Carbon Fiber prosthetic foot - STIFFNESS

[kukogoba]

I have made a FEM model of a prosthetic foot. And i want to obtain the stiffness of the model.
I´m thinking of doing a Force vs Displacement graph, but i´m still not sure how to accomplish my purpose.
I need some help.

Thanks in advance.

 

[Archie264]

There are many links and references I could offer up, including Wikipedia's extensive treatment of the topic, but, in my opinion, this link offers the best treatment of the subject that I've seen. The explanation and the sample spreadsheets are downloadable without enrolling in the course. Also, be aware that the 16 member spreadsheet appears to have an error in its treatment of uniform loads. For learning purposes it's probably better to play with the 5 member version anyway, though. It sounds like you're working on a valuable project. Good luck.

 

[kukogoba]

Wow!
you saved my life!
THANK YOU VERY MUCH!!!

 

[KootK]

It's important to recognize that your prosthetic foot will not have a single stiffness. Stiffness, in this context, is a measure of the rate of change of displacement for a particular degree of freedom as a result of the application of a load applied at a particular location and oriented in a particular fashion.

That's a mouthful, I know. Think of an ordinary beam member. It will have an axial stiffness and an infinite number of flexural and shear stiffnesses depending on where the load is applied and the displacement is measured.

The force vs displacement graph is a great idea. All you'll need to do is isolate what force, and what displacement, will be required to provide you with meaningful results. Then just turn your FEM software loose, hopefully with some way to perform a hand check on the results.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rb1957]

and plan on non-linear (large displacement) analysis.

another day in paradise, or is paradise one day closer ?

 

[kukogoba]

Yes,
I tought over it, i know that there isn´t an only stiffness. Furthermore, the prothesis has a variable thickness, and the non linearity issue is there.
I think that i´m going to plot the results in the point where the load is applied.

on the other hand, i found the way to get the global stiffness matrix in abaqus. But the model has lots of nodes. I´m trying to reduce de model to some nodes and build a condensed stiffness matrix. But i did´t find the way yet.

Thank you very much guys!