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Ted Edwards
 
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Old Nick wrote:

I am not sure if you were among those that guided me through this
maze, but the Elastic modulus is not the whole story, from my
extensive blundering about on this subject. High tensile steel is
"stronger" than mild steel mainly because it can withstand more
distortion before Elastic Modulus (Young's) is overcome and the metal
reaches its permanent deformation point. Young's then no longer
applies.


Exactly so.

So the "tested stress maximum" is often more important than the
Elastic modulus.


How does this apply to aluminium vs say, mild steel? From the Beam.exe
programme it appears about the same using a 100mm tube, with a 6 mm
wall of alum and a 2mm wall of steel. Both reached approixmately the
same proportion of their failure strain under a given load.


Often there are two criteria for a satisfactory design:
It must be strong enough so that it will not fail.
It must be stiff enough so that it doesn't deflect more than an
acceptable ammount.

For example, most building codes state that floor joists must be strong
enough that the floor can withstand so many lbs/sq. ft. They also state
that the deflection must not exceede 1 unit/so_many_units of length.
Frequently this requires far "stronger" joists than would be necessary
for resistance to failure.

Considering your tube (I recall the raw data for an inch system so I'm
adjusting the diameter and wall):
BeamOut ''(10E6,I_tube 100 6÷25.4)Beam 0 120 (60 1000) {Aluminum}
pos'n pt. shear bend. slope def'l
force stress stress

0 -500 -363.1 0 0.01906 0
60 1000 363.1 -12510 0 0.7626
120 -500 0 0 -0.01906 0
BeamOut ''(30E6,I_tube 100 2÷25.4)Beam 0 120 (60 1000) {steel}
pos'n pt. shear bend. slope def'l
force stress stress

0 -500 -1047 0 0.01689 0
60 1000 1047 -33240 0 0.6754
120 -500 0 0 -0.01689 0

The deflection are close but not identical. This is because the tube
wall is not *VERY* much less than the OD. The thinner the wall, the
closer the deflections would be. The stress is roughly 1/3 in the
aluminum tube. An aluminum alloy with a yield strength of (say) 36Ksi
would be stressed to only 1/3 of its yield. A 36Ksi yield mild steel
(water pipe grade) would be stressed to about 90% of its yield but a
100Ksi steel alloy would be stressed to the same fraction of its yield.

Simply put, you must design for both sufficient strength and sufficient
stiffness.

Hope this helps.

Ted