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.

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.

WELDMENT ANGLE BEAM IN ULTRASONIC INSPECTION.

The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principal of refraction and mode conversion to produce refrected shear or longitudinal waves in the test material. This inspection may include the root,sidewall, Crown, and heat- affected zones of a weld.
The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector ( discontinuity)  in the path of the sound beam. Will proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity.
To determine the proper scanning area for the welder,the inspector must first calculator the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the Crown, side wall,and root of the weld.

WELDMENTS ( welded joints) in ultrasonic inspection.

The most commonly occurring defects in welded joints are porosity, Slag inclusions, lack of side - wall fusion, lack of inter - run fusion, lack of  root penetration, undercutting, and longitudinal or transverse cracks.
With the exception of signal gas pores all the defects listed are usually well detectable by ultrasonic. Most applications are on low-alloy construction quality steels,however, world's in aluminium can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding application. This safe,accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology.
Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incident into the test piece, is first used to locate any laminations in important because an angle beam transducer may not able to provide a return signal from a lamina flaw. 

RAIL INSPECTION IN ULTRASONIC INSPECTION.

Rail inspections were initially performed solely means. Ofcourse, visual inspections will only detect external defects and sometimes the subtle signs of large internal problems. The need for a better inspection Methley became a high priority because of aderailment at Manchester, NY in 1911, in which 29 people were killed and 60 were seriously injured. In the U.S bureau of safety's ( now the national transportation safety board)  investigation of the accident, a broken rail was determined to be the cause of the derailment. The bureau established that the rail failure was caused by defect that entirely internal and probably could not have been detected by visual means. The defect was called a transverse fissure ( example shown on the left).  The railroads began investigating the prevalence of this defect and found transverse fissures were widespread.
One of the methods used to inspect rail is ultrasonic inspection. Both normal - and angle - beam techniques are used, as are both pulse - echo and pitch - catch techniques transducer arrangements offer different inspection capabilities. Manual contact testing is done to evaluate small section of rail but the ultrasonic inspection has been automated to allow inspection of large amounts of rail.
Fluid filed wheels or sleds are often used to couple the transducer to the rail. Sperry Rail services,  which is one of the companies that perform rail inspection, uses Roller search units ( RSUs)  comprising a combination of different transducer angles to achieve the best inspection possible. A schematic of an RSU is shown below. 

(DC) BLOCK & (RC) BLOCK IN ULTRASONIC INSPECTION.

Distance calibration Block :-The DC AWS block is a metal path distance and beam exit point calibration standard that conforms to the requirements of the American welding society (AWS) and the American association of state highway and transportation officials ( AASHTO).  Instructions on using the DC block can be found in the annex of American society for testing and materials standard E164, standard practice for ultrasonic contact Examination of weldments.

RESOLUTION CALIBRATION BLOCK:-The RC block is used to determine the resolution of angle beam transducers per the requirements of AWS and AASHTO. engraved index markers are provided for 45,60, and 70 degree refracted angle beams.

ANGLE - BEAM CALIBRATION BLOCK IN ULTRASONIC INSPECTION.

The miniature angle-beam is a calibration Block that was designed for the US Air Force for use in the field for instrument calibration. The block is much smaller and lighter than IIW block but performs many of the same functions. The miniature angle-beam block can be used to check the beam angle and exit point of the transducer. The block can be used to make metal - distance and sensitivity calibrations for both angle and normal - beam inspection setups.

A block that closely resembles the miniature angle-beam block and is used in a similar way is the DSC AWS block. This block is used to determine the beam exit point and refracted angle of angle - beam transducers and to calibration distance and set sensitivity for both normal and angle beam inspection setups. Instructions on using the DSC block can be found in the annex of American society for testing and materials standard E164, standard practice for ultrasonic contact Examination of weldment.