1. Home
  2. ⏣ General Metalworking
  3. Likely Properties of SS Sheet & Stiffness of Different Materials?

Likely Properties of SS Sheet & Stiffness of Different Materials?

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. 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

Reply to
Doug White
Loading thread data ...

...

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.

Reply to
Ned Simmons

Here is a hands-on demonstration of hardness vs stiffness.

Clamp two long drywall screws upright in a vise, held by the tips. Pluck them and adjust the length of grip until they sound the same. Now heat one red with a torch and let it cool. Annealing removes the hardness.

Pluck them again, they should still vibrate at the same frequency which shows that mass and stiffness haven't changed.

Bend them with your fingers. Both show the same resistance until the annealed one reaches its elastic limit and continues to bend with nearly constant force.

Jim Wilkins

Reply to
Jim Wilkins

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.

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.

-- Ed Huntress

Reply to
Ed Huntress

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

Reply to
Doug White

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.