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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