2005_how to make auxetic fibre reinforced composites

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phys. stat. sol. (b) 242 , No. 3, 509518 (2005) / DOI 10.1002/pssb.200460371

2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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How to make auxetic fibre reinforced compositesK. L. Alderson *, 1, V. R. Simkins 1, V. L. Coenen 1, P. J. Davies 1, A. Alderson 1,and K. E. Evans 2 1 Centre for Materials Research and Innovation, Deane Road, Bolton, BL3 5AB, UK2 School of Engineering, Computer Science and Mathematics, Harrison Building,

University of Exeter, Exeter EX4 4QF, UK.

Received 15 June 2004, accepted 8 November 2004Published online 15 February 2005

PACS 81.05.Lg, 81.05.Qk, 81.20.Ev, 81.20.Hy, 81.70.Bt

Auxetic composite materials can be produced either from conventional components via specially designedconfigurations or from auxetic components. This paper reviews manufacturing methods for both thesescenarios. It then looks at the possibility of property enhancements in both low velocity impact and fibrepull out due to the negative Poissons ratio. Tests revealed that auxetic carbon fibre composites madefrom commercially available prepreg show evidence of increased resistance to low velocity impact andstatic indentation with a smaller area of damage. Also, using auxetic fibres in composite materials isshown to produce a higher resistance to fibre pullout.


1 Introduction

Developments in structural engineering design and technology over the past three decades in industriessuch as aircraft, automobile and sports and leisure equipment have demanded the production of new,high-performance materials. Within this class of materials are the fibre-reinforced composites. The pos-sibility of producing composite laminates with Poissons ratios close to 1 was discussed by Milton in1992 [1]. Advances in auxetic materials production and development have led to the possibility of pro-ducing auxetic fibre reinforced composites, thus exploiting property enhancements known to arise inauxetic materials such as energy absorption, improved fracture toughness and better resistance to inden-tation.

There are a number of ways in which auxetic fibre reinforced composites can be made. The routeclosest to conventional manufacture is to use off-the-shelf pre-preg material which, given specific stack-ing sequences, will produce an overall auxetic effect. Several groups have used this route to auxetic fibrereinforced composites [27] and have made some inroads into testing the resultant materials for en-hanced fracture toughness [4] and indentation resistance [5]. Composite laminates can be designed tohave negative in-plane or through-thickness Poissons ratios, . The requirement for an auxetic compos-ite laminate is that the individual ply materials be highly anisotropic. This means that carbon/epoxy [3] isa more suitable choice than either Kevlar/epoxy [8] or glass/epoxy [6], though all three material combi-nations have been investigated. In many cases, the negative Poissons ratios obtained by this route todate have been small i.e. around = 0.17 [26] although Miki and Morotsu [7] did produce a value of = 0.37 for an unbalanced, bi-directional laminate. Methods of increasing the value of havebeen discussed and these include use of a prepreg with increased anisotropy (i.e. using a stiffer fibre or a

* Corresponding author: e-mail: [email protected]


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higher volume fraction) [3]. Another approach would be to investigate different laminate configurationspredicted to give a more negative Poissons ratio [9]. Work towards using specially designed software[10] to achieve this is ongoing, with prediction of stacking sequences now possible to specifically maxi-mise the negative Poissons ratio. For example, for a specified set of properties, it has been possible topredict a value of = 0.25. Clearly this is an area of extreme importance and one in which very inter-esting results are expected.

The work reported here, however, is concerned with the effect of a negative Poissons ratio on me-chanical properties of the laminate. In particular, the properties of static indentation and low velocityimpact damage are examined with an emphasis on the energy absorption and damage sustained byauxetic laminates in comparison with their conventional counterparts.

