Overview

MFront comes with a handy and easy-to-use tool called MTest. This tool handles two types of computations:

• The description of a single material point.
• The description of a pipe, subject to various kinds of loadings.

From various benchmarks, this tool has been found much faster (from ten to several hundred times depending on the test case) than using a full-fledged finite element solver such as Cast3M or Code_Aster.

The mtest python module, described here, allows much finer control of the computations.

Usage

MTest can be used from the command line as follows:

$ mtest [options] inputfile

If the input file has the ptest extension, MTest will assume that the input files describes a test on a pipe. Otherwise, a test on a material point is assumed.

This can be changed by using the --scheme option that currently accept two values:

• --scheme=mtest to specify that the simulation of a single material point is intended.
• --scheme=ptest to specify that the simulation of a pipe is intended.

Getting help

The --help-keywords-list command line option

The list of available keywords are available thanks to the --help-keywords-list option. By default, this command describes the list of keywords associated with the simulation of a single material point. To see the keywords associated with the simulation of a pipe, use --scheme=ptest option before the --help-keywords-list option.

The --help-keyword option

The --help-keyword displays the help associated with a keyword. If this keyword is specific to the simulation of a pipe, use --scheme=ptest option before the --help-keyword option.

Material point

MTest can test the local behaviour of a material, by imposing independent constraints on each component of the driving variables (or deformation gradient or the stress). It is equivalent to the SIMU_POINT_MAT operator available within the Code-Aster finite element solver [1] or to the SiDoLo software [2].

MTest can be used to model various experiments, as far as a stage implying strain localisation is not reached: tensile, compressive or shear tests driven by stresses or deformations, pipe loaded by internal or external pressure, test, etc.

MTest generates a text file containing the evolution of the strains (for small strains behaviours), the stresses and the state variables during the loading history. Other MTest functionalities include:

• the ability to test all the behaviours handled by MFront (small strain and finite strain behaviours, cohesive zone models);
• the ability to test isotropic and orthotropic behaviours;
• the support of various modelling hypotheses, notably the plane stress and axisymmetric generalised imposed plane stress hypotheses;
• many features to evaluate the numerical performances of mechanical behaviours. For example, user can compare the computed tangent consistent operator to a numerical approximation;
• a C++ library and a Python interface [3]. MTest can be embedded in general purpose scientific environment to fit behaviour parameters against experimental data. In particular, MTest can be used in ADAO [4], a module for Data Assimilation and Optimization of the Salome platform [5];
• comparison of the results (strains, stresses, internal state variables) to reference or analytical results. MTest automatically generates XML file using the JUnit format. Those files can be used for reporting using the Jenkins continuous integration application. This functionality is central in the quality assurance procedure of MFront.

Through an appropriate option, a behaviour implementation generated through MFront may create an MTest file in case of integration failure: this MTest file only describes the failed time step with the appropriate initial conditions. This feature is particularly useful to analyze the failure of large simulations which may happen after several hours of computations.

Example

@Behaviour<aster> 'src/libAsterBehaviour.so' 'asterplasticity';
@MaterialProperty<constant> 'YoungModulus'     150.e9;
@MaterialProperty<constant> 'PoissonRatio'       0.3;
@MaterialProperty<constant> 'H'                100.e9;
@MaterialProperty<constant> 's0'               100.e6;
@ExternalStateVariable 'Temperature' {0:293.15,3600.:800};
@ImposedStrain<function> 'EXX' '1.e-3*t';
@Times {0.,1 in 20};

Simulation of a pipe

MTest can also be used to model a pipe subjected to various loadings.

Radial loadings

Tight pipe

Specifying the gas equation of state

The equation of state of the gas can be specified using the @GasEquationOfState keyword.

This keywords expects a function \(f{\left(P,V,T\right)}\) such that the equation of state is:

\[ f{\left(P,V,T, n\right)}=0 \qquad{(1)}\]

where:

• \(P\) is the inner pressure in the pipe.
• \(V\) is the volume of gas in a pipe of unit height. For small strain analyses, \(V\) is equal to the initial volume of the pipe. For finite strain analysis, \(V\) is computed by: \[ V = \pi \, {\left.R\right|_{t+\Delta\,t}}^{2} \, {\left(1 + {\left.\varepsilon_{zz}\right|_{t+\Delta\,t}}\right)} \] where \({\left.R\right|_{t+\Delta\,t}}\) and \({\left.\varepsilon_{zz}\right|_{t+\Delta\,t}}\) are respectively the current estimation of the radius and the current estimation of the linear axial strain at the end of the time.
• \(T\) is the current value of temperature at the inner surface of the pipe (the temperature is assumed homogeneous inside the pipe).
• \(n\) is the amount of substance of the gas. This quantity is assumed constant during the experiment. Its value is deduced from Equation (1) by using:
  • The filling pressure (see the @FillingPressure keyword).
  • The filling temperature (see the @FillingTemperature keyword).
  • The initial inner radius of the pipe¹.

