Magnetic Particle

Magnetic particle inspection (MPI)
- Is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).
Magnetic particle inspection is a method for detecting cracks, laps, seams, voids, pits, subsurface holes, and other surface, or slightly subsurface, discontinuities in ferromagnetic materials. Magnetic particle inspection can be used only on ferro-magnetic materials (iron and steel). It can be performed on raw material, billets, finished and semi-finished materials, welds, and in-service assembled or disassembled parts. Magnetic particles are applied over a surface either dry, as a powder, or wet, as particles in a liquid carrier such as oil or water. Common uses for magnetic particle inspectionare; final inspection, receiving inspection, in process inspection; and quality control, maintenance, and overhaul

A technician performs MPI on a pipeline to check for stress corrosion cracking using what is known as the "black and white" method. No indications of cracking appear in this picture; the only marks are the 'footprints' of the magnetic yoke and drip marks.

A close-up of the surface of a (different) pipeline showing indications of stress corrosion cracking (two clusters of small black lines) revealed by magnetic particle inspection. Cracks which would normally have been invisible are detectable due to the magnetic particles clustering at the crack openings. The scale at the bottom is numbered in centimeters.
The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.
A popular name for magnetic particle inspection is or used to bemagnafluxing.

PRINCIPLES OF OPERATION
Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an applied field to pass through the metal rather than through the air. A defect at or near the metal’s surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface.
a. To locate a defect, it is necessary to control the direction of magnetization, and flux lines must be perpendicular to the longitudinal axes of expected defects. Examination of critical areas for defects may require complete disassembly.
Two methods of magnetization, circular and longitudinal, are used to magnetize the part and induce perpendicular flux paths. Parts of complex configuration may require local magnetization to ensure proper magnetic field direction and adequate removal of surface coatings, sealants, and other similar compounds. Possible adverse influence of the applied or residual magnetic fields on delicate parts such as instruments, bearings, and mechanisms may require removal of these parts before performing the inspection.
b. Certain characteristics inherent in the magnetic particle method may introduce errors in examination results. Non relevant errors are caused by magnetic field distortions due to intentional design features, such as:
(1) Sharp radii, less than 0.10 inch radius, in fillets;
(2) Thread roots, keyways, and drilled holes; and
(3) Abrupt changes in geometry or in magnetic properties within the part.

c. Operators must understand non relevant error indications and recognize them during examination. Proper analysis of indications in these regions will require considerable skill and experience, and supplemental methods may berequired before a final evaluation can be made. Special techniques for examination of these areas are given in subsequent paragraphs.

APPLICATIONS Use magnetic particle inspection on any well-cleaned surface that is accessible for close visual examination. Typical parts deserving magnetic particle examination are: steel fasteners and pins; critical structural elements; linkages; landing gear components; splice and attach fittings; and actuating mechanisms.
a. During field repair operations, disassembly is often not necessary, except when the parts have critical areas or delicate installed components. However, for overhaul operations, amore thorough and critical examination may be obtained with stationary equipment in a shop environment with completely disassembled, and thoroughly cleaned and stripped parts.
b. Magnetic rubber examination material is useful for in-field service examinations of fastener holes in areas where the accessibility is limited or restricted, where particle suspensions may cause unwanted contamination, when a permanent record is desired, and when the examination area cannot be observed visually.

