Portable Magnetizing Equipment For Magnetic Particle Inspection.

To property inspect a part for cracks or other defects, it is important to become familiar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detecting a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degree angle. Flaws that are normal (90 degrees)  to the magnetic field will produce the strongest indication because they disrupt more of the magnet flux.
Therefore, for proper inspection of a component, it is important to be able to establish a magnetic field in at least two directions. A variety of equipment exists to establish the magnetic field for MPI.One way to classify equipment is based on its portability.  Some equipment is designed to be portable so that inspection can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility. Portable equipment will be discussed first.
PERMANENT MAGNETS:-Permanent magnets are sometimes used for magnetic particle inspection as the source of magnetism. The two primary types of permanent magnets are bar magnets and horseshoe (yoke) magnets. These industrial magnets m are usually very strong and may require significant strength to remove them from a piece of metal.Some permanent magnets require over 50 pounds of force to remove them from the surface. Because it is difficult to remove the magnets from the component being inspected and sometimes difficult and dangerous to place the magnets, their use is not particularly popular. However permanent magnets are sometimes used by diverse 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 electromagnetes might not fit.

Magnetic field orientation and flaw detectable.

Magnetic field orientation and flaw detectable. 

The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two direction is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimensions of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. As can be seen in the image, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects)  should be detectable. Alternately, transverse -type defects will not be detectable with circular magnetization. 

THE HYSTERESIS LOOP AND MAGNETIC PROPERTIES.

THE HYSTERESIS LOOP AND MAGNETIC PROPERTIES. 

A great deal of information can be learned about the magnetic properties of a materials by studying it's hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop.
The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been throughly demagnetized will follow the dashed line as H is increased. As the line demonstrates the greater the amount of current applied (H+), the stronger the magnetic field in the component ((B+). At point  "a" almost all of the magnetic Domains are aligned andante additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When H is reduced to zero, the curve will move from point "a" to point "b" At this point, it can ne seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentive on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost their alignment)  As the magnetizing force is reversed, the curve moves to point "c" where the flux has been reduced zero.This is Allen the point of coercivity on the curve.( The revised magnetizing force has flipped enough of the domains so that the net flux within the material is zero.)  The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material. 

MAGNETIZATION USING DIRECT INDUCTION.

MAGNETIZATION USING DIRECT INDUCTION. 

With direct magnetization, current passed directly through the component. Recall that whenever current flows, a magnetic field is produced. Using the right-hand rule, which was introduced earlier, it is known that the magnetic lines of flux form normal to the direction of the current and form a circular field 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 improper contact can result in arcing that may damage the component. It is also possible to overheat component in areas of high resistance such as the contact points and in areas of small cross - sectional area.

  There are several ways that direct magnetization is commonlyaccomplished. One way involves 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 if proportional to the amount of current passed through the component.

MAGNETIZATION USING INDIRECT INDUCTION.

MAGNETIZATION USING INDIRECT INDUCTION. 

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.
The use of permanent magnets is a low cost method of establishing field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.
Electromagnetes in the form of an adjustable horseshoe magnet ( called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnetes 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.
Another way of indirectly inducing a magnetic field in a material is 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 component 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.

MAGNETIZING CURRENT IN MAGNETIC PARTICLE INSPECTION.

MAGNETIZING CURRENT IN MAGNETIC PARTICLE INSPECTION. 

Half wave Alternative current (HWAC):-

When single phase alternative current is passed through a rectifier, current is allowed to flow in only one direction. The reverse half of each cycle is blocked outsourcing 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 HAWC repeat at same rate as the unrectified current (60hertz typical). Since half of the current is blocked out, the amperage is halftone 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 inspection form by vibrating the particles and giving them added mobility. This added mobility is especially important when using dry particles. The pulsation is reported to significantly improve inspection sensitivity. HAWC IS most often used to power Electromagnetic yokes.

Full Wave Rectified Alternating Current (FWAC):-

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.

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. These 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. 

Longitudinal Magnetic Fields in Magnetic Particle Inspection.

Longitudinal Magnetic Fields in Magnetic Particle Inspection. 

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 often 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.
The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much denser inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component. 

magnatic particle test.

When a component is magnetized along its complete length, the flux loss is small alongside length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density wellbeing relatively uniform throughout the component. Flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component.

When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the solenoid and return to the part at a opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid. As a result, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore a long component must be magnetized and inspected at several location along its length for complete inspection coverage. 

Circular Magnetic Field Distribution and Intensity.

