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Re: Crack testing

To: wise@vaxa.cis.uwosh.edu, triumphs@autox.team.net
Subject: Re: Crack testing
From: KVacek@aol.com
Date: Thu, 9 May 1996 15:58:42 -0400
In a message dated 96-05-09 12:09:37 EDT, you write:

>       What can anybody out there tell me about crack testing?  How does
>it work?  What does it cost?  Can an average Joe do it at home?

Well, there's Magnaflux, which is a method in which iron-containing particles
are induced (by an electromagnetic field) to line up along cracks.  Takes
some equipment, but it's a very reliable way to detect cracks in iron or
steel.  Won't work on non-ferritic materials.

Zyglo is a dye penetrant method used on aluminum, magnesium, etc.  It's
basically a thorough cleaning and then a shot of a penetrating material which
will wick into cracks.  The part is then cleaned and the material in the
cracks (which doesn't tend to come out of the cracks too readily)  is viewed
under black light, where it fluoresces and highlights the cracks.  Pretty
good if the parts are cleaned properly.

Closely allied to Zyglo is the home-style aerosol dye penetrant method.  You
clean the parts to the best of your ability, then spray on a primer, a
penetrant, and then a developer.  The penetrant in the cracks tends to bleed
a color after developing, which makes the crack visible.

There's also X-ray, etc., but these methods are generally too far out of the
$$ range to make them practical for our needs with car parts, and generally
overkill.

Any of these methods need careful attention to cleaning before inspection,
proper application of the detection method, and careful observation of the
part afterward.  For a critical part (crank, rods, head, hubs, etc.), IMHO it
pays to have a trusted shop do it.  It's pretty cheap, and if you know the
guy doing the work, you can be fairly sure the part is good.  Magnaflux can
harm parts, particularly thin stuff, if a poor technician does the job.

DIY penetrants are good for quickie checks on things which aren't critical,
or just as a final check on something you're pretty sure is OK anyway.  Also
something to consider if you're driving 150 miles away on Sunday afternoon to
buy a crank from someone, as a quick test before peeling off the cash.

Note that shot peening or sandblasting should NOT be done prior to crack
detection.  The process tends to microscopically peen over the edges of
cracks, and can make them harder to detect, or worse, maybe invisible.  Do
this later.

Hope this helps.
-Karl

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