The Gent hyperelastic material model 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 I m {\displaystyle I_{m}} . The strain energy density function for the Gent model is
W = − μ J m 2 ln ( 1 − I 1 − 3 J m ) {\displaystyle W=-{\cfrac {\mu J_{m}}{2}}\ln \left(1-{\cfrac {I_{1}-3}{J_{m}}}\right)}
where μ {\displaystyle \mu } is the shear modulus and J m = I m − 3 {\displaystyle J_{m}=I_{m}-3} . In the limit where J m → ∞ {\displaystyle J_{m}\rightarrow \infty } , the Gent model reduces to the Neo-Hookean solid model. This can be seen by expressing the Gent model in the form
W = − μ 2 x ln [ 1 − ( I 1 − 3 ) x ] ; x := 1 J m {\displaystyle W=-{\cfrac {\mu }{2x}}\ln \left[1-(I_{1}-3)x\right]~;~~x:={\cfrac {1}{J_{m}}}}
A Taylor series expansion of ln [ 1 − ( I 1 − 3 ) x ] {\displaystyle \ln \left[1-(I_{1}-3)x\right]} around x = 0 {\displaystyle x=0} and taking the limit as x → 0 {\displaystyle x\rightarrow 0} leads to
W = μ 2 ( I 1 − 3 ) {\displaystyle W={\cfrac {\mu }{2}}(I_{1}-3)}
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 (the below strain energy function yields a non zero hydrostatic stress at no deformation, refer for compressible Gent models).
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