Creating MPM Materials

This section explains how to write C++ code create a new material class for use in NairnMPM.

Getting Started

The first steps are to create source files for the new material class, to give the material a unique name, and to allow the main NairnMPM code to instantiate the material class when it is needed for a particle.

Class Source Code

The best way to create the source code is to duplicate the NewMaterial.cpp and NewMaterial.hpp files, which are templates for a new material class. Edit the files and change NewMaterial to the new material's class name (e.g., MyMaterial). The NewMaterial template is a subclass of MaterialBase. Normally the new material will be a subclass of another class. If so, change MaterialBase references, as needed, to the actual parent class. These changes are needed whenever a method in the new material class needs to pass control to its immediate super class.

Material Class Hierarchy

The new material should be inserted into the NairnMPM material class hierarchy with its name (from previous step) and a unique ID (an integer). In the new material's header files, replace NEWMATERIAL with a defined constant representing the new material's ID (which by convention is in UPPERCASE) and replace the number in the new constant's definition with the new material's ID. For example:

#define MYMATERIAL 102

All documented materials use low numbers (starting with 1). To avoid conflicts, those working on custom materials should use large numbers (>100).

Editing Required in Core Code

Almost all coding will be done in the new material class files, but for that material to be recognized as an option in NairnMPM, two places in the core code have to be edited first. These should be the only changes needed outside the new material's class files.

1. Include the new material's header file at the top of Common/Read_XML/MaterialController.cpp:

    #include "Materials/MyMaterial.hpp"
   

   or the appropriate relative path to the new material's header file.

2. In MaterialController::AddMaterial(int matID,char *matName), add a new case in the switch(matID) section to call the default constructor of the new material when matID matches the new material's constant defined above.

   case MYMATERIAL:
      newMaterial=new MyMaterial(matName,matID);
      break;
   

   If needed you can define a custom constructor and use that in this code (this need is very rare).

Compiling with make Command

If you want to be able to do command-line compile using the make command, the new material source files needed to add to the file NairnMPM/build/makefile. The steps are best done by comparing to a similar material's files in that makefile. The main makeFile editing stepsare:

1. Define an alias for relative path from makefile to the new material class files (in the list d)

   MyMaterial = $(src)/Materials/MyMaterial
   

2. Add an object file to the list of "all compiled objects" in the form MyMaterial.o.
3. Add the new material's header file to the header file's needed to compile MaterialController.hpp. Using the alias defined in step 1, the added text will be $(MyMaterial).hpp.
4. Compile the new material's source file with lines such as

   MyMaterial.o : $(MyMaterial).cpp $(dprefix) $(MyMaterial).hpp $(MaterialBase).hpp $(MPMBase).hpp \
   			$(CommonException).hpp
   	$(CC) $(CFLAGS) $(headers) -include $(prefix) $(MyMaterial).cpp
   

   The list of headers must include all headers explicitly (or implicitly) included in the new material's source code. The leading white space for lines 2 (and on) must use only tabs and only one tab for the final compilation line.

Writing the Material Class Source Code

The remaining steps can usually all be done by editing only the material's source code file. The requirements and optional methods are described in the following sections. This stage involves editing the template file created above and customizing the listed methods and/or deleting optional methods that are not needed. All methods that are overridden should be declared as virtual in the header file to insure the correct code is used at run time.

Creating Material Properties

Usually a newly-created material type will have additional material properties. To create such properties, you need to define them, allow them in input command files (i.e., update the DTD file), set them in the material class file, validate them, and finally use them in calculations. The following steps are needed:

1. Define a class variable for the property in the .hpp file (usually int or double). It is best to define these properties in the protected section of the class, although public properties are sometimes needed (and are allowed).
2. To allow the property in input files, select a unique property name (the name may or may not be same as variable in previous step). Define that property by name in the DTD file, usually as a simple XML element such as:

   <!ELEMENT prop (#PCDATA)>
   

   where prop is the new property name.

3. Add that property's name to the list of allowed elements within the Material element definition (beginning in <!ELEMENT Material in the DTD file.
4. Set some default value for the new property variable in the new material's constructor method.

