I just like to add a comment to this conversation, LIAISON_SOLIDE is actually an RBE element yet one needs to understand that it works like a RBE1 (Rigid body connected to an arbitrary number of grid points) and we don’t have the option to use RBE2 (Rigid body with independent DOFs at a grid point and dependent DOFs at an arbitrary number of grid points) in aster. Furthermore, one should not assume high E value in his/her simulation for RBEs because it will act as an RBER element (Rigid bar element).
Regards,
Ahmad Abbas

Thank you all for your responses,
If it is not as easy to create welded joints; I was thinking that we could evaluate the added stiffness of
the welded edge on the overall model by creating different properties for the edge line (thickness,
material properties). See image as attached:
<<Image>>
I am using shell elements for my model (DKT elements), and that doesn’t allow me to have thickness for
the line. Is there any kind of element that I can use to account for that added stiffness?
Thank you for your assistant and inputs in advance,
Ahmad Abbas

It seems like a memory issue, because I did everything the same with the same geometry but a coarser mesh and it ran fine!? .
Thank you,
A.Abbas

Hi,
I have recently started using Code_Aster and really didn’t know that Aster could do XFEM. When I saw your posting  I was very interested and did a quick search for the XFEM modeling in Aster. I have found the following online:

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
“The X-FEM allows to introduce a crack in a sound structure. It is implemented in Code_Aster for several years and began to be used in the engineering of EDF in the analysis of toxicity of default.

The definition of the crack is not through the mesh structure, but the calculation of two level sets: a level set normal to locate the surface crack and a level set tangent to locate the bottom crack. Thus, the simulation of crack propagation is equivalent to simulate the spread of the two level sets.

The new order allows PROPA_FISS to propagate a crack 3d any calculations by successive new level sets by projection on the new discretized geometry of the crack. PROPA_FISS need to have the predetermined values of intensity factors of stress at each point of the bottom crack. In 2d, the results are very satisfactory. In 3d, this method still faces difficulties in propagation of off-plan.”
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
http://www.code-aster.org/V2/doc/v9/man … .82.11.pdf
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

I think what this is saying is that they have a good model for shell elements and under progress (working) for 3d element types. I really could make use of fracture failure modeling in my current work, thus if anyone has an example your help will be appreciated. 

