Magnetic ParticleLevel 3

Review Key Terms and PrinciplesMagnetic DomainsMaterials that can be magnetized have atoms that group into submicroscopic regions called magnetic domains.

Review Key Terms and PrinciplesMagnetic PolesThe ability of a magnet to attract or repel is not uniform but is localized in areas called poles.

Review Key Terms and PrinciplesLines of Force (flux field)

Form continuous loopsThey do not cross one anotherLeave through north pole and enter at the south pole.Density decreases with increasing distance from the poles.Seek the path of least magnetic resistance.

Types of Magnetic MaterialsDiamagneticPermeability slightly less than that of a vacuum, there induced magnetic field is opposite the direction of iron.

ParamagneticPermeability is slightly greater than air, and become slightly magnetized.

FerromagneticPermeability much greater than air, and become highlymagnetized.

ParamagneticLiquid oxygen droplets are paramagnetic

Sources of MagnetismPermanent MagnetsHeat treating special alloys in a strong magnetic field

Magnetic Field of Earth

Mechanically Induced MagnetismCold working of ferromagnetic materials

Electrically Induced Magnetism

Flux Leakage

Field Direction

Circular Magnetism

Longitudinal Magnetism

Prods and Yoke

Units of MeasureMaxwellOne line of flux

Magnetic Flux DensityNumber of lines of flux through a unit area.Gauss = 1Maxwell / square centimeter

SI Units (metric)1 Weber = 108 lines of fluxTesla = 1 Weber/ square meter

Magnetic Flux DensityNikola TeslaCarl Friedrich Gauss1 Tesla = 104 Gauss

Review CalculationsB = /A where = flux (webers)A = area (square meters)B = flux density (weber/m2)

or = flux (Maxwels)A = area (cm2)B = flux density (Gauss)

Magnetic flux density is magnetic flux per unit area.

Review Calculations = B/H where = permeability (Gauss/ Oersteds)(weber/amp-m)B = flux density (Gauss)(weber/m2)H = magnetizing force (Oersteds)

Magnetic permeability is the ease with which a magnetic field can be established in a given circuit.

Flux DensityCentre of an air filled circular coilB = 0 NI 2r N = number of turnsI = current in amps0 = permeability of air (4 x 107 webers/amp-m)r = radius of coil

Flux DensityOn the axis of an air filled circular coilB = 0 NIr2 2(r2 + x2)3/2 N = number of turnsI = current in amps0 = permeability of air (4 x 107 webers/amp-m)r = radius of coilx = distance along axis

Ferromagnetic Material Characteristics

Ferromagnetic Material Characteristics

Ferromagnetic Material Characteristics

Ferromagnetic Material Characteristics

Ferromagnetic Material Characteristics

Ferromagnetic Material Characteristics

Magnetic PermeabilityThe ease of which a material can be magnetized.

Magnetizing Current

Half Wave Direct Current

Full Wave Rectification

Review Calculations= length of part/ diameter of part

L over D ratios of 3 to 15 are appropriate

For ratios lower than 3 a pole piece can be added to to increase the L/D ratio

For ratios higher than 15, a maximum value of 15 should be used for calculations and examine part in sections

Review Calculations= length of part/ effective diameter of part

= L / Deff

where Deff = ((OD)2-(ID)2)1/2

Review CalculationsInside coil area / area of part

Low fill factor = less than 10%

Intermediate fill factor = greater than 10% less than 50%

High fill factor = greater than 50%

Review CalculationsLongitudinal magnetization in a low fill factor coil, part close to the wall of the coil.

NI = 45000/(L/D)(+-10%)

Longitudinal magnetization in a low fill factor coil, part at the centre of the coil.

NI = 43000* R/((6L/D) 5) (+-10%)

Sample QuestionGiven a 10 turn 20 ID coil to longitudinally inspect a 15 inch long part, 3 inches in diameter what current is required if the part is a) at the centre of the coil and b) near the inside circumference of the coil?Fill Factor = area of part / area of coil = 7.065/314 = .0225 = 2.25%

Less than 10% therefore previous formula can be used.

Sample Question - AnswerNI = 43000* R/((6L/D) 5)

= 43000*10((6*15/3)-5

= 17200

I = 17200 / 10 = 1720 amps

NI = 45000 / L/D = 45000 / (15/3) = 9000

I = 9000 / 10 = 900 amps

Review CalculationsLongitudinal magnetization in an intermediate fill factor coil, part close to the wall of the coil.

NI = (NI)HF(10-Y)+(NI)LF(Y-2)/8 (+-10%)

where Y = ratio of cross sectional the coil to part

Review CalculationsLongitudinal magnetization in a high fill factor coil, part close to the wall of the coil.

