Drucker's stability check of MCalibration

Hello Jorgen,

I have a question regarding the stability check of hyperelastic model in MCalibration.

For example, I chose a neo-hooken material with C10=1 and D1=0.02. In the Drucker's stability check of MCalibration, it is shown that the material model with that particular parameters stable except for the simple shear deformation mode---which is unstable for strains larger than 5.85983e-07.

However, the MCalibration Drucker's stability check is inconsistent with two observations I found.

(1) I set a virtual experiments of simple shear in MCalibration. The stress-strain curve is quite normal which does not show negative stiffness, meaning the simple shear deformation mode is stable.

(2) I also defined the exact same neo-hooken material in ABAQUS and evaluated its stability. The results indicate that the material is stable for all deformation modes.

Do you have any comments? Thanks!

-Weilin