TYPES OF ELECTRIC CURRENT USED
There are several types of electrical currents used in MPI. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity one is looking for, and how far the magnetic field needs to penetrate into the part.
• Alternating current (AC) is commonly used to detect surface discontinuities. Using AC to detect subsurface discontinuities is limited due to what is known as the skin effect, where the current runs along the surface of the part. Because the current alternates in polarity at 50 to 60 cycles per second it does not penetrate much past the surface of the test object. This means the magnetic domains will only be aligned equal to the distance AC current penetration into the part. The frequency of the alternating current determines how deep the penetration.
• Direct current (DC, full wave DC) is used to detect subsurface discontinuities where AC can- not penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current through the part.DC is also limited on very large cross-sectional parts in terms of how effectively it will magnetize the part.
• Half wave DC (HWDC, pulsating DC) work similar to full wave DC, but allows for detection of surface breaking indications and has more magnetic penetration into the part than FWDC. HWDC is advantageous for inspection process as it actually helps move the magnetic particles during the bathing of the test object. The aid in particle mobility is caused by the half-wave pulsating current waveform. In a Typical mag pulse of 0.5 seconds there are 15 pulses of current using HWDC. This gives the particle more of an opportunity to come in contact with areas of magnetic flux leakage.
Each method of magnetizing has its pros and cons. AC is generally the best for discontinuities on the surface, while some form of DC is better for subsurface defects.

EQUIPMENT

A wet horizontal MPI machine with a 36 in (910 mm) coil
Using a similar machine, a U.S. Navy technician sprays magnetic particles on a test part under ultraviolet light.

An automatic wet horizontal MPI machine with an external power supply, conveyor, and demagnetizing system; it is used to inspect engine cranks.
• A wet horizontal MPI machine is the most commonly used mass production inspection machine. The machine has a head and tail stock where the part is placed to magnetize it. In between the head and tail stock is typically an induction coil, which is used to change the orientation of the magnetic field by 90° from head stock. Most of the equipment is customized and built for a specific application.
• Mobile power packs are custom-built magnetizing power supplies used in wire wrapping applications.
• Magnetic yoke is a hand-held device that induces a magnetic field between two poles. Common applications are for outdoor use, remote locations, and weld inspection. The draw- back of magnetic yokes is that they only induce a magnetic field between the poles so large-scale inspections using the device can be time-consuming. For proper inspection the yoke needs to be rotated 90 degrees for every inspection area to detect horizontal and vertical discontinuities. Yokes subsurface detection is limited. These systems used dry magnetic powders, wet powders, or aerosol cans.

DEMAGNETIZING PARTS

A pull through AC demagnetizing unit
After the part has been magnetized it needs to be demagnetized. This requires special equipment that works the opposite way of the magnetizing equipment. The magnetization is normally done with a high current pulse that reaches a peak current very quickly and instantaneously turns off leaving the part magnetized. To demagnetize a part, the current or magnetic field needed has to be equal to or greater than the current or magnetic field used to magnetize the part. The current or magnetic field is then slowly reduced to zero, leaving the part demagnetized.

AC DEMAGNETIZING
• Pull through AC demagnetizing coils: seen in the figure to the right are AC powered devices that generate a high magnetic field where the part is slowly pulled through by hand or on a conveyor. The act of pulling the part through and away from the coil's magnetic field slows drops the magnetic field in the part. Note that many AC demagnetizing coils have power cycles of several seconds so the part must be passed through the coil and be several feet (meters) away before the demagnetizing cycle finishes or the part will have residual magnetization.
• AC Decaying demagnetizing: this is built into most single phase MPI equipment. During the process the part is subjected to an equal or greater AC current, after which the current is reduced over a fixed period of time (typically 18 seconds) until zero output current is reached. As AC is alternating from a positive to a negative polarity this will leave the magnetic domains of the part randomized.
• AC Demag does have significant limitations on its ability to demag a part depending on the geometry and the alloys used.

• Reversing Full Wave DC demagnetizing: this is a demagnetizing method that must be built into the machine during manufacturing. It is similar to AC decaying except the DC current is stopped at intervals of half a second, during which the current is reduced by a quantity and its direction is reversed. Then current is passed through the part again. The process of stopping, reducing and reversing the current will leave the magnetic domains randomized. This process is continued until zero current is passed through the part. The normal Reversing DC demag cycle on modern equipment should be 18 seconds or longer. This method of demag was developed to overcome the limitations presented by the AC Demag method where part geometry and certain alloys prevented the AC Demag method from working.
• Halfwave DC demagnetizing (HWDC): this process is identical to Full Wave DC demag except the waveform is Halfwave. This method of demagnetization is new to the industry and only available from a single manufacturer. It was developed to be a cost effective method to demagnetize without needing a Full Wave DC bridge design power supply. This method is only found on single phase AC/HWDC power supplies. HWDC Demag is just as effective as Full Wave DC without the extra cost and added complexity. Of course other limitations do apply due to inductive losses when using HWDC waveform on large diameter parts. Also, HWDC effectiveness is limited past 16 inches in diameter using a 12 volt power supply.