A further method of manufacturing an auxetic fibre reinforced composite is to use auxetic compo-nents, for example, an auxetic matrix, auxetic reinforcement or both. This has been achieved usingauxetic foams previously [11] and has been modelled for some considerable time [12]. A simple press-fitfastener [13] has been developed based on an auxetic copper foam which demonstrates both theoretically

and experimentally that it is much more difficult to remove than would be a conventional copper foam.However, it has only very recently been possible to begin work on auxetic fibre reinforced composites ofthis type with the advances in the manufacture of auxetic fibres [14] and new methods to produce auxeticpolymers [15, 16].

This paper reports current work to assess the possibility of using auxetic polymeric fibres within acomposite. The fibres used are polypropylene and tests have been carried out to assess their pull-outperformance from a specially designed matrix. The results have shown that it is up to 4 times more diffi-cult to pull out an auxetic fibre than a similar conventional fibre and these results and their implicationswill be discussed.

This paper, then, draws together the work in producing and testing auxetic fibre reinforced compos-ites.

2 Auxetic fibre reinforced composites made from conventional materialsThe work reported here is concerned with looking at manufacturing and testing carbon fibre reinforcedlaminates with a negative 13 i.e. through-the-thickness Poissons ratio. The main aim is to compareauxetic laminates with those of matched through thickness modulus, E 3, but with a small positive or nearzero Poissons ratio. This is so that any property difference in this direction can be attributed to the Pois-sons ratio alone. The first stage is, then, to design appropriate laminate configurations and this wasachieved using specially designed software [17, 18], presented by Zhang and Evans. Software packagesgenerally available for this technique predict the mechanical properties of a specific lay-up orientation,allowing the designer to try many different lay-ups rapidly. This can be time consuming and may notresult in optimum matched laminate configurations. A number of workers have tried to address this prob-lem, beginning in the late 1960s by Schmit [19] and Bush [20]. They proposed an analytical approach tocomposite laminate design but these were limited by very specific loading conditions and orientation

assumptions. Since then, several different optimisation procedures for the design of laminated platessubjected to various stiffness and strength constraints have been established. These include work onoptimum design for in-plane loading [2123], maximum stiffness and bending strength [24, 25] andstability [26, 27]. The optimisation approach of Zhang and Evans, however, is more general. Here, nosymmetry is assumed and all layers can be different to each other. The FORTRAN program developedenables the design of laminates with required mechanical properties where only the individual laminalayer properties are known. The program is able to produce an optimised stacking sequence by minimis-ing the difference between properties calculated from, for example, classical laminate theory and thoseproperties required by the designer. For this case, where the aim was to study the effect of a negativePoissons ratio, the stacking sequences predicted are given in Tables 1a and b below. Each panel com-prised 24 layers.


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Table 1a Laminate stacking sequences and software predictions of E 1, E 2, E 3 and G 12.

stacking sequences E 1 (GPa) E 2 (GPa) E 3 (GPa) G 12(GPa)([0/45/5/40] s)3 75.3 20.1 9.6 19.3[+/30] 6s 49.6 11.9 9.5 26.5[35/20/25/40/85/40/25/45/35/15/25/40] s 49.8 25.0 9.6 20.3

Table 1b Laminate stacking sequences and software predictions of 12, 21, 31 and 13.

stacking sequences 12 21 13 31

([0/45/5/40] s)3 0.721 0.192 0.086 0.011[+/30] 6s 1.243 0.298 0.156 0.03[35/20/25/40/85/40/25/45/35/15/25/40] s 0.554 0.278 0.187 0.036

2.1 Manufacture of auxetic fibre reinforced composites

Specimens with the laminate configurations given in Tables 1a and b were prepared as 80 80 3 mmsquare panels from IM7/5882 unidirectional carbon epoxy prepreg. Standard vacuum bagging techniqueswere employed. The laminate was prepared in accordance with the required stacking sequence. Eachsuccessive ply was placed directly on top of the other at the desired orientation on a smooth base platecovered by a PTFE release material to aid removal of the specimen after curing. Once lamination wascomplete, a metal top-plate also covered in PTFE release material was placed on the laminate stack. Alayer of breather fabric to allow air to be evacuated efficiently was placed on top with the vacuum valveattachment taped in a central position. The whole assembly was made airtight using a high temperaturenylon bagging film and vacuum sealant. A vacuum pressure of 0.8 bar was applied over night to consoli-date the laminate. The lay up was placed in a fan oven and the temperature raised at a rate of 23 /min.

