An Introduction to Nondestructive Testing Methods , Tools , And Applications
Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used.
In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples ("lot sampling"), rather than on the materials, components or assemblies actually being put into service.
These destructive tests are often used to determine the physical properties of materials such as impact resistance, ductility, yield and ultimate tensile strength, fracture toughness and fatigue strength, but discontinuities and differences in material characteristics are more effectively found by NDT.
Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public.
It should be noted that while the medical field uses many of the same processes, the term "nondestructive testing" is generally not used to describe medical applications
NDT Test Methods
Test method names often refer to the type of penetrating medium or the equipment used to perform that test. Current NDT methods are: Acoustic Emission Testing (AE), Electromagnetic Testing (ET), Guided Wave Testing (GW), Ground Penetrating Radar (GPR), Laser Testing Methods (LM), Leak Testing (LT), Magnetic Flux Leakage (MFL), Microwave Testing, Liquid Penetrant Testing (PT), Magnetic Particle Testing (MT), Neutron Radiographic Testing (NR), Radiographic Testing (RT), Thermal/Infrared Testing (IR), Ultrasonic Testing (UT), Vibration Analysis (VA) and Visual Testing (VT).
The six most frequently used test methods are MT, PT, RT, UT, ET and VT. Each of these test methods will be described here, followed by the other, less often used test methods:
(3) Penetrant Dwell:
The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.
(4) Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water
(5) Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).
(8) Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.
Magnetic flux lines must run in a direction such that they are interrupted at a defect, causing a flux leakage. So, in order to detect defects, the flux lines should ideally be at 90° to their principal direction. In order to locate all defects within a specimen, we need to generate magnetic fields in two directions at 90° to one another. These are normally:
1-Longitudinal
2-Circular
Longitudinal field
In Fig. 7 the magnetic lines of force are longitudinal in a bar and thus the bar has magnetic poles. Transverse flaws will easily show, but longitudinal defects such as seams, which are very straight, will not. However, it is accepted that flaws up to 45° to the flux lines will also be shown. In fact, longitudinal flaws having a transverse component, such as jagged cracks, will almost certainly show.
Circular field
The longitudinal magnetising field in the bar is now replaced by a longitudinal current, which creates a magnetic field at 90° to itself. In fact, the current has produced a circular non-polar field around the bar. Under normal circumstances the circular field is not detected, due to it having no external poles, but a longitudinal surface flaw at 90° creates a flux leakage, creating miniature poles and is thus detectable with magnetic particles. Figure 8 shows the effect of flaw orientation in a circularly magnetised bar.
In order to detect any cracks in the surface, the material is analyzed based on powder behavior and dispersal. This process can be repeated 2-5 times in which the yoke is placed differently each time to create magnetic fields with different directions.
Equipment for magnetic particle inspection
Electromagnets
Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism. That is, 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 ferromagneticsteel. 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.
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 a 4.5 meter cable. 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 come with wheels so that they can be rolled where needed. These units also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half-wave DC when 9 meters or less of cable is used.
The test specimen is first of all properly cleaned and visually inspected and all the surface imperfections are noted. A properly selected film, usually sandwiched between intensifying screens and enclosed in a light proof cassette is prepared. The source of radiation, the test specimen and the film are arranged as shown in Figure 16
Image quality indicators and lead identification letters are also placed on the source side of the test specimen. From a previously prepared exposure chart for the material of the test specimen, the energy of radiations to be used and the exposure (intensity of radiations x time) to be given are determined. Then the exposure is made. After the source of radiation has been switched off or retrieved
back into the shielding (in case of gamma ray source), the film cassette is removed and taken to the dark room. In the dark room, under safe light conditions, the film is removed from the cassette and the screens and processed. The processing of the film involves mainly four steps. Development reduces the exposed silver bromide crystals to black metallic silver thus making the latent image visible. Development is usually done for 5 minutes at 20°C. After development the film is fixed whereby all the unexposed and undeveloped crystals of film emulsion are removed and the exposed and image-forming emulsion is retained on the film. The fixing is done for approximately 2-6 minutes.
The film is then washed preferably in running water for about 20-30 minutes and dried Finally the film is interpreted for defects and a report compiled. The report includes information about the test specimen, the technique used and the defects. It also sometime says something about acceptance
or rejection of the reported defects. The report is properly signed by responsible persons.
1- Sources for radiographic testing
(i) X ray machine
X rays are generated whenever high energy electrons hit high atomic number materials. Such a phenomenon occurs in the case of X ray tubes, one of which is shown in Figure 17
(ii) Gamma ray sources
These are some elements which are radioactive and emit gamma radiations. There are a number of radioisotopes which in principle can be used for radiographic testing. But of these only a few have been considered to be of practical value.
The characteristics which make a particular radioisotope suitable for radiography include the energy of gamma rays, the half life, source size, specific activity and the availability of the source.
(iii) Radiographic linear accelerators
For the radiography of thick samples, X ray energy in the MeV range is required. This has now become possible with the availability of radiographic linear accelerators. In a linear accelerator the electrons from an electron gun are injected into a series of interconnected cavities which are energized at radio frequency (RF) by a klystron or magnetron
(iv) Betatron
The principle of this machine is to accelerate the electrons in a circular path by using an alternating magnetic field. The electrons are accelerated in a toroidal vacuum chamber or doughnut which is placed between the poles of a powerful electromagnet.
2- Films for radiographic testing
Radiographic film is manufactured by various film companies to meet a very wide diversified demand. Each type of film is designed to meet certain requirements and these are dictated by the circumstances of inspection such as (a) the part (b) the type of radiation used (c) energy of radiation (d) intensity of the radiation and (e) the level of inspection required. No single film is capable of meeting all the demands.
Therefore a number of different types of films are manufactured, all with different characteristics, the choice of which is dictated by what would be the most effective combination of radiographic technique and film to obtain the desired result.
The film consists of a transparent, flexible base of clear cellulose derivative or like material. One or both sides of this base are coated with a light sensitive emulsion of silver bromide suspended in gelatin. The silver bromide is distributed throughout the emulsion as minute crystals and exposure to
radiation such as X rays, gamma rays or visible light, changes its physical structure. This change is of such a nature that it cannot be detected by ordinary physical methods, and is called the latent image.
Types of Radiography
There are numerous types of RT techniques including conventional radiography and multiple forms digital radiographic testing. Each works slightly differently and has its own set of advantages and disadvantages.
