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Monday, 14 November 2016

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.

Sunday, 13 November 2016

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. 

Saturday, 12 November 2016

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.

Friday, 11 November 2016

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.  

Thursday, 10 November 2016

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. 

Wednesday, 9 November 2016

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. 

Tuesday, 8 November 2016

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. 

Monday, 7 November 2016

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.

Sunday, 6 November 2016

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.

Saturday, 5 November 2016

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.