Magnetic Particle Inspection TWI Copyright 2003 TWI Ltd

Magnetism • Some natural materials strongly attract pieces of iron to themselves. • Such

Magnetic Particle Inspection (MT or MPI) • MT is a test method for the

Permeability (μ) • Permeability can be defined as the relative ease with which a

Permeability (μ) • On the basis of their permeability materials can be divided into

Permeability (μ) Diamagnetic: Permeability slightly below 1, weakly repelled by magnets. Examples: Gold, Copper,

Domain theory N- S N-S N- S Copyright © 2003, TWI Ltd N Unmagnetized

Domain Theory N- S S N N-S N- S Copyright © 2003, TWI Ltd

Domain Theory S N-S N Saturated state NS N -S NS N-S All domains

S N N- Un-magnetised S N-S N- S Copyright © 2003, TWI Ltd N

S N S N- N-S N- S Copyright © 2003, TWI Ltd N- N-S

S N S N-S N-S N-S Copyright © 2003, TWI Ltd N- N- N-S

S N S N-S N-S N-S S N-S N Copyright © 2003, TWI Ltd

Lines of Flux Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Lines of flux • By convention they flow from North to South outside and

Electromagnetism • A current flows through a conductor and sets up a magnetic field

Right Hand Rule Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Coil Magnetisation • Changes circular field into longitudinal • Increases the strength of the

Hysteresis Place an un-magnetised piece of ferromagnetic material within a coil Saturation point B+

Hysteresis B+ Residual magnetism H- H+ BCopyright © 2003, TWI Ltd World Centre for

Hysteresis B+ H- H+ Coercive force BCopyright © 2003, TWI Ltd World Centre for

Hysteresis B+ Negative saturation point H- H+ BCopyright © 2003, TWI Ltd World Centre

Hysteresis A B C D Copyright © 2003, TWI Ltd F E World Centre

Hysteresis Hard ferromagnetic Copyright © 2003, TWI Ltd Soft ferromagnetic World Centre for Materials

Permeability (µ) • The ease with which a material can be magnetised • Opposite

Relative Permeability (µr) • Paramagnetics Slightly > 1 • Diamagnetics Slightly < 1 •

Hard v Soft Ferromagnetics Soft Hard • Typically Low carbon steel • High permeability

Definitions • Magnetic field Flux circuit Flux Density Copyright © 2003, TWI Ltd Region

Principle of MPI : Flux Leakage No Defect N Defect S N S Lines

LEAKAGE FIELDS Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Visibility of Flux Leakage Depends on: • Depth of defect • Orientation of defect

Indications Relevant Indications - Indications due to discontinuities or flaws Non-Relevant Indications - Indications

Defect Orientation Defect at 90 degrees to flux : maximum indication Copyright © 2003,

Defect Orientation >30 Degrees to Flux: Acceptable indication Copyright © 2003, TWI Ltd World

Defect Orientation <30 Degrees to Flux Copyright © 2003, TWI Ltd : Weak indication

Defect Orientation Test 1 Test 2 MPI requires 2 tests at 90 o to

Equipment Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permanent Magnet Longitudinal field between poles Maximum sensitivity for defects orientated at 90º to

Permanent Magnet Advantages • No power supply • No electrical contact problems • Inexpensive

Electromagnets Maximum sensitivity for defects orientated at 90º to a line drawn between the

Electromagnets Advantages • AC, DC or rectified • Controllable field strength • No harm

Prods • Current passed between 2 contacts. • Defects detected parallel to contacts Field

PROD METHOD Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Prods Advantages • AC, DC or rectified • Controllable field strength • No poles

Flexible Cable • Flexible, current carrying cable Used as • Adjacent cable • Threading

Flexible Cable Advantages • Simple to operate • No danger of burning • AC,

Current Flow Current passed through sample Defects Current Circular Field Copyright © 2003, TWI

Threading Bar • Current passed through brass bar placed between heads of bench unit

Magnetic Flow Magnetism passed through sample Defects Magnetism Copyright © 2003, TWI Ltd World