Conventional Radiography
Conventional radiography uses a sensitive film which reacts to the emitted radiation to capture an image of the part being tested. This image can then be examined for evidence of damage or flaws. The biggest limitation to this technique is that films can only be used once and they take a long time to process and interpret.
Digital Radiography
Unlike conventional radiography, digital radiography doesn’t require film. Instead, it uses a digital detector to display radiographic images on a computer screen almost instantaneously. It allows for a much shorter exposure time so that the images can be interpreted more quickly. Furthermore, the digital images are much higher quality when compared to conventional radiographic images. With the ability to capture highly quality images, the technology can be utilized to identify flaws in a material, foreign objects in a system, examine weld repairs, and inspect for corrosion under insulation
ULTRASONIC TESTING
Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more.
General procedure for ultrasonic testing
Ultrasonics are the sound waves whose frequency is greater than 20kHz. Due to the high frequency they have a very good penetrating power. When sound waves propagate from one medium to another, a part of the sound energy is reflected and the rest is transmitted at the interface seperating the two media as shown in Figure 20 . This property is made use to detect flaws because not only interfaces also the flaws can reflect the ultrasonic sound energy
he interaction of the sound energy is stronger for higher frequencies. Hence high frequency ultrasound in the frequency range 0.5 MHz to 25MHz is found suitable for the testing. The waves are generated by using either a Piezo-electric energised crystal cut in a particular fashion to generate the desired wave mode or an Electromagnetic accoustic transducer. The relation among the intensities of the incident and reflected sound energy is given in equation
The intensity of the sound wave reflected from the interface generally depends upon the difference in the densities of the pair of media ( 𝝆1 𝝆2 ) for the given incident wave intensity. Here 𝝆1 and 𝝆2 are the densities of the two media 1 and 2 respectively through which the sound wave is propagating. Thus, if the ultrasonic wave propagates from a medium of higher density into a medium of lower density then maximum reflection of intensity takes place at the interface seperating the two media. The flaw in the medium results in the reflection of sound energy due to the variation of density and hence their detection is made possible. Reflections are analysed electrically and the reflection is called echo.
General procedure for ultrasonic testing
Figure 21 shows that the typical ultrasonic testing system. It consists of several functional units, such as the pulser/receiver, transducer and display devices.
A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. The longitudinal ultrasonic pulses are generated using the probe. For each generated pulse the echoes are observed on the oscilloscope as shown in the Figure 21 . The first echo corresponds to the reflection from the upper surface of the part. If there exists a flaw, a second echo is observed with a lower pulse height due to smaller reflection intensity. A third echo is observed due to the reflection from the back surface. The intensity of the echo from the back surface reflection is less due to attenuation of sound energy in the medium
Applications of ultrasonic testing
Thickness measurements
Thickness measurements using ultrasonics can be applied using either the pulse echo or resonance techniques. Some typical applications are:
(i) Wall thickness measurement in pressure vessels, pipelines, gas holders, storage tanks for chemicals and accurate estimate of the effect of wear and corrosion without having to dismantle the plant.
(ii) Measurement of the thickness of ship hulls for corrosion control.
(iii) Control of machining operations, such as final grinding of hollow propellers.
(iv) Ultrasonic thickness gauging of materials during manufacture.
(v) Measurement of wall thickness of hollow aluminium extrusions.
(vi) Measurement of the thickness of lead sheath and insulating material extruded over a core of wire.
(vii) Inspection of heat exchanger tubing in nuclear reactors.
(viii) Measurement of the wall thickness of small bore tubing including the
canning tubes for reactor fuel elements.
Flaw detection
Typical flaws encountered in industrial materials are cracks, porosity, laminations, inclusions, lack of root penetration, lack of fusion, cavities, laps, seams, corrosion, etc. Some examples of the detection of these defects are as follows:
(i) Examination of welded joints in pressure vessels, containers for industrial liquids and gases, pipelines, steel bridges, pipelines, steel or aluminium columns, frames and roofs (during manufacturing, pre-service and inservice).
(ii) Inspection of steel, aluminium and other castings,
(iii) Inspection of rolled billets, bars and sections.
(iv) Inspection of small bore tubes including the canning tubes for nuclear fuel elements.
(v) Ultrasonic testing of alloy steel forgings for large turbine rotors,
(vi) Testing of turbine rotors and blades for aircraft engines.
(vii) Early stage inspection in the production of steel and aluminium blocks and slabs, plates, bar sections, tubes, sheets and wires.
(viii) Detection of unbonded surfaces in ceramics, refractories, rubber, plastics and laminates.
(ix) Detection of honeycomb bond in the aircraft industry.
(x) Inspection of jet engine rotors.
(xi) Detection of caustic embrittlement failure in riveted boiler drums in the power generation industry.
(xii) Detection of cracks in the fish plate holes in railway lines and in locomotive and bogey axles.
(xiii) Detection of hydrogen cracks in roller bearings resulting from improper heat treatment.
(xiv) In service automatic monitoring of fatigue crack growth.
(xv) Detection of stress corrosion cracking.
(xvi) Detection of fatigue cracks in parts working under fluctuating stress.
(xvii) Inspection of fine quality wire.
(xviii)Testing of wooden components such as utility poles.
(xix) Application of ultrasonics to monitor material characteristics in the space environment.
(xx) Determination of lack of bonding in clad fuel elements,
(xxi) Detection of flaws in grinding wheels.
(xxii) Varieties of glass which are not sufficiently transparent to allow optical inspection can be tested ultrasonically.
(xxiii)Quality control in the manufacture of rubber tyres by locating voids, etc.
(xxiv)Inspection of engine crankshafts.
Miscellaneous applications
In addition to the applications already mentioned there are numerous others. Notable among these are those based on the measurement of acoustic velocity and the attenuation of acoustic energy in materials. Some of these applications are as follows:
(1) Assessment of the density and tensile strength of ceramic products such as high tension porcelain insulators.
(2) Determination of the difference between various types of alloys.
(3) Detection of grain growth due to excessive heating.
(4) Estimation of the values of the elastic moduli of metals over a wide range of temperature and stress.
(5) Tensile strength of high grade cast iron can be estimated by measuring its coefficient of acoustical damping.
(6) Crushing strength of concrete can be measured from the transit time of an ultrasonic pulse.
(7) Quarrying can be made more efficient by the measurement of pulse velocity or attenuation in rock strata.
(8) To find the nature of formations in geophysical surveys without having to undertake boring operations.
(9) Detection of bore hole eccentricity in the exploration for mineral ores and oil.
(10) Study of press fits.
(11) Metallurgical structure analysis and control of case depth and hardness, precipitation of alloy constituents and grain refinement.
