Impact Attenuator Calculations

[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