9 dof triangular element

[kamini]

Can anybody help with regards using triangle elements with 9 dofs. I am using green strain and I get curious values out (sometimes negative shear strain for positive values of the other strains). I have struggled to get models to converge using hyperelastic models (neo hookean and mooney rivlin) using an array of different methods for getting the material jacobian. I wonder is the fundamental problem in the strains. Any help on this would be much appreciated asap as it is for an assignment at uni that is due very soon. thanks.

[Jorgen]

I am not quite sure I understand the problem that you are trying to figure out. Are you attempting to formulate a user-element? Or something else?

- Jorgen

[kamini]

I am writing my own finite element code for analysing thin rubber membranes. I am looking at both the inflation of a circular disk and simple elongation. The main difficulty I have been experiencing is in the material jacobian does not seem to be correct. I have used three different techniques for this (2 analytical) and none yield the correct relationships between stress and strain, although others have used them with success. I cant understand why none of them work correctly (although in a few cases solutions converge anyway), and thought the problem may lie in the formulation of the element itself.

[Jorgen]

OK, I see. I would start with a linear elastic material model. In that case the Jacobian is trivial. Then I would try a neo-Hookean material model. Also in that case you can find closed-form approaximations in the literature.

- Jorgen