NI = 35000/((L/D) +2) (+-10%)

Sample QuestionDetermine current required to inspect (longitudinally) a part 4 diameter, 15 long, the coil has 10 turns and is 5.5 IDCheck L/D ratio

Check fill factor

Determine which current formula

Determine current required

Sample Question - AnswerL/D = 15/4 = 3.75Fill factor = Ap / Ac = 12.56 / 23.75 = .53High fill factorNI = 35000/((L/D) +2) = 35000/ (3.75+2) = 6087 I = 6087/10 = 609 Amps

Magnetic Field DistributionsDirect Current, Nonmagnetic Solid Conductor

Magnetic Field DistributionsDirect Current, Nonmagnetic Hollow Conductor

Magnetic Field DistributionsDirect Current, Magnetic Solid ConductorWhere = permeability

Magnetic Field DistributionsDirect Current, Magnetic Hollow Conductor

Magnetic Field DistributionsDirect Current, Nonmagnetic Central Conductor, Hollow Magnetic Part

Magnetic Field DistributionsAlternating Current, Magnetic, Solid Conductor

Skin EffectMagnetic materials and nonmagnetic materials at high frequencies display the skin effect.

Alternating current tends to flow along the surface of a conductor and consequently the magnetic field is Strongest at the surface.

Magnetic Field DistributionsAlternating Current, Magnetic Hollow Conductor

Magnetic Field DistributionsDirect current, square shapes, circular magnetization

Magnetic Field DistributionsDirect current, rectangular shapes, circular magnetization

Prod Current LevelsSpacing: 2 to 8

Material or less90 to 115 Amps/ inch

Material greater than 100 to 125 Amps/inch

Yoke Lifting CapacityAC 2 to 6 spacing 10 poundsDC 2 to 4 spacing 30 poundsDC 4 to 6 spacing 50 pounds

Head Shot Current Requirements300 to 800 amps per inch diameter

Avoid overheating and arcing

Verify field strength

Central ConductorAC is suitable for ID only

If centrally located 300 to 800 amps per inch OD

Verify field strength

MagnetizingCurrent

MagnetizingCurrent

MagnetizingCurrentHalf Wave DCThe average current in the conductive cycle is representative of the effective magnetization.

The magnetizing ampere for half wave DC is then twice the current measured on a conventional DC meter.

Wet vs. DryAdvantages:Sensitive to very fine & shallow defectsGood surface coverageEasier to mechanize or automateFast for small partsGood reproducibilityMaterial often recovered and reusedGood particle mobility

Wet vs. DryDisadvantagesMessySensitivityPost cleaningExpensive

Wet vs. DryAdvantagesGeneral sensitivityEasy to use for portable testingGood particle mobility with ACSimpler & cheaperDisadvantagesNot as sensitive for fine cracksNot easily automatedSurface coverage

Wet vs. Dry Direct Current

Wet vs. Dry

Wet Bath (Water Vs. Oil Base)More costlyFlammability AvailabilityOdorMust be treated to:Improve surface wettingPrevent corrosion & eliminate foamingImprove dispersion of particles

Wet Bath ConsiderationsViscosityFlash PointOdourReactivityCorrosivenessFluorescents (Background noise)CostContaminationParticle concentration

Fluorescent Particle MethodUnder proper conditions even very small amounts of fluorescent material is easily seen. This results in an apparent increase in sensitivity.

Even on larger defects, indications are more easily seen resulting in apparent increase in reliability.

Inside drilled holes or roots of threads are more easily seen than visible colour indications.

Recording IndicationsSketches

Photographic visible or fluorescent

Tape transfer usually with dry particles

Magnetic Rubber

Fixing coatings

Continuous Vs. Residual MethodsMust have sufficient retentivity, used only for surface discontinuities.High retentivity usually associated with hardened steels which by there nature require higher magnetizing currents.

Continuous Vs. Residual MethodsWet or dryWet maybe immersed or curtain spray.Increased time can improve sensitivity.Particle build up greatest on upper horizontal surfaces.Rapid removal from immersion can wash off particles.More sensitive due to higher strength fieldUsually faster.Best for softer materials

Field Verification

Black Light (UV-A)3200 to 4000 Ao (Angstroms) Optimum 3650 AoUsually produced by Mercury vapour lamp with filter.Minimum intensity at 15 1000mW/cm2

Remove white light & harmful short wave ultra violet Clean, free from physical damage, properly fitting.

