Magnetic Particle Testing

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Magnetic Particle Testing

Magnetic particle testing is one of the most widely utilized NDT

Magnetic particle testing is one of the most widely utilized NDT methodsmethods since it is fast and relatively easy to

since it is fast and relatively easy to apply and part surface preparation isapply and part surface preparation is not as critical as it is

not as critical as it is for some other methods. This mithod uses for some other methods. This mithod uses magneticmagnetic fields and small magnetic particles (

fields and small magnetic particles (i.e.iron filingsi.e.iron filings) to detect flaws in) to detect flaws in components. The only requirement from an inspectability standpoint is components. The only requirement from an inspectability standpoint is that the component being inspected must be made

that the component being inspected must be made of a ferromagneticof a ferromagnetic material (

material (a materials that can be magnetized a materials that can be magnetized ) such as iron, nickel, cobalt,) such as iron, nickel, cobalt, or some of their alloys.

or some of their alloys. The method is used to

The method is used to inspect a variety of product forms includinginspect a variety of product forms including castings, forgings, and weldments. Many different industries

castings, forgings, and weldments. Many different industries useuse magnetic particle inspection such as

magnetic particle inspection such as structural steel, automotive,structural steel, automotive,

petrochemical, power generation, and aerospace industries. Underwater petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines. 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 In theory, magnetic particle testing has a relatively simple concept. It can be

considered as a combination of two nondestructive testing methods: magnetic flux considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. For the case of a

leakage testing and visual testing. For the case of a barbar magnet, the magnetic field is in and

magnet, the magnetic field is in and around the magnet.around the magnet. Any place that a magnetic line of

Any place that a magnetic line of force exits or enters theforce exits or enters the magnet is called a

magnet is called a ““polepole” (” (magnetic lines of force exit themagnetic lines of force exit the magnet from north pole and enter from the south pole magnet from north pole and enter from the south pole).). When a bar magnet is

When a bar magnet is broken in the center of broken in the center of its length, two complete bar magnetsits length, two complete bar magnets with magnetic poles on each end of

with magnetic poles on each end of each piece will result. If each piece will result. If the magnet is just crackedthe magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the 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

crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters

magnetic field spreads out when it encounters thethe small air gap created by the

small air gap created by the crack because the aircrack because the air cannot support as much magnetic field per

cannot support as much magnetic field per unitunit volume as the magnet can. When the

volume as the magnet can. When the field spreadsfield spreads out, it appears to leak out of

out, it appears to leak out of the material and, thusthe material and, thus is called a flux leakage field.

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If iron particles are sprinkled on

If iron particles are sprinkled on a cracked magnet, the particles will a cracked magnet, the particles will be attracted tobe attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the 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

edges of the crack. This cluster of particles is much easier to see than the cluster of particles is much easier to see than the actual crackactual crack and this is the basis

and this is the basis for magnetic particle inspection.for magnetic particle inspection. The first step in a

The first step in a magnetic particle testing is to magnetize the component that is to magnetic particle testing is to magnetize the component that is to bebe inspected. If any defects on or

inspected. If any defects on or near thenear the surface are present, the defects will surface are present, the defects will create a leakage field. After the create a leakage field. After the

component has been magnetized, iron component has been magnetized, iron particles, either in a dry or wet

particles, either in a dry or wet suspended form, are applied to the suspended form, are applied to the surface of the magnetized part. The surface of the magnetized part. The particles will be attracted and cluster particles will be attracted and cluster at the flux leakage fields, thus forming at the flux leakage fields, thus forming

a visible indication that the inspector can detect. a visible indication that the inspector can detect.

Advantages and Disadvantages

The primary advantages and disadvantages when compared to other NDT

The primary advantages and disadvantages when compared to other NDT methodsmethods are:

are:

Advantages Advantages



 High sensitivity (High sensitivity (small discontinuities can be detected small discontinuities can be detected ).). 

 Indications are produced directly on the surface of Indications are produced directly on the surface of the part and constitute a visualthe part and constitute a visual

representation of the flaw. representation of the flaw.



 Minimal surface preparation (Minimal surface preparation (no need for paint removal no need for paint removal )) 

 Portable (Portable (materials are available in aerosol spray cansmaterials are available in aerosol spray cans)) 

 Low cost (Low cost (materials and associated equipment are relatively inexpensivematerials and associated equipment are relatively inexpensive))

Disadvantages Disadvantages



 Only surface and near surface defects can be detected.Only surface and near surface defects can be detected. 

 Only applicable to ferromagnetic materials.Only applicable to ferromagnetic materials. 

 Relatively small area can be inspected at a time.Relatively small area can be inspected at a time. 

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

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Magnetism

The concept of magnetism centers around the magnetic field and what is known as a The concept of magnetism centers around the magnetic field and what is known as a dipole. The term "

dipole. The term "magnetic field magnetic field " simply describes a volume of space where there is " simply describes a volume of space where there is aa change in energy within that volume. T

change in energy within that volume. The location where a magnetic field exits orhe location where a magnetic field exits or enters a material is called a magnetic pole. Magnetic poles have never been detected enters a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in

in isolation but always occur in pairs, hence the name dipole. Therefore, a pairs, hence the name dipole. Therefore, a dipole is andipole is an object that has a magnetic pole on one end and a second, equal but opposite,

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 magnetic pole on the other. A bar magnet is a dipole with a north pole at one end and south pole at the other.

south pole at the other.

The source of magnetism lies in the basic

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

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

move through space. A magnetic field is produced whenever anoduced whenever an electrical charge is in motion. The strength of

electrical charge is in motion. The strength of this field is called thethis field is called the magnetic moment.

magnetic moment.

When an electric current flows through

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

through the conductor causes a magnetic field to form around the conductor. Theform around the conductor. The magnetic field can be detected using a compass. Since all

magnetic field can be detected using a compass. Since all matter is comprised of matter is comprised of atoms, all materials are affected in some way by a

atoms, all materials are affected in some way by a magnetic field; however, materialsmagnetic field; however, materials do not react the same way

do not react the same way to the magnetic field.to the magnetic field.

Reaction of Materials to Magnetic Field

When a material is placed within a

When a material is placed within a magnetic field, the magnetic forces of the material'smagnetic field, the magnetic forces of the material's electrons will be affected. This effect is known

electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction.as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an external magnetic 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 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 motion, the change in motion caused by an external magnetic field, and the spin of the electrons.

electrons.

