Expansion of Stent 2

[pradeepburla]

Hello,
I will try to be as concise as possible .
I am working on stent expansion using a baloon ,all the units I mention below will be based on N/(micrometer)**2.
So initially I expanded my stent by giving radial displacement boundary condition to the baloon ande i could achieve uniform expansion of my stent.
Now I am trying to expand my stent using pressure loading.
Baloon-Y=0.000920N/(Micrometer)**2
Stent- Y=0.203N/(micrometer)**2
Dog boning takes place when i apply internal pressure to baloon and the sides of baloon expand the way I want to but the stent doesnot.How can i achieve uniform expansion of the baloon along with stent?
[link Your file's link is:

http://files.engineering.com/getfil...6-64ae-40aa-9866-c082e23b8b6c&file=view_1.png

]Link[/url]
[link Your file's link is:

http://files.engineering.com/getfil...c-e869-41cb-8042-03b5a2a76d2a&file=view_2.png

]Link[/url]

Any suggestion would be of great help

 

[IceBreakerSours]

Are you sure your material properties for the balloon are correct? They tend to be "rigid" (in relation to the stents).

*********************************************************
Are you new to this forum? If so, please read these FAQs:

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[pradeepburla]

When I looked for material properties of stent and baloon I guess the values I found were prety consistent.
Baloon - Y=920 MPA
Stent - Y=203 GPA
Reference Link

 

[pradeepburla]

Another doubt I have is in the link I attached(Section 2.3-Boundary and loading conditions) he says he applied increasing uniform pressure from 0-1.5MPA .When I apply the same to my baloon I get a radial displacement of 22micrometers but according to the paper he gets a radial displacement of about 1000 micrometers.
I created a step -
*Step, name=Step-1, nlgeom=YES, inc=1000000
*Dynamic,application=QUASI-STATIC,initial=NO
0.1,1.,1e-08
**
** LOADS
**
** Name: Load-1 Type: Pressure
*Dsload,
Surf-1, P, 1.013e-06 ( 1.013 MPA)

Am I doing something wrong here?

 

[Dave442]

Hi,

Some points:

(1) Have you accounted for plasticity in the stent material model? The paper you linked includes plasticity. Plasticity will be critical in this type of analysis.

(2) In the paper you linked the authors have used a quasi-static explicit analysis, whereas you are using a quasi-static implicit analysis. I assume the authors of the paper used an explicit analysis due to the complex contact between the folded balloon/stent/artery. If your model is just a simple cylindrical balloon geometry and no artery, a static analysis should be fine. As IceBreakerSours stated, if your balloon model is just cylindrical, you could get away with using a rigid surface and drive stent expansion using a prescribed displacement with contact. Using a static analysis with this approach could dramatically reduce the cost of your analysis.

(3) The geometry of your stent differs pretty significantly from that of the stent shown in the paper. Why do you expect to observe uniform expansion at the same pressure as that used in the paper? In reality, different pressures are required to fully inflate stents with different geometries.

(4) Why are you working in micrometers? Seems unnecessary - use mm/MPa and avoid issues with inconsistent units. See this table:

http://idac.co.uk/services/downloads/Consistent.pdf

Good luck
Dave

 

[pradeepburla]

Thanks Dave for the elaborate reply .

1.I am using a Co-Cr alloy based stent and i just wanted to first run it for elastic case and then include plasticity.

2.I actually completed the baloon,stent and artery analysis by prescribing radial displacement to the baloon.It didnot work with static and then i tried to do the same with dynamic implicit , it worked.Now I just started the same problem with pressure loading.So first I want to make sure my baloon stent system is working,then i will introduce artery into the system.

3.I modeled my stent based on a real Stent and got it expanded in real with increasing uniform pressure upto 1.103 MPA.And the recorded video suggested that from dog boning to uniform expansion would be the end result.By uniform i mean dog boning to be minimal.

4.I modeled my geometry in micrometers.Probably its a good idea to scale my geometry to meters and run simulation based on consistent units.But even while using micrometers i make sure all the units are consistent.

Do you think running the same simulation in explicit would help instead of standard?

THanks again!

 

[Dave442]

Hi again,

If you just want the expanded geometry of the stent, then you can neglect the the balloon and expand the stent using a rigid surface with SFM3D4R elements. Assign a radial displacement to the surface and set up contact with the inner surface of the stent. It will run quick in static implicit and should also run with a simple model of an artery.

If you are interested in the dynamic behavior of the stent you need to include the balloon in your FEA model and perform a dynamic analysis. To get agreement between your experimental test and your FEA results however, you will also need to make sure your geometry is correct and your material models adequately characterise the behavior of the stent/balloon in the experimental test. Your stent geometry looks good but is your balloon geometry correct? Initially, they are often folded about a shaft in some manner.

