MPI Flashcards
Name 3 types of magnetism
There are Three Types of Magnetism
Ferromagnetism - Materials which can be strongly magnetised & which show good magnetic properties
Paramagnetism - Materials which are weakly attracted by strong magnetic fields
Diamagnetism - Materials that are repelled by a strong magnetic field. An externally applied magnetic field induces a “like” magnetic field within the material & repulsion occurs.
Explain the theory of magnetism
In ferromagnetic materials, the atoms are gathered together in groups called Domains.
These domains have a magnetic moment, one end acting as a North pole, the other as a South pole.
Created by the combined effort of the motion of electrons around the nucleus of the atom & by the electron spin around its own axis.
When a material is un-magnetised, the domains are distributed randomly & their magnetic effects cancel
each other out
When an external magnetic field is introduced, the domains align themselves North to South in a common direction.
When all the domains in a material are fully aligned, the material is said to be magnetically saturated.
Even after the external magnetic field is removed, there will be some residual magnetism left in the material, as the domains will not be totally randomised again.
Describe a magnetic field
The Magnetic Field Is described as the area surrounding the magnet, in which the magnetic forces exist.
Lines of force or lines of magnetic flux, represent the magnetic field.
These purely imaginary lines were introduced by Faraday as a means of visualising the distribution &
density (flux density) of a magnetic field.
The SI unit used to measure flux density is the Tesla (T).
For practical MPI the min. flux density required is 1.0(T).
Describe the characteristics of the magnetic lines of force
Magnetic lines of force :
Have direction as if flowing, but no actual movement occurs.
Travel North to South externally, South to North internally.
Form a closed loop.
All have the same strength.
Do not cross each other.
Seek the path of least resistance.
Are in constant tension.
Decrease in density with increasing distance from the poles.
Decrease in density when moving from an area of higher
permeability to an area of lower permeability.
Prefer to travel in materials that easily accept magnetic
fields.
Describe Longitudinal Magnetism
Magnetic field around a bar magnet produces longitudinal magnetism
Describe Electromagnetism
In electromagnetism, when an electric current flows through a conductor (copper wire or rod), a magnetic field is set up around the conductor, in a direction at 90o to the electric current
When a conductor carries an electrical current, strong magnetic flux lines are created this is called Circular Magnetism.
It is not polar, so cannot be detected externally on a round symmetrical specimen.
If the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction.
The field within the loop has direction:
One side will be a North pole, the other a South pole.
By increasing the number of loops, a coil or solenoid is created.
The strength of the field passing through the coil is proportional to the current passing through the conductor in amperes, multiplied by the number of turns in the solenoid.
When a ferromagnetic specimen is placed in an energised coil, the magnetic field is concentrated in the specimen.
One end is a North pole, the other a South pole.
This is called Longitudinal Magnetism.
It has polarity & is, therefore, readily detectable.
Describe Magnetic Hysteresis
When a ferromagnetic material is influenced by an alternating magnetising force (H), the variation of magnetic flux density (B) in it is related to a phenomenon known as Magnetic Hysteresis.
Hysteresis, is derived from the Greek word for delayed & is used to describe one quantity lagging behind another.
The variation of B-H follows a Hysteresis Loop & is characteristic to particular ferromagnetic materials.
Low Permeability
Hard Ferromagnetic
1) High Retentivity
2) High Remanence/Residual Magnetism
3) High Reluctance
4) High Coercivity to demagnetise
High Permeability
Soft Ferromagnetic
1) Low Retentivity
2) Low Remanence/Residual Magnetism
3) Low Reluctance
4) Low Coercivity to demagnetise
Magnetic hysteresis terms
Flux Density (B) - Number of magnetic flux lines per unit area.
Magnetising Force (H) – Force used to set up a magnetic circuit.
Permeability – Ease with which a material can be magnetised.
Saturation - Stage at which any increase in H produces no gain in B.
Coercive Force – Measure of the amount of reverse magnetic force needed to return a material’s magnetic field to zero.
Residual Magnetism or Remanence –Magnetic flux density remaining in a material when the magnetising force is zero.
