I am trying to implement a custom hyperelastic constitutive model in Abaqus. The model itself is derived on the basis of the in-plane stretch ratios and assumes the normal stretch ratio is fixed at 1 (i.e. constant thickness). I started with UHYPER, but decided eventually that this could not work since there was no way to imply directionality of the stretch ratios from the invariants.

I have therefore been trying to implement a UANISOHYPER_STRAIN model. However, I get an error that an orientation is required. Upon assigning an orientation, regardless of what that orientation might be, I get an error: "ORIENTATION,SYSTEM=ZRECTANGULAR is not valid for XX axisymmetric elements." Does this mean it is not possible to assign a suitable orientation to axisymmetric membrane elements?

Edit: I found a statement in the documentation, "You can define other directions by defining a local orientation, except for MAX1 and MAX2 elements (Axisymmetric membrane element library), which do not support orientations." This would seem to imply that it is not possible to use an anisotropic model with MAXn elements.

Abaqus ​​​​​​​