Impact Attenuator Calculations

[bacoa]

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

[Matt Herset]

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 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/85510167411/r/85510167411#85510167411)

Matt H.

[Jersey Tom]

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!!

[Matt Herset]

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.

[Jersey Tom]

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.

[Mike Cook]

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.

[Matt Herset]

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.

[James Waltman]

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.

[bacoa]

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?

[Jersey Tom]

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.

[BuffRacing]

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...

[HenningO]

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.

[Jersey Tom]

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

[HenningO]

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.

[adrial]

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.

[CMURacing - Prometheus]

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.

[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.

[CrazyDave]

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.

[CMURacing - Prometheus]

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.

[Bill Kunst]

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

[CappyUMD]

Has there ever been a 2 car collision at the competitions? I'm not worried about it. The course speeds are so pathetic that if a T-bone was immiment, you could probably stop or go around. There would be something to worry about if a car's throttle sticks open and the brakes fail and the brake overtravel switch fails and the car is in one of the faster gears and the driver panics instead of killing the engine. I'm willing to take that chance.

[Bill Kunst]

Yeah, your right. It is about as likely as having a head on colision with anything at almost every competition. I guess someone might go through the same scenario as what you said, and drive a quarter mile off track and hit a curb.

On a more realistic design note, the test tracks of most teams do have more risks than the competition. Ask the teams that have had a crash, better yet, all those who have had a crash should make a post with pics of the damage. If my memory serves me right, a few years back Cornell took a front-on collision with a curb and had significant damage.

I guess, with the amount of poles in the parking lots and the inexperianced drivers, is there a chance of a sliding impact into a pole? I think there is as much a chance of this as there is a front impact.

And my question still stands, has anyone made accomodations in design for the chance of a side impact.

Thanks much,
Bill

[absolutepressure]

Don't go givin people new ideas (rules (obstacles)) for us Bill. But to add a more blood curdling picture to your crushed pelvis scenario, think of what would happen if someone with side pods put the radiators in them. Not only would the crushing blow cause you to scream out in extreme pain, but the boiling water would now be spraying all over you, causing you to get 2nd degree burns. If your lucky, you'll go into shock or pass out from the pain.

[Jersey Tom]

ap - worse than that, there was a year (way back whe) our team's frontal impact attenuator WAS the radiator.

We werent so smart back then..

[absolutepressure]

Originally posted by Jersey Tom:
ap - worse than that, there was a year (way back whe) our team's frontal impact attenuator WAS the radiator.

Wow...

[Jersey Tom]

Our team has definately done embarassingly dumb stuff well before my time.

The 07 car has potential to be an unbelieveable step up from our 06 even, which placed 22nd.

[Luke_Morrow]

Hi all,

I am from The Newcastle of University in Australia, and we made our impact attenuator out of 3 layers of aerolan, each separated by a sheet of aluminium. This was bonded with glue, with wire also used to keep the sandwich together.

We did physical testing at uni with beyond standard results. It then decided (without a vote) to do some destructive testing at the 2006 Australian competition.

http://www.eng.newcastle.edu.au/~fsae/page_files/gallery_files/Front_view.jpg http://www.eng.newcastle.edu.au/~fsae/page_files/gallery_files/Chassis_member.jpg

Unfortunately the barriers in this area were made from 6 metre lengths of steel guardrail, with a gap in them at the height of every teams impact attenuator. That said, the nose of our car went through the gap and the chassis was effectively the first point of impact. I hope that the event organisers for future events refuse to use this style of barrier where frontal impacts could be expected to occur (ie small skid pans and practice tracks). This style of accident has happened on practice tracks before in Australia, but the other incident was with a water filled plastic barrier, with no chassis damage.

This is no defence of our own design, as every team at this comp in the same scenario would have had their nose go straight through this hole and damage their own chassis. Fortunately with a steel space frame it is easy to cut and weld. Good luck to those of you with a composite chassis.

BIG thanks again to UOA, UOW, RMIT, TAFE & Bill McBride for offering active assistance. We were back and running again for our next event.


Luke Morrow
UN Racing 2004-2007

[Buckingham]

ap - worse than that, there was a year (way back whe) our team's frontal impact attenuator WAS the radiator.

We werent so smart back then..


Way way way back when I guess there was a year we did the same and also decided to use the frame tubes themselves as the coolant lines back to the engine ( I imagine there was some sort of rubber line pickup at each end).

[drivetrainUW-Platt]

We thought about that, use the rollbar is a radiator, untill someone grabbed it to push the car it might work ok.

[Ian McMurdo]

From what I can tell, the impact attenuator report doesn't actually count for points. Its more of a pass / fail thing. Am I correct in proceeding with this assumption?

[Running Mike]

Somebody asked for a video of Al honeycomb being tested. You can find one in the bottom of this page:

http://www.unicorn.aau.dk/nyheder/2002811

We tested different types and concluded that honeycomb is perfect if you use the right type. The video shows a pre test experiment with Al comb from a wing of a F16 fighter jet.

And yes, the report is a pass or fail thing.

[vane]

Hi, My name is Vanessa. I was wondering how you guys test you impact attenuator and wich method do you use to measure deformation and desaceleration. Thanks for your help

[Luke_Morrow]

Originally posted by vane:
Hi, My name is Vanessa. I was wondering how you guys test you impact attenuator and wich method do you use to measure deformation and desaceleration. Thanks for your help

After testing a number of ways over a few years, we found that dropping a 300kg 44-gallon drum half full of reinforcement and cement from a forklift onto the test piece proved the most effective results. A quick release was used to drop the drum, and a safety rope was used to prevent the drum from falling over. A high speed camera at 1000 fps and rule in the background were used to record data (knowing time and displacement its easy to get initial velocity, then deformation displacement and then average acceleration is easy).

A sheet aluminium fabrication (riveted or welded) absorbed next to no energy, expanded foam absorbed a small amount of energy while it disintegrated like an imploding building without stopping the drum and an aerolam and sheet aluminium alternating stack ~ 200 mm thick stopped the barrel before it hit the ground, absorbing 108 kJ of energy at a constant 16G.

This is also a great opportunity for a risk assessment! Remember, safety first.

[Ricardo Avila Ibarra]

Hi everyone.
I was wondering if any of you knows if we are allowed to present impact attenuator test data from a scale model.

I was actually planing to use only 10 percent of the load determined for the test. Lets say build an impact attenuator scale model 1:10 and drop 30 kilograms instead of 300 kilograms. Obviously using the same drop height.

Thanks in advance

[Luke_Morrow]

Scale testing is easiest when you scale the energy (in your case the mass) and use a cross section of the same scale (1/10th). The IA length needs to be representative. So, yes you can scale the energy, but don't scale the length. This is because your IAD should also validate that the mass comes to rest in less than the IA length.

I am putting together a little document on IAs for FSAE-A and hope to make this available for all teams at the event, to take into consideration next year. Watch this space.

[Tech Guy]

I think the answer is in Question and Answer # 4 of FAQ # 104, which can be found at:
http://www.formulasae.org/forums/formula/dispatch.cgi/r...%20Test%20Method.doc (http://www.formulasae.org/forums/formula/dispatch.cgi/rules/docProfile/100214/d20081029201720/No/B.3.21.1%20IA%20Test%20Method.doc)

[mittalrdx]

Hi,
I just wanted to know that for the destructive testing of the impact attenuator can we give more than 7350J of energy and have some bounce back i.e.(give approx 9000J of energy and have a bounce back energy of 1600J) and still say that the impact attenuator absorbs 7340J of energy thus it works fine?


cheers
shivam