Magnetic Particle Inspection
TWI
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
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 ()
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) =B/H
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Permeability ()
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 ()
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
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 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 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 90o 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
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 field
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Hysteresis
Place an un-magnetised piece of ferromagnetic material within a coil B+
Saturation point Virgin curve
HBCopyright 2003, TWI Ltd
H+
World Centre for Materials Joining Technology
Hysteresis
B+ Residual magnetism HH+
BCopyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Hysteresis
B+
HCoercive force BCopyright 2003, TWI Ltd
H+
World Centre for Materials Joining Technology
Hysteresis
B+
Negative saturation point
H-
H+
BCopyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Hysteresis
B A
D
Copyright 2003, TWI Ltd
E
World Centre for Materials Joining Technology
Hysteresis
Hard ferromagnetic Soft ferromagnetic
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Permeability ()
The ease with which a material can be magnetised Opposite of reluctance (difficulty with which a material can be magnetised) =B/H Permeability of free space = o Relative Permeability (r) = / o
World Centre for Materials Joining Technology
Copyright 2003, TWI Ltd
Relative Permeability (r)
Paramagnetics Diamagnetics Ferromagnetics Slightly > 1 Slightly < 1 240 +
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Hard v Soft Ferromagnetics
Soft
Typically Low carbon steel High permeability Easy to magnetise Low residual magnetism
Hard
Typically high carbon steel Lower permeability More difficult to magnetise High levels of residual magnetism
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Definitions
Magnetic field Region in which magnetic forces exist
Flux circuit Flux Density
Total number of lines existing in a magnetic
area Tesla) Tesla)
Magnetic flux per unit (measured in
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Principle of MPI : Flux Leakage
No Defect Defect
Lines of flux follow the path of least resistance
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
LEAKAGE FIELDS
Copyright 2003, TWI Ltd
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
Defect Orientation
Defect at 90 degrees to flux : maximum indication
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Defect Orientation
>30 Degrees to Flux:
Copyright 2003, TWI Ltd
Acceptable indication
World Centre for Materials Joining Technology
Defect Orientation
<30 Degrees to Flux
Copyright 2003, TWI Ltd
: Weak indication
World Centre for Materials Joining Technology
Defect Orientation
Test 1 Test 2
MPI requires 2 tests at 90o to one another
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Equipment
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Permanent Magnet
Longitudinal field between poles Maximum sensitivity for defects orientated at 90 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 Disadvantages Direct field only Deteriorate over time No control over field strength Poles attract detecting media Tiring to use
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 90o 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
Coil Magnetisation
Changes circular field into longitudinal Increases the strength of the field
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Electromagnets
Maximum sensitivity for defects orientated at 90 to a line drawn between the poles
Copyright 2003, TWI Ltd
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 Disadvantages Power supply required Longitudinal field only Electrical hazard Poles attract particles Legs must have area contact
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Current Flow
Current passed through sample Defects
Current Circular Field
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
PROD METHOD
Copyright 2003, TWI Ltd
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
Magnetic Flow
Magnetism passed through sample Defects
Magnetism
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Coil Magnetisation
Changes circular field into longitudinal Increases the strength of the field
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Rigid Coil
Current passed through coil to generate a longitudinal field Defects
Magnetism
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
MPI Equipment
Portable Permanent magnet Electromagnet Prods Flexible coil Flexible cable Clamps and leeches Fixed Current flow Magnetic flow Threader Bar Rigid coil Induced current
Copyright 2003, TWI Ltd
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 Sub Availability from batteries Disadvantages No agitation Less sensitive to surface defects
Copyright 2003, TWI Ltd
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 subdefects
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
Disadvantages Sensitivity to surface defects lower than AC
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
3 - PHASE FW RECTIFIED
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Direct Current: Field distribution
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
A.C. : Field distribution
SKIN EFFECT
Copyright 2003, TWI Ltd
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
Permanent Magnet and DC Electromagnet
Use the Lift Test For pole spacing from 75 to 150mm - 18 kg
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
AC Electromagnets
Use the Lift Test For pole spacing no more than 300mm - 4.5kg
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Current Flow
Current passed through sample, typically: 7.5 Amps / mm diameter or 2.4 Amps / mm perimeter For L/D = 1.5 or less, one shot only reqd req
Defects Current Circular Field
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
PROD METHOD
Current passed through sample, typically: 5 Amps (rms) per mm of prod spacing
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Threading Bar
R (mm) = I / 15 engineering R (mm) = I / 56 for General for aerospace Increase the current (I) to increase R, the radius of the test zone. R=I/15 is equivalent to 7.5A per mm of diameter. R=I/56 is equivalent to 28A per mm of diameter.
