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.

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.

The IIW Type Calibration Block in ultrasonic inspection.

The standard shown in the above figure is commonly known in the US as an IIW Type reference block. IIW is an acronym for the international Institute of welding. It is referred to as an IIW Type reference block because it was patterned after the true IIW block but does not conform to IIW requirements in IIS/IIW-23-59. True IIW blocks are only made out of steel (to be precise, killed, open hearth or electric furnace, low - carbon steel in the normalize condition with a grain size of McQuaid-ehn #8) where IIW Type blocks can be commercially obtained in a selection of materials. The dimensions of  true IIW blocks are in metric units while IIW  type blocks usually have English units. IIW type blocks may also include additional calibration and references features such as notches, circular groves, and scales that are not specified by IIW. There are two full -sized and a mini versions of the IIW Type blocks. The mini version is about one - half the size of the full-sized block and weighs only only about one-fourt as much. The IIW type US-1 block was derived the basic true IIW block and is shown below in the figure on the left. The IIW type US-2 block was developed for US air force application and is shown below in the center. The mini version is shown on the right.

IIW type blocks are used to calibrate instruments for both angle beam and normal incident inspections. Some of their uses include setting metal-distance and sensitivity settings,determining the sound exit point and refracted angle of angle beam transducers, and evaluating depth resolution of normal Beam inspection setups. 

Distance Amplitude correction (DAC) in ultrasonic inspection.

Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer. Distance Amplitude correction (DAC)  provides a means of establishing a graphic reference level sensitivity as a function of sweep distance on the A - scan display. The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. Most often DAC will allow for loss in amplitude over material depth  (time), graphically on the A - scan display but can also be done electronically by certain instruments. Because near field length and beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC cure must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.
A distance Amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. A-scan echoes are displayed at their non - electronically compensated height and peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH),or notches whereby the reflectors are located at varying Depths are commonly used.

INTRODUCTION TO THE COMMON STANDARDS IN ULTRASONIC INSPECTION.

Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirements is a major limitations of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more realistic defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allows the inspector to creat computer simulated model of the part and flaw may one day lessen this limitation.

CALIBRATION METHODS IN ULTRASONIC INSPECTION.

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a user calibration of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigure for use in a large variety of applications. The user must calibration the system which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually  only used when an absolute value is measured and in many cases,the standards are traceable back to standards at the national institute for standards and technology.
In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in measurements and to help interpret and quantity the information contained in the received signal. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal.

Attenuation measurements in ultrasonic inspection.

Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the absorption and the scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantity..

Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructure characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation.
Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude. The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance. By varying the separation distance, the attenuation can be measured from the changes in the amplitude. To get accurate results, the influence of coupling conditions must be carefully addressed. To overcome the problems related to conventional ultrasonic attenuation measurements, ultrasonic spectral parameters for frequency - dependent attenuation measurements, which are independent from coupling conditions are also used. For example, the ratio of the amplitudes of higher frequency peak to the lower frequency peak, has been used for microstructure characterization of some materials.