Black Light IntensityKnow the specificationDo not use damaged lights or filtersWarm up timeBackground white light (22 lux or 2 foot candles max)Position of the lampMeasure at the part (or specified distance)(watts/cm2)Record the information

White Light IntensityKnow the specificationDo not use damaged lightsWarm up timePosition of the lampMeasure at the part (or specified distance)(lux or foot candles)Record the information

Nonrelevant IndicationsCaused by magnetic field leakage not caused by discontinuities

Mask indications from actual discontinuitiesInterpreted as discontinuitiesDiscontinuities can be interpreted as nonrelevant

Nonrelevant IndicationsVariations in hardness from cold workingHeat affected zonesHigh internal or external stresses

Nonrelevant IndicationsObjects touching during magnetization(magnetic writing)Metal stampings that have been removed by grindingResidual magnetic fields

Nonrelevant IndicationsAbrupt changes in cross sectionSharp corners & keywaysShrink fitsRoots of threads

Nonrelevant IndicationsDissimilar metals fused togetherHeat affected zones

False IndicationsForeign materialScale from a forming operation

From damage or previous operationsUsing distinguished through visual exam

DemagnetizationMay affect instruments (aircraft, positioning equipment)Particles that adhere may cause damage to lubricants, coating, etc.Problems during subsequent machiningAttracts metallic debrisProblems during subsequent weldingMay cause problems with subsequent MPI testing in a different direction

DemagnetizationCurrie Point Heating

DemagnetizationElectromagnetic

DemagnetizationElectromagnetic

Field strength must start high enough to over come the residual field.Each cycle reduction must be small enough so that the reverse magnetic field coercive force.There are sufficient reversals to reduce the residual field to an acceptable level.

DemagnetizationAC Demagnetization

Pass object slowly through an AC Coil and away from the coil (usually two coil diameters)

Placing the component in an AC field and gradually decaying the field.

May not be successful on large objects because of the skin effect.

DemagnetizationDC Magnetization

Identical to the AC method of reversing and reducing the field.

Field usually reversed at 1 Hz

Overcomes the skin effect problem on larger parts.

DemagnetizationYoke Demagnetization

AC or reversing DC.

Yoke placed on surface, moved in a circular pattern and then withdrawn.

Care must be taken not to magnetize adjacent areas.

Multi Directional Magnetization Combined Direct Current

Combined Direct Current and Alternating Current

Combined Alternating Current

Multi Directional Magnetization

Multi Directional Magnetization

Multi Directional Magnetization

Multi Directional MagnetizationTest media must be applied during magnetization.

Excellent sensitivity because the field is perpendicular to all cracks at some point in its cycle.

Economic because only one magnetization is required.

For hollow objects a laminated steel copper centre conductor can be used to establish a field on the inside and outside surfaces.

AutomationType of field and direction, most often multidirectionalStrength and directionSequencing TimingApplication of the bathComponent HandlingSafety & damage to partsSpace requirementsAccurate repeatable control

Automation (Continued)System DesignPLC, inputs and outputsSystem monitoringMalfunction alarmsCurrent monitoringBath monitoringVerification of cleaning processVerification of magnetization

Automation (Continued)ObservationSource of UV (flood light or moving beam laser)Photo detector (photo tube, photo multiplier, vidicon camera)Imaging apparatus (television photo detector or flying spot system)Amplification and discrimination Signal processingOptical pattern recognitionAlgorithms Enhancement

Automated Systems350 Ford struts per hourSt. Catharines, OntarioFluorescent wet continuousMultidirectional, 4000 ampsPart comes in on conveyor, medium is applied and part magnetized and presented to an inspector. Rejects are separated by the inspector.

Automated Systems500 Connecting rods per hourRidgeway, PA, USA2000 amp multi directionalTurntable holds 8 parts, parts are advanced one at a time, each time a parts is removed for inspection.

Automated SystemsGun tube inspection10,000 amp coil and central conductorUnit is 66 feet longCentral conductor is perforated to apply bathHead stock rotation, coil position and hood movement are motorized.

Under Water MPIMarine growthHigh pressure water cleaningCoatingsAC YokeTest mediaDry powder delivered in a slurryGeometry of connections being testedSingle leg yoke sometimes used

Under Water MPI

ASME Section V, Article 7General requirements for magnetic particle examination

Used in conjunction with SE-709 (ASTM E709-95)

Appendix I, MPI on Coated Ferritic Materials Using AC Yoke

No acceptance or rejection criteria

ASTM E709-95Standard Guide for Magnetic Particle Examination

Intended as a reference to aid in the preparation of specifications, procedures and Techniques

No acceptance or rejection criteria

ASME B31.3, Chapter 6Process PipingInspection, Examination and Testing

Specific inspection requirements and acceptance criteria

ASME Section VIII, Appendix 6Boiler and Pressure Vessel Code

Methods for Magnetic Particle Examination

Requires the use of Section V, Article 7

Includes acceptance criteria

Last Slide!

*Discuss the relationship between the variables*Discuss the relationship between the variables

*Hysteresis Curve