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{{Continuum mechanics|cTopic=[[Solid mechanics]]}} | |||
The ''' [[Alan N. Gent|Gent]]''' [[hyperelastic material]] model <ref name=Gent/> is a phenomenological model of [[rubber elasticity]] that is based on the concept of limiting chain extensibility. In this model, the [[strain energy density function]] is designed such that it has a [[mathematical singularity|singularity]] when the first invariant of the left Cauchy-Green deformation tensor reaches a limiting value <math>I_m</math>. | |||
The strain energy density function for the Gent model is <ref name=Gent>Gent, A.N., 1996, '' A new constitutive relation for rubber'', Rubber Chemistry Tech., 69, pp. 59-61.</ref> | |||
:<math> | |||
W = -\cfrac{\mu J_m}{2} \ln\left(1 - \cfrac{I_1-3}{J_m}\right) | |||
</math> | |||
where <math>\mu</math> is the [[shear modulus]] and <math>J_m = I_m -3</math>. | |||
In the limit where <math>I_m \rightarrow \infty</math>, the Gent model reduces to the [[Neo-Hookean solid]] model. This can be seen by expressing the Gent model in the form | |||
:<math> | |||
W = \cfrac{\mu}{2x}\ln\left[1 - (I_1-3)x\right] ~;~~ x := \cfrac{1}{J_m} | |||
</math> | |||
A [[Taylor series expansion]] of <math>\ln\left[1 - (I_1-3)x\right]</math> around <math>x = 0</math> and taking the limit as <math>x\rightarrow 0</math> leads to | |||
:<math> | |||
W = \cfrac{\mu}{2} (I_1-3) | |||
</math> | |||
which is the expression for the strain energy density of a Neo-Hookean solid. | |||
Several '''compressible''' versions of the Gent model have been designed. One such model has the form<ref>Mac Donald, B. J., 2007, '''Practical stress analysis with finite elements''', Glasnevin, Ireland.</ref> | |||
:<math> | |||
W = -\cfrac{\mu J_m}{2} \ln\left(1 - \cfrac{I_1-3}{J_m}\right) + \cfrac{\kappa}{2}\left(\cfrac{J^2-1}{2} - \ln J\right)^4 | |||
</math> | |||
where <math>J = \det(\boldsymbol{F})</math>, <math>\kappa</math> is the [[bulk modulus]], and <math>\boldsymbol{F}</math> is the [[deformation gradient]]. | |||
== Consistency condition == | |||
We may alternatively express the Gent model in the form | |||
:<math> | |||
W = C_0 \ln\left(1 - \cfrac{I_1-3}{J_m}\right) | |||
</math> | |||
For the model to be consistent with [[linear elasticity]], the [[Hyperelastic_material#Consistency_conditions_for_incompressible_I1_based_rubber_materials|following condition]] has to be satisfied: | |||
:<math> | |||
2\cfrac{\partial W}{\partial I_1}(3) = \mu | |||
</math> | |||
where <math>\mu</math> is the [[shear modulus]] of the material. | |||
Now, at <math>I_1 = 3 (\lambda_i = \lambda_j = 1)</math>, | |||
:<math> | |||
\cfrac{\partial W}{\partial I_1} = -\cfrac{C_0}{J_m} | |||
</math> | |||
Therefore, the consistency condition for the Gent model is | |||
:<math> | |||
-\cfrac{2C_0}{J_m} = \mu\, \qquad \implies \qquad C_0 = -\cfrac{\mu J_m}{2} | |||
</math> | |||
The Gent model assumes that <math>J_m \gg 1</math> | |||
== Stress-deformation relations == | |||
The Cauchy stress for the incompressible Gent model is given by | |||
:<math> | |||
\boldsymbol{\sigma} = -p~\boldsymbol{\mathit{1}} + | |||
2~\cfrac{\partial W}{\partial I_1}~\boldsymbol{B} | |||
= -p~\boldsymbol{\mathit{1}} + \cfrac{\mu J_m}{J_m - I_1 + 3}~\boldsymbol{B} | |||
</math> | |||
=== Uniaxial extension === | |||
[[Image:Hyperelastic.svg|thumb|350px|right|Stress-strain curves under uniaxial extension for Gent model compared with various hyperelastic material models.]] | |||
For uniaxial extension in the <math>\mathbf{n}_1</math>-direction, the [[finite strain theory|principal stretches]] are <math>\lambda_1 = \lambda,~ \lambda_2=\lambda_3</math>. From incompressibility <math>\lambda_1~\lambda_2~\lambda_3=1</math>. Hence <math>\lambda_2^2=\lambda_3^2=1/\lambda</math>. | |||
Therefore, | |||
:<math> | |||
I_1 = \lambda_1^2+\lambda_2^2+\lambda_3^2 = \lambda^2 + \cfrac{2}{\lambda} ~. | |||
</math> | |||
The [[finite strain theory|left Cauchy-Green deformation tensor]] can then be expressed as | |||
:<math> | |||
\boldsymbol{B} = \lambda^2~\mathbf{n}_1\otimes\mathbf{n}_1 + \cfrac{1}{\lambda}~(\mathbf{n}_2\otimes\mathbf{n}_2+\mathbf{n}_3\otimes\mathbf{n}_3) ~. | |||
</math> | |||
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have | |||