In most atoms, electrons occur in pairs where

In most atoms, electrons occur in pairs where these pairs spin inthese pairs spin in opposite directions. The opposite spin directions of electron pairs opposite directions. The opposite spin directions of electron pairs cause their magnetic fields to cancel each other. T

cause their magnetic fields to cancel each other. Therefore, no netherefore, no net magnetic field exists. Alternately, materials with some unpaired magnetic field exists. Alternately, materials with some unpaired

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electrons will have a net magnetic field and

electrons will have a net magnetic field and will react more to an external field.will react more to an external field. According to their interaction with a magnetic field, materials can be

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

Diamagnetic materials which have a weak, negative susceptibility to magneticwhich have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelle

fields. Diamagnetic materials are slightly repelled by a d by a magnetic field and themagnetic field and the material does not retain the magnetic properties when the external field material does not retain the magnetic properties when the external field isis removed. In diamagnetic materials all the electrons are paired so there is removed. In diamagnetic materials all the electrons are paired so there is nono

permanent net magnetic moment per atom. Most elements in the periodic table, permanent net magnetic moment per atom. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

including copper, silver, and gold, are diamagnetic. Paramagneti

Paramagnetic c materialsmaterials which have a small, positive susceptibility to magneticwhich have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the

fields. These materials are slightly attracted by a magnetic field and the materialmaterial does not retain the magnetic properties when the external field is

does not retain the magnetic properties when the external field is removed.removed. Paramagnetic have some unpaired electrons. Examples of

Paramagnetic have some unpaired electrons. Examples of paramagnetic materialsparamagnetic materials include magnesium, molybdenum, and lithium.

include magnesium, molybdenum, and lithium. Ferromagnetic materials

Ferromagnetic materials have a large, positive susceptibility to an externalhave a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and

magnetic field. They exhibit a strong attraction to magnetic fields and are able toare able to retain their magnetic properties after the external field has been removed.

retain their magnetic properties after the external field has been removed. Ferromagnetic mater

Ferromagnetic materials have some unpaired electrons so their atoms have a ials have some unpaired electrons so their atoms have a netnet magnetic moment. They get their strong magnetic properties due to the presence 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 of magnetic domains. In these domains, large numbers of atom's moments are aligned parallel so that the magnetic force within the d

aligned parallel so that the magnetic force within the domain is strong (omain is strong (this happensthis happens during the solidification of the material where the atom m

during the solidification of the material where the atom moments are aligned withinoments are aligned within each crystal

each crystal ”i.e., grain” ”i.e., grain” causing a strong magnetic force in one causing a strong magnetic force in one directiondirection). When a). When a ferromagnetic material is in the

ferromagnetic material is in the

unmagnetized state, the domains are nearly unmagnetized state, the domains are nearly randomly organized (

randomly organized (since the crystals are insince the crystals are in arbitrary directions

arbitrary directions) and the net magnetic) and the net magnetic field for the part as a whole is zero. When a field for the part as a whole is zero. When a magnetizing force is applied, the domains magnetizing force is applied, the domains become aligned to produce a strong

become aligned to produce a strong

magnetic field within the part. Iron, nickel, magnetic field within the part. Iron, nickel, and cobalt are examples

and cobalt are examples of ferromagneticof ferromagnetic materials. Components made of these materials. Components made of these

materials are commonly inspected using the materials are commonly inspected using the magnetic particle method.

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Magnetic Field Characteristics

Magnetic Field In and Around a Bar Magnet Magnetic Field In and Around a Bar Magnet The magnetic field surrounding a bar

The magnetic field surrounding a bar magnet can be seen in thmagnet can be seen in the magnetograph below.e magnetograph below. A magnetograph can be created by placing a

A magnetograph can be created by placing a piecepiece of paper over a magnet and sprinkling the paper of paper over a magnet and sprinkling the paper with iron filings. The

with iron filings. The particles align themselvesparticles align themselves with the lines of magnetic force produced by with the lines of magnetic force produced by thethe magnet. It can be seen in

magnet. It can be seen in the magnetograph thatthe magnetograph that there are poles all along the length

there are poles all along the length of the magnetof the magnet but that the poles are

but that the poles are concentrated at the ends of concentrated at the ends of the magnet (

the magnet (the north and south polesthe north and south poles).).

Magnetic Fields in and around Horseshoe and Ring Magnets Magnetic Fields in and around Horseshoe and Ring Magnets Magnets come in a variety of shapes and

Magnets come in a variety of shapes and one of the more common isone of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane.

poles lie in the same plane. The magnetic lines of force flow from poleThe magnetic lines of force flow from pole to pole just like in

to pole just like in the bar magnet. However, since the poles arethe bar magnet. However, since the poles are located closer together and a more direct path exists for

located closer together and a more direct path exists for the lines of the lines of flux to travel between the poles, the magnetic field is

flux to travel between the poles, the magnetic field is concentratedconcentrated between the poles.

between the poles.

General Properties of Magnetic Lines of Force General Properties of Magnetic Lines of Force Magnetic lines of force have a number of

Magnetic lines of force have a number of important properties, which include:important properties, which include:



 They seek the path of They seek the path of least resistance between oppositeleast resistance between opposite

magnetic poles (

magnetic poles (in a single bar main a single bar magnet shown, they attempt tognet shown, they attempt to form closed loops from pole to pole

form closed loops from pole to pole).).



 They never cross one another.They never cross one another. 

 They all have the same strength.They all have the same strength. 

 Their density decreases with increasing distance from the poles.Their density decreases with increasing distance from the poles. 

 Their density decreases (Their density decreases (they spread out they spread out ) when they move from) when they move from

an area of higher permeability to an

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

 They are considered to have direction as They are considered to have direction as if flowing, though no actual movementif flowing, though no actual movement

occurs. occurs.



 They flow from the south pole to the north pole within a material and north poleThey flow from the south pole to the north pole within a material and north pole

to south pole in air. to south pole in air.