(12) Determination of intensity and direction of residual stresses in structural metal components.
(13) Detection of honeycomb debonds and the regions in which the adhesive fails to develop its nominal strength in the aerospace industry.
(14) Measurement of liquid level of industrial liquids in containers.
NDT Method Summary
No single NDT method will work for all flaw detection or measurement applications. Each of the methods has advantages and disadvantages when compared to other methods. The table below summarizes the scientific principles, common uses and the advantages and disadvantages for some of the most often used NDT methods.
Introduction
Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used.
In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples ("lot sampling"), rather than on the materials, components or assemblies actually being put into service.
These destructive tests are often used to determine the physical properties of materials such as impact resistance, ductility, yield and ultimate tensile strength, fracture toughness and fatigue strength, but discontinuities and differences in material characteristics are more effectively found by NDT.
Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public.
It should be noted that while the medical field uses many of the same processes, the term "nondestructive testing" is generally not used to describe medical applications
NDT Test Methods
Test method names often refer to the type of penetrating medium or the equipment used to perform that test. Current NDT methods are: Acoustic Emission Testing (AE), Electromagnetic Testing (ET), Guided Wave Testing (GW), Ground Penetrating Radar (GPR), Laser Testing Methods (LM), Leak Testing (LT), Magnetic Flux Leakage (MFL), Microwave Testing, Liquid Penetrant Testing (PT), Magnetic Particle Testing (MT), Neutron Radiographic Testing (NR), Radiographic Testing (RT), Thermal/Infrared Testing (IR), Ultrasonic Testing (UT), Vibration Analysis (VA) and Visual Testing (VT).
The six most frequently used test methods are MT, PT, RT, UT, ET and VT. Each of these test methods will be described here, followed by the other, less often used test methods:
1- magnetic particle testing (MT).
2- liquid penetrant testing (PT).
3- rdiographic testing (RT).
4- ultrasonic testing (UT).
5- eddy current testing (ET).
6- visual testing (VT).
VISUAL TESTING (VT)
Visual testing is the first NDT method that should be considered before applying more sophisticated and expensive methods. In this method direct visual and optically aided inspection is applied to the surface of object to detect flaws and anomalies. If significant flaws are detected during visual inspection then the part being inspected can be rejected on that basis. There is then hardly any need or
justification for applying the other NDT methods.
Tools for visual inspection
The human eye is the most frequently used tool for visual inspection. It can be aided by lenses and magnifiers. At places where direct vision is not possible horoscopes can be used. The images can be observed under visible light or ultraviolet light may be used for fluorescent materials. Video and film cameras have also been employed for remote visual inspection, hi fact liquid penetrant testing and magnetic particle testing are only more advanced forms of visual inspection
Applications of visual inspection
Visual inspection can be applied to all sorts of materials for the detection of surface cracks, voids, pores, inclusions and for the assessment of surface roughness.
LIQUID PENETRANT TESTING (PT)
This is a method which can be employed for the detection of open-to-surface discontinuities in any industrial product which is made from a non-porous material. In this method a liquid penetrant is applied to the surface of the product for a certain predetermined time after which the excess penetrant is removed from the surface.
The surface is then dried and a developer is applied to it. The penetrant which remains in the discontinuity is absorbed by the developer to indicate the presence as well as the location, size and nature of the discontinuity.
General procedure for liquid penetrant inspection
(1) Cleaning the surface to be examined
One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.
Visual testing is the first NDT method that should be considered before applying more sophisticated and expensive methods. In this method direct visual and optically aided inspection is applied to the surface of object to detect flaws and anomalies. If significant flaws are detected during visual inspection then the part being inspected can be rejected on that basis. There is then hardly any need or
justification for applying the other NDT methods.
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| FIG 1 |
Tools for visual inspection
The human eye is the most frequently used tool for visual inspection. It can be aided by lenses and magnifiers. At places where direct vision is not possible horoscopes can be used. The images can be observed under visible light or ultraviolet light may be used for fluorescent materials. Video and film cameras have also been employed for remote visual inspection, hi fact liquid penetrant testing and magnetic particle testing are only more advanced forms of visual inspection
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| FIG 2 |
Applications of visual inspection
Visual inspection can be applied to all sorts of materials for the detection of surface cracks, voids, pores, inclusions and for the assessment of surface roughness.
LIQUID PENETRANT TESTING (PT)
This is a method which can be employed for the detection of open-to-surface discontinuities in any industrial product which is made from a non-porous material. In this method a liquid penetrant is applied to the surface of the product for a certain predetermined time after which the excess penetrant is removed from the surface.
The surface is then dried and a developer is applied to it. The penetrant which remains in the discontinuity is absorbed by the developer to indicate the presence as well as the location, size and nature of the discontinuity.
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| FIG 3 |
General procedure for liquid penetrant inspection
(1) Cleaning the surface to be examined
One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.
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| FIG 4 |
(2) Penetrant Application:
Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.
Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.
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| FIG 5 |
The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.
(5) Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).
(6) Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks.
(7) Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.(8) Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.
Areas of application of liquid penetrants
Liquid penetrants can be used for the inspection of all types of materials such as ferrous and non ferrous, conductors and non-conductors, magnetic and non-magnetic and all sorts of alloys and plastics. Most common applications are in castings, forgings and welding.
MAGNETIC PARTICLE TESTING (MT)
Magnetic particle testing is used for the testing of materials which can be easily magnetized. This method is capable of detecting open-to-surface and just below-the-surface flaws, hi this method the test specimen is first magnetized either by using a permanent magnet or an electric current through or around the specimen.
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| FIG 6 |
The magnetic field thus introduced into the specimen is composed of magnetic lines of force. Whenever there is a flaw which interrupts the flow of magnetic lines of force, some of these lines must exit and re-enter the specimen. These points of exit and re-entry form opposite magnetic poles and whenever minute magnetic particles are sprinkled onto the surface of the specimen, these particles are attracted by these magnetic poles to create a visual indication approximating the size and shape of the flaw.
Methods of magnetization
1-Longitudinal
2-Circular
3- Magnetization of irregular parts
In Fig. 7 the magnetic lines of force are longitudinal in a bar and thus the bar has magnetic poles. Transverse flaws will easily show, but longitudinal defects such as seams, which are very straight, will not. However, it is accepted that flaws up to 45° to the flux lines will also be shown. In fact, longitudinal flaws having a transverse component, such as jagged cracks, will almost certainly show.
