Interesting, thanks for that.
However, we don't know at which point precisely the failure mode changes so it's as unwise to suggest that we can't rely on them as it is to suggest that we can?
Presumably a fracture during impact at the helmet's rated speeds would mean the helmets do not absorb enough energy to pass the required standard, which might well be costly for manufacturers. As such, are these helmets built in with a safety factor?
Additionally, would it be fair to suggest that the first head impact would tend to be most severe and any further impact would likely be less so?
I feel you are being quite reserved in not over-stating the real truth of the materials science behind helmets; are you merely against the blind faith that many have in helmets or do you think there is a good reason to avoid wearing one?
One argument commonly brought up is that helmets increase the risk of rotational/strain injuries to the neck. The weight of a human head being around 4-5kg, compared with that of a helmet (250g for mine, maybe a bit more for cheap £20 shells though), seems to make this quite unlikely. Do you have an opinion on this?
Many thanks mate
Material scientists and metallurgists tend to classify materials into two categories: "tough" and "brittle". A tough material retards crack propagation while a brittle material is susceptible to crack propagation (contrast the results when you drop a steel enamelled mug (tough) - it bounces - with a brittle glass). Even tough materials will behave in brittle manner under certain circumstances and it's difficult to predict just when that will happen.
The more upmarket helmets are sold on being light and less bulky than cheaper ones. The only way that can be achieved is to use stiffer foam. Though that means the forces in any impact will be higher, it does have the benefit that the stiffer foam will provide more protection if the impact is against something pointed such as a kerb. Unfortunately, stiffer foam is also more brittle so is more likely to fail in a brittle fashion in high energy impacts. The significance of this is and the exact energy that this happens is impossible to say - it also depends on the shape of the helmet, the shell and the object that impacts it.
Helmets are designed down to the standards. To design in a 50% safety margin over the standard requires 50% more material. With something that whose selling points are lightness and small bulk, this is something of a disadvantage. It is a route that a manufacturer is unlikely to take because it won't help them sell helmets.
Whether the first is worse than subsequent impacts depends very much on the exact circumstances of the accident. Though in my most recent crash, it wasn't the
first impact that broke my ankle...
It is not the weight of the helmet that increases the probability of rotational injury, it is its bulk. A helmet makes your head larger, thereby making a head impact more likely. Further, the bulk means that impact forces are applied further out from essential structures such as the spinal cord: this has a leverage effect that increases rotational forces by 10-20%. The types of injury caused by rotational forces - spinal, basal skull fractures and diffuse axonal injury - are all amongst the most serious.
