On Jun 13, 7:30 pm, Orval Fairbairn wrote:
In article ,



GeorgeB wrote:
On Tue, 12 Jun 2007 03:05:30 GMT, Orval Fairbairn
wrote:

In article . com,
Lou wrote:

For all or you that are interested. I've made the brackets out
of 2024 and the 2024 bends harder, but doesn't seem to snap
as easy.
Lou

Of course, 2024-T3 doesn't snap as easily as 6061-T6! It is the "T
number" that determines brittleness (and stiffness).

Orval, stiffness is the same for the same alloy, and substantially the
same for all aluminum alloys, independent of heat treatment
conditions. "T-number" has nothing to do with stiffness.

No, it isn't! Try to bend some .025 6061-T3 and then some 6061-T6. You
will find the T2 bends more easily.

Wrong. GeorgeB was correct.

You are confusing yield strength with stiffness.

Stiffness refers only to elastic deformation.
Strength refers only to plastic deformation.

Elastic deformation is when it bends and
then springs back to exactly the same
shape it was before. Plastic deformation
is when it stays bent.

The stiffness of a material is the ratio of stress to strain.
The formal name for that material property is the spring
constant or Young's modulus.

Ignoring allotropes for the sake of simplicity, the
stiffness of a material depends only on gross
composition and is independent of temper.

Thus all low-alloy steels have the same stiffness
which is just about the same as plain old iron.

Yield strength is the minimum stress that will
result in plastic deformation (e.g. a permanent
bend.)

Heat treatment affects yield (and ultimate strength.

So if you make a spring out of annealed 4130
and another out of hardened 4130 they will both
stretch the same amount for the same applied
force, so long as you do not apply so much force
that either is permanently stretched. (e.g. the
applied stress is in the elastic range) They are
equally stiff.

However the force that leaves the annealed spring
permanently deformed (yielded) is much less than
that which leaves the hardened spring deformed.
The hardened material is stronger, and so it
remains 'springy' over a range of stress that would
leave the other spring 'sprung'.

The difference between 'spring' steel and ordinary
steel is the strength, not the stiffness.

The same is true for all alloys, not just steel.



Yield strengths (and tensile, but yield is the one we USUALLY care
about) vary amont alloys and heat treatments.

That is why "T-number" controls stiffness, since bending is the result
of inelastic deformation. It takes more torque to bend a piece of T6 vs
T3 of the same alloy and thickness.

Wrong. It takes equal torque to bend each
_in the elastic rage_. It takes more torque to
_yield_ the stronger piece.

'Bend' is ambiguous. You can bend the piece
and have it spring back all the way (elastic
deformation) or you can bend it and have it stay
bent (plastic deformation).

Stiffness refers only to the elastic deformation.

Strength refers only to the plastic deformation.



2024 is about 5% "stiffer" than 6061. 2024-T4 yields about 40% more
in tension than 6061-T6 before breaking.

Yes.

Yes.

--

FF