2001 rev.0 - magnetic particle testing level 1 & 2 combined - note book

MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED NOTE BOOKNASA-MT-2001 REV.0

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TABLE OF CONTENTS

CHAPTER 1 QUALIFICATION, CERTIFICATION AND AUTHORISATION .............................. 2CHAPTER 2 BASIC PRINCIPLES .......................................................................................... 5CHAPTER 3 MAGNETIC PROPERTIES .............................................................................. 14CHAPTER 4 CURRENT TYPES........................................................................................... 25CHAPTER 5 MAGNETIZING METHODS............................................................................ 27CHAPTER 6 DEMAGNETISATION .................................................................................... 46CHAPTER 7 DETECTION MEDIUM................................................................................... 48CHAPTER 8 VIEWING CONDITIONS ................................................................................ 56CHAPTER 9 MAGNETIC FIELD INDICATORS .................................................................... 61CHAPTER 10 PERFORMANCE CHECKS ............................................................................ 65CHAPTER 11 INTERPRETATION VS. EVALUATION .......................................................... 68


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CHAPTER 1 QUALIFICATION, CERTIFICATION AND AUTHORISATION

SNT-TC-1A & ISO 9712It is important that the technician be qualified and certified in the NDT method before thetechnique is used and the test results evaluated.

The American Society for Nondestructive Testing recommends the use of their documentRecommended Practice No SNT-TC-1A.

The International Standards Organisation requires the use of their Specification, namely ISO9712.

These documents provides the employer with the necessary guidelines to properly qualify andcertify the NDT technician in all methods.

To comply with these documents, the employer must establish a written practice which describesin detail how the technician will be trained, examined and certified.

These documents specifies the initial number of hours of classroom instruction and months orhours of experience necessary to be certified as an NDT testing technician. The main differencebetween these documents are that:

SNT-TC-1A requires Company (Employer) Certification, and

ISO 9712 requires Certification by a Body such as PCN or CSWIP.


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LEVELS OF QUALIFICATIONLevel 1

An individual certified to Level 1 has demonstrated competence to carry out NDT according towritten instructions and under the supervision of Level 2 or Level 3 personnel. Within the scopeof the competence defined on the certificate, Level 1 personnel may be authorized by theemployer to perform the following in accordance with NDT instructions:

a) set up NDT equipment;b) perform the tests;c) record and classify the results of the tests according to written criteria;d) report the results.

Level 1 certified personnel shall neither be responsible for the choice of test method ortechnique to be used, nor for the evaluation of test results.

Level 2

An individual certified to Level 2 has demonstrated competence to perform NDT according toNDT procedures. Within the scope of the competence defined on the certificate, Level 2personnel may be authorized by the employer to:

a) select the NDT technique for the testing method to be used;b) define the limitations of application of the testing method;c) translate NDT codes, standards, specifications, and procedures into NDT instructions

adapted to the actual working conditions;d) set up and verify equipment settings;e) perform and supervise tests;f) interpret and evaluate results according to applicable standards, codes, specifications or

procedures;g) carry out and supervise all tasks at or below Level 2;h) provide guidance for personnel at or below Level 2;i) report the results of NDT.

Level 3

An individual certified to Level 3 has demonstrated competence to perform and direct NDToperations for which he is certified. Level 3 personnel have demonstrated:

a) the competence to evaluate and interpret results in terms of existing standards, codes,and specifications;

b) sufficient practical knowledge of applicable materials, fabrication, process, and producttechnology to select NDT methods, establish NDT techniques, and assist in establishingacceptance criteria where none are otherwise available;

c) a general familiarity with other NDT methods.


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Within the scope of the competence defined on the certificate, Level 3 personnel may beauthorized to:

a) assume full responsibility for a test facility or examination centre and staff;b) establish, review for editorial and technical correctness, and validate NDT instructions

and procedures;c) interpret standards, codes, specifications, and procedures;d) designate the particular test methods, procedures, and NDT instructions to be used;e) carry out and supervise all tasks at all levels;f) provide guidance for NDT personnel at all levels.

EXAMINATION BREAKDOWNThe end of Course examination (SNT-TC-1A), at NASA will comprise of the following:General examination:

Closed book. 40 Multi-choice questions. 1 Hour.

Specific examination:

20 Multi-choice questions. 5 Open Code Book Questions. (10 points) 1 Hour.

Practical examination:

Pre-Test Calibrations: Complete a Calibration Procedure as allocated by examiner. 1 Hour.

Practical 1: Visible, wet, continuous method with AC Yoke on welded sample. Complete a Written Instruction. Fill out a Test Report. 3 Hours.

Practical 2: Fluorescent, wet, continuous method with AC Yoke on welded sample. Complete a Technique sheet. Fill out a Test Report. 2 Hours.

A minimum of 70% must be scored on each segment of the exam with an aggregate of 80% inorder to pass.


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CHAPTER 2 BASIC PRINCIPLES

INTRODUCTIONMagnetic Particle Testing (Inspection) (MT or MPI) is a Non-Destructive Testing method used fordefect detection. MPI is fast and relatively easy to apply, and part surface preparation is not ascritical as it is for some other NDT methods. These characteristics make MPI one of the mostwidely utilized non-destructive testing methods.MPI uses magnetic fields and small magnetic particles (i.e. iron filings) to detect flaws incomponents. The only requirement from an inspectability standpoint is that the componentbeing inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some oftheir alloys. Ferromagnetic materials are materials that can be magnetized to a level that willallow the inspection to be effective.The method is used to inspect a variety of product forms including castings, forgings, andweldments. Many different industries use magnetic particle inspection for determining acomponent's fitness-for-use. Some examples of industries that use magnetic particle inspectionare the structural steel, automotive, petrochemical, power generation, and aerospace industries.Underwater inspection is another area where magnetic particle inspection may be used to testitems such as offshore structures and underwater pipelines.

BASIC PRINCIPLESA magnetic field is introduces into a specimen to be tested, then fine particles of ferromagneticpowder, or ferromagnetic particles in a liquid suspension, are applied to the area being tested.Any discontinuity in the test area which cuts across the magnetic field creates a leakage field. Aleakage field has a north and South Pole on either side of it, and therefore will attract theferromagnetic particles in great numbers.

There are many ways to apply a magnetic field, e.g. by the use of permanent magnets,electromagnetic yokes, coils, prods, cables and other devices.

A basic sequence of operations for the examination of a weld using MPI with a permanentmagnet and black ink is shown below:

1. Clean area using a wire brush if required.2. Apply a thin layer of white contrast paint.3. When the paint is dry, straddle the magnet over the weld at 90 to the weld axis.4. Apply the magnetic particles.5. Interpret the area. Look for indications with their length lying along the same axis as the

weld. Evaluate in accordance with the relevant specification.6. To look for transverse weld discontinuities, turn magnet approximately 90 and re-apply

the ink.7. Interpret the area. Look for indication with their length perpendicular to the weld axis.8. Evaluate in accordance with the relevant specification.


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HISTORYMagnetism is the ability of matter to attract other matter to itself. The ancient Greeks were thefirst to discover this phenomenon in a mineral they named magnetite. Later on Bergmann,Becquerel, and Faraday discovered that all matter including liquids and gasses were affected bymagnetism, but only a few responded to a noticeable extent.The earliest known use of magnetism to inspect an object took place as early as 1868. Cannonbarrels were checked for defects by magnetizing the barrel then sliding a magnetic compassalong the barrel's length. These early inspectors were able to locate flaws in the barrels bymonitoring the needle of the compass. This was a form of Non-Destructive Testing but the termwas not commonly used until some time after World War I.In the early 1920s, William Hoke realized that magnetic particles (coloured metal shavings) couldbe used with magnetism as a means of locating defects. Hoke discovered that a surface orsubsurface flaw in a magnetized material caused the magnetic field to distort and extend beyondthe part. This discovery was brought to his attention in the machine shop. He noticed that themetallic grindings from hard steel parts (held by a magnetic chuck while being ground) formedpatterns on the face of the parts which corresponded to the cracks in the surface. Applying a fineferromagnetic powder to the parts caused a build-up of powder over flaws and formed a visibleindication. The image shows a 1928 Electro-Magnetic Steel Testing Device (MPI) made by theEquipment and Engineering Company Ltd. (ECO) of Strand, England.