Electromagnetic Fields

Magnets are not the only source of

Magnets are not the only source of magnetic fields. The flow of electric currentmagnetic fields. The flow of electric current through a conductor generates a magnetic field. When

through a conductor generates a magnetic field. When electric current flows in a longelectric current flows in a long straight wire, a circular magnetic field is generated around the

straight wire, a circular magnetic field is generated around the wire and the intensity of wire and the intensity of this magnetic field is directly proportional to the amount of

this magnetic field is directly proportional to the amount of currentcurrent carried by the wire. The strength of the field is strongest next to carried by the wire. The strength of the field is strongest next to the wire and diminishes with

the wire and diminishes with distance. In most conductors, thedistance. In most conductors, the magnetic field exists only as long as the current

magnetic field exists only as long as the current is flowing.is flowing. However, in ferromagnetic materials the electric current will

However, in ferromagnetic materials the electric current will causecause some or all of the magnetic domains to align

some or all of the magnetic domains to align and a residualand a residual magnetic field will remain.

magnetic field will remain.

Also, the direction of the magnetic field is dependent on

Also, the direction of the magnetic field is dependent on the direction of the electricalthe direction of the electrical current in the wire. The direction of the magnetic field around a conductor can be current in the wire. The direction of the magnetic field around a conductor can be determined using a simple rule called the

determined using a simple rule called the ““right-hand clasp ruleright-hand clasp rule””. If a person grasps a. If a person grasps a conductor in one's right hand with the thumb pointing in the direction of the current, conductor in one's right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in

the fingers will circle the conductor in the direction of the magnetic field.the direction of the magnetic field. Note:

Note: remember that current flows from the positive terminal to the negativeremember that current flows from the positive terminal to the negative terminal (electrons flow in the opposite direction).

terminal (electrons flow in the opposite direction).

Magnetic Field Produced by a Coil

When a current carrying wire is formed into When a current carrying wire is formed into several loops to form a coil, the magnetic field several loops to form a coil, the magnetic field circling each loop combines with the fields from circling each loop combines with the fields from the other loops to produce a

the other loops to produce a concentrated fieldconcentrated field through the center of the coil (

through the center of the coil (the field flows alongthe field flows along the longitudinal axis and circles back around the the longitudinal axis and circles back around the outside of the coil

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When the coil loops are tightly

When the coil loops are tightly wound, a uniform magnetic field is wound, a uniform magnetic field is developeddeveloped throughout the length of the coil.

throughout the length of the coil. The strength of the magnetic field increases not onlyThe strength of the magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long,

with increasing current but also with each loop that is added to the coil. A long, straight coil of wire is called a

straight coil of wire is called a solenoid solenoid and can be used to generate a nearly uniformand can be used to generate a nearly uniform magnetic field similar to that of a bar

magnetic field similar to that of a bar magnet. The concentrated magnetic field inside amagnet. The concentrated magnetic field inside a coil is very useful in

coil is very useful in magnetizing ferromagnetmagnetizing ferromagnetic materials for inspection using theic materials for inspection using the magnetic particle testing method.

magnetic particle testing method.

Quantifying Magnetic Properties

The various characteristics of magnetism can be measured and expressed The various characteristics of magnetism can be measured and expressed quantitatively. Differen

quantitatively. Different systems of units can be t systems of units can be used for quantifying magneticused for quantifying magnetic

properties. SI units will be used in this material. The advantage of using SI units is that properties. SI units will be used in this material. The advantage of using SI units is that they are traceable back to an agreed set of

they are traceable back to an agreed set of four base units; meter, kilogram, second,four base units; meter, kilogram, second, and Ampere.

and Ampere.



 The unit for magneticThe unit for magnetic field strength field strength HH isis ampere/meter ampere/meter

(( A/m A/m). A magnetic field strength of ). A magnetic field strength of 1 A/m1 A/m is producedis produced at the center of a single circular conductor with a at the center of a single circular conductor with a 11 meter

meter diameter carrying a steady current of diameter carrying a steady current of 1 ampere1 ampere..



 The number of magnetic lines of force cutting through a plane of a given area atThe number of magnetic lines of force cutting through a plane of a given area at

a right angle is known as the magnetic

a right angle is known as the magnetic flux density flux density ,, BB. The flux density or. The flux density or magnetic induction has the

magnetic induction has the teslatesla as its unit. Oneas its unit. One teslatesla is equal tois equal to 11 Newton/(A/m)

Newton/(A/m). From these units, it can be seen that the flux density is a. From these units, it can be seen that the flux density is a measure of the force applied to a

measure of the force applied to a particle by the magnetic field.particle by the magnetic field.



 The total number of lines The total number of lines of magnetic force in a material is called magneticof magnetic force in a material is called magnetic flux flux ,,

ɸ

ɸ. The strength of the . The strength of the flux is determined by the flux is determined by the number of magnetic domainsnumber of magnetic domains that are aligned within a material. The

that are aligned within a material. The total flux total flux is simply the flux density appliedis simply the flux density applied over an area. Flux carries the unit of a

over an area. Flux carries the unit of a weber weber , which is simply a, which is simply a tesla-meter tesla-meter 22..



 TheThe magnetizationmagnetization MMis a measure of the extent to is a measure of the extent to which an object iswhich an object is

magnetized. It is a measure of the

magnetized. It is a measure of the magnetic dipole moment per unit volume of magnetic dipole moment per unit volume of the object. Magnetization carries the same units as a magnetic field

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Quantity

Quantity SI SI UnitsUnits

(Sommerfeld) (Sommerfeld) SI Units SI Units (Kennelly) (Kennelly) CGS Units CGS Units (Gaussian) (Gaussian) Field Field ((Magnetization Magnetization Force Force )) H

H A/m A/m A/m A/m oerstedsoersteds

Flux Density Flux Density ((Magnetic Magnetic Induction Induction )) B

B tesla tesla tesla tesla gaussgauss

Flux

Flux ɸɸ weber weber weber weber maxwellmaxwell

Magnetization M

Magnetization M A/m A/m - - erg/Oe-cmerg/Oe-cm33

The Hysteresis Loop and Magnetic Properties

A great deal of information can be

A great deal of information can be learned about the magnetic properties of a materiallearned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the

by studying its hysteresis loop. A hysteresis loop shows the relationship between therelationship between the induced magnetic flux density (

induced magnetic flux density (BB) and the ) and the magnetizing force (magnetizing force (HH). It is often referred to). It is often referred to as the

as the B-HB-H loop. An example hysteresis loop is shown below.loop. An example hysteresis loop is shown below.