Once a property variable and name are defined, you set that variable, check it setting, and use it with the following methods:

Required Initialization Methods

char *InputMaterialProperty(char *xName,int &input,double &gScaling)

If xName string matches a property name for this material, set input to DOUBLE_NUM or INT_NUM (depending on the type of variable) and return a pointer to the class variable for that property. If gScaling is set, the input value will be multiplied by gScaling after it is read. If xName is not a recognized property return Parent::InputMat(xName,input) to allow the super class to look for their valid property types (replace Parent with the name of the superclass).

const char *VerifyAndLoadProperties(int np) (optional)

This method is called after the input file is read but before the new material is printed to the results file. You can use this method to verify the input material properties are physically allowed. If not, return a string with an error message and the simulation will be aborted. If there are no errors, return Parent::VerifyAndLoadProperties(np) to let super class check properties as well (replace Parent with the name of the superclass) The input np is a constant for the analysis type (e.g., plane stress, plane strain, 3D, etc.). If the properties are valid, but maybe not allowed in current MPM mode, that check should be done instead in ValidateForUse() below. This method is only called once at the beginning. For efficient, it is therefore a good place to to calculate material properties that are independent of the particle state and thus will remain constant throughout the calculation (e.g., to calculate specific properties by dividing by density or to convert to units more convenient to the constitutive law).

void PrintMechanicalProperties(void)

This method should print all mechanical properties or call a super class and thenprint just the new mechanical properties. Use a style similar to other materials. To help in formatting, you can use the MaterialBase class method PrintProperty(label,prop,units) to print a label, a property numeric value, and units within one column. You can use several calls in sequence to get several properties on the same line. You can also call PrintProperty(text,right) to print text in a column and right or left justified if right is true or false. This method need not pass on to MaterialBase because it does not print any mechanical properties. This method is called after VerifyAndLoadProperties() is done.

Optional Initialization Methods

void ValidateForUse(int np)

This method is called just before the first MPM time step and only for materials used by one or more materials. Throw a CommonException if this material cannot by used in current MPM mode (type specified in np which will be PLANE_STRAIN_MPM, PLANE_STRESS_MPM, AXISYMMETRIC_MPM, or THREED_MPM). If no exceptions, must call the same method in the parent class. Basic material properties are usually checked in VerifyAndLoadProperties() instead; this method is for materials that may have valid properties, but may be contingent on other MPM settings or for materials only implement for certain styles of simulations.

FillTransportProperties(TransportProperties *)

This method is called by MaterialBase::VerifyAndLoadProperties() and can be used calculate transport properties that are independent of the particle state and thus will remain constant throughout the calculation. The MaterialBase class automatically finds diffusion and conductivity tensors for isotropic materials by using material properties D (in diffusionCon) for diffusion constant and kCond (in kCond) for thermal conductivity. It only needs to be overridden if something more is needed.

void PrintTransportProperties(void)

This method should print all transport properties in format similar to other materials (see help PrintProperty() methods described above). It should only print them if transport is activated (i.e., if(DiffusionTask::active) or if(ConductionTask::active)). The MaterialBase class prints isotropic properties and the ORTHO and TRANSISO1(2) classes print anisotropic properties. No additional printing is needed if one of these classes handles the task.

void SetInitialParticleState(MPMBase *mptr,int np,int offset) const

This method is called in preliminary calculations before a simulation states. It can be used, if needed, to set and material properties for each material point (in the mptr pointer).

Basic Class Accessors

These methods (required unless specified as optional) return basic facts about the material:

Required Accessors

const char *MaterialType(void)

Return a short name to describe the material. This string is printed with material properties in the output of simulations.

double WaveSpeed(bool,MPMBase *)

Return the maximum wave speed for material in velocity units. This method is called once for each material point at start of calculation and after material properties have been defined. If the wave speed might change during the simulation, be conservative and return the maximum possible save speed. The first parameter is true for 3D calculations or false for 2D. This speed is needed to pick the time step for explicit calculations. The second parameter is pointer to a material point in case the wave speed depends on the initial particle state. This method is also used in some crack propagation methods. If the material supports crack propagation, make sure it gives good results for these uses.