Reluctance – Opposition that a ferromagnetic material shows to the establishment of a magnetic field.
Retentivity – Flux density remaining when H is at zero
Explain flux leakage
A flux leakage is a discontinuity in a magnetic circuit.
Any abrupt change of permeability within a magnetic specimen will change the number of flux lines that can flow, there will be a diversion of the field.
Magnetic particle inspection relies on flux leakage fields being seen on the surface of a ferromagnetic specimen.
All defects produce flux leakage, but not all flux leakage fields are created by defects.
A flux leakage is a discontinuity in a magnetic circuit.
Magnetic Particle Inspection relies on:
Magnetising the specimen to an adequate flux density.
Applying fine ferromagnetic particles over the surface.
Being able to see the magnetic particles that gather at flux leakage fields.
Why is the direction of the magnetic field important when looking for flux leakage
The magnetic field.
Must run in a direction in which, it can be interrupted by the defect, thus producing a flux leakage field.
The degree of distortion at the leakage must allow the magnetic particles to provide an adequate degree of contrast between the leakage & the adjacent material surface, so that it is readily visible.
Flux lines flowing in a ferromagnetic bar but having to divert around an air gap, create a flux leakage
If ferromagnetic particles are sprinkled on the bar they’ll start to form a magnetic bridge across the
flux leakage
What are the factors that make flux leakage visible
Whether a flux leakage is made into a visual indication depends on a number of factors: Size of defect. Shape of defect. Volume of defect. Orientation of defect. Depth below surface. Permeability of material.
What are indications
Indications are any particle indications that are seen on the specimen under test.
Just as not all flux leakage fields are defects, not all indications are due to flux leakage.
Indications can be further subdivided into:
Relevant.
Non-relevant.
Spurious.
What is relevant indications
Relevant Indications are discontinuities or flaws, which in turn, are unwanted imperfections.
When it is considered that a relevant indication will affect the fitness-for-purpose of a test
specimen, then it is classified as a defect, but not all defects are cracks
What are non relevant indications
Non-Relevant Indications
True magnetic particle patterns formed & held in place by leakage fields.
Caused by design features & the structure of the specimen.
Only in exceptional cases will they affect the fitness-for-purpose.
A non-exhaustive list:
Tool marks.
Abrupt changes of geometry.
Dissimilar magnetic material.
What are Spurious Indications
Indications that are not held on the surface by a flux leakage are termed Spurious, i.e. scale or dirt.
Magnetic Writing is when two pieces of steel touch when one of them is in a magnetised condition, local poles are created at the areas of contact.
What direction must the indications be to be seen
Indications that are Transverse will show (90 deg to the lines of magnetism)
Indications that are up to at least 30 deg will show
Defects running in the line of magnetism will not show
Methods of Magnetisation
The equipment used for MPI can be divided according to size & purpose.
Electricity used to magnetise, is transformed into a low voltage, high amperage supply.
There is no danger from electrocution.
Magnetising equipment must meet the requirements of & be used in accordance with, BS EN ISO 9934-3.
Permanent Magnets
Permanent Magnets
Produce a longitudinal magnetic field between the poles.
Horseshoe magnets have adjustable arms & may have variable-geometry, removable pole ends.
The optimum defect detectability is at 90deg to the poles.
On straight work pieces, like plates & cylinders, good contact between the pole pieces & the work piece is easily obtained by having shaped pole pieces, flat for plate & radiused for cylindrical-shaped work pieces.
For more complicated shapes, e.g., the weld at 12o’c on the joint of a VDM on a node, the pole pieces need to rotate as well as being shaped in order to make good contact
BS EN ISO 9934-1 states that a lift test should be carried out before a magnet is used for MPI.
The lift test should confirm that the magnet can lift 18kg with a pole spacing of between 75 & 150mm.
The inspection area is between the poles & half the pole spacing either side.
