High Intensity Ultraviolet Light

The 400 watt metal halide bulbs or "super light" can be found in some facilities. This super bright will provide adequate lighting over an area of up to ten times that covered by the 100 watt bulb. Due to their high intensity, excessive light reflecting from the surface of a component is a concern. Moving the light a greater distance from the inspection area will generally reduce this glare. Another type of high intensity light available is the micro - discharge light. This particular light produces up to ten times the amount of UV light conventional lights produce. Reading of up to 60,000 uW/cm2 at 15 inches can be achieved.
Determining whether a magnetic field is of adequate strength and in the proper direction is critical when performing magnetic particle testing. As discussed previously, knowing the direction of the field is important because the field should be as close to perpendicular to the defect as possible and no more than 45degrees from normal. Being able to evaluate the field direction and strength is especially important when inspecting with a multidirectional machine, because when the fields are not balanced property, a vector field will be produced that may not detect some defects.
There is actually no easy -to-apply method that permits an exact measurement of field intensity at a given point with in a material. In order to measure the field strength, it is necessary to intercept the flux lines. This is impossible without cutting into the material and cutting the material would immediately change the field within the part. However, cutting a small slot or hole into the material and measuring the leakage field that crosses the air gap with a Gauss meter is probably the best way to get an estimate of the actual field strength within a part. Nevertheless, there are a number of tools and methods available that are used to determine the presence and direction of the field surrounding a component. 

MAGNETIC FIELD PRODUCED BY A COIL.

When a current carrying conductor is formed into a loop or several loops to from a coil, a magnetic field develops that flows through the center of the loop or coil along its longitudinal axis and circles back around the outside of the loop or coil. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. A loosely wound coil is illustrated below to show the interaction of the magnetic field. The magnetic field is essentially uniform down the length of the coil when it is wound tighter.
The strength of a coils magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long, straight coil of wire is called a solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection using the magnetic particle testing method. please be aware that the field outside the coil is weak and is not suitable for magnetizing ferromagnetic materials. 

ELECTROMAGNETIC FIELDS IN MAGNETIC PARTICLE INSPECTION.

Magnets are not the only source of magnetic field. In 1820, Hans christian oersted discovered that an electric current flowing through a wire caused a nearby compass to deflect. This indicated that the current in the wire was generating a magnetic field. Oersted studied the nature of the magnetic field around the long straight wire. He found that the magnetic field existed in circular From around the wire and that the intensity of the field was directly proportional to the amount of current carried by the wire. He also found that the strength of the field was strongest next to the wire and diminished with distance from the conductor until it could no longer be detected. In most conductors,the magnetic field exists only as long as the current is flow (i.e.an electrical charge is in motion).  However, in ferromagnetic materials the electric current will cause some or all of the magnetic Domains to align and a residual magnetic field will remain.

Oersted also noticed that the direction of the magnetic field was dependent on the direction of the electrical current in the wire. A three - dimmensional representation of the magnetic field is shown below. There is a simple rule for remembering the direction of the magnetic field around a conductor. It is called the right-hand rule. If a person grasps a conductor in one's right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field. 

Magnetic fields in and around horseshoe and ring magnets.

Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The  magnetic lines of force flow from Pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.
If a bar magnet was placed across the end of a horseshoe magnet or if a magnet was formed in the shape of a ring, the lines of magnetic force would not even need to enter the air.The value of such a magnet where the magnetic field is completely contained with the material probably has limited use. However, it is important to understand that the magnetic field can flow in loop within a material. 

MAGNETIC FIELD IN AND AROUND A BAR MAGNET.

A magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over over a magnet and sprinkling the paper with iron filings. The particles align themselves with the line of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exits in three dimensions but are only seen in two dimensions in the image.
It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. The area where the exit poles are concentrated is called the magnets north pole and the area where the entrance poles are concentrated is called the magnets south pole.

Magnetic Domains in magnetic particle inspection.

Ferromagnetic materials get their magnetic properties not only because their atoms carry a magnetic moment but also because the material is made up of small regions known as magnetic Domains.  In each domain, all of the atomic dipoles are coupled together in a preferential direction. This alignment develops as the material develop ls it's crystalline structure during solidification from the molten state. Magnetic Domains can be detected using magnetic force Microscopy (Mum)and images of the domains like the one shown below can be constructed.
during solidification, a trillion or more moments are aligned parallel so that the magnetic force within the domain is strong in one direction. Ferromagnetic materials are said to be characterized by spontaneousm magnetization since they obtain saturation magnetization in each of the domains without an external magnetic field being applied.  Even though the domains are magnetically saturated  the bulk material may not show Amy signs of magnetism because the domains.develop themselves and are randomly arrested relative tutor each other. 

The source of magnetism in magnetic particle inspection.

All matter is composed of atoms, and atoms are composed of protons and electrons.The protons and neutrons are located in the atoms nucleus and the electrons are in constant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through space..A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.
This may be hard to visualize on a subatomic scale but consider electric current flowing through a conductor. When the electrons ( electric current)  are flowing through the conductor, a magnetic field forms around the conductor. The magne field can be detected using a compass. The magnetic field will place a force on the compass needle, which is another example of a dipole. Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field. However, not all materials react the same way. This will be explored more in the next section.

MAGNETISM IN MAGNETIC PARTICLE INSPECTION.

Magnets are very common items in the workplace and household. Uses of magnets range from holding pictures on the refrigerator to the causing torque in electric motors. Most people are familiar with the general properties of magnets but are less familiar with the source of magnetism. The traditional concept of magnetism centres around the magnetic field and what is know as a dipole. The term magnetic field, simply describes a volume of space where there is a change in energy within that volume. This change in energy can be detected and measured. The location where a magnetic field can be detected existing or entering a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in pairs, hence the name dipole. Therefore, a dipole is an object that has a magnetic pole on one end a second, equal but opposite, magnetic pole on the other
A bar magnet can be considered a dipole with a north pole at one end south pole at the other. A magnetic field can be measured leaving the dipole at the north pole and returning the magnet at the south pole. If a magnet is cut in two, two magnets or dipoles are created out of one. This sectioning and creation of dipoles can continue to the atomic level. Therefor, the source of magnetism lies in the basic building block of all matter...the atom.

HISTORY OF MAGNETIC PARTICLE INSPECTION

Magnetism is the ability of matter to attract other to itself. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becharee, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.
The earliest known use of magnetism to inspect took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrels length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not commonly used until some time after world war 1.
In the early 1920s, William Hoke realized that magnetic particle ( colored metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that metallic grindings from hard steel parts (held by a magnetic chuk while being ground)  formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. The image shows a 1928 Electyro-magnetic steel Testing Device (MPI) made by the Equipment and Engineering company Ltd.(ECO) of standard, England.

BASIC PRINCIPLES OF MAGNETIC PARTICLE TESTING.

In the theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnetic is called a north pole and a pole where a line of force enters the magnet is called a south pole.
When a bar magnet is broken in the center of its length, two complete bar magnet with magnetic poles on each end of each piece will result. If the magnetic is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and,thus is called a flux leakage field.

If iron particle are sprinkle on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

INTRODUCTION TO MAGNETIC PARTICLE INSPECTION.

Magnetic particle inspection (MPI) is a nondestructive testing method used for defect detection MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.
MPI uses magnetic field and small magnetic particle (I.e.iron filings) to detect flaws in component. The only requirement from inspectability standpoint is that the component being must be made of a ferromagnetic material such as iron, nickel, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be affective.
The method is used to inspect a variety of product including castings,forgings,and weldments. Many different industries use magnetic particle inspection for determining a components fitness - for - use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such offshore structure and underwater pipelines.