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| FIG 7 |
Circular field
The longitudinal magnetising field in the bar is now replaced by a longitudinal current, which creates a magnetic field at 90° to itself. In fact, the current has produced a circular non-polar field around the bar. Under normal circumstances the circular field is not detected, due to it having no external poles, but a longitudinal surface flaw at 90° creates a flux leakage, creating miniature poles and is thus detectable with magnetic particles. Figure 8 shows the effect of flaw orientation in a circularly magnetised bar.
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| FIG 8 |
Magnetization of irregular parts
Parts of irregular shape sometimes have to be tested. They can be tested using the magnetic particle inspection method. Local magnetization is created by applying prods to the area to be tested. The area is magnetized circularly and any defect in the path of the magnetic lines of force can be indicated as shown in Figure 9
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| FIG 9 |
General procedure for magnetic testing
The material [1] to be tested must first be cleaned so the powder which will then be applied to the surface is not absorbed by the contamination. A magnetic yoke [2] creates a magnetic field in the surface of the material and in a ferromagnetic powder sprinkled gently over the surface and then blown gently away. The powder collects at defects [3] as a result of disturbances in the direction of the magnetic field lines. |
| FIG 10 |
In order to detect any cracks in the surface, the material is analyzed based on powder behavior and dispersal. This process can be repeated 2-5 times in which the yoke is placed differently each time to create magnetic fields with different directions.
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 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 divers 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 electromagnets might not fit.
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 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 divers 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 electromagnets might not fit.
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| FIG 11 |
Electromagnets
Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism. That is, 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 ferromagneticsteel. 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
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 to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.
Portable Power SuppliesProds 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 to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.
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 a 4.5 meter cable. 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 come with wheels so that they can be rolled where needed. These units also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half-wave DC when 9 meters or less of cable is used.
Applications of the magnetic method of testing
In general engineering practice, a large proportion of components are made of steel or iron which are capable of being magnetized. This is fortunate because this testing method is not expensive and it can reveal all the surface faults in parts which are subjected to light stresses and fatigue and in those which have been cast, welded or heat treated during fabrication. Many inspection specifications for aerospace, atomic and other critical work specially call for this type of test
EDDY CURRENT TESTING
An alternating current of known frequency is applied to an electric coil placed adjacent to the material to be inspected. This current will produce its own magnetic field known as the excitation field and will also induce currents in the metal part known as eddy currents according to Faraday's law of electromagnetic induction.
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| FIG12 |
These eddy currents will produce their own magnetic field which will oppose the excitation field. The resultant field is thus reduced which will change the coil impedance.
In Figure 12 an alternating current of a given frequency is generated in the primary or exciting coil. An alternating magnetic flux is consequently produced. This induces an alternating current of the same frequency in the secondary coil. With the introduction of the specimen, the alternating flux of the primary induces in it (the specimen) an eddy current flow which gives rise to an alternating magnetic flux in the opposite direction. The current in the secondary coil is consequently reduced.
For given conditions the reduction in current should be equal for all identical specimens placed in the same position relative to the coils. Any observed inequality in the value of the reduced current could indicate the presence of a defect, a change in dimensions, or a variation in the electrical conductivity or in the magnetic permeability of the test specimen due perhaps, to a change in its physical or
chemical structure
The coil impedance, which is usually measured in practice instead of the current or flux, is a vector quantity having resistive and inductive components. These are 90° out of phase with each other. The other quantity that may be measured in practice is the voltage across the coil. The coil impedance as well as voltage is related to the effective permeability of the test specimen, the test frequency of the coil, the limiting or boundary frequency of the test specimen and the fill factor of the coil.
Physical Principles
• Alternating magnetic fields are generated by alternating current excitation – Maxwell Ampere Law
• Magnetic field induces currents (eddy currents) in test specimen – Maxwell Faraday Law
• Eddy currents establish secondary fields which oppose the primary fields
• Changes net flux linkage and hence the impedance of the coil Changes net flux linkage and hence the impedance of the coil • Anomalies in the test specimen affect the induced field, changing the net impedance of the coil
• Test coil characteristics
Inductive reactance
Ohmic resistance R
f : excitation frequency
L : self-inductance of the coil
Significant properties of test specimen
Electrical conductivity (𝝈)
Dimensions (such as depth of conducting plate )
Magnetic permeability (µ)
Material discontinuities (such as cracks or corrosion's)
Detecting cracks in the material
In crack detection, the surface of the part to be tested is scanned – without physical contact – by one or more eddy current probes. For this purpose, the test item is rotated by a specially-adapted mechanism and then scanned by a stationary probe. Alternatively, a rotating probe scans the stationary test item. The system can incorporate either one test track along the sample’s circumference with a single probe or several test tracks with probes arranged in parallel. Alternatively, the surface to be tested can be scanned with a probe that tracks its contour. The choice of probe depends on the geometry of the component, the cycle time and the defect specification.
Checking materials for their properties
For testing of material properties, the specimens are passed through an encircling testing coil. For particular applications, e.g. local checks for surface hardness or hardening depth, specimen-specific sensor systems are used. The voltage detected by the individual sensors results from the magnetic and electric properties of the specimen. The exact voltage is displayed as a measuring point. During calibration, a sorting limit is automatically created through the statistical evaluation of multiple measured values. In subsequent serial testing, all further measuring points are compared against the specified tolerance limits. The parts are sorted according to the respective test results.
Equipment and procedure for eddy current testing
Preliminary NDT system
To perform the experimental validation of the probe concept, a preliminary version of the NDT system was developed in collaboration with the IST Solid-State Welding Group. The system provided a test bench to quickly bring the probe into operation and is composed by the following components: 1-XY table to move the probe along the test material;
2-Signal generator;
3- IOnic eddy currents probe prototype;
4-Analog electronics;
5-Analysis software
Eddy Current Sensors
Absolute probe
Differential Bobbin probe
Plus Point & Array Probe
Meandering coil Meandering coil
Eddy Current – Magneto-optic (MOI) sensor
Eddy Current – Magneto-resistive (MR) sensor
Applications of eddy current testing
Eddy current testing is employed for the detection and measurement of defects such as cracks, porosity, blowholes, inclusions, overlaps, shrinkages and soft spots in a wide variety of test specimens in solid cylindrical, hollow cylindrical or other complex shapes. Corrosion and cracking due to stress corrosion can also be detected. Changes in electrical conductivity and permeability can be measured which in turn have a bearing upon the material properties such as hardness,
homogeneity, degree of heat treatment, existence of internal stresses, decarburization, diffusion, alloy composition, presence of impurities, etc. Thickness measurements can be made on metallic plates, foils, sheets, strips, tubes and cylinders. Typically it is possible to determine the thickness of non-metallic coatings on metals such as for example the insulating layers on cables, nonconducting
paints on some aircraft castings and anodic coating on aluminium alloy surfaces. Dimensions such as diameters of cylindrical specimens can also be determined. The materials can be automatically sorted in a production process.
