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Rod Rigging - Hype?
It's not necessary for the material to be ferrous to do magnetic testing.
All it has to be is conductive. You can induce currents into the metal by means of an electric field (and yes, it can have circulation ie it is magnetic). The discontinuities in the induced current (due to cracks) will cause a net increase in the responsive magnetic field (even though it is not magnetic material - think - copper wire produces magnetic fields but it is non ferrous) or changes in the E field on the surface of the metal. There are films responsive to E fields, these can be used on the metal surface or you can use small electric field probes. You can even use the old standby of iron filings if you know what to look for. I prefer RCS measurements to look for cracks. Amen! wrote in message ps.com... The cracks shown in that link are exactly like the ones I found in my 20 yr old rigging. The cracks in mine were very difficult to find until I used fine sandpaper. They were all on the lower swaged end. Most of the cracks were under small brown stains that were the result of the stainless in the crack becoming non-stainless and rusting. They required a 10X magnifier to find. At work, I ground the fittings away to see how deep they went and they went through the entire fitting. I even used dye penetrant with no luck. I replaced all the rigging and lifelines last year. NOW, although thefittings and wire and rod are stainless, stainless can be slightly ferromagnetic and can be tested via flux leakage methods (I used to do magnetic NDT research). Furthermore, the area around a fatigue crack is less stainless and more ferromagnetic than the bulk material. This may allow testing via mag particle inspection (so-called magnafluxing). It is entirely that nobody has done this because they simply assumed that teh stainless material wouldnt allow this to work. Unfortunately, I threw the old rigging away so I cannot try this. |
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