[Texasbowhunter]

I Have read where there were some information/conversations out there as to the steel vs. aluminum materials in strength...
I was curious if I had a 12" x 12" x 1/4" thick piece of hot rolled steel, where would the thickness come into place with aluminum of the same 12" x 12" outer dimensions?
Also, what are the approximate percentages I could use for this scenario?
Example: If I was going to use 1" thick aluminum, what can I get away with if I was to do in steel hot rolled? or vice versa? If I was using 1/4" thick steel, what would the thickness be in aluminum? Roughly percentage...add 75% thickness if using aluminum? Subtract 75% if using steel from aluminum thickness?
Hope this makes sense...
Thanks
Paul

[pippin88]

Aluminium part needs to be about 45 thicker than the steel part.

Steel 3x as stiff as aluminium (per unit / at a fixed thickness).

Stiffness is proportional to the cube of thickness. Twice as thick = 8 times as stiff (for same material). Increasing thickness makes a big difference to stiffness.

[Texasbowhunter]

Thank You Pippin
So let me understand this and gather my feeble little brain around what you just explained...
With my scenario in original post
12 x 12 x 1/4" thick Hot Rolled steel = 12 x 12 x 3/4" thick aluminum which would be 3x...Correct?
Thanks
Paul

[pippin88]

No
Sorry, typo, % dropped.
Should have said "Aluminium part needs to be about 45 percent thicker than the steel part."

Imperial fractions too hard ( 1.45x 1/2.... ). Let's work in something sensible. Either mm or at least decimal inches

10mm steel = stiffness A
20mm steel = stiffness A times 8. Doubling the thickness (same material) results in the stiffness increasing 8 times.

The point being section size (thickness) makes a massive difference. This difference has a huge impact even when using materials that are not as stiff.

10mm steel is 3x as stiff as 10mm aluminium

But

10mm steel has the same stiffness as 14.5mm thick aluminium.

[peteeng]

Hello Paul - Your first sentence says "strength". There are many steels that are less strong than aluminium and vice versa. Depends on alloy temper and heat treat. I expect you mean rigidity or stiffness. We engineers use stiffness to describe a material property and rigidity to describe a structural property. So I have attached the calculation to enable comparing sectional rigidities. Steel has stiffness of 200GPa and Aluminium is 70GPa so thickness to thickness steel is always 3x more rigid. So 10mm of steel is 3x more rigid then 10mm of Al. But a sections geometric inertia is 1/12 x width x thickness^3. The cubic part means that aluminium can be more rigid and less heavy if you take it into account. The rigidity of a section is its material stiffness (youngs modulus) x its geometric inertia. Once these are understood you can compare different materials to get the same rigidity section. Being more than one variable you can't say 75% thicker as a general thing, You need to do the math to determine the answer.

So in the example below a 14mm thick aluminium plate is the same bending rigidly as a 10mm thick steel plate. Peter

[Texasbowhunter]

Great break down for us laymen to comprehend...
Thanks
Paul