In the early 1930s, magnetic particle inspection was quickly replacing the oil-and-whiting method(an early form of the liquid penetrant inspection) as the method of choice by the railroad industryto inspect steam engine boilers, wheels, axles, and tracks. Today, the MT inspection method isused extensively to check for flaws in a large variety of manufactured materials and components.


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TEST PROCEDURESApproved procedures for magnetic particle testing are formulated from analysis of the testspecimen, review of its past history, experience and information available concerningdiscontinuities in like or similar articles. It is the responsibility of personnel conducting orchecking tests to ensure that the test procedures are adequately performed, and that the testobjective is accomplished. Procedures found incorrect or inadequate must be brought to theattention of responsible supervision for correction.

TEST OBJECTIVEThe objective of magnetic particle testing is to ensure maximum reliability by providing a meansof:

Obtaining a visual image of an indication related to a discontinuity below the surface orat the surface of a material.

Disclosing the nature of discontinuities without impairing the material. Separating acceptable and unacceptable material in accordance with predetermined

standards.

ADVANTAGES

The magnetic particle method has a number of outstanding advantages within its field ofusefulness that is, on ferromagnetic materials. Some of these are the following:

It is the best and most reliable method available for finding surface cracks, especiallyvery fine and shallow ones.

It will detect cracks filled with foreign material. No elaborate pre-cleaning is ordinarily necessary. It will work well through thin coatings of paint, or other nonmagnetic coverings such as

plating.

DISADVANTAGES

Although the method has many desirable and attractive advantages, it has, as does everymethod, certain limitations. These, the operator must be aware of, and take into account byobserving the precautions which they dictate. Some of these are the following:

It will work only on ferromagnetic materials. It is not in all cases reliable for locating discontinuities which lie wholly below the

surface. Exceedingly heavy currents are sometimes required for the testing of very large castings

and forgings. Care is required to avoid local heating and burning of highly finished parts or surfaces at

the points of electrical contact.


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TERMINOLOGYAir Gap When a magnetic circuit contains a small gap, which the magnetic flux must cross, thespace is referred to as an air gap. Cracks produce small air gaps on the surface of an article.Alternating Current Electric current periodically reversing in polarity or direction of flow.Ampere The unit of electrical current. One ampere is the current that flows through aconductor having a resistance of one ohm at a potential of one volt.Ampere Turns The product of the number of turns in a coil and the number of amperes flowingthrough it. A measure of the magnetizing or demagnetizing strength of the coil.Bath The suspension of iron oxide particles in a liquid vehicle (light oil or water).Black light Radiant energy in the near ultraviolet range. This light has a wavelength of 3200 to4000 angstrom units (A), peaking at 3650 A, on the spectrum. This is between visible light andultraviolet light.Black light filter A filter that transmits black light while suppressing the transmission of visiblelight and harmful ultraviolet radiation.Carbon Steel Steel which does not contain significant amounts of alloying elements other thancarbon and manganese.Carrier Fluid The fluid in which fluorescent and non-fluorescent magnetic particles aresuspended to facilitate their application in the wet method.Central Conductor An electrical conductor that is passed through the opening in a ring or tube,or any hole in an article, for the purpose of creating a circular field in the ring or tube, or aroundthe hole.Circular Field See Field, circular Magnetic.Coercive Force The reverse magnetizing force necessary to remove residual magnetism indemagnetizing an article.Coil Shot A pulse of magnetizing current passed through a coil surrounding an article for thepurpose of longitudinal magnetization.Contact Head The electrode, fixed to the magnetic particle testing unit, through which themagnetizing current is drawn.Contact Pads Replaceable metal pad, usually of copper braid, placed on contact heads to givegood electrical contact thereby preventing damage to the article under test.Continuous Method An inspection method in which ample amounts of magnetic particles areapplied, or are present on the piece, during the time the magnetizing current is applied.Circular Magnetization A method of inducing a magnetic field in an article so that the magneticlines of force take the form of concentric rings about the axis of the current. This is accomplishedby passing the current directly through the article or through a conductor which passes into orthrough a hole in the article. The circular method is applicable for the detection ofdiscontinuities with axes approximately parallel to the axis of the current through the article.


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Core That part of the magnetic circuit which is within the electrical winding.Curie Point The temperature at which ferromagnetic materials can no longer be magnetized byoutside forces, and at which they lose their residual magnetism: approximately 1200 to 1600F(649 to 871C) for many metals.Current Flow Method A method of circular magnetization by passing a currant through an articlevia prods or contact heads. The current may be alternating, half-wave rectified alternating, ordirect.Current Induction Method A method of magnetization in which a circulating current is inducedin a ring-shaped component by fluctuating magnetic field.Defect A discontinuity that interferes with the usefulness of an article or exceeds acceptabilitylimits established by applicable specifications. A fault in any material or part which isdetrimental to its serviceability. Note that all cracks, seams, laps, etc., are not necessarily defectsas they may not affect the serviceability of the part in which they exist.Demagnetization The reduction in the degree of residual magnetism in ferromagnetic materialsto an acceptable level.Diffuse Indications Indications that are not clearly defined as, for example, indications ofsubsurface defects.Direct Current An electric current which flows steadily in one direction.Discontinuity An interruption (cracks, forging laps, seams, inclusions, porosity, etc.) in thenormal physical structure of configuration of an article. A discontinuity may or may not affect theusefulness of the article.Distorted Field The direction of a magnetic field in a symmetrical object will be substantiallyuniform if produced by a uniformly applied magnetizing force. But if the article being magnetizedis irregular in shape, the field is distorted and does not follow a straight path or have a uniformdistribution.Dry Method Magnetic particle inspection in which the particles employed are in the dry powderform.Dry Powder (Dry Method) Finely divided ferromagnetic particles suitably selected and preparedfor magnetic particle inspection by the dry method.Electromagnet A magnet created by inserting a suitable metal core within, or near, amagnetizing field formed by passing electric current through a coil of insulated wire.Etching The process of exposing subsurface conditions of metal articles by removal of theoutside surface through the use of chemical agents. Due to the action of the chemicals in eatingaway the surface, various surface or subsurface conditions are exposed or exaggerated and madevisible to the eye. For example forging flow lines, discontinuities, and defects.Ferromagnetic A term applied to materials which can be magnetized and strongly attracted by amagnetic field.Field, Bipolar Longitudinal magnetic field within an article that creates two poles.


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Fields, Circular Magnetic Generally, the magnetic field in and surrounding any electricalconductor or article resulting from a current being passed through the conductor or article orfrom prods.Field, Magnetic Leakage The magnetic field that leaves or enters the surface of an article at amagnetic pole.Field, Longitudinal Magnetic A magnetic field where in the flux lines traverse the component in adirection essentially parallel with the axis of the magnetizing coil or to a line connecting the twopoles at the magnetizing yoke.Field, Magnetic The space within and surrounding a magnetized article, or a conductor carryingcurrent, in which the magnetic force is present.Field, Residual Magnetic The field that remains in magnetizable material after the magnetizingforce has been removed.Field Resultant Magnetic A magnetic field that is the result of two magnetic forces impressedupon the same area of a magnetizable object at the same timesometimes called a vectorfield.Field, Vector See Field, Resultant Magnetic.Flash Magnetization Magnetization by current flow of very brief duration.Fluorescence The emission of visible radiation by a substance as the result of, and only during,the absorption of black light radiation.Fluorescent Magnetic Particle Inspection The magnetic particle inspection process employing afinely divided fluorescent ferromagnetic Inspection medium that fluoresces when activated byblack light of 3200 to 4000 Angstroms.Flux Density This is the flux-per-unit area through an element which cuts the unit area at rightangles to the direction of the flux. Flux density is usually designated by the letter B and its unit isthe gauss.Flux Leakage Magnetic lines of force which leave and enter an article at poles on the surface.Flux Lines Imaginary magnetic lines used as a means of explaining the behavior of magneticfields. Their conception is based on the pattern of lines produced when iron filings are sprinkledover a piece of paper laid over a permanent magnet. Also called lines of force, the unit is asingle line of force called the Maxwell designated by the Greek letter Phi().Flux Penetration, Magnetic The depth to which a magnetic flux is present in an article.Furring Build-up, or bristling, of magnetic particles due to excessive magnetization of the articleunder examination resulting in a furry appearance also referred to as Fur or Grass.Gauss The unit of flux density. Numerically, one gauss is one line of flux per square centimeterof area and is designated by the letter B.Heads The clamping contacts on a stationary magnetizing unit.


