Hi guys,
Could anyone of you give me a hint on how to do the calculations on the impact attenuator to calculate the average deceleration without doing any destructive testing. All i'm doing right now is a finite element analysis. Thanks in advance.

Cheers,
dave

Dave

Here is a place to start looking for information, I know there are a couple of other threads on the topic.

http://fsae.com/eve/forums/a/tpc/f/125607348/m/85510167...10167411#85510167411

Matt H.

Destructive testing is the way to do it if you can figure out a good way. We used the Civil Engineernig lab. Dropped some hundreds of pounds of steel on it from 8 feet or somethin.

I'll tell you this - about 1/3 of the crush zones on FSAE cars at competition this year, including some top name schools, were total BS. Aluminum honeycomb will NOT work!!

Well I don't know if I would completely with Jersey Tom, from the calculations that I did, I found that AL Honeycomb was sufficient for the requirements set by SAE. I was never able to do a dynamic testing of AL Honeycomb to prove my calculations. I would agree that most of the crushzones on the FSAE cars @ Detriot could be considered insufficient.

In 05 we made an aluminum honeycomb impact attenuator which we thought should have worked just fine. Looked like pretty much every other AL honeycomb crush zone. Also had some foam in there.

Dropped 650lb of steel on it, instantly flattened like a pancake and bounced the stack of steel blocks back in the air. Absorbed almost no energy.

Come on Tom, thats crazy talk. If you do the calculations right, and you actually test it and it didn't work, it was because either a) your test was wrong, or b) the data on your aluminum was wrong.

Tom

What was the set up for you testing? How was data collect for the experiment? In your test the instantly flattened the honeycomb, well when the flatteneing of the honeycomb is doing what it is design to do, absorb energy. From off the top of my head, the total impact time is something on the order of 0.38 seconds, so collecting this data is difficult.

quote:

Originally posted by Jersey Tom:
...Looked like pretty much every other AL honeycomb crush zone...

Not all honeycomb is the same. Cell size and wall thickness make an enormous difference. To say that no aluminum honeycomb works because of a test like that is a little silly.

Have a read through that thread Matt linked to above. There's some guy on there that rambles on about honeycomb for an impact attenuator.

bacoa/Dave,
Do a search for "Crush" or "Attenuator" and you'll find that this has come up before. There have been some good tips too.
I think that FEA for this sort of thing is a complete waste of time and I would be extremely skeptical of any results from it.

thanks for the info guys,
I found some high density foam lying around in the lab, planning to use it. I think its called divnylcell by diab, hopefully it'll work. If not then i'm a dead man.

I saw from one of the forums that general plastics last-a-foam FR3700 is suitable to act as an impact attenuator as well, any of u guys using it?

I'll admit, the guy who ran the calculations probably was not doing things correctly. In any case, we tried a couple different honeycomb constructions, and they were all toast. The AL honeycomb constructions I saw at competition were for the most part very similar to what we had used. And after seeing the sheer decimation of those things in our testing, I'd be very skeptical of most of them. You'd have to show me test video to prove it. As James was saying I wouldnt trust any FEA on this one.

If the test slug lands on you crush zone and bounces up in a vertical drop test.. it has not absorbed all the energy. If it crushes and the slug just sits there, the energy has been absorbed. We tested a bunch of attenuators before settling on a thick (5 layer?) carbon shell with a steel structure interior.

We video'd all the drops. Didn't have an accelerometer handy. But the rules required 20g average decel, so if it absorbed all the energy in X inches of crush, we called it good.

In any case it was friggin fun as hell.

Tom is very right. Al honeycomb offered practically no protection. I was there for the test and we almost broke our weight stack on that one since it impacted the concrete so hard. Our test of steel structure with high density foam and multi-layer carbon fiber was barely adequite.

I'd think twice in cutting corners on a safety device, you never know what could happen...

quote:

I'd think twice in cutting corners on a safety device, you never know what could happen...

So true, but I think there are areas on a FSAE car that are more important than the impact attenuator from a safety point of view. Because the rules would actually allow anything, the rules say (opposite to what Tom says) a maximum deceleration of 20 g (p. 34 in the rules). So a 0.1 g deceleration of the impact attenuator would fulfill the rules.

