada

Manufacture and characterization of auxetic foams

Royale S. UnderhillDRDC – Atlantic Research Centre

Defence Research and Development Canada

Scientific ReportDRDC-RDDC-2017-R099September 2017

Page 2: Manufacture and characterization of auxetic foams · Manufacture and characterization of auxetic foams Royale S. Underhill DRDC – Atlantic Research Centre ... September 2017. Manufacture
Page 3: Manufacture and characterization of auxetic foams · Manufacture and characterization of auxetic foams Royale S. Underhill DRDC – Atlantic Research Centre ... September 2017. Manufacture

Manufacture and characterization of auxeticfoams

Royale S. UnderhillDRDC – Atlantic Research Centre

Defence Research and Development CanadaScientific ReportDRDC-RDDC-2017-R099September 2017

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c⃝ Her Majesty the Queen in Right of Canada, as represented by the Minister of NationalDefence, 2017

c⃝ Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de laDéfense nationale, 2017

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Abstract

In the thirty years since the first reports of auxetic foams (i.e., foams exhibiting anegative Poisson’s ratio), there have been no significant advances in their productionmethods. With the ultimate goal of developing new lightweight armour materials,Defence Research and Development Canada (DRDC) had a program to explore thecreation and testing of polymeric auxetic materials, including foams. The goals of thework presented here were to establish methodologies reproducibly consistent with theliterature, and to assess measurement techniques. Part of this process involved theconstruction of a novel mold that achieved uniform compression simultaneously inthe three axes.

Three foams with differing cell sizes (30, 60, and 90 pores per inch (PPI)) were suc-cessfully compressed to form auxetic materials. All three exhibited negative Poisson’sratios at compression factors of 2.0 or greater. The 90 PPI foam yielded the mostnegative Poisson’s ratio (-0.16), at a compression factor of 3.2.

Significance for defence and security

The Canadian Army’s Soldier System 2030 plan includes the use of advanced ma-terials to provide the most effective equipment for the dismounted soldier. NegativePoisson’s ratio (auxetic) materials have shown increased shear modulus, increasedfracture toughness, and improved impact resistance. All properties beneficial for ar-mour systems. It has been postulated that negative Poisson’s ratio materials could beincorporated into helmets, bullet proof vests, shin, elbow and knee pads. Impartingimproved material properties, while remaining lightweight.

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Résumé

Dans les trente années qui se sont écoulées depuis les premiers rapports de moussesauxétiques (c’est-à-dire des mousses présentant un coefficient de Poisson négatif), iln’y a pas eu d’avancées significatives dans leurs méthodes de production. Dans le butultime de développer de nouveaux matériaux de blindage léger, Recherche et déve-loppement pour la défense Canada (RDDC) a poursuivi un programme pour explorerla création et l’essai de matériaux auxériques polymériques, dont les mousses. Lesobjectifs du travail présenté ici sont d’établir des méthodologies compatibles avecla littérature et d’évaluer les techniques de mesure. Une partie de ce processus im-pliquait la construction d’un nouveau moule permettant une compression uniformesimultanée dans les trois axes.

Trois mousses présentant différentes tailles de cellules (30, 60 et 90 pores par pouce(PPP)) ont été comprimées avec succès pour former des matériaux auxétiques. Lestrois matériaux obtenus présentaient des coefficients de Poisson négatifs à des facteursde compression de 2.0 ou plus. La mousse de 90 PPP a donné le meilleur coefficientde Poisson négatif (-0,16), avec un facteur de compression de 3.2.

Importance pour la défense et la sécurité

Le plan pour le Système du soldat de l’Armée canadienne 2030 comprend l’utilisationde matériaux avancés pour fournir l’équipement le plus efficace qui soit pour le sol-dat à pied. Les matériaux auxétiques (présentant un coefficient de Poisson négatif)présentent un module de résistance au cisaillement accru, une ténacité de ruptureaccrue et une résistance aux chocs améliorée. Toutes ces propriétés sont bénéfiquespour les systèmes d’armures. On postule que les matériaux présentant un coefficientde Poisson négatif pourraient être incorporés dans des casques, des gilets pare-balles,des protège-tibias, des protège-coudes et des genouillères puisqu’ils présentent despropriétés améliorées tout en restant légers.