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Head Shot A short pulse of magnetizing current passed through an article or a central conductorwhile clamped between the head contacts of a stationary magnetizing unit for the purpose ofcircularly magnetizing the article.Horseshoe Magnet A bar magnet, bent into the shape of a horseshoe so that the two poles areadjacent. Usually the term applies to a permanent magnet.Hysteresis1) The lagging of the magnetic effect when the magnetic force acting upon a ferromagnetic

body is changed.2) The phenomenon exhibited by a magnetic system wherein its state is influenced by its

previous magnetic history.Hysteresis Loop A curve showing the flux density, B, plotted as a function of magnetizing force,H. As the magnetizing force is increased to the saturation point in both the positive, negative,and positive direction sequentially, the curve forms a characteristic S-shaped loop. Intercepts ofthe loop with the B and H axes and the points of maximum and minimum magnetizing forcedefine important magnetic characteristics of the material.Indication Any magnetically held magnetic particle pattern on the surface of an article beingtested.Inductance The magnetism produced in a ferromagnetic body by some outside magnetizingforce. The magnetism is not the result of passing current through the article.Inspection The process of examining and checking materials and articles for possible defects orfor deviation from established standards.Interpretation The determining of the cause and significance of indications of discontinuitiesfrom the standpoint of whether they are detrimental defects or false or non-relevant indications.Leakage Field The magnetic field forced out into the air by the distortion of the field within anarticle.Lines of Force. See Flux Lines.Longitudinal Field See Field, Longitudinal Magnetic.Longitudinal Magnetization The process of inducting a magnetic field into the article such thatthe magnetic lines of force extending through the article are approximately parallel to the axis ofthe magnetizing coil or to a line connecting the two poles when yokes (electromagnets) are used.Magnet, Permanent A highly retentive metal that has been strongly magnetized for example,the alloy Alnico.Magnetic Field See Field, magnetic.Magnetic Field Meter An instrument designed to detect and/or measure the flux density andpolarity of magnetic fields.Magnetic Field Strength The measured intensity of a magnetic field at a point always external tothe magnet or conductor usually expressed in Oersted.


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Magnetic Material Some materials are attracted by a magnet while others are repelled. Fromthe definition of magnetism it follows that magnetic materials are those that are attracted bymagnetism. These materials are known as paramagnetic materials, whereas materials whichrepel are known as diamagnetic materials. The subdivision of paramagnetic, calledferromagnetic, is a main concern as only ferromagnetic materials can be strongly magnetized.Magnetic Particle Inspection A nondestructive inspection method for locating discontinuities inferromagnetic materials. It utilizes flux leakage that forms magnetic poles to attract finelydivided magnetic particles which mark the discontinuity.Magnetic Particle Inspection Indications The accumulation of ferromagnetic particles that maybe either true indications of discontinuities, or may be false or non-relevant indications.Magnetic Writing A form of non-relevant indications caused when the surface of a magnetizedpart comes in contact with ;another piece of ferromagnetic material that is magnetized to adifferent value.Magnetizing Current The flow of either alternating, rectified alternating or direct current used toinduce magnetism into the article being inspected.Magnetizing Force This is the total force tending to set up a magnetic flux by a magnetizingcurrent. It is usually designated by the letter H and its unit is the Oersted.Non-relevant Indication A magnetic particle indication due to a leakage magnetic field which isnot caused by an actual discontinuity in the magnetized material, but by some other conditionwhich does not affect the usefulness of the article (such as a change of section).Oersted A unit of field strength which produces magnetic induction and is designated by theletter H.Paramagnetic Materials which are slightly attracted by a magnetic field. Examples arechromium, manganese, and aluminum.Paste (Slurry) Finely divided, ferromagnetic particles in paste form used in preparing wetsuspensions.Permeability

1) The ease with which a material can become magnetized.2) The ratio between field strength produced and the magnetizing force (B/H).3) The ratio of flux density produced to magnetizing force.

Pole The area on a magnetized article from which the magnetic field is leaving or returning tothe article.Prods Hand-held electrodes attached to cables used to transmit the magnetizing current fromthe source to the article under inspection.Rectified Alternating Current Alternating current which has been converted into direct current.Reluctance The opposition of a magnetic material to the establishment of magnetic flux. Thereluctance of the material determines the magnitude of the flux produced by a given magneticforce. Reluctance is analogous to the resistance; in an electric circuit.


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Residual Field See Field, Residual Magnetic.Residual Magnetism The amount of magnetism that a magnetic material retains after themagnetizing force is removed also called residual field.Residual Method A procedure in which the indicating material is applied after the magnetizingforce has been discontinued.Resultant Field See Field, Resultant Magnetic.Retentivity The ability of a material to retain a portion of the magnetic force induced in it afterthe magnetizing force has been removed.Saturation The point in the magnetization of a magnetizable article at which an increase in themagnetizing force produces no increase in the magnetic field within the article.Sensitivity The capacity of degree of responsiveness to magnetic particle inspection.Solenoid (Coil) An electric conductor formed into a coil; often wrapped around a central core ofhighly permeable material.Subsurface Discontinuity Any discontinuity which does not open onto the surface of the articlein which it exists.Suspension The correct term applied to the liquid bath in which is suspended the ferromagneticparticles used in the wet magnetic particle inspection method.Swinging Field Magnetization Magnetic fields induced in two different directions alternatelyand quickly to detect, more accurately, defects oriented in different directions in an article.Test Piece An article containing known artificial or natural defects used for checking theefficiency of magnetic particle flaw detection processes.Vector Field See Field, Resultant Magnetic.Wet Method The inspection method employing ferromagnetic particles suspended in a liquid(oil or water) as a vehicle.Yoke A U or C shaped piece of highly permeable magnetic material, either solid orlaminated, sometimes with adjustable pole pieces, around which is wound a coil carrying themagnetizing current.Yoke Magnetization A longitudinal magnetic field induced in an article, or in an area of anarticle, by means of an external electromagnet shaped like a yoke.

CONVERSION TABLE1 m 1 000 mm 1 000 000 m 1 000 000 000 nm1 Bar 14.5 Psi 100 000 Pascal3650 365 nm10 W/m 1 000 w/cm

1 fc (foot-candle) 10,76 lx (lux)1 C (? F - 32) x 5/9


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CHAPTER 3 MAGNETIC PROPERTIESMAGNETISM

All materials consist of atoms and molecules which may or may not have a permanent magneticinfluence depending on the electron configuration within the material.Atoms in magnetic materials group together in regions called magnetic domains; each domainhas its own north and South Pole. When these domains are randomly positioned, the material isunmagnetized. If the domains are aligned in a common direction, then the material will bemagnetised and the material itself will have its own north and South Pole.

The domains can be aligned by bringing them within an existing magnetic field. If the domainsremain aligned when they are removed from the influence of the magnetic field, then thematerial is said to be permanently magnetised.The poles of magnetised materials have an inherent attraction/ repulsion effect. If two pieces ofmagnetised material are placed with their dissimilar poles end to end there is an attraction, but ifthe poles are alike then there is a repulsion, therefore: like poles repel, unlike poles attract.

When two magnetizing fields are imposed simultaneously in the same part, the object is notmagnetized in two directions at the same time. A vector field is formed which is the resultantdirection and strength of the two imposed fields. This is illustrated in below where A is the firstmagnetizing force, B is the second force, and C equals the resultant magnetizing force.


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LINES OF FORCE

Faraday used the concept of lines of force to explain what happens in the space between twomagnets. He suggested properties for these lines of force, which he imagined as spreading outfrom all magnetic poles into the surrounding space.

It can be seen in themagnetograph that there are poles all along the length of the magnet butthat the poles are concentrated at the ends of the magnet. The area where the exit Poles areconcentrated is called the magnet's North Pole and the area where the entrance poles areconcentrated is called the magnet's South Pole.

By assuming the lines were in tension, like pieces of stretched elastic, he could account for theattraction of unlike poles, since the lines of force stretch from one pole to another.

