MATERIALS

STANDARD DESCRIPTION

Warning

The number of materials and the number of zones (per material), provided in the “material.xml” file, must be consistent with the description given by the vtk file(s).

Warning

if ID in materialID.vtk starts to zero, the material number numM in the “material.xml” file corresponds to ID+1

Example

This material.xml file below provides in a single (and dummy!) example different syntaxes to associate the geometry to material properties

<?xml version="1.0" encoding="UTF-8"?>
<Materials>

    <!-- REFERENCE MATERIAL -->
    <Reference_Material Lambda0="2.0952e+10" Mu0="1.5014e+10"/>


    <!-- MATERIAL "2", elastic isotropic behavior                                      -->
    <!--           prefer the syntax Lib="" for native behavior Law (see Material "1") -->
    <Material numM="2" Lib="(amitex)/libAmitex/src/materiaux/libUmatAmitex.so" Law="elasiso">
        <!-- Two constant coefficients (Lamé coefficients Lambda and Mu) -->
        <Coeff Index="1" Type="Constant" Value="4.0385e+10"/>
        <Coeff Index="2" Type="Constant" Value="2.6923e+10"/>
    </Material>


    <!-- MATERIAL "1", linear viscoelastic behavior                                     -->
    <!--               Lib="" possible for 'native' behavior Law                        -->
    <!--               possible if AMITEX_PATH is defined (export AMITEX_PATH=(amitex)) -->
    <Material numM="1" Lib="" Law="viscoelas_maxwell">

        <!-- coefficient 1: constant per zone -->
        <Coeff Index="1" Type="Constant_Zone">
            <!-- value within zone 1 -->
            <Zone numZ="1" Value="5.235e+10" />
            <!-- value within zone 2 -->
            <Zone numZ="2" Value="3.912e+10" />
        </Coeff>

        <!-- coefficient 2: constant per zone, values defined in the ASCII file "Coeff2" -->
        <Coeff Index="2" Type="Constant_Zone " File="Coeff2" Format="ASCII" />

        <!-- coefficient 3: constant per zone, values defined in the binary file "Coeff3.bin" -->
        <Coeff Index="3" Type="Constant_Zone " File="Coeff3.bin" Format="binary" />


        <!-- Internal variables are defined similarly-->
        <IntVar Index="1" Type="Constant" Value="0."/>

        <IntVar Index="2" Type="Constant_Zone">

            <Zone numZ="1" Value="0." />
            <Zone numZ="2" Value="10." />

        </IntVar>

        <IntVar Index="3" Type="Constant_Zone" File="Var3"/>

        <IntVar Index="4" Type="Variable" File="(rep_to_vtk)/coeff.vtk" Format="vtk"/>

    </Material>

</Materials>

Begin and End

The file must begin and end with

<?xml version="1.0" encoding="UTF-8"?>
<Materials>
        .
        .
</Materials>

Reference Material

The reference material is defined by its Lamé coefficients, given by

<Reference_Material Lambda0="2.0952e+10" Mu0="1.5014e+10"/>

For an optimal choice with the basic scheme, Moulinec proposed to use \(X_0=(min(X)+max(X))/2\) (with \(X=\lambda\) or \(\mu\)). If convergence acceleration is used (see ALGORITHM), the same choice can be used but the influence on the convergence is much less significant.

Materials

Each material is associated to the geometry by the material number numM which directly refers to a material domain in materialID.vtk.

Materials numbers numM starts to one (if ID in materialID.vtk starts to zero : ID = 0 corresponds to numM = 1, ID = 1 to numM = 2 and so on)

<Material numM="2" Lib="(amitex)/libAmitex/src/materiaux/libUmatAmitex.so" Law="elasiso">
   .
   .
</Material>
<Material numM="1" Lib="" Law="elasiso">
   .
   .
</Material>

One <Material.. > .. </Material> section must be defined per material present in the materialID.vtk. If the file materialID.vtk is omitted when launching amitex_fftp (see HOW TO LAUNCH AMITEX_FFTP), a unique material is assumed, and a unique section must be given.

The material behaviour laws are implemented through UMAT compatible procedures gathered in a dynamic library (see UMAT behaviors).

The fields Lib and Law provide the complete path to the library and the name of the procedure, respectively.

NATIVE BEHAVIORS

In the standard version of AMITEX_FFTP, only a few native behavior laws are provided within the library (amitex)/libAmitex/src/materiaux/libUmatAmitex.so.

