Modeling balloon expansion inside coronary artery

[Andre1212]

As suggested by the title, I am attempting to model a balloon expansion inside a coronary artery. The balloon is a uniform cylinder, whereas the artery is a non-uniform cylinder (See attached picture). I want to apply a given pressure inside of the balloon, causing it to expand until it touches the vessel. Then, through contact, it should cause the vessel itself to expand with it.

I have specified a contact interaction between the outer surface of the balloon and the inner surface of the vessel. However, when I apply a Pressure Load on the inner surface of the balloon, it ends up ignoring any contact with the vessel and simply expanding as if it were alone.

When I try to apply a Body Force Load instead of Pressure Load, the balloon moves in the specified direction and interacts with the vessel as it should, causing the vessel to also move in the same direction through contact. However, this is not ideal since I cannot make a Body Force act in all directions (as the Pressure Load does).

Does anybody have any tips? Either in how to make the pressure load work or how to apply another type of force that would act in all directions?

I would be happy to provide more details if needed. Thanks!

 

[Dave442]

are you using standard or explicit?
and how have you defined the interaction between the balloon/artery?
also, can you describe how you are modelling the balloon - they are usually very thin and folded about themselves?

 

[Andre1212]

I am using standard and have defined the interaction as surface to surface with small sliding and all default parameters.
The interaction properties are hard contact for normal behavior and penalty with specified friction coefficient for tangential behavior.
As of now I am modelling the balloon as a uniform, thin, isotropic elastic cylinder with specified parameters.

 

[Dave442]

when you define the contact surfaces are the surface normals pointing toward each other?

 

[Andre1212]

Yes they are

 

[Mustaine3]

- Use finite sliding
- Check if you have positive results for CPRESS. If yes, then the contact works and your penalty stiffness might be too low.

 

[IceBreakerSours]

Break the analysis in to steps and start with the balloon whose OD is very close to the min. ID of the vessel and make sure the max. time step in the first step is small to allow for the contact algorithm to detect the opposing facets.

By the way, it may be that the perspective is making me think the geometry has sharp transitions so take this with a grain of salt. You might want to consider smoothening the geometry a bit. It will help the contact solver. If I were doing this analysis, I'd start with a much smaller and simpler geometry to have the workflow down first.

By the way, no one seems to take into account the moving elastic foundation of the heart in to account in these analyses. Perhaps, BCs are challenging to account for, I do not know. However, it may be an interesting avenue to explore given the rapid advances tagged MRI seems to have made in the last few years.

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[Dave442]

@Andre1212

Like Mustaine suggested, I would check the contact output and diagnostics to figure out your issue. You would also see severe discontinuity iterations in the .sta file in any increments where contact constraints change state (open/closed).


@IceBreakerSours

this paper incorporates the movement of the cardiac tissue. Only one I'm aware of:

https://doi.org/10.1016/j.jbiomech.2014.01.007

 

[Andre1212]

Thanks everybody, really useful advice from everyone.

It turns out that only the first section that comes into contact has a positive CPRESS value. Everywhere else, the balloon goes right through the vessel and the respective CPRESS value is 0. My guess is the vessel geometry needs to be smoothed as @IceBreakerSours mentioned.

 

[IceBreakerSours]

Thanks for the link, Dave. I wonder how different the strains/stresses are be when you account for the elastic foundation effects and if that has any influence on the predicted fatigue life. I will look at the paper at some point.

@Andre1212

Contact solver likes the opposing surfaces to be "similar". The closer in "similarity" they are, the better it is for the solver. By "similar", I mean, in stiffness (material and geometric contributions) and discretization (mesh). The more "dissimilar" the opposing surfaces are, the trickier the contact problem starts to become. Metallic stents coming to contact with a soft vessel falls in the latter category. When you introduce geometric features such as sharp edges at the interface, you are just making life a hell for the contact solver and, consequently, for yourself

By the way, I am not entirely convinced if finite sliding is required while you are still getting your model to work. Friction coefficients seem to appear out of thin air in these analyses so I'd rather get the small sliding model running first to have something meaningful to report and then consider whether finite sliding is a more appropriate interaction or not.

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[Mustaine3]

Other question from my side would be:

Is NLGEOM active?
What happens with General Contact?

And no, I don't think that Small Sliding is applicable here - independent of friction. The displacements and deformations will be too large and also be relative to each other.

I also guess, that the overall goal of the simulation might be easier to achieve with quasi-static in A/Explicit. With the complex contact situation, especially when both expand while being in contact, it will be hard to get it running smooth in an implicit solver.

What happens in the z-directions? Maybe it is possible to run it in 2D...

 

[Dave442]

In your first response you mentioned that "As of now I am modelling the balloon as a uniform, thin, isotropic elastic cylinder". If you intend to generate a more sophisticated model in the future that incorporates the shaft, the folded layers of the balloon and all of the associated contact, I would agree with Mustaine and recommend you run these analyses as quasi-static in A/Explicit.

 

[IceBreakerSours]

Sure, both displacements and deformations are expected to be large but if the element size of the vessel mesh is coarse enough, you can get away with small sliding - and I repeat - while you are in the method development stage. Besides, the focus in these analyses is on stent fatigue and I am not sure how much difference finite vs. small sliding interaction with a vessel - which happens to be orders of magnitude relatively compliant - is going to make to the fatigue life of a stent. It might but I wouldn't worry about it in my first pass while I am ironing out a whole host of kinks.

Assuming surfaces and meshes are smooth enough, /Standard can get the job done. While I will admit it is not easy, it is do-able and I mention this because of the benefits of running these jobs in implicit. That said, if you can not get over the hump, I would follow the advice above and switch over to /Explicit.

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