More math stuff

[Thomas MacFinn]

It occured to me that since I-beams are stronger than square beams by weight, that shape should matter for weapon cores as well.

If two cores are made from the same materials - one 1/2" square and the other 5/8" round - the square core would be slightly lighter. Which core would be stronger?

I know square cores have other advantages, even if they aren't stronger than 5/8" round.

[Derian]

For a bit of math:

Assuming two cores of 48" length, the square 0.5" core will have a volume of 12.0 cubic inches. The round core will have a volume of 14.7 cubic inches. If we also assume that the densities are consistent throughout the core, the square core will weigh 81.63% of the round core.

So, it's definitely lighter. Pretty significantly so, in fact. I don't have weights of the cores themselves handy, but I'd estimate a savings of 2-3 oz, which, if you're making a min. weight red, is about 10% of your overall weight.

Perhaps someone with little regard for their personal safety, some hugeass clamps and a **** ton of free weights can try to measure how strong they actually are?

[Thomas MacFinn]

I'm specifically thinking of polearms in the 5-6 ft length range.

The 4 ft length of 1/2" round I bought recently weighs 11.0 ounces. I don't have any 5/8" yet, but I'm picking up some 1/2" square from the same manufacturer tomarrow so, in my case at least, the actual fiberglass formula used is exactly the same for all of my new (and potentially new) cores.

I was just hoping somebody knew some type of structural enginerring formula so we didn't have to break any cores experimenting.

[Thomas MacFinn]

I hadn't calculated the volumes but, before making my first post, had come up with 81.43% by comparing area of the cross-sections with a hand calculator and rounding a few numbers (like pi). I had also calculated that the 1/2" square would be narrower across the flats than the 5/8" round but thicker across the diagonal. ... a lot of numbers that don't really tell me anything yet.

By the way, the two five foot sections of 1/2" square fiberglass I picked up today weighed 17.9 and 18.0 ounces.

[Arrakis]

You need to pick up a mechanics of materials book and look at shear stress distributions across beams of different cross-sectional shape.

[Thomas MacFinn]

Thanks Arrakis. "Shear stress distributions across beams" was just enough jargon for me to do some good web searches and I'm now slogging my way though terms like "Euler Bernoulli beam equation". I'm still trying to understand the formulas, but I have them in front of me now.

[Arrakis]

If you have any questions, just holler.