When any material is magnetized it has a magnetic field that will attract certain metals and other magnetic fields. It is possible to reveal discontinuities by using a medium having magnetic attraction.
The magnetic particle media will be built up on the discontinuity section (which is creating new poles: North & South) in the magnetized part. A crack in the part will disrupt the flow of lines of force and create a FLUX LEAKAGE. LEAKAGE FIELD WILL ATTRACT MAGNETIC PARTICLES
Leakage field (flux leakage) are actually magnetic lines of force that leave the part and pass through air from one pole to the other of opposite polarity.
- They form closed loops between north and south poles.
- They do not cross one another.
- They seek paths of least magnetic resistance.
- Their density decreases with increasing distance from the poles.
- They are considered to have direction; by convention, from NORTH POLE TO SOUTH POLE EXTERNAL to the magnet, and from south to north internally
FLUX DENSITY/ INDUCTION: usually designated in “GAUSS” units and refers to the FLUX PER-UNIT AREA at right angles to the direction of the flux.
- PERMEABILITY : this refers to the ease with which a magnetic flux is established in the article being inspected. High permeability –easy to magnetize Low permeability–difficult to magnetize
- RELUCTANCE : this is the opposition of a magnetic material to the establishment of a magnetic flux. A material with high permeability will have a low reluctance.
- RESIDUAL MAGNETISM : this refers to the amount of magnetism retained after the magnetizing force is removed.
- RETENTIVITY : refers to the ability of the material to retain a certain amount of residual magnetism.
- COERCIVE FORCE : refers to the reverse magnetizing force necessary to remove the residual magnetism from the part.
TYPE OF MAGNETIZATION
1. CIRCULAR MAGNETIZATION
Circular magnetization will detect discontinuities that are between 45 and 90 degrees to the lines of force. A circular magnetic field is induced into a specimen by:
- DIRECT INDUCTION of a circular field into an article (using HEAD SHOT or PRODS)
- INDIRECT INDUCTION of a circular fields by placing a current carrying conductor into the specimen. This method is known as the CENTRAL CONDUCTOR technique.
In a nonmagnetic material, the lines of force will NOT stay in the material. For example, when a copper bar is used, the magnetic field is established AROUND the bar.
In a ferromagnetic material, the lines of force are established WITHIN the material. Iron is permeable and readily conducts the magnetic field.
2. LONGITUDINAL MAGNETIZATION A longitudinal magnetic field is induced into a specimen by:
- The YOKES, is essentially a temporary horseshoe magnet. It is made of soft, low retentivity iron, which is magnetized by a small coil wound around its horizontal bar
- COIL (SOLENOID), When the length of the specimen is several times its diameter or cross section, the specimen can be successfully magnetized by placing it lengthwise in the field of the coil. Field strength developed within a coil depends on 3 factors: The number of turns in the coil, the current flowing through the coil, the diameter of the coil.
- By CABLE. Sometimes test articles are too big to fit into the ordinary coil. When this happens, an insulated copper cable can be used to form a coil for longitudinal magnetization of the article. Here is an example. The effective distance of the longitudinal magnetic field created by the cable is the same as the effective distance of a stationary coil (6 to 9 inches on both sides of the cables).
CURRENT
Direct current (DC), alternating current (AC), half-wave rectified current (HWDC), and full-wave rectified current (FWDC) are used as magnetizing currents in magnetic particle testing. Only one type is required for a test.
Alternating Current (AC)
- can be readily converted to the low voltages used in magnetic particle inspection by the use of transformers.
- has little penetrating power and provides the best detection of surface discontinuities due to skin effect (NOT effective for subsurface).
- since AC is continuously reversing direction, the magnetic field has a tendency to agitate or make the iron particles more mobile. This causes the iron particles to be more responsive to the flux leakage current reversal.
HWDC is best suited for locating near-surface discontinuities.
HWDC means that the reverse polarity or negative portion of the sine curve is eliminate.
FWDC rectification inverts the negative current to positive current so that the number of positive pulses is doubled
The objective of magnetic particle testing is to insure product reliability by providing a means of:
- obtaining a visual image of an indication on the surface of a material.
- disclosing the nature of discontinuities without impairing the material.
- separating acceptable and unacceptable material in accordance with predetermined standards.
- magnetic flux: the concept that the magnetic field is flowing along the lines of force suggests that these lines are therefore “flux” lines, and they are called magnetic flux. The strength of the field is defined by the number of flux lines crossing a unit area taken at right angles to the direction of the lines.
- magnetic field : the volume within and surrounding either a magnetized part or a current-carrying conductor wherein a magnetic force is exerted.
- magnetic particle examination flaw indications: the accumulatio of ferromagnetic particles along the areas of flaws or discontinuities due to the distortion of the magnetic lines of force in those area
- magnetic pole: one of two or more areas of flux leakage on a part