I am having some difficulty using the INISTATE feature in Ansys 13.0. Lets say I have a square sheet modelled using shell elements. I am using a non-linear hyperelastic law. The edge of the sheet is fixed in all DOF. I apply an anisotropic initial stress field to the sheet where Sx=0.01 and Sy-0.02 using:
I check that the stress field is what I want it to be by typing:
I run two analyses where I displace the centre node of the domain and calculate the reaction force on the node. In the first analysis, I displace the node in the positive X direction. In the second analysis, I displace the same node in the positive Y direction. As the material is non-linear, I expect the stiffness of the sheet in the Y direction to be greater than in the X direction due to the initial stress state. Therefore, the reaction force in the second analysis should be higher for a given displacement than the reaction force for the same displacement in the first analysis. However, this is not what I get. The reaction force is higher when the node is displaced in the X direction.
Instead of using INISTATE, if I apply the initial stress by applying appropriate forces along the edge of the sheet in a loadstep prior to displacing the nodes, the reaction force results are what I expect. I have run equivalent ABAQUS analyses in the past using the *INITIAL CONDITIONS, TYPE=STRESS feature and the results were what I expected also.
Am I misunderstanding/misusing the INISTATE feature in Ansys? Or is my logic wrong?
Thanks for any input.