In 06, it was maximum average deceleration for sure.

I thought maximum was implied in my earlier post.

In any case.. as Brad (BuffRacing) was saying.. the 5 layer carbon shell with steel internal structure and foam.. barely worked. After seeing the straight up decimation of that honeycomb.. I just have a hard time believing it works. Wonder if I can dig up the video..

Edit - was 8 layers of carbon

Yeah, it was average, missed that :P.

Another thing that's quite interesting with the rules, there's no limit for peak deceleration. In the impact attenuator I designed, the peak value was -25 g and the average was -18 g. But making something that peaks at 100 g and then levels around 1 g could also mean an average deceleration of 20 g...

We used thin-walled aluminum tubes that had notches to lower the inital force required to initate buckling. From a theoretical point of view the buckling of tubes is an lightweight and cheap solution, but in real life, they have quite poor off axis properties.

We didn't do any dynamic testing, but we did a steady crush of a 4"x4" test section of Al honeycomb (I dont remember exactly which one...it was one of the group deals on here) and it looked like it would work great...numbers matched those supplied by the manufacturer almost exactly. I dont recall the name of the machine we used, but the output was a force vs. displacement curve. We used it at competition and scored well in the safety area of design.

Another worthy item of note is how you mount the attenuator. Honeycomb is well and good (I did the calcs, tested some pieces in a press, and am satisfied that my model matched the real situation), but the backing setup we used was far from acceptable. Do a quick FEA of a thin sheet with 120psi of pressure on it, and see what happens. I'll bet you a boatload of cash (courtesy of my employer) that your mounting setup won't handle that sort of stuff without forcing the impact attenuator right into the footwell.

Unless, of course, you did some funky tube thing that already transfers the loads into the frame. That would probably work better from this standpoint.

Last year our capstone project was on the impact attenuator and we did many evaluations of materials first. We used a honeycomb/foam combo with a carbon shell that was very effective. We have high speed video and accelerometer data from each drop and found we had actually overbuilt it originally. Our reason for using the foam was to get some off-axis protection even though the energy absorbed/density was unfavorable. Tests did reveal that the weakest part of the system is the mounting, and it took a lot of reinforcement to come up with an acceptable solution. That is the one area I agree that teams overlook most and should be in the rules or at least evaluated by the judges in the report.

quote:

Originally posted by Greg H:
Last year our capstone project was on the impact attenuator and we did many evaluations of materials first. We used a honeycomb/foam combo with a carbon shell that was very effective. We have high speed video and accelerometer data from each drop and found we had actually overbuilt it originally. Our reason for using the foam was to get some off-axis protection even though the energy absorbed/density was unfavorable. Tests did reveal that the weakest part of the system is the mounting, and it took a lot of reinforcement to come up with an acceptable solution. That is the one area I agree that teams overlook most and should be in the rules or at least evaluated by the judges in the report.

I second the mounting problem, "with out a strong foundation....."

http://www.plascore.com/energy/pdf/nhtsa.pdf#search='nhtsa%20aluminum%20honeycomb'

this might be intresting for you all, these are the barriers that are used in crash tests.

quote:

Originally posted by Greg H:
That is the one area I agree that teams overlook most and should be in the rules or at least evaluated by the judges in the report.

Greg-

I agree entirely, but its gets into the realm of chassis design and FEA (i.e., there's too many permutations to require each team to document their backing.) But the rules can require a good-faith effort on the part of the teams, which is all we ended up putting in to it. We did get a complement from our design judge, though, for actually considering the mounting, and installing braces in our front bulkhead.

OK,
Maybe I am crazy. The front impact zone is for hitting solid barriers. A glancing blow will most likely take out the wheel and crush zone. But what happens when another car t-bones you. I know this is why we have the "side impact structure", but many of the teams I saw back when I competed, and the more recent pictures from competitions, show the frame members up against the drivers.

I am not trying to stir the pot, but has anyone brought in a design for body work with the intent of it attenuating any type of crash? What would a crash like this look like. Does anyone design their side impact farther away from the driver for this reason?

Sorry if I cause future design implications,
Bill