How about sending an acoustic pulse along the rod rigging the same as a time
domain reflectometer works. The pulse speed is a property of material and
tension and the reflectance a function of acoustic impedance. Cracks would
raise the impedance. The cracks could be located along the length of the rod
by simply applying the pulse signal at one point.

http://www.tscm.com/riprcop.html




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Bob:

Yes, that is eddy current testing. I have not yet considered it. I'd
prefer to use mag particles (iron filings) as this does not require
runing a probe along all possible surfaces.