The Design and Structure of Warp Knit Auxetic Fabrics

Samuel C. Ugbolue, Steven B. Warner, Yong K. Kim, Qinguo Fan, Chen Lu Yang*, Olena Kyzymchuk** and Yani Feng (Graduate

student)

Department of Materials and Textiles, University of Massachusetts Dartmouth, MA 02747, USA

* Advanced Technology and Manufacturing Center, UMassD, USA

** Kyiv National University of Technologies and Design, Ukraine.

WHAT IS AUXETIC STRUCTURE?

• The Greek word “auxetos” means “that which grows”.• Auxetic materials exhibit the unexpected feature of becoming

fatter when stretched and narrower when compressedIn other words, they exhibit a negative Poisson’s ratio.

• Auxetic materials have been formed as polymer gels, carbon fiber composite laminates, metallic foams, honeycombs and microporous polymers as affine structures.

CONVENTIONAL AUXETIC(Positive Poisson's Ratio) (Negative Poisson's Ratio)

Poisson’s Ratio is the ratio of transverse contraction strain to longitudinal

extension strain in the direction of stretching force

Typical Values of Poisson’s Ratio for various materials

Materials Poisson’s ratio Materials Poisson’s ratio

Rubbers 0.5Typical polymer

foams0.1-0.4

Soft biological tissues

0.5 Polyester 0.37-0.44

Lead 0.45 Nylon 6.6 0.41

Aluminum 0.33 Acrylic 0.37-0.45

Common Steels 0.27 Cork Nearly zero

A whole range of synthetic auxetic materials, including carbon fiber composites, honeycomb structures and microporous

polymers have been produced

The multifilament construction Ken Evans and Patrick Hook, the University Exeter

MOTIVATION FOR DEVELOPING AUXETIC TEXTILES

The use of auxetic materials has been limited because of problems with deploying them in their fabricated forms.

Auxetic fibers in an engineered textile structure will exhibit the very unusual, interesting and useful property of becoming wider when stretched and thinner when compressed. Such a process will revolutionize the protective clothing industry.

The novel fabrics will offer improved shear stiffness, increased plane strain fracture toughness and increased indentation resistance.

In terms of cost and performance, the new auxetic textiles will be technically superior and environmentally viable, providing United States companies with a competitive advantage.

POTENTIAL APPLICATIONS

Auxetic yarns and filaments could be used for sutures and fiber-reinforced composite applications.

Auxetic structures could be used as composite materials, personal protective appliances, fibrous materials and biomedical filtration materials.

As textile structures for bandages for compression therapy (where the bandage would react to compress swelling of the limb while also improving breathability as required).

The use of auxetic filaments, yarns or fabric structures to deliver active agents as intelligent textiles having anti-inflammatory, anti-odor, or drug-release capabilities.

Objectives

• The main objective of this research is to study how fabric geometry and structural design can be integrated to engineer novel auxetic warp knit fabrics.

• The technical approaches are to:

1.Design and investigate warp knitted structures as auxetic textiles offering optimum performance.

2. Investigate the influence of knitting parameters such as tension, cover factor and stitch density on the mechanical performance of the developed auxetic structures;

3. Develop appropriate models and validate with experimental data for the developed auxetic structures.

DESIGN of AUXETIC WARP KNITS

The warp knit structures formed by wales of chain and inlay yarns

The model of auxetic structure IFor the system, the strains in the

Oxi directions (i=1,2) for the modes of deformation are given by:

where X1 and X2 are dimensions of the unit cell along the Ox1 and Ox2 directions which are given by:

The analytical equation for the Poisson’s ratio is:

1( 1,2)i

i

dXd i

X d

1

2

2 sin

2( cos )

X l

X h l

2 1

1 2

tanxy

X

X

The fillet knitting structure with inlay yarns

The model of auxetic structure IIThe principal assumptions for the

deformation are: the angles between ribs deform elastically; no change of length of the individual ribs is allowed; the translational symmetry of the net is kept throughout deformation.

The engineering strain of such model:

1cos

1cos4

00

00

k

rx

1

sin

sin4

0

ny r

000

00000

cossinsin

sincos1cos

nyx

k

The in-lay warp knit Auxetic structure

FABRIC PRODUCTION

Knitting machine

Jakob Muller Crochet Knitting machine (RD3MT3/630) equipped with 8 guide bars is used in this study to produce fabric samples of 10wpi,

630 millimeters wide.