What are the advantages and disadvantages of permanent magnets
Advantages No power supply needed Clings to vertical surfaces No electrical contact problems Cheap & ready available No damage to test piece Lightweight
Disadvantages Direct longitudinal field only Deteriorate with wear Have to be pulled from test piece No control over field strength Ink particles attracted to poles Poles must have good contact
Electromagnets
Electromagnets are made from soft iron laminates to reduce eddy current losses, if powered by alternating
current (AC)
The yoke laminates are encased in a multi-turn coil, usually powered by mains electricity, stepped down to
6 or 12v
The legs of modern equipment are normally articulated to allow contact on uneven surfaces
Electromagnets produce a longitudinal field, with the test area being a circle inscribed by the poles.
BS EN ISO 9934-1 states that a lift test should be carried out prior to using for MPI.
The lift test should confirm that the electromagnet can lift 4.5kg with a pole spacing of up to 300mm.
Defect orientation is the same as with a permanent magnet.
What are the advantages and disadvantages of electromagnets
Advantages AC, or rectified DC operation Controllable field strength Can be switched off allowing easy removal No harm to test piece Lightweight Can be used to demagnetise
Disadvantages Needs a power supply Longitudinal field only Poles may attract magnetic particles Poles must have good contact
Current Flow Prods
Current flow Prods induce a circular magnetic field by sending a high amperage current (typically 1000A)
through the test piece.
Prods produce a circular magnetic field with defects showing at a maximum when orientated along a line
between the prod tips.
Advantages and disadvantages of prods
Advantages
Either AC or DC may be used
Low voltage used (Typically 3V)
Field strength can be adjusted
No collection of ink around the prods
Disadvantages Good prod contact is required Heavy duty transformer required Danger of arcing on test piece If arcing does occur inclusions from the prod tips may result
Flexible Coil
Flexible Coil Technique
A current-carrying cable is wound around the component.
It is a longitudinal magnetisation method & will find defects lying parallel to the cable.
The area to be tested is considered to lie between the turns of the coil.
Evenly Spaced Coil
A flexible cable is wound around the test piece so that the area under inspection is contained within the encircling coil. Individual wraps of the coil are spaced equidistant to each other.
Close Wrapped Coil
In this technique, the inspection area does not have to lie inside the encircling coils, however, the test zone must still lie close enough to the ends of the coil to ensure an adequate flux density.
Parallel Loops
This produces a transverse field between the two sides of the loop. The loop has to be positioned so that the current in the two sides is moving in the same direction, otherwise the magnetic fields will tend to cancel instead of reinforce.
Advantages and disadvantages of parallel loops
Advantages
Cables can be positioned relatively easily
Large areas can be inspected with one placement of the coil
Field strength & flux density can be adjusted
Demagnetising is easily achieved
Disadvantages
A Relatively weak Field is produced
Large, heavy step-down transformers are required
Isolating transformers are required on deck
Long cables are required
Continuous & Residual Magnetisation
Continuous & Residual Magnetisation Techniques
Both techniques are used for land base inspections.
The continuous technique is used almost exclusively for underwater applications, because of the possibility of wash out or dilution of the indicating medium in the water.
Continuous Magnetisation Technique
The work piece is magnetised at the same time as the indicating medium is applied.
The area under inspection is then examined while the magnetic field is maintained.
This method is generally regarded as being the more sensitive, but indications of defects other than actual damage can be given.
Residual Magnetisation Technique
The work-piece is magnetised then made cold before the indicating medium is applied.
The inspection is completed with the magnetising force removed; the work piece is examined with only any residual field producing the flux leakage.
These fields are much weaker than those by the continuous method, so much lower in sensitivity.
Detecting Media
What factors are involved when considering the sensitivity of MPI
Detecting Media
The effectiveness of the magnetic particle inspection is determined by the sensitivity with which it is possible to detect the change in the magnetic field due to the presence of a defect causing flux leakage.
This is done by visual observation, using an indicating medium to show the distortion.
Visual Detection
How is the defect seen
Visual Detection
Achieved by observing the distortion of the magnetic field as shown by the patterns in
magnetic particles distributed over the test surface.
Magnetic particles are available in dry powder form, as a liquid slurry or fully mixed with water & ready to use.
The Particles
Fine ferrous-oxides that are much lighter than iron filings & will go into suspension in the water more easily & remain suspended much longer.
A fluorescent powder (usually green coloured) is mixed with the particles so that they will fluoresce under ultraviolet light.
