In materials science and solid mechanics, Poisson's ratioν (nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Poisson's ratio is the negative of the ratio of transverse strain to axial strain. For small values of these changes, ν is the amount of transversal elongation divided by the amount of axial compression. Most materials have Poisson's ratio values ranging between 0.0 and 0.5. For soft materials,^{[1]} such as rubber, where the bulk modulus is much higher than the shear modulus, Poisson's ratio is near 0.5. For open-cell polymer foams, Poisson's ratio is near zero, since the cells tend to collapse in compression. Many typical solids have Poisson's ratios in the range of 0.2 to 0.3. The ratio is named after the French mathematician and physicist Siméon Poisson.
Originedit
Poisson's ratio is a measure of the Poisson effect, the phenomenon in which a material tends to expand in directions perpendicular to the direction of compression. Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions transverse to the direction of stretching. It is a common observation when a rubber band is stretched, it becomes noticeably thinner. Again, the Poisson ratio will be the ratio of relative contraction to relative expansion and will have the same value as above. In certain rare cases,^{[2]} a material will actually shrink in the transverse direction when compressed (or expand when stretched) which will yield a negative value of the Poisson ratio.
The Poisson's ratio of a stable, isotropic, linear elastic material must be between −1.0 and +0.5 because of the requirement for Young's modulus, the shear modulus and bulk modulus to have positive values.^{[3]} Most materials have Poisson's ratio values ranging between 0.0 and 0.5. A perfectly incompressible isotropic material deformed elastically at small strains would have a Poisson's ratio of exactly 0.5. Most steels and rigid polymers when used within their design limits (before yield) exhibit values of about 0.3, increasing to 0.5 for post-yield deformation which occurs largely at constant volume.^{[4]} Rubber has a Poisson ratio of nearly 0.5. Cork's Poisson ratio is close to 0, showing very little lateral expansion when compressed and glass is between 0.18 and 0.30. Some materials, e.g. some polymer foams, origami folds,^{[5]}^{[6]} and certain cells can exhibit negative Poisson's ratio, and are referred to as auxetic materials. If these auxetic materials are stretched in one direction, they become thicker in the perpendicular direction. In contrast, some anisotropic materials, such as carbon nanotubes, zigzag-based folded sheet materials,^{[7]}^{[8]} and honeycomb auxetic metamaterials^{[9]} to name a few, can exhibit one or more Poisson's ratios above 0.5 in certain directions.
Assuming that the material is stretched or compressed in only one direction (the x axis in the diagram below):
and positive strain indicates extension and negative strain indicates contraction.
Poisson's ratio from geometry changesedit
Length changeedit
For a cube stretched in the x-direction (see Figure 1) with a length increase of ΔL in the x-direction, and a length decrease of ΔL′ in the y- and z-directions, the infinitesimal diagonal strains are given by
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 it decreases with increase of length
Characteristic materialsedit
Isotropicedit
For a linear isotropic material subjected only to compressive (i.e. normal) forces, the deformation of a material in the direction of one axis will produce a deformation of the material along the other axis in three dimensions. Thus it is possible to generalize Hooke's Law (for compressive forces) into three dimensions:
Here ν is Poisson's ratio, E is Young's modulus, n is a unit vector directed along the direction of extension, m is a unit vector directed perpendicular to the direction of extension. Poisson's ratio has a different number of special directions depending on the type of anisotropy.^{[11]}^{[12]}
Orthotropicedit
Orthotropic materials have three mutually perpendicular planes of symmetry in their material properties. An example is wood, which is most stiff (and strong) along the grain, and less so in the other directions.
G_{ij} is the shear modulus in direction j on the plane whose normal is in direction i
ν_{ij} is the Poisson ratio that corresponds to a contraction in direction j when an extension is applied in direction i.
The Poisson ratio of an orthotropic material is different in each direction (x, y and z). However, the symmetry of the stress and strain tensors implies that not all the six Poisson's ratios in the equation are independent. There are only nine independent material properties: three elastic moduli, three shear moduli, and three Poisson's ratios. The remaining three Poisson's ratios can be obtained from the relations
From the above relations we can see that if E_{x} > E_{y} then ν_{xy} > ν_{yx}. The larger ratio (in this case ν_{xy}) is called the major Poisson ratio while the smaller one (in this case ν_{yx}) is called the minor Poisson ratio. We can find similar relations between the other Poisson ratios.
Transversely isotropicedit
Transversely isotropic materials have a plane of isotropy in which the elastic properties are isotropic. If we assume that this plane of isotropy is the yz-plane, then Hooke's law takes the form^{[15]}
This leaves us with six independent constants E_{x}, E_{y}, G_{xy}, G_{yz}, ν_{xy}, ν_{yz}. However, transverse isotropy gives rise to a further constraint between G_{yz} and E_{y}, ν_{yz} which is
Therefore, there are five independent elastic material properties two of which are Poisson's ratios. For the assumed plane of symmetry, the larger of ν_{xy} and ν_{yx} is the major Poisson ratio. The other major and minor Poisson ratios are equal.
