NON- DESTRUCTIVE TESTING

B. Tech VI Semester

BY

Mr. Vishnu Pratap Singh, Assistant Professor

DEPARTMENT OF MECHANICAL ENGINEERING

BUDDHA INSTITUTE OF TECHNOLOGY

GIDA GORAKHPUR

Magnetic Particle Inspection(MPI)

UNIT-II

Introduction to MPI

• In theory, magnetic particle testing has a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing.
• MPI is governed by the laws of magnetism and is therefore restricted to the inspection of materials that can support magnetic flux lines, metals can be classified as

​

• Ferromagnetic
• Paramagnetic
• Diamagnetic.

Introduction to MPI

• Ferromagnetic metals are those that are strongly attracted to a

magnet and can become easily magnetized.

• Ex: iron, nickel, cobalt.
• Paramagnetic metals such as austenitic stainless steel are very weakly attracted by magnetic forces of attraction and cannot be magnetized.
• Ex: Aluminum
• Diamagnetic metals which have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed.
• Ex: Copper, Silver, Gold

Introduction to MPI

• The only requirement For the inspection through this technique is that the components being inspected must be made of a ferromagnetic material (a materials that can be magnetized) such as iron, nickel, cobalt, or some of their alloys.
• The method is used to inspect a variety of product forms including castings, forgings, and weldments.
• Many different industries use MPI such as structural steel, automotive, petrochemical, power generation, and aerospace industries.

Introduction to MPI

• Underwater inspection is another area where magnetic particle

inspection may be used to test items such as offshore structures

​

and underwater pipelines.

• MPI is very cheap, robust, easy to perform and can be handle by semi-skilled personnel without requiring much protection as needed for other NDT Methods

Magnetism

Magnetism

• Magnetism is the ability of matter to attract other matter to itself.
• Objects that possess the property of magnetism are said to be magnetic or magnetized and magnetic lines of force can be found in and around the objects.
• A magnetic pole is a point where the a magnetic line of force exits or enters a material.

Magnetism

• Magnetic field lines:
• Form complete loops.
• Do not cross.
• Follow the path of least resistance.
• All have the same strength.
• Have a direction such that they cause poles to attract or repel.

Introduction to Magnetism

Magnetic lines of force around a bar magnet

Opposite poles attracting

Similar poles repelling

Ferromagnetic Materials

• A material is considered ferromagnetic if it can be magnetized. Materials with a significant Iron, nickel or cobalt content are generally ferromagnetic.
• Ferromagnetic materials are made up of many regions in which the magnetic fields of atoms are aligned. These regions are call magnetic domains.
• Magnetic domains point randomly in demagnetized material, but can be aligned using electrical current or an external magnetic field to magnetize the material.

Demagnetized

Magnetized

12

How Does Magnetic Particle Inspection Work?

A ferromagnetic test specimen is magnetized with

a strong

magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, the discontinuity will interrupt the magnetic field flowing through the specimen and a leakage field will occur.

Contd.

• Finely milled iron particles coated with a dye pigment are applied to the test specimen.
• These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity.

​

• This indication can be visually detected under proper

lighting conditions.

Contd.

Basics terms in magnetism

16

Basics terms in magnetism

1. Polarity:- In magnetism, polarity refers to the orintation of north and south poles in space
2. Magnetic force:- It is a force of attraction or repulsion that one body has upon another.
3. Magnetic field:- It is the area around a magnet in which the magnetic forces are observable.
4. Permeability:- It is the case with which a material can be magnetized. It is expressed with the symbol µ.

µ = ^𝐵

𝐻

B= Flux density

H= magnetizing force

Contd..

5. Flux density:- It is defined as the number of lines of force per unit area. It is measured in Gauss and represented with the symbol “B”.
6. Magnetizing force:- The total number of lines making up a

magnetic field determines the strength of the force of attraction or repulsion that can be exerted by the magnet and can be represented by the symbol “H”.

7.Flux:- The total number of lines of magnetic force in a

​

material is called magnetic flux.

Contd..

Contd..

8. Retentivity:- Ability of a coil to retain some of its magnetism

within the core after magnetization process has stopped.