The Method
Apply the magnetic field & then apply the ink.
Any flaws cause flux leakage with a resultant build-up in flux density.
The ferrous-oxide in the ink is attracted to this strong magnetic field & will collect there.
Using a suitable ultraviolet light, the indication can be viewed, interpreted & reported.
What properties must the ink have
The standard for MPI ink is BS EN ISO 9934-2
Ink must be non-toxic, free from contaminants, not cause discomfort to users & be non-corrosive to the
work piece.
Grains should be fine enough to reduce gravitational effects.
Go easily into suspension, but not coagulate in the liquid.
Have elongated shape to facilitate polarisation.
Have high permeability so as to be easily magnetised.
Have low retentivity to facilitate removal.
Have good contrast against the test background.
The concentration of ferrous-oxide particles, additives & fluorescent powder is specified in the standard, as is the method for testing the correct concentration of ink.
The ink or powder must be supplied as certified manufactured to the ISO standard with a batch number that should be recorded.
The correct quantity of powder should be measured & put into a suitable mixing vessel
Inks are available which include wetting agents.
It’s more common to add such an agent at this stage.
Enough liquid is added to make it up into a slurry, which is put into a bucket & made up to the correct quantity with water.
Once the ink is mixed, it should be constantly agitated to ensure the suspension is maintained & a 100ml sample drawn off.The sample is required to undergo a settling test to confirm the ink has been made up to the correct concentration.
Explain the settlement test
Draw off the 100 ml sample into a calibrated settling flask.
Either a Sutherland or Thistle type flask is suitable.
Allow the sample to settle for 60 minutes.
Record the quantity of solids collected in the bottom.
Of paramount importance is the maintenance of the ink strength.
The solid content must be constantly monitored.
The ink must be constantly agitated to keep the solid content in suspension. Not stated in current standards, but previously that the solid content should be between:
0.1-0.3% by volume for fluorescent ink.
1.25-3.5% for visible (black) ink.
No more than 10% by weight of other solids.
What are the requirements for lighting and viewing conditions
As the chosen method for detecting MPI indications is vision, the ambient light conditions & the quality
& quantity of ultraviolet light must be at correct levels
Standards for lighting are stated in BS EN ISO 3059. The minimum requirements for ambient & ultraviolet
light as stated in the standard are:
Visible Light Inspection (Using non-fluorescent Inks).
Minimum of 500 lux of white light in the viewing area.
Background & Ultraviolet Light Levels (Fluorescent Inks).
Ambient light must not exceed 20 lux, so that the fluorescence from the ink particles has sufficient contrast to be easily seen.
The ultraviolet light must be a minimum of 1000µW/cm2.
The wavelength of the light should be between 365 to 400nm (A nanometre (nm) is 1 millionth of a mm).
What are the safety considerations when using UV light
Ultraviolet light covers a range of frequencies
These are split into UVA & UVB with UVA being the less harmful wavelengths, which is why they are used for MPI.
Mercury vapour bulbs, which produce light by discharging an arc in mercury vapour, produce a large amount of ultraviolet light along with visible white light.
This type of bulb is put into a housing with a Woods filter in front of it, which blocks the harmful UVB wavelengths.
Testing the UV light
Reasons to test ultraviolet lights used in MPI:
Intensity must be high enough.
Wavelength must be in the correct part of the
spectrum.
Mercury vapour bulbs degenerate with use.
Requirements stated in the standard must be met.
The procedure for testing the ultraviolet light:
Switch on & allow it to warm up for 15 minutes.
Lamps for underwater use must be immersed in cooling water during this test, otherwise the heat generated will damage the seals.
Shine the light onto a photometer (Black Light Meter) holding it at a distance of about
400mm.
Record the results.
What is Residual Magnetic Field
At the end of a magnetic particle inspection when the magnetising force is removed, a residual field will remain
The strength will depend on the material retentivity.
Applying a field strength indicator to the work piece will assess the magnitude of the residual field.
Also known as gauss meters.
Offshore structures are fixed in Earth’s magnetic field, subject to vibration so may be weakly magnetic.
