5. Auxetic Materials:


5.1 : Characteristics:
- Auxetics are materials that have a negative Poisson's ratio.

- Since Poisson’s ration is the ratio between the contractile lateral strain and the tensile axial strain “for a material stretched axially” , So it is normally + ve.
But in Auxtics, it is - ve as during stretching the material from one dimension, the other perpendicular dimension becomes thicker.

conv-auxetic.jpg






5.2 : History:
- The term auxetic derives from the Greek word αξητικός (auxetikos) which means "that which tends to increase" and has its root in the word αξησις, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.
- Scientists have known about auxetic materials for over 100 years, but have only recently given them special attention. The earliest published example of a synthetic auxetic material was in Science in 1987, entitled "Foam structures with a Negative Poisson's Ratio" by R.S. Lakes from the University of Iowa. The use of the word auxetic to refer to this property probably began in 1991.

5.3 :Advantages of Auxetic materials:
Apart from the scientific value of having materials with a negative Poisson`s ration, auxetic materials show enhanced mechanical properties such as:
    • increased shear stiffness
    • increased plane strain fracture toughness
    • increased indentation resistance
Which make them superior to classical materials for many practical applications.


5.4 : Applications:
5.4.1: Biomedical Industry:
Key areas of application are seen in the biomedical field. Prosthetic materials, surgical implants, suture/muscle/ligament anchors and a dilator to open up blood vessels during heart surgery are all possible. Another area relates to the use of auxetic materials in piezoelectric sensors and actuators.
5.4.2: 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. 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. 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 Poisson’s ratio is dependent on the stretching direction. The studies clearly demonstrate that de-fouling is enhanced when the filter is loaded in the direction with the largest negative Poisson’s ratio.
5.4.3:Auxetic Fibers:
The breakthrough development of a continuous process to produce auxetic materials in fibrous form has created the opportunity to apply their unique characteristics in a wide range of applications previously not possible. Fibers can be used in single or multiple filament structures and can be used to produce a woven structure. Typical performance characteristics expected of auxetic fibers and structures are detailed in the table of applications, together with a list of the applications in which these characteristics could offer significant benefits.




Table 1 (Part A). Application-performance characteristic matrix for auxetic fibers and auxetic fiber based structures.
|| Application
Fiber

Pull-Out

Resist.
Fiber

Fracture

Tough
Energy

Absorp.
Densif.

& Indent.

Resist
Impact

Resist
Composite Materials

Auxetic fibre reinforcement in composites

X

X
X
X
X
Personal Protection Clothing

Crash helmets, body armour, sports clothing


X
X
X
Fitration

Woven structures using auxetic fibres





Mechanical Lungs

Microporous hollow auxetic structures





Ropes, Cords & Fishnets

High strength, lower weight

X



Upholstery Fabrics

Enhanced abrasion properties & entrapment of fire retardant components


X
X

Biomedical

Controlled release of drugs





Medical Bandages

Prevents swelling of wound by application of wound healing agent





Fibrous Seals


X
X
X







Table 1 (Part B). Application-performance characteristic matrix for auxetic fibers and auxetic fiber based structures.
|| Application
Release

Entrapped

Particles
Wear

Resist.
Microporous

Breathable

Structure
Constant

Pressure

Structure
Composite Materials

Auxetic fibre reinforcement in composites




Personal Protection Clothing

Crash helmets, body armour, sports clothing


X
X
Fitration

Woven structures using auxetic fibres
X

X
X
Mechanical Lungs

Microporous hollow auxetic structures
X

X

Ropes, Cords & Fishnets

High strength, lower weight

X


Upholstery Fabrics

Enhanced abrasion properties & entrapment of fire retardant components
X
X


Biomedical

Controlled release of drugs
X

X

Medical Bandages

Prevents swelling of wound by application of wound healing agent
X

X

X

Fibrous Seals

X

X




5.5: Links to external material for further reading:

http://www.azom.com/article.aspx?ArticleID=167

http://www.azom.com/article.aspx?ArticleID=168

http://en.wikipedia.org/wiki/Auxetics

http://groups.exeter.ac.uk/auxetic/auxetic_f2.html



Video Link:

http://www.youtube.com/watch?v=nDuR9hHIpZM