Magnetic particle testing (MT):
Magnetic particle testing is used to test materials that are susceptible to magnetism. This method is capable of detecting defects that are open on the surface as well as located just below the surface. In this method, the test object must first be magnetized using a permanent magnet or an electromagnet, either through direct current or through a coil surrounding the test object. The magnetic field induced into the test object consists of magnetic field lines. Where there is a defect that will disturb the lines of force, some of these lines must go out and back into the object. These entry and exit points form opposite magnetic poles known as leakage magnetic fields. Magnetic powder is sprinkled or sprayed on the surface of the test object, these magnetic poles or areas with magnetic leakage will attract the magnetic powder to form a visible indicator similar to the size and shape of the defect. Figure 4.3 illustrates the basic principles of this method. Depending on the specific application, there are different magnetization techniques.
These techniques are divided into the following two categories:
- Direct Current Magnetization Techniques: This technique is performed by passing an electric current directly through the test specimen, which creates a magnetic field around the specimen and this field is used to detect show defects. This technique is depicted in Figure 4.4 (a,b&c).
- Indirect magnetization techniques: in these techniques a magnetic flux is generated in the test object using a permanent magnet, an electromagnet, or by passing a current through a coil or a conductor. These techniques are depicted in Figure 4.4 (d,g).
The advantages of magnetic particle testing method:
(1) Open defects on the surface as well as defects located near the surface of the test object can be detected.
(2) Can be used without scraping off thin protective coatings on the surface of the test object.
(3) There is no strict requirement for surface cleaning before inspection.
(4) Fast execution.
(5) For high sensitivity.
(6) Less processing so the possibility of error by the operator is low.
Some limitations of the magnetic particle testing method:
(1) Not suitable for non-magnetic materials.
(2) Sensitive only to defects with an angle between 45 0 and 90 0 with respect to the direction of the magnetic field lines.
(3) The equipment used in this method is more expensive.
Video clip for reference: https://www.youtube.com/watch?v=zuH8IazmRzw