The loop is generated by measuring the

The loop is generated by measuring the magnetic flux of a ferromagnetic materialmagnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed previously magnetized or has been thoroughly demagnetized will follow the dashed line as

line as HH is increased. As the line demonstrates, the greater the amount of currentis increased. As the line demonstrates, the greater the amount of current applied (

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almost all of the magnetic domains are aligned and an

almost all of the magnetic domains are aligned and an additional increase in theadditional increase in the magnetizing force will produce very little increase in magnetic flux. The material has magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When

reached the point of magnetic saturation. When HH is reduced to zero, the curve willis reduced to zero, the curve will move from point "

move from point "aa" to point "" to point "bb". At this point, it ". At this point, it can be seen that some magnetic fluxcan be seen that some magnetic flux remains in the material even though the magnetizing force is

remains in the material even though the magnetizing force is zero. This is referred to aszero. This is referred to as the point of retentivity on the graph

the point of retentivity on the graph and indicates the level of residual magnetism inand indicates the level of residual magnetism in the material (

the material (Some of the magnetic domains remain aligned but some have lost their Some of the magnetic domains remain aligned but some have lost their alignment

alignment ). As the magnetizing force is reversed, the curve ). As the magnetizing force is reversed, the curve moves to point "moves to point "c c ", where", where the flux has been reduced to zero. This is called the point of coercivity on the curve the flux has been reduced to zero. This is called the point of coercivity on the curve ((the reversed magnetizing force has flipped enough of the domains so that the net flux the reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero

within the material is zero). The force required ). The force required to remove the residual magnetism fromto remove the residual magnetism from the material is called the coercive force or coercivity of the

the material is called the coercive force or coercivity of the material.material. As the magnetizing force is increased in the

As the magnetizing force is increased in the negative direction, the material will againnegative direction, the material will again become magneticall

become magnetically saturated but in y saturated but in the opposite direction, point "the opposite direction, point "d d ". Reducing". Reducing HH toto zero brings the curve to point "

zero brings the curve to point "ee". It will ". It will have a level of residual magnetism equal tohave a level of residual magnetism equal to that achieved in the other direction.

that achieved in the other direction. IncreasingIncreasingHH back in the positive direction willback in the positive direction will return

returnBBto zero. Notice that the curve did not return to the origin of the graph becauseto zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The

some force is required to remove the residual magnetism. The curve will take acurve will take a different path from point "

different path from point " f f " back to the saturation point " back to the saturation point where it with complete thewhere it with complete the loop.

loop.

From the hysteresis loop, a number of primary

From the hysteresis loop, a number of primary magnetic properties of a material canmagnetic properties of a material can be determined:

be determined: 1.

1. Retentivity Retentivity - A measure of the residual flux - A measure of the residual flux density corresponding to thedensity corresponding to the saturation induction of a magnetic material. In other words, it

saturation induction of a magnetic material. In other words, it is a material'sis a material's ability to retain a certain amount of residual

ability to retain a certain amount of residual magnetic field when the magnetizingmagnetic field when the magnetizing force is removed after achieving saturation (The value of

force is removed after achieving saturation (The value of BBat pointat point bb on theon the hysteresis curve).

hysteresis curve). 2.

2. Residual Magnetism or Residual Flux Residual Magnetism or Residual Flux - The magnetic flux density that remains in - The magnetic flux density that remains in aa material when the magnetizing force is zero. Note that residual magnetism and material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the

retentivity are the same when the material has been magnetized to the saturationsaturation point. However, the level of residual magnetism may be lower than

point. However, the level of residual magnetism may be lower than the retentivitythe retentivity value when the magnetizing force did not

value when the magnetizing force did not reach the saturation level.reach the saturation level. 3.

3. Coercive ForceCoercive Force - The amount of reverse magnetic field which must be applied to a- The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero

magnetic material to make the magnetic flux return to zero (The value of (The value of HH atat point

point c c on the hysteresis curve).on the hysteresis curve). 4.

4. PermeabilityPermeability, , µµ - A property of a - A property of a material that describes the ease with which amaterial that describes the ease with which a magnetic flux is established in the material.

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5.

5. ReluctanceReluctance - Is the opposition that a - Is the opposition that a ferromagnetic mateferromagnetic material shows to therial shows to the

establishment of a magnetic field. Reluctance is analogous to the resistance in an establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

electrical circuit.

Permeability

As previously mentioned, permeability (

As previously mentioned, permeability ( µ µ) is a ) is a material property that describes thematerial property that describes the ease with which a magnetic flux is

ease with which a magnetic flux is established in a component. It is the ratio of established in a component. It is the ratio of the fluxthe flux density (B) created within a material to

density (B) created within a material to the magnetizing field (H) and is represented bythe magnetizing field (H) and is represented by the following equation:

the following equation: µ

µ = B/H= B/H

This equation describes the slope of the curve at any This equation describes the slope of the curve at any point on the hysteresis loop. The

point on the hysteresis loop. The permeability valuepermeability value given in letrature for materials is usually the

given in letrature for materials is usually the maximum permeability or the maximum relative maximum permeability or the maximum relative permeability. The maximum permeability is the point permeability. The maximum permeability is the point where the slope of the B/H curve for the

where the slope of the B/H curve for the

unmagnetized material is the greatest. This point is unmagnetized material is the greatest. This point is often taken as the point where a

often taken as the point where a straight line fromstraight line from the origin is tangent to the

the origin is tangent to the B/H curve.B/H curve. The shape of the hysteresis loop tells a

The shape of the hysteresis loop tells a great deal about the material beinggreat deal about the material being

magnetized. The hysteresis curves of two different materials are shown in the graph. magnetized. The hysteresis curves of two different materials are shown in the graph.



 Relative to other materials, a material with a widerRelative to other materials, a material with a wider

hysteresis loop has: hysteresis loop has: -- Lower PermeabilityLower Permeability -- Higher RetentivityHigher Retentivity -- Higher CoercivityHigher Coercivity -- Higher ReluctanceHigher Reluctance

-- Higher Residual Higher Residual MagnetismMagnetism



 Relative to other materials, a material with a narrowerRelative to other materials, a material with a narrower

hysteresis loop has: hysteresis loop has: -- Higher PermeabilityHigher Permeability -- Lower RetentivityLower Retentivity -- Lower CoercivityLower Coercivity -- Lower ReluctanceLower Reluctance

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-- Lower Residual MagnetismLower Residual Magnetism

In magnetic particle testing, the level of residual magnetism is important. Residual In magnetic particle testing, the level of residual magnetism is important. Residual magnetic fields are affected by the permeability, which can be r

magnetic fields are affected by the permeability, which can be r elated to the carbonelated to the carbon content and alloying of the material. A

content and alloying of the material. A component with high carbon content will component with high carbon content will havehave low permeability and will retain more magnetic flux than a

low permeability and will retain more magnetic flux than a material with low carbonmaterial with low carbon content.

content.