As discussed previously, when current is passed through a solid conductor, a magnetic field forms in and around the conductor. The following statements can be made about the distribution and Intensity of the magnetic field.
1)    The field strength varies from zero at the centre of the component to a maximum at the surface.
2)    The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant.( However, a larger conductor is capable of carrying more current.)
3)    The field strength outside the conductor is directly proportional to the current strength. Inside the conductor, the field strength is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B-H curve.
4)    The field strength outside the conductor decreases with distance from the conductor.
In the images below, the magnetic field strength is graphed versus distance from the centre of the conductor. It can be seen that in a nonmagnetic carrying DC, the internal field strength rises from zero at the centre to a maximum value at the surface of the conductor. The external field strength decreases with distance from the surface of the conductor. When the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic conductor. This is due to permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.  

The Magnetic Field Distribution In Alternative Current.

The Magnetic Field Distribution In Alternative Current. 

When the conductor is carrying Alternating Current, the internal magnetic field strength rises from zero at the centre to a maximum at the surface. However, the field is concentrated in a thin layer near the surface of  the conductor. This is known as the "skin effect."  The skin effect is evident in the field strength versus distance graph for a magnetic conductor shown to the right. The external field decreases with increasing distance from the surface as it does with DC. It should be remembered that with AC the fields constantly varying in strength and direction.
In a hollow circular conductor there is no magnetic field in the void area. The magnetic field is zero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. As with a solid conductor, when the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic conductor due to the permeability of the magnetic material. The external field strength decreases with distance from the surface of the conductor. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

The Magnetic Field In Distribution In Direct Current.

As can be seen in the field distribution images, the field strength at the inside surface of hollow conductor carrying magnetic field produced by direct magnetization is very low. Therefore, the direct method of magnetization is not recommended when inspecting the inside diameter wall of a hollow component for shallow defects. The field strength increase rather rapidly as one moves in from the ID,so if the defect has significant depth, it may be detectable.However a much better method of magnetizing hollow component for inspection of the ID and OD surfaces is with the use of a central conductor. As can be seen in the field distribution image to the right, when current is passed through a nonmagnetic central conductor (copper bar), the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface. After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can.
1)    Affect machining by causing to cling to a component.
2)    Interfere with electronic equipment such as a compass.
3)    Create a condition known as arc blow in the welding process. Arc blow may cause the weld arc wonder or filler metal to be repelled from the weld.
4)    Cause abrasive particles to cling to bearing or flying surfaces and increase wear.
Removal of a field may be accomplished in several ways. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. The curie temperature for a low carbon steel is 770 C or 1390 F. When steel is heated above its curie temperature, it will become austenitic and loses its magnetic properties. When it is cooled backdown, it will go through a reverse transformation and will contain no magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the earth's magnetic field. 

ELECTROMAGNETS IN MAGNETIC PARTICLE INSPECTION.

Magnetic particle inspection.

Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism. That's, using an electrical current to produce the magnetic field. An Electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field.

It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. 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 Alternating Current from a wall socket or by direct current 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. Some yokes can lift weights in excess of 40 pounds.

PRODS IN MAGNETIC PARTICLE INSPECTION

Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods that can be used in magnetic particle inspection. 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 (as shown in the image)  to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.
If proper contact is not maintained between the prods and the component surface. electrical arcing can occur and cause damage to the component. For this reason, the use of prods are not allowed when inspecting aerospace and other critical components. To help prevent arcing, the prod tips should be inspected frequently to ensure that they are not oxidized, covered with scale or other contaminant,or damaged. 

PORTABLE COILS AND CONDUCTIVE CABLES.

Coils and conductive cables are used to establish a longitudinal Magnetic field within a component. When a preformed coils 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 is typically 00 extra flexible or 0000 extra flexible. 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 direct current or Alternating Current when used with 4.5meter 0000cable. 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 also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half - wave DC when 9meters or less of 0000 cable is used 

Stationery Equipment for Magnetic Particle Inspection.

Stationary magnetic particle inspection equipment is designed for use in laboratory or production environment. The most 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 lathe) with electrical contact that the part can be clamped between. A circular magnetic field is produced with direct magnetization.  The tail stock can be moved and locked into place to accommodate parts of various lengths. To assist the operator in clamping the parts, the contact on the headstock can be moved pneumatically via a foot switch.
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 component being inspected. Either the visible or fluorescent particles can be used. Some of the system offer a variety of options in electrical current used for magnetizing the component. The operator has the option to use AC, half wave DC, or full wave DC.In some units, a demagnetization features is built in, which uses the coil and decaying AC.

Equipment Of Magnetic Particle Inspection.

