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The thermal expansion coefficient of aluminum alloys is 0.00001244 per degree F.
In other words it expands that much of its length for each degree you heat it,
so if it is 10" long and you heat it 500° it would expand 0.062 or 1/16"
The coefficient for steel is quite a bit lower, 0.00000633, so if you heat a
steel part 10" long 500°, it would expand only 0.0315, or 1/32".
This is reversible expansion. Now if your forged parts have a lot of residual
stresses from the forging, and have not been stress relieved, they will stress
relieve upon heating and, depending on the magnitude and direction of the
stresses, might grow longer, and STAY longer when cooled.
The elastic modulus of steel is about 30,000,000 psi (per inch per inch, if you must). That means that for 100% elongation from stress (rarely happens but
handy for relative calculations) it takes a tensile stress of 30 million psi.
More rationally for a 1% expansion in length you would need 1%of 30 million
or 300,000 psi. Typically most steels won't go much more than half of that
before they reach their elastic limit, and you usually figure on some strength
of 60 to 100,000 psi as an allowable maximum stress for high strength steel
and 32,000 for mild steel.
Aluminum has a much lower elastic modulus (not to mention elastic limit).
Most aluminum alloys have an elastic modulus of about 11,000,000 psi so they
stretch almost 3 times as far under the same stress as steel PLUS most are
not nearly as strong as steel so you can't put as much stress on them without
permanently deforming them either. Usually permanent deformation in service
= failure.
So you can see that steel does indeed stretch but not very much. But it is
extremely elastic, far more so than rubber or plastic. That is, the amount
it stretches is exactly proportional to the load you put on it (although very
small). That is the definition of elasticity, at least in the strict sense.
So if you measure your pistons and rods, and know temp differentials and loads,
you can calculate the reversible expansion of pistons and rods (and your engine
block too, but if cooling system works, it doesn't get very hot, and it is so
thick and big the stresses are smaller). Growth due to stress relaxation has
to be determined experimentally.
--
George Downs Bartlesville, Heart of the USA!
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