Adding mass to femur model.... 1

[jdoggk40]

I am trying to simulate the femur falling on the floor while including the flesh around the hip and a hip protector outside of that. The femur is pinned at the distal end and falls similar to that of a pendulum, and impacts an "impact plane".

The problem I am running into is adding mass on top to represent the body's weight. I created a solid and molded it around, and gave it a high enough density to represent a human. Is there a better way, through boundary conditions, do create the same effect of the body mass. I looked into the "lumped mass" feature, but it seems to just add it at a single node and removes the "bounce" of the femur at impact.

If anyone has any other ideas, or a better way to use "lumped mass", that would help a lot.

Thanks,
Jesse Krisher

 

[GBor]

Hello, Jesse,

I'm glad to see you are still working on this. I think your approach is the right one for what you are trying to do. The lumped mass should also work, but make sure you are applying it in all three directions. It will look funny on screen because it will give you a single vector pointing off axis. As for removing the bouncing, shouldn't it? If someone falls, wouldn't their body weight stop most of the bounce?

One other thought, don't you need to tie the body weight to the center of gravity? By placing all of the mass on one end, you are removing the moment effect of the body not being centered over the femur after the bounce. Perhaps I'm misunderstanding what you are trying to simulate, but I think you are on the right track. My thought here is that you place a fairly flexible beam from where you are currently pinned to the center of mass of the body. You could even just attach the beam to a single node (this forms a natural, analytical hinge). Apply the lumped mass to the other end of the beam which would be located near the center of the body, then let the whole model "fall" to the impact plane.

If you need to center the mass, the lumped mass will probably work better than it does now.

One other thought, you could increase gravity so that your "pendulum" would fall faster.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[jdoggk40]

Garland,

Sounds good, I'll keep playing around with the lumped mass feature. As of now, I'm playing around with a more "stripped down" simulation, just to see how close I can get without accounting for all of the forces and moments experienced at impact. You are right in saying I am missing the moment due to the center of gravity not being centered over the femur. Hopefully I can eventually account for that.

One of the biggest problems I am having right now is getting the desired results (force at the hip) at impact. Algor is unable to acquire an interior force (between the hip and flesh), so I'm still not sure how I can present some results. Most papers published report their hip protector findings as a function of the force reduction they provide at the bone. Another idea I had was if I could measure the distance that the protector compresses, I could make a graph of the stiffness of the protector versus its compression distance. Do you know if that measurement is possible within ALGOR?

Thanks a lot,
Jesse

 

[GBor]

Jesse,

There is a checkbox to "calculate reaction force" and you can also view internal forces...have to think about this one for a few minutes, but you can also place "probes" anywhere in the model to determine various values. In SuperView there is a Max and Min probe, but if you right click on a node, you can place a probe on it.

Garland

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[jdoggk40]

Alright, I gave those a shot, but I think the problem might be that I'm still on version 16.1, which doesn't seem to calculate internal forces (even when a probe is used). Reaction forces seem to only occur where the flesh contacts the impact plane. I'm going to try it with surface-to-surface contact to see if anything works.

-Jesse

 

[jdoggk40]

Garland,

One other thing that is important for me to determine is what the velocity of the model is at impact. I checked the velocity of nodes checkbox in the output settings before running the analysis. I think its supposed to write it to some output file? Is there a way to just track the velocity of the model similar to the way the stress is shown as a distribution after running the simulation?

Thanks,
Jesse

 

[GBor]

Since you are working with v. 16, I'm not sure what is available. There are two possibilities:

Right click on a node in FEMPRO and see if you can "add to graph". If you can, then there is a way to take the derivative of the displacement (Velocity).

If graphing from FEMPRO isn't an option, then check the node number that you are interested in. Look for a program called "Monitor" in your Algor installation files. This is a program that you can use much like any graphing program. You can select a node to input into a dialog box and select the derivative of position with respect to time.

I'm confident that Monitor should be in v.16. It's a little difficult to use, but it works pretty well. If Neither of these options appear to be available, the final suggestion would be to import the displacement output into MSExcel. Filter node column on the node that you are looking for, graph it and do the velocity calculation in MSExcel, but this shouldn't be necessary.