This is an extremely important and powerful interface, allowing the user to define his/her own mechanical material behavior. The subroutine “umat_main.f” is a driver subroutine, calling user-defined routines similar to “umat_user.f”, depending on the kind of material present in the model. To create a new material law, a “umat_user.f” routine must be written and an appropriate call must be inserted in routine “umat_main.f”.

For instance, assume you want to write a material user routine for a Drucker-Prager material model. Let us call this routine umat_drucker_prager.f. To write the routine, you can use the umat_user.f routine as a template. The header of this routine shows you the fields which you have at your disposal.

When you finished writing the routine you have to make it available for selection in routine umat_main.f. The selection is done based on the material name. Let us take DRUCKER-PRAGER for the material name, i.e. if the user wants to select this model with a *USER MATERIAL card, he has to use a name for his material starting with DRUCKER-PRAGER. Material names in CalculiX can be 80 characters long, so the remaining 66 characters after DRUCKER-PRAGER can be used to distinguish between several Drucker-Prager materials used within one and the same input deck, e.g. the user could use DRUCKER-PRAGER1 and DRUCKER-PRAGER2 if he has two different Drucker-Prager materials in his model. Using the block

elseif(amat(1:4).eq.'USER') then ! amatloc(1:76)=amat(5:80) amatloc(77:80)=' ' call umat_user(amatloc,iel,iint,kode,elconloc,emec,emec0, & beta,xikl,vij,xkl,vj,ithermal,t1l,dtime,time,ttime, & icmd,ielas,mi(1),nstate_,xstateini,xstate,stre,stiff, & iorien,pgauss,orab,pnewdt,ipkon)

as template we arrive at:

elseif(amat(1:14).eq.'DRUCKER-PRAGER') then ! amatloc(1:66)=amat(15:80) amatloc(67:80)=' ' call umat_drucker-prager(amatloc,iel,iint,kode,elconloc,emec, & emec0,beta,xikl,vij,xkl,vj,ithermal,t1l,dtime,time,ttime, & icmd,ielas,mi(1),nstate_,xstateini,xstate,stre,stiff, & iorien,pgauss,orab,pnewdt,ipkon)

which has to be inserted in routine umat_main.f. Notice that the DRUCKER-PRAGER part is removed from the material name before entering the subroutine, i.e. if the user has named his material DRUCKER-PRAGER1 only 1 will be transferred to the user subroutine.

After storing umat_main.f and umat_drucker_prager.f, umat_drucker_prager.f has to be added to the FORTRAN routines in Makefile.inc and CalculiX has to be recompiled, i.e. a new executable has to be generated.

After this, the Drucker-Prager material routine is at the disposal of the user. To select it, he has to use the *USER MATERIAL card and start the name of this material with DRUCKER-PRAGER. Furthermore, he has to know how many constants he has to define for this material (should be in the documentation of the material model) and how many internal variables there are (to be inserted underneath the *DEPVAR card; should also be in the documentation of the material model). If our Drucker-Prager material is characterized by 4 constants and 2 internal variables (this is out-of-the-blue) the input should look like:

*MATERIAL,NAME=DRUCKER-PRAGEREXAMPLE *USER MATERIAL,CONSTANTS=4 constant1,constant2,constant3,constant4 *DEPVAR 2

The header and input/output variables of the umat_user routine are as follows:

