Contact Mechanics - Friction Test

Numerical methods and mathematical models of Elmer
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maike151
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Contact Mechanics - Friction Test

Post by maike151 » 10 Jul 2019, 12:13

Hello ELMER-users,

I am trying to simulate a simple friction test, where I pull a cube over the floor. I want to validate this simulation by using COULOMBS friction law: F_R = mu * F_N, where F_R is the dynamic Friction load (in opposite direction of the direction in which the cube is pulled), mu the dynamic friction coefficient and F_N the normal load of the cube. I use displacement as a BC and save the displacement loads in the direction in which I pull. Usually after a few timesteps, when the speed of the cube becomes constant, these loads should correspond to the dynamic friction load, which is easy to calculate by hand.

I used displacement as a BC at the y-side of the cube. Unfortunately, the contact was only recognized on the floor in the area of this boundary. Then I tried many things and it turned out that it works best if I put displacement as a BC on EVERY boundary of the cube and prohibit the displacement of the top floor boundary (Master boundary) (pretty sure this is not physically correct anymore).

The cube and the floor consist of austenitic steel, so I chose the dynamic friction coefficient mu=0,2. However, the solution looks good and the contact is well-recognized. The displacement loads I receive are also in the correct magnitude, but when I change the friction coefficient for testing-reasons, the loads do not change at all.

Is simulation of friction contact in ELMER even possible yet? Why is the friction coefficient not taken into account? Or did I miss anything or make a mistake?

I attached my sif and a picture that illustrates my problem.

Thanks in advance
Maike
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kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 11:22


maike151
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Re: Contact Mechanics - Friction Test

Post by maike151 » 11 Jul 2019, 11:28

Hello Kevin,

thanks for your reply! I've seen this test case before. Unfortunately It didn't really help me since they do not use "Apply Contact BCs" and "Friction Contact".

Sincerely,
Maike

kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 12:22

Where is the normal force defined in your sif file?

maike151
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Re: Contact Mechanics - Friction Test

Post by maike151 » 11 Jul 2019, 12:28

Do you mean the normal force of the cube? I didn't define it, because I assumed ELMER would calculate it automatically. In which section would you define it?

kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 14:52

Yes the normal force of the cube, the friction force is the normal force on the contact area times the coefficient of friction. I do not believe there are any automatic forces in Elmer. See the loaded elastic beam tutorial for a method to apply a gravity body force.

kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 15:07

See the test case of the loaded elastic beam. the only vector sif defines the gravity vector but the body force is blank. It produces no load and no deflection. The case.sif file has a body load defined using gravity, it produces deflection.

I think you are missing a normal force in your case.
onlyvectorcase.sif
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case.sif
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maike151
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Re: Contact Mechanics - Friction Test

Post by maike151 » 11 Jul 2019, 16:11

Thank you! That makes sense. But wouldn't it be

Stress Bodyforce 2 = $ -9.81 * 7870.0 * "Volume" (Normal Force = 9,81 * mass instead of density)

I don't know the measurements of the beam, but in my case (the cube is 1cm³) I would put

Stress Bodyforce 3 = $ -9.81 * 7810.0*10e-6. Do you agree?

I tried this with Dynamic Friction Coefficient 0,2 and 0,01 (Static 0,3 and 0,03, respectively), but I still get the same values for both.. Maybe I am saving the wrong data? Right now I am saving

Variable 1 = Displacement Loads 2
Operator 1 = boundary sum

at the y-side of the cube (direction in which the displacement is defined).

kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 18:42

I will have to do some test cases, but the volume may be included since it is a body force.

kevinarden
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Re: Contact Mechanics - Friction Test

Post by kevinarden » 11 Jul 2019, 18:56

I checked the Elmer solution versus theory and Elmer is close using the equation for load I have. Since it is a body force the volume is automatic.
theory.PNG
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delection.PNG
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