Hacker News Reader

Comment by jfoutz | Hacker News Reader

Oh, i think i get it. The materials are different, but not different enough for a whole different design - you can't generally simplify out parts. Some strut being 1cm across vs 1.2 cm across doesn't buy you much.

Thanks!

View on HN

To better understand the comment above you need to understand stiffness. When you take a wire of steel, which is very strong, you'll find you can actually bend it very easily. If this kind of stress is what the material will face, you could replace the wire by a thin tube which will be much more rigid -- you probably don't want your structure bending and deforming. In this case, where stiffness is more important, it's more important that the material is light so you can make it larger yet proportionally stiffer per unit weight by using an adequate structure -- e.g. a tube or large diameter (this is because the bending stiffness increases more-than-linearly with diameter).

Now suppose instead you were trying to traction the wire (pull it). There's no structural change that will make it stiffer -- in this case all you may want is that it is very strong -- in which case you simply go for the best strength/weight or just pure strength.

> which is very strong

How are you defining strength? Tensile strength?

In this example, yes.

If you went for a compression stress however you'd probably want that same bending stiffness (by making a hollow tube) because rarely the force is perfectly axial.

Just the opposite. 1.2cm vs 1cm might be a world of difference. And if weight is a concern, you might be stuck with a 1cm tube, which might be less stiff than the 1.2 cm tube even if it's made out of a stiffer material.

The idea is that design is material dependent. You can't assume you can just swap out parts for lighter/stronger/stiffer parts and still have everything work.

Reply on news.ycombinator.com