9. Coercive Force:- It is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized.
10. Reluctance :- Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

Contd..

12. Residual magnetism:- It is defined as the amount of magnetism left behind after removing the external magnetic field from the circuit.

• Residual magnetic fields are affected by the permeability, which can be related to the carbon content and alloying of the material.
• A component with high carbon content will have low permeability and will retain more magnetic flux than a material with low carbon steel

​

​

Contd..

13.Hysteresis:-

Contd..

1. The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed.
2. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased.
3. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+).
4. At point "a" almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux.
5. The material has reached the point of magnetic saturation.

Contd..

8.

6. When H is reduced to zero, the curve will move from point "a" to point "b". At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentivity on the graph and indicates the level of residual magnetism in the material.
7. As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve.

The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material.

9. As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction, point "d".

Contd..

10. Reducing H to zero brings the curve to point "e". It will have a level of residual magnetism equal to that achieved in the other direction.

​

• Increasing H back in the positive direction will return B to zero.
• Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back to the saturation point where it with complete the loop.

Basic Procedure- MPI

Basic Procedure- MPI

Basic steps involved:

1. Component pre-cleaning
2. Introduction of magnetic field
3. Application of magnetic media
4. Interpretation of magnetic particle indications

1. Pre-cleaning

• When inspecting a test part with the magnetic particle method it is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields alike.
• The part’s surface should be clean and dry before inspection.
• Contaminants such as oil, grease, or scale may not only prevent particles from being attracted to leakage fields

1. Pre-cleaning

2. Introduction of the Magnetic Field

The required magnetic field can be introduced into a component in a number of different ways.

1. Using a permanent magnet or an electromagnet that contacts the test piece

​

• Flowing an electrical current through the specimen
• Flowing an electrical current through a coil of wire around the part or through a central conductor running near the part.

2. Introduction of the Magnetic Field

3. Application of Magnetic Media (Wet Versus Dry)

• MPI can be performed using either dry particles, or particles suspended in a liquid. With the dry method, the particles are lightly dusted on to the surface. With the wet method, the part is flooded with a solution carrying the particles.

​

• The dry method is more portable.
• The wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility.

3. Application of Magnetic Media (Wet Versus Dry)

33

4. Interpretation of Indications

After applying the magnetic field, indications that form must interpreted. This process requires that the inspector distinguish between relevant and non-relevant indications.

Basic Procedure- MPI Dry powder method

35

Basic Procedure- MPI

Crane Hook with Service Induced Crack

Fluorescent, Wet Particle Method

Gear with Service Induced Crack

Fluorescent, Wet Particle Method

38

Drive Shaft with Heat Treatment Induced Cracks

Fluorescent, Wet Particle Method

Vineet Kumar Rai (HOD M.E.)

39

Large Bolt with Service Induced Crack

Fluorescent, Wet Particle Method

Magnetic particles

Magnetic Particles

• The particles used in magnetic particle testing are made of ferromagnetic materials, usually combinations of iron and iron oxides, having a high permeability and low retentivity.

• Particles having high permeability

are easily attracted to and magnetized by the low-

level leakage fields at discontinuities.

• Low retentivity is required to prevent the particles from being

permanently magnetized.

• Strongly retentive particles will cling together and to any magnetic surface, resulting in reduced particle mobility and increased backgr ound accumulation.
• A common particle used to detect cracks is iron oxide, for both dry and wet system/ style.

Dry Magnetic Particles

• Magnetic particles come in a variety of colors.
• A color that produces a high level of contrast against the background should be used.

Wet Magnetic Particles

• Wet particles are typically supplied as

visible or fluorescent.

• Visible particles are viewed under normal white light and fluorescent particles are viewed under black light.

Difference between Dry & Wet Magnetic Particles

Dry

• Common, relatively cheap
• Generally applied to

rougher surfaces

• Particle types
• Elongated - align well

with magnetic fields

• Rounded - move freely across a surface

Wet

• More expensive, accurate
• Painted or sprayed onto

surfaces

• Wet dye classifications
• Light (UV or

fluorescent)

• Removal type (water, solvent, or emulsifier)

Methods of magnetization

46

Methods of magnetization

• The basic principle of magnetization is to produce magnetic lines

of force across the expected direction of cracks.