Since the method is adaptable to automation high speed inspection of small diameter tubings such as those used in steam generators, heat exchangers and as cladding for nuclear reactor fuel elements is possible. Here the characteristics of fuel tubing such as inner and outer diameters, eccentricity, wall thickness and the presence of defects are determined. It is also possible to inspect welded small bore
piping. By using encircling type probes large diameter pipes can be inspected.
Similarly long bars and wires can be speedily inspected. In tube testing the eddy current method also allows high speed detection of intergranular corrosion on the inside surface. In some applications round metallic spheres and balls are inspected by eddy currents.
RADIOGRAPHIC TESTING
The method of radiographic testing involves the use of a source of radiation from which the radiations hit the test specimen, pass through it and are detected by a suitable radiation detector placed on the side opposite to that of the source. This is schematically shown in the Figure 15 . While passing through the test specimen the radiations are absorbed in accordance with the thickness, physical density and the internal defects of the specimen and the detector system therefore receives the differential radiations from different parts of a defective specimen which are recorded onto the detector.
X-rays and gamma rays are electromagnetic radiations which have the following common properties.
(i) They are invisible.
(ii) They cannot be felt by human senses.
(iii) They cause materials to fluoresce. Fluorescent materials are zinc sulfide, calcium tungstate, diamond, barium platinocyanide, napthalene, anthracene, stilbene, thalium activated sodium iodide etc.
(iv) They travel at the speed of light i.e. 3 x 10¹⁰cm/sec.
(v) They are harmful to living cells.
(vi) They can cause ionization. They can detach electrons from the atoms of a gas, producing positive and negative ions.
(vii) They travel in a straight line. Being electromagnetic waves, X-rays can also be reflected, refracted and diffracted.
(viii) They obey the inverse square law according to which intensity of X-rays at a point is inversely proportional to the square of the distance between the source and the point. Mathematically I 𝜶 1/r² where I is the intensity at a point distant r from the source of radiation.
(ix) They can penetrate even the materials through which light cannot. Penetration depends upon the energy of the rays, the density and thickness of the material. A monoenergetic beam of X-rays obeys the well known absorption law, I = Io exp (-𝜇x) where Io = the incident intensity of X-rays and I= the intensity of X-rays transmitted through a thickness x of material having attenuation coefficient 𝜇 .
(x) They affect photographic emulsions.
(xi) While passing through a material they are either absorbed or scattered.
Equipment and procedure for radiographic testing
In Figure 12 an alternating current of a given frequency is generated in the primary or exciting coil. An alternating magnetic flux is consequently produced. This induces an alternating current of the same frequency in the secondary coil. With the introduction of the specimen, the alternating flux of the primary induces in it (the specimen) an eddy current flow which gives rise to an alternating magnetic flux in the opposite direction. The current in the secondary coil is consequently reduced.
For given conditions the reduction in current should be equal for all identical specimens placed in the same position relative to the coils. Any observed inequality in the value of the reduced current could indicate the presence of a defect, a change in dimensions, or a variation in the electrical conductivity or in the magnetic permeability of the test specimen due perhaps, to a change in its physical or
chemical structure
Physical Principles
• Alternating magnetic fields are generated by alternating current excitation – Maxwell Ampere Law
• Magnetic field induces currents (eddy currents) in test specimen – Maxwell Faraday Law
• Eddy currents establish secondary fields which oppose the primary fields
• Changes net flux linkage and hence the impedance of the coil Changes net flux linkage and hence the impedance of the coil • Anomalies in the test specimen affect the induced field, changing the net impedance of the coil
• Test coil characteristics
Inductive reactance
f : excitation frequency
L : self-inductance of the coil
Significant properties of test specimen
Electrical conductivity (𝝈)
Dimensions (such as depth of conducting plate )
Magnetic permeability (µ)
Material discontinuities (such as cracks or corrosion's)
Detecting cracks in the material
In crack detection, the surface of the part to be tested is scanned – without physical contact – by one or more eddy current probes. For this purpose, the test item is rotated by a specially-adapted mechanism and then scanned by a stationary probe. Alternatively, a rotating probe scans the stationary test item. The system can incorporate either one test track along the sample’s circumference with a single probe or several test tracks with probes arranged in parallel. Alternatively, the surface to be tested can be scanned with a probe that tracks its contour. The choice of probe depends on the geometry of the component, the cycle time and the defect specification.
 |
| FIG 13 |
For testing of material properties, the specimens are passed through an encircling testing coil. For particular applications, e.g. local checks for surface hardness or hardening depth, specimen-specific sensor systems are used. The voltage detected by the individual sensors results from the magnetic and electric properties of the specimen. The exact voltage is displayed as a measuring point. During calibration, a sorting limit is automatically created through the statistical evaluation of multiple measured values. In subsequent serial testing, all further measuring points are compared against the specified tolerance limits. The parts are sorted according to the respective test results.
 |
| FIG 14 |
Equipment and procedure for eddy current testing
Preliminary NDT system
To perform the experimental validation of the probe concept, a preliminary version of the NDT system was developed in collaboration with the IST Solid-State Welding Group. The system provided a test bench to quickly bring the probe into operation and is composed by the following components: 1-XY table to move the probe along the test material;
2-Signal generator;
3- IOnic eddy currents probe prototype;
4-Analog electronics;
5-Analysis software
Eddy Current Sensors
Absolute probe
Differential Bobbin probe
Plus Point & Array Probe
Meandering coil Meandering coil
Eddy Current – Magneto-optic (MOI) sensor
Eddy Current – Magneto-resistive (MR) sensor
Applications of eddy current testing
Eddy current testing is employed for the detection and measurement of defects such as cracks, porosity, blowholes, inclusions, overlaps, shrinkages and soft spots in a wide variety of test specimens in solid cylindrical, hollow cylindrical or other complex shapes. Corrosion and cracking due to stress corrosion can also be detected. Changes in electrical conductivity and permeability can be measured which in turn have a bearing upon the material properties such as hardness,
homogeneity, degree of heat treatment, existence of internal stresses, decarburization, diffusion, alloy composition, presence of impurities, etc. Thickness measurements can be made on metallic plates, foils, sheets, strips, tubes and cylinders. Typically it is possible to determine the thickness of non-metallic coatings on metals such as for example the insulating layers on cables, nonconducting
paints on some aircraft castings and anodic coating on aluminium alloy surfaces. Dimensions such as diameters of cylindrical specimens can also be determined. The materials can be automatically sorted in a production process.
