Helmet failure modes: cracking vs deformation

[annirak]

Who sent that email?

A company rep.

 

[glenn forger]

You seem anxious to avoid naming the helmet manufacturer that boasts a cracked helmet means it worked?

 

[G3CWI]

The more of these threads that I read the less I understand. Everyone's an expert but which one to believe?

 

[Dan B]

So, based on your experience of using polystyrene in any other setting (e.g. packaging items sent through the post) how much energy do you think you use when cracking a lump of the stuff in two?

 

[ufkacbln]

There was a recent article on fake helmets.

When they tested the fake, it broke in two.

The engineer stated:

That's a terrible result, the helmet has simply split into two

 

[ufkacbln]

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

Especially for children, and the elderly

Walking can be so dangerous

 

[ufkacbln]

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?

The protection would save many pedestrian head injuries, and IIRC there is no evidence of pedestrians being killed ny helmets either

Inconclusive proof for pedestrian helmets?

 

[ufkacbln]

The problem is that there is no "evidence" either way

I know people who (like the OP) suffered spinal injuries and blamed them on the rotational moments of the impact being increased by the helmet

Now whether that is truly the case is as debatable as whether the helmet saved someone's life.

However if we accept the beliefs of one as "proof" of the effectiveness of a helmet, surely the belief of the other as "proof" that it caused an injury?

 

[annirak]

You seem anxious to avoid naming the helmet manufacturer that boasts a cracked helmet means it worked?

There isn't some grand conspiracy. I thought that if you wanted to know more about the helmet, you would have followed the link I provided to the original conversation, in which you commented.

Since that appears to be too much effort, it was a Catlike Whisper.

 

[glenn forger]

They sent that email?

 

[Wobblers]

Whoever wrote this:

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.

clearly is not a professional engineer or scientist - not least because any professional scientist or engineer who stated this - without proof or any meaningful process - would not be a professional for very long.

It is also clear that the author does not understand basic materials science. A material can absorb energy in one of two ways. It can deform or it can crack. A ductile material such as most metals will bend - this is called plastic deformation. This requires energy to break the metallic bonds (technically, you're generating large numbers of dislocations within the crystal lattice of the metal). It is a highly effective means to absorb energy, as the whole bulk of the material is involved, hence a very large number of bonds are being broken.. Think of how much effort it takes to bend even something small like a spoon. The crumple zone in a car works in this way; if this quote was correct, crumple zones in vehicles could not function. Therefore, it is entirely wrong, ignoring as it does everything that we've learned about how materials deform and fail.

The other mechanism of absorbing energy is through cracking. Yes, indeed, some chemical bonds are broken, and some more energy is used to create a fresh surface (all surfaces have energy associated with them, the surface tension of water is the physical manifestation of this phenomenon). This means that the energy absorbed is very closely related to the area of the freshly created surface that cracking - but cracks have very little surface area as a rule and involve far less of the material than bulk deformation, hence there is less scope for energy absorption. The end result is that cracking is very much less effective in absorbing energy than deformation.

I would suggest that you buy your helmet from a company that actually understands the materials science. At least you can be sure that they'll know what they're doing...

 

[Wobblers]

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?

Is that a fact? I haven't come across any evidence supporting that statement in peer reviewed medical journals.

 

[classic33]

The protection would save many pedestrian head injuries, and IIRC there is no evidence of pedestrians being killed ny helmets either

Inconclusive proof for pedestrian helmets?

Source: Kuratorium ZNS (Hannelore Kohl Stiftfung fur verlaetze mit Scaden des zentretralen Nervenstems) Geschafsbreicht 2004, DE
So a bit out of date.

 

[benb]

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 best proof that the helmet worked properly is that you are 100% safe and didn’t have any brain damage

Whoever wrote that is talking utter bollox.