Impact Attenuator problem

[bacoa]

Hi guys,
I was wondering if anyone of you could give me some advice on how to start with the calculations on the impact attenuator data.

We currently do not have any money for destructive testing so, i did finite element analysis on impact attenuator, is that sufficient enough? Thanks you guys in advance.

Cheers,
Dave

[bacoa]

Hi guys,
I was wondering if anyone of you could give me some advice on how to start with the calculations on the impact attenuator data.

We currently do not have any money for destructive testing so, i did finite element analysis on impact attenuator, is that sufficient enough? Thanks you guys in advance.

Cheers,
Dave

[DH]

If you can get your hands on or can measure compressive dynamic stress-strain data for whatever material you're planning to use, the calculations are fairly basic. The initial and final velocity and the mass of the object to be attenuated yield the energy that needs to be absorbed, and manipulation/integration of the stress-strain curve will tell you if the average deceleration requirement has been met after that amount of energy has been absorbed.

An FEA model would have to account for the non-linearity of the problem, as we are well into the plastic region of the material by the time the car stops. In my opinion the analytical solution is just as easy.

There were several papers published by General Plastics that were really helpful in analyzing the impact problem. I'm not sure if they're still on their website but it might pay to check. They also contained stress-strain curves for some of their foams.

[Schultz]

I notice that you did fea on the impact attenuator. What software did you use to simulate the drop test? I have tried Cosmos but i am unable to run the test. Please let me know if you have any tips. Thanks

[rjwoods77]

Do a search. Travis Waltman laid it all completely out for anyone and everyone to see.

[mtg]

Interesting, I wasn't aware that James Waltman and Travis Garrison had mysteriously morphed into a uni-dude over the weekend. That's going to make Monday morning at work a bit odd.

[Travis Garrison]

I feel violated.

FEA for an impact attenuator is mental masturbation IMHO. Look up the properties for aluminum honeycomb and be done with it. The only thing worth analyzing is the support for the attenuator, and if you did your job properly on the attenuator the force it has to support should be easily predictable.

http://www.hexcel.com/Markets/Automotive/Crash.htm

http://www.hexcel.com/Products/Downloads/Hexweb%20Honeycomb%20DataSheets

That's a start they have more if you're willing to dig.

-Travis GARRISON

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by DH:An FEA model would have to account for the non-linearity of the problem, as we are well into the plastic region of the material by the time the car stops. In my opinion the analytical solution is just as easy. </div></BLOCKQUOTE>

Do you know how the FEA program is performing the non-linear analysis? There are two types of non-linear analysis, geometrical and material. An impact analysis needs both to properly simulate the situation. You have to make sure that the program you're running is running both and not just one or the other during the analysis.

[rjwoods77]

I am such a dolt. I am sorry guys. I definitely know the difference. Shouldnt talk when I am tired,

[James Waltman]

I figured that Rob heard that Travis and I were getting married and assumed that it was to each other and not to different girls. Since Colorado just banned gay marriage, Monday should be a normal day at work Matt.
No worries Rob. That was funny.

Anyway, I'm not sure if the topic starter will see this but it may help Schultz.
I think that this is the topic Rob was talking about:
I need all the info about the crush zone!!! - aka: Waltman's Dissertation on Impact Attenuators (http://fsae.com/eve/forums/a/tpc/f/125607348/m/85510167411?r=85510167411#85510167411)

This one has some good info too:
New 2006 Rules for Frontal Impact attenuator (http://fsae.com/eve/forums/a/tpc/f/125607348/m/30710525721?r=30710525721#30710525721)

I wouldn't trust FEA for the Impact Attenuator. In fact, I think that you'll see that it's just a bad idea once you realize what's going on.
If you do a little thinking (and choose an appropriate material) you can do the calculations on your cell phone's calculator. Actually, you could get a 12 year old to do it on paper without a calculator.
Then verify it with a physical test.

[mtg]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by James Waltman:
If you do a little thinking (and choose an appropriate material) you can do the calculations on your cell phone's calculator. </div></BLOCKQUOTE>

Remember, not everybody has a space phone that spontaneously splits and flies back to the mother ship.

[Parker]

Well, I only know of one 12 year old that could do the calculation on paper. The only reason he can is because he designed and built a log-splitter for a middle school science fair project.

For those still scratching their head, think apples...