Your material models are your big issue I think - you absolutely must include plasticity in you FEA model to get agreement with you test results. That is how these devices are expanded - they are deformed elastically until the material yields. Then they rapidly expand to a nominal balloon diameter due to plastic deformation at the crowns.

If you are not seeing uniform expansion in your FEA model at the same pressures as your experimental test, its because you have neglected plasticity. To check, look at the equivalent stress contour plot in your FEA model at 1.103 MPa inflation pressure. I'll bet it's above the yield stress for CoCr (as measured from stent strut specimens). So include an elastic-plastic material model. Its pointless modelling the behavior as linear elastic when its elastic-plastic in reality. There is plenty of tensile data in the literature for CoCr and its easy to set up an elastic-plastic material model. Check the Getting Started guide in the documentation for a detailed explanation.

It may also be pointless taking material propeties for the balloon from that paper you linked earlier. The properties given in that paper are for one particular balloon and may differ significantly from the balloon used in your experimental tests. Find out what its made from and get suitable properties from tests/literature.

Also, if your happy with your units stick with them. I just found working in mm/MPa a lot easier with these devices.

Good luck!
Dave

 

[pradeepburla]

Thanks Dave ,

Instead of D4R elements for radial displacement it makes sense to use rigid surface if I do not care about baloon.

I modeled stent geometry from the actual stent but I have not modeled 3 fold baloon or 6 fold baloon, I am just working with a normal cylindrical baloon.I would like to work on 3-6 fold baloon if I succeed in this.But even with a normal baloon I guess I should be able to expand the stent atleast.

I guess your suggestion is right to include plasticity and you were right regarding the stresses, yield stress in the stent is above 550MPA just after 200 micrometers of radial displacement(I need about 1000micrometers).

Even I feel my material properties need some corrections .I will sit down and do some literature review on it.

Thanks a lot for giving a search direction .Will let you know if the suggestion worked.

Cheers!

 

[IceBreakerSours]

a) I completely forgot about plasticity; Dave is absolutely correct (no surprise there!). All stent analyses have elastic-plastic materials assigned to them.

b) Please use consistent units (i.e., there should not be any numerical conversion factor.) You will have one less thing to worry about.

c) By the way, why didn't the static analysis work?

*********************************************************
Are you new to this forum? If so, please read these FAQs:

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[Dave442]

One last thing:

when you say that you are working with a normal cylindrical balloon what do you mean exactly?

Did you model the balloon as a simple cylinder with a diameter lower than that of the initial stent diameter?

Dave

 

[pradeepburla]

@IceBreakerSours General,static didn't work as it failed to converge as it needed smaller time steps<1e-10. I changed the material properties of the Stent to elasto plastic but i still have some issues.

@Dave Exactly.With a simple baloon i meant cylindrical shell with a diameter lower that the stent.

Boundary conditions are that the ends of the balloon are constrained in longitudinal direction and internal pressure is applied to the inner surface of the balloon (shell here).But i see that one end of the balloon expands a lot and the other end does not. Even when i give uniform pressure to the balloon. What can be the reason for that?The balloon has simple elastic properties.I want to make sure that once balloon reaches 1000 magnitude of displacement it waits for the other portions of the balloon to expand.I am not sure how I can ensure that.

Link

Link

[link

http://files.engineering.com/getfil...4-8384-4ae4-9971-c4b2cc17b59b&file=inc_66.png

]Link[/url]

 

[Dave442]

Hi again,

Unfortunately, the approach you are using will never provide realistic results in terms of the pressure/diameter behaviour of the stent.

When a balloon is manufactured, it has an initial diameter considerably larger than that of the stent. The balloon is then folded down around the shaft so that its profile is reduced significantly. If you forget about the stent for a minute, the deployment of the balloon can be split into two stages. (1) As pressure is first applied, the balloon diameter increases rapidly as it unfolds about the shaft to its initial "as manufactured" diameter. (2) At this point, the balloon material is first loaded and any further increase in balloon diameter is much more gradual and is due to the mechanical response of the balloon material.

In your analysis, you have neglected the balloon folding and modelled a cylinder with a small diameter. When you apply your pressure to the the cylinder, it does not unfold and, as a result, its diameter does not rapidly increase and the material is loaded immediately. As the analysis progresses, the diameter of your balloon will gradually increase since you have assigned a linear elastic material model. However, you will not achieve similar pressure/diameter results to your experimental test and you will require a much larger pressure than observed in reality to expand your stent to its nominal diameter.

Dave

 

[pradeepburla]

Hi Dave ,
You were bang on when you recommended inclusion of elasto plastic material model.Now my model works even for a General,Static which didnot earlier.
You are also right regarding modelling of the baloo0n .I modeled a 3 fold baloon and ran a couple of simulations.That works fine as well and quite similar to what has been said in the papers.
Thank you for the help.
Owe you a beer.
Cheers!