• If the likely crack direction is unknown, then test must be

performed in two directions at right angles.

​

• Basic magnetization methods are

1. Magnetic flow:- to make the component of a magnetic circuit by effectively using it as the bridge of a permanent or electromagnet.

Contd..

2. Current flow:- to pass an electric current through the specimen, broadly along the direction and through the region in which cracks are to be expected.

​

• Induced current flow
• Electromagnetic induction:- to pass an electric current through a conductor which is threaded through a hollow specimen or placed adjacent or wrapped around it.

Magnetization of Ferromagnetic Materials

• There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection.

​

• It is common to classify the magnetizing methods as either
1. Direct method of magnetization
2. Indirect method of magnetization

Direct method of magnetization

• Current is passed directly through the component.
• The flow of current causes a circular magnetic field to form in and

around the conductor.

• When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component to avoid damage of the component (due to arcing or overheating at high resistance points).
• The magnetic field formed during this method is at right angles to the

direction of electric flow.

Contd..

• There are several ways that direct magnetization is commonly accomplished.
• Clamping the component between two electrical contacts in a special piece of equipment.

​

2. Using clamps or prods

Contd..

1. Clamping the component between two electrical contacts in a special piece of equipment.

Contd..

1. Clamping the component between two electrical contacts in a special piece of equipment.

• Current is passed through the component and a circular magnetic field is established in and around the component. When the magnetizing current is stopped, a residual magnetic field will remain within the component.
• The strength of the induced magnetic field is proportional to the amount of current passed through the component.

Contd..

2. Using clamps or prods

• Which are attached or placed in contact with the component. Electrical current flows through the component from contact to contact.
• The current sets up a circular magnetic field around the path of the current.

Contd..

2. Using clamps or prods

Indirect method of magnetization

• Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component.
• As with direct magnetization, there are several ways that indirect magnetization can be accomplished.

​

• Use of permanent magnets
• Electromagnets
• Central conductor magnetization
• Using coils and solenoids

1. Permanent magnets

1.Use of permanent magnets

• The use of permanent magnets is a low cost method of establishing a magnetic field.
• However, their use is limited due to lack of control of the field strength and the difficulty of placing

permanent magnets from the componen

and removing strong

t.

Contd..

• The use of industrial magnets is not popular because they are very strong (they require significant strength to remove them from the surface, about 250 N for some magnets) and thus they are difficult and sometimes dangerous to handle.
• However, permanent magnets are sometimes used by divers for inspection in underwater environments or other areas, such as explosive environments, where electromagnets cannot be used.
• Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit.

2. Electromagnets(using yokes)

• Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry.
• Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core.
• When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.

Contd..

• An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off.
• They can be powered with AC from a wall socket or by DC from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected.

3. Central conductor magnet

ization

• Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor.
• A circular magnetic field can be established in cylindrical components by using a central conductor.
• Typically, one or more cylindrical components are hung from a solid

copper bar running through the inside diameter.

​

• Current is passed through the copper bar and the resulting circular

magnetic field establishes a magnetic field within the test

components.

4. Use of coils and solenoids

• When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component.
• The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot".

Longitudinal & Circular Magnetic Fields

Circular

1. Clamping between

the component two electrical

contacts (HEAD SHOT)

​

2. Using clamps or prods
3. Central conductor Magnetization

Longitudinal

​

1. Permanent magnets

​

• Electromagnetic yokes

​

• Using a Coil (COIL SHOT)

Types of Magnetizing Current

64

Types of Magnetizing Current

• Electric current is used to establish the magnetic field in

​

components during magnetic particle inspection.

​

• Commonly types of current are

​

1. Alternating current (AC)

​

• Direct current (DC)

1. Direct Current

• Direct current (DC) flows continuously in one direction at a constant

voltage. A battery is the most common source of direct current.

• The current is said to flow from the positive to the negative terminal,

though electrons flow in the opposite direction.

• DC is very desirable when inspecting for subsurface defects because DC generates a magnetic field that penetrates deeper into the material.
• In ferromagnetic materials, the magnetic field produced by DC generally penetrates the entire cross-section of the component.

