Helmet failure modes: cracking vs deformation

[annirak]

I recently was in in accident (see here but please keep the helmet discussion to this thread).

There were some comments about how my helmet failed me because it cracked, rather than deforming.

Your helmet catastrophically failed.

Cycle helmets fail catastrophically, not gradually, in high impact crashes- the forces are so great that a helmet will compress and break in around 1/1000th of a second. The absorption of the initial forces during this very short period of time is unlikely to make a significant difference to the likelihood of serious injury or death.

A helmet that worked properly in an accident would have a permanent depression at the point of impact, but would still be in one piece.

Your helmet failed.

This disturbed me. I landed head-first into the road at 17.7mph (extracted from speed/cadence sensor log), cracked my helmet, got three compression fractures in my T spine and a fracture in my C spine, but not even a bruise on my head; no concussion, no loss of consciousness, no memory problems. Something didn't add up.

Suspecting that the user who posted said comment might not be an expert in helmet design, testing, or validation, I contacted the manufacturer. Here is their reply:

The fact that the helmet cracked is probably one of the best things that can happen as it means that it absorbed the impact and liberated the energy, if it wouldn’t crack, the energy of the impact could go to your head/brain. Its meant to be cracked, not deforming. All helmets need to past a specific homologation and rigorous test. The best proof that the helmet worked properly is that you are 100% safe and didn’t have any brain damage.

I'm left with no conclusion other than that the cracking vs deformation debate is just FUD. A cracked helmet is one that worked at high speed. A deformed helmet is one that worked at low speed.

 

[Drago]

Hold on, let me just grab a Guinness and open these Doritos...

 

[mcshroom]

The term "they would say that wouldn't they" I think sums up that response. It definitely isn't written by a scientist or an engineer

Ask yourself how the helmet is supposed to protect your head, it's mechanism of action. A helmet is designed to compress and absorb energy. This compression allows for your head to decelerate more slowly so the forces exerted on your head is reduced. In the event of the shell fracturing, a fault is opened through the helmet, and the compression does not occur. Therefore the forces exerted on your head are greater than if it had worked as designed.

See http://www.cyclehelmets.org/1209.html

As for the "rigorous testing" I find that rather amusing. try this - http://www.cyclehelmets.org/1081.html

If a helmet manufacturer is telling you that their helmet is supposed to crack, then I would advise you try looking for a different manufacturer's helmet. If you can, get one that meets the Snell standards rather than the lower EN1078.

 

[glenn forger]

Who sent you that email? Ask them if the helmet complies with BHRF, 1081, ask them how the direct linear force was absorbed if there is no compression. Ask them whether their helmet is designed to compress at a level of force less than that which might be expected to lead to brain injury. That's a mickey mouse email from someone who doesn't understand basic principles.

 

[blazed]

Surely it's better for the skull to hit against the inside of a helmet than tarmac/concrete.

 

[Drago]

To be fair, the information presented in that link was also not written by a scientist or engineer. They cherry pick the data to support their position, while vaguely dismissing data that undermines it with phrases such as "not likely to..."

Your position may well be the correct one Mr Shroom, but that web page does as much to negate your contention as it does support it.

 

[glenn forger]

To be fair, the information presented in that link was also not written by a scientist or engineer.

A major helmet manufacturer collected damaged childrens' helmets for investigation over several months. According to their senior engineer, in that time they did not see any helmet showing signs of crushing on the inside (Sundahl, 1998). Helmet foam does not 'rebound' after compression to any significant extent. If the styrofoam does not compress, it cannot reduce linear acceleration of the brain. The most protection that it can give to the wearer is to prevent focal damage of the skull and prevent minor wounds to the scalp. It is not likely to prevent serious brain injury.

 

[blazed]

Even if a helmet did not protect against brain damage the fact it can prevent cracks in the skull stil means they are potentially life saving. How many recorded incidents are there of people being killed because they wore a helmet?

 

[Drago]

Read the whole thing Glenn. The things that support their claim get quote in detail. The things that oppose it are vaguely dismissed.

My sympathies lie largely in the Shrooms corner, but that link leads to a very poorly presented and selective summary of other people's research.

 

[annirak]

I think you're looking at the physics in too narrow of a way: Any Kinetic Energy absorbed by the helmet is kinetic energy NOT absorbed by YOU. So cracking the material of the helmet takes a lot of energy. Energy absorbed, less absorbed by me.

Again, I need to point out that I have actual evidence. I hit head-first into tarmac with enough force to sustain compression fractures in three vertebrae, and I didn't even have a bruise on my head. At the very least, the helmet saved me from bruising and abrasion; that's just obvious. While I can't prove that I would have sustained anything worse, a concussion seems likely.

 

[Drago]

Kinetic AND potential energy. Most riders tend to fall downwards as well as forwards. Those lucky few that fall upwards in a gravity defying arc tend not to need helmets.

Energy is never 'absorbed', it just takes on other forms, noise, heat, whatever.

 

[swansonj]

How many recorded incidents are there of people being killed because they wore a helmet?

A non-zero number of completely unambiguous incidents actually, kids getting strangled when their helmet got caught in openings (half open car window was one instance).

 

[glenn forger]

Who sent that email?

 

[Drago]

To be fair again, what about the kids who've died trapped in car windows who weren't wearing lids?

Everyone is falling into the trap of whoever summarised the article in Shrooms link, that of parading evidence to support their standpoint while failing to account for and explain evidence that apparently undermines their position.

 

[annirak]

Kinetic AND potential energy. Most riders tend to fall downwards as well as forwards. Those lucky few that fall upwards in a gravity defying arc tend not to need helmets.

Energy is never 'absorbed', it just takes on other forms, noise, heat, whatever.

Good point about the PE.

Energy can be stored in chemical bonds or crystal lattices. Does that count as absorbtion? I think its a reasonable approximation. And breaking chemical bonds can take energy. Is it unreasonable to call breaking chemical bonds absorbing energy?