World Centre for Materials Joining Technology
R R
Copyright 2003, TWI Ltd
Threading Bar
Component placed within field and rotated for complete coverage
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Magnetic Flow
Magnetism passed through sample Defects
Magnetism
Field strength can be assessed using a flux indicator.
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Rigid Coil
K NI = L/ D
K I= L / D N
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
ASME V MAGNETIC FLUX INDICATOR
CONSISTS OF 8 STEEL PIE SEGMENTS BRAZED TOGETHER WITH COPPER FACEPLATE
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
ASME V MAGNETIC FLUX INDICATOR
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
FLUX INDICATORS - COMMON TYPES
ASME BERTHOLD PENETRAMETER BURMAH CASTROL STRIPS
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Detecting Media
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
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
How to Demagnetise?
A constantly reversing and reducing magnetic field
Flux
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 770C for steel)
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
MPI Practices
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Test Methods
Continuous or Residual Fluorescent or Visible Wet or Dry
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
Visible No special lighting required Higher concentration of particles Background paint may be required
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
But not all indications are caused by defects Relevant indicationsLinear 3:1
Non-relevant indications NonDue to flux leakage but arising from design features Changes in section Changes in permeability Grain boundaries Forging flow lines Spurious indications Not due to flux leakage Lint Scale Dirt Hairs Magnetic writing
NB All surface defects form indications
Copyright 2003, TWI Ltd
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
100
100 ml
4.0 3.0 2.0
1.0
1.0 ml 0.5 ml
0.5
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Ink Settlement Test
Fluorescent Ink 0.1 - 0.3 % Non-Fluorescent Ink 1.25 - 3.5 %
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Control and Maintenance Checks
Ink settlement Fluorescent ink check Equipment performance check
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Equipment Performance Checks
Current flow test piece Magnetic flow test piece Cracked component
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Equipment Performance Checks
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
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 Viewing efficiency Magnetising unit Unit tank levels Unit ammeters Demagnetiser
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Control Check Frequency
Settlement test Fluorescent intensity Test piece Viewing efficiency Daily Weekly Daily Monthly
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Maintenance Check Frequency
Magnetising units Tank levels UV lamp Ammeters Demagnetiser Weekly Daily Monthly 6 monthly 6 monthly
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
UV(A)
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Electromagnetic Spectrum
X-rays & Gamma Microwaves Ultra Infra violet red Light TV Electric Waves
10-10 10-8
10-6
10-4
10-2
1cm
102
104
106
108
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
Wavelength
Electromagnetic Spectrum
UV-C
UV-B UV-A
A Damaged Black Light UV-B&C
10
100
200 LIGHT
300
400
500 500
600
700
ULTRAVIOLET
VISIBLE LIGHT
Copyright 2003, TWI Ltd
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
Fluorescence and the Electromagnetic Spectrum
Absorbs Emits
10
100
200 LIGHT
300
400
500 500
600
700
ULTRAVIOLET
VISIBLE LIGHT
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
Fluorescent v Colour Contrast
Black Particles Fluorescent Particles
Copyright 2003, TWI Ltd
World Centre for Materials Joining Technology