Optional Accessors

double CurrentWaveSpeed(bool,MPMBase *)

Return the current wave speed, which might depend on particle state. This method is only used by the AdjustTimeStep task to change time step as needed; without this method, the time step will not adjust for this new material.

double ShearWaveSpeed(bool)

Return the shear wave speed for the material. This method is only used by silent boundary conditions for force. The MaterialBase class returns WaveSpeed()/sqrt(3). You only need to override this method if you want a better value. Also note that silent boundary conditions only work for isotropic materials.

double MaximumDiffusion(void)

Return maximum diffusion constant in diffusion units. This method is called once for each material at the start of the calculations and after material properties are defined. The MaterialBase class returns the appropriate result for isotropic materials. You only need to override if you need a different result.

double MaximumDiffusivity(void)

Return maximum thermal diffusivity in diffusion units = k/(100 rho Cp). This method is called once for each material at the start of the calculations and after material properties are defined. The MaterialBase class returns the appropriate result for isotropic materials. You only need to override if you need a different result.

double GetCurrentRelativeVolume(MPMBase *)

Return current relative volume to be used to convert tracked stress to true stress. Low strain materials track stress/initial density and they should return 1 (which is what base material class returns). Large deformation materials should be tracking specific Cauchy stress (= Kirchoff stress/initial density) and therefore should return relative volume (i.e., initial density/current density).

Tensor GetStress(Tensor *sp,double pressure)

Return current stress. Most materials just return the contents of sp which is the particle stress. Materials that track pressure and deviatoric stress separately (or use some other stress tracking scheme), should reconstruct and return the full stress in this method.

bool SupportsArtificialViscosity(void)

Materials that implement artificial viscosity must override this method to return TRUE and then implement the calculations in the constitutive law methods.

bool SupportsDiffusion(void) const

if a material does not support diffusion or poroelasticity, override and return false only called when diffusion is active. The MaterialBase class returns true.

double GetMaterialConcentrationSaturation(MPMBase *mptr) const

Return the saturation concentration for the material. Only needed if the value depends on state of the material point. The MaterialBase returns the saturation concentration property.

double GetRho(MPMBase *) const

Return the density for the material. Only needed if the value depends on state of the material point. The MaterialBase returns the density property.

Alternate Particle Strain

The particle information can store two strains - total strain and an alternate strain. A given material type can use the alternate strain anyway it needs, but only certain styles are fully supported by visualization tools. The following table lists supported alternate strains and how visualization options plots elastic and plastic strains.

Type	Stored Alternate Strain	Viz Elastic Strain	Viz Plastic/Alternate Strain
NOTHING	nothing	total strain	0
ENG_BIOT_PLASTIC_STRAIN	plastic strain	total - plastic strain	plastic strain
LEFT_CAUCHY_ELASTIC_B_STRAIN	elastic B tensor	elastic from B	total - elastic from B
LEFT_CAUCHY_TOTAL_B_STRAIN	total B tensor	total strain	0
MEMBRANE_DEFORMATION	membrane deformation	total strain	0

Whenever strain is archived, total rotational strain is also archived. Thus all materials can plot total strain and any components of the total deformation gradient. Note that versions of NairnMPM prior to 11.0 used to have option for materials to partition elastic and plastic strains into the two particle strains. Such materials should only be visualized in the NairnFEAMPMViz tool provided with that old version. The use of current visualization tools on output from old calculations will not correctly plot elastic and plastic strains.

When creating a new material, the following method should return how it uses the alternate strain:

int AltStrainContains(void) (optional)

If the alternate strain tensor is used, return the quantity stored by the material. The supported options are in the "Type" column in the above table. If you choose to use that alternate strain for some other purpose, the material can return NOTHING to avoid confusing visualization calculations.

This above method is only needed in calculations to support correct global archiving and VTK archiving of strain quantities. The separate NairnFEAMPM and NairnFEAMPMViz tools evaluate strains from raw total and alternate strains depending on material type. A problem arises when adding your own materials. These tools will not know what is in the plastic strain. The current tools assume all unknown materials store ENG_BIOT_PLASTIC_STRAIN in the alternate strain. You can visualize results as follows:

1. To see what is in particle strain, plot total strain.
2. To see what is in alternate strain, plot plastic strain.
3. Plots of elastic strain (which are difference of the two strains) may not make sense.

