Comparison of Neo-Hookean Materials
Introduction
The definition of an isotropic, neo-Hookean is an elastic material that depends ot two elastic constants and in the limit of small deformations is identical to a small-strain, linear-elastic, Hookean material. The currently available MPM Materials that a neo-Hookean material options are:
The first is always a neo-Hookean material. The second has three properties for bulk modulus (K) and two shear modulus (G1 and G2). Is can be considered as neo-Hooken that reduce to small strain material with same bulk modulus and shear modulus G = G1+ G2. The third allows plastic deformation by limiting the amount of elongation in tension or compressure. By turning off the clamping, the clamped neo-Hookean material represents a modified corotated elasticity law.
Elastic Energy Functions
The elastic energy functions for these three material compared are:
- Neo-Hookean Material
[math]\displaystyle{ W ={\lambda\over 2 }({1\over 2 }(J^2-1)-\ln J) + {G\over 2 } (I_{1}-3-2\ln J) }[/math]
The first term can be changed to two other options to provide more neo-Hookean material models. - Mooney Material
[math]\displaystyle{ W ={\kappa\over 2 }({1\over 2 }(J^2-1)-\ln J) + {G_{1} \over 2 } (\bar I_{1}-3) + {G_{2} \over 2 }(\bar I_{2}-3) }[/math]
The first term can be changed to two other options to provide more neo-Hookean material models.This material is not strictly neo-Hookean (because of three material properties), but choosing G2=0 would make it a neo-Hookean material. If all parameters are used, it is neo-Hookean extension of small strain material with G = G1+G2. - Clamped Neo-Hookean Material
[math]\displaystyle{ W = {\lambda\over 2}(J_E-1)^2 + G\sum_k (\lambda_k-1)^2 }[/math]
To use this neo-Hookean material, the material properties must set either CritComp or CritTens to less than zero and set Elastic is 0.
Stress Strain Response
The following figure plots stress vs. strain from compression to tension for the above neo-Hookean materials. For the Mooney Material