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A question on Residual Load Vector in Abaqus User Element Subroutine implementation

Gurudutt Chandrashekar's picture

Hello all,

 

I am implementing a Finite Shell element in Abaqus as a User Element.  I have a question regarding the Residual Lod vector that has to be defined in the User Element Subroutine.  

We know that K(du)=R-f       where R is the external force vector at time t+dt

                                                  f is the internal force vector at time t which depends on the present stress state

                                                  K is the stiffness matrix in the present configuration and du is the displacement incremen from time (t) to (t+dt)

 The above equation is for a non linear analysis.

Now, if I am doing a non linear analysis ,how exactly do I specify the external force vector in the UEL subroutine?. The external forces are sent as an input from the input file.  If I need to write the code for the residual load vector then I must be aware of the external forces as well. The two examples in the Abaqus documentation on UEL subroutine just multiply the stiffness matrix with the displacements and equate it to RHS (residual force vector). I do not totally understand this point.

  • If I am doing a displacement based analysis,where I constrain some nodes and apply displacements at other nodes and apply no external forces at all,then how exactly do I define the residual force. I know that the solver solves for the Reaction forces but I would not be aware of those values to implement them in my Fortran code.
  • If I am doing a linear analysis where there is no residual,how exactly do I define the RHS term in my UEL subroutine. Is it again RHS(i,1)=RHS(i,1)-AMATRX(i,j)*U(i)  ?     (where RHS(i,1) is assigned zero values initially)

 

 

 

 

 

 

 

 

 

                                                  

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