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks

To properly inspect a component for cracks or other defects, it is important toor other defects, it is important to understand that the orientation of the

understand that the orientation of the crack relative to the magnetic lines of forcecrack relative to the magnetic lines of force determinies if the crack can or cannot be

determinies if the crack can or cannot be detected. There are two general types of detected. There are two general types of magnetic fields that can be established within a

magnetic fields that can be established within a component.component.



 A longitudinal magnetic field A longitudinal magnetic field has magnetic lines of forcehas magnetic lines of force

that run parallel to the

that run parallel to the long axis of the part. long axis of the part. LongitudinalLongitudinal magnetization of a component can be accomplished using magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or

be accomplished using permanent magnets or electromagnets.

electromagnets.



 A circular magnetic field A circular magnetic field has magnetic lines of force thathas magnetic lines of force that

run circumferentially around the perimeter of a part. run circumferentially around the perimeter of a part. AA circular magnetic field is induced in an

circular magnetic field is induced in an article by eitherarticle by either passing current through the component or by passing passing current through the component or by passing current through a conductor surrounded by the

current through a conductor surrounded by the component.

component.

The type of magnetic field established is determined by the

The type of magnetic field established is determined by the method used to magnetizemethod used to magnetize the specimen. Being able to magnetize the part

the specimen. Being able to magnetize the part in two directions is important becausein two directions is important because the best detection of defects occurs when the

the best detection of defects occurs when the lines of magnetic force are establishedlines of magnetic force are established at right angles to the longest dimension of the

at right angles to the longest dimension of the defect. Thisdefect. This orientation creates the largest disruption of

orientation creates the largest disruption of the magnetic fieldthe magnetic field within the part and t

within the part and the greatest flux leakage at the surface of thehe greatest flux leakage at the surface of the part. If the magnetic field is

part. If the magnetic field is parallel to the defect, the field willparallel to the defect, the field will see little disruption and no flux

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An orientation of 45 to 90 degrees between the

An orientation of 45 to 90 degrees between the magnetic field and the defect ismagnetic field and the defect is necessary to form an indication. Since defects may occur in

necessary to form an indication. Since defects may occur in various and unknownvarious and unknown directions, each part is normally magnetized in two directions at right angles to

directions, each part is normally magnetized in two directions at right angles to eacheach other. If the component shown is

other. If the component shown is considered, it is known that passing current throuconsidered, it is known that passing current throughgh the part from end to

the part from end to end will establish a circular magnetic field that will end will establish a circular magnetic field that will be 90 degreesbe 90 degrees to the direction of the current.

to the direction of the current. Therefore, defects that have a Therefore, defects that have a significant dimension in the significant dimension in the direction of the current direction of the current

((longitudinal defectslongitudinal defects) should be) should be detectable, while transverse-type detectable, while transverse-type defects will not be detectable defects will not be detectable with circular

with circular magnetization.magnetization.

Magnetization of Ferromagnetic Materials

There are a variety of methods that can

There are a variety of methods that can be used to establish a magnetic field in be used to establish a magnetic field in aa component for evaluation using magnetic particle inspection. It is common to classify component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect.

the magnetizing methods as either direct or indirect.

Magnetization Using Direct Induction (Direct Magnetization) Magnetization Using Direct Induction (Direct Magnetization)

With direct magnetization, current is passed directly through the component. The With direct magnetization, current is passed directly through the component. The flowflow of current causes a circular magnetic field to form in and around the conductor. When of current causes a circular magnetic field to form in and around the conductor. When using the direct magnetization method, care must be taken to

using the direct magnetization method, care must be taken to ensure that goodensure that good

electrical contact is established and maintained between the test equipment and the electrical contact is established and maintained between the test equipment and the test component to avoid damage of the the component (

test component to avoid damage of the the component (due to arcing or overheatingdue to arcing or overheating at high resistance ponts

at high resistance ponts).). There are several ways

There are several ways that direct magnetization is commonly accomplished.that direct magnetization is commonly accomplished. -- One way involvesOne way involves clamping the component between twoclamping the component between two

electrical contacts

electrical contacts in a special piece of equipment.in a special piece of equipment.

Current is passed through the component and a circular Current is passed through the component and a circular magnetic field is established in and around the

magnetic field is established in and around the

component. When the magnetizing current is stopped, a component. When the magnetizing current is stopped, a residual magnetic field will remain within the component. residual magnetic field will remain within the component. The strength of the induced magnetic field is

The strength of the induced magnetic field is proportionalproportional to the amount of current passed through

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-- A second technique involves usingA second technique involves using clamps or prodsclamps or prods, which are, which are attached or placed in contact with the

attached or placed in contact with the component. Electricalcomponent. Electrical current flows through the component from contact to contact. current flows through the component from contact to contact. The current sets up a circular magnetic field around the path of The current sets up a circular magnetic field around the path of the current.

the current.

Magnetization Using Indirect Induction (Indirect

Magnetization Using Indirect Induction (Indirect Magnetization)Magnetization)

Indirect magnetization is accomplished by using a strong external magnetic field to Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with

establish a magnetic field within the component. As with direct magnetization, theredirect magnetization, there are several ways that

are several ways that indirect magnetization can be accomplished.indirect magnetization can be accomplished. -- The use of The use of permanent magnets permanent magnets is a low cost method of is a low cost method of

establishing a magnetic field. However, their use is limited due establishing a magnetic field. However, their use is limited due to lack of control of the field

to lack of control of the field strength and the difficulty of strength and the difficulty of placing and removing strong permanent magnets from the placing and removing strong permanent magnets from the component.

component.

-- ElectromagnetsElectromagnets in the form of an adjustablein the form of an adjustable horseshoe magnet (

horseshoe magnet (called a yokecalled a yoke) eliminate) eliminate the problems associated with permanent the problems associated with permanent magnets and are used extensively in industry. magnets and are used extensively in industry. Electromagnet

Electromagnets only s only exhibit a magnetic fluxexhibit a magnetic flux when electric current is flowing around the when electric current is flowing around the soft iron core. When the magnet is

soft iron core. When the magnet is placed onplaced on the component, a magnetic field

the component, a magnetic field is establishedis established between the north and south poles of the between the north and south poles of the magnet.

magnet.

