Out of range HIC re...
 
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Out of range HIC results  

 

dummy-error
(@dummy-error)
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September 8, 2020 8:12 pm  

Hi everyone, hope you are all doing ok

I couldn’t find any related topic, sorry it that’s already answered. Sorry if it's a bit confusing.

I’m trying to demonstrate the difference of the impact of the head against a deformable plate and against a rigid one. This study has been done before with a rigid dummy, but now I’m using the deformable one (all LSTC standard 50th p. male) and a few extra “complications” to bring the problem closer to reality – most of them geometry related, nothing fancy.

Turns out that in all cases, the impact is way worse than the previous studies (HIC is like 2x higher) and the difference is even higher when the head impacts the rigid plate. HIC in this case reached 15.000 over and over, when I was expecting a value around 1500. For comparison, the rigid dummy in the new analysis reached 2100, which is acceptable.

I double checked everything with the previous analysis, and everything seems fine. I already tried to slightly move the dummy around, also I have tried different types of contacts, but none of them seemed to make any difference whatsoever. There are no errors in the dummy.

 

Do any of you guys have any idea on why is this happening? Or what I shall try next to solve it?

If you want more information about my analysis in order to help me, please, feel free to ask. I’m not sure what information you may need.

Thanks!


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Negative Volume
(@negativevolume)
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Posts: 326
September 8, 2020 9:32 pm  

Hi @dummy-error

Your username checks out for sure 🤣 How are you calculating HIC? If you are using LS-PrePost, make sure that you select "Apply pre-filter" and make any necessary time or gravity adjustments to account for your unit system. Also, is "15.000" fifteen-thousand? I know this is a nomenclature sometimes used in Europe so I want to make sure that I understand the discrepancy. 

If all of that checks out, could you lay out your comparison a little more clearly? Maybe make a table like below and fill in the HIC values for each case. Also, is this HIC 15ms?

1599597165-hic_table.png

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dummy-error
(@dummy-error)
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September 9, 2020 11:43 pm  

@negativevolume, thank you for your attention! Sorry for the late response, by the way.

I am using LSPP to calculate HIC, always following the instructions on the dummy manual. The last column shows the filter I used for each dummy model. Gravity and unit systems are correct. The difference in HIC values for the rigid and deformable plate are expected. On the other hand, the difference between the dummies is not. I don’t understand why such discrepancy in the results.

The numbers stated in original topic were from the top of my head, this ones (at the table) are real values taken from exactly the same analysis, except for the dummy. Please, forgive me if my English is causing some confusion - I am not a native speaker (but you probably got that already hehe)

1599691430-Screen-Shot-2020-09-09-at-31254-PM.png

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Negative Volume
(@negativevolume)
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September 10, 2020 5:08 pm  

@dummy-error thanks for providing this. I’m not familiar with these particular versions of the HIII head models, but I would think that the one labeled “rigid” would be stiffer and yield higher HIC values. Were these freely available through LSTC? If so I may take a look at them which would help figure out why these differences may be.


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dummy-error
(@dummy-error)
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September 11, 2020 2:31 pm  

@negativevolume, I cannot thank you enough

You can download it at http://www.lstc.com/download/dummy_models?CachedSimilar. They are "free of charge to cur­rent LS-DY­NA® li­censees". You may need the ftp user and pass.

I am not quite sure if the dummy is the problem... More likely I am doing something wrong in my model, maybe I'm using some feature or some config (contacts, control, database) that cannot be used along with the dummy. Anyway, I ask my University if I could give you access to the dummies and they deny it.

If there's any way we can communicate easily, please, I will be happy to talk more about the project. Unfortunately, there are a few topics I could not bring up in public.

Really hope you can download the dummies!
Thanks!!!


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Negative Volume
(@negativevolume)
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September 11, 2020 4:50 pm  

@dummy-error Okay I'll see if I can take a look at them. Oh no worries, I wouldn't ask you to share anything through this that would be proprietary information.

So you set up the impact conditions? How are you retrieving your force response?


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dummy-error
(@dummy-error)
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September 13, 2020 2:26 pm  

@negativevolume

Thanks! Hope you can check the dummies out!

I am putting the dummy on a seat (belted) and accelerating the seat in the opposite direction. The dummy, then, will hit the parts in front of it. The process of accelerating the seat is performed with PRESCRIBED_MOTION_RIGID, where I apply velocity to the parts (actually A Part linked to other parts by RIGID_BODIES).

I'm not sure how I'm retrieving force response. The only outputs I'm interested in are within the ASCII data, mainly HIC. The dummy model already contains a few accelerometers and node history requests - and every time I plot some variable I always use a filter. Same goes for displacements, velocities, accelerations, forces and moments at dummy's head, neck and chest. I'm not sure if that's the information you're expecting...

That's pretty much the whole process, putting it this way seems easy heheh


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Negative Volume
(@negativevolume)
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September 16, 2020 2:04 pm  

@dummy-error Sorry, I didn't mean to ask about force response as you are only looking at HIC. Another thing you can do to spot check the different simulations is to plot head acceleration of the different dummy headforms. HIC is calculated from resultant head acceleration so it would be good to look at that underlying data to make sure that everything makes sense. 


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dummy-error
(@dummy-error)
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September 21, 2020 7:55 pm  

Hi, @negativevolume!

Sorry again for the late response.

I think this will help you to understand better what's going on. Let's call the dummies Rigid and Fast - as it is in their names. Here are the plots from head velocity and acceleration (part data, not accelerometer data). There are also HIC values and head accelerometer data on the right hand side. This refers to the case where the head hit a rigid plate.

Despite small differences in positioning that creates a small difference between hitting times, everything is the same. There is a higher delta in velocity (I don't know why) that causes a higher HIC at Fast Dummy case. Also, the delta time is pretty similar between them, not sufficient to cause such a huge difference in HIC.

Yes, the part acceleration (green curve) is greater in the Fast Dummy case, but the difference in HIC shouldn't be 15883. Physically, it doesn't make sense for me. It does for you? Also, any idea why the difference in velocity if the BCs are the same?

Thanks again!

1600714541-Dummies.jpg

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Negative Volume
(@negativevolume)
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September 24, 2020 7:42 pm  

@dummy-error what are your units for acceleration?


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dummy-error
(@dummy-error)
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September 25, 2020 2:55 am  

@negativevolume, it's G*e-3. I'm using kg-mm-ms-kN units. The HIC graph is adjusted by that factor we have to enter in the ASII window. 


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Negative Volume
(@negativevolume)
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September 25, 2020 1:47 pm  

@dummy-error if you are going to calculate HIC, you need to make sure head acceleration is in G's. You are seeing huge HIC values because you are calculating HIC from G*e-3. 

Also, since you are dealing with head impacts, generally HIC 15ms is used as an injury criteria, not HIC 36ms. 

Try both of these changes and lets see how it looks. 


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