Facetjoint in the model “SpineFixationWithFDK”

Main Forum
1

Hi AnyBody,
I have some problems when using the example of the facetjoint in the model “SpineFixationWithFDK”. The algorithm of the facetjoint is stiffness multiplied by the penetration distance, and the penetration distance is a negative number.
Should the penetration distance be added to the absolute value in the formula for calculating the facet joint force?
Thanks and
Best regards,
Shaotian

2

Hi @Shaotian and welcome to the forum!

I'm not entirely sure what you mean by your question.
Are you changing the facet joint force calculation?
Or are you just experiencing negative penetration values?

The contact forces defined will first be active when there penetration distance is negative which indicate contact.

Best regards,
Bjørn
AnyBody Technology

3

Hi Bjorn，Thank you for your reply！
My problem is that：I don’t know the direction of the facet joint force.I want to know the direction of the facet joint force on L1Seg.
Can I understand the facet joint force as spring force? If possible, the formula for calculating the spring force is stiffness multiplied by the deformation. The deformation should be a number greater than zero. In this model, ".meas.Pos[i]" in the calculation formula for the facet joint force is a negative value. Is this problematic? Still, I don't understand it correctly.

image
image929×52 1.87 KB

Best regards,
Shaotian

4

Hi @Shaotian

Yes the forces are spring like as you can see from your image the force of each component will either be zero, the negative stiffness divided by a fraction times the penetration or simply the negative stiffness times the penetration.

The formula is set up so that negative penetration values (which indicate the two surfaces are penetrating ) causes the contact forces to be active and produce a positive force.

The contact forces can be visualized by enabling the viewForce.Visibility property of the force class.

Best regards,
Bjørn

5

Yes,The formula causes the contact forces to be active and produce a positive force,but I think if it is like a spring, it should generate a negative force.Because the direction of spring force and displacement are opposite, is my understanding right?
Best regards,
Shaotian

6

Hi @Shaotian

The force and displacement are opposite, just with positive force and negative displacement.

Best regards,
Bjørn

7

I understand, thank you very much.
Best regards,
Shaotian

8

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