Poisson's ratio values for different materialsedit
Some materials known as auxetic materials display a negative Poisson's ratio. When subjected to positive strain in a longitudinal axis, the transverse strain in the material will actually be positive (i.e. it would increase the 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.^{[19]}
This can also be done in a structured way and lead to new aspects in material design as for mechanical metamaterials.
Studies have shown that certain solid wood types display negative Poisson's ratio exclusively during a compression creep test.^{[20]}^{[21]} Initially, the compression creep test shows positive Poisson's ratios, but gradually decreases until it reaches negative values. Consequently, this also shows that Poisson's ratio for wood is time-dependent during constant loading, meaning that the strain in the axial and transverse direction do not increase in the same rate.
Media with engineered microstructure may exhibit negative Poisson's ratio. In a simple case auxeticity is obtained removing material and creating a periodic porous media.^{[22]} Lattices can reach lower values of Poisson's ratio,^{[23]} which can be indefinitely close to the limiting value −1 in the isotropic case.^{[24]}
More than three hundred crystalline materials have negative Poisson's ratio.^{[25]}^{[26]}^{[27]} For example, Li, Na, K, Cu, Rb, Ag, Fe, Ni, Co, Cs, Au, Be, Ca, Zn Sr, Sb, MoS_{2} and others.
Poisson functionedit
At finite strains, the relationship between the transverse and axial strains ε_{trans} and ε_{axial} is typically not well described by the Poisson ratio. In fact, the Poisson ratio is often considered a function of the applied strain in the large strain regime. In such instances, the Poisson ratio is replaced by the Poisson function, for which there are several competing definitions.^{[28]} Defining the transverse stretch λ_{trans} = ε_{trans} + 1 and axial stretch λ_{axial} = ε_{axial} + 1, where the transverse stretch is a function of the axial stretch, the most common are the Hencky, Biot, Green, and Almansi functions:
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 hoop stress within the pipe material. Due to Poisson's effect, this hoop stress will cause the pipe to increase in diameter and slightly 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.^{[citation needed]}
Another area of application for Poisson's effect is in the realm of structural geology. Rocks, like most materials, are subject to Poisson's effect while under stress. 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.^{[29]}
Although cork was historically chosen to seal wine bottle for other reasons (including its inert nature, impermeability, flexibility, sealing ability, and resilience),^{[30]} cork's Poisson's ratio of zero provides another advantage. As the cork is inserted into the bottle, the upper part which is not yet inserted does not expand in diameter as it is compressed axially. The force needed to insert a cork into a bottle arises only from the friction between the cork and the bottle due to the radial compression of the cork. If the stopper were made of rubber, for example, (with a Poisson's ratio of about +0.5), there would be a relatively large additional force required to overcome the radial expansion of the upper part of the rubber stopper.
Most car mechanics are aware that it is hard to pull a rubber hose (such as a coolant hose) off a metal pipe stub, as the tension of pulling causes the diameter of the hose to shrink, gripping the stub tightly. (This is the same effect as shown in a Chinese finger trap.) Hoses can more easily be pushed off stubs instead using a wide flat blade.
^For soft materials, the bulk modulus (K) is typically large compared to the shear modulus (G) so that they can be regarded as incompressible, since it is easier to change shape than to compress. This results in the Young's modulus (E) being E = 3G and hence ν = 0.5.Jastrzebski, D. (1959). Nature and Properties of Engineering Materials (Wiley International ed.). John Wiley & Sons, Inc.
^Lakes, R.; Wojciechowski, K. W. (2008). "Negative compressibility, negative Poisson's ratio, and stability". Physica Status Solidi B. 245 (3): 545–551. Bibcode:2008PSSBR.245..545L. doi:10.1002/pssb.200777708.
^Gercek, H. (January 2007). "Poisson's ratio values for rocks". International Journal of Rock Mechanics and Mining Sciences. 44 (1): 1–13. Bibcode:2007IJRMM..44....1G. doi:10.1016/j.ijrmms.2006.04.011.
^Park, R. J. T. Seismic Performance of Steel-Encased Concrete Piles.^{[full citation needed]}
^Mark, Schenk (2011). Folded Shell Structures, PhD Thesis(PDF). University of Cambridge, Clare College.
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^Epishin, A. I.; Lisovenko, D. S. (2016). "Extreme values of Poisson's ratio of cubic crystals". Technical Physics. 61 (10): 1516–1524. Bibcode:2016JTePh..61.1516E. doi:10.1016/j.mechmat.2019.03.017. S2CID 140493258.
^Gorodtsov, V.A.; Lisovenko, D.S. (2019). "Extreme values of Young's modulus and Poisson's ratio of hexagonal crystals". Mechanics of Materials. 134: 1–8. Bibcode:2019MechM.134....1G. doi:10.1016/j.mechmat.2019.03.017. S2CID 140493258.
^Boresi, A. P; Schmidt, R. J.; Sidebottom, O. M. (1993). Advanced Mechanics of Materials. Wiley.^{[page needed]}
^Lekhnitskii, S. G. (1981). Theory of elasticity of an anisotropic elastic body. Mir Publishing. p. 36.
