[Solved] thermal strain / stress in MFront

Yutakachan

Hello everyone,

I'm investigating the usage of MFront in Code-Aster, to implement plasticity model with temperature-dependent yield stress.

And I become wondering how the thermal strain is handled in Code-Aster + MFront framework.

MFront examples I found in astest or tfel site do not consider thermal strain. i.e. model routine neglects thermal in elastic strain computation (= total strain - nonelastic model strain).

And provides stress as (stiffness matrix)*(elastic strain), where stiffness matrix is built from the E, nu given by DEFI_MATERIAL/MFRONT.

Then I found:

* if I set non-zero CTE to ELAS (or ELAS_ORTH) and ramp the temperature, result shows thermal expansion / shrinkage.

* if I set different E to ELAS and MFRONT in DEFI_MATERIAL, those specified to MFRONT seem applied to compute stress.

Now I'm totally confused... could anyone please teach me how the thermal strain / stress is computed, when we use MFront?

Thanks in advance and a Happy New Year,

Yutaka Nishizawa

AsterO'dactyle

Hello,

For small strains (\epsi infinitesimal strains, additive decomposition hypothesis)

\epsi = \epsi_mech + \epsi_ther

MFront use mechanical strains at input \epsi_mech

1/ code_aster compute \epsi

2/ code_aster compute \epsi_ther = \alpha \Delta T where alpha is the coefficient of thermal expansion from DEFI_MATERIAU/ELAS/ALPHA and \Delta T the temperature increment (from AFFE_MATERIAU/AFFE_VARC/TEMP, known as external state variable in MFront). Thermal stress is \stress_ther = Young \epsi_ther with Young from DEFI_MATERIAU/ELAS/E

3/ code_aster send \epsi_mech = \epsi - \epsi_ther to MFront

4/ MFront compute stress, internal state variable and tangent matrix

For thermal strains, code_aster use only DEFI_MATERIAU(ELAS/E) not Young modulus from MFront .

There is normally a small consistency check between the elastic coefficients given by DEFI_MATERIAU/ELAS in aster and those given for MFront

Yutakachan

Hello,

Thank you very much for detailed explanation!

So Code-Aster (i) sends mechanical strain and total stress as "strain" and "stress" to MFront respectively, (ii) accepts mechanical stress from MFront, (iii) adds thermal stress and (iv) solve the principle of equilibrium.

I think now I get it.

Thanks.