magnetic

pole, magnetic: see magnetic pole.

flux, magnetic, in physics, term used to describe the total amount of magnetic field in a given region. The term flux was chosen because the power of a magnet seems to "flow" out of the magnet at one pole and return at the other pole in a circulating pattern, as suggested by the patterns formed by iron filings sprinkled on a paper placed over a magnet or a conductor carrying an electric current. These patterns are called lines of induction. Although there is no actual physical flow, the lines of induction suggest the correct mathematical description of magnetism in terms of a field of force. The lines of induction originate on the north pole of the magnet and end on the south pole; their direction at any point is the direction of the magnetic field, and their density (the number of lines passing through a unit area) gives the strength of the field. Near the poles where the lines converge, the field and the force it produces are large; away from the poles where the lines diverge, the field and force are progressively weaker.

permeability, magnetic: see magnetism; flux, magnetic.

Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field. Nuclei that have at least one unpaired proton or neutron act like tiny magnets. When a strong magnetic field acts on such nuclei, it sets them into precession. When the natural frequency of the precessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed by the nuclei at a frequency called the resonant frequency. NMR is used to study the molecular structure of various solids and liquids. Magnetic resonance imaging, or MRI, is a version of NMR used in medicine to view soft tissues of the human body in a hazard-free, noninvasive way.

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Absorption or emission of electromagnetic radiation by electrons or atomic nuclei in response to certain magnetic fields. The principles of magnetic resonance are used to study the atomic and nuclear properties of matter; two common laboratory techniques are nuclear magnetic resonance and electron spin resonance. In medicine, magnetic resonance imaging is used to produce images of human tissue.

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Relative increase or decrease in the magnetic field inside a material compared with the magnetic field in which the material is located. In empty space the magnetic permeability is 1, because there is no matter to modify the field. Materials may be classified by the value of their magnetic permeability. Diamagnetic materials (see diamagnetism) have constant relative permeabilities of slightly less than 1. Paramagnetic materials (see paramagnetism) have constant relative permeabilities of slightly more than 1. The relative permeability of ferromagnetic materials (see ferromagnetism) increases as the magnetizing field increases, reaches a maximum, and then decreases. Pure iron and some alloys have relative permeabilities of 100,000 or more.

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Attraction or repulsion that arises between electrically charged particles that are in motion. While only electric forces exist among stationary electric charges, both electric and magnetic forces exist among moving electric charges. The magnetic force between two moving charges is the force exerted on one charge by a magnetic field created by the other. This force is zero if the second charge is traveling in the direction of the magnetic field due to the first and is greatest if it travels at right angles to the magnetic field. Magnetic force is responsible for the action of electric motors and the attraction between magnets and iron.

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Region around a magnet, electric current, or changing electric field in which magnetic forces are observable. The field around a permanent magnet or wire carrying a steady direct current is stationary, while that around an alternating current or changing direct current is continuously changing. Magnetic fields are commonly represented by continuous lines of force, or magnetic flux, that emerge from north-seeking magnetic poles and enter south-seeking poles. The density of the lines indicates the magnitude of the field, the lines being crowded together where the magnetic field is strong. The SI unit for magnetic flux is the weber.

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Tiny magnet with subatomic dimensions, equivalent to the flow of electric charge around a loop. Examples include electrons circulating around atomic nuclei, rotating atomic nuclei, and single subatomic particles with spin. On a large scale, these effects may add together, as in iron atoms, to make magnetic compass needles and bar magnets, which are macroscopic magnetic dipoles. The strength of a magnetic dipole, its magnetic moment, is a measure of its ability to turn itself into alignment with a given external magnetic field. When free to rotate, dipoles align themselves so that their moments point predominantly in the direction of the magnetic field. The SI unit for dipole moment is the ampere-square metre.

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Magnetic particle inspection processes are non-destructive methods for the detection of defects in ferrous materials. They make use of an externally applied magnetic field or half-wave DC (rectified AC) current through the material, and the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material.

The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields.

The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles.

This inspection can be applied to raw material in a steel mill (billets or slabs), in the early stages of manufacturing (forgings, castings), or most commonly to machined parts before they are put into service. It is also very commonly used for inspecting structural parts (e.g., landing gear) that have been in-service for some time to find fatigue cracks.

Usually tested pieces needs to be demagnetizated by a degaussing tool before use. Parts are demagnetized by applying AC current through the part which scrambles the magnetic domains causing it to demagnetize

It is a quite economic non-destructive test because it is easy to do and much faster than ultrasonic testing and radiographic testing.

Common test methods used include BS6072:1981 (1986 - AMD 4843) - Magnetic Flow - Colour Contrast Method, and ASTM E709.

There are two different ways of magnetizing a part Longitudinal and Circular magnetization. Longitudinal Magnetization passes current through a coil and the magnetic flux lines go through the part. Circular magnetization passes current through the part and establishes a magnetic field around the part. The two different methods are used because cracks can only be seen 45 to 90 degrees to the magnetic flux lines.