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Friction energy dissipation in LS DYNA

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Posts: 6
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(@shfkyl666)
Active Member
Joined: 15 years ago

Im relatively new to LS-DYNA and Im working with a simple contact model of a softball. Its a solid ball (with an initial velocity of 60-120 MPH) directly impacting a fixed solid cylinder. The aluminum cylinder is very stiff relative to the ball, so only the ball deforms. The homogeneous ball is the standard linear viscoelastic material (LS DYNA mat_006) and the cylinder is elastic.

I adjusted the ball material properties to match experimental data (rebound velocity, impact force and contact time). However, I have an interesting problem. The experimental results show that when the ball impacts a cylinder, the rebound velocity is less than that of an impact with a rigid flat plate. However, the LS DYNA model shows the opposite trend: the rebound velocity for the cylinder impact is higher. I believe that the viscoelastic material is adequate and not causing this discrepancy.

My current hypothesis is that the frictional energy loss on the contact surfaces is significant and larger for the cylinder-ball impact. The ball impacting a cylinder tends to have significant sliding on the contact surfaces due to the large deformation of the ball. When I add friction (set the static friction coefficient SF=0.1) in the contact properties (Automatic Surface-to-Surface), there is less sliding between the contact surfaces, but the rebound velocity does not change significantly.

[B]

How to I ensure that the frictional energy dissipation is being included in the energy calculations? Also, how do I plot the frictional energy dissipation to see if it is a significant form of energy loss?[/B]

After reading online support websites, I tried changing the following cards:

Control_Contact: set FRCENG =1

Control_Energy: set SLNTEN=2

But this did not change my rebound velocity results.

In post-processing, I output GLSTAT, which shows kinetic energy, total energy, internal energy, etc. These plots seem to make sense. Also, I output SLEOUT, which shows plots of the slave and master contact energy and Frictional Energy. But the plot of Frictional Energy is always zero. I would expect the Frictional Energy to have some value other than zero, even if it is relatively small and insignificant. So it seems that the frictional energy dissipation is either not being included in the energy calculations or not being outputted. [B]Is SLEOUT the correct output for frictional energy dissipation, or is it outputted elsewhere?[/B]

Thanks. -Ryan

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Posts: 3998
(@jorgen)
Member
Joined: 4 years ago

Interesting problem and questions.

First, have you tried the following cases:
- Zero friction for the cylinder case and for the flat plate case
- High friction (about 0.4?) for the cylinder case and the flat plat case

Based on your description is sounds like you have done something simular and that the results are indicating that friction is not a major contributer to the rebound velocity. That would also be my guess.

I would carefully confirm that the friction is not important in your case.

Then I would look at the boundary conditions and mass of the cylinder and the flat plate. Is there something going on there to explain your results?

-Jorgen

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Posts: 6
Topic starter
(@shfkyl666)
Active Member
Joined: 15 years ago

Thanks for the response. I have already tried what you suggested. The zero friction models show sliding between the plate and the ball. When I add friction, there is no sliding between the plate and the ball. There is a VERY slight change in rebound velocity when friction is added, but this is probably caused by the slight change in the deformation of the ball (due to no sliding in the contact). All of the energy loss seems to still be in the viscous material of the ball.

I tried very small values of FS (0.05) and very large values of FS (0.5). There is no relative sliding for any value of FS and the rebound velocity doesnt change. I also tried imputting other friction constants like VC, which supposedly limits the friction force to a maximum, but nothing I input seems to make a difference. There is always no relative sliding and no change in my model.

Im assuming that frictional energy loss only occurs when there is relative sliding, since work has to be done. (?)

The boundary conditions of the model are pretty easy. I just fixed the cylinder at both ends and the ball has an initial velocity. I am only modeling a quarter of the ball and cylinder, so I am also using symmetry constraints (constrained in the normal direction). The mass of the ball and cylinder appear to be correct. I inputted densities to match phyically measured masses.

In the model, there is less energy loss when impacting a cylinder (vs. flat plate), simply because there is less deformation in the ball. I have tried all kinds of different material properties in an attempt to correct this trend, but nothing worked. In actual experiments, there must be some form of friction energy loss at the contact, or maybe between the leather cover (not modeled) and the inside of the ball that causes this trend.

So I still need to find a way to output the frictional energy loss in LS DYNA. Then I need to learn how to configure the contact friction so that friction is a significant form of energy loss.

I appreciate any suggestions you have.

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Posts: 3998
(@jorgen)
Member
Joined: 4 years ago

I am not sure what is going on. Do you use the same mass scaling? Do you use small enough time increments?

-Jorgen

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Posts: 6
Topic starter
(@shfkyl666)
Active Member
Joined: 15 years ago

I figured it out mostly. The sliding energy (plotted in GLSTAT or SLEOUT) is the sum of the normal (contact) energy and the shear (friction) energy. So for problems without friction, the sliding energy should be relatively small during contact and then return to nearly zero after the contact. If there is friction, then the sliding energy is a positive value after the contact. This positive value is the friction energy lost.

The frictional energy lost in my analysis was very small and did not change the results significantly, so I have to try something else.

Thanks.

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