until a temperature of 180 C was reached. This was maintained for 130 minutes. The oven and contentswere then slowly cooled to room temperature, the vacuum pressure removed and the bag opened. Thethrough-the-thickness Poissons ratio values were measured using the technique of video extensometryand good agreement was found with the predicted values shown in Table 1b [28].

2.2 Static indentation testing

A specially designed flexural indentation test configuration was adapted for this work. The reason forthis was that it was desirable to allow the specimen to flex without the confines of clamping at this stageand to allow the indentation nose to penetrate through the specimen, giving the full damage capability ofthe specimen. This was to build on standard indentation testing of the auxetic carbon fibre laminatewhich gave an indication of an enhancement in performance in the elastic region only [5].

Tests were conducted on a Dartec Universal Hydraulic testing facility in compression mode. Fourspecimens of each configuration were tested to a penetration depth of 5 mm (i.e. to full specimen dam-age) at a strain rate of 2 mm/min. The indentor nose, a 12.7 mm diameter hemisphere, was applied at thecentre of the specimen. From the resultant load/displacement plots, the load, displacement and energyabsorbed at first failure and to peak load were obtained.

2.3 Low velocity impact testing

The starting point for the low velocity impact testing was the analysis of the load/displacement curvesfrom the static indentation tests. An in-house designed and built fully instrumented drop weight impactmachine was used in conjunction with a data capture system with medium range sampling rate per chan-nel and in-built signal conditioning. Eight channels were available, allowing accelerometer, voltage and


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Fig. 1 Typical static load/displacement curves for the laminates with 13 = 0.156, 13 = +0.187 and

13 = +0.086. The three energy levels selected for impact testing are indicated as 1, 2 and 3.

strain gauge inputs. This system was modified substantially in house and allowed a million samples persecond to be captured. A single bounce mechanism was fitted to ensure clear analysis of the impactevent.

Three energy levels corresponding to specific points in the damage process were selected (see Fig. 1).These were 2 J, 7 J and 12 J. A fixed mass impactor with an equivalent 12.7 mm hemispherical nose wasused with the same support conditions employed as in the static indentation testing. The impactorweighed 1.76 kg and, correspondingly, resulted in impact velocities of 2 m/s, 3 m/s and 4 m/s.

Force/time histories were recorded and the same properties were evaluated for comparison with the staticindentation tests.

2.4 Damage observation after testing

The specimens were all sectioned to evaluate the internal damage using fractographic analysis. Images ofthe damage were recorded photographically for ease of comparison.

2.5 Experimental results and discussion

2.5.1 Static indentation testing

Typical load/displacement curves for each specimen type are shown in Fig. 1. The extrapolated points

from the curves are given in Table 2a for the first failure point and Table 2b for the peak load data. It canbe seen that the auxetic specimens had the highest properties in each case i.e. a higher load sustained andmore energy absorbed to first failure and to peak load. This points to an enhancement in resistance toindentation.

The fractographic damage was extremely interesting with this in mind. The laminate with 13 = 0.086had damage which was dominated by several large delaminations through the thickness. The laminatewith 13 = 0.187 also had several large delaminations but this time towards the back face of the laminate.However, the damage in the auxetic specimen is characterised by very localised fibre breakage directlyunder the indentor with very few delaminations. These findings can be clearly seen in Figs. 2, 3 and 4below which show the comparison between the laminates with 13 = 0.086, 13 = 0.156 and 13 = 0.187,respectively. Basically, the auxetic laminate will contract under the indentor as a direct consequence of


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Table 2a Experimental first failure values for the static indentation tests. Poissons ratio refers to 13.