But, the tension of the lines of force must increase as they shorten, since the repulsive forcebetween two like poles increases as the poles approach. With like poles, the lines of force alsotend to curve outwards, seeming to suggest that they repel each other. Faraday used thishypotheses to account for the repulsion of like poles.


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The properties ofmagnetic lines of force are as follows:

They form closed loops between north and south poles. They do not cross one another. (Repel each other laterally) They seek paths of least magnetic resistance. Their density decreases with increasing distance from the poles, i.e. the number of lines

of force in a unit area decreases. They are considered to have direction, that is: from the north pole to South Pole

external to the magnet, and from the South Pole to the North Pole within the magnet. They are in a constant state of tension.


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MATERIAL PROPERTIES

The degree to which materials are capable of being influenced by a magnetic field varies greatlyfrom material to material, however, they fall into three specific categories defined by theirbehaviour in the magnetic field.

Diamagnetic materialsThese are, to a very slight degree, repelled by a magnetic field and include copper, titanium andmost non-metals. ( < 1)

Paramagnetic materials

These are very weakly attracted by a magnetic field and include oxygen and most metalsincluding austenitic stainless steel, magnesium, molybdenum, lithium and tantalum. ( 1)

Ferromagnetic materials

These are strongly attracted by a magnetic field and include iron, cobalt, nickel and many of theiralloys. They also exhibit permanent magnetism and can themselves be magnetized. ( > 1 ; 240or more)


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PERMEABILITY ()

For magnetic particle inspection, the only materials of interest are those which areferromagnetic. Within this group, some materials are more easily magnetised than others, that isto say, more permeable.

To permeate means to spread through. In this context it refers to the ease by which the magneticlines of force are spread through the material. Soft iron and low carbon steel have a highpermeability, i.e. they are easy to magnetise. Hard iron and high carbon steel have a lowpermeability, i.e. they are difficult to magnetise.

An alternative description favoured in the USA would be, the ability to concentrate magneticfields and it is shown on the 'slope' of the B/H curve which varies continuously.Permeability () may be calculated by dividing the flux density (B) achieved by the magnetisingforce applied (H).

=

The permeability of a material may be given a value based on a ratio when compared with freespace. These values vary depending on alloy composition, heat treatment and any workingapplied.

RELUCTANCE (R)

Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magneticcircuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electricenergy it stores magnetic energy. In likeness to the way an electric field causes an electric currentto follow the path of least resistance, a magnetic field causes magnetic flux to follow the path ofleast magnetic reluctance.

Reluctance is the reciprocal of permeability, i.e.

R =

RETENTIVITY

When a magnetising force is removed from a ferromagnetic material the amount of magnetismremaining will vary between materials and depends upon the permeability of the material. Theremaining magnetism is termed residual magnetism and the material is said to have retentivity orretained magnetism.

If a material has high permeability it is very difficult to magnetise, that is to say it has highmagnetic reluctance, but once magnetisation has been achieved then it does not give up themagnetic force easily, therefore it has high retentivity.

High permeability = Low retentivityLow permeability = High retentivity


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RESIDUAL MAGNETISM

Remanence or remanent magnetization is the magnetization left behind in a ferromagneticmaterial after an external magnetic field is removed.

The equivalent term residual magnetization is generally used in engineering applications. Intransformers, electric motors and generators a large residual magnetization is not desirable as itis an unwanted contamination, for example a magnetization remaining in an electromagnet afterthe current in the coil is turned off. Where it is unwanted, it can be removed by demagnetisation.

COERCIVE FORCE

Coerce means to forcibly control; in this context it relates to the reversed magnetising forcewhich is necessary to remove remnant or residual magnetism for demagnetisation of a part.

To summarise:

PERMEABILITY This refers to the ease with which a magnetic flux is established inthe article being inspected.

RELUCTANCE This is the opposition of a magnetic material to the establishment ofa magnetic flux. A material with a high permeability will have a lowreluctance.

RESIDUAL MAGNETISM This refers to the amount of magnetism retained after themagnetizing force is removed.

RETENTIVITY Refers to the ability of the material to retain a certain amount ofresidual magnetism.

COERCIVE FORCE Refers to the reverse magnetizing force necessary to remove theresidual magnetism from the part.

MAGNETIC FLUX () AND MAGNETIC FLUX DENSITY (B)

When a specimen is magnetised, lines of force or flux exist within the specimen the stronger themagnetising force applied, the greater the amount of flux produced. The magnetising force maybe applied by using a permanent magnet or electrically operated magnetic flow apparatus, or bypassing an electric current through the specimen.

Magnetic flux is measured in Webers (Wb).

1 Wb = 108 lines of force

The number of lines of force (or flux) passing transversely through a given cross-sectional area isknown as the flux density (B)

Flux density (B) = Where: = flux

A = Area


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Flux density is measured in Tesla (T)

1 Wb/m2 = 1 Tesla (T).

The old (cgs) unit for flux density which is still widely encountered is the Gauss:

1 Gauss = 1 line of force/cm2.10,000 (104) Gauss = 1 Tesla1 Gauss = 0.1 mT

MAGNETIC FIELD STRENGTH (H)

The magnetic field strength or magnetising force is that which is needed to induce a flux in amagnetic circuit and is measured in amperes per metre (A/m), or in old (cgs) units, the Oersted.

1 Oersted =79.58 Amperes per metre.

MAGNETIC HYSTERESISA great deal of information can be learned about the magnetic properties of a material bystudying its hysteresis loop. A hysteresis loop shows the relationship between the inducedmagnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop. Anexample hysteresis loop is shown below.

The loop is generated by measuring the magnetic flux of a ferromagnetic material while themagnetizing force is changed. A ferromagnetic material that has never been previouslymagnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. Asthe line demonstrates, the greater the amount of current applied (H+), the stronger the magneticfield in the component (B+). At point "a" almost all of the magnetic domains are aligned and an


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additional increase in the magnetizing force will produce very little increase in magnetic flux. Thematerial has reached the point of magnetic saturation. When H is reduced to zero, the curve willmove from point "a" to point "b." At this point, it can be seen that some magnetic flux remains inthe material even though the magnetizing force is zero. This is referred to as the point ofretentivity on the graph and indicates the Remanence or level of residual magnetism in thematerial. (Some of the magnetic domains remain aligned but some have lost their alignment.)As the magnetizing force is reversed, the curve moves to point "c", where the flux has beenreduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizingforce has flipped enough of the domains so that the net flux within the material is zero.) Theforce required to remove the residual magnetism from the material is called the coercive force orcoercivity of the material.As the magnetizing force is increased in the negative direction, the material will again becomemagnetically saturated but in the opposite direction (point "d"). Reducing H to zero brings thecurve to point "e." It will have a level of residual magnetism equal to that achieved in the otherdirection. Increasing H back in the positive direction will return B to zero. Notice that the curvedid not return to the origin of the graph because some force is required to remove the residualmagnetism. The curve will take a different path from point "f" back to the saturation point whereit with complete the loop.When AC is used for magnetising a specimen, a complete hysteresis loop is produced with eachcycle of current; in the U.K. this is 50 times per second.*A material which exhibits a hysteresis loop with a wide appearance will have high retentivity andtherefore may be useful for making permanent magnets. A material which exhibits a hysteresisloop with a narrow appearance will have low retentivity and therefore may be useful for makingmagnetic particles.

The gradient of the loop also gives information regarding the usefulness of materials for use inmagnetising apparatus, for making magnetic particles or for magnetising purposes. For example,a material which exhibits a steep gradient will attain a high flux density when using a lowmagnetising force.


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FLUX LEAKAGEWhen a magnetic field is created within a ferromagnetic within a ferromagnetic specimen, linesof magnetic flux are developed and flow through and around the material completing a circuit.Magnetic particle inspection relies on a leakage of flux occurring within this circuit, this may becaused by a break or discontinuity in the material.Because it is a change in magnetic permeability that causes a leakage field, flux leakages may alsobe caused by changes in metallurgy.

Opposite poles attract, therefore any break or discontinuity causing a flux leakage will, becauseof the magnetic poles, attract a ferromagnetic material such as iron powder. This wouldaccumulate at the area of the leakage field and give an indication of the defect's existence.

Where the flux leaves the circuit a North Pole is created.

Where the flux re-enters the circuit a South Pole is created.

