MAGNETIC TESTING

Magnetic Particle Testing

Magnetic particle testing is one of the most widely utilized NDT methods since it is fast and relatively easy to apply and part surface preparation is not as critical as it is for some other methods. This mithod uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components.
 The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material (a materials that can be magnetized) such as iron, nickel, cobalt, or some of their alloys

The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection such as structural steel, automotive, petrochemical, power generation, and aerospace industries
. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines
Basic Principles
In theory, magnetic particle testing has a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. For the case of a bar magnet, the magnetic field is in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a “pole” (magnetic lines of force exit the magnet from north pole and enter from the south pole)

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. 
The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field. 

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.


The first step in a magnetic particle testing is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect

Advantages and Disadvantages
The primary advantages and disadvantages when compared to other NDT methods are:

Advantages

 High sensitivity (small discontinuities can be detected).

 Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

 Minimal surface preparation (no need for paint removal)

 Portable (materials are available in aerosol spray cans)


 Low cost (materials and associated equipment are relatively inexpensive)

Disadvantages
 Only surface and near surface defects can be detected.

 Only applicable to ferromagnetic materials.

 Relatively small area can be inspected at a time.

 Only materials with a relatively nonporous surface can be inspected.


 The inspector must have direct access to the surface being inspected.

Magnetism

The concept of magnetism centers around the magnetic field and what is known as a dipole. The term "magnetic field" simply describes a volume of space where there is a change in energy within that volume. 

The location where a magnetic field exits or enters a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in pairs, hence the name dipole.

 Therefore, a dipole is an object that has a magnetic pole on one end and a second, equal but opposite, magnetic pole on the other.
 A bar magnet is a dipole with a north pole at one end and south pole at the other.

The source of magnetism lies in the basic building block of all matter, the atom. Atoms are composed of protons, neutrons and electrons.

 The protons and neutrons are located in the atom's nucleus and the electrons are in constant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through space.

A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.

When an electric current flows through a conductor, the movement of electrons through the conductor causes a magnetic field to form around the conductor. 

The magnetic field can be detected using a compass. Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field; however, materials do not react the same way to the magnetic field.

Reaction of Materials to Magnetic Field
When a material is placed within a magnetic field, the magnetic forces of the material's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an external magnetic field

. The magnetic moments associated with atoms have three origins: the electron motion, the change in motion caused by an external magnetic field, and the spin of the electrons.


In most atoms, electrons occur in pairs where these pairs spin in opposite directions. The opposite spin directions of electron pairs cause their magnetic fields to cancel each other. Therefore, no net magnetic field exists. Alternately, materials with some unpaired

electrons will have a net magnetic field and will react more to an external field.

According to their interaction with a magnetic field, materials can be classified as:

Diamagnetic materials which have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. In diamagnetic materials all the electrons are paired so there is no permanent net magnetic moment per atom. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic materials which have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic have some unpaired electrons. Examples of paramagnetic materials include magnesium, molybdenum, and lithium.


Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. 

They get their strong magnetic properties due to the presence of magnetic domains.

 In these domains, large numbers of atom's moments are aligned parallel so that the magnetic force within the domain is strong (this happens during the solidification of the material where the atom moments are aligned within each crystal ”i.e., grain” causing a strong magnetic force in one direction)

When a ferromagnetic material is in the unmagnetized state, the domains are nearly randomly organized (since the crystals are in arbitrary directions) and the net magnetic field for the part as a whole is zero.

 When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Components made of these materials are commonly inspected using the magnetic particle method.

Magnetic Field Characteristics

Magnetic Field In and Around a Bar Magnet

The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet (the north and south poles).

Magnetic Fields in and around Horseshoe and Ring Magnets


Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles


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