Abaqus: Unit of von mises stress? What's maximum stress for soft materials?

[Mohammed Nagy]

Hello,
I'm modeling a soft pneumatic actuator on abaqus. I wrote the dimensions in mm and the pressure in MPa. Is the von mises stress from simulation in MPa also? If yes, is the maximum von mises stress of value = 600 reasonable with soft materials (dragon skin, eco-flex, etc...)?

 

[FEA way]

With dimensions in mm and pressure in MPa stress will also be in MPa. But be careful with other units (for example density if you use it). Von Mises stress of 600 MPa is quite a lot but it all depends on your material model. Which one do you use ? If it's linear elastic then I would definitely switch to more advanced model for this structure, possibly hyperelastic.

 

[Mohammed Nagy]

The model is hyperelastic neo-hookean.
What is the acceptable range of maximum von-mises?

 

[FEA way]

You could compare maximum von Mises stress value with ultimate tensile strength of your material. However in case of hyperelastic materials it might be better to use other output variables for evaluation of model's response to load. Particularly, strain and strain energy density values can be useful. Very good choice is NE (nominal strain) and its principal values.

Also keep in mind that in case of hyperelastic materials test data input is defined in terms of nominal stress while variable S used by default in Abaqus is true stress. Thus the calculated stress results can be much higher than values specified in material definition.