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Consolidating Multiple Contact Definitions to a Single Contact


Over the last few years, the simplicity of defining a global AUTOMATIC_SINGLE_SURFACE contact to treat the interactions between multiple parts of varying stiffnesses and element types has changed the way we model contact interfaces. They are not only simple to define but also promote better modeling since they hugely eliminate the need to manually identify contact surfaces which may be prone to errors. Legacy models which could have several hundred contact definitions (one-way and two-way types) can be extremely difficult to convert to a single contact definition since it requires a thorough review of manually defined segments. Here are some approaches that could be followed to migrate the legacy models to use a single consolidated contacts.

Approach 1
The simplest approach is to identify all the segments and use the parent parts to define a single surface definition. The main disadvantage is that segments which may not have been included in original contact definitions will now be included which may introduce new errrors especially if the newer segements are inter-penetrating.

Approach 2
The second approach, which is the recommended approach, is to take advantage of the output from D3HSP file. For every contact defintion (one-way or two-way) in the legacy models, LS-DYNA outputs the list of slave-segements (two-way) and a list of master-segments into the D3HSP file. We can consolidate all the slave and the master segments from the D3HSP file into one global segment list and use this segment list in a single surface definition. This avoids the addition of new segments. If we need the forces of interface as in the original legacy model, we can simple change the old contact defintions to a FORCE_TRANSDUCER contact which will provide a way to extract crucial interaction forces.
In some situations, the frictional constants can be different between various older contact interfaces. To model this, we can use the DEFINE_PART_FRICTION along with FS<0 to model part-pair based frictional coefficients.

  • Sven says:

    Hi Suri,

    When consolidating many surf-surf contacts into one single surface contact, is your aim always to reduce it to a single definition?

    I understand that this post shows a method to do this in a segment-wise fashion, but I’m thinking about a more general rule of thumb (as defining one big segment list will not adapt easily to added/removed/remeshed parts).

    For example, consider a simulation of two (shell) soccer balls colliding with each other. Inside each soccer ball is a bunch of smaller (shell) golf balls.

    In this case, I think there are two approaches:
    1. Define a single auto-single-surface contact containing all soccer/golf ball parts
    2. Define 3 contacts as follows:
    – An auto-single-surface containing one soccer ball and all its contained golf balls
    – An equivalent auto-single-surface contact for the other soccer ball
    – A surface-to-surface contact between only the two soccer ball shells

    For me, the second approach seems quite an intuitive one, although I can’t really offer any concrete reasons why it would be better than the first (when used with appropriate force-transducer contacts). One difference is that the second approach will show the cpu time used during dyna’s calculation in the d3hsp file (which may be useful for troubleshooting/optimisation). I’m curious if there are any other differences, and if you have any reasons why you’d choose one approach over the other.

    Cheers,
    Sven.

  • Suri says:

    Hi Sven,

    Not sure how I missed replying to this comment. In your example, both approaches are equally good with the exception the second approach requires a manual input. As you mentioned using a single surface with appropriate transducers is a more elegant approach since it requires less knowledge about who is impacting whom as this is all figured out internally.

    Computationally, both boils down to a node-segment pair checking with some subtle differences that is outside of what your concerns are.

    Suri

  • Sujit Ojha says:

    Hi Suri,

    I used single surface contact between two plates with tied contact between each other.
    Is it a good practice or suggest me some good way to model following problem:

    Two plates are tied together. An impactor hitting the top plate and taking away bulge of elements from the first plate. Then finally the failed elements are coming in contact with bottom plates directly.

    Thanks,
    Sujit Ojha

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