Hyperelastic PMMA - convergence issues

[herminator]

Hello,

I try to simulate the thermoforming of a PMMA sheet in Ansys 14.

Having done some research I used the hyper-elastic Mooney-Rivlin material model using the following parameters:

C10 = -0,03
C01 = 0,453

My first simple simulation (in Ansys transient structural) only contains a rectangular sheet which is fixed at the green highlighted border areas and a single pressure load.
ansys.jpg
The simulation only coverges if the pressure load is very low (e.g. 1E-5 MPa).
I enabled the "large displacement" setting, but if I try to simulate a higher pressure, the simulation does not converge.

Can anybody suggest what I need to change in order to get the simulation to converge?

Thanks in advance!

[Jorgen]

Does it work better if you make C10 = 0.03?

-Jorgen

[matsgd]

Does it work better if you make C10 = 0.03?

-Jorgen

I agree! Consider the case of imposing a deformation such that I2=0. Does it seem alright to produce a negative strain energy..?
Mats

[herminator]

Thanks for your help Jorgen and Mats!
The simulation did not converge when I used Mooney-Rivlin even with positive constants. Using Neo-Hookean material model (setting C10=0) I got a stable convergent system.
Since the approximation of NH is also acceptable I did further development of my project with this hyperelastic model.
My developed project uses two layers of two different hyperelastic materials. The upper layer is still a PMMA sheet. The other layer is made out of silicone. Both layers are in permanent contact (rough)
The sheets are fixed on the edges and bulge in the center due to pressure on the upper layer.
The simulation converges when I use the same material (PMMA) for both layers. It does not converge, when I use the materialdata I got for my silicone sheet (Mu=0,34 Pa and D1=0) nor does it when I enter the values from my PMMA data (Mu=0,5 Pa and D1=0) in the silicone material data.

Do you have an idea what might be wrong with my settings?
Thanks