How to remove initial penetrations in a braided cord with a large number of contact regions?

We are being challenged with removing initial penetrations in an LS-DYNA finite element model of a braided cord. The model of the braided cord is built at the level of the 16 tows that comprise the cord. The model is relatively complex in that contact can happen at multiple locations among the 16 tows.

The geometry of the model is developed in one program, and that geometry is then imported into LS-Prepost in preparation for analyzing in LS-DYNA. There are initially some minor penetrations between the individual tows of the braid in the model that comes from that outside program. For running the analysis in LS-DYNA, we have turned on PENCHECK in the SURFACE_TO_SURFACE CONTACT keyword, but we are not seeing that this option is removing these penetrations.

We have run an implicit dynamic relaxation of the braid with the contact keyword SURFACE_TO_SURFACE_INTERFERENCE where we can ramp up the contact stiffness during the dynamic relaxation phase to remove penetrations. Our goal is to remove the penetrations by using dynamic relaxation to get the new nodal locations of the tows to update the model to be one with no penetrations. Despite all of our efforts, we have yet to get the relaxation to clean up all of the initial penetrations.

The boundary conditions that are used on the braid are that all of the nodes on the top are constrained to have the same x-displacement and the same boundary constraint for the nodes on the bottom. These boundary conditions keep the ends flat (Fig 1). We ran the implicit dynamic relaxation analysis of the model, and it was able to remove some penetrations -- but not all (Fig 2). This outcome of not removing all penetrations did not change whether we increased the contact stiffness or changed the tolerance in the dynamic relaxation keyword. We also ran it in explicit and did not get as much penetration removal as we did when we ran the model in implicit.

We are seeking help on what analysis options can be used to remove these initial penetrations.

A copy of the *.k file is available upon request.

[email protected]

[email protected]


Fig. 1 Side view of braid showing left and right ends of the cord to be flat

Fig. 2 Close-up of initial penetrations

Answers

  • cxquancxquan Berkeley, CAForum Coordinator

    You have used the right procedures to remove initial penetrations. You may also use LSPP to find the penetrations and use FIX to remove them.

    If problem persists, you may contact ANSYS Tech Support with your model and ask for help.




  • Dear James,

    First since you are using LS-DYNA implicit solver, please make sure your model adheres to the following recommendations https://www.lstc.com/sdb/361

    To remove penetrations via simulation, you can use MORTAR contacts (*CONTACT_AUTOMATIC_SINGLE_SURFACE_MORTAR, include all the parts for simplicity) and set IGNORE=3 or -3 (negative value tells the code to ignore self-penetrations), with MPAR1 would be time value when all these penetrations gets removed. You may want to experiment with scaling up contact stiffness (SFS) to make sure the penetrations are removed as expected. Please let us know if you have follow up questions.

    -Satish

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