Let's not confuse stiffness with strength. All three materials have
essentially the same stiffness, but the major practical difference
between them are the yield and ultimate strengths. Until they yield,
they will deform about the same amount for a given load. After they
yield, of course, they might deform much, much more. But when designed
properly for aircraft, they should not yield at the maximum flight or
landing load. Yield, of course, is a "permanent detrimental
deformation," according the practical definition that the FAA uses,
while ultimate is defined as failure.
3003 aluminum is a very soft material. It's not used for structure, and
has limited use for formed non-structural parts. I could not find
information about this material.
6061, especially in the T6 temper, is a common material for aircraft and
non-aerospace applications such as flashlight housings. Its yield
strength is 35,000 psi and its ultimate tensile strength is 42,000 psi
when used for aircraft. It's a rugged, durable, practical material
that's relatively inexpensive. Because of these properties and its
reluctance to grow cracks, it's a common aluminum for some spacecraft
structures.
2024-T3 is a common aircraft material. The strength varies slightly
depending on its shape, for example, tube, extrusion, sheet or bar. For
sheet, its yield strength in tension is 42,000 psi and its ultimate
strength is 63,000 psi. It's a strong aluminum alloy, readily available
but more expensive than 6061-T6. It is more susceptible to cracking and
crack growth than 6061-T6, but that's a matter of degree, so don't let
that frighten you away from it.
The stiffness and strength are two entirely different properties. Since
I've just supplied the strength numbers, let's briefly discuss
stiffness. All these materials have about the same Young's modulus of
elasticity, which is the term for the property of stiffness. As I
mentioned, I found no data for 3003, but 6061-T6 has a stiffness of 9.9
million psi and 2024-T3 sheet has a stiffness of 10.5 million psi, or
about 6% stiffer than the 6061.
One of the interesting factoids of stress analysis is that light
aluminum structures are often critical in buckling. And buckling turns
out to be primarily sensitive to the stiffness, rather than strength, at
least until it is highly loaded. Of course a structual engineer has to
check strength as well as buckling, because you can't just expect that
one or the other will dominate.
David Paule
B-T-W
3003 is very soft.
6061 is twice as stiff
and 2024 is three time stiffer than 6061.
Regards