For native behaviors, if the environment variable AMITEX_PATH is defined, the complete path can be avoided, using Lib="".

This is the recommended way to use native behaviors

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

They are described in NATIVE behaviors.

USER-DEFINED BEHAVIORS

Introducing a new behaviour law is quite simple and discribed in UMAT behaviors. In that case, filling Lib="path_to_library" with the correct path towards the dynamic library is mandatory.

Material Coefficients

The material coefficient’s index is the position of each coefficient within the vector COEFF used as an input of the UMAT compatible procedure for the behaviour law (see UMAT behaviors).

For an isotropic behavior (Law="elasiso"), the coefficients COEFF(1) and COEFF(2) are the Lamé coefficients \(\lambda\) and \(\mu\), respectively.

In the example above, four different ways are used to define these values

1. <Coeff Index="1" Type="Constant" Value="4.0385e+10"/>

2. <Coeff Index="1" Type="Constant_Zone">
          <Zone numZ="1" Value="5.235e+10" />
          <Zone numZ="2" Value="3.912e+10" />
   </Coeff>

3. <Coeff Index="2" Type="Constant_Zone " File="Coeff2" Format="ASCII"/>

4. <Coeff Index="3" Type="Constant_Zone " File="Coeff3.bin" Format="binary"/>

  1. is used if the coefficient is constant within the Material

  2. and 3. and 4. are used if the coefficient is different from one Zone to another

  3. is used to import coefficients from a text file, which can be useful when considering a relatively large number of zones. To define a coefficient for two zones, the file must contain at least two lines (one value per line) see an example of the file Coeff2:

    1.429430417
3.905850691

    If the Format tag is not specified, the program will assume that the file given is written in the ASCII format.

4. is used to import coefficients from a binary file. This is very useful when there is a very large number of zones. The file format is described in BIN files and the precision must be float or double.

Initial Internal Variables

Initial internal variables are assigned according to the same procedure. Here, the index is the position of each internal variable within the vector STATEV (used as an input/output of the UMAT compatible procedure for the behaviour law, see UMAT behaviors).

A fifth possibility is proposed to assign internal variables with binary vtk files (format float or double)

5. <IntVar Index="4" Type="Variable" File="(rep_to_vtk)/variable.vtk" Format="vtk"/>

The vtk files definition is given in VTK files with precision float or double.

This is especially interesting to assign a continuously variable field (a random field for example).

If the Material do not cover the whole unit-cell (which occurs with more than one Material in the unit-cell), only the corresponding part of the complete field given in ‘variable.vtk’ is used. The rest of the field is not considered.

COMPOSITE VOXELS (ADVANCED USERS)

Important

  • The important idea is that the same vtk files can be used with or without composite voxels.
  • The introduction of composite voxels in the simulation is done by additionnal informations within the material.xml file.
  • The definition of composite voxels requires a pre-treatment to precisely identify where they are located and what they consist of (which materials, which volume fractions…)

Introduced progressively since version 5.0.0, the final goal is to be able to account for:

  1. different averaging rules (at least Voigt, Reuss and Laminate)
  2. any number of phases
  3. any linear or non-linear behavior
  4. Small Strains and Finite Srains frameworks

In v8.17.1, Finite Strain extension is not yet available. The other points are fullfilled.

General Description

The introduction of composite voxels in the file material.xml requires the modification or addition of the 3 following xml nodes :

1. <Material.. > .. </Material>

All the materials present in the unit-cell are defined as previously (see STANDARD DESCRIPTION), whether they are present or not as homogeneous voxels. A node <Coeff_composite .. /> must be added to provide an elastic isotropic behavior for the numerical integration of the average behavior (Laminate or Reuss). The two parameters are respectively the Lamé coefficients \((\lambda,\mu)\)

2. <Material_composite> .. </Material_composite>

The path of the directory were the complete description of composite voxels is given according to a precise format given below.

3. <Interphase> .. </Interphase> (if required)

List of the materials and/or zones which do not appear as homogeneous voxels. Actually, sometimes, a specific ID of materialID.vtk or a specific zone of zoneID.vtk can be completely overlaid by the composite voxels so that it doesn’t appear anymore as homogeneous voxels. This is especially the case when dealing with thin interphases.