Poissons ratio displacement(mm)

load(kN)

energy absorbed(J)

initial gradient(kN/mm)

0.086 1.13 2.42 1.4 2.140.156 1.37 3.48 2.4 0.1 2.540.187 1.21 3.02 1.8 0.1 2.50

Table 2b Experimental peak load values for the static indentation tests. Poissons ratio refers to 13.


load(kN)

energy absorbed(J)

0.086 3.3 0.2 6.2 0.6 10 20.156 3.6 0.3 7.7 0.7 14 2

0.187 3.6 0.5 6.8 0.1 12 2

Fig. 2 Damage induced in the specimen with 13 = 0.086 by the flexural indentation test.




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its through-the-thickness (i.e. 13 and 31) auxetic nature. As the damage is not dissipated over a largearea as in the creation and subsequent growth of delaminations, the majority of the damage created oc-curs directly under the indentor, where it is more severe but highly localised. This type of response hasbeen seen in other auxetic materials, and has been studied systematically in foams [29]. Here, the densi-fication under the indentor was suggested to be due to an enhanced shear modulus for auxetics causingan improved material response to shear stress and allowing a wider distribution of strain. The suppres-sion of delaminations observed suggests a similar densification mechanism in the composite laminatestested here added to a better distribution of strain. In addition, the consistent mismatch between individ-ual lamina layers in the auxetic laminate which have a stacking sequence of ( 30) s leads to an orderlyprogression of damage through the specimen thickness and a greater communication of shear strain,causing higher resistance to delamination [28]. Thus, there are two effects contributing to this response,the mismatch and the auxetic effect, and the relative roles of each are currently being examined, as is therole of the positive in-plane Poissons ratio ( 12).

2.5.2 Low velocity impact testing

The results from the low velocity impact testing of the specimens are presented in Table 3a for results tofirst failure (easily isolated from the impact plot) and in Table 3b to the end of the impact event for the 7Jimpact. Similar results were obtained for the 12J impact. It should be noted that at the 2J level, no dam-age was induced in the specimens.

It can be seen that the enhancement at first failure observed in the auxetic samples is found under bothimpact and static conditions. It is, however, interesting to note that for the auxetic specimens, the firstfailure load is actually the maximum load measured during the impact. This suggests that the first failureonset is a much more significant event for auxetic specimens. The fractographic examination revealedthat in the specimen with 13 = 0.086, there were several large delaminations in the upper half of thelaminate. The laminates with 13 = 0.187 revealed a number of delaminations in the upper half of the

laminate accompanied by shear cracking under the indentor. The auxetic specimen, however, showedonly a small amount of back face failure (even at such low velocities) but no discernible delaminations atall. This was very surprising and seems to add weight to the possibility of the localised mechanism de-scribed above occurring in the auxetic laminates compared to the other two laminates.

In conclusion, static and low velocity indentation resistance of auxetic composite laminates has beenevaluated in comparison with laminates having near zero ( 13 = 0.086) and large positive ( 13 = 0.187)

Table 3a Experimental first failure values for the low velocity impact tests. Poissons ratio refers to 13.


applied load(kN)

energy absorbed(J)

initial gradient(kN/mm)

0.086 1.3 4.1 0.1 2.6 0.2 3.3 0.10.156 1.6

4.8

0.1 3.8

0.2 2.9

0.10.187 1.3 0.1 4.3 0.3 2.7 0.4 3.2 0.2

Table 3b Experimental end of impact event values for the low velocity impact tests. Poissons ratio re-fers to 13.

Poissons ratio max. displacement(mm)

max. load(kN)

energy absorbed(J)

duration(ms)

0.086 2.3 4.8 0.1 6.2 2.90.156 2.3 4.8 0.1 6.1 3.10.187 2.3 4.7 0.2 6.1 2.9 0.1


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Poissons ratio values. The auxetic laminates showed higher loads to first failure with enhanced energyabsorption in both cases. The statically tested specimen also sustained higher loads with the onset ofdamage and absorbed more energy to catastrophic failure.