Since the method is adaptable to automation high speed inspection of small diameter tubings such as those used in steam generators, heat exchangers and as cladding for nuclear reactor fuel elements is possible. Here the characteristics of fuel tubing such as inner and outer diameters, eccentricity, wall thickness and the presence of defects are determined. It is also possible to inspect welded small bore
piping. By using encircling type probes large diameter pipes can be inspected.
Similarly long bars and wires can be speedily inspected. In tube testing the eddy current method also allows high speed detection of intergranular corrosion on the inside surface. In some applications round metallic spheres and balls are inspected by eddy currents.
RADIOGRAPHIC TESTING
The method of radiographic testing involves the use of a source of radiation from which the radiations hit the test specimen, pass through it and are detected by a suitable radiation detector placed on the side opposite to that of the source. This is schematically shown in the Figure 15 . While passing through the test specimen the radiations are absorbed in accordance with the thickness, physical density and the internal defects of the specimen and the detector system therefore receives the differential radiations from different parts of a defective specimen which are recorded onto the detector.
 |
| FIG 15 |
X-rays and gamma rays are electromagnetic radiations which have the following common properties.
(i) They are invisible.
(ii) They cannot be felt by human senses.
(iii) They cause materials to fluoresce. Fluorescent materials are zinc sulfide, calcium tungstate, diamond, barium platinocyanide, napthalene, anthracene, stilbene, thalium activated sodium iodide etc.
(iv) They travel at the speed of light i.e. 3 x 10¹⁰cm/sec.
(v) They are harmful to living cells.
(vi) They can cause ionization. They can detach electrons from the atoms of a gas, producing positive and negative ions.
(vii) They travel in a straight line. Being electromagnetic waves, X-rays can also be reflected, refracted and diffracted.
(viii) They obey the inverse square law according to which intensity of X-rays at a point is inversely proportional to the square of the distance between the source and the point. Mathematically I 𝜶 1/r² where I is the intensity at a point distant r from the source of radiation.
(ix) They can penetrate even the materials through which light cannot. Penetration depends upon the energy of the rays, the density and thickness of the material. A monoenergetic beam of X-rays obeys the well known absorption law, I = Io exp (-𝜇x) where Io = the incident intensity of X-rays and I= the intensity of X-rays transmitted through a thickness x of material having attenuation coefficient 𝜇 .
(x) They affect photographic emulsions.
(xi) While passing through a material they are either absorbed or scattered.
Equipment and procedure for radiographic testing
The test specimen is first of all properly cleaned and visually inspected and all the surface imperfections are noted. A properly selected film, usually sandwiched between intensifying screens and enclosed in a light proof cassette is prepared. The source of radiation, the test specimen and the film are arranged as shown in Figure 16
 |
| FIG 16 |
back into the shielding (in case of gamma ray source), the film cassette is removed and taken to the dark room. In the dark room, under safe light conditions, the film is removed from the cassette and the screens and processed. The processing of the film involves mainly four steps. Development reduces the exposed silver bromide crystals to black metallic silver thus making the latent image visible. Development is usually done for 5 minutes at 20°C. After development the film is fixed whereby all the unexposed and undeveloped crystals of film emulsion are removed and the exposed and image-forming emulsion is retained on the film. The fixing is done for approximately 2-6 minutes.
The film is then washed preferably in running water for about 20-30 minutes and dried Finally the film is interpreted for defects and a report compiled. The report includes information about the test specimen, the technique used and the defects. It also sometime says something about acceptance
or rejection of the reported defects. The report is properly signed by responsible persons.
1- Sources for radiographic testing
(i) X ray machine
X rays are generated whenever high energy electrons hit high atomic number materials. Such a phenomenon occurs in the case of X ray tubes, one of which is shown in Figure 17
 |
| FIG 17 |
These are some elements which are radioactive and emit gamma radiations. There are a number of radioisotopes which in principle can be used for radiographic testing. But of these only a few have been considered to be of practical value.
The characteristics which make a particular radioisotope suitable for radiography include the energy of gamma rays, the half life, source size, specific activity and the availability of the source.
(iii) Radiographic linear accelerators
For the radiography of thick samples, X ray energy in the MeV range is required. This has now become possible with the availability of radiographic linear accelerators. In a linear accelerator the electrons from an electron gun are injected into a series of interconnected cavities which are energized at radio frequency (RF) by a klystron or magnetron
(iv) Betatron
The principle of this machine is to accelerate the electrons in a circular path by using an alternating magnetic field. The electrons are accelerated in a toroidal vacuum chamber or doughnut which is placed between the poles of a powerful electromagnet.
2- Films for radiographic testing
Radiographic film is manufactured by various film companies to meet a very wide diversified demand. Each type of film is designed to meet certain requirements and these are dictated by the circumstances of inspection such as (a) the part (b) the type of radiation used (c) energy of radiation (d) intensity of the radiation and (e) the level of inspection required. No single film is capable of meeting all the demands.
Therefore a number of different types of films are manufactured, all with different characteristics, the choice of which is dictated by what would be the most effective combination of radiographic technique and film to obtain the desired result.
 |
| FIG 18 |
The film consists of a transparent, flexible base of clear cellulose derivative or like material. One or both sides of this base are coated with a light sensitive emulsion of silver bromide suspended in gelatin. The silver bromide is distributed throughout the emulsion as minute crystals and exposure to
radiation such as X rays, gamma rays or visible light, changes its physical structure. This change is of such a nature that it cannot be detected by ordinary physical methods, and is called the latent image.
Types of Radiography
There are numerous types of RT techniques including conventional radiography and multiple forms digital radiographic testing. Each works slightly differently and has its own set of advantages and disadvantages.
Conventional Radiography
Conventional radiography uses a sensitive film which reacts to the emitted radiation to capture an image of the part being tested. This image can then be examined for evidence of damage or flaws. The biggest limitation to this technique is that films can only be used once and they take a long time to process and interpret.