:<math> | |||
\sigma_{11} = -p + \cfrac{\lambda^2\mu J_m}{J_m - I_1 + 3} ~;~~ | |||
\sigma_{22} = -p + \cfrac{\mu J_m}{\lambda(J_m - I_1 + 3)} = \sigma_{33} ~. | |||
</math> | |||
If <math>\sigma_{22} = \sigma_{33} = 0</math>, we have | |||
:<math> | |||
p = \cfrac{\mu J_m}{\lambda(J_m - I_1 + 3)}~. | |||
</math> | |||
Therefore, | |||
:<math> | |||
\sigma_{11} = \left(\lambda^2 - \cfrac{1}{\lambda}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right)~. | |||
</math> | |||
The [[stress (physics)|engineering strain]] is <math>\lambda-1\,</math>. The [[stress (physics)|engineering stress]] is | |||
:<math> | |||
T_{11} = \sigma_{11}/\lambda = | |||
\left(\lambda - \cfrac{1}{\lambda^2}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right)~. | |||
</math> | |||
=== Equibiaxial extension === | |||
For equibiaxial extension in the <math>\mathbf{n}_1</math> and <math>\mathbf{n}_2</math> directions, the [[finite strain theory|principal stretches]] are <math>\lambda_1 = \lambda_2 = \lambda\,</math>. From incompressibility <math>\lambda_1~\lambda_2~\lambda_3=1</math>. Hence <math>\lambda_3=1/\lambda^2\,</math>. | |||
Therefore, | |||
:<math> | |||
I_1 = \lambda_1^2+\lambda_2^2+\lambda_3^2 = 2~\lambda^2 + \cfrac{1}{\lambda^4} ~. | |||
</math> | |||
The [[finite strain theory|left Cauchy-Green deformation tensor]] can then be expressed as | |||
:<math> | |||
\boldsymbol{B} = \lambda^2~\mathbf{n}_1\otimes\mathbf{n}_1 + \lambda^2~\mathbf{n}_2\otimes\mathbf{n}_2+ \cfrac{1}{\lambda^4}~\mathbf{n}_3\otimes\mathbf{n}_3 ~. | |||
</math> | |||
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have | |||
:<math> | |||
\sigma_{11} = \left(\lambda^2 - \cfrac{1}{\lambda^4}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right) = \sigma_{22} ~. | |||
</math> | |||
The [[stress (physics)|engineering strain]] is <math>\lambda-1\,</math>. The [[stress (physics)|engineering stress]] is | |||
:<math> | |||
T_{11} = \cfrac{\sigma_{11}}{\lambda} = | |||
\left(\lambda - \cfrac{1}{\lambda^5}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right) = T_{22}~. | |||
</math> | |||
=== Planar extension === | |||
Planar extension tests are carried out on thin specimens which are constrained from deforming in one direction. For planar extension in the <math>\mathbf{n}_1</math> directions with the <math>\mathbf{n}_3</math> direction constrained, the [[finite strain theory|principal stretches]] are <math>\lambda_1=\lambda, ~\lambda_3=1</math>. From incompressibility <math>\lambda_1~\lambda_2~\lambda_3=1</math>. Hence <math>\lambda_2=1/\lambda\,</math>. | |||
Therefore, | |||
:<math> | |||
I_1 = \lambda_1^2+\lambda_2^2+\lambda_3^2 = \lambda^2 + \cfrac{1}{\lambda^2} + 1 ~. | |||
</math> | |||
The [[finite strain theory|left Cauchy-Green deformation tensor]] can then be expressed as | |||
:<math> | |||
\boldsymbol{B} = \lambda^2~\mathbf{n}_1\otimes\mathbf{n}_1 + \cfrac{1}{\lambda^2}~\mathbf{n}_2\otimes\mathbf{n}_2+ \mathbf{n}_3\otimes\mathbf{n}_3 ~. | |||
</math> | |||
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have | |||
:<math> | |||
\sigma_{11} = \left(\lambda^2 - \cfrac{1}{\lambda^2}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right) ~;~~ \sigma_{22} = 0 ~;~~ \sigma_{33} = \left(1 - \cfrac{1}{\lambda^2}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right)~. | |||
</math> | |||
The [[stress (physics)|engineering strain]] is <math>\lambda-1\,</math>. The [[stress (physics)|engineering stress]] is | |||
:<math> | |||
T_{11} = \cfrac{\sigma_{11}}{\lambda} = | |||
\left(\lambda - \cfrac{1}{\lambda^3}\right)\left(\cfrac{\mu J_m}{J_m - I_1 + 3}\right)~. | |||
</math> | |||
=== Simple shear === | |||
The deformation gradient for a [[simple shear]] deformation has the form<ref name=Ogden>Ogden, R. W., 1984, '''Non-linear elastic deformations''', Dover.</ref> | |||
:<math> | |||
\boldsymbol{F} = \boldsymbol{1} + \gamma~\mathbf{e}_1\otimes\mathbf{e}_2 | |||
</math> | |||
where <math>\mathbf{e}_1,\mathbf{e}_2</math> are reference orthonormal basis vectors in the plane of deformation and the shear deformation is given by | |||
:<math> | |||
\gamma = \lambda - \cfrac{1}{\lambda} ~;~~ \lambda_1 = \lambda ~;~~ \lambda_2 = \cfrac{1}{\lambda} ~;~~ \lambda_3 = 1 | |||