For a discontinuity to be detected by MPI it must interrupt the lines of force. Any lineardiscontinuities running parallel with the flux or small non-linear discontinuities, i.e. equiaxeddefects do not break the lines of force, they bend around these defects taking the path of leastresistance; these discontinuities therefore remain undetected.

MPI is most effective in detecting discontinuities with their major axis at 90 to the lines of force,but will usually remain effective down to about 45 of this axis (BS EN ISO 17638: 2009 and BS ENISO 9934: 2001 quotes 60). Below that it is unlikely that the discontinuity will be found,therefore in order to examine a specimen completely, the lines of force must be applied indifferent directions.

Magnetic particle inspection used for the detection of surface breaking discontinuities and only inferromagnetic materials. This is because the magnetic fields induced are concentrated at thesurface of the components. However, sub-surface discontinuities may be detected if usingpermanent magnets or electrical systems using direct or rectified current, because the magneticfield penetrants much further into the test specimen in comparison with MPI test methods whichuse alternating current. It is unlikely that any form of MPI would be used to detect discontinuitiesdeeper than 2 mm or 3 mm below the surface.

MPI test equipment using alternating current as an output produces a high density magnetic fluxat the surface of the test component. This phenomenon, known as the skin effect, produces a farstronger flux leakage field on the surface breaking, or near surface discontinuities, compared topermanent magnets or direct current test equipment.


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The depth of flux penetration is governed by the wave frequency of the alternating current, theconductivity of the test material and its permeability. If any of these variables increase, the depthof penetration will decrease.

It is difficult to try and interpret very weak and diffused MPI indications which could be fromsources other than defects, e.g. caused by rough/ uneven surfaces or changes in permeability.

Because of this problem, sub-surface, or body defects, would normally be located by othermethods of NDT, assuming the detection of sub-surface defects is a requirement.

ELECTROMAGNETISMThere is a fundamental relationship between electricity and magnetism; the movement of anelectric charge will create a magnetic force field around it, in a plane perpendicular to thedirection of travel of the electric charge.


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Electrons that are moving in a current carrying conductor set up a magnetic field, the directionand orientation of which are given by the right hand rule if we assume the current flow, byconvention, is opposite to electron flow; or the left hand rule if we assume the direction ofelectron flow.

Current Flow Theory = Right Hand Rule = + to Electron Flow Theory = Left Hand Rule = to +

When a current carrying conductor is formed into a loop or several loops to form a coil, amagnetic field develops that flows through the center of the loop or coil along its longitudinalaxis and circles back around the outside of the loop or coil. The magnetic field circling each loopof wire combines with the fields from the other loops to produce a concentrated field down thecenter of the coil. A loosely wound coil is illustrated below to show the interaction of themagnetic field. The magnetic field is essentially uniform down the length of the coil when it iswound tighter.

The strength of a coil's magnetic field increases not only with increasing current but also witheach loop that is added to the coil. A long, straight coil of wire is called a solenoid and can beused to generate a nearly uniform magnetic field similar to that of a bar magnet. Theconcentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials forinspection using the magnetic particle testing method. Please be aware that the field outside thecoil is weak and is not suitable for magnetizing ferromagnetic materials.


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CHAPTER 4 CURRENT TYPESAs seen in the previous pages, electric current is often used to establish the magnetic field incomponents during magnetic particle inspection. Alternating current and direct current are thetwo basic types of current commonly used. Current from single phase 110 volts, to three phase440 volts, are used when generating an electric field in a component. Current flow is oftenmodified to provide the appropriate field within the part. The type of current used can have aneffect on the inspection results, so the types of currents commonly used will be briefly reviewed.

Direct CurrentDirect current (DC) flows continuously in one direction at a constant voltage. A battery is themost common source of direct current. As previously mentioned, current is said to flow from thepositive to the negative terminal. In actuality, the electrons flow in the opposite direction. DC isvery desirable when inspecting for subsurface defects because DC generates a magnetic field thatpenetrates deeper into the material. In ferromagnetic materials, the magnetic field produced byDC generally penetrates the entire cross-section of the component. Conversely, the fieldproduced using alternating current is concentrated in a thin layer at the surface of thecomponent.

Alternating CurrentAlternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. In the UnitedStates, 60 cycle current is the commercial norm but 50 cycle current is common in manycountries. Since AC is readily available in most facilities, it is convenient to make use of it formagnetic particle inspection. However, when AC is used to induce a magnetic field inferromagnetic materials, the magnetic field will be limited to narrow region at the surface of thecomponent. This phenomenon is known as the "skin effect" and occurs because the changingmagnetic field generates eddy currents in the test object. The eddy currents produce a magneticfield that opposes the primary field, thus reducing the net magnetic flux below the surface.Therefore, it is recommended that AC be used only when the inspection is limited to surfacedefects.

Rectified Alternating CurrentClearly, the skin effect limits the use of AC since many inspection applications call for thedetection of subsurface defects. However, the convenient access to AC, drives its use beyondsurface flaw inspections. Luckily, AC can be converted to current that is very much like DCthrough the process of rectification. With the use of rectifiers, the reversing AC can be convertedto a one directional current. The three commonly used types of rectified current are describedbelow.


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Half Wave Rectified Alternating Current (HWAC)When single phase alternating current is passed through a rectifier, current is allowed to flow inonly one direction. The reverse half of each cycle is blocked out so that a one directional,pulsating current is produced. The current rises from zero to a maximum and then returns tozero. No current flows during the time when the reverse cycle is blocked out. The HWAC repeatsat same rate as the unrectified current (60 hertz typical). Since half of the current is blocked out,the amperage is half of the unaltered AC.This type of current is often referred to as half wave DC or pulsating DC. The pulsation of theHWAC helps magnetic particle indications form by vibrating the particles and giving them addedmobility. This added mobility is especially important when using dry particles. The pulsation isreported to significantly improve inspection sensitivity. HWAC is most often used to powerelectromagnetic yokes.

Full Wave Rectified Alternating Current (FWAC) (Single Phase)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. Filtering is usually performedto soften the sharp polarity switching in the rectified current. While particle mobility is not asgood as half-wave AC due to the reduction in pulsation, the depth of the subsurface magneticfield is improved.

Three Phase Full Wave Rectified Alternating CurrentThree phase current is often used to power industrial equipment because it has more favorablepower transmission and line loading characteristics. This type of electrical current is also highlydesirable for magnetic particle testing because when it is rectified and filtered, the resultingcurrent very closely resembles direct current. Stationary magnetic particle equipment wired withthree phase AC will usually have the ability to magnetize with AC or DC (three phase full waverectified), providing the inspector with the advantages of each current form.


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CHAPTER 5 MAGNETIZING METHODS

CONTINUOUS MAGNETISATION METHODThe continuous method is a testing method by which the magnetic ink or powder is applied toprior to and during energisation and the test area is viewed whilst the magnetising force isapplied. This is always considered to be the most sensitive method, whatever apparatus is usedto magnetise, due to the fact that the induced magnetic field is always the strongest whilst themagnetising force is being applied.

RESIDUAL MAGNETISATION METHODThe residual method is a testing method where the magnetic ink or powder is applied and thetest area is viewed after the magnetising force has been removed. The test is performed usingthe residual magnetism left in the sample. This method is generally only used on materials with ahigh retentivity. When multiple items are being tested by the residual method, care must betaken to ensure that the components do not come into contact with each other before thedetecting media is applied otherwise a phenomena know asmagnetic writing will occur.

LONGITUDINAL MAGNETISATIONWhen the length of a component is several times larger than its diameter, a longitudinalmagnetic field can be established in the component. The component is often placedlongitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. Thismagnetization technique is often referred to as a "coil shot." It can be accomplished by placing apart in a fixed coil or wrapping the part with flexible cable.

Another method of longitudinally magnetising a part or rather a section of the part is by usingPermanent magnets or Electromagnetic Yokes. These methods is by far the most widely usedtoday, especially for site inspections.


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PERMANENT MAGNETSPermanent magnets are so called because they are able to maintain a magnetic field in thesurrounding space. The field strength can vary considerably, depending on the flux density in themagnet and its shape.The simplest form of penetrant magnet is a bar magnet, which is basically a piece offerromagnetic material with a magnetic pole at each end.If the bar was formed into a closed loop, then the magnetic field would be fully contained withina closed circuit and no external field would exist. If a defect was present in the loop, a fluxleakage would still occur.Neither bar magnets or ring (looped) magnets have any use in MPI. But, if a bar magnet is simplyformed into a U shape, the magnetic lines of force will be concentrated in the gap between themagnetic poles; this provides an ideal configuration for magnetic particle inspection.