-- Another way of indirectly inducting a magnetic field in Another way of indirectly inducting a magnetic field in a material isa material is by using the magnetic field of a current carrying conductor. A

by using the magnetic field of a current carrying conductor. A

circular magnetic field can be established in cylindrical components circular magnetic field can be established in cylindrical components by using a

by using a central conductor central conductor . Typically, one or more cylindrical. Typically, one or more cylindrical components are hung from a solid copper bar running through the components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field

resulting circular magnetic field establishes a magnetic field withinwithin the test components.

the test components.

-- The use of The use of coils and solenoidscoils and solenoids is a third method of is a third method of indirect magnetization. When theindirect magnetization. When the length of a component is several times larger than its

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magnetic field can be established in the magnetic field can be established in the component. The component is placed component. The component is placed longitudinally in the concentrated longitudinally in the concentrated

magnetic field that fills the center of a coil magnetic field that fills the center of a coil or solenoid. This

or solenoid. This magnetization techniquemagnetization technique is often referred to as a "

is often referred to as a "coil shot coil shot ".".

Types of Magnetizing Current

As mentioned previously, electric current is often used to establish the magnetic field As mentioned previously, electric current is often used to establish the magnetic field in components during magnetic particle inspection. Alternating current (AC)

in components during magnetic particle inspection. Alternating current (AC) and directand direct current (DC) are the two basic types of current commonly used. The type of current current (DC) are the two basic types of current commonly used. The type of current used can have an effect on

used can have an effect on the inspection results, so the types of the inspection results, so the types of currents commonlycurrents commonly used are briefly discussed here.

used are briefly discussed here. Direct Current

Direct Current

Direct current (DC) flows continuously in one direction

Direct current (DC) flows continuously in one direction at a constant voltage. A batteryat a constant voltage. A battery is the most common source of direct current. The current is said to flow from the

is the most common source of direct current. The current is said to flow from the positive to the negative terminal, though electrons flow in the opposite direction. DC positive to the negative terminal, though electrons flow in the opposite direction. DC isis very desirable when inspecting for subsurface defects because DC generates a

very desirable when inspecting for subsurface defects because DC generates a magnetic field that penetrates deeper into

magnetic field that penetrates deeper into the material. In the material. In ferromagnetic materials,ferromagnetic materials, the magnetic field produced by DC

the magnetic field produced by DC generally penetrates the entire cross-section of thegenerally penetrates the entire cross-section of the component.

component. Alternating

Alternating Current Current

Alternating current (AC) reverses in direction at a

Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second.rate of 50 or 60 cycles per second. Since AC is readily available in most facilities, it is

Since AC is readily available in most facilities, it is convenient to make use of it forconvenient to make use of it for magnetic particle inspection. However, when AC is used to induce

magnetic particle inspection. However, when AC is used to induce a magnetic field ina magnetic field in ferromagnetic materia

ferromagnetic materials, the magnetic field will be limited to ls, the magnetic field will be limited to a thin layer at the a thin layer at the surfacesurface of the component. This phenomenon is known as the "

of the component. This phenomenon is known as the "skin effect skin effect " and occurs because" and occurs because the changing magnetic field generates eddy currents in the test object.

the changing magnetic field generates eddy currents in the test object. The eddyThe eddy currents produce a magnetic field that opposes the primary field, thus

currents produce a magnetic field that opposes the primary field, thus reducing the netreducing the net magnetic flux below the surface. Therefore, it is recommended that AC

magnetic flux below the surface. Therefore, it is recommended that AC be used onlybe used only when the inspection is limited to surface defects.

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Rectified Alternating Current Rectified Alternating Current

Clearly, the skin effect limits the use of AC

Clearly, the skin effect limits the use of AC since many inspection applications call forsince many inspection applications call for the detection of subsurface defects. Luckily, AC can be converted

the detection of subsurface defects. Luckily, AC can be converted to current that is veryto current that is very much like DC through the

much like DC through the process of rectification. With the use of process of rectification. With the use of rectifiers, therectifiers, the reversing AC can be converted to a

reversing AC can be converted to a one directional current. The three commonly usedone directional current. The three commonly used types of rectified current are described below.

types of rectified current are described below.

Half Wave Rectified Alternating Current (HWAC) Half Wave Rectified Alternating Current (HWAC)

When single phase alternating current is passed through a

When single phase alternating current is passed through a rectifier, current is allowedrectifier, current is allowed to flow in only one direction. The reverse half of each cycle is blocked out so that a one to flow in only one direction. The reverse half of each cycle is blocked out so that a one directional, pulsating current is produced. The current rises

directional, pulsating current is produced. The current rises from zero to a maximumfrom zero to a maximum and then returns to zero.

and then returns to zero. No current flows during the time when No current flows during the time when the reverse cycle isthe reverse cycle is blocked out. The HWAC

blocked out. The HWAC repeats at same rate as the unrectified current (50 or repeats at same rate as the unrectified current (50 or 60 Hz).60 Hz). Since half of the current is blocked out, the amperage is half of the unaltered AC. This Since half of the current is blocked out, the amperage is half of the unaltered AC. This type of current is often referred to

type of current is often referred to asas half wave DC or pulsating DC half wave DC or pulsating DC . The pulsation of . The pulsation of the HWAC helps magnetic particle indications form by vibrating the

the HWAC helps magnetic particle indications form by vibrating the particles and givingparticles and giving them added mobility where that is

them added mobility where that is especially important when using dry particles.especially important when using dry particles. HWAC is most often used to power

HWAC is most often used to power electromagnetelectromagnetic yokes.ic yokes. Full Wave Rectified Alternating Current (FWAC)

Full Wave Rectified Alternating Current (FWAC) ((Single PhaseSingle Phase))

Full wave rectification inverts the negative current to positive current rather than Full wave rectification inverts the negative current to positive current rather than blocking it out. This produces a pulsating DC with no interval between the pulses. blocking it out. This produces a pulsating DC with no interval between the pulses. Filtering is usually performed to soften the sharp polarity switching in the

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current. While particle mobility is not as

current. While particle mobility is not as good as half-wave AC due to the good as half-wave AC due to the reduction inreduction in pulsation, the depth of the subsurface magnetic field is

pulsation, the depth of the subsurface magnetic field is improved.improved. Three Phase Full Wave Rectified Alternating Current

