5. Auxetic Materials:
5.1) Characterizes the material from other smart and ordinary materials:
Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecules or a particular structure of macroscopic matter. Such materials are expected to have mechanical properties such as high energy absorption and fracture resistance.
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An Explaining video



5.2)Thehistory of the material:
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.
The term auxetic derives from the Greek word αὐξητικός (auxetikos) which means "that which tends to increase" and has its root in the word αὔξησις, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.





5.3) Advantages do auxetic materials offer:

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.
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5.4)Current industrial applications of the material:
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. Auxetic metals could be used as electrodes sandwiching a piezoelectric polymer, or piezoelectric ceramic 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 and electromechanical devices is also being investigated.
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. 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 n 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 v.

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, (table 1), together with a list of the applications in which these characteristics could offer significant benefits. For example, by analogy with the filter de-fouling scenario of figure 4, biomedical fibrous drug-release materials could be made from auxetic fibres. Extending the fibers opens the microspores and a specific dose of drug is released.
Advanced auxetic fibers will include multi-filament yarns in which an auxetic filament is wrapped with one or more other yarns, perhaps high stiffness/strength, dye able or conductive filaments, so that the benefits of the auxetic material are combined with other beneficial properties for smart technical textiles applications. This will lead to the possibility of hierarchical composites displaying auxetic behavior at more than one length scale. Current research on auxetic composites is concentrated on the use of non-auxetic constituents and so benefits due to the auxetic effect occur at a macro structural level. Employing auxetic fibers as the reinforcement will enable benefits, such as impact energy and acoustic energy absorption, to be achieved at the micro structural level.






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
Absorb.
Densif.
& Indent.
Resist
Impact
Resist
Composite Materials
Auxetic fiber reinforcement in composites
X
X
X
X
X
Personal Protection Clothing
Crash helmets, body armor, sports clothing


X
X
X
Filtration
Woven structures using auxetic fibers





Mechanical Lungs
Micro porous 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.
Micro porous
Breathable
Structure
Constant
Pressure
Structure
Composite Materials
Auxetic fiber reinforcement in composites




Personal Protection Clothing
Crash helmets, body armor, sports clothing


X
X
Filtration
Woven structures using auxetic fibers
X

X
X
Mechanical Lungs
Micro porous 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 readings:
For:
Types Of Materials Can Exhibit Auxetic Behavior

Production of Auxetic Materials


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

References:
5.1) 2 December 2011 .Retrieved February 26, 2012 ,from
http://en.wikipedia.org/wiki/Auxetics
Retrieved February 26, 2012,from
www.youtube.com
5.2)Mar 9, 2001
2 December 2011 Mar 9, 2001
Mar 9, 2001
.Retrieved February 26, 2012, from
http://en.wikipedia.org/wiki/Auxetics
5.3) J.N. Grima, March 2000. Retrieved February 26, 2012, from
http://groups.exeter.ac.uk/auxetic/auxetic_f2.html
5.4.1), 5.4.2) &5.4.3)
P.J. Stott, R. Mitchell, K. Alderson and A. Alderson. March 9, 2001. Auxetic Materials - An Introduction
Auxetic Materials - An Introduction
Auxetic material-introduction.Retrieved February 26, 2012, fromAuxetic Materials - An IntroductionMar 9, 2001 Mar 9, 2001
http://www.azom.com/article.aspx?ArticleID=168
Done by :Fady Nagy