Magnetic Particle Testing Basic Knowledge

Magnetic particle testing is used for the testing of materials which can be easily magnetized. This method is capable of detecting open to surface and just below the surface flaws. In this method the test specimen is first magnetized either by using a permanent or an electromagnet or by passing electric current through or around the specimen. The magnetic field thus introduced into the specimen is composed of magnetic lines of force. Whenever there is a flaw which interrupts the flow of magnetic lines of force, some of these lines must exit and reenter the specimen. These points of exit and re-entry form opposite magnetic poles. Whenever minute magnetic particles are sprinkled onto the surface of such a specimen, these particles are attracted by these magnetic poles to create a visual indication approximating the size and shape of the flaw.

Depending on the application, there are different magnetization techniques used in magnetic particle testing. These techniques can be grouped into the following two categories:

(a) Direct current techniques: These are the techniques in which the current flows through the test specimen and the magnetic field produced by this flow of current is used for the detection of defects. These techniques are shown in Figure 1.4 (a, b & c).

(b) Magnetic flux flow techniques: In these techniques magnetic flux is induced into the specimen either by the use of a permanent magnet or by flowing current through a coil or a conductor. These techniques are shown in Figure 1.4 (d–g).

Advantages of magnetic particle testing include the following:

(1) It does not need very stringent pre-cleaning operation.

(2) Best method for the detection of fine, shallow surface cracks in ferromagnetic material.

(3) Fast and relatively simple NDT method.

(4) Generally inexpensive.

(5) Will work through thin coating.

(6) Few limitations regarding the size/shape of test specimens.

(7) Highly portable NDT method.

(8) It is quicker.

Some of the limitations of magnetic particle testing include the following:

(1) Material must be ferromagnetic.

(2) Orientation and strength of magnetic field is critical.

(3) Detects surface and near-to-surface discontinuities only.

(4) Large currents sometimes required.

(5) “Burning” of test parts a possibility.

(6) Parts must often be demagnetized, which may be difficult.