Clarification of Non-Linear Spring Load Curve

[orsialex]

Hello FEBio,

I am running a simulation with some non linear springs, and I just wanted some clarification as to how the load curve works. In the manual it doesnt explicitly say it is a force vs. displacement relationship. So I just wanted to make sure this is correct, rather than force vs. strain.

Thanks!

-Alex

[maas]

Hi Alex,

Yes, for (nonlinear) springs the load curve represents a force-displacement curve. Keep in mind that the actual force value is the product of the value for "F" and the value of the loadcurve at the current simulation time.

Cheers,

Steve

[orsialex]

Thanks!

I am running into an issue with the non linear springs. is there a limit on how many data points you can apply to the curve? for some reason when I use NL springs, my simulations run into a negative jacobian...however, when I run the same simulation with linear springs, and just plugging in a random stiffness value for the spring, it runs fine...Any hints on how to debug NL spring issues?

Thanks again,

-Alex

[maas]

Hi Alex,

It may depends on how nonlinear your springs are. If they are highly nonlinear, then they can cause inverted elements if the load is applied too quickly. You can try decreasing the time step size and see if that helps. I would also first try to replicate a linear spring, with the nonlinear springs to make sure that they are working properly. If you still run into problems, could you send me the input file?

Cheers,

Steve

[orsialex]

Hi Steve,

They are pretty basic quadratic to linear (ligament like) springs. Here is a link to the input file. let me know what you can do/see. I really appreciate you taking a look at it!

https://www.dropbox.com/s/o9atpxsax5m1ssr/Nonlinear_Spring_Ligament_Model.feb

-Alex

[maas]

Hi Alex,

I took a look at your model, and I'm not sure what the problem is. This is a fairly complex model and it can be a combination of features that is causing the problem. One thing I noticed is that there is a fair amount of initial penetration in your model, meaning the surfaces intersect before the model starts. This may lead to an instability and I would recommend trying to eliminate this. I do understand that this is not always easy. The sliding_with_gaps contact algorithm has a feature called node_reloc that you can use to automatically push the nodes out of contact before the analysis start. Although this may solve the problem in many causes, in other cases, where the initial penetration is too much, it may actually make things worse since it may invert elements. In any case, it might be worth trying to see if this is part of the problem. As noted, you'll have to use sliding_with_gaps since facet-to-facet sliding does not support this feature.

Cheers,

Steve

[Alireza2]

Curve editor for non-linear springs

Hello
I want to simulate non-linear springs at preview. After creating the springs I don't know how can I use curve editor for them.
In the curve editor tree there is not any choice for "discretes".
Can you explain in detail how can I use curve editor for the non-linear springs.

Thanks,

Alireza

[maas]

Hi Alireza,

Unfortunately, I don't think you can edit the load curves of nonlinear springs in PreView. We will have support for this in FEBio Studio, which replaces PreView. In fact, if you reach out to me via email (steve dot maas at Utah dot edu), I'm happy to give you our latest development build, so you can give it a try.

Cheers,

Steve