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Table of contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Significance for defence and security . . . . . . . . . . . . . . . . . . . . . . . i

Résumé . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Importance pour la défense et la sécurité . . . . . . . . . . . . . . . . . . . . . ii

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Compression mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3 Heat treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.4 Coupon preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.5 Poisson’s ratio measurement . . . . . . . . . . . . . . . . . . . . . . 5

2.5.1 Digital image correlation . . . . . . . . . . . . . . . . . . . . 7

2.5.2 Clip-on extensometers . . . . . . . . . . . . . . . . . . . . . 7

2.5.3 Optical comparator . . . . . . . . . . . . . . . . . . . . . . . 7

3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Clip-on Extensometers . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 Optical comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

List of figures

Figure 1: Schematic of compression into a re-entrant structure. . . . . . . . 2

Figure 2: Compression mold. . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3: Foam specimens before and after compression. . . . . . . . . . . . 6

Figure 4: Image of cell structure of polyurethane polymer foam, cell density30 PPI. (left) before compression, (right) after compression, c.f. 2.7. 6

Figure 5: Image of cell structure of polyurethane polymer foam, cell density60 PPI. (left) before compression, (right) after compression, c.f. 2.7. 6

Figure 6: Foam specimen mounted in a United SSTM screw driven loadframe, with Epsilon axial and transverse clip-on extensometers. . . 8

Figure 7: Caliper gauge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 8: Optical comparator with caliper gauge from Figure 7 in situ. . . . 9

Figure 9: Poisson’s ratio as determined by axial and transverseextensometers for the 30 PPI and 60 PPI foams. . . . . . . . . . . 11

Figure 10: Auxetic response of S60 foam after triaxial compression and heattreatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 11: Poisson’s ratio as calculated using optical comparitor for 30 PPI,60 PPI and 90 PPI polyurethane open cell foams. . . . . . . . . . 13

List of tables

Table 1: Literature values for polyurethane auxetic foams. . . . . . . . . . 12

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Acknowledgements

• Thanks are given to the Fleet Maintenance Facility Cape Scott, for use of theiroptical comparator.

• Foams were prepared by Heather Smiley, Irv Keough and Nancy Hervé.

• Poisson’s ratio measurements were performed by Tomasz Lemczyk, HeatherSmiley and Nancy Hervé.

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1 Introduction

The Canadian Army’s Soldier System 2030 plan will take advantage of advancementsin technologies and materials to provide the most effective equipment to the dis-mounted soldier. This includes lightweight armour with improved performance. Idealmaterials for armour systems must absorb energy locally and be able to spread thatenergy out fast and efficiently [1]. As discussed in the literature review “Defense Ap-plications of Auxetic Materials” [2], it has been postulated that negative Poisson’sratio, auxetic materials may show exceptional resistance to blast and ballistic threats.

Auxetic materials become thicker laterally when stretched. Conversely they will con-tract in the directions orthogonal to a compressive load. It has been shown thata negative Poisson’s ratio material, when impacted, demonstrated increased shearmodulus [3], fracture toughness [4] and impact resistance [5–8]. Equipment that maybenefit from the unique properties of auxetic materials include helmets, bullet proofvests, shin, elbow and knee pads [9]. Helmets, elbow and knee pads may also benefitfrom another property of auxetic materials, synclastic curvature. That is, they bendin the same direction in both perpendicular planes, producing a dome which doesn’thave folds or seams.

Negative Poisson’s ratio foams may be of use in armour systems. Polymeric foamsare too soft for structural applications, but may lend themselves to armour padding[10]. Deformation is part of an armour’s energy absorption/dissipation mechanism.Unfortunately, if there is insufficient stand-off distance between the armour and theperson, then behind-armour blunt trauma (BABT) can occur. BABT can result incontusions, lacerations, and bone fractures. On occasion resulting rib fractures canlead to contusions of the lungs, kidneys or spleen. In the future, an escalation of theavailable energy of bullets and the desire for armour designers to minimize weightand bulk may increase the risk of BABT. Understanding how to make auxetic foamsis the first step towards evaluating them for possible use in minimizing BABT.

