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Looking For Leakers: Crack Detection Technology
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The direction of the magnetic field is also important. The lines of force must cross the crack at an angle to reveal its presence (45 degrees usually works best). If the magnetic field is parallel to a crack, there may not be enough distortion in the field to attract any particles. So if you don’t find a crack when holding the magnets or part one way, rotate or reposition the part or magnets 45 to 90 degrees if possible and repeat the test.
With nonfluorescent particles, you want plenty of light on the part to improve visibility. With fluorescent particles, you need to aim the black light so it illuminates the test area completely. Fluorescent particles are usually easier to see than the nonfluorescent ones. Either way, be careful to keep the light away from the magnets or magnetic coil because the powerful magnetic field may bend and break the filament inside the bulb.
Sometimes cracks can be obscured by casting lines or a rough surface finish on the component that’s being inspected. Parts should have no dirt, oil, grease or carbon on the surface. Parts can be chemically cleaned, spray washed or baked in an oven, but should not be shot blasted prior to inspection because blasting may peen shut small cracks that could reopen later.
Often, castings will show various surface flaws that are not really cracks but only minor surface scratches or imperfections. On these types of defects, you have to make a judgment call as to whether or not what you’re looking at is actually a crack and if it is, if it is worth worrying about or not. A small hairline crack in a noncritical area of a cylinder head may not be a problem, but a hairline crack in a crankshaft journal or between a spark plug hole and valve seat in a combustion chamber could mean trouble.
Parts should be checked for cracks after they have been cleaned and before they are machined or used for assembly. A second check for cracks should also be made after certain kinds of reconditioning operations have been performed to make sure any original cracks have been eliminated and/or that no new cracks have appeared. This might include rechecking a cast iron head after pinning or welding an existing crack or installing a machined-in valve seat, checking a block after installing a cylinder sleeve, checking a crankshaft after it has been straightened, etc. It’s just added insurance to make sure there are no cracks that might cause problems down the road.
Once parts have been magnetized, they need to be demagnetized after testing. This can be done with bench equipment by passing the part back through the ring a second time. Other times, the field will weaken and dissipate after a little while. Even so, parts should be checked with a "field indicator" to make sure there is no residual magnetism. The last thing you want is a magnetized crankshaft that attracts wear particles to its journal surfaces! One "trick" machinists sometimes use to demagnetize a crankshaft is to "ring" the crank by tapping on the journals.
Another method for finding surface cracks and flaws is to use a penetrating dye. Though used mostly on aluminum parts, this technique also works equally well on cast iron, steel, composite materials and even plastic.
The theory behind this technique is that a very light oil will wick into a crack. It’s the same idea as using penetrating oil to loosen a fastener except that the oil contains a dye. If the oil finds its way into a crack, the dye should then make the crack visible. Some penetrating dyes use fluorescent dyes and a black light to make the cracks stand out while others use a chemical developer to make the dye more visible.
If you’re using a UV light and fluorescent dye, a shroud that blocks ambient light will make it easier to see the cracks. Cracks will glow green under the black light. With ordinary dyes, no special light is needed. Cracks usually stand out as a stark red line against the bright aluminum metal.
Multi-stage penetrating dyes typically use a three-step process to highlight cracks. The surface is first cleaned with a spray-on or wipe-on chemical cleaner. The part is allowed to dry, then the penetrant that contains the dye is sprayed or wiped on and allowed to stand for several minutes. The excess penetrant is then wiped or washed off, leaving behind any penetrant that has found its way into a crack. A developer is then applied to the surface, which lifts and separates the dye from the fissure to reveal and highlight the crack.
Penetrating dyes are an excellent tool for finding surface defects in aluminum cylinder heads and blocks, aluminum connecting rods and other aluminum castings such as intake manifolds and timing covers. It will even work on plastic intake manifolds. Cracks that are hidden in intake or exhaust ports or deeply recessed areas may be difficult to see, so for these kind of situations using a fluorescent dye with a small UV penlight light to illuminate the powder will make the job easier.
