Poisson’s ratio measures how stretching or compressing an object in one direction affects its contraction or expansion in other directions. It varies and can even be negative. It is used in different fields of science and has important uses in geology.
Poisson’s ratio concerns how stretching or compressing an object in one direction causes it to compress or stretch in the other direction. The ratio measures the magnitude of this effect in a particular substance. This can vary considerably and the relationship can even be negative, usually in artificial substances.
The technical definition of Poisson’s ratio is “the ratio of transverse contraction strain to longitudinal expansion strain”. It sounds complicated, but it describes a fairly simple effect. To picture this, imagine a piece of rubber, like that used in a rubber band. As you lengthen the band, it becomes both narrower and longer. The relationship between these two changes is what is measured by Poisson’s ratio.
Actually, Poisson’s ratio applies in three dimensions. In the rubber band example, the thickness of the rubber band also decreases—it’s just harder to see. To imagine the effect in three dimensions, imagine taking a cube-shaped pet toy and squeezing two opposite sides. The cube will contract in the direction between these two sides, but expand in the other two directions.
In most cases, Poisson’s ratio is positive, which means that a material stretches in one direction more than it contracts in other directions. There are a couple of explanations for this, using different scientific approaches. One simple explanation is that most materials are better able to resist compression than stretching. A more complicated explanation is that the bonds between the atoms in the structure realign during the process of stretching and compressing.
In most cases, a material’s Poisson’s ratio will be between 0 and 0.5. Among common materials, rubber has a Poisson’s ratio very close to 0.5, while steel has one of 0.3 and cork is much closer to 0. This is why wine stoppers are made of cork: it can withstand the bottle neck pressure without vertically stretching and locking into place.
It is possible to have a negative Poisson’s ratio. Materials that exhibit this quality are known as auxetics. With such materials, stretching them in one direction will cause them to expand in other directions. Living bone tissue is suspected to be an auxetic, although it is difficult to prove. There are also several man-made auxetic substances, most notably the polymers used in Gore-Tex waterproof clothing.
Poisson’s ratio is used in more complicated ways in different fields of science. When you bend an object in one direction, Poisson’s ratio affects how well the object curves in the perpendicular direction. The ratio also affects how stress waves travel through substances like rock, meaning it has some important uses in geology.
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