Stress
Properties of Matter

Stretching the truth

Stories from Physics for 11-14 14-16 IOP RESOURCES

Strange sorts of elasticity 
Typically, materials become thinner when they are stretched and thicker when they are squashed. However, auxetic materials, or anti-rubber, display the opposite property, becoming thicker when stretched and thinner when squashed. Poisson’s ratio is the ratio of strains perpendicular and parallel to the applied stress for a given sample. Typically, for materials like rubber, Poisson’s ratio is positive, meaning that they get thinner when stretched. By contrast, auxetic materials have a negative Poisson’s ratio. The structure of some auxetic materials consists of two-dimensional bow-tie-shaped cells. When the cells are stretched, the centre of the bow-tie-like structure is forced outwards, causing the material to get thicker.

It is thought auxetic materials may have a number of potential applications because they have excellent sound absorption properties. Alternatively, they may be used to manufacture smart bandages which, if impregnated with a drug, can release a dose that varies depending on the force of the wound on the bandage. 

Negative stiffness
Researchers have developed a material with negative stiffness. A pure tin matrix with inclusions of ferroelastic vanadium dioxide behaves in a counterintuitive way — the direction of deformation of the material can be in the opposite direction to the deforming force. The effect arises because vanadium dioxide at equilibrium stores elastic potential energy so when compressed continues to collapse in the direction of the applied force (imagine a spring that, when compressed, continues to compress by itself). An alternative way to produce the effect of negative stiffness is with a magnetic device: engineers have developed magnetic negative stiffness dampers, which consist of an arrangement of permanent magnets in a conductive pipe. Negative stiffness materials may be used in vibration damping to dissipate mechanical energy

Super stretchy seaweed 
By adding a seaweed extract to a hydrogel, Harvard scientists have developed a substance which can stretch to 20 times its original length. The material is being considered as a replacement for cartilage and for covering wounds.

Elastic conductors 
New types of electrical circuits will be possible in the future due to the development of elastic conductors. The material consists of silver flakes in a rubber matrix that can retain their conductivity even when subjected to strains of 215%.

Stretch bolts
Bolts are designed to be elastic so that they exert a clamping force on the material they are fastening. Bolts are therefore normally designed to be tightened to just below their yield point so that they exert the maximum force, yet retain their elasticity. By contrast, torque-to-yield fasteners, sometimes called stretch bolts, are designed to be tightened so that they exceed their yield point, undergoing plastic deformation to become permanently elongated thus strengthening the joint.

Molecular car tyres   A single molecule of a polymer can contain thousands or even millions of atoms. In the case of rubber, the crosslinking between different polymer molecules can be so extensive that a macroscopic object, for example, a car tyre might be considered a single molecule

References

Stress
appears in the relation σ=F/A E=σ/ε
can be represented by Stress-Strain Graphs
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