Permanent magnets provide magnetic flow only in the specimen and produce a longitudinalmagnetic field between the poles.The degree of magnetisation in permanent magnets is determined by the amount of pullrequired to lift the magnet clear of the work piece, or by its lifting power.The pull off force is the force that has to be applied to one pole to break its attraction to thesurface, whilst leaving the other pole attracted.The lifting power is the ability of the magnet to lift a piece of ferromagnetic material by attractionalone.Certain specifications will state the minimum requirements for the strengths of permanentmagnets. When not in use a permanent magnet should have a keeper placed across the poles toprevent loss of magnetism.Some permanent magnets may have adjustable arms, others may have rollers attached to thepoles; the rollers are set to keep the magnet just clear of the surface and enable it to be movedover the work piece with relative ease.


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Advantages of permanent magnets include: No power supply required. Inexpensive. No damage to the test piece from arcing. Relatively lightweight (easily portable). They cling to vertical and overhead surfaces. Both hands free after the magnet is placed onto the surface.

Disadvantages include: Deterioration with wear/ abuse. Have to be pulled from the test surface. Magnetic particles attracted to poles. Limited application on awkward shapes. No control over field strength (unless adjustable arms are used). Only small areas examined in each position. Keeper required when not in use. Not recommended to be used in conjunction with flux indicators. Toxic material when machined. Very hard. Low flux density unless rare earth magnets used.

ELECTROMAGNETIC YOKESElectromagnetic yokes or electromagnets require a source of electrical energy which may be ACor DC. The test method used is sometimes referred to as the magnetic flow or magnetic flux pathmethod, producing a longitudinal magnetic field.

The yoke is made from highly permeable, low retentive steel, which is laminated to reduceinduction caused by eddy current flow (associated only with alternating current) this also helps toprevent the yoke becoming permanently magnetized.Magnetism is induced into the yoke by encircling it with a coil through which a current is passed,the strength of the field produced can be varied in one of two ways:

1. By adjusting the current (amperage) flowing through the yoke - only certainelectromagnets allow for this.

2. By varying the distance between the poles - most electromagnets allow for this but notat all.


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Certain specifications will state the minimum requirements for the lifting power of Yokes.Electromagnets may operate direct from the mains supply of 240V but are available at 110V(battery packs are now available at 12/ 24/ 36V for more flexibility) when required for site use.The field produced from an electromagnetic yoke is longitudinal, travelling from pole to pole aswith permanent magnets, however the depth of the field within the test piece will depend uponthe type of current used to induce magnetism.Surface discontinuities will be more readily found using AC, sub-surface defects will be moreeasily located using DC, but Yokes is usually accepted as a surface technique only. The magnetwill have a much greater pull on DC but the flux will be less at the surface of the componentbeing tested.The area of inspection for electromagnets is a rectangular area between the poles of themagnet(s).

Advantages of electromagnetic yokes include: AC or rectified DC operation. Controllable field strength (not in all cases). Can be switched on/off as required. No damage done to test piece. Relatively lightweight. AC yokes can be used to demagnetise.

Disadvantages include: Power supply required. Only small areas can be examined at each magnet location. Leaves only one hand free. DC yokes are not recommended to be used in conjunction with flux indicators.


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Typical Permanent Magnet and Yoke placement when testing welds.


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COIL TECHNIQUE

RIGID (FIXED) COILThis technique consists of placing the specimen inside a coil of tubular or solid construction,through which a low voltage, high amperage current is passed. The magnetic field passingthrough the centre if the coil - typically three or five turns - creates longitudinal magnetizationand is therefore used to detect discontinuities which lie transverse to the components major axis,e.g. good for detecting circumferential discontinuities in shafts or the bores of tubes.

For practical purposes only defects which lie within the confines of the coil should be interpretedalthough the field will extend for 100 to 225 mm beyond either end.If the specimen being tested has a small diameter in relation to the inside diameter if the coil, itshould ideally be positioned close to one side of the coil and rotated to obtain the best results.NB. The strongest magnetic field is on the inside edge of the coil.

When using any of the current flow or threaded bar methods, the fields strength is largelydetermined by the current (amperes) flowing in the circuit. When using any form of coil the fieldstrength is determined by the current flowing in the circuit and by the number of turns in thatcoil, thereby obtaining ampere/ turns. These requirements will be specified by the ProcedureCode used.


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FLEXIBLE CABLESWhen a single conductor is used, the magnetic field reduces rapidly at increasing distance fromthe conductor; this restricts the production of an adequate test area with a sufficient constantmagnetic field. If the current is made to flow in the same direction through conductors spacedsome distance apart, a relatively constant field is produced.Flexible cable techniques can be used on a considerable variety of component shapes.Configurations used are normally obtained with a heavy insulated flexible cable which is placedthrough, on, or around the specimen. A current passed through the cable will then induce amagnetic field into the test piece.

Defects lying parallel to the cable will be the most readily detected. On complex shapes theposition and method in which the cable is wound may have to be found by experimentation toensure an adequate field in all areas.

Current values to be used shall be specified by the Procedure Code. Values shall be calculatedconsidering the Length/Diameter Ratio of the part and the number of turns in the coil.

Advantages of using a Coil: Slightly Subsurface discontinuities may be found when using DC, HWDC or FWDC. AC energised equipment may be used for demagnetisation operations. Large areas inspected with each set-up. No poles to attract magnetic particles. Field strength can be altered. Predictable field strengths.

Disadvantages include: Cumbersome long heavy cables required. Longer setting up times. Heavy transformers required for large amperages. Expensive equipment.


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CIRCULAR MAGNETISATION

As discussed previously, when current is passed through a solid conductor, a magnetic field formsin and around the conductor. The following statements can be made about the distribution andintensity of the magnetic field.

The field strength varies from zero at the center of the component to a maximum at thesurface.

The field strength at the surface of the conductor decreases as the radius of the conductorincreases when the current strength is held constant. (However, a larger conductor iscapable of carrying more current.)

The field strength outside the conductor is directly proportional to the current strength.Inside the conductor, the field strength is dependent on the current strength, magneticpermeability of the material, and if magnetic, the location on the B-H curve.

The field strength outside the conductor decreases with distance from the conductor.

In the images below, the magnetic field strength is graphed versus distance from the center ofthe conductor. It can be seen that in a nonmagnetic conductor carrying DC, the internal fieldstrength rises from zero at the center to a maximum value at the surface of the conductor. Theexternal field strength decrease with distance from the surface of the conductor.When the conductor is a magnetic material, the field strength within the conductor is muchgreater than it is in the nonmagnetic conductor. This is due to the permeability of the magneticmaterial. The external field is exactly the same for the two materials provided the current leveland conductor radius are the same.


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When the conductor is carrying alternating current, the internal magnetic field strength risesfrom zero at the center to a maximum at the surface. However, the field is concentrated in a thinlayer near the surface of the conductor. This is known as the "skin effect." The skin effect isevident in the field strength versus distance graph for a magnetic conductor shown to the right.The external field decreases with increasing distance from the surface as it does with DC. Itshould be remembered that with AC the field is constantly varying in strength and direction.

In a hollow circular conductor there is no magnetic field in the void area. The magnetic field iszero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. Aswith a solid conductor, when the conductor is a magnetic material, the field strength within theconductor is much greater than it was in the nonmagnetic conductor due to the permeability ofthe magnetic material. The external field strength decreases with distance from the surface ofthe conductor. The external field is exactly the same for the two materials provided the currentlevel and conductor radius are the same.


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When AC is passed through a hollow circular conductor, the skin effect concentrates themagnetic field at the outside diameter of the component.As can be learned from these three field distribution images, the field strength at the insidesurface of hollow conductor is very low when a circular magnetic field was established by directmagnetization. Therefore, the direct method of magnetization is not recommended wheninspecting the inside diameter wall of a hollow component for shallow defects. The field strengthincreases rapidly as one moves out (into the material) from the ID, so if the defect has significantdepth, it may be detectable.

