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Multimaterial MPM
Multimaterial MPM Calculations
NairnMPM can use multimaterial mode where each material has its own velocity field and intersections between materials on nodes are resolved using various contact mechanics models. The commands to set up the mode are:
- MultimaterialMode: Sets to use multimaterial modes and sets some options.
- ContactPosition: Determines method for detecting contact when using particle displacements.
The commands to define the contact mechanics:
- ContactMM: Set default multimaterial mode contact properties.
- Contact: This material property can set custom material-material contact properties.
- shareMatField (matID): When this command is used within a material definition, it allows two or more materials to share the same velocity field.
A few deprecated commands have old options for setting friction and interface properties.
Multimaterial Mode Concepts
In multimaterial MPM, each material extrapolates to a separate velocity field on the grid. Nodes with a single velocity field proceed by normal MPM methods. Nodes with velocity fields from more than one material need to adjust the momenta to match the modeled physics. The important concepts are:
- Detection of Contact
- The first step is to decide if the materials at the node are actually in contact. The details are given in the OSUPDocs wiki. This check proceeds by checking velocity of approach along with some combination of criteria, which can be picked using the
(Vmin)
and (dispCheck)
parameters on the MultimaterialMode command and using the ContactPosition command.
- Adjust Nodal Momenta or Add Internal Forces
- Once contact is detected, the nodal momenta for each velocity field are adjusted to reflect the contact mechanics using any availablce contact law. The contact law is set using the ContactMM command or a
Contact
material property.
- Evaluation of Surface Normals
- The surface normals are needed for both the above steps. Their evaluation is a crucial part of successulf multimaterial mode simulations and more details are given on the OSUPDocs wiki. The method used is determined by the
(normals)
and (rigidBias)
parameters of the MultimaterialMode command. The logistic regress methods is preferred.
Notes
- The general principles of multimaterial contact are described in Bardenhagen et. al (2002). The new options of detecting contact by displacements and finding normals by new methods are unique to
NairnMPM
and are described in Lemiale et al. (2010) and Nairn (2013). The current state-of-the-art are the logistic regression methods described in Nairn et al. (2020)
- Multimaterial mode works best (i.e., has most rigorous physics) when the simulation has only two materials or when no nodes ever see velocity fields from more than two materials. The calculations will continue when 3 or more materials intersect at a single node and will do something reasonable. Such simulations, however, may not be accurate and may display problems.