- Poisson's ratio
Poisson's ratio ("ν"), named after
Simeon Poisson, is the ratio of the relative contraction strain, or transverse strain (normal to the applied load), divided by the relative extension strain, or axial strain (in the direction of the applied load).
When a sample of material is stretched in one direction, it tends to contract (or rarely, expand) in the other two directions. Conversely, when a sample of material is compressed in one direction, it tends to expand (or rarely, contract) in the other two directions. Poisson's ratio (ν) is a measure of this tendency.
The Poisson's ratio of a stable material cannot be less than −1.0 nor greater than 0.5 due to the requirement that the
shear modulusand bulk modulushave positive values. Most materials have between 0.0 and 0.5. Cork is close to 0.0, showing almost no Poisson contraction, most steels are around 0.3, and rubber is nearly incompressible and so has a Poisson ratio of nearly 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.
Assuming that the material is compressed along the axial direction:
where: is the resulting Poisson's ratio,: is transverse strain (negative for axial tension, positive for axial compression): is axial strain (positive for axial tension, negative for axial compression).
Cause of Poisson’s effect
On the molecular level, Poisson’s effect is caused by slight movements between molecules and the stretching of molecular bonds within the material lattice to accommodate the stress. When the bonds elongate in the stress direction, they shorten in the other directions. This behavior multiplied millions of times throughout the material lattice is what drives the phenomenon.
Generalized Hooke's law
For an isotropic material, the deformation of a material in the direction of one axis will produce a deformation of the material along the other axes in three dimensions. Thus it is possible to generalize
Hooke's Lawinto three dimensions::
:where:, and are strain in the direction of , and axis: , and are stress in the direction of , and axis: is
Young's modulus(the same in all directions: , and for isotropic materials): is Poisson's ratio (the same in all directions: , and for isotropic materials)
The relative change of volume "ΔV"/"V" due to the stretch of the material can be calculated using a simplified formula (only for small deformations):
where: is material volume: is material volume change: is original length, before stretch: is the change of length:
If a rod with diameter (or width, or thickness) "d" and length "L" is subject to tension so that its length will change by "ΔL" then its diameter "d" will change by (the value is negative, because the diameter will decrease with increasing length):
The above formula is true only in the case of small deformations; if deformations are large then the following (more precise) formula can be used:
where: is original diameter: is rod diameter change: is Poisson's ratio: is original length, before stretch: is the change of length.
Orthotropic material, such as wood in which Poisson's ratio is different in each direction (x, y and z axis) the relation between Young's modulus and Poisson's ratio is described as follows:
where: is a
Young's modulusalong axis i: is a Poisson's ratio in plane jk
Poisson's ratio values for different materials
glasscomponent additions on Poisson's ratio of a specific base glass. [ [http://www.glassproperties.com/poisson_ratio/ Poisson's ratio calculation of glasses] ] ]
Negative Poisson's ratio materials
Some materials known as
auxeticmaterials display a negative Poisson’s ratio. When subjected to strain in a longitudinal axis, the transverse strain in the material will actually be positive (i.e. it would increase in cross sectional area). For these materials, it is usually due to uniquely oriented, hinged molecular bonds. In order for these bonds to stretch in the longitudinal direction, the hinges must ‘open’ in the transverse direction, effectively exhibiting a positive strain. [ [http://silver.neep.wisc.edu/~lakes/Poisson.html Negative Poisson's ratio ] ]
Applications of Poisson's effect
One area in which Poisson's effect has a considerable influence is in pressurized pipe flow. When the air or liquid inside a pipe is highly pressurized it exerts a uniform force on the inside of the pipe, resulting in a radial stress within the pipe material. Due to Poisson's effect, this radial stress will cause the pipe to slightly increase in diameter and decrease in length. The decrease in length, in particular, can have a noticeable effect upon the pipe joints, as the effect will accumulate for each section of pipe joined in series. A restrained joint may be pulled apart or otherwise prone to failure. [http://www.cpchem.com/hb/getdocanon.asp?doc=135&lib=CPC-Portal]
Another area of application for Poisson's effect is in the realm of
structural geology. Rocks, just as most materials, are subject to Poisson's effect while under stress and strain. In a geological timescale, excessive erosion or sedimentation of Earth's crust can either create or remove large vertical stresses upon the underlying rock. This rock will expand or contract in the vertical direction as a direct result of the applied stress, and it will also deform in the horizontal direction as a result of Poisson's effect. This change in strain in the horizontal direction can affect or form joints and dormant stresses in the rock. [http://www.geosc.psu.edu/~engelder/geosc465/lect18.rtf]
Impulse excitation technique
Coefficient of thermal expansion
* [http://silver.neep.wisc.edu/~lakes/PoissonIntro.html Meaning of Poisson's ratio]
* [http://silver.neep.wisc.edu/~lakes/Poisson.html Negative Poisson's ratio materials]
* [http://home.um.edu.mt/auxetic More on negative Poisson's ratio materials (auxetic)]
* [http://www.webelements.com/webelements/properties/text/definitions/poissons-ratio.html Poisson's ratio]
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