2. Alternating Current

defects.

• Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. Since AC is readily available in most facilities, it is convenient to make use of it for magnetic particle inspection.
• However, when AC is used to induce a magnetic field in ferromagnetic materials, the magnetic field will be limited to a thin layer at the surface of the component. This phenomenon is known as the "skin effect" and occurs because the changing magnetic field generates eddy currents in the test object.
• The eddy currents produce a magnetic field that opposes the primary field, thus reducing the net magnetic flux below the surface. Therefore, it is recommended that AC be used only when the inspection is limited to surface

3. Rectified Alternating Current

• Clearly, the skin effect limits the use of AC since many inspection applications call for the detection of subsurface defects.
• Luckily, AC can be converted to current that is very much like DC through the process of rectification. With the use of rectifiers, the reversing AC can be converted to a one directional current.

3. Rectified Alternating Current

(a)

(b)

(c)

69

(a) Half Wave Rectified Alternating Current (HWAC)

• When single phase alternating current is passed through a rectifier, current is

allowed to flow in only one direction.

• The reverse half of each cycle is blocked out so that a one directional, pulsating current is produced.
• The current rises from zero to a maximum and then returns to zero. No current flows during the time when the reverse cycle is blocked out.
• The HWAC repeats at same rate as the unrectified current (50 or 60 Hz). Since half of the current is blocked out, the amperage is half of the unaltered AC. This type of current is often referred to as half wave DC or pulsating DC. The pulsation of the HWAC helps magnetic particle indications form by vibrating the particles and giving them added mobility where that is especially important when using dry particles. HWAC is most often used to power electromagnetic yokes.

(b) Full Wave Rectified Alternating Current (FWAC) (Single Phase)

• Full wave rectification inverts the negative current to positive current rather than blocking it out. This produces a pulsating DC with no interval between the pulses. Filtering is usually performed to soften the sharp polarity switching in the rectified current.
• While particle mobility is not as good as half-wave AC due to the reduction in pulsation, the depth of the subsurface magnetic field is improved.

( c ) Three Phase Full Wave Rectified Alternating Current

• Three phase current is often used to power industrial equipment because it has more favorable power transmission and line loading characteristics.
• This type of electrical current is also highly desirable for magnetic particle testing because when it is rectified and filtered, the resulting current very closely resembles direct current.
• Stationary magnetic particle equipment wired with three phase AC will usually have the ability to magnetize with AC or DC (three phase full wave rectified), providing the inspector with the advantages of each current form.

Equipment's used for MPI

73

Equipment's used for MPI

• MPI equipment serves the following purposes, it provides

Sufficient power of right type Suitable contact and coils

Convenient means for accomplishing proper magnetization with respect to field strength and direction.

Means of applying the magnetic particles

Well lighted space for careful examination of the part of

indication.

Contd..

1. Simple equipment
1. Permanent Magnets
2. Electromagnetic Yokes
2. Large portable equipment
• Prods
• Portable Coils and Conductive Cables
• Portable Power Supplies
3. Stationary magnetizing equipment
4. Large heavy duty DC equipment

1. Simple equipment

For occasional testing of small casting or machine parts for

detection of surface cracks, small and easily portable equipment

​

is most convenient.

​

1. Permanent Magnets (already discussed)

​

• Electromagnetic Yokes (already discussed)

2. Large portable equipment

it is used where higher power is required or heavier duty cycles make the small kits inadequate.

One of the smallest of this series operates at 120V AC and delivers up to 700amperes, either AC or half wave DC.

​

1. Prods (already discussed)
2. Portable Coils and Conductive Cables
3. Portable Power Supplies

Prods

• Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current (AC or DC) through the metal.
• Prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off.
• Sometimes the two prods are connected by any insulator, to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.

Portable Coils and Conductive Cables

• Coils and conductive cables are used to establish a longitudinal magnetic field within a component. When a preformed coil is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used to energize the coil.
• Also, flexible conductive cables can be wrapped around a component to form a coil. The number of wraps is determined by the magnetizing force needed and of course, the length of the cable. Normally, the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere-turns. Ampere-turns is the amperage shown on the amp meter times the

number of turns in the coil.

