INCOmpressible Command For The Hyperelastic Model

[mehrangh2016]

Dear All,

I have a question about using the INCOmpressible command for the hyperelastic model. In a simple test problem attached below, the displacement control loading is applied on the upper plane of the cubic specimen. The cubic body is completely fixed, which means that all the displacement degrees of freedom are constrained. The result in terms of the deformation demonstrates that the body inflates in the middle of the specimen. I want to know the cause of the phenomenon. The result is uploaded here.

Note that the applied loading is tension.

Best,
Mehran

[Prof. R.L. Taylor]

In linear elasticity the bulge happens due to the Poisson ratio effect on strains and the constraint on constant volume.  The same applies to finite elasticity.  If the volume is constant and you change the displacement in the z-direction the volume needs to increase.   If you constrain the displacements on the top/bottom in the z-direction only and add enough constraints to prevent any rigid translation or rotation it should remain a rectangular brick.