Keywords:
In article , "Ed Huntress" wrote:

"Doug White" wrote in message
.. .
I have a metal piece that I would like to replace with plastic (long
story). The original piece is 34 mil thick non-magnetic stainless sheet,
bent in an L shape with about a 3/4" radius. It's not structural, but I
would like the plastic version to be as rigid as the original. I'm not
sure what sort of SS it might be, and 34 mils is an odd thickness. I
don't know if it's 1/32" & a bit thick, or 20 gauge & a little thin.

I know that the bend would have work hardened the metal a bit, and I
would think it's a bit stiffer as a result. The part that always baffles
me is that the bending formulas are all in terms of the Modulus of
Elasticity, which doesn't vary much. I expect work hardening & the like
to make a difference, but the formulas say not.

Right. Modulus of elasticity doesn't change with hardness. It's
counter-intuitive, but it's true. The modulus for a piece of 1008 carbon
steel is the same, roughly, as for a piece of fully hardened 1090.

Stainless has a slightly lower modulus than carbon steel, and it doesn't
matter if the stainless is work-hardened or not.

I'm basically trying to
figure out how thick a piece of plastic would be required to match (or
slightly exceed) the original stiffness. I have the modulus of
elasticity for the plastic, but no real idea of what the steel might be.


Is it likely to be 304 SS or something similar? I found
properties on-line, and the modulus of elasticity for 304 is around 200
GPa. It doesn't vary signficantly for nay of the 300 series steels. The
plastic is only 2.5 GPa. If the stiffness varies as the thickness cubed,
that says I need the plastic to be roughly 4.3 times as thick, or about
0.15". This is do-able, although 1/8" thick is probably easier to get.

Does this make sense, or am I missing something?

Thanks!

Doug White

You got it right, with the caveats regarding sectional shape that others
have pointed out here. If it's just a plate and you're bending it across the
plate thickness, your calculations are correct. If you're bending it in
another direction, you have to analyze how the section is loaded.

Thanks to everyone for their comments.

The two pieces will be identical, except for the thickness, and the
bending will be in a direction along the existing bend, i.e. it would
tend to make the 90 degree bend more acute. I think the "cube
of the thickness" correction should work well enough.

I'm going to see if I can get an 1/8" thick piece of the plastic (Kydex)
to play with. Once I get it bent (using heat), I can compare the
stiffness of the steel directly against the plastic. On ething I'm not
sure is whether bending the Kydex modifies it's mechanical properties
much, but I'm sure the vendor can tell me.

Doug White