Rigid Coil Current passed through coil to generate a longitudinal field Defects Magnetism Copyright

MPI Equipment Portable • Permanent magnet • Electromagnet • Prods • Flexible coil •

The Swinging Field Method • Uses two magnetic fields at 90º to each other

Testing Values Page 64 Copyright © 2003, TWI Ltd World Centre for Materials Joining

Current Types • Direct current (DC) • Alternating current (AC) • Half wave rectified

Direct Current + Advantages • Sub-surface defects • Availability from batteries Copyright © 2003,

Alternating Current Advantages • Availability • Sensitivity to surface defects • Agitation of particles

Half Wave Rectified Current Advantages • Penetration like DC • Agitation • Ease of

Full Wave Rectified Current Advantages • Penetration like DC • Agitation Copyright © 2003,

3 - PHASE FW RECTIFIED Copyright © 2003, TWI Ltd World Centre for Materials

RMS 16 12 8 4 0 -4 Copyright © 2003, TWI Ltd World Centre

Direct Current: Field distribution Copyright © 2003, TWI Ltd World Centre for Materials Joining

A. C. : Field distribution SKIN EFFECT Copyright © 2003, TWI Ltd World Centre

SKIN EFFECT In order to achieve the same sensitivity to shallow defects a DC

Permanent Magnet and DC Electromagnet Use the Lift Test For pole spacing from 75

AC Electromagnets Use the Lift Test For pole spacing no more than 300 mm

PROD METHOD Current passed through sample, typically: 5 Amps (rms) per mm of prod

Flexible Coil • • I = 3 H(T + Y 2 / 4 T)

Adjacent Cable • Defects located parallel to cable • I = 4 d H

Current Flow Current passed through sample, typically: • I = H diameter or •

Threading Bar • I = H x perimeter R Increase the current (I) to

Threading Bar Component placed within field and rotated for complete coverage Copyright © 2003,

Magnetic Flow Magnetism passed through sample Defects Magnetism Field strength can be assessed using

Rigid Coil Conditions • Cross section of test piece <10% of Coil (the fill

FLUX INDICATORS Check for adequate flux density and correct orientation with Flux Indicators. (Do

FLUX INDICATORS - COMMON TYPES • ASME • BERTHOLD PENETRAMETER • BURMAH CASTROL STRIPS

ASME V MAGNETIC FLUX INDICATOR CONSISTS OF 8 STEEL PIE SEGMENTS BRAZED TOGETHER WITH

ASME V MAGNETIC FLUX INDICATOR Copyright © 2003, TWI Ltd World Centre for Materials

Detecting Media Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Dry Magnetic Particles • • Iron powder or magnetic iron oxide (magnetite). 5 -

Wet Magnetic Particles Magnetic iron oxide (magnetite) or iron powder 0. 1 - 100

Detecting Media • Magnetic Rubber • Fluorescence may degrade under UV(A), when exposed to

Demagnetisation Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Demagnetisation Removal of residual magnetisation Required for: • Aircraft parts • Rotating parts •

How to Demagnetise? • A constantly reversing and reducing magnetic field Flux Copyright ©

Methods of Demagnetisation • Aperture type coil reversing stepped DC • Aperture type coil

MPI Practices Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Test Methods • Continuous or Residual • Fluorescent or Visible • Wet or Dry

Continuous or Residual? Continuous Method • Detecting media applied immediately prior to & during

Fluorescent or Visible? Fluorescent Detecting media dye coated More sensitive Less tiring for operators

NB All surface defects form indications But not all indications are caused by defects

Control and Maintenance Checks To ensure equipment, ancillaries and materials are up to standard

Control and Maintenance Checks • Ink settlement 100 ml 100 4. 0 3. 0

Ink Settlement Test Fluorescent Ink • 0. 1 - 0. 3 % Copyright ©

Control and Maintenance Checks • • • Ink settlement Fluorescent ink check Equipment performance

Equipment Performance Checks • • • Current flow test piece Magnetic flow test piece

