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 > > >
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