Format for Composite Voxels description

The whole set of composite voxels is divided into composite materials. Each composite material is defined by:

The complete definition of composite voxels is given in a single directory containing:

  • one file
  • one directory per composite material

One file

The file list_composite_materials.txt lists the different composite materials. Each line has the form I J K ... P averaging_rule where I, J, K … P are the material IDs present in the composite material.

Example for list_composite_materials.txt:

1 2 4 6 laminate
1 1 voigt

In the example above, two composite materials are defined.

The first one :

  • contains 4 phases, associated to the material IDs 1, 2, 4 and 6 (corresponding to numM in STANDARD DESCRIPTION). Here each phase has a different material behavior law.
  • uses laminate averaging rule.

The second one :

  • contains two phases associated to the same material ID “1” (corresponding to numM in STANDARD DESCRIPTION). In that case, the two phases have the same behavior law but may have different material coefficients associated to different zones (see numZ in STANDARD DESCRIPTION)
  • uses the voigt avering rule,

One directory per composite material

Each directory rep_I_J_K..._P describes the geometry of each composite material listed in list_composite_materials.txt. In the example above, two directories, rep_1_1 and rep_1_2_4_6 are required. Within each directory, a local phase renumbering is considered : I \(\leftrightarrow\) 1, J \(\leftrightarrow\) 2, K \(\leftrightarrow\) 3… and the following files are contained :

  • pos.bin (for i =1 .. number of phases) : the linearized positions of the composite voxels
  • zonei.bin (for i =1 .. number of phases) : their zone IDs for phase i (local phase number)
  • fvi.bin (for i =1 .. number of phases) : their volume fractions of phase i (local phase number)
  • Nijx.bin, Nijy.bin, Nijz.bin (for i, j =1 .. number of phases and i \(\ne\) j) : their components of the normal vector to the interface between phases i and j.
  • Sij.bin (for i, j = .. number of phases and i \(\ne\) j) : their surfaces of intersection between the voxel and the interface between phases i and j.

Each file contains a list of binary values, one for each voxel composite of the family, respecting the format BIN files.

Note

In practice, the mandatory files are:

  • For all averaging rules : pos.bin, zonei.bin (for i =1 .. number of phase), fvi.bin (for i =1 .. number of phase -1)
  • For laminate : N12x.bin, N12y.bin, N12z.bin (an arbitrary choice is made to keep the normal N12 for the laminate model). In addition, the laminate rule requires the components of the tangent vectors given in files Tx.bin, Ty.bin and Tz.bin.

However, all the normals Nij and surfaces Sij could be used in the futur, and can be usefull for evaluating interfaces average quantities in post-treatment.

Interphase Definition

The section <Interphase> .. <\Interphase> must be added to the file material.xml IF AND ONLY IF, after adding the composite voxel’s definition:

1. At least one material ID, matID, (numM in STANDARD DESCRIPTION) do not appear anymore as a homogeneous voxel. Such an “Interphase” material must be identified through:

<Interphase_material  numM="matID" Nzones="number_of_zones"/>

As all the materials in amitex_fftp, its coefficients can be associated to different zones so that the number of zones Nzones must be specified (unlike the other materials, the number of zones can’t be deduced from the files materialID.vtk and zoneID.vtk associated to homogeneous voxels).

2. At least one zone ID of a given material ID, matID, do not appear anymore as a homogeneous voxel. Such “Interphase” zones must be identified through:

<Interphase_zone_list numM="matID" Nzones="number_of_zones">
        zoneID1 zoneID2 ... zoneIDnumber_of_zones
</Interphase_zone_list>

Below is an example where material 2, which consists of 4 zones (associated to different material coefficients) does not appear anywhere as a homogeneous voxel. In addition, material 1 appears as a homogeneous voxel but not its three zones 2, 4 and 6.:

<Interphase>
        <Interphase_material  numM="2" Nzones="4">
        <Interphase_zone_list numM="1" Nzones="3">
                2 4 6
        </Interphase_zone_list>
</Interphase>

Example 1 : a voronoï polycrystal with isotropic grains

Note

In this exemple, and in any case where the definition of “interphase” is not required (all the material and material’s zones are present as homogeneous voxels), the simulation can be run without composite voxels by simply commenting the section <Material_composite   > .. </Material_composite>.