Equipment Performance Checks Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Control and Maintenance Checks • • Ink settlement Fluorescent ink check Equipment performance check

Control Check Frequency • • Settlement test Fluorescent intensity Test piece Viewing efficiency Copyright

Maintenance Check Frequency • • • Magnetising units Tank levels UV lamp Ammeters Demagnetiser

UV(A) Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Electromagnetic Spectrum X-rays & Gamma Electric Waves Microwaves Ultra Infra violet red Light 10

Electromagnetic Spectrum UV-C 10 100 UV-B UV-A 200 ULTRAVIOLET LIGHT Copyright © 2003, TWI

Fluorescence UV-A Source : Mercury vapour arc lamp + Filter Precautions • Avoid looking

Fluorescence and the Electromagnetic Spectrum Absorbs 10 100 200 ULTRAVIOLET LIGHT Copyright © 2003,

Fluorescent v Colour Contrast • • Fluorescent methods are more sensitive. Less operator fatigue

Fluorescent v Colour Contrast Black Particles Copyright © 2003, TWI Ltd Fluorescent Particles World

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Magnetism • Some natural materials strongly attract pieces of iron to themselves. • Such materials were first discovered in the ancient Greek city of Magnesia. • Magnets were utilised in navigation. • Oersted found a link between electricity and magnetism. • Faraday proved that electrical and magnetic energy could be interchanged. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Magnetic Particle Inspection (MT or MPI) • MT is a test method for the detection of surface and near surface defects in ferromagnetic materials. • Magnetic field induced in component • Defects disrupt the magnetic flux causing “flux leakage”. • Flux leakage can be detected by applying ferromagnetic particles Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permeability (μ) • Permeability can be defined as the relative ease with which a material may be magnetised. • It is defined as the ratio of the flux density (B) produced within a material under the influence of an applied field to the applied field strength (H) • μ =B/H Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permeability (μ) • On the basis of their permeability materials can be divided into 3 groups: • Diamagnetic • Paramagnetic • Ferromagnetic Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permeability (μ) Diamagnetic: Permeability slightly below 1, weakly repelled by magnets. Examples: Gold, Copper, Water • Paramagnetic: Permeability slightly greater than 1, weakly attracted by magnets. • Examples: Aluminium, Tungsten Ferromagnetic: Very high permeability, strongly attracted by magnets. Examples: Iron, Cobalt, Nickel Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Domain theory N- S N-S N- S Copyright © 2003, TWI Ltd N Unmagnetized state -S S N-S N NS N-S N- S N • A domain is a minute internal magnet • Each domain comprises 1015 to 1020 atoms Domains randomly orientated World Centre for Materials Joining Technology

Domain Theory N- S S N N-S N- S Copyright © 2003, TWI Ltd S N-S N Domains orientated in external magnetic field NS N -S NS N-S S N- N-S N Magnetized state -S S N-S N NS N-S N- N-S World Centre for Materials Joining Technology

Domain Theory S N-S N Saturated state NS N -S NS N-S All domains orientated in strong external field N-S N-S Copyright © 2003, TWI Ltd N-S S N- N-S N-S N-S World Centre for Materials Joining Technology

S N S N-S N-S N-S S N-S N Copyright © 2003, TWI Ltd N- N-S N-S N-S S N- N-S S N N- N-S -S S N-S N N-S S N- N-S S N-S N NS N -S S N-S N N- N -S S -S Un-magnetised Magnetised Saturated Residual World Centre for Materials Joining Technology

Lines of flux • By convention they flow from North to South outside and South to North inside • They form closed loops • They never cross • They follow path of least resistance • Flux density is the number of lines of flux passing through a unit area. • Field strength is highest where flux density is highest. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Electromagnetism • A current flows through a conductor and sets up a magnetic field around it • Field is at 90 o to the direction of the electrical current Direction of current flow Direction of magnetic field Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permeability (µ) • The ease with which a material can be magnetised • Opposite of reluctance (difficulty with which a material can be magnetised) • µ=B/H • Permeability of free space = µo • Relative Permeability (µr) = µ / µo Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Hard v Soft Ferromagnetics Soft Hard • Typically Low carbon steel • High permeability • Easy to magnetise • Low residual magnetism Copyright © 2003, TWI Ltd • Typically high carbon steel • Lower permeability • More difficult to magnetise • High levels of residual magnetism World Centre for Materials Joining Technology

