VUMAT

[dmeloni]

hi everybody.
it's the first time form me in this Forum.
i'm trying to compile a user subroutine Vumat in Abaqus.
can anybody tell me if I have to explicitly write any instructions for the data check at time=0 in wich Abaqus determines the critical time step for the analysis, or is it something automatically provided .
thanks

[Jorgen]

Hi dmeloni!

This is what ABAQUS states:

In the data check phase of the analysis ABAQUS/Explicit calls user subroutine VUMAT with a set of fictitious strains and a totalTime and stepTime both equal to 0.0. This is done as a check on the user's constitutive relation and to calculate the equivalent initial material properties, based upon which the initial elastic wave speeds are computed.

My experience is that you do not need to do anything different during the first time increment at time=0. If you simply use the fictitious strain to calculate the new stress state then you are typically OK.

Best of luck,
Jorgen

[dmeloni]

dear Dr. Bergstrom
thanks so much for your attention, your answers will be very useful to me.
i just want to add something about the data check in Abaqus, the doubts about the need of writing data check instructions in user subroutines rose from this warning given me by the program:

***WARNING: A material defined in user subroutine VUMAT must be defined as
purely elastic (using the initial elastic modulus) at the
beginning of the analysis (stepTime=0). This is an informative
message. It does not necessarily indicate that user subroutine
VUMAT is incorrectly defined.

i think it's a little ambiguous
thanks[/u]

[Jorgen]

I have also seen that warning message. From what I know it does not mean that your subroutine is incorrect, and you can probably ignore the message.

You should always, however, validate the results from your material model subroutine by direct comparison with some other solution method in order to build confidence in your implementation and the finite element results.

Best of luck,
Jorgen