In this example, there is only one material and the file zoneID.vtk consists of a voronoï tessellation (one grain per zone): all the grains have an elastic isotropic behavior but different Lamé coefficients. In the material.xml file below, the unique material (numM=1) is defined with constant per zone Lamé coefficients and the Lamé coefficients for the numerical integration of the averaging rule are the same. The introduction of composite voxels is simply done by adding the section <Material_composite> .. <\Material_composite> to specify the directory where the composite voxels are fully defined (see Format-for-composite-voxels-definition)

<?xml version="1.0" encoding="UTF-8"?>
<Materials>

<!-- REFERENCE MATERIAL -->
<Reference_Material Lambda0="5.76923076923077e8" Mu0="3.84615384615385e8"/>

<!-- MATERIAL 1  -->
<Material numM="1" Lib="/home/gelebart/amitex_fftp/libAmitex/src/materiaux/libUmatAmitex.so" Law="elasiso" >

                <Coeff Index="1" Type="Constant_Zone" File="materiaux/coefficients/Lambda1_polyx27G_R21.bin" Format="binary"/>
                <Coeff Index="2" Type="Constant_Zone" File="materiaux/coefficients/Mu1_polyx27G_R21.bin" Format="binary"/>

                <Coeff_composite Index="1" Type="Constant_Zone" File="materiaux/coefficients/Lambda1_polyx27G_R21.bin" Format="binary" />
                <Coeff_composite Index="2" Type="Constant_Zone" File="materiaux/coefficients/Mu1_polyx27G_R21.bin" Format="binary"/>
</Material>

<!-- DIRECTORY FOR THE DEFINITION OF COMPOSITE VOXELS -->
<Material_composite>
                <Coeff_composite directory="microstructures/voxcomp/polyX_27G_R21_reuss"/>
</Material_composite>

</Materials>

Example 2 : a single composite voxel

Note

In this exemple, and in any case where the definition of “interphase” is required, the simulation can not be run without composite voxels by simply comenting the section <Material_composite   > .. </Material_composite>.

In this example, the file materialID.vtk consists of a single voxel. The composite voxel definition (not described here) identifies this voxel as a composite voxel between two different material (1 and 2). Obviously, in this specific case, there is no homogeneous voxel at all and these two materials must be introduced as “interphase” materials. The corresponding material.xml file is given below:

<?xml version="1.0" encoding="UTF-8"?>
<Materials>

        <!-- REFERENCE MATERIAL -->
    <Reference_Material Lambda0="22.21175e+10" Mu0="14.80765e+10"/>

    <!-- MATERIAL 1 -->
    <Material numM="1" Lib="(amitex)/libAmitex/src/materiaux/libUmatAmitex.so" Law="elasiso" >
                <Coeff Index="1" Type="Constant" Value="4.0385e+10"/>
                <Coeff Index="2" Type="Constant" Value="2.6923e+10"/>
                <Coeff_composite Index="1" Type="Constant" Value="4.0385e+10"/>
                <Coeff_composite Index="2" Type="Constant" Value="2.6923e+10"/>
    </Material>

    <!-- MATERIAL 2 -->
    <Material numM="2" Lib="(amitex)/libAmitex/src/materiaux/libUmatAmitex.so" Law="elasiso" >
                <Coeff Index="1" Type="Constant" Value="4.0385e+11"/>
                <Coeff Index="2" Type="Constant" Value="2.6923e+11"/>
                <Coeff_composite Index="1" Type="Constant" Value="4.0385e+11"/>
                <Coeff_composite Index="2" Type="Constant" Value="2.6923e+11"/>
    </Material>

    <!-- DIRECTORY FOR THE DEFINITION OF COMPOSITE VOXELS -->
    <Material_composite>
                <Coeff_composite directory="microstructures/VoxComp"/>
    </Material_composite>

    <!-- IDENTIFICATION OF MATERIALS WHICH ARE NOT PRESENT AS "HOMOGENEOUS" VOXELS -->
    <Interphase>
                <Interphase_material  numM="1" Nzones="1"/>
                <Interphase_material  numM="2" Nzones="1"/>
    </Interphase>

</Materials>

The two materials are desribed in the two sections <Material.. > .. </Material>. Note that Lamé coefficients are given, if necessary, for the numerical integration of the averaging rule (<Coeff_composite ../>).

As already mentionned, the two materials do not appear as homogeneous voxels and they are identified as “interphase” <Interphase> .. </Interphase>.