tension-compression rigid deformable contact in PreView 2.0

[scottho]

I couldn't find the option to set tension-compression contact between deformable and rigid bodies in PreView 2.0 when pulling down the contact menu. Is this option available under a different heading in PreView 2.0? It seems that if I substitute the text for tension-compression contact in the .feb file format for 2.5 is still runs fine in FEBio 2.7. I just don't see the option to set it up in PreView 2.0

[maas]

Hi,

The tension-compression contact formulation was renamed, as well as many others, to provide a more consistent naming convention. It is now called sliding-elastic.

Cheers,

Steve

[scottho]

Thanks. I had previously used tension-compression contact option with Parameter Optimization to fit constitutive models. Should I now substitute the sliding-elastic option when solving the rigid-deformable contact problem in Parameter Optimization for FEBio2.7?

[maas]

FEBio 2.7 still understands the old contact names, so if you want, or you still have files that use those names, you could still use them. Going forward, I do recommend that you use the new naming conventions for new any new models you create.

Cheers,

Steve

[mkaz010]

Hi,
I'm modeling the knee joint contact, defining two sliding-elastic contacts with "tension" flag as true for medial and lateral tibia plateau as slave body. The tibia and femur bones (including cartilage surfaces) defined as rigid bodies. Viewing the results in the PostView for contact pressure and contact gap are exactly the same. Could you please give me some explanation for this? Thanks in advance.

Cheers
Mousa

[ateshian]

Hi Mousa,

In the simplest case (i.e., when laugon=0, penalty=1, auto_penalty=0), the contact pressure is equal to the contact gap, since (contact pressure) = penalty*(contact gap). In practice, one should use a sufficiently high value of the penalty to minimize the magnitude of the contact gap. Turning auto penalty on (auto_penalty=1) will also scale the penalty using the material properties and element dimensions at the contact interface, in which case the contact pressure and contact gap are unlikely to produce the same values.

Best,

Gerard

[mkaz010]

Hi Gerard,

Thanks for the prompt reply and happy New Year. I increased the penalty factor to 100000 and I have significant different figures for the contact pressure. According to the mentioned formula, I expected that the value of contact pressure should remain unchanged and the more value of penalty the less value of contact gap. But, the value of contact pressure is proportionally changed when the value of penalty is increased which is physically meaningless. Could you please explain why it happens and what my possible mistake would be? For example when a constant traction vector of 450 MPa (equivalent to the patient's half body weight) was distally applied on a single triangle face at femoral head (with approximately 1mm^2 area) with penalty factor of 10, the peak contact pressure was about 53 MPa, however, if the penalty factor 10^5 was chosen, then the value of contact pressure peak increase to 53 times 10^5 MPa. Although the BC of these two configurations are the same why the results are significantly different?

Furthermore, auto-penalty did not work, so, I am wondering while I do not use auto-penalty, febio still uses the material properties to calculate contact pressure or not. Please consider that I use rigid body material with density of 1, E = 0 and v =0. The mesh elements are surface triangles with thickness of zero.

Regards
Mousa

[ateshian]

Hi Mousa,

Happy New Year to you as well.

Sorry, I had missed the part where you said that both contact surfaces are rigid. In that case auto-penalty should not be used (it won't work since there is no modulus that can be calculated from the material properties). Also, since neither contact surface deforms, increasing the penalty will not produce a contact pressure that converges to a unique value. In fact, increasing the penalty will reduce the contact area until (in the limit of infinite penalty) the contact area reduces to a discrete set of contact points (for arbitrary surface geometries). The results you are getting are consistent with the expected behavior for rigid contact. The quantity that should converge to a constant value with increasing penalty is the contact force. So, evidently, the contact pressure should go to infinity, since the force becomes constant while the contact area reduces to zero. I hope this explanation clears up your concern. Using deformable surfaces will produce a substantially different behavior.

Best,

Gerard