However, a much better method of magnetizing hollow components for inspection of the ID andOD surfaces is with the use of a central conductor. As can be seen in the field distribution imageto the right, when current is passed through a nonmagnetic central conductor (copper bar), themagnetic field produced on the inside diameter surface of a magnetic tube is much greater andthe field is still strong enough for defect detection on the OD surface.


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PROD TECHNIQUEWith this technique the current is introduced into the item under test by using electrical contactsknown as prods. Prods induce a circular magnetic field within the specimen using current valuestypically in the region of 1000 amps; at this current level arcing can occur between the electrodesand the test surface causing damage. To prevent this possible damage, the prod contact tips andthe test surface must be kept clear of any contamination and the current must not be switchedon until firm contact has been established, likewise, the current should be switched off beforelifting the prods.Precautions when using current flow in respect to prods/clamps shall be taken to preventexcessive heating, burning or arcing. Certain metals including copper and zinc (includinggalvanised prod tips) may, if used as prod material, contaminate and cause metallurgical damageto the component if arcing occurs. For this reason and the fact that perfect contact is difficult toachieve with prods, ideally they shall be made of steel or aluminium. Zinc shall not be used andcopper or copper-tipped prods shall be used only in applications where complete assurance canbe given that metallurgical damage will not occur. The cleanliness of both prod contact faces andthe component shall be such to ensure good electrical contact. Prods shall have a minimumdimension of 10 mm and shall have as large a contact area as possible. Arcing or excessiveheating shall be regarded as a defect requiring a verdict of acceptability. If further testing isrequired on such affected areas, it shall be carried out using a different technique.Note: Lead contact pads may be used, but only in well ventilated conditions, because they maygenerate harmful vapour which may cause headaches and/or dizziness.Since the lines of force radiate from prods, correct positioning is essential to ensure that allpossible defects are located. Ideally the prods should be in a line parallel to, and on the sameaxis, as the defects being sought.

Current requirements shall be specified by the Procedure Code and is based on the thickness ofthe part and the distance between the prods.


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Advantages of the prod technique include: AC, DC, HWDC, and FWDC equipment available. AC energised equipment may be used for demagnetisation operations. Low voltage output. No poles to attract magnetic particles. Variable field strength, on/off control. Can be used in confined spaces. Relatively fast coverage of area under test.

Disadvantage include: Risk of creating arc strikes (forming localised hard spots which may contain cracking). Heavy transformer required. Classed as a two person operation. Contacts and small test items can overheat. Careful positioning and spacing of prods required. Possible to switch on without creating a field. May leave residual field which interfere with next prod positions. Expensive equipment.


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CURRENT FLOW TECHNIQUE (DIRECT MAGNETISATION) (HEAD SHOT OR CLAMPS)

Current flow techniques produce a circular magnetic field by passing a current through the testpiece, i.e. concentric rings of magnetic lines of force radiate at 90 (perpendicular) to the currentflow.

The current flow is normally obtained from a transformer DC, HWDC, FWDC or 3 PHASE FWDC.The output voltage of current flow equipment is so low that there is no risk of electrical shock tothe operator from the equipment's specimen contact points or test specimen.The choice of power supply depends on the test requirements, i.e. using AC will reveal onlysurface discontinuities, thus not recommended, using DC or rectified DC will revealdiscontinuities typically up to 2 mm to 3 mm below the surface.An ammeter is usually incorporated in the equipment to indicate the amount if current flowingthrough the work piece.In fixed installations, i.e. bench equipment, the component is firmly clamped between contactheads. With portable equipment, electrical contact is made by the use of prods and/or clamps.Current flow can also be achieved in regularly shaped items, e.g. bar or tube, by applying contactsto the ends of a test piece and passing a high amperage, low voltage current through it. This setsup a circular field in the ferromagnetic material in a direction at 90 to the current flow, thereforethe technique is used for detecting defects parallel to, and up to 45, of current flow, e.g.longitudinal defects in bar. Copper gauze is usually placed between the contracts and the testpiece to increase the contract area and reduce the possibility of burning.Irregular shaped items may also be tested by contact heads, although, depending on thecomponent's shape and dimensions, it may be preferable to use an alternative method.Because the current values are dependant only on the test piece perimeter, the length of the testpiece is of no importance, i.e. on a test piece of 25 mm diameter, the same current value wouldbe used whether it was 10 cm long or 1 m long, therefore, if two test pieces of differingdiameters using the same current, the magnetic field would be stronger in the smaller diametertest piece. Specifications usually stipulates the current requirements.


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Advantages of the Head Shot technique include: AC, DC, HWDC, and FWDC equipment available. AC energised equipment may be used for demagnetisation operations. Low voltage output. No poles to attract magnetic particles. Variable field strength, on/off control. Relatively fast coverage of area under test.

Disadvantage include: Risk of creating arc strikes (forming localised hard spots which may contain cracking). Heavy transformer required. Contacts and small test items can overheat. Possible to switch on without creating a field. May leave residual fields. Expensive equipment.


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CENTRAL CONDUCTOR TECHNIQUE (INDUCTION METHOD)

Induction MPI methods do not necessarily require any contact between the magnetisingapparatus and the test specimen.Sometimes known as the central conductor method, although the conductor need not always becentral.The object being examined must be of hollow section and access must be available to both ends,providing these limitations are met, then a conductor - typically made of brass, copper oraluminium - is threaded through the bore, or aperture, and a current passed through it.This sets up a circular field in the surrounding ferromagnetic material in a direction at 90 to thecurrent flow, therefore the technique is used for detecting defects parallel to, and up to 45, ofthe current flow, e.g. longitudinal pipe defects.

Conductor may be located centrally to the specimen, but on larger diameters the conductor isoften placed to one side to ensure sufficient flux strengths and the test piece rotated to allow forsurface inspection. Alternatively, two conductors may be used on larger diameter test pieces.The threaded bar technique is ideal for the testing of ring like specimens, especially becausenumerous samples may be tested at the same time; lengths of pipe may also be examined by thismethod. A hollow part can be examined for discontinuities on the inside diameter of the part aswell as on the outside, when practical.On site work, this technique is not widely encountered, but could not be modified by using aflexible cable instead of rigid conductor.When using a bar that is not covered with insulating material, care should be taken to ensurethat components in contact with the bar cannot touch any part of the magnetic equipment atearth potential.

Current requirements is based on the outside diameter of the part, similarly to when using theheadshot and will be specified by the Procedure Code.


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Advantages of the Central Conductor technique include: AC, DC, HWDC, and FWDC equipment available. AC energised equipment may be used for demagnetisation operations. Low voltage output. No poles to attract magnetic particles. Variable field strength, on/off control. Relatively fast coverage of area under test. No current through part.

Disadvantages include: Heavy transformer required. May leave residual fields. Expensive equipment.


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MULTIDIRECTIONAL TECHNIQUEFor this technique, magnetization is accomplished by high amperage power packs operating asmany as three circuits that are energized one at a time in rapid succession. The effect of theserapidly alternating magnetizing currents is to produce an overall magnetization of the part inmultiple directions. Circular or longitudinal magnetic elds may be generated in any combinationusing the various techniques.


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Whichever technique or combination of techniques are used to produce the magneticux in the part, maximum sensitivity will be to linear discontinuities oriented perpendicular tothe lines of ux.For optimum effectiveness in detecting all types of discontinuities, each area is to be examined atleast twice, with the lines of ux during one examination being approximately perpendicular tothe lines of ux during the other.

One or more of the following techniques may be used:

LONGITUDINAL MAGNETISATION CIRCULAR MAGNETISATIONPERMANENT MAGNET PRODS

AC YOKE HEAD SHOTDC YOKE CLAMPSRIGID COIL BAR CENTRAL CONDUCTOR

CABLE WRAP COIL CABLE CENTRAL CONDUCTORMULTI DIRECTIONAL YOKE OR BENCH UNIT

TYPES OF POWER SUPPLIES

PORTABLE EQUIPMENT

Portable equipment is lighter and less expensive than theother types of magnetic particle testing equipment.Typical portable equipment operates on 220 volts AC withan output of between 500 and 3 000 amperes.These units usually have a choice of either AC or HWDC.As with mobile equipment, the cables can be used forprods, wrapping into a coil, or connecting to a centralconductor.