Digital Radiography
Unlike conventional radiography, digital radiography doesn’t require film. Instead, it uses a digital detector to display radiographic images on a computer screen almost instantaneously. It allows for a much shorter exposure time so that the images can be interpreted more quickly. Furthermore, the digital images are much higher quality when compared to conventional radiographic images. With the ability to capture highly quality images, the technology can be utilized to identify flaws in a material, foreign objects in a system, examine weld repairs, and inspect for corrosion under insulation
The four most commonly utilized digital radiography techniques in the oil & gas and chemical processing industries are computed radiography, direct radiography, real-time radiography, and computed tomography.
1) Computed Radiography
Computed radiography (CR) uses a phosphor imaging plate that replaces film in conventional radiography techniques. This technique is much quicker than film radiography but slower than direct radiography. CR requires several extra steps compared to direct radiography. First, it indirectly captures the image of a component on a phosphor plate, then converts the image into a digital signal that can be visualized on a computer monitor. Image quality is fair but can be enhanced using appropriate tools and techniques (i.e, adjusting contrast, brightness, etc. without compromising integrity). It’s important to know how tools, such as adjusting contrast, effect the image. Care should also be taken to make sure minor defects are not hidden after enhancements are made.
2) Direct Radiography
Direct Radiography (DR) is also a form of digital radiography and very similar to computed radiography. The key difference lies in how the image is captured. In DR, a flat panel detector is used to directly capture an image and display that image on a computer screen. Although this technique is fast and produce higher quality images, it is more costly than computed radiography.
3) Real-Time Radiography
Real-time radiography (RTR), like it’s name suggests, is a form of digital radiography that occurs in real time. RTR works by emitting radiation through an object. These rays then interact with either a special phosphor screen or flat panel detector containing micro-electronic sensors. The interaction between the panel and the radiation creates a digital image that can be viewed and analyzed in real time.
The brighter areas on the image are a result of higher levels of radiation that contact the screen. This corresponds to the thinner or less dense section of the component. Conversely, darker areas are a result of less radiation interacting with the screen and indicate where the component is thicker.
Aside from being able to make the images available more quickly and analyze them in real time, RTR has several other advantages. One being that digital images don’t require physical storage space and thus are easier to store, transfer, and archive than film.
On the other hand, this method has several disadvantages as well. Compared to conventional radiography, RTR has a lower contrast sensitivity and limited image resolution. Images created via RTR often suffer from uneven illumination, limited resolution, a lack of sharpness, and noise. These factors have a major impact on image quality.
4) Computed Tomography
Computed tomography (CT) is a technique that takes hundreds to thousands (depending on the size of the component) of 2D radiography scans and superimposes them to create a 3D radiographic image.
In an industrial setting, CT can be achieved in two ways. In one method, the component to be inspected remains stationary while the radiation source and x-ray detector rotate around the component. This technique is more likely to be utilized for large components. The second method consists of the radiation source and x-ray detector remaining stationary while the component is rotated 360 degrees. This second technique is more useful when the component is small or has complex geometry.
Although this technology is timely, expensive, and requires a large amount of data storage, CT provides highly accurate images, is repeatable and reproducible, and minimizes human error.
Applications of radiographic testing method
1) Computed Radiography
Computed radiography (CR) uses a phosphor imaging plate that replaces film in conventional radiography techniques. This technique is much quicker than film radiography but slower than direct radiography. CR requires several extra steps compared to direct radiography. First, it indirectly captures the image of a component on a phosphor plate, then converts the image into a digital signal that can be visualized on a computer monitor. Image quality is fair but can be enhanced using appropriate tools and techniques (i.e, adjusting contrast, brightness, etc. without compromising integrity). It’s important to know how tools, such as adjusting contrast, effect the image. Care should also be taken to make sure minor defects are not hidden after enhancements are made.
2) Direct Radiography
Direct Radiography (DR) is also a form of digital radiography and very similar to computed radiography. The key difference lies in how the image is captured. In DR, a flat panel detector is used to directly capture an image and display that image on a computer screen. Although this technique is fast and produce higher quality images, it is more costly than computed radiography.
3) Real-Time Radiography
Real-time radiography (RTR), like it’s name suggests, is a form of digital radiography that occurs in real time. RTR works by emitting radiation through an object. These rays then interact with either a special phosphor screen or flat panel detector containing micro-electronic sensors. The interaction between the panel and the radiation creates a digital image that can be viewed and analyzed in real time.
The brighter areas on the image are a result of higher levels of radiation that contact the screen. This corresponds to the thinner or less dense section of the component. Conversely, darker areas are a result of less radiation interacting with the screen and indicate where the component is thicker.
Aside from being able to make the images available more quickly and analyze them in real time, RTR has several other advantages. One being that digital images don’t require physical storage space and thus are easier to store, transfer, and archive than film.
On the other hand, this method has several disadvantages as well. Compared to conventional radiography, RTR has a lower contrast sensitivity and limited image resolution. Images created via RTR often suffer from uneven illumination, limited resolution, a lack of sharpness, and noise. These factors have a major impact on image quality.
4) Computed Tomography
Computed tomography (CT) is a technique that takes hundreds to thousands (depending on the size of the component) of 2D radiography scans and superimposes them to create a 3D radiographic image.
In an industrial setting, CT can be achieved in two ways. In one method, the component to be inspected remains stationary while the radiation source and x-ray detector rotate around the component. This technique is more likely to be utilized for large components. The second method consists of the radiation source and x-ray detector remaining stationary while the component is rotated 360 degrees. This second technique is more useful when the component is small or has complex geometry.
Although this technology is timely, expensive, and requires a large amount of data storage, CT provides highly accurate images, is repeatable and reproducible, and minimizes human error.
Applications of radiographic testing method
Radiographic testing is mainly applied for the detection of flaws such as cracks, porosity, inclusions, lack of root penetration, lack of fusion, laps, seams, shrinkage, corrosion, etc. in weldments and castings, in pressure vessels, containers for industrial liquids and gases, pipelines, steel bridges, steel and aluminium columns and frames and roofs, nuclear reactors and nuclear fuel cycle, boiler tubes, ships and submarines, aircraft and armaments. In most of these cases weld inspection is involved.
Radiography is also extensively used for the inspection of castings and forgings. The regular shaped and uniformly thick specimens can be inspected as usual like welds in plates while special considerations need to be made for testing of specimens of varying thickness. Double film technique is usually employed wherein two films of different speeds are used for a single exposure. In this way
correct density is obtained under the thick sections on the faster film whereas the
slower films record correct images of the thin sections.
Radiography is used in inspection of explosives contained within casings, sealed boxes and equipment. In the field of electronics it is employed for the inspection of printed circuit boards and assemblies for checking adequacy of connections.