^Tan, S. C. (1994). Stress Concentrations in Laminated Composites. Lancaster, PA: Technomic Publishing Company.^{[page needed]}
^Fluegel, Alexander. "Poisson's Ratio Calculation for Glasses". www.glassproperties.com. Archived from the original on 23 October 2017. Retrieved 28 April 2018.
^P. H. Mott; C. M. Roland (20 October 2009). "Limits to Poisson's ratio in isotropic materials" (PDF). Physical Review B. 80 (13): 132104. arXiv:0909.4697. Bibcode:2009PhRvB..80m2104M. doi:10.1103/PhysRevB.80.132104. Archived (PDF) from the original on 2014-10-31. Retrieved 2014-09-24.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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^Ozyhar, Tomasz; Hering, Stefan; Niemz, Peter (March 2013). "Viscoelastic characterization of wood: Time dependence of the orthotropic compliance in tension and compression". Journal of Rheology. 57 (2): 699–717. Bibcode:2013JRheo..57..699O. doi:10.1122/1.4790170. ISSN 0148-6055.
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External linksedit
Meaning of Poisson's ratio
Negative Poisson's ratio materials
More on negative Poisson's ratio materials (auxetic) Archived 2018-02-08 at the Wayback Machine
Conversion formulae
Homogeneous isotropic linear elastic materials have their elastic properties uniquely determined by any two moduli among these; thus, given any two, any other of the elastic moduli can be calculated according to these formulas, provided both for 3D materials (first part of the table) and for 2D materials (second part).
3D formulae
$K=\,$
$E=\,$
$\lambda =\,$
$G=\,$
$\nu =\,$
$M=\,$
Notes
$(K,\,E)$
${\tfrac {3K(3K-E)}{9K-E}}$
${\tfrac {3KE}{9K-E}}$
${\tfrac {3K-E}{6K}}$
${\tfrac {3K(3K+E)}{9K-E}}$
$(K,\,\lambda )$
${\tfrac {9K(K-\lambda )}{3K-\lambda }}$
${\tfrac {3(K-\lambda )}{2}}$
${\tfrac {\lambda }{3K-\lambda }}$
$3K-2\lambda \,$
$(K,\,G)$
${\tfrac {9KG}{3K+G}}$
$K-{\tfrac {2G}{3}}$
${\tfrac {3K-2G}{2(3K+G)}}$
$K+{\tfrac {4G}{3}}$
$(K,\,\nu )$
$3K(1-2\nu )\,$
${\tfrac {3K\nu }{1+\nu }}$
${\tfrac {3K(1-2\nu )}{2(1+\nu )}}$
${\tfrac {3K(1-\nu )}{1+\nu }}$
$(K,\,M)$
${\tfrac {9K(M-K)}{3K+M}}$
${\tfrac {3K-M}{2}}$
${\tfrac {3(M-K)}{4}}$
${\tfrac {3K-M}{3K+M}}$
$(E,\,\lambda )$
${\tfrac {E+3\lambda +R}{6}}$
${\tfrac {E-3\lambda +R}{4}}$
${\tfrac {2\lambda }{E+\lambda +R}}$
${\tfrac {E-\lambda +R}{2}}$
$R={\sqrt {E^{2}+9\lambda ^{2}+2E\lambda }}$
$(E,\,G)$
${\tfrac {EG}{3(3G-E)}}$
${\tfrac {G(E-2G)}{3G-E}}$
${\tfrac {E}{2G}}-1$
${\tfrac {G(4G-E)}{3G-E}}$
$(E,\,\nu )$
${\tfrac {E}{3(1-2\nu )}}$
${\tfrac {E\nu }{(1+\nu )(1-2\nu )}}$
${\tfrac {E}{2(1+\nu )}}$
${\tfrac {E(1-\nu )}{(1+\nu )(1-2\nu )}}$
$(E,\,M)$
${\tfrac {3M-E+S}{6}}$
${\tfrac {M-E+S}{4}}$
${\tfrac {3M+E-S}{8}}$
${\tfrac {E-M+S}{4M}}$
$S=\pm {\sqrt {E^{2}+9M^{2}-10EM}}$
There are two valid solutions.
The plus sign leads to $\nu \geq 0$.
The minus sign leads to $\nu \leq 0$.
$(\lambda ,\,G)$
$\lambda +{\tfrac {2G}{3}}$
${\tfrac {G(3\lambda +2G)}{\lambda +G}}$
${\tfrac {\lambda }{2(\lambda +G)}}$
$\lambda +2G\,$
$(\lambda ,\,\nu )$
${\tfrac {\lambda (1+\nu )}{3\nu }}$
${\tfrac {\lambda (1+\nu )(1-2\nu )}{\nu }}$
${\tfrac {\lambda (1-2\nu )}{2\nu }}$
${\tfrac {\lambda (1-\nu )}{\nu }}$
Cannot be used when $\nu =0\Leftrightarrow \lambda =0$