🚀 go-pugleaf

5916rahmank@vms.csd.mu.edu (K. M. A. Rahman) writes:

>	I like to add one more question. I have seen that, time-dependent
>	degradation in materials is termed as 'aging'. Since degradation
>	lead to failure, is there any substantial difference between
>	these two terms? Or just a matter of terminology?

Aging (or ageing, depending on which sode of the Pacific you come from)
does not generally refer to _degradation_ of materials, but rather a
microstructural change occurring under the influence of thermal activation
and time.  A classic example is in precipitation hardening (age hardening)
alloy systems  --  second phase particles nucleate and grow over a period
of time, and faster at elevated temperatures.

Whether or not this is regarded as "degradation" depends on (a) what the
final microstructure is, and (b) what properties you _want_.
  |  Of course, there's no such thing as the "final" microstructure  --  it
  |  will continue to change  --  but if the aging is carried out at elevated
  |  temp, then cooling to room temp will typically slow the process down to
  |  negligable levels.
After quenching to retain a supersaturated solid solution, you deliberately
age these alloys to form a fine distribution of second phase particles,
in order to inhibit dislocation motion.  This is what you want  --  not
degradation.  But if, for example, the alloy ages at room temp (or in hot
sunny conditions or whatever), then the alloy will _over-age_, the particles
coarsen and the yield strngth will decrease.  Assuming you want maximal yield
strength, the material can be said to have been degraded.  Note, however, that
this does not lead _directly_ to failure;  failure only occurs after a high
stress, exceeding the (reduced) yield strength, is applied.

There are many such failure modes associated with material change.
But fatigue is not one of these  --  fatigue is a failure mode in itself,
and, as Philip Sargent (pms@eng.cam.ac.uk) pointed out a couple of weeks ago,
does _not_ involve any change in the "molecular structure" of metals, as was
thought in the '40s and perpetuated in novels like No Highway (Hammond Innes,
I think).

|  jeff@minmet.uq.oz.au	       |  Jeff Gates
|			       |  Dept Mining & Metallurgical	       _--_|\
|  Ph. direct: +61 7 365 3726  |		     Engineering      /      B
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