MOBILE EQUIPMENT

Typical mobile equipment usually operates on 220 /380 volts AC and will produce about 8 000 amperes.Mobile equipment will usually produce both AC andHWDC magnetizing current.The cables used on mobile equipment vary from 5meters to 30 meters. Shorter cables will permit themaximum current output. Prods and cables areusually used with the mobile equipment. However,longitudinal magnetization can be produced bywrapping the cable into the coil. It is also possible touse a central conductor clamped between the twocables to produce circular magnetization.


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STATIONARY EQUIPMENT

Stationary magnetic particle inspection equipment is designed for use in laboratory or productionenvironment. The most common stationary system is the wet horizontal (bench) unit. Wethorizontal units are designed to allow for batch inspections of a variety of components. The unitshave head and tail stocks (similar to a lathe) with electrical contact that the part can be clampedbetween. A circular magnetic field is produced with direct magnetization. The tail stock can bemoved and locked into place to accommodate parts of various lengths. To assist the operator inclamping the parts, the contact on the headstock can be moved pneumatically via a foot switch.

Most units also have a movable coil that can be moved into place so the indirect magnetizationcan be used to produce a longitudinal magnetic field. Most coils have five turns and can beobtained in a variety of sizes. The wet magnetic particle solution is collected and held in a tank. Apump and hose system is used to apply the particle solution to the components being inspected.Either the visible or fluorescent particles can be used. Some of the systems offer a variety ofoptions in electrical current used for magnetizing the component. The operator has the option touse AC, half wave DC, or full wave DC. In some units, a demagnetization feature is built in, whichuses the coil and decaying AC.


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CHAPTER 6 DEMAGNETISATIONAfter conducting a magnetic particle inspection, it is usually necessary to demagnetize thecomponent.Remanent magnetic fields can:

Affect machining by causing cuttings to cling to a component. Interfere with electronic equipment such as a compass. Create a condition known as "arc blow" in the welding process. Arc blow may cause the

weld arc to wonder or filler metal to be repelled from the weld. Cause abrasive particles to cling to bearing or faying surfaces and increase wear.

Each time the magnetizing field is reduced and reversed, the residual field is reduced.

REVERSING THE MAGNETIC FIELD Reversing the part in the magnetic field. Reversing the current through the coil. Reversing the coil (turn the coil 180).

REDUCING THE MAGNETIC FIELD Reduce the magnet current. Move the part away from the coil / yoke. Move the coil / yoke away from the part.

Any method of demagnetization will combine one of the methods to reduce the magnetizing fieldwith one of the methods to reverse the magnetizing field.

Removal of a field may be accomplished in several ways. This random orientation of the magneticdomains can be achieved most effectively by heating the material above its curie temperature.The Curie temperature for a low carbon steel is 770oC or 1390oF. When steel is heated above itscurie temperature, it will become austenitic and loses its magnetic properties. When it is cooledback down, it will go through a reverse transformation and will contain no residual magneticfield. The material should also be placed with it long axis in an east-west orientation to avoid anyinfluence of the Earth's magnetic field.


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It is often inconvenient to heat a material above its curie temperature to demagnetize it, soanother method that returns the material to a nearly unmagnetized state is commonly used.Subjecting the component to a reversing and decreasing magnetic field will return the dipoles toa nearly random orientation throughout the material. This can be accomplished by pulling acomponent out and away from a coil with AC passing through it. The same can also beaccomplished using an electromagnetic yoke with AC selected. Also, many stationary magneticparticle inspection units come with a demagnetization feature that slowly reduces the AC in a coilin which the component is placed.A field meter is often used to verify that the residual flux has been removed from a component.Industry standards usually require that the magnetic flux be reduced to less than 3 gauss aftercompleting a magnetic particle inspection. The Procedure Code will specify limits and differentprocedures as required.


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CHAPTER 7 DETECTION MEDIUMAs mentioned previously, the particles that are used for magnetic particle inspection are a keyingredient as they form the indications that alert the inspector to defects. Particles start out astiny milled (a machining process) pieces of iron or iron oxide. A pigment (somewhat like paint) isbonded to their surfaces to give the particles colour. The metal used for the particles has highmagnetic permeability and low retentivity. High magnetic permeability is important because itmakes the particles attract easily to small magnetic leakage fields from discontinuities, such asflaws. Low retentivity is important because the particles themselves never become stronglymagnetized so they do not stick to each other or the surface of the part. Particles are available ina dry mix or a wet solution.

DRY MAGNETIC PARTICLES

Dry magnetic particles can typically be purchased in red, black, gray, yellow and several othercolours so that a high level of contrast between the particles and the part being inspected can beachieved. The size of the magnetic particles is also very important. Dry magnetic particle productsare produced to include a range of particle sizes. The fine particles are around 0.05 mm (0.002inch) in size, and are about three times smaller in diameter and more than 20 times lighter thanthe coarse particles (0.15 mm). This make them more sensitive to the leakage fields from verysmall discontinuities. However, dry testing particles cannot be made exclusively of the fineparticles. Coarser particles are needed to bridge large discontinuities and to reduce the powder'sdusty nature. Additionally, small particles easily adhere to surface contamination, such asremnant dirt or moisture, and get trapped in surface roughness features. It should also berecognized that finer particles will be more easily blown away by the wind; therefore, windyconditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is notrecommended because the small particles are less likely to be recaptured and the "once used"mix will result in less sensitive inspections.


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The particle shape is also important. Long, slender particles tend align themselves along the linesof magnetic force. However, research has shown that if dry powder consists only of long, slenderparticles, the application process would be less than desirable. Elongated particles come from thedispenser in clumps and lack the ability to flow freely and form the desired "cloud" of particlesfloating on the component. Therefore, globular particles are added that are shorter. The mix ofglobular and elongated particles result in a dry powder that flows well and maintains goodsensitivity. Most dry particle mixes have particles with L/D ratios between one and two.

WET MAGNETIC PARTICLES

Magnetic particles are also supplied in a wet suspension such as water or oil. The wet magneticparticle testing method is generally more sensitive than the dry because the suspension providesthe particles with more mobility and makes it possible for smaller particles to be used since dustand adherence to surface contamination is reduced or eliminated. The wet method also makes iteasy to apply the particles uniformly to a relatively large area.

Wet method magnetic particles products differ from dry powder products in a number of ways.One way is that both visible and fluorescent particles are available. Most non-fluorescentparticles are ferromagnetic iron oxides, which are either black or brown in colour. Fluorescentparticles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles thatfluoresce green-yellow are most common to take advantage of the peak colour sensitivity of theeye but other fluorescent colours are also available.

The particles used with the wet method are smaller in size than those used in the dry method forthe reasons mentioned above. The particles are typically 0.01 mm and smaller and the syntheticiron oxides have particle diameters around 0.0001 mm. This very small size is a result of theprocess used to form the particles and is not particularly desirable, as the particles are almost toofine to settle out of suspension. However, due to their slight residual magnetism, the oxideparticles are present mostly in clusters that settle out of suspension much faster than theindividual particles. This makes it possible to see and measure the concentration of the particlesfor process control purposes. Wet particles are also a mix of long slender and globular particles.The carrier solutions can be water or oil-based. Water-based carriers form quicker indications,are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, andare easier to clean from the part. Water-based solutions are usually formulated with a corrosioninhibitor to offer some corrosion protection. However, oil-based carrier solutions offer superiorcorrosion and hydrogen embrittlement protection to those materials that are prone to attack bythese mechanisms.


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Suspension liquids used in the wet magnetic particle inspection method can be either a wellrefined light petroleum distillate or water containing additives. Petroleum-based liquids are themost desirable carriers because they provided good wetting of the surface of metallic parts.However, water-based carriers are used more because of low cost, low fire hazard, and theability to form indications quicker than solvent-based carriers. Water-based carriers must containwetting agents to disrupt surface films of oil that may exist on the part and to aid in thedispersion of magnetic particles in the carrier. The wetting agents create foaming as the solutionis moved about, so anti-foaming agents must be added. Also, since water promotes corrosion inferrous materials, corrosion inhibitors are usually added as well.Petroleum based carriers are primarily used in systems where maintaining the proper particleconcentration is a concern. The petroleum based carriers require less maintenance because theyevaporate at a slower rate than the water-based carriers. Therefore, petroleum based carriersmight be a better choice for a system that gets only occasional use or when regularly adjustingthe carrier volume is undes

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