Typically, negative Possion’s ratio is a result of an engineered structure. For example,a re-entrant cell structure in foams (re-entrant means “angles pointing inward”), seeFigure 1. Polymeric foams with re-entrant cell structures are formed by triaxiallycompressing regular foams. The compression deforms the polyhedron open- or closed-cells into collapsed re-entrant structures. In the literature, polymeric auxetic foamsare created using a mold of fixed shape, usually a segment of tube [11, 12]. Themethodology involves two-steps to achieve triaxial compression. A non-processed foamsample of predetermined dimensions is inserted into the mold tube, compressing intwo dimensions. Plungers apply the pressure resulting in compression in the thirddimension.

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Figure 1: Schematic of how compression deforms a foam cellinto a re-entrant structure.

The goals of the work presented here were to produce auxetic foams along the linesof the methodologies reported in the literature, and to use these materials to assessPoisson’s ratio measurement techniques. In evaluating Lakes’ methodology, it wasnoted that the compression and insertion of the foam into the tube resulted in wrinklesand kinks that must be smoothed prior to longitudinal compression. This led to thedesign of a novel mold that would achieve uniform, simultaneous, compression onthree axes.

This report will outline the foam fabrication method using the new mold, as well asthree characterization methods for determining Poisson’s ratio. The Poisson’s ratiosof three polyurethane foams at a number of compression factors will be analysed,evaluating which characterization methodology worked best for foams.

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2 Methodology2.1 Materials

Two open cell ether polyurethane foams (30 pores per inch (PPI) and 60 PPI) werepurchased from Vitafoam Canada and used as provided (designated NA30CHAR andS60 respectively). A 90 PPI ester polyurethane foam (designated S90) was purchasedfrom New England Foam and used as provided. All had densities between 1.7 and2.0 lbs/cu.ft. (0.034–0.040 g/cm3)

2.2 Compression mold

The details of the triaxial compression apparatus are documented in the contractreport, “Development of Equipment for the Manufacture of Auxetic Foam for DRDC”,DRDC Atlantic CR2013-042 [13]. A brief summary will be given here.

The mold design consists of a static frame (Figure 2a) with eight movable segments(Figure 2b). Figure 2c shows the static frame fully assembled into a rectangular boxwith the movable corner segments in place. The components were all machined fromlow alloy steel. The non-compressed foam coupon fits in the cavity of the static framewhich provides structural support, as well as a series of planar predetermined pathsto guide the movable segments.

The angular inner surfaces of the movable corner segments (Figure 2b) mate withthose on the static frame, and guide the compression of the foam coupon. The externalsurfaces of the corner segments have V-grooves that engage with wedges located ona load frame which provides the compressive force. The load frame is an externalsupport frame with upper and lower compression mold engagement fixtures. Theplanar surfaces, in combination with the V-slot/wedge configuration was designedto impose a diagonal displacement path on each mold section. The diagonal pathimposed on the mold sections results in a foam compression of approximately 1/3 uniton the lateral faces for each unit of longitudinal deflection. Using an initial ratio of3:1 for the length:width of the foam coupon, the diagonal compression path imposedby this mold results in an equal applied strain in the XYZ coordinate directionssimultaneously. The final volume (Vf ) of the foam coupon will always be 75 mm ×25 mm × 25 mm, or 46,875 mm3. One can back calculate the initial dimensions basedon the desired volumetric compression factor (c.f.).

Vi = c.f. × Vf (1)

With the inital volume, one can calculate the initial dimensions of the coupon.

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(a) Static frame.

(b) Movable corner segment.

(c) Fully assembled.

Figure 2: Compression mold.

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Vi = l × w × h (2)w = h (3)l = 3w (4)

2.3 Heat treatment

Heating of the compressed samples was achieved using a commercial gravity con-vection oven. The oven had a temperature range between room temp and 300◦C, amicroprocessor based PID temperature control algorithm, and process control pro-gramming/monitoring digital displays.

During the heating process, three type T thermocouples where used to monitor thetemperature. Two of the thermocouples were attached to the exterior surface of themold, while the third was inserted into the centre of the foam in the mold. The ther-mocouple in the foam is used to verify that the core of the coupon has reached thefoam softening temperature, which was approximately 160◦C for these polyurethanefoams. The thermocouple inserted into the foam left a 3 mm hole down the longitu-dinal axis of the coupon. The effect of this hole was minimized when the coupon wascut in half lengthwise to produce two specimens for analysis.