Materials

• Structure I

• Polyester The linear density is 250 den x 2. It is

manufactured by DuPont.

• Polyester / Spandex 96 filaments of polyester are

wrapped with 1 end Spandex. The linear density of polyester is

150 den, while that of Spandex is 40den. The yarn is supplied by Unifi Inc

• Structure II

• Polyester The linear density is 250 den x 2. It is

manufactured by DuPont.

• Nomex The linear density is 200 den x 2.

EXPERIMENTAL RESULTS

The methodology for measuring the Poisson’s ratio of the fabrics

Videoextensometry along with micro-tensile testing were used:

• Instron 5569 Mechanical Tester

ASTM D5034-95(2001) for Breaking Strength and Elongation of Textile Fabrics (Grab Test) is followed.

• Sensicamera QE has high resolution (1376 x 1040 pixel).

Poisson’s ratio test results of the fillet knitting structure with inlay yarns

Photo of initial Photo of specimen specimen during test

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

0,1

0,2

0 10 20 30 40 50 60 70 80 90 100

Strain, %

Poi

sson

rat

io

1

2

3

Average

Min Poisson’s ratio of fillet warp knit structures (wales direction)

3

5 1

2

3

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

- Ex/Ey

Number of chain courses

Number of tricot courses

Poisson’s ratio test results of the in-lay warp knit Auxetic structure

Photo of specimen during test

Photo of initial specimen

Conclusion• Poisson’s ratio test results of auxetic warp knitted

structures indicate that auxetic properties are achieved in all structures.

• Further studies on the influence of knit parameters such as tension, architecture and cover factor/density on the mechanical performance of the auxetic structures are in progress.

REFERENCES• Andy Alderson, Kim Alderson, Expanding materials and applications: exploiting auxetic textiles,

Technical Textiles International, 777, September 2005, 29-34.• Ravirala N., Alderson K., Davies P., Simkins V., Alderson A., Negative Poisson’s ratio polyester

fibers, Textile Research Journal, 2006, 76(7), 540-546.• Philip J. McMullan, Satish Kumar, Anselm C. Griffin, Textile Fibres Engineered from Molecular

Auxetic Polymers, Project M04-GT21 National Textile Center, Annual Report 2006, p1-10. • Mouritz A., Bannister M., Falson P., Leong K., Review of applications for advanced three dimensional

fibre textile composites, Composites. Part : Applied Science and Manufacturing, 1999, 30, 1445-1461.• Smith C., Grima J., Evans K., A novel mechanism for generating auxetic behaviour in reticulated

foam: Missing rib foam model, Acta Materiala, 2000, 48, 4349-4356.• Leong K., Ramakrishna S., Huang Z., Bibo G., The potential of knitting for engineering composites,

Composites. Part A, 2000, 31, 197-220.• B. Gommers, I. Verpoest , P. Van Houtte, Analysis of knitted fabric reinforced composites: Part I.

Fibre orientation distribution, Composites. Part : Applied Science and Manufacturing, 1998, 29, 1579-1588.

• Rangaswamy Venkatraj, Net Fabrics. The Indian Textile Journal, 1996, Sept., 46-51.• Whitty J.P.M., Alderson A., Myler P., Kandola B., Towards the design of sandwich panel composites

with enhanced mechanical and thermal properties by variation of the in-plane Poisson’s ratios. Composites. Part : Applied Science and Manufacturing, 2003, 34, 525-534.

• Gaspar N., Ren X.J., Smith C.W., Grima J.N., Evans K.E., Novel honeycombs with auxetic behaviour, Acta Materiala, 2005, 53, 2439-2445.

• Ugbolue, S.C., Warner, S. B., Kim, Y.K., Fan, Q., and Yang, Chen Lu, The Formation and Performance of Auxetic Textiles, NTC Project F06-MD09 , National Textile Center Annual Report, November 2006.

• Ugbolue, S.C., Warner, S. B., Kim, Y.K., Fan, Q., and Yang, Chen Lu, Olena Kyzymchuk and Yani Feng, The Formation and Performance of Auxetic Textiles, NTC Project F06-MD09 , National Textile Center Annual Report, September 2007.

Thanks!