Definitions • Magnetic field Flux circuit Flux Density Copyright © 2003, TWI Ltd Region in which magnetic forces exist Total number of lines existing in a magnetic area Tesla) Magnetic flux per unit (measured in World Centre for Materials Joining Technology

Visibility of Flux Leakage Depends on: • Depth of defect • Orientation of defect shape of defect • Size of defect • Permeability of material • Applied Field Strength • Contrast Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Indications Relevant Indications - Indications due to discontinuities or flaws Non-Relevant Indications - Indications due to flux leakage from design features Spurious Indications - Indications due incorrect inspection procedures Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permanent Magnet Longitudinal field between poles Maximum sensitivity for defects orientated at 90º to a line drawn between poles Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Permanent Magnet Advantages • No power supply • No electrical contact problems • Inexpensive • No damage to test piece • Lightweight Copyright © 2003, TWI Ltd Disadvantages • Direct field only • Deteriorate over time • No control over field strength • Poles attract detecting media • Tiring to use World Centre for Materials Joining Technology

Electromagnets Advantages • AC, DC or rectified • Controllable field strength • No harm to test piece • Can be used to demagnetise • Easily removed Copyright © 2003, TWI Ltd Disadvantages • Power supply required • Longitudinal field only • Electrical hazard • Poles attract particles • Legs must have area contact World Centre for Materials Joining Technology

Prods Advantages • AC, DC or rectified • Controllable field strength • No poles attract particles • Control of amperage Copyright © 2003, TWI Ltd Disadvantages • Arcing / damage to work piece • Transformer required • Current can be switched on without creating field • Good contact required • 2 man operation World Centre for Materials Joining Technology

Flexible Cable Advantages • Simple to operate • No danger of burning • AC, DC or rectified • Current adjustable Copyright © 2003, TWI Ltd Disadvantages • Difficult to keep cables in place • High currents required • Transformer required World Centre for Materials Joining Technology

Threading Bar • Current passed through brass bar placed between heads of bench unit • Circular field generated around bar • Sample hung from bar Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

MPI Equipment Portable • Permanent magnet • Electromagnet • Prods • Flexible coil • Flexible cable • Clamps and leeches Copyright © 2003, TWI Ltd Fixed Current flow Magnetic flow Threader Bar Rigid coil Induced current World Centre for Materials Joining Technology

Current Types • Direct current (DC) • Alternating current (AC) • Half wave rectified current (HWDC or HWRAC) • Full wave rectified (FWDC or FWRAC) Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Direct Current + Advantages • Sub-surface defects • Availability from batteries Copyright © 2003, TWI Ltd Disadvantages • No agitation • Less sensitive to surface defects World Centre for Materials Joining Technology

Alternating Current Advantages • Availability • Sensitivity to surface defects • Agitation of particles • Demagnetisation Copyright © 2003, TWI Ltd Disadvantages • Will not detect sub-surface defects World Centre for Materials Joining Technology

Half Wave Rectified Current Advantages • Penetration like DC • Agitation • Ease of production • High flux density for less power Copyright © 2003, TWI Ltd Disadvantages • Sensitivity to surface defects lower than AC World Centre for Materials Joining Technology

Full Wave Rectified Current Advantages • Penetration like DC • Agitation Copyright © 2003, TWI Ltd Disadvantages • Sensitivity to surface defects lower than AC World Centre for Materials Joining Technology

