Question about UMAT in feap

[YunfeiWang]

Dear all

      I want to writer a user material model according to a umat of abaqus. So there are two questions

(1) For small deformation, the strains at point would store in array eps(*). But for finite deformation, which item could denote strain？In other words， which item

in feap could instead of the “stran(ntens)” in abaqus for  finite deformation？

(2) For  finite deformation， how to denote the rigid body rotations in FEAP's umat?

      Thank you very much
   
     Best regards!

[Prof. R.L. Taylor]

Feap gives deformation gradient and displacement gradient at current and previous time, you have to compute every thing you need from these. 

[Prof. S. Govindjee]

From umatl1.f

Code: [Select]

      subroutine umatl1(eps,theta,td,d,ud,hn,h1,nh,ii,istrt, sig,dd,isw)
!     subroutine umatl1( f , detf,td,d,ud,hn,h1,nh,ii,istrt, sig,dd,isw)

!          eps(*)  -  Current strains at point      (small deformation)
!          f(3,3,*)-  Deformation gradient at point (finite deformation)

Comment or uncomment the subroutine declaration as desired (small or finite).  Either way the deformation information comes to you in the first slot, either as eps(1:6) or f(1:3,1:3).  If you want the rotation, you can compute it from f( , ) in the finite deformation case using the polar decomposition.  In the small deformation case it is not available (since you never need it).

[YunfeiWang]

Feap gives deformation gradient and displacement gradient at current and previous time, you have to compute every thing you need from these.

Dear Professor

        Thank you for your answer.  Could you tell me how to denote the increments of strains for finite deformation in Feap?

Thank you again

Best regards

[Prof. S. Govindjee]

Which version of FEAP are you using?

[Prof. R.L. Taylor]

You need to use the Abaqus theory manual to get the description of how they compute the "strain" you need; If you are using ver8.4 or later you have the gradient of displacement and deformation gradient.  Earlier versions only have the deformation gradient.  Use the FEAP programmer manual and the Abaqus manuals to compute what you need.

[YunfeiWang]

Which version of FEAP are you using?

Dear Professor
 
       I am using the ver8.4

Thank you

[FEAP_Admin]

Look at the  section of the programmers manual on finite deformation models.  It explains how to get the deformation gradient and the displacement gradient at time n and time n+1.