Any residual field should be removed:
Prior to MPI to prevent the possibility of vector fields.
May interfere with sensitive electronic equipment.
Prior to welding so as to avoid arc blow.
Explain demagnetism and why it’s important
Looking at a typical Hysteresis Loop, after the initial magnetising force is applied & then removed, it is virtually impossible to end the test with zero flux density.
Even if a negative coercive force is applied it will only keep the flux density at zero, as long as it continues to be applied.
The following diagram shows that the key to demagnetisation is that a reversing & reducing magnetising force must be applied.
The energised yoke is pulled over & off the component, to a distance of about 450mm & then
switched off The operation is repeated in the same way & direction until the residual field is removed
Alternative Forms of Current Applied in MPI
Inducing a magnetic field was examined earlier where it was stated that either AC or DC may be used.
The type of current must be considered as the induced field strength & characteristics are determined by this.
Either current or voltage may be measured & for the purpose of inducing a magnetic field, the amperage is the more important, up to 1000amps may well be used.
Underwater, the voltage is kept as low as possible for safety, only about 4 volts potential is applied.
Talk about using DC current for MPI
DC is an electrical current flowing in one direction only & effectively free from pulsation
After a small build-up period, the current is at a constant peak value & this is what the meter reads
Direct current is either supplied from a permanent magnet, a battery pack or a DC generator.
Early days of MPI, DC was almost universally used.
Advantages
Finds sub-surface defects
Availability from batteries
Disadvantages
No agitation
Less sensitive to surface defects
Talk about using AC current for MPI
Alternating current
In the UK, 50Hz is the commercial norm, & as AC is readily available, it is convenient to make use of it MPI.
When AC is used; the magnetic field will be limited to a narrow region at the surface of the component.
This is known as the ‘Skin Effect’.
Induction is not a spontaneous reaction & the rapidly reversing current does not allow the domains down in the material time to align.
As the current is constantly reversing, no single value for the current can be measured.
To determine a single value for an AC current the Root Mean Square (RMS) value is calculated.
To calculate the RMS value, all that is required is to divide the peak value by the constant 1.414.
Advantages Availability Sensitivity to surface defects Agitation of particles Demagnetisation
Disadvantages
Will not detect sub-surface defects
How do we verify if there’s enough magnetism
BS EN ISO 9934-1 specifies that the adequacy of the surface flux density should be established.
Flux Indicators, such as the Burmah Castrol Strip are common & simple to use, giving a clear visual indication of the direction of the surface field.
They’re a rough guide to the magnitude of the field.
Flux Indicators consist of a magnetic material that is interrupted by non-magnetic spacers.
When the flux indicator is placed on the surface of a magnetised specimen, flux is induced in it.
The non-magnetic spacers behave as artificial flaws.
If the magnetic field at the surface of the specimen is sufficiently high, flux leakage above the artificial flaws can be detected.
What are BC foil strips made of
Flux Indicators are made with high permeability magnetic materials with low coercivity & low remanence, so that a flux can be easily induced into them, yet without permanently magnetising them.
Flux indicators may be divided into two main types:
Segment type
Foil type
Explain Berthold Penetrameter
Berthold Penetrameter
A four section indicator with an adjustable foil, giving a varying air gap between them.
Designed to indicate flux direction & sensitivity.
The central, cylindrical iron piece is cut into quadrants to provide indications at 0 & 90o.
When the penetrameter is placed on a magnetised test surface, magnetic lines of flux will pass through the cut quadrants of the iron cylinder.
These lines will be visible when using MPI testing media.
Explain Burma’s Castro Strips
The most common foil type indicator is the Burmah Castrol Strip or as it is more correctly now called, a magnetic flux indicator.
Consist of a magnetic foil containing linear slots of different widths to simulate discontinuities, sandwiched between non-magnetic foils.
The simulated discontinuities are arranged in 3 parallel lines.
These foils are less than 0.2mm thick & flexible, which gives them a significant advantage over the segment type.
BCS’s are placed on the test object as it is inspected, ideally at 90o to the possible defect orientation.
The number of linear indications gives the Inspector a general idea of the magnetic field strength.