2.4 Coupon preparation

The compression factors (c.f) explored were 0 (uncompressed), 2.0, 2.7 and 3.2. Eachcoupon was cut from the foam, compressed using the specialized mold and load frame,then heated until the thermocouple inserted in the middle of the foam coupon reg-istered the softening temperature (160◦C), and allowed to bake at that temperaturefor 10 minutes. The coupon was left in the mold and allowed to cool to room temper-ature before being removed. Each coupon was cut in half lengthwise to produce twospecimens. Figure 3 shows the foam coupons before and after compression. Figures 4and 5 show optical microscopy images of typical 30 PPI and 60 PPI foam structuresbefore and after compression.

2.5 Poisson’s ratio measurement

Poisson’s ratio is determined by measuring the change in volume of a material as itdeforms, typically under tension or compression. The most common technique usesa strain measuring device such as a load frame, and extensometers to measure thedimensional change. In the current work, three methods were explored to measurePoisson’s ratio.

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Figure 3: Foam specimens before and after compression.

1 mm 1 mm

Figure 4: Image of cell structure of polyurethane polymer foam, cell density30 PPI. (left) before compression, (right) after compression, c.f. 2.7.

1 mm 1 mm1 mm

Figure 5: Image of cell structure of polyurethane polymer foam, cell density60 PPI. (left) before compression, (right) after compression, c.f. 2.7.

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2.5.1 Digital image correlation

Digital Image Correlation (DIC) is an optical technique which tracks registration dotson an object’s surface to measure dimensional changes. The technique is often used toquantify deformation, displacement, strain or optical flow. Because it is a contact-freetechnique, it was thought to be ideal for analysing foams. The pliability of the foamsmeant that contact techniques such as bonded strain gauges would have deformedthe specimens.

The foam specimens were glued to steel mounts with 5-minute epoxy, well away fromthe measurement area. They were then mounted in an MTS servo hydraulic loadframe. A white paint pen was used to put registration dots on the surface of thespecimens. The load frame was manually incremented through a series of increasingtensions in the vertical direction, while the DIC was used to dermine the dimensionalchanges in the perpendicular directions.

Unfortunately it was found that the surface roughness of the foam interfered withthe DIC, resulting in large scatter in the data. This technique could not be used todetermine Poisson’s ratio accurately. It will not be discussed further in this report.

2.5.2 Clip-on extensometers

Clip-on axial and transverse extensometers are designed for soft materials. Figure 6shows a foam sample in the United SSTM screw driven load frame with Epsilon axialand transverse clip-on extensometers. This set-up provided data on tensile deforma-tion, and was used to evaluate NA30CHAR and S60 specimens.

2.5.3 Optical comparator

An optical comparator is typically used in the inspection of manufactured parts.It operates by projecting a magnified silhouette onto a screen. The dimensions canthen be accurately measured by superimposing graduations on the image. By usinga magnified image, even small dimensional changes can be measured.

Each specimen was mounted to a specially prepared caliper gauge, fitted with a posi-tive mechanical stop and a dial gauge to measure translational movement (Figure 7).The caliper gauge was used to incrementally stretch the foam in the horizontal direc-tion, and measured the displacement with an acccuracy of to 0.0254 mm. The wholegauge was placed in the comparator (Figure 8), and the change in perpendicular di-mensions was measured as a function of horizontal displacement of the caliper gauge.This technique is also contact-free.

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Figure 6: Foam specimen mounted in a United SSTM screw driven load frame,with Epsilon axial and transverse clip-on extensometers.

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Figure 7: Caliper gauge.

Figure 8: Optical comparator with caliper gauge from Figure 7 in situ.

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3 Results and discussion

The degree of the re-entrant angle is theoretically determined by the compressionfactor used. Therefore a range of compression factors should result in a range ofnegative Poisson’s ratios. Practically, there are other variables that affect the Poisson’sratio; foams have a distribution of cell sizes, and the cells are not perfectly isotropic.

