Bill:
getting back to some interesting ideas you brought-up w/ respect to rod
strength, fatigue life, etc--
you said...
Then too, which steels and irons, out of hundreds of varieties in use,
have an infinite fatigue life under 25% of yield stress, and which under
60%? Here again we don't know, and probably can't find out.
-- this data is published for many alloys. By ASM, among others. Usually
no problem at all to find what you need.
you said...
And does 'age' have much, or anything, to do with it. Is something going on
in
old metal other than detectable cracks, that affects its
serviceablility? I have to take that seriously because the immortal
P.E.Irving, the Vincent mc designer, once referred to "metal decay".
Was he kidding? What metal was he referring to? Perhaps the aluminum
and magnesium families, we hear of "intercrystalline corrosion" or
something, in them,
-- I think you are on the right track. Although Irving used an archaic
term, he was probably talking about "intergranular stress corrosion" (IGSC).
I have little knowledge of how it effects the light alloys (they are used
little in my industry), but it is a very common failure mechanism of
300-series stainless steels in coastal environments, and it stands to reason
that the same sort of thing could cause parts made of less "noble" metals to
fail. It eventually results in parts that are interlaced with fields of
very fine cracks-- so fine that their "probability of detection" (POD) is
very low with the dye-penetrant techniques typically used to evaluate those
alloys. For this "metal decay" [sic-- w/ tongue-in-cheek) to happen, two
factors must exist:
1. the metal must be exposed to a liquid or gas that is corrosive--
typically not corrosive to the alloy "grains" of the metal, but to the
intergranular "glue" that holds the grains together (for 300SS on the gulf
coast, this corrosion agent is the chlorine in the coastal air)
2. the part must be under stress, so that there is a force trying to pull
the grains apart, exposing more and more of the intergranular boundary to
the corrosive effect.
You might think that parts "sitting on the shelf" would be immune, because
there is nothing putting any force on them; not so, Tonto! Most parts have
"residual" stresses in them, from the manufacturing processes. For
example-- a complex casting is full of residual stresses, because (after the
"pour" of the liquid metal) different parts of the casting solidify at
different rates, and the cooler areas "pull" at the hotter ones. Unless the
parts are thoroughly stress-relieved afterward, those residual stresses stay
in. And they are sufficient to affect intergranular stress corrosion. (You
could say that the intergranular corrosion "stress-relieves" the part-- in a
very NON-helpful way!)
Interestingly, some experts prefer well-aged parts-- weren't we talking
about the Pettys' preference for old engine blocks? (These are alloys that
are apparently much more immune to IGSC.) This preference is also related to
the residual stresses left in from manufacture of the block: as a block
heats-up under heavy use, it will distort-- due to many factors. The
distortion is bad, because it makes it more difficult to machine the (cold,
unstressed) block so that the dimensions will be correct under operating
conditions. Many experts believe that one of the worst factors in this
distortion is the way the residual stresses in the block react to heating.
Therefore, they prefer blocks with the least amount of residual stress.
It has been proven that-- even at ambient temperature (in a junkyard, for
example) manufacturing process residual stresses reduce on their own--
without significant heat, or other external energy input. But they do it
very gradually. Without help, it takes MANY years for residual stresses to
reduce significantly.
(Personally, I don't understand why folks like the Pettys don't just get a
new "raw" block-- before machining-- and then carefully stress relieve it in
a furnace. THEN machine it.)
Hope I haven bored the poo out of everybody!
Russ, #1226B
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