Auxetic material

Surprisingly, many cubic metals behave in a similar fashion, although the effect has been masked by the fact that the mechanical properties are most often measured on poly crystalline solids. For example, the commonplace metallic alloy /3-brass, CuZn, which has the CsCl structure, is noticeably auxetic. A tensile stress applied along the [001] direction 

It has been found that many cubic metals with stmctures related to the body-centred cubic stmcture are, in fact, auxetic. A number of silicates also show this unusual property. In each case, the relationship between the bonding and stmcture controls the response to the tensile stress. 

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In this section, three illustrative examples of the impact of scale on mechanical properties are outlined. First, solid lubricants are discussed, underlining the connection between crystal structure and the observed mechanical properties. Second, auxetic materials, in which crystal structure and microsttuc-ture combine to produce materials with negative values of Poisson s ratio, are discussed. Last, thin films, in which mechanical properties are measured by methods similar to that used in the bulk, are considered. 

The negative sign is to ensure that the numerical value of Poisson s ratio is positive for a normal material that becomes thinner as it is stressed. Auxetic materials have a negative value of is, and become fatter when stressed. 

Auxetic materials have a unique property— when they are stretched, they actually become thicker in the directions perpendicular to the applied force Think about this in comparison to anything else you stretch it is really quite a stremge phenomenon. This occurs as a result of hinged arrangements within the materied that flex apart when a... 

In a realistic situation the adhesive filament will not act as a perfect elastic body uniformly stressed up to fracture. Uneven stress distributions and plastic yielding would be expected to increase the energy dissipation observed beyond that calculated for the ideal elastic model. It will be very interesting to see whether in the future auxetic materials can be developed to an extent that they can be used as coatings for such porous substrates. Even greater increases in fracture energy can then be anticipated. 

R H Baughman, Auxetic materials Avoiding the shrink. Nature 425, 667 (16 October 2003) doi 10.1038/425667a... 

Figure 2.53. Illustration of the deformation modes exhibited by an auxetic material. These materials possess hinge-like structures that flex upon elongation.

The work on auxetic polymers arising from microporous structure has continued. Aider-son et al. [120], in an attempt to produce auxetic material in a more easily usable form, have used melt-spinning to produce auxetic polypropylene fibres. This work was developed further, by way of a study of the processing parameters for melt-spinning of auxetic polypropylene, polyester and nylon fibres [121]. Ravirala et al. [122] have produced auxetic polypropylene film using melt extrusion. Less conventionally, Alderson et al. [123] have produced auxetic polyethylene by a combination of powder compaction and sintering, without an extrusion step. 

Fig. A6. A model of an auxetic material, i material with negative Poisson ratio...

Most materials have o between 0.0 and 0.5. Cork is close to 0.0, polysilicon is around 0.22, single crystal silicon is around 0.28, most steels are around 0.3, and rubber is almost 0.5. A perfectly incompressible material deformed elastically at small strains would have a Poisson s ratio of exactly 0.5. Some materials, mostly polymer foams, have a negative Poisson s ratio if these auxetic materials are stretched in one direction, they become thicker in perpendicular directions. 

Table 15.1 shows the Poisson s ratios of some materials. An incompressible material that does not change volmne when elongates would have a Poisson s ratio of exactly 0.5. For example, rabber has a Poisson s ratio of nearly 0.5. Cork s Poisson ratio is close to 0, indicating very little transverse contract when stretched. Auxetic materials, such as polymer foams, have negative Poisson s ratios, and they become thicker in perpendicular directions when stretched. Most other materials, including metals, ceramics, and polymers, have Poisson s ratios ranging from 0 to 5. 

Materials with NPRs have superior properties when compared with conventional materials (Choi and Lakes 1996 Wang and Lakes 2002 Scarpa et al. 2005 Bezazi and Scarpa 2009). For example, upon impact, the material in auxetic systems flows toward the point of impact (Chan and Evans 1998) resulting in a more dense system. This is in opposition to conventional materials where the material tends to flow away from the point of impact resulting in a less dense system. This means that auxetic materials tend to show a higher impact resistance (Alderson 1999) (see Figure 10.2a). This feature is potentially applicable in personal protection equipment such as... 

Owing to these enhanced material properties, auxetic materials can be used in a number of applications. Some proposed applications include the use of auxetic materials in rivets, gaskets, and fasteners to provide a stiffer grip upon loading (Evans 1991) and their use in fiber-reinforced composites. The strength of such composites... 

FIGURE 10.2 (a) Indentation resistance of auxetic materials compared with a conventional... 

Grima, J. N. New auxetic materials, Ph.D. Thesis, University of Exeter, Exeter, UK, 2000. 

Wojciechowski, K. W. and Brartka, A. C. Auxetics — Materials and models with negative Poisson s ratios. Mol. Phys. Rep. 6, 1994, 71-85. 

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