To inspect a part using a head - shot, the part is clamped between two electrical contact pads. The magnetic solution,called a bath, is then flowed over the surface of the part. The bath is then interrupted and magnetizing current is applied to the part for a short duration. typically 0.5 to 1.5 seconds.(precautions should be taken to prevent burning or overheating of the part)  A circular field flowing around the circumstances of the part is created. Leakage fields from defects then attract the particles to form indications.
When the coil is used to establish a longitudinal Magnetic field within the part is placed on the inside surface of the coil. Just as done with a head shot, the bath is then flowed over the surface of the part. A magnetizing current is applied to the part for a short duration, typically 0.5 to 1.5 seconds, just after coverage with the bath is interrupted. (Precautions should be taken to prevent burning or overheating of the part) Leakage fields from defects attract the particles to form visible indications.
Theoretical horizontal unit can also be used to establish a circular magnetic field using a central conductor. This type of a setup is used to inspect parts  that have an open centre, such as gears, tubes, and other ring - shaped objects. A central conductor is an electrically conductive bar that is usually made of copper or aluminium. The bar is inserted through the opening and the bar is then clamped between the contact pads  When current is passed through the central conductor, a circular magnetic field flows around the bar and enters into the part or parts being inspected. 

Multidirectional Equipment for Magnetic Particle Inspection.

Multidirectional units allow the component to be magnetized in two directions, longitudinally and circumferentially, in rapid succession.  Therefore, inspections are conducted without the need for a second shot in multidirectional units, the two fields are balanced so that the field strength are equal in both directions. These quickly changing balanced fields a multidirectional field in the component detection of defects lying in more than one direction.
Just as conventional wet-hirizontal systems the electrical current used in multidirectional magnetization may be Alternating half-wave direct, or full-sized. It is also possible to use a combination of currents depending on the test applications.Multidirectional magnetization can be used for a large number of production application, and high volume inspections.
To determine adequate field strength and balance of the rapidly changing fields,techniques development requires a little more effort when multirectional equipment is used. It is desirable to develop the technique using a component with known defects oriented in at least two directions, or a manufactured defect standard. Quantitative Quality Indicators (QQI)  are also often used to verify the strength and direction of magnetic fields. 

Lights for Magnetic Particle Inspection.

Magnetic particle inspection can be performed using particles that are highly visible under white light conditions or particles that are highly visible under ultraviolet light conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is required as long as the area of inspection is well lit.A light intensity of at least 1000 lux (100fc) is recommended when visible particles are used, but a variety of light sources can be used.
When fluorescent particles are used, special ultraviolet light must be used. Fluorescence is defined as the property. of emitting radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic particle inspections are coated with a material that produces light in the visible spectrum when exposed to near - ultraviolet light. This "particle glow" provides high contrast indications on the component anywhere particles collect. Particles that fluoresce yellow - green are most common because this color matches the peak sensitivity of the human eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available. 

Ultraviolet Light in Magnetic Particle Inspection.

Ultraviolet light or "black light"  is light in the 1000 to 4000 Angstroms (100 to 400nm) wavelength range in the Electromagnetic spectrum. It is a very energetic form of light that is invisible to the human eye.wavelengths above 4000A fall into the visible spectrum and are seen as the color violet. UV is separated according to wavelength into three classes : A, B' and C. The shorter the wavelength, the more energy that is carried in the light and the more dangerous it is to the human cells.
The desired wavelength range for use in nondestructive testing is between 3500 and 3800A with a peak wavelength at about 3650A. This wavelength range is used because it is in the UV-A range, which is the safest to work with. UV-B will do an effective job of causing substances to fluoresce, however, it should not be used because harmful effects such as skin burns and eye damage can occur. This wavelength of radiation is found in the arc created during the welding process. UV-C (1000 to 2800A) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings.
The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the bulb. The output of a UV bulb spans a wide range of wavelengths.The short wavelength of 3120 to 3340A are produced in low levels. A peak wavelength of 3650A is produced at a very high intensity Wavelength in the visible violet range (4050A to 4350A),green- yellow (5460A), yellow (6770A) are also usually produced. The filter allows only radiation in the range of 3200 to 4000A and a little visible dark purple to pass. 

Gauss Meter or Hall Effect Gage in Magnetic particle Inspection.

A Gauss meter with a Hall Effect probe is commonly used to measure the tangential field strength on the surface of the part. As discussed in some detail on the measuring magnetic fields page the Hall Effect is the transverse electric field created in a conductor when placed in a magnetic field. Gauss meters, also called Tesla meters, are used to measure the strength of a field tangential to the surface of the magnetized test object. The meters measure the intensity of the field in the air adjacent to the component when a magnetic field is applied.
The advantages of Hall effect devices are: they provide a quantitative measure of the strength of magnetizing force tangential to the surface of a test piece, they can be used for measurement of residual magnetic fields, and they can be used repetitively.
Their main disadvantages are that they must be periodically calibrated and they cannot be used to establish the balance of fields in multidirectional applications.