The initial damage sustained was found to be much more localised for both static and low velocityimpact testing in the auxetic laminates, with a distinct lack of large delaminations in each case. Thispoints to a different mechanism operating in response to indentation for auxetic laminates, which is acombination of the auxetic response and mismatch between layers causing a localised response and sup-pressing delamination growth.

3 Auxetic fibre reinforced composites made from auxetic components

Though it has been possible to make auxetic foam composites for a number of years [1113], it is nowpossible to make composites using auxetic fibres. Previously, it was suggested [30] that an auxetic fibrewithin a composite would resist fibre pullout. This effect is shown schematically in Fig. 5 and would

appear to indicate that as the fibre is pulled, it will expand and lock into the matrix rather than contract-ing and pulling out easily as a conventional fibre would do. This is also the principle behind the designof a press-fit fastener using auxetic copper foam reported previously [13]. This section of this paperreports work to investigate this idea using auxetic PP fibres as the reinforcement to a specially adaptedmatrix.

3.1 Manufacture of model single fibre composites with auxetic fibres

In order to assess the single fibre pullout behaviour of composites using auxetic fibres, a model systemmust be produced. It is not sensible to produce a fibre reinforced composite with a large volume fractionof fibres to try to gather this type of data so, in accordance with normal practise, a model system consist-ing of a single fibre embedded in a matrix has been manufactured. The fibres currently produced [14] arerelatively weak so the matrix material was carefully selected and modified so that the fibre may be re-moved from the matrix before it necked. Also, in order to rule out thermal effects, the matrix was re-quired to be a cold-cure resin.

The resin selected was Araldite LY 5052 with hardener HY 5084 and dibutyl phthalate used as thesoftener. After an extensive period of experimentation [31], the matrix combination found to be the mostsuitable was 40 ml of resin, 9 ml of hardener and 25 ml of dibutyl phthalate. The fibre was embedded inthe resin using a method developed in-house [31] and shown schematically in Fig. 6. A gel time of 6hours at room temperature (24 2 C) was followed by seven days in the same conditions. The finalspecimen was 80 mm long including an embedded fibre length of 20 mm, a free fibre length of 50 mmand 10 mm spare fibre for handling and gripping. The specimen diameter was 15 mm. For compari-

Fig. 5 Schematic of the predicted response of auxetic reinforcement fibres.


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Fig. 6 Schematic illustrating the manufacture of the fibre pull out samples.

son purposes, both auxetic and carefully matched conventional fibres were prepared for testing. Thematching ensured that the fibres had the same modulus (1.3 GPa) and diameter (227 m) i.e. the maindifference between them was the Poissons ratio. The auxetic fibre had a Poissons ratio of +0.34whereas the auxetic fibre had a Poissons ratio of 0.60. The Poissons ratio values were determinedusing the technique of video extensometry and further details are available in a previous paper [20].

3.2 Fibre pullout tests

The tests were conducted using an Instron 4200 tensile testing machine fitted with a 100 N load cell. Thespecimen was inverted and gripped in a collet grip attached to the crosshead of the machine. The freefibre was clamped in the lower jaw. The distance from the upper end of the inverted resin cylinder downto the upper edge of the lower jaw was 70 mm, and was set as the grip distance. The free fibre length(between the lower surface of the inverted resin and the upper edge of the lower jaw) was 50 mm andthis was set as the gauge length. The test was conducted at a loading rate of 5 mm/min.

In order to assess the fibre/matrix interface for any evidence of surface interaction, small samples ofresin of dimensions 10 50 2 mm were cast with fibres of each type laid in. These were allowed to set


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and cure as above. The samples were then fractured using liquid nitrogen and the broken fragments ex-amined microscopically. No evidence was found of any surface interaction between the two materials ineither auxetic or conventional specimens.