The reported auxetic foam producing methodologies transform open cell foams througha thermo-mechanical process. They use a two-step triaxial compression. Alderson etal. [14] filed a patent for a biaxial compression process that could be performed asa continuous process, contrary to the two-step triaxial batch process. The biaxialcompression leads to elastomerically anisotropic materials, meaning that they exhibita difference between the in-plane properties and the through-thickness properties.Grima et al. [15] introduced a chemical-mechanical variant to the triaxial method-ology that added a solvent treatment step (e.g., acetone for polyurethanes). Thesolvent step has the same effect as heating above the polymer softening temperature,but only works for open cell structure where the solvent can flow throughout thesample. The mold reported here for simultaneous triaxial compression is unique fromthe previously reported methods.

3.1 Clip-on extensometers

The data obtained from the Epsilon axial and transverse extensometers show a mini-mum (Figure 9) in the Poisson’s ratio for the 30 PPI and 60 PPI polyurethane foams.For both foams, the minimum is at a compression factor of between 2.0 and 2.7, butthe Poisson’s ratio is not negative. When we slice the specimens into thin (1 cm thick-ness) slices and observe their behaviour visually, there is an obvious perpendicularexpansion when extended (Figure 10). There are possible areas for error that canaccount for the contradiction between the extensometer data and what is witnessedvisually. The foam is an inherently compliant material and the transverse extensome-ter, which is spring loaded, may be applying a small load to the foam. If the loadis on the same order of magnitude as the auxetic response, then they would canceleach other. Because of this data, we did not continue with this technique, and didnot analyse the 90 PPI samples.

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5 3 3.5 4

Poiss

on's

ratio

compression factor

30 ppi polyurethane60 ppi polyurethane

Figure 9: Poisson’s ratio as determined by axial and transverse extensometers forthe 30 PPI and 60 PPI foams.

(a) Relaxed. (b) Extended.

Figure 10: Auxetic response of S60 foam after triaxial compressionand heat treatment.

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3.2 Optical comparator

The optical comparator is a contact free method. It uses visual inspection, and quan-tifies it. Figure 11 shows the Poisson’s ratios of the 30 PPI, 60 PPI and 90 PPI foams.All show similar results, with negative Poisson’s ratios being realized at a compressionfactor of approximately 2.0 and above.

All three foams had similar densities, with only the size of the pores varying. The sizeof the pores had a large effect on the Poisson’s ratio of the untreated foams wherethe 90 PPI was approximately 0.65, while the 30 PPI and 60 PPI where around 0.2.Once triaxially compressed, all the foams responded similarily, with Poisson’s ratiosapproaching zero around a compression factor of 2. Wang et al. [16] showed that thePoisson’s ratio increases as the cell size decreases (i.e. -0.8, -0.5 and -0.4 for 20 PPI,65 PPI and 100 PPI foams respectively, with a compression factor of 2.5). The datashown in Figure 11 follows the same trend at C.F. 2.7 and C.F. 3.2. With C.F of 2.0or less, this trend is not seen.

Table 1 shows the results for auxetic foams from the literature. The 60 PPI foamsprepared by Evans et al. [12] show more negative Poisson’s ratios than found in theresults reported here (green circles in Figure 11). The difference in results is likelydue to specimen surface effects. The specimen may have inhomogeneous distributionof the cell collapse throughout their volume [18]. In fact, the surface may experiencemore cell collapse than the core, thus the surface may have a lower Poisson’s ratio.The measurement technique looks at the bulk Poisson’s ratio, which would be anaverage over the whole volume of the specimen.

It is difficult to compare data across different foams, different pore sizes and differentcompression factors.

Table 1: Literature values for polyurethane auxetic foams.PPI C.F. ν reference20 2.7 -0.6 Lakes 1987 [11] (used polyester)20 2.5 -0.8 Wang 2001 [16]30 3.5 -0.04 Scarpa 2002 [17]60 1.4 +0.1 Evans 1997 [12]60 2.0 -0.3460 2.4 -0.5765 2.5 -0.5 Wang 2001 [16]100 2.5 -0.4

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-0.3-0.2-0.1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 0.5 1 1.5 2 2.5 3 3.5 4

Poiss

on's

ratio

compression factor

30 ppi polyurethane60 ppi polyurethane90 ppi polyurethane

Figure 11: Poisson’s ratio as calculated using optical comparitor for 30 PPI,60 PPI and 90 PPI polyurethane open cell foams.

