[mtest] Add to ability to define failure criteria in `ptest`

[leopeuziat]

Is it possible to define the failure criteria in an external MFront files ?

[thelfer]

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

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 freezed and do 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};

Note This example assumes that a failure criterion named ElongationAtBreak has been loaded from an external library.