Auxetic material
Auxetic materials (from ancient Greek αὐξητός auxetos , German 'stretchable' ) have the unusual property of expanding at right angles to the direction of stretching. They are therefore characterized by a negative Poisson's number ( Poisson's ratio):
description
Auxetic materials include highly stretched Teflon and the skin of cow teats . Auxetic behavior has been known since the beginning of the 20th century.
One of the first artificially produced auxetic materials, the RFS structure (diamond-fold structure), was invented in 1978 by the Berlin researcher Karsten Pietsch. Although he did not use the term auxetics, he describes for the first time the underlying lever mechanism and its non-linear mechanical reactions and is therefore considered the inventor of the auxetic network shown in the graphic above and the formula of auxetic transverse expansion derived from the Pythagorean theorem :
Theorem of auxetic transverse expansion: (q = transverse expansion, c = length of the lever arm, z = the length of the acting pulling movement)
The dynamics and kinematics of the auxetic lever mechanics from the smallest to the largest expansion can be derived from the theorem of auxetic transverse expansion, taking into account the laws of levers (toggle levers) .
In later publications he relativized the term auxetic by stating that there cannot be a solid material made from a basic material with a negative Poisson's number, since an auxetic material consists of at least two different basic materials and its auxetic effect is always due to a lever-mechanical reaction . This is why the calculation of the transverse contraction according to Siméon Denis Poisson in strength theory cannot simply be applied to auxetic materials. This is only defined for solid materials that consist of a single basic material.
Auxetic materials usually consist of basic materials that themselves have a neutral or positive Poisson's number.
The negative Poisson's number has only limited applications, as it is usually only found in porous or composite materials that allow a change in volume and whose lever mechanical reaction shows a force / path inversion. A negative Poisson number can be calculated, but it has no meaningful relevance. The principle of auxetic materials was first described in detail in 1987 in the science magazine Science .
Auxetic behavior can arise at the molecular or macro level. It can be seen in various mineral cuts. These include molybdenum (IV) sulfide , graphite , labradorite, and augite . Auxetic behavior can also be demonstrated with appropriately cut cristobalite thin sections, zinc and polypropylene .
construction
Normal materials, when pulled apart, get thinner in the middle as they stretch in the direction of the pull. Auxetic materials, on the other hand, increase in cross-section when pulled; they widen transversely to the direction of pull.
Three-dimensional auxetic materials expand in all directions across the direction of pull. These are mostly macro-structures that give the material the auxetic properties, not a property at the molecular level. There are materials that naturally have such structures and materials that are artificially brought into an auxetic structure, such as the diamond-fold structure (RFS), which can be made from various basic materials.
application
Auxetic materials are particularly suitable for lightweight construction applications due to their mechanically determined intelligent structural behavior.
Based on the auxetic mechanics, which significantly the mechanical structural parameters such as z. B. Stiffness, thermal and vibration behavior, energy absorption capacity or toughness characterizes and has an above-average positive influence, one achieves performance levels with the same or lighter weight that cannot be achieved with conventional construction methods.
In addition, auxetic mechanics enable completely new functionalities and design solutions for a large number of innovative products with specifically adjustable functional properties. Therefore, auxetic construction approaches are of great importance from a scientific as well as economic and design point of view.
Artificial lungs made from auxetic materials can take in more oxygen and give off carbon dioxide. Also in the pharmaceutical industry are conceivable applications: If installed in a plaster one filled with drugs mini-depot, would at a swelling released the drug of the wound because the diminishing by the pressure section would push out the substance contained.
It is conceivable to manufacture bulletproof vests from auxetic materials. Conventional safety vests are made of fibers that distribute the force of the projectile over a large area and thereby reduce the penetration power. Vests made of auxetic materials would suddenly harden upon impact and thus distribute the impulse over an even larger area.
Web links
- Negative Poisson's ratio material (English)
- Auxetic molecules
- Report about auxetics on ORF.at
- Wunder-Stoff fascinates with bizarre properties on welt.de on November 27, 2011
- About auxetic materials (English)
Individual evidence
- ↑ RFS structure (Rauten-Falt-Structure) , In: Materialblog.de
- ^ Maria Burke: A stretch of the imagination . In: New Scientist . tape 154 , no. 2085 , 1997, pp. 36-39 ( HTML [accessed December 6, 2011]). HTML ( Memento of the original from August 26, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Roderic Lakes: Foam Structures with a Negative Poisson's Ratio . In: Science . tape 235 , no. 4792 , 1987, pp. 1038-1040 , doi : 10.1126 / science.235.4792.1038 .
- ↑ S. P Tokmakova: Stereographic projections of Poisson's ratio in auxetic crystals . In: physica status solidi (b) . tape 242 , no. 3 , 2005, p. 721-729 , doi : 10.1002 / pssb.200460389 .
