CAPABILITIES OF MAGNETIC PARTICLE TESTING
Magnetic particle testing can reveal surface discontinuities, including those too small or too tight to be seen with the unaided eye. Magnetic particle indications form on an object’s surface in the area of a discontinuity and show the location and approximate size of the discontinuity. Magnetic particle tests can also reveal discontinuities that are slightly below the surface.
There are limits to this ability to locate subsurface discontinuities. These are determined by the discontinuity’s depth, size, type and shape; the strength of the applied field; and the type of current used.
In some cases, special techniques or equipment can improve the test’s ability to detect subsurface discontinuities. Magnetic particle testing cannot be used on nonmagnetic
materials, including glass, ceramics, plastics or such common metals as aluminum, magnesium, copper and austenitic stainless steel alloys. In addition, there are certain positional limitations: a magnetic field is directional, and for best results must be oriented perpendicular to the discontinuity. This generally requires two complete magnetizing operations to detect discontinuities parallel and perpendicular to the test object’s axis. Objects with large cross
sections require a very high current to generate a magnetic field adequate for magnetic particle tests. A final limitation is that a demagnetization procedure is usually required following the
magnetic particle process.
Personnel Qualifications
Personnel performing nondestructive testing should, at a minimum, be qualified in accordance with Recommended Practice No. SNT-TC-JA. Only personnel qualified for Level I nondestructive testing, and working under the direct supervision of a Level II or Level III nondestructive testing technician ( or personnel qualified to Level II or Level III), may perform nondestructive tests. The Level I, Level II or Level III magnetic particle testing technician should be qualified through specific and practical examinations based on the requirements of the applicable code. The technician should have successfully demonstrated the ability to accurately detect and evaluate discontinuities in accordance with the applicable code. Qualified personnel are required by many jurisdictions, codes, standards and specifications to be certified by an organized corporate, national or international agency.
This text provides an overview of the magnetic particle testing process and is introductory to the detailed treatments in subsequent sections. Topics covered here include the following.
1. The fundamental theory of magnetism, magnetic flux and types of magnetic fields.
2. Principles of electrically induced magnetism and magnetizing current.
3. Testing media, equipment and processes.
4. Process controls.
5. The basic principles and methods of demagnetization.
6. Reporting of magnetic particle tests.
Such data can be helpful to managers, supervisors and personnel outside of nondestructive testing who require general information on the magnetic particle testing process. It may also be helpful for introductory studies by individuals already using magnetic particle testing, or those preparing for advanced training in the technique.