A first example

In this example, the equation of state of a perfect gas is specified:

@FillingPressure      1.e5;
@FillingTemperature 293.15;
@Real 'R' 8.314;
@GasEquationOfState 'P*V-n*R*T';

A second example

In this example, the equation of state of a perfect gas is specified, but one imagine that the gas can only occupy a fraction \(\alpha\,V\) of the volume inside the pipe:

@FillingPressure      1.e5;
@FillingTemperature 293.15;
@Real 'R' 8.314;
@Real 'a' '0.8';
@GasEquationOfState 'P*a*V-n*R*T';

Axial loadings

Modelling a non-deformable mandrel inside

The effect of a mandrel located inside the pipe can be modelled by defining the evolution of its radius \(R_{m}\) using the @MandrelRadiusEvolution keyword. The inner radius \(R_{i}\) will then satisfy the following unilateral boundary condition:

\[ R_{i} - R_{m} \geq 0 \]

This boundary condition is imposed by using a Lagrange multiplier. Its value is given by the contact pressure in the output file. The total pressure applied to the inner surface of the pipe is given by the sum of the imposed pressure (if any) and the contact pressure. Only the imposed inner pressure is used to compute the end cap effect.

This boundary condition is not compatible with:

• boundary conditions imposing the evolution of the inner radius of the pipe or the outer radius of the pipe.
• the modelling of a tight pipe.

Axial binding

If the evolution of the axial growth of the mandrel is defined using the @MandrelAxialGrowthEvolution keyword, an axial binding between the mandrel and the pipe is assumed, i.e. the difference between the axial growth of the pipe \(\varepsilon^{p}_{zz}\) and the axial growth of the mandrel \(\varepsilon^{p}_{zz}\) is assumed to be constant to its value when the contact between the mandrel and the pipe is detected:

\[ \varepsilon^{p}_{zz}-\varepsilon^{m}_{zz}=\textrm{Cste} \]

This axial boundary condition is not compatible with the boundary condition imposing the evolution of the axial growth of the pipe.

Support for failure criteria for pipes

Failure criteria can be added to pipe modelling using the @FailureCriterion keyword. Note that no failure criterion is currently shipped with MTest.

A failure criterion is called at the end of each time step to detect failure of the pipe.

Each failure criterion adds a column to the output file giving the status of the criterion:

• 0 means that the criterion is not met, i.e. no failure is detected and the pipe is sound.
• 1 means that the criterion is met, i.e. failure is detected and the pipe is broken.

In case of failure, three policies can be selected using the @FailurePolicy keyword:

• ReportOnly: failure does not affect computation. The evaluation of the failure criteria only affects the output file.
• StopComputation: failure leads to reject the current time step. If substepping is enabled, the time step is divided by two. With this policy, one can thus only approach the failure time, but never go beyond.
• FreezeState (or FreezeStateUntilEndOfComputation): if a failure is detected, the state of the structure is frozen and does not evolve. No equilibrium is performed, the behaviour is no more called and PipeTest will output the same results again and again until the end of computation. This option may be useful when optimizing material parameters.

Example of usage

@FailurePolicy 'FreezeState';
@FailureCriterion 'ElongationAtBreak' {maximum_value : 1e-4};

Support for oxidation models for pipes

The @OxidationModel keyword introduces a model which computes an oxidation length at either the inner boundary or the outer boundary of the pipe. This keyword must be followed by a data map with the following entries:

• model: the name of model.
• library: the name of the library in which the model is available.
• boundary: the name of boundary on which the model is defined. This variable must be equal to inner_boundary or outer_boundary.

An oxidation model must define an internal state variable named OxidationLength. The values of the material properties and external state variables passed to an oxidation model are computed on the boundary on which the model is defined.

The definition of at least one oxidation model automatically defines an evolution named OxidationStatus which states if an integration point is inside an oxidation layer.

Example of usage

@OxidationModel{
  model : 'umatm5deziroxoxidationmodel_srma2020b',
  library : 'src/libUmatM5.so',
  boundary : 'outer_boundary'
};

References