ULTRASONIC TESTING
Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more.
 |
| FIG 19 |
General procedure for ultrasonic testing
Ultrasonics are the sound waves whose frequency is greater than 20kHz. Due to the high frequency they have a very good penetrating power. When sound waves propagate from one medium to another, a part of the sound energy is reflected and the rest is transmitted at the interface seperating the two media as shown in Figure 20 . This property is made use to detect flaws because not only interfaces also the flaws can reflect the ultrasonic sound energy
 |
| FIG 20 |
he interaction of the sound energy is stronger for higher frequencies. Hence high frequency ultrasound in the frequency range 0.5 MHz to 25MHz is found suitable for the testing. The waves are generated by using either a Piezo-electric energised crystal cut in a particular fashion to generate the desired wave mode or an Electromagnetic accoustic transducer. The relation among the intensities of the incident and reflected sound energy is given in equation
The intensity of the sound wave reflected from the interface generally depends upon the difference in the densities of the pair of media ( 𝝆1 𝝆2 ) for the given incident wave intensity. Here 𝝆1 and 𝝆2 are the densities of the two media 1 and 2 respectively through which the sound wave is propagating. Thus, if the ultrasonic wave propagates from a medium of higher density into a medium of lower density then maximum reflection of intensity takes place at the interface seperating the two media. The flaw in the medium results in the reflection of sound energy due to the variation of density and hence their detection is made possible. Reflections are analysed electrically and the reflection is called echo.
General procedure for ultrasonic testing
Figure 21 shows that the typical ultrasonic testing system. It consists of several functional units, such as the pulser/receiver, transducer and display devices.
 |
| FIG 21 |
A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. The longitudinal ultrasonic pulses are generated using the probe. For each generated pulse the echoes are observed on the oscilloscope as shown in the Figure 21 . The first echo corresponds to the reflection from the upper surface of the part. If there exists a flaw, a second echo is observed with a lower pulse height due to smaller reflection intensity. A third echo is observed due to the reflection from the back surface. The intensity of the echo from the back surface reflection is less due to attenuation of sound energy in the medium
Applications of ultrasonic testing
Thickness measurements
Thickness measurements using ultrasonics can be applied using either the pulse echo or resonance techniques. Some typical applications are:
(i) Wall thickness measurement in pressure vessels, pipelines, gas holders, storage tanks for chemicals and accurate estimate of the effect of wear and corrosion without having to dismantle the plant.
(ii) Measurement of the thickness of ship hulls for corrosion control.
(iii) Control of machining operations, such as final grinding of hollow propellers.
(iv) Ultrasonic thickness gauging of materials during manufacture.
(v) Measurement of wall thickness of hollow aluminium extrusions.
(vi) Measurement of the thickness of lead sheath and insulating material extruded over a core of wire.
(vii) Inspection of heat exchanger tubing in nuclear reactors.
(viii) Measurement of the wall thickness of small bore tubing including the
canning tubes for reactor fuel elements.
Flaw detection
Typical flaws encountered in industrial materials are cracks, porosity, laminations, inclusions, lack of root penetration, lack of fusion, cavities, laps, seams, corrosion, etc. Some examples of the detection of these defects are as follows:
(i) Examination of welded joints in pressure vessels, containers for industrial liquids and gases, pipelines, steel bridges, pipelines, steel or aluminium columns, frames and roofs (during manufacturing, pre-service and inservice).
(ii) Inspection of steel, aluminium and other castings,
(iii) Inspection of rolled billets, bars and sections.
(iv) Inspection of small bore tubes including the canning tubes for nuclear fuel elements.
(v) Ultrasonic testing of alloy steel forgings for large turbine rotors,
(vi) Testing of turbine rotors and blades for aircraft engines.
(vii) Early stage inspection in the production of steel and aluminium blocks and slabs, plates, bar sections, tubes, sheets and wires.
(viii) Detection of unbonded surfaces in ceramics, refractories, rubber, plastics and laminates.
(ix) Detection of honeycomb bond in the aircraft industry.
(x) Inspection of jet engine rotors.
(xi) Detection of caustic embrittlement failure in riveted boiler drums in the power generation industry.
(xii) Detection of cracks in the fish plate holes in railway lines and in locomotive and bogey axles.
(xiii) Detection of hydrogen cracks in roller bearings resulting from improper heat treatment.
(xiv) In service automatic monitoring of fatigue crack growth.
(xv) Detection of stress corrosion cracking.
(xvi) Detection of fatigue cracks in parts working under fluctuating stress.
(xvii) Inspection of fine quality wire.
(xviii)Testing of wooden components such as utility poles.
(xix) Application of ultrasonics to monitor material characteristics in the space environment.
(xx) Determination of lack of bonding in clad fuel elements,
(xxi) Detection of flaws in grinding wheels.
(xxii) Varieties of glass which are not sufficiently transparent to allow optical inspection can be tested ultrasonically.
(xxiii)Quality control in the manufacture of rubber tyres by locating voids, etc.
(xxiv)Inspection of engine crankshafts.
Miscellaneous applications
In addition to the applications already mentioned there are numerous others. Notable among these are those based on the measurement of acoustic velocity and the attenuation of acoustic energy in materials. Some of these applications are as follows:
(1) Assessment of the density and tensile strength of ceramic products such as high tension porcelain insulators.
(2) Determination of the difference between various types of alloys.
(3) Detection of grain growth due to excessive heating.
(4) Estimation of the values of the elastic moduli of metals over a wide range of temperature and stress.
(5) Tensile strength of high grade cast iron can be estimated by measuring its coefficient of acoustical damping.
(6) Crushing strength of concrete can be measured from the transit time of an ultrasonic pulse.
(7) Quarrying can be made more efficient by the measurement of pulse velocity or attenuation in rock strata.
(8) To find the nature of formations in geophysical surveys without having to undertake boring operations.
(9) Detection of bore hole eccentricity in the exploration for mineral ores and oil.
(10) Study of press fits.
(11) Metallurgical structure analysis and control of case depth and hardness, precipitation of alloy constituents and grain refinement.
(12) Determination of intensity and direction of residual stresses in structural metal components.
(13) Detection of honeycomb debonds and the regions in which the adhesive fails to develop its nominal strength in the aerospace industry.
(14) Measurement of liquid level of industrial liquids in containers.
NDT Method Summary
No single NDT method will work for all flaw detection or measurement applications. Each of the methods has advantages and disadvantages when compared to other methods. The table below summarizes the scientific principles, common uses and the advantages and disadvantages for some of the most often used NDT methods.
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