Portable Power Supplies

• Portable power supplies are used to provide the

necessary electricity to the prods, coils or cables. Power supplies are commercially available in a variety of sizes. Small power supplies generally provide up to 1,500A of half-wave DC or AC. They are small and light enough to be carried and operate on either 120V or 240V electrical service.

• When more power is necessary, mobile power supplies can be used. These units come with wheels so that they can be rolled where needed. These units also operate on

120V or 240V electrical service and can provide up to

6,000A of AC or half-waveDC.

3. Stationary magnetizing equipment

• A large variety of stationary , bench-type units is available, with various characteristics to fit different testing requirements.
• The smaller size equipment is used for small parts that can be easily transported and handled by hand.
• The larger ones are used for heavy parts such as long diesel engine crankshafts, where handling must be crane.

Contd..

• Stationary magnetic particle inspection equipment is designed for use in laboratory or production environment.

• The most common stationary system is the wet horizontal (bench) unit.
• Wet horizontal units are designed to allow for batch inspections of a variety of components.
• The units have head and tail stocks (similar to a lathe) with electrical contact that the part can be clamped between. A circular magnetic field is produced with direct magnetization.

Contd..

• Most units also have a movable coil that can be moved into place so the indirect magnetization can be used to produce a longitudinal magnetic field.
• Most coils have five turns and can be obtained in a variety of sizes. The wet magnetic particle solution is collected and held in a tank.
• A pump and hose system is used to apply the particle solution to the components being inspected.

​

Contd..

Vineet Kumar Rai (HOD M.E.)

• Some of the systems offer a variety of options in electrical current used for magnetizing the component (AC, half wave DC, or full wave DC). In some units, a demagnetization feature is built in, which uses the coil and decaying AC.

4. Large heavy duty DC equipment

• Very versatile types of heavy duty stationary equipment are those DC units designed for application of the “overall” method of magnetizing , for the inspection of very large and complicated castings.
• Rectified three-phase AC is delivered with current values

running as high as 20,000 amperes.

System Sensitivity

System Sensitivity

• MPI Methods are sensitive means of locating small and shallow surface cracks in ferromagnetic components. Many incipient fatigue cracks and fine grinding cracks having size less than 0.02mm deep and surface openings of one tenth of that or less can be located using MPI.
• Detectability generally involves a relation b/w surface opening and depth.
• If defects sought are usual cracks, comparatively low level of magnetic force will give sufficient build-up.

Contd…

• However, to find minute cracks or subsurface flaws, the flux level should be high. Theoretically, a level just below saturation would give the most sensitive results. But this is impractical owing to the non-regular shapes of the components encountered.

​

• Sensitivity also depends on the type of current used.

Contd…

• Various tests have conclusively proved the following information:

AC magnetization is most effective for surface defects. AC magnetization is not effective for subsurface defects.

DC( straight or half wave) must be used for subsurface defects. Half wave DC gives superior penetration as compared to straight DC

​

Half wave DC dry method gives the greatest penetration.

Checking devices of MPI- Magnetic field indicators

90

Magnetic field indicators

is of adequate

is critical when

• Determining whether a magnetic field

strength and in the proper direction performing magnetic particle testing.

• There is actually no easy-to-apply method that permits an

exact measurement of field intensity at a given point within a material.

Contd..

• Tools and methods that are used to determine the presence and direction of the field surrounding a component are
1. Hall-Effect Meter (Gauss Meter)
2. Quantitative Quality Indicator (QQI)
3. Pie Gage
4. Slotted Strips

1. Hall-Effect Meter (Gauss Meter)

• A Gauss meter is commonly used to measure the tangential

​

field strength on the surface of the part.

​

• By placing the probe next to the surface, the meter measures

the intensity of the field in the air adjacent to the component when a magnetic field is applied.

Contd..

• The advantages of this device are:

It provides a quantitative measure of the strength of magnetizing force tangential to the surface of a test piece.

It can be used for measurement of residual magnetic

fields, and it can be used repetitively.

• The main disadvantage is that such devices must be periodically calibrated.