Thank you,
Ahmad Abbas

Thomas,
Thank you for your response, I have not changed the TOUT='OUI' in many places to “GROUP_MA” yet I don’t know what I am doing wrong in regards of calculating the elastic energy on the deforming plate. I basically need to know how to use “Post ELEM” to calculate “ENER_ELAS”. 
Model:
•    1x1 m plate steel plate
•    Thickness .01 m
•    Boundary, clamed at all edges at the line.
•    Pressure, uniform pressure on the plate.
So, I understand that by using:
****************************************************************************************************
PRE_GMSH();
mesh=LIRE_MAILLAGE();
mesh=DEFI_GROUP(reuse =mesh,
                MAILLAGE=mesh,
                CREA_GROUP_MA=(_F(NOM='Surface',
                                  GROUP_MA='GM500',),
                               _F(NOM='Constraint_group',
                                  GROUP_MA='GM15',),),);
mesh=MODI_MAILLAGE(reuse =mesh,
                   MAILLAGE=mesh,
                   ORIE_NORM_COQUE=_F(GROUP_MA='GM500',),);
model=AFFE_MODELE(MAILLAGE=mesh,
                  AFFE=_F(GROUP_MA='GM500',
                          PHENOMENE='MECANIQUE',
                          MODELISATION='DKT',),);
steel=DEFI_MATERIAU(ELAS=_F(E=210000000000L,
                            NU=.3,
                            RHO=7800,),);
ele=AFFE_CARA_ELEM(MODELE=model,
                   COQUE=_F(GROUP_MA='GM500',
                            EPAIS=0.01,),);
mat=AFFE_MATERIAU(MAILLAGE=mesh,
                  AFFE=_F(GROUP_MA='GM500',
                          MATER=steel,),);
pressure=AFFE_CHAR_MECA(MODELE=model,
                        PRES_REP=_F(GROUP_MA='GM500',
                                    PRES=100000,),);
Fix=AFFE_CHAR_MECA(MODELE=model,
                   DDL_IMPO=_F(GROUP_MA='GM15',
                               DX=0,
                               DY=0,
                               DZ=0,
                               DRX=0,
                               DRY=0,
                               DRZ=0,),);
time=DEFI_LIST_REEL(DEBUT=0,
                    INTERVALLE=_F(JUSQU_A=.5,
                                  PAS=.1,),);
Static=STAT_NON_LINE(MODELE=model,
                     CHAM_MATER=mat,
                     CARA_ELEM=ele,
                     EXCIT=(_F(CHARGE=pressure,),
                            _F(CHARGE=Fix,),),
                     COMP_INCR=_F(RELATION='ELAS',),
                     INCREMENT=_F(LIST_INST=time,),
                     CONVERGENCE=_F(ITER_GLOB_MAXI=20,
                                    ITER_GLOB_ELAS=25,),);
Static=CALC_ELEM(reuse =Static,
                 CHAM_MATER=mat,
                 CARA_ELEM=ele,
                 RESULTAT=Static,
                 REPE_COQUE=_F(NIVE_COUCHE='SUP',),
                 OPTION=('SIGM_ELNO_COQU','EQUI_ELNO_SIGM','EPSI_ELNO_DEPL',),);
Static=CALC_NO(reuse =Static,
               RESULTAT=Static,
               TOUT_ORDRE='OUI',
               OPTION=('EQUI_NOEU_SIGM','SIGM_NOEU_COQU',),
               CHAM_MATER=mat,
               CARA_ELEM=ele,);
IMPR_RESU (MODELE=model,
          FORMAT='GMSH',
          UNITE=37,
          RESU=_F(RESULTAT=Static,
                  NOM_CHAM=('DEPL','EQUI_NOEU_SIGM','EPSI_ELNO_DEPL',),),);
FIN();
************************************************************************************************************
I we will get the Equivalent nodal stress and other important results like the strain tensor, yet how could I get the elastic energy, can I use:

*******************************************************************************
  Energy=POST_ELEM(ENER_ELAS=_F(GROUP_MA='GM500',),
                 MODELE=model,
                 CHAM_MATER=mat,
                 CARA_ELEM=ele,
                 RESULTAT=Static,);
If yes, I am getting an error, please note Group”15” is the fixed line in the side and Group”500” is the plate: I get the error:
   Energy=POST_ELEM(INFO=1,
                   CRITERE='RELATIF',
                   MODE_FOURIER=0,
                   PRECISION=1.E-3,
                   NUME_COUCHE=1,
                   RESULTAT=Static,
                   CARA_ELEM=ele,
                   NIVE_COUCHE='MOY',
                   ENER_ELAS=_F(GROUP_MA='GM500'),
                   MODELE=model,
                   CHAM_MATER=mat,
                   );

   
   !--------------------------------------------------------------------------!
   ! <F> <CALCULEL_34>                                                        !
   !  le calcul de l'option :  ENER_ELAS  n'est possible pour aucun des types !
   ! d'elements  du ligrel.                                                   !
   ! Cette erreur est fatale. Le code s'arrete.                               !
   !--------------------------------------------------------------------------!
***********************************************************************************************
Please also how do I write the energy to the result file?