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4 Conclusions

The goal of this work was to reproduce the seminal work of Lakes [11], and createfoams with negative Poisson’s ratio. In this work three cell sizes were triaxially com-pressed, heat treated and characterized. The three cell sizes were consistent with thesizes used in the literature. All three yielded auxetic foams when a compression factorgreater than 2 was used. This was consistent with the literature [11]. The smallestpore size (90 PPI) gave the lowest Poisson’s ratio (-0.16), at a compression factorof 3.2.

An experimental assembly was designed, fabricated and tested for the manufactureof auxetic foams from conventional foams. The basic design of the apparatus is anovel compression mold which simultaneously applies equal strain to the six sides ofa rectangular foam coupon. The simultaneous compression of foam coupons hasn’tbeen reported before. This apparatus was designed to avoid the wrinkling seen in themore traditional tube/plunger assembly.

Three techniques were explored to measure Poisson’s ratio. The two somewhat auto-mated techniques (digital image correlation and clip-on extensometers) were exploredin an effort to “modernize” data collection. The literature reports mostly visual datacollection using pictures taken with a graduated background. Unfortunately the au-tomated techniques were not accurate enough due to the foams being compliant andhaving rough surfaces. The most accurate collection technique used here was a varia-tion on the visual techniques in the literature. It used an optical comparator combinedwith a caliper gauge.

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5 Future work

Future work will need to scale up the mold to make larger coupons. The current setup yields coupons that are 75 mm × 25 mm × 25 mm. Larger sizes will be needed formore advanced characterization. For example, impact tests typically require samplesthat are 4 inches on two sides.

The proposed uses for auxetic foam all rely on its performance in compression. Thecurrent work was all performed in tension. Characterization under compression isneeded. See for example the compression test apparatus of Lisiecki at the Air ForceInstitute of Technology in Poland [18].

There is currently a project at DRDC Valcartier Research Centre on behind armourmaterials. Part of this project is looking into novel foams to address BABT, includingauxetic foams. To support this new work, and to find an exploitation route for aux-etic foams, the triaxial compression apparatus has been loaned to Natural ResourcesCanada, Canmet Energy. They are testing different foams as armour backing mate-rials. These tests will include basic material evaluation (such as density, hardness,tensile strength, and elongation), followed by impact testing using ASTM D5420(Standard Test Method for Impact Resistance of Flat, Rigid Plastic Specimen byMeans of a Striker Impacted by a Falling Weight), and impact tests with plastic bul-lets. Finally V50 ballistic tests and ballistic tests of body armour (using NIJ standard0101.06) will be performed.

The use of auxetic foams as liners for athletic equipment is being explored by Aldersonat Bolton University in the U.K. [9,10]. As this is related to the use of auxetic foamsin soldier personal protective equipment, a tech watch on Alderson’s research shouldbe maintained.

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References

[1] Jacobs, M. J. N. and Dingenen, J. L. J. V. (2001), Ballistic protectionmechanisms in personal armour, Journal of Materials Science, 36(13),3137–3142.

[2] Underhill, R. (2014), Defense Applications of Auxetic Materials, DSIACJournal, 1, 7–13.

[3] Alderson, A. and Alderson, K. L. (2007), Auxetic materials, In Proceedings ofthe Institution of Mechanical Engineers Part G, Vol. 221 of J. AerospaceEngineering, pp. 565–575.

[4] Yang, W., Li, Z.-M., Shi, W., Xie, B.-H., and Yang, M.-B. (2004), Review onauxetic materials, Journal of Materials Science, 39, 3269–3279.

[5] Hou, Y., Neville, R., Scarpa, F., Remillat, C., Gu, B., and Ruzzene, M. (2014),Graded conventional-auxetic Kirigami sandwich structures: Flatwisecompression and edgewise loading, Composites Part B: Engineering, 59, 33–42.

[6] Alderson, K. and Coenen, V. (2008), The low velocity impact response ofauxetic carbon fibre laminates, physica status solidi (b), 245(3), 489–496.

[7] Alderson, K. L. and Evans, K. E. (2000), The strain dependent indentationresilience of auxetic microporous polyethylene, Journal of Materials Science,35, 4039–4047.

[8] Kocer, C., McKenzie, D., and Bilek, M. (2009), Elastic properties of a materialcomposed of alternating layers of negative and positive Poisson’s ratio,Materials Science and Engineering: A, 505(1), 111–115.