SKIN EFFECT In order to achieve the same sensitivity to shallow defects a DC field must be far more powerful than a corresponding AC field Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Current Flow Current passed through sample, typically: • I = H diameter or • I = H x perimeter • For D/d = 1. 5 or less, one shot only req’d Defects Current Circular Field Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Rigid Coil NI = 0. 4 H K L/ D N = 0. 4 H K L/ D x I • N = Number of turns in coil • K = 32, 000 for DC (typical) 22, 000 for AC or FWR (typical) 11, 000 for HWR (typical) • L/D = Length / Diameter Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Rigid Coil Conditions • Cross section of test piece <10% of Coil (the fill factor) • Test piece must lie against side or bottom • The test zone is the part of the component which lies within the coil • L / D must be between 5 and 20 Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

FLUX INDICATORS Check for adequate flux density and correct orientation with Flux Indicators. (Do not use with permanent magnets or DC electromagnets. ) Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Dry Magnetic Particles • • Iron powder or magnetic iron oxide (magnetite). 5 - 200 microns, rounded and elongated shapes Colours vary for contrast against component Can be used on hot surfaces Poor particle mobility, HWDC best, DC or permanent magnets must never be used Greater operator skill required Difficult to apply to overhead surfaces especially in field conditions Generally less sensitive than wet particles Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Wet Magnetic Particles Magnetic iron oxide (magnetite) or iron powder 0. 1 - 100 microns rounded and elongated shapes Colour contrast or fluorescent Water or kerosene based Concentration important Good particle mobility Easier to use More sensitive Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Demagnetisation Removal of residual magnetisation Required for: • Aircraft parts • Rotating parts • Components to be welded, machined or electroplated Check for removal with Field strength meter (magnetometer) Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Methods of Demagnetisation • Aperture type coil reversing stepped DC • Aperture type coil reducing AC • AC or reversing DC aperture type coil, withdraw component along the coil axis • AC electromagnet • Heating to above the Curie point (about 770 C for steel) Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Continuous or Residual? Continuous Method • Detecting media applied immediately prior to & during magnetisation. Residual • Detecting media used after the applied field has been removed. • Requires high retentivity. • Less sensitive than continuous. • Useful for components like ball bearings Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Fluorescent or Visible? Fluorescent Detecting media dye coated More sensitive Less tiring for operators Better for batch inspections Copyright © 2003, TWI Ltd Visible No special lighting required Higher concentration of particles Background paint may be required World Centre for Materials Joining Technology

NB All surface defects form indications But not all indications are caused by defects Relevant indications…Linear 3: 1 Non-relevant indications Due to flux leakage but arising from design features Changes in section Changes in permeability Grain boundaries Forging flow lines Copyright © 2003, TWI Ltd Spurious indications Not due to flux leakage • Lint • Scale • Dirt • Hairs • Magnetic writing World Centre for Materials Joining Technology

Control and Maintenance Checks To ensure equipment, ancillaries and materials are up to standard • Ink • Lighting conditions • Magnetising units Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Control and Maintenance Checks • • Ink settlement Fluorescent ink check Equipment performance check Viewing efficiency Magnetising unit Unit tank levels Unit ammeters Demagnetiser Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Electromagnetic Spectrum X-rays & Gamma Electric Waves Microwaves Ultra Infra violet red Light 10 -10 10 -8 10 -6 Copyright © 2003, TWI Ltd 10 -4 10 -2 TV 1 cm 102 Wavelength 104 106 108 World Centre for Materials Joining Technology

Electromagnetic Spectrum UV-C 10 100 UV-B UV-A 200 ULTRAVIOLET LIGHT Copyright © 2003, TWI Ltd 300 400 A Damaged Black Light UV-B&C 500 600 700 VISIBLE LIGHT World Centre for Materials Joining Technology

Fluorescence UV-A Source : Mercury vapour arc lamp + Filter Precautions • Avoid looking directly at the lamp • Do not use if filter is cracked, damaged or incorrectly fitted Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Fluorescent v Colour Contrast • • Fluorescent methods are more sensitive. Less operator fatigue with fluorescent. Background lacquer is not required. Fluorescent properties will degrade if exposed to UV light, acids, alkalis or high temperature. • Background fluorescence is a problem on rough surfaces. • Some oils will produce strong background fluorescence. • Low background light levels are required. Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology