Magnetic Particle Inspection Gets Revitalized

February 23, 2006
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A direct contact method of magnetizing ring-shaped parts may be used to locate circumferential defects. Source: Magnaflux

The difficulty with processing ring-shaped parts for aerospace applications is a risk of deformation, or burning or arcing of the part when clamped between contact plates. Correct part processing requires the part to be clamped twice, thus doubling the risk. The part must be turned 90 degrees between the two shots in order to inspect the area that comes in contact with the clamps because an indication may not develop in this area.

For the MPI process, current machinery can be designed as stand alone (integrated power supply), or as a slave unit (separate power source needed, such as a wet bench). Source: Magnaflux
Ring-shaped parts, saucers and some gears are ideal for an induced current fixture. Most people do not use induced current fixtures because they have never been exposed to them or do not understand how they work. The technology, as applied to the magnetic particle inspection (MPI) process, has been around since the 1940s. Current MPI machinery can be designed as stand-alone units with an integrated power supply or as a slave unit with a separate power source needed, such as a wet bench. Induced current fixtures can be used in wet or dry applications.

The theory of operation for an induced current fixture includes several different rationalizations. The induced current fixture can find indications traditionally found with a direct current shot. Processing a round part with direct current requires a minimum of two orientations and can subject sensitive parts to pressures and heat that many designers try to avoid. The advantage of the induced current fixture is a noncontact method, and if running bearing races, or fine gears, contact surface can be a significant issue. Also, the full part can be processed in one step, saving time.

So what makes an induced current fixture different from a normal coil? The answer is simple: the laminated core. Laminated, soft iron works best as a core material.

The core makes the system more efficient. First, consider traditional formulas for the calculation of current through the coil. There are several variations to the formula depending on factors such as fill and types of current, but it is important to consider the current calculation formulas.

The induced current fixture is different from a normal coil because it has a laminated core. Source: Magnaflux

The bearing race

The bearing race requires a lot of current because of the poor length-to-diameter (l/d) ratio; this is true of all the shapes mentioned earlier. One trick to demagnetize parts with poor l/d ratios is to stack them, thus changing the l/d ratio. Adding the iron core does the same thing. Adding extra iron to the magnetic field changes the formula because the l/d ratio changes. The iron core needs to be included in the calculation. By calculating the mass distribution of the system-core and bearing race-a different l/d ratio is derived and lowers the current requirements for a coil shot. A simple method to do this is:
  • Calculate the l/d ratio for the core.
  • Calculate the l/d ratio for the ring.
  • Weigh each part and add the weights together.
  • Figure a percentage of weight for each part.
  • Use the weight percentage for each part and multiply it by the l/d ratio for the part.
  • Add the new l/d ratios together for a weighted average l/d ratio for the system.
  • Now apply traditional formulas using this new l/d ratio.

Another approach is to look at the ring as the inner winding of a transformer. The coil on most machines is five turns; the ring becomes a one-turn coil. Some manufacturer's unit coils are considered the primary winding, while the ring becomes the secondary winding.

The induced current fixture is different from a normal coil because it has a laminated core. Source: Magnaflux
Transformers come in many forms. Air core and iron core are two common types. The laminated soft-iron core in this case can be thought of as the iron core of the transformer. It acts as a magnetic bridge to make the field jump between windings more efficiently. In this case, the system is analogous to an air core transformer vs. an iron core transformer. The iron core is more efficient.

For example, if this system were 100% efficient, the current in the MPI unit coil is 1,000 amps, 5 turns; the current in a one-turn bearing race is 5,000 amps. The system is far from 100% efficient and this loss in efficiency translates to lower amperage in the ring. A typical power transformer is 80% efficient, the induced current fixture is closer to 25% efficient and a coil alone would be close to 5% efficient. Based on this, it is easy to see how much the iron core adds to the system. After the current is induced into the ring, it will magnetize and react just like a direct current shot.

These two explanations are simplified for ease of understanding. Operators should understand that the induced current fixture is only one of many tools developed through the years to make the MPI process easier and more repeatable. The induced current fixture can be used in AC or DC circuits.

When used in DC circuits, it is best and most efficient when coupled with three-phase full wave DC. It also should be noted that the quick break is essential for the induced current fixture to work correctly when used with three-phase full-wave DC. AC should be used with an induced current fixture, only when the part under test is a soft iron.

The MPL yoke

Another version of the induced current fixture is known as a yoke. The MPI yoke incorporates the same techniques and principles as the induced current fixture. Care should be taken when comparing yokes to induced current fixtures because placement of the part under test is different for the two techniques. As in all cases, the direction and strength of the magnetic field is key to obtaining reliable, repeatable results.

Manufacturers offer many configurations of induced current fixtures. Standard versions can be placed on top of wet horizontal units. Standard slave units have quick break built into the electronics and self-powered units come equipped with the same functions as standard wet benches. Core sizes can range from 0.25 inch diameter to more than 12 inches.

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Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
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