Bob Engelhardt wrote:
It doesn't have to be hard in the sense of holding an edge (no cutting
involved). But it doesn't seem to me (by intuition ONLY) that mild
steel would be "strong" enough. It seems like it would bend too easily.

But not-bending is a matter of tensile strength, yes? And mild steel
and hard steel have the same tensile strength, right? Or, is it a
matter of yield verses ultimate strength?

Tensile strength -- AFAIK this is a vague term vs. yield and ultimate
strength.

yield strength -- the 'tensile strength' at which the material begins to
permanently deform.

ultimate strength -- the 'tensile strength' at which the material
really-o truly-o breaks.

Mild and medium steels have different yield and ultimate strengths, more
or less related by elongation. Hard steels, and other brittle materials
like glass, stone, ceramic, etc., don't elongate, so their yield and
ultimate strengths are the same tensile strength.

You left out the Young's modulus, which, together with the dimensions of
the piece, determine the spring rate. _This_ is the thing that is
practically the same between mild and hard steel. It's what says "if
you stress it so much, it'll deform so much, if you stress it twice as
hard, it'll deform twice as much" -- as long as nothing's getting
permanently bent.

Bottom line - what should I use use to make a pry bar? (It will have
very specific geometry, so store-bought is not an option.)

Something that won't bend easily, but _will_ bend before it breaks. You
don't want it to just go "blorp" when you stress it, but you'd much
rather it bends when you're really reefing on it than to have it snap
with a ton of force behind the little bits that are still prying.

Model T Ford drive shafts make superlative pry bars, for just this reason.

As Ed alluded, it's going to be hard to make a pry bar without heat
treating facilities. I rather suspect that you want to start with
material in an annealed condition and/or bend it up using heat -- but
then you need to harden the thing, which means that you have to get the
whole mess up to a uniform temperature and quench it. And that wouldn't
be easy.

If you just need the "handle" end to have weird kinks, but the
"business" end can be normal pry bar, consider bending a normal pry bar
to shape (with generous use of a rosebud torch). You'll lose temper at
the bends, but if they're not as stressed as the ends maybe you'll get
away with it.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com