[9] Sanami, M., Ravirala, N., Alderson, K., and Alderson, A. (2014), Auxeticmaterials for sports applications, Procedia Engineering, 72, 453–458.

[10] Allen, T., Martinello, N., Zampieri, D., Hewage, T., Senior, T., Foster, L., andAlderson, A. (2015), Auxetic materials for sports safety applications, ProcediaEngineering, 112, 104–109.

[11] Lakes, R. S. (1987), Foam structures with a negative Poisson’s ratio, Science,235, 1038–1040.

[12] Chan, N. and Evans, K. (1997), Fabrication Methods for Auxetic Foams,Journal of Materials Science, 32, 5945–5953.

[13] KarisAllen, K. (2013), Development of Equipment for the Manufacture ofAuxetic Foam for DRDC, (DRDC Atlantic CR2013-042) FACTS EngineeringInc.

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[14] Alderson, A., Alderson, K., Davies, P., and Smart, G. (2012), Process for thepreparation of auxetic foams. Patent number: 8277719.

[15] Grima, J. N., Attard, D., Gatt, R., and Cassar, R. N. (2009), A Novel Processfor the Manufacture of Auxetic Foams and for Their re-Conversion toConventional Form, Adv. Eng. Mater., 11, 533–535.

[16] Wang, Y.-C., Lakes, R., and Butenhoff, A. (2001), Influence of Cell Size onRe-Entrant Transformation of Negative Poisson’s Ratio ReticulatedPolyurethane Foams, Cellular Polymers, 20, 373–385.

[17] Scarpa, F., Yates, J., Ciffo, L., and Patsias, S. (2002), Dynamic Crushin ofAuxetic Open-cell Polyurethane Foam, Proc. Instn. Mech Engrs Part C: J.Mechanical Engineering Science, 216, 1153–1156.

[18] Liesiecki, J., Klysz, S., Blazejewicz, T., Gmurczyk, G., and Reymer, P. (2014),Tomographic examination of auxetic polyurethane foam structures, PhysicaStatus Solidi B, 251, 314–320.

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DOCUMENT CONTROL DATA(Security markings for the title, abstract and indexing annotation must be entered when the document is Classified or Protected.)

1. ORIGINATOR (The name and address of the organization preparingthe document. Organizations for whom the document was prepared,e.g. Centre sponsoring a contractor’s report, or tasking agency, areentered in section 8.)

DRDC – Atlantic Research CentrePO Box 1012, Dartmouth NS B2Y 3Z7, Canada

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UNCLASSIFIED

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(NON-CONTROLLED GOODS)DMC AREVIEW: GCEC DECEMBER 2013

3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriateabbreviation (S, C or U) in parentheses after the title.)

Manufacture and characterization of auxetic foams

4. AUTHORS (Last name, followed by initials – ranks, titles, etc. not to be used.)

Underhill, R. S.

5. DATE OF PUBLICATION (Month and year of publication ofdocument.)

September 2017

6a. NO. OF PAGES (Totalcontaining information.Include Annexes,Appendices, etc.)

17

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DRDC-RDDC-2017-R099

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13. ABSTRACT (A brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highlydesirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of thesecurity classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), or (U). It isnot necessary to include here abstracts in both official languages unless the text is bilingual.)

In the thirty years since the first reports of auxetic foams (i.e., foams exhibiting a negative Pois-son’s ratio), there have been no significant advances in their production methods. With the ulti-mate goal of developing new lightweight armour materials, Defence Research and DevelopmentCanada (DRDC) had a program to explore the creation and testing of polymeric auxetic mate-rials, including foams. The goals of the work presented here were to establish methodologiesreproducibly consistent with the literature, and to assess measurement techniques. Part of thisprocess involved the construction of a novel mold that achieved uniform compression simultane-ously in the three axes.

Three foams with differing cell sizes (30, 60, and 90 pores per inch (PPI)) were successfully com-pressed to form auxetic materials. All three exhibited negative Poisson’s ratios at compressionfactors of 2.0 or greater. The 90 PPI foam yielded the most negative Poisson’s ratio (-0.16), at acompression factor of 3.2.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and couldbe helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such asequipment model designation, trade name, military project code name, geographic location may also be included. If possible keywordsshould be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified.If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

auxetic; negative Poisson’s ratio; armour; personal protective equipment; PPE; foam; BABT

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