MAGNETIC PATICLE POWDER
A common particle used to detect cracks is iron oxide, for both dry and wet systems.
• Wet system particle range in size from less than 0.5 to 10 micrometers for use with water or oil carriers. Particles used in wet systems have pigments applied that fluoresce at 365 nm (ultraviolet A) requiring 1000 µW/cm2 (10 W/m2) at the surface of the part for proper inspection. If the particles do not have the correct light applied in a darkroom the particles cannot be detected/seen. It is industry practice to use UV goggles/glasses to filter the UV light and amplify the visible light spectrum (normally green and yellow) created by the fluorescing particles. Green and yellow fluorescence was chosen because the human eye reacts best to these colors.

After applying wet magnetic particles, a U.S. navy technician examines a bolt for cracks under ultraviolet light.
• Dry particle powders range in size from 5 to 170 micrometers, designed to be seen in white light conditions. The particles are not designed to be used in wet environments. Dry powders are normally applied using hand operated air powder applicators.
• Aerosol applied particles are similar to wet systems, sold in premixed aerosol cans similar to hair spray.

MAGNETIC PATICLE CARRIERS
It is common industry practice to use specifically designed oil and water-based carriers for magnetic particles. Deodorized kerosene and mineral spirits have not been commonly used in the industry for 40 years. It is very dangerous to use kerosene or mineral spirits as a carrier due to their low flash points, and inhalation of fumes by the operators.

INSPECTION
The following are general steps for inspecting on a wet horizontal machine:
1. Part is cleaned of oil and other contaminants
2. Necessary calculations done to know the amount of current required to magnetize the part. See ASTM E1444/E1444M for formulas.
3. The magnetizing pulse is applied for 0.5 seconds during which the operator washes the part with the particle, stopping before the magnetic pulse is completed. Failure to Stop prior to end of the magnetic pulse will wash away indications.
4. UV light is applied while the operator looks for indications of defects that are 0 to +/- 45 degrees from path the current flowed through the part. Indications only appear 45 to 90 degrees of the magnetic field applied. The easiest way to quickly figure out which way the magnetic field is running is grab the part with either hand between the head stocks laying your thumb against the part (do not wrap your thumb around the part) this is called either left or right thumb rule or right hand grip rule. The direction the thumb points tell us the direction current is flowing, the Magnetic field will be running 90 degrees from the current path. On complex geometry like an engine crank the operator needs to visualize the changing direction of the current and magnetic field created. The current starts at 0 degrees then 45 degrees to 90 degree back to 45 degrees to 0 then -45 to -90 to -45 to 0 and repeats this for crankpin. So inspection can be time consuming to carefully look for indications that are only 45 to 90 degrees from the magnetic field.
5. The part is either accepted or rejected based on pre-defined accept and reject criteria
6. The part is demagnetized
7. Depending on requirements the orientation of the magnetic field may need to be changed 90 degrees to inspect for indications that cannot be detected from steps 3 to 5. The most common way to change magnetic field orientation is to use a Coil Shot. In Fig 1 a 36 inch Coil can be seen then steps 4, 5, and 6 are repeated

