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Contact type 24 and 7 errors

I have a model of a tigh bone and the hip, three concentrated loads are in the model to press the tigh bone into the hip. We use contact type 7 or 24 between the hip (blue) and the cartilage of the femur (orange), and contact 2 between the tigh bone (yellow) and the cartilage (orange).

That's how my model looks like:

If you hide the hip bone:

But as soon as the concentrated load comes into play, it looks like this:

The problem shows up with both, contact type 24 and 7. I've played around with several contact parameters but haven't found a solution yet. Does anyone have an idea what the problem could be?

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Pranav Hari

Hi rea2

Try updating the Type 7 contact (Cartilage cartilage 2) from your model as recommended below and check whether it works

image.png.f9a9e855bde7686b1fd963c9c38143f9.png

image.png.a703c2262a879c18bf39106c9cb07c8c.png

image.png.e8daa7e7ab2aa2ee2f1a2e10f0bb681c.png

Simon Križnik

Hi @rea2

it seems the loads are causing trouble, because only imposed motion works fine. In particular, the problem is with the compressibility of cartilage. So I have set up cartilage material like incompressible rubber (nu=0.495) and property:

isolid=14

ismstr=10

icpre=1

i=2

j=2

k=2

I have increased the Gapmin to 0.6, so contact engages sooner thereby avoiding deep penetration. Note the timestep is dropping and added mass increases due to contact. This is expected since you have relatively soft cartilage attached to rigid surfaces and there is a lot of force applied so contacts have to provide stiffness. There is also the possibility that somewhere along the simulation there will be edge penetration and if the direction changes it can lead to deep penetrations. This can be avoided by using an additional type 11 interface (or type 19 which is a combination of type 7 and 11).

There might also be some issues due to an insufficient ramp-up of loading: 3000 Newtons applied over a period of 20 ms could introduce some unwanted inertial effects.

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