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Engine Building Fundamentals: Crack Testing

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Crack Testing

02.52

00:00 - There's a huge amount of stress applied to the internal components of an engine and particularly for those components that are being retained or reused during a build.
00:10 It's always nice to have some piece of mind that they're in good condition and safe to use.
00:16 Particularly with highly stressed components, such as crankshafts, it's not uncommon for stress cracks to start forming in the fillet radius of the journals.
00:26 These will start out invisible to the eye, but if the crankshaft is reused when it's cracked, it's quite possible for it to fail in use, resulting in expensive and catastrophic engine failure.
00:40 Crack testing is an effective way of testing components to ensure they are crack free and in a condition where they're safe to fit to your engine.
00:50 The two common processes used for crack testing include dye penetrant testing and magnetic particle testing.
00:58 While it's possible to purchase dye penetrant test kits to perform your own crack testing at home, it's much more common to have an engine machinist perform this work for you.
01:10 Dye penetrant testing involves spraying the part to be tested with a special dye and allowing this to soak into the part before removing any excess.
01:21 The dye penetrant will soak into any cracks, or surface discontinuities, in the middle.
01:27 Once the dye has been allowed sufficient time to soak, the part is sprayed with a developer that withdraws any remaining dye left behind.
01:36 Depending on the type of dye penetrant used, the part may be inspected by eye, or may require inspection under a blacklight where the cracks will become obvious.
01:47 Magnetic particle testing, on the other hand, requires the part to be tested to first be magnetised and this limits the testing to ferrous materials such as crankshafts, con rods and perhaps cylinder blocks.
02:02 Once a magnetic field is established in the part, magnetic particles are then applied.
02:08 The principle behind this testing is that, when there is a crack in the surface of the part being tested, this will set up north and south poles at the edge of the discontinuity and this attracts the magnetic particles.
02:22 When viewed under a blacklight, cracks are visible by bright lines where the magnetic particles are concentrated.
02:29 It's important that the magnetic field is removed from the part after magnetic testing is complete, as we don't want engine components to be magnetised while in use.