State variable time derivative

[adp0014]

Hello everybody,
I am writing a UMATHT and I am using it in a coupled stress-thermal analaysis. I have stored the equivalent plastic strain (ep) as a state variable (STATEV) and I want to compute its rate, i.e. I want to use the plastic equivalent strain rate (dep/dt). I suppose that I should store the STATEV value in the previous increment and make:

dep/det = (STATEV(t+DTIME)-STATEV(t))/DTIME

but I am not sure how to implement this scheme. Someone could help me?

Thanks in advanced!

 

[akabarten]

Hi,

I am not sure but I think that implementation of your scheme can not be done.
In each increment of analysis you have access to current and previous equivalent plastic strain.
Your scheme requires information about equivalent plastic strain from next increment (STATEV(t+DTIME)).

I think you can change the scheme to:
dep/det = (STATEV(t)-STATEV(t-DTIME))/DTIME

I think in this case DTIME has to also from previous increment so we have to store with state variable as well.

This way it can be implemented.

! set eq. plastic strain for current increment
rPeeq = ...
! calc rate
rPeeqRate = (rPeeq-statev(1))/statev(2)
! store current peeq to use in next increment
statev(1) = rPeeq
! store dtime to use in next increment
statev(2) = dtime

Regrads,
Bartosz



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[adp0014]

You are totally right, I was confused because I was using the current increment DTIME. Storing the DTIME from the previous increment as a STATEV, as you have suggested, I have solved the problem. Thank you so much!!

Regards,
Andres