MATERIALS USED IN MAGNETIC PARTICLE INSPECTION

The particles used in magnetic particle inspection are finely divided Ferro-magnetic materials that have been treated with color or fluorescent dyes to improve visibility against the various surface backgrounds of the parts under inspection. Magnetic Particles, particle-suspension vehicles, and cleaners are required for conducting magnetic particle inspection. Requirements for magnetic particle inspection materials, other than cleaners, are contained in the aerospace industry standard, ASTM-E1444, Inspection, Magnetic Particle (as revised). A certification statement which will certify that the material meets applicable specification requirements will generally be received when a magnetic particle inspection material is purchased. Magnetic particle inspection materials for use on a specific part or component will generally be specified by the aircraft or component manufacturer or the FAA in documents such as; maintenance or overhaul manuals, AD’s, SSID’s, or manufacturer’s SB’s. However, if the magnetic particle inspection materials are not specified for the specific part or component to be inspected, it is recommended that personnel use materials meeting the aircraft or component manufacturers’ specifications or materials meeting the requirements of ASTM-E1444. Other FAA engineeringapproved materials may also be used. Table 5-2 provides a partial listing of commonly accepted standards and specifications for magnetic particle inspecti STANDARDS

International Organization for Standardization (ISO)
ISO 3059, Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions
ISO 9934-1, Non-destructive testing - Magnetic particle testing - Part 1: General principles
ISO 9934-2, Non-destructive testing - Magnetic particle testing - Part 2: Detection media
ISO 9934-3, Non-destructive testing - Magnetic particle testing - Part 3: Equipment
ISO 10893-5, Non-destructive testing of steel tubes. Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections
ISO 17638, Non-destructive testing of welds - Magnetic particle testing
ISO 23279, Non-destructive testing of welds - Magnetic particle testing of welds - Acceptance levels
European Committee for Standardization (CEN)
EN 1330-7, Non-destructive testing - Terminology - Part 7: Terms used in magnetic particle testing
EN 1369, Founding - Magnetic particle inspection
EN 10228-1, Non-destructive testing of steel forgings - Part 1: Magnetic particle inspection
American Society of Testing and Materials (ASTM)
ASTM E1444/E1444M Standard Practice for Magnetic Particle Testing
ASTM A 275/A 275M Test Method for Magnetic Particle Examination of Steel Forgings
ASTM A456 Specification for Magnetic Particle Inspection of Large Crankshaft Forgings
ASTM E543 Practice Standard Specification for Evaluating Agencies that Performing Nondestructive Testing
ASTM E 709 Guide for Magnetic Particle Testing Examination
ASTM E 1316 Terminology for Nondestructive Examinations
ASTM E 2297 Standard Guide for Use of UV-A and Visible Light Sources and Meters used in the Liquid Penetrant and Magnetic Particle Methods
Canadian Standards Association (CSA)
CSA W59
Society of Automotive Engineers (SAE)
AMS 2641 Magnetic Particle Inspection Vehicle
AMS 3040 Magnetic Particles, Nonfluorescent, Dry Method
AMS 3041 Magnetic Particles, Nonfluorescent,Wet Method, Oil Vehicle, Ready-To-Use
AMS 3042 Magnetic Particles, Nonfluorescent, Wet Method, Dry Powder
AMS 3043 Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Aerosol Packaged
AMS 044 Magnetic Particles, Fluorescent, Wet Method, Dry Powder
AMS 3045 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Ready-To-Use
AMS 3046 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Aerosol Packaged5
AMS 5062 Steel, Low Carbon Bars, Forgings, Tubing, Sheet, Strip, and Plate 0.25 Carbon, Maximum
AMS 5355 Investment Castings
AMS I-83387 Inspection Process, Magnetic Rubber
AMS-STD-2175 Castings, Classification and Inspection of AS 4792 Water Conditioning Agents for Aqueous Magnetic Particle Inspection AS 5282 Tool Steel Ring Standard for Magnetic Particle Inspection AS5371 Reference Standards Notched Shims for Magnetic Particle Inspection
United States Military Standard
A-A-59230 Fluid, Magnetic Particle Inspection, Suspension