The results are fairly repeatable as long as the same orientation of the magnetic field maintained.
Disadvantages of these strips are that they cannot be bent to complex shapes & are not suitable for multi-
directional field systems as they indicate defect indications in one direction only
Explain Gauss meters
An electric powered meter that will read the field strength
directly from the input of a probe that is applied to the
test area. The meter gives a digital readout in Tesla.
List the surface checks when doing MPI
Surface Checks:
Obtain the necessary work permits.
Ensure an isolating transformer is in the circuit.
Test all circuit breakers or residual current devices .
Check all electric cables for integrity.
Confirm rigging & buoyancy is correct.
Ensure the UV light is to BS EN ISO 3059 specifications.
Ensure the ink is to BS EN ISO 9934-2 (settling test).
Confirm ink distribution system is functioning correctly.
Function test all the MPI equipment.
Ensure all ancillary equipment is ready.
Prepare the recording equipment.
What is the in water preparation and pre inspection procedures
In Water Preparation:
Establish the down lines on the correct site
Clean the site to SA2½ for 75mm either side
Pre-inspection:
Establish a datum & place the tape, mark up the weld.
Complete a CVI to identify any areas that may cause spurious indication during the MPI.
What must be done before testing can take place
Magnetic Particle Inspection:
Rig the transformer as close as possible to the test site.
Switch on the UV light.
Confirm the ambient light is less than 20lux.
Rig & lay out the coils, or magnet on the test site.
Demagnetise the work site.
Confirm the ink is constantly agitated.
Position the gauss meter to monitor the induced field strength.
Switch on the current on the diver’s command, record current.
Record the field strength on the gauss meter (1.0T minimum).
Apply ink as required to the weld & complete the inspection.
Confirm adequate field strength at each clock position.
What is the procedure when testing
Magnetic Particle Inspection If any indications are identified, re-inspect to confirm With confirmed indications record: Location (distance from datum). Length Orientation & position on weld Continuous or intermittent Branching or not Weak or strong indication Complete remedial grinding as per the procedure. Retest after grinding. Mark the ends of the feature with a punch mark if required.
What are the post inspection procedures
Post Inspection: Demagnetise. De-rig and recover all equipment to the surface. Wash all equipment with fresh water. Flush out the ink system. Report the results to the client. Cancel work permits.
In what ways can indications be recorded
MPI indications can be recorded in a number of ways.
Ultraviolet Photography. A conventional photograph can be taken using either:
An ultraviolet lamp with a long exposure time on the camera.
An appropriate ultraviolet filter on the strobe.
Foil Packets (Magfoil)
Supplied in ready-made, two-compartment packets.
These contain magnetic particles & the mixing liquid.
A flux indicator is also mounted inside the packet.
Break the internal barrier & mix the contents.
After 45 seconds, the contents will take on a grey colour.
Apply the bag to the test site.
Contents remain liquid for 100 seconds.
Apply the magnetising force during this period.
Indication will be recorded as a white mark in the packet.
Leave the packet in place until the liquid sets (5 minutes).
Remove & measure the indication length & breadth.
Video
Some video cameras will capture the visible light of the fluorescent particles.
Rubberised Tape Transfer:
Rubberised tape can be applied to a dry indication.
This cannot be applied underwater.
It may have a possible use in a welding habitat.
Name Factors Affecting MPI Sensitivity
The sensitivity of a magnetic particle inspection is the effectiveness with which the test will discover defects & crack-like features in the material.
This will depend, in the main, on three factors:
Diver Inspector.
Equipment being used.
Conditions on site.
For maximum efficiency, the diver inspector should, as well as being comfortable & as well equipped for the dive as possible:
Have confidence in his ability to use the equipment.
Be confident in his own ability to detect defects with it.
Be sure of the value of his contribution to the efficiency & safety of the structure he is inspecting.
Those last three points are acquired from the competence & confidence imparted by good training & experience.
The sensitivity of the MPI will depend on several factors, some within the control of the inspector, others not.
Sensitivity of detection depends ultimately on the contrast produced between the defect & its surroundings & the definition, which tells the size, shape & orientation.