Three Phase Full Wave Rectified Alternating Current Three phase current is often used to

Three phase current is often used to power industrial equipment because it has morepower industrial equipment because it has more favorable power transmission and line

favorable power transmission and line loading characteristics. This type of loading characteristics. This type of electricalelectrical current is also highly desirable for

current is also highly desirable for magnetic particle testmagnetic particle testing because when it is ing because when it is rectifiedrectified and filtered, the resulting current very closely resembles direct current. Stationary

and filtered, the resulting current very closely resembles direct current. Stationary magnetic particle equipment wired with three phase AC will

magnetic particle equipment wired with three phase AC will usually have the ability tousually have the ability to magnetize with AC or DC (

magnetize with AC or DC (three phase full wave rectified three phase full wave rectified ), providing the inspector with), providing the inspector with the advantages of each current form.

the advantages of each current form.

Magnetic Fields Distribution and Intensity

Longitudinal Fields Longitudinal Fields

When a long component is magnetized using a

When a long component is magnetized using a solenoid having a shorter length, onlysolenoid having a shorter length, only the material within the solenoid and

the material within the solenoid and about the same length on each side of about the same length on each side of the solenoid will be strongly magnetized. the solenoid will be strongly magnetized. This occurs because the

This occurs because the magnetizingmagnetizing force diminishes with increasing distance force diminishes with increasing distance from the solenoid. Therefore, a long

from the solenoid. Therefore, a long component must be magnetized and component must be magnetized and inspected at several locations along its inspected at several locations along its length for complete inspection coverage. length for complete inspection coverage.

Circular Fields Circular Fields

When a circular magnetic field is forms in and around a conductor due to the passage When a circular magnetic field is forms in and around a conductor due to the passage of electric current through it, the

of electric current through it, the following can be said about the following can be said about the distribution anddistribution and intensity of the magnetic field:

intensity of the magnetic field:

-- The field strength varies from zero at The field strength varies from zero at the center of the component to a the center of the component to a maximummaximum at the surface.

at the surface.

-- The field strength at the surface of The field strength at the surface of the conductor decreases as the radius of thethe conductor decreases as the radius of the conductor increases (

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-- The field strength inside the conductor is dependent on the current strength,The field strength inside the conductor is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B magnetic permeability of the material, and if magnetic, the location on the B -H-H curve.

curve.

-- The field strength outside the conductor iThe field strength outside the conductor is directly proportional to the currents directly proportional to the current strength and it decreases with distance from the conductor.

strength and it decreases with distance from the conductor. The images below show the magnetic field strength graphed v

The images below show the magnetic field strength graphed v ersus distance from theersus distance from the center of the conductor when current

center of the conductor when current passes through a solid circular conductor.passes through a solid circular conductor.



 In a nonmagnetic conductor carrying DC, tIn a nonmagnetic conductor carrying DC, the internal field strength rises from zerohe internal field strength rises from zero

at the center to a maximum value at

at the center to a maximum value at the surface of the conductor.the surface of the conductor.



 In a magnetic conductor carrying DC, In a magnetic conductor carrying DC, the field strength within the conductor isthe field strength within the conductor is

much greater than it is in the nonmagnetic conductor. This is due to the much greater than it is in the nonmagnetic conductor. This is due to the

permeability of the magnetic material. The external field is exactly the same for the permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius

two materials provided the current level and conductor radius are the same.are the same.



 When the magnetic conductor is carrying AC, When the magnetic conductor is carrying AC, the internal magnetic field will bethe internal magnetic field will be

concentrated in a thin layer near

concentrated in a thin layer near the surface of the conductor (the surface of the conductor (skin effect skin effect ). The). The external field decreases with increasing distance from the surface same as with DC. external field decreases with increasing distance from the surface same as with DC.

The magnetic field distribution in The magnetic field distribution in and around a solid conductor of a and around a solid conductor of a nonmagnetic material carrying nonmagnetic material carrying direct current.

direct current.

The magnetic field distribution in The magnetic field distribution in and around a solid conductor of a and around a solid conductor of a magnetic material carrying direct magnetic material carrying direct current.

current.

The magnetic field distribution in The magnetic field distribution in and around a solid conductor of a and around a solid conductor of a magnetic material carrying

magnetic material carrying alternating current.

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In a hollow circular conductor there is no magnetic field in the void area. The magnetic In a hollow circular conductor there is no magnetic field in the void area. The magnetic field is zero at the inn

field is zero at the inner surface and rises until it er surface and rises until it reaches a maximum at the outerreaches a maximum at the outer surface.

surface.



 Same as with a solid conductor, when Same as with a solid conductor, when DC current is DC current is passed through a magneticpassed through a magnetic

conductor, the field strength within the conductor is

conductor, the field strength within the conductor is much greater than inmuch greater than in nonmagnetic conductor due to the permeability of the magnetic material. The nonmagnetic conductor due to the permeability of the magnetic material. The external field strength decreases with distance from the surface of the conductor. external field strength decreases with distance from the surface of the conductor. The external field is exactly the same for

The external field is exactly the same for the two materials provided the currentthe two materials provided the current level and conductor radius are the same.

level and conductor radius are the same.



 When AC current is When AC current is passed through a hollow circular passed through a hollow circular magnetic conductor, the skinmagnetic conductor, the skin

effect concentrate

effect concentrates the magnetic field at the s the magnetic field at the outside diameter of the component.outside diameter of the component.

The magnetic field distribution in The magnetic field distribution in and around a hollow conductor of a and around a hollow conductor of a nonmagnetic material carrying nonmagnetic material carrying direct current.

direct current.

The magnetic field distribution in The magnetic field distribution in and around a hollow conductor of and around a hollow conductor of a magnetic material carrying direct a magnetic material carrying direct current.

current.

The magnetic field distribution in The magnetic field distribution in and around a hollow conductor of and around a hollow conductor of a magnetic material carrying a magnetic material carrying alternating current.

alternating current.

