DIYbanter - View Single Post - Likely Properties of SS Sheet & Stiffness of Different Materials?

Ned Simmons

On Sun, 25 May 2008 23:17:00 GMT, (Doug White)
wrote:

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.

....

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?

You're right if you simply want a flat section of the plastic to be as
stiff as flat piece of SS. But if you want the plastic angle as stiff
in bending as the original SS angle, the entire section has to be
deeper. How much deeper will depend on how you change the other
dimensions. Machinery's Handboook has the formula for the moment of
inertia of an angle's cross section. It's pretty ugly, but I'll bet
with a little head scratching it can be simplified if all you need is
an approximation. The section "Moment of Inertia of Built-up Sections"
may provide some additional insight.

--
Ned Simmons