2. Quantitative Quality Indicator (QQI)

• The Quantitative Quality Indicator (QQI) or Artificial Flaw Standard is often the preferred method of assuring proper field direction and adequate field strength (it is used with the wet method only).
• The QQI is a thin strip (0.05 or 0.1 mm thick) of AISI 1005 steel with a specific pattern, such as concentric circles or a plus sign, etched on it.
• The QQI is placed directly on the surface, with the itched side facing the surface, and it is usually fixed to the surface using a tape then the component is magnetized and particles applied.

Contd..

adequate, the particles

• When the field strength is

will

adhere over the engraved pattern and provide information about the field direction.

3. Pie Gauge

• The pie gage is a disk of highly permeable material divided into four, six, or eight sections by non-ferromagnetic material.
• The divisions serve as artificial defects that radiate out in different directions from the center.

​

• The sections are furnace brazed and copper plated.
• The gage is placed on the test piece copper side up and the test piece is magnetized.
• After particles are applied and the excess removed, the indications

provide the magnetic field.

Contd..

• Pie gages are mainly used on flat surfaces such as weldments or steel castings where dry powder is used with a yoke or prods.
• The pie gage is not recommended for precision parts with complex shapes, for wet-method applications, or for proving field magnitude.
• The gage should be demagnetized between readings.

4. Slotted Strips

• Slotted strips are pieces of highly permeable ferromagnetic material with slots of different widths.
• These strips can be used with the wet or dry method.
• They are placed on the test object as it is inspected.
• The indications produced on the strips give the inspector a

general idea of the field strength in a particular area.

Interpretations of MPI and Indications

Interpretations of MPI

• It involves the testing of inspection by a trained NDT engineer whose skills and experience is needed most to perform the test correctly.

• During MPI, the magnetic particles adhere to the test object wherever leakage flux is present and the interpreter has to properly interpret the adherence of magnetic particles.

​

• Proper interpretation involves finding out whether

1. Adherence is due to the prescience of cracks
2. Adherence is because of the change in the cross section

Contd..

• Wrong interpretation may lead to rejection of the material, thus resulting in material wastage. Thus an NDT engineer who interprets the results, determine the success or failure of any NDT methods.

Indications in MPI

• Indications in MPI relies greatly on the qualification of the

inspector (Interpreter).

• After proper interpretation, the interpreter classify the indications

as False, Non relevant or relevant.

• 1. False Indications:- can be produced due to improper handling, use of excessively high magnetizing currents, inadequate pre- cleaning of the parts to remove oil, grease, corrosion products and other surface contaminates.

Contd..

2. Non-relevant Indications:- these indications are the result of flux

leakage due to geometrical or permeability changes of the test object.

• Ex:- Geometrical changes include splines, thread roots, gear teeth

etc.

3. Relevant indications:- are produced by flux leakages due to discontinuities in the part.

Note;- Discontinuities which cause the part to fail from its orgibal purpose are classified as Defects.

Advantages & Limitations of MPI

Advantages of MPI

• Fast, simple and inexpensive
• Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
• Unaffected by possible deposits, e.g. oil, grease or other

metals chips, in the cracks

​

• Minimal surface preparation (no need for paint removal)
• Results readily documented with photo or tape impression
• Post-cleaning generally not necessary.

Advantages of MPI

materials generally not

size and shape of the

• A relatively safe technique; combustible or hazardous.
• Indications can show relative discontinuity.

• Easy to use and requires minimal amount of training.
• Portable (materials are available in aerosol spray cans)
• Low cost (materials and associated equipment are relatively inexpensive)

Limitations of MPI

• Non-ferrous materials, such as aluminum, magnesium, or most stainless steels, cannot be inspected - Only applicable to ferromagnetic materials.

• Examination of large parts may require use of equipment with special power requirements - Relatively small area can be inspected at a time.
• May require removal of coating or plating to achieve desired sensitivity.

​

• Limited subsurface discontinuity detection capabilities.

Limitations of MPI

• Post-demagnetization is often necessary.
• Alignment between magnetic flux and indications is important.
• Each part needs to be examined in two different directions.
• Relatively small area can be inspected at a time.
• Only materials with a relatively nonporous surface can be inspected.
• The inspector must have direct access to the surface being

inspected.

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Thank You