Learning Forum

[Oleg Makarov]

Is it possible to use a user-defined material model (UMAT) with Smoothed particle Galerkin (SPG) method?

I want to model the impact problem with a material fracture but I have to use my own material model. What are the options to implement such a task?

For example, if I use MAT_ADD_ERROSION can it be combined with any other material during SPG modeling?

Or SPG method allows only a limited list of material models even with MAT_ADD_ERROSION?

[Andreas Koutras]

Hello Oleg,

User-defned materials are supported for SPG in the same manner as for solid elements. *MAT_ADD_EROSION is also supported for SPG. However, please use LS-DYNA R14.

Note that the function of *MAT_ADD_EROSION is different with that of FEM. This keyword will delete an element in FEM when the criterion is fulfilled. In SPG, the connection of two particles (the bond) will be cut when the criterion is fulfilled, that means you will not see particle erosion.

I hope this helps.

[Oleg Makarov]

Mr. Andreas Kourtras, thank you for your answer!

Could you please clarify a few moments?:

1. Does UMAT subroutine require any special design/commands to use with SPG or I can use the same UMAT for FEM elements and SPG particles in the same model?
2. How can I implement user-defined failure criteria for SPG in this case? I have special constitutive model with Failure criterion in 3D stress domain. Therefore MAT_ADD_EROSION is not suitable, right? Is it possible to implement failure criterion inside UMAT for SPG particles?

Thank you in advance!

[Andreas Koutras]

Hello Oleg,

SPG uses bond-failure mechanism to represent ductile material failure. In SECTION_SOLID_SPG, FS and stretch is the failure criteria . The value of FS and stretch depend on the damage type: IDAM.  

The developer mentioned that the SPG failure with UMAT has not been fully tested.

What type of material are you modeling?

 

[Oleg Makarov]

I want to model brittle material (ice) with elliptical failure criterion (figure below), which is both temperature and strain rate dependent.

For current step I don't need any material yielding / plastic deformations. I want to model the material with orthotropic elastisicty and failure according to criterion in Fig. below.

If it is possible with SPG and UMAT it would be great.

1. Or may be I can modify some special parameter in UMAT that will address creterion in MAT_ADD_EROSION when failure criterion is met? Therefore bond failure will occure because some MAT_ADD_EROSION criterion is met, but this quantity will be met because of some parameter inside UMAT
2. Or can I set this failure criterion as yield criterion (yield surface) and use very small critical value for effective plastic strain (IDAM=1) to model material failure in this way?

[Andreas Koutras]

Hello Oleg,

Here are the suggestions of the developer for your problem:

"SPG can use effective plastic strain, first principle stress, maximum shear strain or GISSMO damage value as bond failure criterion.  According the description from the user, my suggestions are:

1.  MAT_ADD_EROSION with UMAT's history variable is not suggested. SPG doesn't use element erosion, but bond failure. 
2. User can use MAT_ADD_DAMAGE_GISSMO uses HISVN to define the history variable to accumulate damage value. Then use IDAM=0 to use damage value to failure bond. Set dtype=0, don't delete elements. As I mentioned, this way is not verified with SPG. 
3. IDAM=1 with very small EPS should work. It becomes a ductile failure.
4. Alternative way, user can try to set the stress of material points to zero when criterion match. Then use IDAM=3 and bigger stretch to fail bond. I cannot grantee the performance since it is brittle failure. The brittle failure is very sensitive."

[Oleg Makarov]

Thank you for your answer. Now I see the possible ways.

I want to use option #4 that you mentioned but with another implementation. Instead of setting the stress to zero and using of stretch to model material failure, can I set the first principle stress to an extremely high value, which is not normal for ice-like material (for example 500 MPa), and use IDAM=2 to model the failure? Will be brittle failure more stable in that way?

[Andreas Koutras]

Hello, for IDAM=2 you can use the tensile strength of your material. Once this is reached bond failure will occur preventing the principal stress to increase further.

[Andreas Koutras]

Consider also Option 2 which provides additional flexibility given that you specify a history variable from the umat to control the damage.