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tdtyl@hubcap.clemson.edu (theodore d taylor) writes:

>Static fatigue refers to crack growth at a stress intensity level below
>critical.  There is not enough stress intensity for the crack to
>propagate on it's own; instead it is 'assisted' by some chemical agent
>such as moisture.  The moisture diffuses to the crack tip and reduces
>the fracture surface energy required for the crack to propagate (the
>crack tip region is under considerable stress and thus in a more
>chemically reactive state).  The change in crack length is related to

In metals we would call this stress corrosion cracking, and in polymers
environmental stress cracking.  I guess it's just a matter of differing
terminology  --  it's a pity we can't be consistant about it, though.
Anyway, I conclude from what you say that, in ceramics, "fatigue" can be
used to denote any form of sub-critical (i.e. below KIc) crack growth,
and you differentiate by adding terms cyclic, static etc.

>the stress intensity factor by:
>	(dc/dt)=A*K^n = A*(stress*geometry*c^.5)^n
>where:
>c = crack length
>t = time
>A = experimentally determined constant
>This is not the same as creep and does not necessarily occur at elevated
>temperatures.  It does not necessarily involve the motion of
>dislocations (glass undergoes static fatigue).

Yes, there are a number of possible mechanisms of SCC in metals, too,
and not all involve dislocation motion.

>	Please note that clical stress application is not part of the
>above description.  Also note that when the crack length becomes large
>enough, catastrophic failure will occur.  An examples of 'static fatigue'

Of course.

>is the "exploding windows" in an automobile.

>			T. D. Taylor
>			Department of Ceramic Engineering
>			Clemson University
>			Clemson, SC USA

Thanks for clarifying this for me.

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