3.3 Experimental results and discussions

The average curves obtained from performing pull out tests on conventional and auxetic embedded fibresare shown in Fig. 7. It can clearly be seen that the auxetic fibres have a much higher peak debondingload (0.96 N as opposed to 0.44 N) which remains higher over a great displacement. This would appearto indicate a much larger debonding event than the conventional fibre. The energy used to pull out thefibre is 8.3 mJ for the auxetic fibre which is over three times greater than that required to pull out a con-ventional fibre i.e. 2.5 mJ. This appears to show that enhanced fibre pullout as has been suggested does

indeed occur. It should be noted that high resistance to fibre pullout is not always beneficial, since en-ergy dissipation during pullout can confer a toughness on the material. A conceptual model of the fibrepullout process has been developed and is the subject of a further paper [31]. To complement this de-scription of the pullout process, existing analytical models for fibre pullout and in particular those focus-sed on considering auxetic behaviour [13] have been studied.

These are now being adopted to the specific situation of an auxetic fibre in a conventional ma-trix to further aid analysis of the process. The results, however, do have implications for the use ofauxetic fibres in such applications as fibre reinforced composites and in biomedical applications as su-tures.

4 Conclusion

This paper has presented two very different ways in which an auxetic composite can be made. The firstuses off-the-shelf prepregs and, by variations of the stacking sequence employed, can be designed toproduce a through thickness or inplane negative Poissons ratio. This has resulted in enhanced mechani-cal properties with static indentation and low velocity impact testing reported here. The second usesauxetic constituents in this case, auxetic fibres as part of the composite. Here, fibre pullout is re-ported to have been resisted due to the auxetic deformation of the fibres.

References

[1] G. Milton, J. Mech. Phys. Solids 40, 1105 (1992).[2] C. T. Herakovich, J. Compos. Mater. 18, 447 (1984).[3] J. F. Clarke, R. A. Duckett, P. J. Hine, I. J. Hucthinson, and I. M. Ward, Composites 25(9), 863 (1994).

Fig. 7 Comparison of average force-displacement dataobtained from auxetic and non-auxetic single fibre pull-out.


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[4] J. Chandhuri and Q. Jang, Proc. Am. Soc. Compos. 3rd Tech. Conf. edited by J. C. Serais and J. T. Quilow,(Tech. Pub. Co. Inc., Seattle WA, 1988), p. 701.

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Brussels, Belgium 1990).[19] L. A. Schmit, in: Mechanics of Composite materials: Proc. 5th Symp. Naval Structural Mechanics 1967, Penn-

sylvania USA, edited by F. W. Wendt, H. Liebowitz, and H. Perrone, 533 (Pergamon Press, Oxford, 1970).[20] H. G. Bush, Proc. 6th St. Louis Symp. on Composite Materials in Engineering Design, St. Louis, USA (1972),

p. 391.[21] Y. Hirano, AIAA J. 17, 1017 (1979).[22] Y. Hirano, J. Compos. Mater. 13 , 329 (1979).[23] W. J. Park, J. Compos. Mater. 16, 341 (1982).[24] T. R. Tauchert and S. Abidhatla, Eng. Optim. 8, 238 (1984).[25] T. R. Tauchert and S. Abidhatla, J. Compos. Mater. 18, 58 (1984).[26] H. Fukunga and Y. Hirano, in: Progress in Science and Engineering of Composites: Proc. 4th Int. Conf. Com-

pos. Mater., Tokyo, edited by T. Hayashi, K. Kawata, and S. Umekawa (Jpn. Soc. for Compos. Mater., Japan,1982), p. 505.

[27] J. Onoda, AIAA Journal 23, 1093 (1985).[28] V. L. Coenen, PhD Thesis, Bolton Institute (2003).[29] C. W. Smith, F. Lehman, R. J. Wootten, and K. E. Evans, Cell. Polym. 18(2), 79 (1999).[30] K. E. Evans, Chem. Ind. 654 (1990).[31] V. R. Simkins, A. Alderson, P. J. Davies, and K. L. Alderson, submitted to J. Mater. Sci. (2004).

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