Thank you in advance for any help,
A.    Abbas

Hi all,
I am trying to understand the elasto-plastic molding in Aster. My final goal is to model large strain with either DKT or Coque-3D. Yet I have failed to model that until now.
According to posting:
http://www.code-aster.org/forum2/viewtopic.php?id=12033
“lf you have to modelize large strains, then (with DEFORMATION='SIMO_MIEHE') stress-strain curve is the true one : cauchy stress and logarithmic strains. The when you compute the stesses (with STAT_NON_LINE) they are Caychy stresses, and the strains (with CALC_ELEM) are computes with the hypothesis of large strains. “

I have remodeled and ran a simple specimen, yet when I just get little above the yield point my solution diverges and I cannot solve the problem!

INSTANT DE CALCUL :  1.000000000E-01

---------------------------------------------------------------------
|   ITERATIONS   |     RESIDU     |     RESIDU     |     OPTION     |
|     NEWTON     |     RELATIF    |     ABSOLU     |   ASSEMBLAGE   |
|                | RESI_GLOB_RELA | RESI_GLOB_MAXI |                |
---------------------------------------------------------------------
|     0        X | 4.86406E-01  X | 1.19943E+05    |TANGENTE        |
|     1        X | 2.54900E+00  X | 4.39760E+04    |TANGENTE        |
|     2        X | 4.82148E+00  X | 3.87162E+05    |TANGENTE        |
---------------------------------------------------------------------

ECHEC DANS L'INTEGRATION DE LA LOI DE COMPORTEMENT                   
VERIFIEZ VOTRE MODELE OU  ESSAYEZ DE SUBDIVISER LE PAS DE TEMPS       

   ! <EXCEPTION> <COMPOR1_9>                                           !
   ! Arret suite à l'échec de l'intégration de la loi de comportement. !
   !-------------------------------------------------------------------!
   

Why is my solution failing for large strain?
Why I am seeing no necking at all?
Why the user in the posted example used the engineering stress-strain curve? Shouldn’t the strain be in terms of true strain?
e = ln(L / Lo)  ~  el = ln (1+e)   
Thank you in advance for any input,
A.    Abbas

Ok I think this problem is solved by extra effort from user side in post processing using python or any other code….
When we calculate “EPSI_ELNO_DEPL “ using “CALC_ELEM”  for example we get:

********************************************************************
CHAMP PAR ELEMENT AUX NOEUDS DE NOM SYMBOLIQUE  EPSI_ELNO_DEPL                 
NUMERO D'ORDRE: 1 INST:  1.00000E-01                                           
M158      EPXX         EPYY         EPZZ         EPXY         EPXZ         EPYZ       
N294      1.15881E-05  3.86876E-05  0.00000E+00 -3.08932E-05 -4.37125E-06  3.50328E-06
N155     -1.94818E-20  4.11023E-05  0.00000E+00 -4.62066E-06 -4.37125E-06  3.50328E-06
N156      6.56451E-21 -8.15851E-07  0.00000E+00  2.80464E-06 -4.37125E-06  3.50328E-06
********************************************************************

Then by using “CALC_NO” and calculating “EPSI_NOEU_DEPL “we will get for example:
**************************************************************************************
NOEUD        EPXX         EPYY         EPZZ         EPXY         EPXZ         EPYZ   
N1       -1.07706E-06 -1.85632E-06  0.00000E+00 -1.49834E-06  1.31977E-06  3.82515E-07
**************************************************************************************
Where:
o   EPXX = EPSX = X Normal strain
o   EPYY =EPSY = Y Normal strain
o   EPZZ= EPSZ = Z Normal strain
o    EPXY= GMXY = Shear strain in Y direction on YZ plane
o   EPXZ= GMXZ = Shear strain in Z direction on YZ plane
o    EPYZ= GMYZ = Shear strain in Z direction on XZ plane
o    ESTRN = Equivalent nodal strain
Regards,
A.    Abbas

I think I had a similar problem to what I see here, I basically call it Aster is tired , I been looking for any reason for this as well, but did not really have anything out of my research. But one thing is known, this problem is some how mesh dependent, by small change in my mesh file the problem sometimes just doesn’t appear anymore or happens in different number of iteration.
Regards,
A Abbas

Are you using a shell or 3D?