Is that plate broken?

[Jim Coleman, Sr.]

Hi Glen:

Congratulations on a very fine article on plate crack analyzing.

Jim Coleman, Sr.



On Sat, 27 Sep 1997 Glen_Deligdisch@sil.org wrote:

>      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
> 
> 
>