</math> | |||
In matrix form, the deformation gradient and the left Cauchy-Green deformation tensor may then be expressed as | |||
:<math> | |||
\boldsymbol{F} = \begin{bmatrix} 1 & \gamma & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{bmatrix} ~;~~ | |||
\boldsymbol{B} = \boldsymbol{F}\cdot\boldsymbol{F}^T = \begin{bmatrix} 1+\gamma^2 & \gamma & 0 \\ \gamma & 1 & 0 \\ 0 & 0 & 1 \end{bmatrix} | |||
</math> | |||
Therefore, | |||
:<math> | |||
I_1 = \mathrm{tr}(\boldsymbol{B}) = 3 + \gamma^2 | |||
</math> | |||
and the Cauchy stress is given by | |||
:<math> | |||
\boldsymbol{\sigma} = -p~\boldsymbol{\mathit{1}} + \cfrac{\mu J_m}{J_m - \gamma^2}~\boldsymbol{B} | |||
</math> | |||
In matrix form, | |||
:<math> | |||
\boldsymbol{\sigma} = \begin{bmatrix} -p +\cfrac{\mu J_m (1+\gamma^2)}{J_m - \gamma^2} & \cfrac{\mu J_m \gamma}{J_m - \gamma^2} & 0 \\ \cfrac{\mu J_m \gamma}{J_m - \gamma^2} & -p + \cfrac{\mu J_m}{J_m - \gamma^2} & 0 \\ 0 & 0 & -p + \cfrac{\mu J_m}{J_m - \gamma^2} | |||
\end{bmatrix} | |||
</math> | |||
==References== | |||
<references/> | |||
== See also == | |||
* [[Hyperelastic material]] | |||
* [[Strain energy density function]] | |||
* [[Mooney-Rivlin solid]] | |||
* [[Finite strain theory]] | |||
* [[Stress measures]] | |||
[[Category:Continuum mechanics]] | |||
[[Category:Elasticity (physics)]] | |||
[[Category:Non-Newtonian fluids]] | |||
[[Category:Rubber properties]] | |||
[[Category:Solid mechanics]] |
Revision as of 23:02, 8 August 2013
Template:Continuum mechanics The Gent hyperelastic material model [1] is a phenomenological model of rubber elasticity that is based on the concept of limiting chain extensibility. In this model, the strain energy density function is designed such that it has a singularity when the first invariant of the left Cauchy-Green deformation tensor reaches a limiting value .
The strain energy density function for the Gent model is [1]
where is the shear modulus and .
In the limit where , the Gent model reduces to the Neo-Hookean solid model. This can be seen by expressing the Gent model in the form
A Taylor series expansion of around and taking the limit as leads to
which is the expression for the strain energy density of a Neo-Hookean solid.
Several compressible versions of the Gent model have been designed. One such model has the form[2]
where , is the bulk modulus, and is the deformation gradient.
Consistency condition
We may alternatively express the Gent model in the form
For the model to be consistent with linear elasticity, the following condition has to be satisfied:
where is the shear modulus of the material. Now, at ,
Therefore, the consistency condition for the Gent model is
Stress-deformation relations
The Cauchy stress for the incompressible Gent model is given by
Uniaxial extension
For uniaxial extension in the -direction, the principal stretches are . From incompressibility . Hence . Therefore,
The left Cauchy-Green deformation tensor can then be expressed as
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have
Therefore,
The engineering strain is . The engineering stress is
Equibiaxial extension
For equibiaxial extension in the and directions, the principal stretches are . From incompressibility . Hence . Therefore,
The left Cauchy-Green deformation tensor can then be expressed as
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have
The engineering strain is . The engineering stress is
Planar extension
Planar extension tests are carried out on thin specimens which are constrained from deforming in one direction. For planar extension in the directions with the direction constrained, the principal stretches are . From incompressibility . Hence . Therefore,
The left Cauchy-Green deformation tensor can then be expressed as
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have
The engineering strain is . The engineering stress is
Simple shear
The deformation gradient for a simple shear deformation has the form[3]
where are reference orthonormal basis vectors in the plane of deformation and the shear deformation is given by
In matrix form, the deformation gradient and the left Cauchy-Green deformation tensor may then be expressed as
Therefore,
and the Cauchy stress is given by
In matrix form,