Engineering Help Needed

[Gary Slahor]

Here's one for all you Mechanical or Structural engineers.

I have 2 different sheets of Aluminum available to me to use as floor pan material. Which one will provide the greater stiffness to chassis twisting ? Assume that the bonding and attachment is the same for both pieces.

Choice #1: Sheet of 6061 T6 material .080" thick

Choice #2: Sheet of 7075 T6 Alclad (for corrosion resistance) .062" thick

Thanks in advance...............Gary

[S Lathrop]

.080 Thick 6061. 6061 is possibly a better choice because of the rough service of the belly pan. Also there could be corrosion problems with 7075 after you get through the Alclad surface protection.


.080 x 6061 is stronger than .065 x 7075 because of the greater thickness. The difference is a geometric function of the difference in thickness.

[DaveW]

As Steve said, as long as they are the same material, the stiffness of 2 different thickness metal sheets is directly proportional to the thickness, since they both have the same modulus. The structural difference between 2 different grades of the same material is the yield point, or how much they can be stretched, before they will not return to their original length. This results in the higher-grade material being stronger and less subject to permanent deformation, but does nothing to increase its stiffness.

[Gary Slahor]

If the modulus (of elacticity) is only an indication of the overall strength....what is the indicator of stiffness ?

I assume (from your posting) that 7075 and 6061 have the same value of this indicator of stiffness ? thus the governing factor becomes the thickness of the material ?

G

[TimW]

The modulus of elasticity is the indicator of stiffness, as well as the geometric factors. Pretty much all alluminums have the same stiffness given the same geometry. The yield point is what determines its strength and is what the heat treatment primarily improves. The stiffness does not really change by heat treatment, it can only be varied by type of material (steel versus ally) and by dimensional changes.

[Gary Slahor]

Very informative. Thanks for the input. I think I got it.......

So....even though the yield strength of the 7075 is much higher than the 6061, the modulus of elasticity (which determines the stiffness) is virtually the same (10,000 vs 10,400 ksi) so the stiffer material would be the thicker one ! ! !

G

[Steve Demeter]

Yield strength is the stress a material will plastically deform (take a permanent set) at.

It is typically not closely tied to modulus of elasticity, which is the measure of stiffness.

[JimLill]

also, IIRC, 7000 series is not a good option if you need to weld it.