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# A question about large deformation in UMAT

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Posts: 7
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(@donngall84)
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Joined: 15 years ago

Hi everyone,

I am now using ABAQUS UMAT to carry out a constitutive model.

As the model is considering the large deformation, firstly I have to caculate the time rate of change of the deformation gradient to obtain celocity gradient. The following is my confusion:

The ABAQUS gives the deformation gradient at the beginning of the increment in the array of DFGRD0(3,3), as well as that at the end of the increment, i.e. DFGRD1(3,3). What I want to make sure is: could I then caculate the time rate of change of the deformation gradient by the equation (DFGRD1(3,3)-DFGRD0(3,3))/DTIME?

If negative, how should I get the variable?

Thank you very much~

Okyle

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Posts: 3998
(@jorgen)
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Joined: 5 years ago

Your finite different approach should give an approximate time rate of change of the deformationg gradient.

-Jorgen

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Posts: 7
Topic starter
(@donngall84)
Active Member
Joined: 15 years ago

Thank you, Jorgen, very much.

You mean I can make use of the equation mentioned above to obtain an approximate result of time rate of change of the deformation gradient . right?

And, what I am worrying about is:

DFGRD0(3,3), which is also called the deformation gradient by ABAQUS, is the same with the deformation gradient (i.e. F) that involved in a common consitutive model?

Posts: 3998
(@jorgen)
Member
Joined: 5 years ago

DFGRD0 is the deformation gradient at time t
DFGRD1 is the deformation gradient at time t+dt

-Jorgen

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