Like magnetic particle inspection, penetrating dyes only reveal surface cracks. Neither technique will reveal cracks that are below the surface or that are hidden inside a casting. Also, neither of these two techniques can tell you if a crack extends all the way through a casting to the water jacket inside, or if a crack will leak or won’t leak when the cooling system is under pressure. Only pressure testing can do that. Pressure testing is often used in conjunction with these other methods of crack detection to check the integrity of the cooling jackets in the cylinder head and block, and to see if visible cracks are really leakers or not.
A cylinder head or block is pressure tested by first sealing all the water outlets with plugs or cover plates. The casting is then pressurized with air to simulate water pressure inside the engine. The casting may then be submerged in a water tank or sprayed with soapy water to reveal any air leaks. If there are no bubbles, the part is assumed to hold pressure. If there are bubbles, you follow the bubbles to find the leak.
The main advantage of this technique is that it can find leaks other techniques can’t (such as porosity leaks in aluminum castings) and it can verify the integrity of crack repairs that have been made in heads and blocks to make sure they hold water.
A typical wet (submersible) pressure testing system starts at about $7,000, while the dry systems will cost about $5,500. According to one equipment supplier which makes both types of systems, 99 out of 100 buyers go for the wet system because it’s easier and faster to find the leaks.
With pressure testing, it takes some time to seal up a head or block. You must have the proper equipment to plug or block off all the water outlets in the casting. If you’re only working on certain engines, a pressure tester with dedicated fixturing or plates may be all you need. But if you’re working on a wide range of engines, you’ll need a setup with universal fixturing or a wider assortment of plates and plugs.
One question many engine builders have with regard to pressure testing is how much air pressure should they use? Most experts say 30 to 40 psi is all that’s needed for an accurate test. If a casting is going to leak at all, it’s going to leak at 30 to 40 psi just as it would at a higher pressure. Using excessive air pressure is dangerous because it increases the risk of blowing out an expansion plug. Besides, most cooling systems never see the high side of 15 to 18 psi anyway.
Those who use submersible tanks sometimes have problems keeping the tank water clean after repeated use. Oil and scum from dirty parts can accumulate in the water, and algae may find a hospitable environment in the tank. Using a biocide can kill off the green stuff, and a filtration system can keep the water clean.
Vacuum testing is the same basic idea as pressure testing, except in reverse. Instead of using air pressure to test the cooling jackets for leaks, vacuum is pulled on a cylinder head or block after the water outlets have been plugged. If the casting holds vacuum, there are no leaks. But if it doesn’t, you’ve found a leaker.
Unfortunately, this technique does not use water or dye to pinpoint the leak so you still have to use one of the other techniques to find the leak. It’s mostly a quick check for verifying the integrity of a casting.
This is a technique that is more commonly used in industrial and aviation applications, but it can also be used to find internal flaws in castings and other parts. The technology uses sound waves to find cracks. A transponder generates an acoustic signal (up to 25 MHz) that passes into and through the part. Cracks or flaws will reflect some of the sound waves back to the detector, which allows the information to be displayed on the tester.
The advantages of this method of crack detection is that it can find hidden flaws that the other commonly used techniques can’t. As we said earlier, magnetic particle inspection and penetrating dye can only reveal surface defects, and pressure testing and vacuum testing can only reveal cracks and porosity leaks in cooling jackets. A crankshaft with an internal defect could easily pass a Magnaflux test, yet fail on a race track when the stresses of racing expanded the crack and caused the crank to snap, for example.
The best applications for ultrasonic testing include heavy castings, large shafts and expensive parts that may be used for racing or extreme-duty service. Ultrasonics can also be used to check the integrity of welds and welded castings.
Hand-held ultrasonic testers are available for $6,000 to $7,000, and some are available with special pen-like probes for finding cracks in hard-to-reach places.
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