How to make successful helmet from scratch

[Madmace10]

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

We have to make a helmet in engineering class that will survive a bludgeoning from the teacher with a hockey stick. I am not sure what the best materials to use would be or the best way to make it. Additional info is that we are trying to make a helmet for rugby that will be safe.

 

[rumborak]

Any constraints to the design? You could go Dark Helmet, that would certainly be safe (but obviously rather impractical).

 

[Trevorman]

One important aspect to overview is the impulse. You should try to have a dampening in the helmet so the force is spread out over time. From there you could look at different foams or other materials with low weight.

 

[Madmace10]

Any constraints to the design? You could go Dark Helmet, that would certainly be safe (but obviously rather impractical).

only restraints are the outside has to be paper machete (to add decoration) the helmet has to protect a watermelon

 

[rumborak]

The thing to optimize in the helmet will be the deceleration of the head. So, to some degree you will be bound by very basic physics. E.g. if the helmet is of thickness d, the smallest deceleration from speed v will be a continuous declaration across that distance d, the deceleration given by

a = v^2/(2s)

So, on the one hand you're looking for a material that will compress linearly under such deceleration, but at the same time you're also looking for a material that will distribute a sharp impact across the head area (so you don't get impaled by something). Those two are somewhat opposed to each other.

 

[haruspex]

There are three things a helmet needs to do.
As others have noted, you want the impulse spread as evenly as possible over time. An elastIc material won't do that well because the resistance increases linearly with compression. A material that yields plastically is better, offering a more or less constant resistance as it collapses. The tricky part is that you need to get the resistive force right. Too high, and too much force will be transferred to the head, too low and it will collapse completely before enough of the blow has been absorbed. So the ideal is a resistance just a bit less than the natural strength of what you are trying to protect - in this case, the tissues that hold the brain together.
As also noted, you need to be able to cope with a blow from a sharp object. That implies a strong and stiff outer shell.
The third is to spread the blow evenly over an area of the skull so as to avoid localised crushing, so a softer lining (but this is the least important of the three).
As regards onslaught by the teacher, an Apprehended Violence Order should also be effective. Consult your lawyer.