MAGNETIZATION

a. Circular. Circular magnetization is induced in the part by the central-conductor method or the direct-contact method. (See figure 5-11.)
(1) Indirect Induction (central-conduc-tor method). Pass the current through a central conductor that passes through the part. When several small parts are examined at one time, provide sufficient space between each piece to permit satisfactory coverage (with particles), magnetization, and examination.
(2) Direct Induction (contact method). Pass current through the part mounted horizontally between contact plates. As an example, circular magnetization of a round steel bar would be produced by placing the ends of the steel bar between the heads of the magnetic inspection machine and passing a current through the bars.Magnetic particles applied either during or after passage of the current, or after passage of the current in magnetically-retentive steels, would disclose discontinuities parallel to the axis of the bar.
NOTE: Exercise extreme caution to prevent burning of the part at the electrode contact areas. Some causes of overheating and arcing are: insufficient contact area, insufficient contact pressure, dirty or coated contact areas,electrode removal during current flow, and too high an amperage setting.
b. Longitudinal. Longitudinal magnetization is induced in a part by placing the part in a strong magnetic field, such as the center of a coil or between the poles of an electromagnetic yoke. When using a coil, optimum results are obtained when the following conditions are met. (1) The part to be examined is at least twice as long as it is wide.
(2) The long axis of the part is parallel to the axis of the coil opening.
(3) The area of the coil opening is at least 10 times the cross-sectional areas of the part.
(4) The part is positioned against the inner wall of the coil.
(5) Three to five turns are employed for hand-held coils formed with cables.
(6) For the 10-to-1 fill factor, the effective region of inspection is 1 coil radius on either side of the coil with 10 percent overlap.
(7) The intensity of the longitudinal shots is kept just below the level at which leakage fields develop across sharp changes of section, such as radii under bolt heads, threads, and other sharp angles in parts. This does not apply when checking chrome-plated parts for grinding cracks.
(a) For example, longitudinal magnetization of a round steel bar would be produced by placing the DC coil around the bar.
After application of the magnetic particles, either during or subsequent to magnetization, discontinuities perpendicular to the longitudinal axis of the bar would be disclosed.
(b) When a yoke is used, the portion of the part between the ends of the yoke completes the path of the magnetic lines of force. This results in a magnetic field between the points of contact.
c. Permanent Magnets and Electromagnetic Yoke. The stability of the magnetic field generated by permanent magnets requires some agitation of the oxide particles within the field.
The wet method is considered most satisfactory.
Use a well-agitated plastic squirt bottle for the most effective application of the magnetic particle suspension. When the direction of possible cracks in a suspect area is not known, or would not necessarily be normal to the lines of force between the poles of the magnet, reposition the magnet to the best advantage and recheck. Usually, two shots, 90 degrees apart are required.
The part must be demagnetized between each magnetization when the field direction is changed unless the next shot is at least 10 percent stronger than the previous shot, if this is the case demagnetization is not necessary.

DETERMINATION OF FIELD
STRENGTH.

Factors such as part size, shape, magnetic properties of the material, and the method of magnetization will affect thefield strength induced within a part by a given applied magnetizing force. The factors vary considerably, making it difficult to establish rules for magnetizing during examination. Technique requirements are best determined on actual parts having known defects.
a. A magnetization indicator, such as a Quantitative Quality Indicator (QQI), should be used to verify that adequate magnetic flux strength is being used. It effectively indicates the internally-induced field, the field direction, and the quality of particle suspension during magnetization.
b. The level of magnetization required for detection of service-related defects in most cases can be lower than that required for material and manufacturing control. Contact the manufacturer for correct specifications.
NOTE: If the examination must be performed with less current than is desired because of part size or equipment limitations, the lower field strength can be partially accommodated by reducing the area of examination for each magnetization, or the examination can be supplemented by using electromagnetic yokes. Examine only 4 inches on either side of a coil instead of 6, or apply additional magnetization around the periphery of a hollow cylinder when using an internal conductor.