subroutine umat_user(amat,iel,iint,kode,elconloc,emec,emec0, & beta,xokl,voj,xkl,vj,ithermal,t1l,dtime,time,ttime, & icmd,ielas,mi,nstate_,xstateini,xstate,stre,stiff, & iorien,pgauss,orab,pnewdt,ipkon) ! ! calculates stiffness and stresses for a user defined material ! law ! ! icmd=3: calcutates stress at mechanical strain ! else: calculates stress at mechanical strain and the stiffness ! matrix ! ! INPUT: ! ! amat material name ! iel element number ! iint integration point number ! ! kode material type (-100-#of constants entered ! under *USER MATERIAL): can be used for materials ! with varying number of constants ! ! elconloc(21) user defined constants defined by the keyword ! card *USER MATERIAL (max. 21, actual # = ! -kode-100), interpolated for the ! actual temperature t1l ! ! emec(6) Lagrange mechanical strain tensor (component order: ! 11,22,33,12,13,23) at the end of the increment ! (thermal strains are subtracted) ! emec0(6) Lagrange mechanical strain tensor at the start of the ! increment (thermal strains are subtracted) ! beta(6) residual stress tensor (the stress entered under ! the keyword *INITIAL CONDITIONS,TYPE=STRESS) ! ! xokl(3,3) deformation gradient at the start of the increment ! voj Jacobian at the start of the increment ! xkl(3,3) deformation gradient at the end of the increment ! vj Jacobian at the end of the increment ! ! ithermal 0: no thermal effects are taken into account ! >0: thermal effects are taken into account (triggered ! by the keyword *INITIAL CONDITIONS,TYPE=TEMPERATURE) ! t1l temperature at the end of the increment ! dtime time length of the increment ! time step time at the end of the current increment ! ttime total time at the start of the current step ! ! icmd not equal to 3: calculate stress and stiffness ! 3: calculate only stress ! ielas 0: no elastic iteration: irreversible effects ! are allowed ! 1: elastic iteration, i.e. no irreversible ! deformation allowed ! ! mi(1) max. # of integration points per element in the ! model ! nstate_ max. # of state variables in the model ! ! xstateini(nstate_,mi(1),# of elements) ! state variables at the start of the increment ! xstate(nstate_,mi(1),# of elements) ! state variables at the end of the increment ! ! stre(6) Piola-Kirchhoff stress of the second kind ! at the start of the increment ! ! iorien number of the local coordinate axis system ! in the integration point at stake (takes the value ! 0 if no local system applies) ! pgauss(3) global coordinates of the integration point ! orab(7,*) description of all local coordinate systems. ! If a local coordinate system applies the global ! tensors can be obtained by premultiplying the local ! tensors with skl(3,3). skl is determined by calling ! the subroutine transformatrix: ! call transformatrix(orab(1,iorien),pgauss,skl) ! ! ! OUTPUT: ! ! xstate(nstate_,mi(1),# of elements) ! updated state variables at the end of the increment ! stre(6) Piola-Kirchhoff stress of the second kind at the ! end of the increment ! stiff(21): consistent tangent stiffness matrix in the material ! frame of reference at the end of the increment. In ! other words: the derivative of the PK2 stress with ! respect to the Lagrangian strain tensor. The matrix ! is supposed to be symmetric, only the upper half is ! to be given in the same order as for a fully ! anisotropic elastic material (*ELASTIC,TYPE=ANISO). ! Notice that the matrix is an integral part of the ! fourth order material tensor, i.e. the Voigt notation ! is not used. ! pnewdt to be specified by the user if the material ! routine is unable to return the stiffness matrix ! and/or the stress due to divergence within the ! routine. pnewdt is the factor by which the time ! increment is to be multiplied in the next ! trial and should exceed zero but be less than 1. ! Default is -1 indicating that the user routine ! has converged. ! ipkon(*) ipkon(iel) points towards the position in field ! kon prior to the first node of the element's ! topology. If ipkon(iel) is smaller than 0, the ! element is not used.

The parameter ielas indicates whether irreversible effects should be taken into account. Forced displacements can lead to huge strains in the first iteration. Therefore, convergence in quasistatic calculations is often enhanced if the first iteration is completely linear, i.e. material and geometric nonlinearities are turned off. The parameter ielas is the appropriate flag.

Two extra routines are at the user's disposal for conversion purposes. “str2mat.f” can be used to convert Lagrangian strain into Eulerian strain, Cauchy stress into PK2 stress, or Kirchhoff stress into PK2 stress. The header and a short description are as follows:

subroutine str2mat(str,ckl,vj,cauchy) ! ! converts the stress in spatial coordinates into material coordinates ! or the strain in material coordinates into spatial coordinates. ! ! INPUT: ! ! str(6): Cauchy stress, Kirchhoff stress or Lagrange strain ! component order: 11,22,33,12,13,23 ! ckl(3,3): the inverse deformation gradient ! vj: Jakobian determinant ! cauchy: logical variable ! if true: str contains the Cauchy stress ! if false: str contains the Kirchhoff stress or ! Lagrange strain ! ! OUTPUT: ! ! str(6): Piola-Kirchhoff stress of the second kind (PK2) or ! Euler strain !

The second routine, “stiff2mat.f” converts the tangent stiffness matrix from spatial coordinates into material coordinates.

subroutine stiff2mat(elas,ckl,vj,cauchy) ! ! converts an element stiffness matrix in spatial coordinates into ! an element stiffness matrix in material coordinates. ! ! INPUT: ! ! elas(21): stiffness constants in the spatial description, i.e. ! the derivative of the Cauchy stress or the Kirchhoff ! stress with respect to the Eulerian strain ! ckl(3,3): inverse deformation gradient ! vj: Jacobian determinant ! cauchy: logical variable ! if true: elas is written in terms of Cauchy stress ! if false: elas is written in terms of Kirchhoff stress ! ! OUTPUT: ! ! elas(21): stiffness constants in the material description,i.e. ! the derivative of the second Piola-Kirchhoff stress (PK2) ! with respect to the Lagrangian strain !