BEHAVIORS¶

UMAT procedures¶

UMAT procedures implement the behaviors used in the simulation. They are a corner stone of the code as they are called at every iteration, on every voxels. They allow for compatibility with :

Finite Element codes such as CAST3M (http://www-cast3m.cea.fr/) or ABAQUS
The MFront (http://tfel.sourceforge.net/index.html) code generator, whic simplifies the process of behavior law implementation.

The UMAT signature of the procdure reads

SUBROUTINE umat_my_behavior( STRESS, STATEV, DDSDDE, SSE, SPD, SCD,&
                    RPL, DDSDDT, DRPLDE, DRPLDT,&
                    STRAN, DSTRAN, TIME, DTIME,&
                    TEMP, DTEMP, PREDEF, DPRED,&
                    CMNAME, NDI, NSHR, NTENS, NSTATV,&
                    PROPS, NPROPS, COORDS,&
                    DROT, PNEWDT, CELENT, DFGRD0, DFGRD1,&
                    NOEL, NPT, LAYER, KSPT, KSTEP, KINC )

Among the different variables usefull for ABAQUS, AMITEX_FFTP uses :

TIME : [in], double, array(2)

time(2) = \(t\), time at the begining of the load increment, time(1) = 0 (unused here)
DTIME : [in], double

time increment \(dt\)
STRAN : [in], double, array(6)

Strain components at \(t\) (11,22,33,12,13,23, Voigt notation). For Small Strain Simulations.
DSTRAN : [in],double, array(6)

Strain increment components during \(dt\) (11,22,33,12,13,23, Voigt notation). For Small Strain Simulations.
DFGRD0 : [in], double, array(3,3)

Transformation gradient (\(F=1+grad(U)\)) at \(t\). For Finite Strains Simulations.
DFGRD1 : [in], double, array(3,3)

Transformation gradient (\(F=1+grad(U)\)) at \(t+dt\). For Finite Strains simulations.
TEMP : [in], double

Temperature at \(t\).
DTEMP : [in], double

Temperature increment during \(dt\).
PREDEF : [in], double, array(*)

External parameters at \(dt\).
DPRED : [in], double, array(*)

External parameters increment during \(dt\)
STRESS : [in/out], double, array(6)

Cauchy stress components (11,22,33,12,13,23). [in] : value at \(t\). [out] : value at \(t+dt\).
STATEV : [in/out], double, array(NSTATEV)

Internal Variables. [in] : value at \(t\). [out] : value at \(t+dt\).
PROPS : [in], double, array(NPROPS)

Material coefficients

These UMAT procedure are used in AMITEX_FFTP to implement native behaviors. In addition, with these definitions in mind, users can implement their own behavior whether directly (see UMAT behaviors) or with the MFRONT code generator.

The links between these procedures with XML files are :

PROPS : in material.xml (see STANDARD DESCRIPTION)

the coefficients <Coeff Index="i" assigns PROPS(i)
STATEV : in material.xml (see STANDARD DESCRIPTION)

the initial internal variable <IntVar Index="i" assigns to STATEV(i)
TEMP : in loading.xml (see EXTERNAL LOADING)

the temperatue <T assigns TEMP and DTEMP
PREDEF : in loading.xml (see EXTERNAL LOADING)

the external parameters <Param Index="i" assigns PREDEF(i) and DPRED(i)

NATIVE behaviors¶

Only a few native behavior laws are provided with AMITEX_FFTP.

Their implementation can be found in the files (amitex)/libAmitex/src/materiaux/*.f90.

Each file gives a description of the coefficients and internal variables.

The available native behavior laws are

elasiso             : isotropic elasticity
elasiso_eigs        : isotropic elasticity with eigenstrain
elasiso_GD          : Lagrangian isotropic elasticity
elasaniso           : orthotropic elasticity
thermoelasiso       : isotropic thermoelasticity
paramextelasiso     : isotropic 'thermo'elasticity but with an external parameter governing the dilatation (instead of the temperature)
contrainte_imposee  : imposed stress
viscoelas_maxwell   : Maxwell linear visco-elasticity

UMAT behaviors¶

amitex_fftp offers the possibility to introduce user material behaviors through a UMAT compatible procedure.

An example is available in (amitex_fftp)/cas_tests/comportements/polyxCC/comportement_umat. The user can copy-paste this folder on his home directory (denoted here as home) and follow the procedure below.

The folder contains user material files and an additionnal Makefile. The name of the umat compatible procedure used for the integration of the behaviour law corresponds to the field Law to be filled in the material.xml file (see STANDARD DESCRIPTION).

The Makefile launch the compilation and create in the current folder a dynamic library libUmatAmitex.so. The full path towards this library must be used to fill the Lib field in the material.xml file (see STANDARD DESCRIPTION).

Adjust the following first lines of the Makefile

# Compiler (ifort or gfortran)
FC=ifort

# Path to the librairie 2decomp_fft, (amitex)/lib_extern/2decomp_fft-1.5.847, for example:
lDecomp=/home/gelebart/amitex_fftp/lib_extern/2decomp_fft-1.5.847

Launch the compilation to create the dynamic library
```
make clean
make
```
Adjust the Law and Lib fields of the desired material.xml files.

MFRONT behaviors¶

Interfacing AMITEX_FFTP with MFront libraries is straightforward.

Of course, it assumes that you have installed MFront on your machine and that the LD_LIBRARY_PATH contains the MFront library path.

An example with a dynamic library created by MFront, specifying the umat interface , is given in (amitex_fftp)/cas_tests/mazars.

The dynamic library (amitex_fftp)/cas_tests/mazars/src/libUmatBehaviour.so is an output of MFront using the command:

mfront --obuild --interface=umat mazars.mfront

where mazars.mfront is the Mazars damage behavior law delivered with MFront (see http://tfel.sourceforge.net/documentations.html for more information).

The new behaviour law can be used by filling correctly the Law and Lib fields of the desired material.xml files.

Note

The ordering of the coefficients and internal variables in the material.xml file corresponds to their ordering in the .dgibi file (CAST3M finite element code input) located in the local castem folder generated together with the src folder when launching mfront --obuild --interface=umat file.mfront.