As can be seen from

As can be seen from these three field distribution images, the field strength at thethese three field distribution images, the field strength at the inside surface of hollow conductor is very low wh

inside surface of hollow conductor is very low when a circular magnetic field isen a circular magnetic field is

established by direct magnetization. Therefore, the direct method of magnetization is established by direct magnetization. Therefore, the direct method of magnetization is not recommended when inspecting the inside diameter wall of

not recommended when inspecting the inside diameter wall of a hollow componenta hollow component for shallow defects (

for shallow defects (if the defect has significant depth, it may be detectable using DC if the defect has significant depth, it may be detectable using DC since the field strength increases rapidly as one moves from the inner towards the outer since the field strength increases rapidly as one moves from the inner towards the outer surface

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

 A much better method of magnetizing hollowA much better method of magnetizing hollow

components for inspection of the ID and

components for inspection of the ID and OD surfacesOD surfaces is with the use of a central conductor. As can be seen is with the use of a central conductor. As can be seen in the field distribution image, when current

in the field distribution image, when current is passedis passed through a nonmagnetic central conductor (copper through a nonmagnetic central conductor (copper bar), the magnetic field produced on the inside bar), the magnetic field produced on the inside

diameter surface of a magnetic tube is much greater diameter surface of a magnetic tube is much greater and the field is still

and the field is still strong enough for defect detectionstrong enough for defect detection on the OD surface.

on the OD surface.

Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can:

the component. Remanent magnetic fields can:

-- affect machining by causing cuttings to cling to affect machining by causing cuttings to cling to a component.a component. -- interfere with electronic equipment such as a compass.interfere with electronic equipment such as a compass.

-- create a condition known as "arc blow" create a condition known as "arc blow" in the welding process. Arc blow in the welding process. Arc blow maymay cause the weld arc to wonder

cause the weld arc to wonder or filler metal to be repelled from the or filler metal to be repelled from the weld.weld.

-- cause abrasive particles to cling to bearing or faying cause abrasive particles to cling to bearing or faying surfaces and increase wear.surfaces and increase wear. Removal of a field may be accomplished in several ways.

Removal of a field may be accomplished in several ways. The most effective way toThe most effective way to demagnetize a material is by heating the material above its curie temperature ( demagnetize a material is by heating the material above its curie temperature ( for for instance, the curie temperature for a low carbon steel is 770°C

instance, the curie temperature for a low carbon steel is 770°C ). When steel is heated). When steel is heated above its curie temperature then it is

above its curie temperature then it is cooled back down, the the orientation of cooled back down, the the orientation of thethe magnetic domains of the individual grains will become randomized again and

magnetic domains of the individual grains will become randomized again and thus thethus the component will contain no residual magnetic field. The material should

component will contain no residual magnetic field. The material should also be placedalso be placed with it long axis in

with it long axis in an east-west orientation to avoid any influence of the Earth'san east-west orientation to avoid any influence of the Earth's magnetic field.

magnetic field.

However, it is often inconvenient to heat a material above its

However, it is often inconvenient to heat a material above its curie temperature tocurie temperature to demagnetize it, so another method that returns the material to a

demagnetize it, so another method that returns the material to a nearly unmagnetizednearly unmagnetized state is commonly used.

state is commonly used.

Subjecting the component to a reversing and decreasing magnetic field will

Subjecting the component to a reversing and decreasing magnetic field will return thereturn the dipoles to a nearly random orientation throughout the material. This can

dipoles to a nearly random orientation throughout the material. This can bebe accomplished by pulling a component out and away from a coil with AC passing accomplished by pulling a component out and away from a coil with AC passing

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through it. With AC

through it. With AC Yokes, demagnetization of localYokes, demagnetization of local areas may be accomplished by placing the yoke contacts areas may be accomplished by placing the yoke contacts on the surface, moving them in circular

on the surface, moving them in circular patterns aroundpatterns around the area, and slowly withdrawing the yoke wh

the area, and slowly withdrawing the yoke while theile the current is applied. The same can

current is applied. The same can also be accomplishedalso be accomplished using an electromagnetic yoke with AC selected. Also, using an electromagnetic yoke with AC selected. Also, many stationary magnetic particle inspection units come many stationary magnetic particle inspection units come with a demagnetization feature that slowly reduces the with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.

AC in a coil in which the component is placed. A field meter is often used to

A field meter is often used to verify that the residual flux has been rverify that the residual flux has been removed from aemoved from a component. Industry standards usually require that the magnetic flux be

component. Industry standards usually require that the magnetic flux be reduced toreduced to less than 3 Gauss (

less than 3 Gauss (3x103x10-4-4TeslaTesla) after completing a magnetic particle inspection.) after completing a magnetic particle inspection.

Measuring Magnetic Fields

When performing a magnetic particle inspection, it is very important to

When performing a magnetic particle inspection, it is very important to be able tobe able to determine the direction and intensity of the magnetic field. T

determine the direction and intensity of the magnetic field. T he field intensity must behe field intensity must be high enough to cause an

high enough to cause an indication to form, but not too indication to form, but not too high to cause nonrelevanthigh to cause nonrelevant indications to mask relevant indications. Also, after magnetic inspection it is often indications to mask relevant indications. Also, after magnetic inspection it is often needed to measure the level of residual magnetezm.

needed to measure the level of residual magnetezm.

Since it is impractical to measure the actual field strength within

Since it is impractical to measure the actual field strength within the material, all thethe material, all the devices measure the magnetic field that is outside of the material. The

devices measure the magnetic field that is outside of the material. The two devicestwo devices commonly used for quantitative measurement of magnetic fields

commonly used for quantitative measurement of magnetic fields n magnetic particlen magnetic particle inspection are the field indicator and the

inspection are the field indicator and the Hall-effect meter, which is also called a gaussHall-effect meter, which is also called a gauss meter.

meter.

Field Indicators Field Indicators

Field indicators are small mechanical devices that utilize a soft iron Field indicators are small mechanical devices that utilize a soft iron vane that is deflected by a magnetic field. The

vane that is deflected by a magnetic field. The vane is attached to avane is attached to a needle that rotates and moves the pointer for

needle that rotates and moves the pointer for the scale. Fieldthe scale. Field indicators can be adjusted and calibrated so that

indicators can be adjusted and calibrated so that quantitativequantitative

information can be obtained. However, the measurement range of information can be obtained. However, the measurement range of field indicators is usually small due to

field indicators is usually small due to the mechanics of the devicethe mechanics of the device ((the one shown in the image has a range from plus 20 to minus 20the one shown in the image has a range from plus 20 to minus 20 Gauss

Gauss). This limited range makes them best ). This limited range makes them best suited for measuringsuited for measuring the residual magnetic field after