History Dependent Properties

For some materials, the constitutive law will depend on the state of the particle in the form of history-dependent data (e.g. cumulative plastic strain for plasticity or strain history for viscoelasticity). Such properties are implemented in a material by using history data. The methods in the material class are as follows:

char *InitHistoryData(char *pchr,MPMBase *mptr)

Tthis method is called once for each material point using this material at the start of the calculations. If pchr==NULL, It should allocate memory to store history data on the particle, otherwise assume pchr points to enough memory for history data. Fill history data and return pointer to the data. It can be as simple as a single variable or a pointer to an array of any number of variables.

double GetHistory(int num,char *ptr)

This method should extract history variable number num from the allocated data pointer in ptr. The MaterialBase class assumes history is an array of doubles of length equal to NumberOfHistoryDoubles().

int NumberOfHistoryDoubles(void) const

Return number of doubles used in a simple array of doubles.

MPM Step Calculations

The bulk of MPM calculations for a material will be done in the material-dependent code that is called during each MPM time step. Below is a summary of the key methods. These methods should be made as efficient as possible.

Prepare for Updates

These methods are called prior to constitutive law methods to allow update of material properties when those material properties depend on the current state of the particle (e.g., properties that depend on temperature, moisture, orientation, or current particle's stress or stain) and to support parallel code for strain updates.

Need to explain particle-dependent variables

int SizeOfMechanicalProperties(int &altBufferSize) const

The size

void *GetCopyOfMechanicalProps(MPMBase *mptr,int np,void *matBuffer,void *altBuffer) const

This method is called just before the constitutive law on each time step. You can set any parameters for the material that depend on the current state of the particle. Things that never change (i.e., properties that are independent of particle state) should be calculated in VerifyAndLoadProperties() instead. Some materials also used in FEA make use of LoadMechProps() in this task, but that method is limited to 2D because it only inputs a single rotation about the z axis.

void GetTransportProps(MPMBase *,int,TransportProperties *) const (optional)

This method is called when looping over material points to store parameters needed in transport calculations (i.e., diffusion and conductivity tensors). It is called prior to the transport task to AddForces(). It is only needed for anistropic materials or for materials whose transport properties change depending on particle state. This method should load the required values into diffusionTensor and kCondTensor variables. It is automatically handled for subclasses of TRANSISO1(2) if the only effect is the current orientation. Transport properties that never change (i.e., independent of particle state) should be calculated in FillTransportProperties() instead.

double GetHeatCapacity(MPMBase *) (optional)

Return current constant-strain heat capacity in the material (and code should always call this method when heat capacity is needed rather then directly using the heat capacity variable). This call allows materials to implement a state-dependent heat capacity. Return current heat capacity heat capacity units or in of nJ/(g-K) of using Legacy units. This method can be omitted if heat capacity is a constant.

double GetCpMinusCv(MPMBase *mptr) (optional)

Return the difference between constant-stress ( C_p) and constant-strain ( C_v) heat capacities (and code should always call this method when heat capacity is needed rather then directly using the heat capacity variable). Return the difference inheat capacity units or in of nJ/(g-K) of using Legacy units. This call is only used by conduction calculations because heat flow is based on C_p instead of on C_v. For an elastic material, this difference is given by -M.αT/ρ, where M is stress-temperature tensor and α is thermal expansion tensor. For an isotropic material, this reduces to 3Kα²T/ρ where K is bulk modulus and α is linear thermal expansion coefficient. For an ideal gas, it reduces to nR/ρ. If this method is omitted, the difference defaults to zero (which is often close enough for most solid materials).

Constitutive Law Methods

Once the properties are set (from previous section), the most important method is the one that implements the constitutive law:

void MPMConstitutiveLaw(MPMBase *mptr,Matrix3 du,double delTime,int np)

This method applies constitutive law for the material and updates all needed particle properties. The required updates include stress (should be a specific stress), strain, plastic strain, rotational strain, work energy, residual energy, plastic energy, heat energy and any history dependent variables defined for the material. To support thermal and solvent expansion, include their effect on the constitutive law. The input du is the velocity gradient times the time step, which gives displacement gradient for this time step. See note below on particle temperature updates.

The update should never change particle temperature, because that would invalidate some thermodynamics calculations in NairnMPM. To cause a particle temperature change, call IncrementHeatEnergy() with a temperature or energy increment. The code will take care of translating this result into particle temperature rise when appropriate. The particle temperature will rise during adiabatic calculations. For isothermal calculations, the temperature will not rise, but the corresponding energy that was released will be reflected in heat energy. See thermodynamics modes for more details.