Is that plate broken?

[Glen_Deligdisch@sil.org]

     Dear Ken and List,
     
     I have been following this thread with some interest as some of the 
     methods for checking this plate for cracks are ones that I use in the 
     aviation industry.
     
     I use Magnaflux to check aircraft parts (engine, transmission, landing 
     gear, etc) for defects.  As I listened to the suggestion to Magnaflux 
     a 6' grand plate I had to sit down and think, "with the equipment we 
     have, how would I do that?"  
     
     Let me comment on some of the problems and then make a few alternate 
     suggestions.
     
     The limitation is the size of the magnetizing coil (24 inches) or the 
     distance between the pressure heads (42 inches) on our machine.  We 
     magnetize parts by two different methods and a plate of this size is 
     too big.  I'm sure there are Magnaflux machines that are bigger and 
     could handle it.  I'm sure you could arrange an appointment with one 
     but you might stagger at the cost.
     
     There are two other problems with this method:
     
     1.  We require all parts to be bare metal, the plate would have to be 
     stripped.  I suppose you could Magnaflux it with the paint on, but 
     then I would not guarantee the results.
     
     2.  I don't know how I would demagnetize the plate.  To demagnetize 
     the plate properly it would have to fit through the coil.  There are 
     methods to do it after all, the US Navy demagnetizes a whole ship 
     after construction before commissioning.
     
     I don't think Magnaflux should be the first line method for checking 
     the plate.  Let me suggest 3 other non destructive crack detection 
     methods that we also use in our shop:
     
     1.  Visual inspection with a 10X magnifying glass and bright light 
     (ie. a Maglite which can focus the light beam).  This is always our 
     first line of attack.  Focus on obvious stress points, weak spots, 
     where strength changes because the cast thickness changes or heads off 
     in a new direction.  God has made the human eye incredible just like 
     our ears.  We can detect a gaps as small as 1 millionth of an inch if 
     under the right conditions!  A lot of cracks are not going to escape a 
     good visual inspection.
     
     Let me make an aside here.  We place hard paints on aircraft parts for 
     two reasons; for protection and to indicate cracks.  Seldom do we find 
     a crack in the metal part that did not first indicate it's presence in 
     the paint.  From what I have been reading on the list about what we 
     paint our plates with, a crack should be observable with the above 
     method by looking for it in the paint.  CHEAPEST
     
     2.  Dye Penetrant inspection.  This method uses a dye that gets 
     absorbed into the unseeable crack and then indicates when the 
     developer is applied.  The plate is cleaned thoroughly, the dye is 
     applied, the dye is cleaned off again (but the dye in the crack 
     remains) and then the plate is developed.  The developer is a dry 
     powdery substance that is sprayed on the plate and now sucks the 
     bright red dye into it's white powder.  SMALL EXPENSE
     
     3.  Zyglo Penetrant inspection.  This method is a little bit more 
     elaborate as it requires the penetrant and a black light (ultraviolet) 
     for detection.  It uses at florescent chemical instead of dye and is 
     basically the same method as number 2 above.  KIND OF EXPENSIVE
     
     We use these 3 methods to guarantee aircraft parts as being free of 
     defect upon which people are placing their lives every day.  I know 
     they work.
     
     A word of caution:  If you ever find what you think is a crack with 
     the above methods, don't stop there.  Crack indications show up for a 
     host of reasons but not all are cracks.  If you find a crack, remove 
     the plate paint by desolving the paint.  Don't sand it as you can pack 
     paint into a crack and cause it to stop indicating.  Removing the 
     paint with strippers, MEK (Methyl Ethyl Ketone) or other paint 
     desolving product would be the way to go.
     
     Sorry this is so long.  I hope it is helpful to some.
     
     Glen
     
     ---------------------------------------------------------------
     
     Glen and Ruth Deligdisch
     P.O. Box 248
     Waxhaw, NC  28173
     
     Tel:  (704) 843-9089
     
     E-mail:  Glen_Deligdisch@SIL.ORG