Non destructive Testing Techniques
Non destructive Testing Techniques
There can be inherent microscopic flaws in materials due to crystal lattice imperfections. Also, manufacturing processes such as welding, casting, forging, and surface treatment, etc. can cause further flaws or defects. Further, materials are used under various conditions of stress, fatigue, and corrosion, which can create additional defects or aggravate present ones. The material failures normally occur when these imperfections reach dangerous proportions such that remaining part of the material cannot withstand the stress it is subjected to, thus become ductile or brittle. Hence, there a need to detect these imperfections in materials and to evaluate them in terms of their nature, size and location. Further steps are needed to assess the severity of the imperfections in order to decide whether the material is accepted, is accepted after repairing, or is to be rejected and scrapped.
Non-destructive testing (NDT) is the technique of inspecting, testing, or evaluating materials, components, or assemblies for imperfections also known as 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. The technique can be applied on a sampling basis for individual investigation or can be used for 100 % checking of material in a production quality control system. It is possible to inspect and / or measure the materials or structures without destroying their surface texture, product integrity and future usefulness.
Though NDT is a high technology concept, the evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture. Its application ranges from steel production to site inspection of components already in service. A certain degree of skill is needed to apply NDT techniques properly in order to achieve the maximum amount of information concerning the product, with consequent feed back to the production facility. NDT is not just a method for rejecting substandard material but it is also an assurance which determines the supposed good material is good. The technique uses a variety of principles. There is no single method around which a black box can be built to satisfy all requirements in all circumstances
The field of NDT is a very broad and interdisciplinary which plays a critical role in inspection of structural component and systems so that they perform their function in a reliable manner. Certain standards have been also made to assure the reliability of the NDT tests and prevent certain errors due to either the fault in the equipment used, the misapplication of the methods or the skill and the knowledge of the inspectors. Successful NDT tests allow locating and characterizing material conditions and flaws. The NDT techniques normally need considerable operator skill and interpreting of the test results accurately can be difficult since the results can be subjective.
NDT Technique name frequently refer to the type of penetrating medium or the equipment used to perform the test. NDT techniques can be classified as conventional techniques and as non-conventional techniques. The conventional NDT techniques include (i) visual or optical inspection, (ii) liquid penetrant testing, (iii) magnetic particle testing, (iv) eddy current testing, (v) radiographic testing, and (vi) ultrasonic testing. The non-conventional NDT techniques are used only for specialized applications and include neutron radiography, acoustic emission, infrared testing, microwave techniques, leak testing, holography, guided wave testing, ground penetrating radar, and laser testing etc.
The essential elements common to most of the NDT techniques are (i) a probing medium, (ii) a test sample which is appropriate for the medium being used so that discontinuities can be detected, (iii) a detector capable of measuring the distributions or alterations in the media, (iv) a technique for recording or displaying information received from the detector which is suitable for evaluation, and (v) the operator who is trained to interpret detector feedback to evaluate results.
NDA technique provides an indication during the testing. The definition of the term ‘indication’ as it applies to NDT is ‘a response or evidence of a response disclosed through NDT which needs further evaluation to determine its true significance’. When a specific NDT technique is applied on a part then there is a response. This response is an indication. The term ‘response’ is intended to mean (i) a ‘bleed out’ when performing a liquid penetrant testing, (ii) a particle buildup when performing a magnetic particle testing, (iii) a change in density on the radiographic film in case of a radiographic testing, (iv) a signal when performing ultrasonic testing, and (v) a meter deflection, signal, or digital change when performing eddy current testing. Once the response is observed, the operator performing the test is required to interpret it, and then categorize it into anyone of the groups of indications namely (i) false, (ii) non-relevant, or (iii) relevant or true discontinuity.
None of the NDT techniques provide solutions to all the possible problems, i.e. they are not optional alternatives but rather complementary to each other. The basic principles, typical applications, advantages and limitations of the conventional techniques are described below.
Visual or optical inspection
Visual and optical inspection techniques (Fig 1) are used to examine the surface condition of a component. Visual testing is widely used for just about every conceivable surface condition. By its very nature, visual and optical testing can be simple and straight forward. At its simplest, a clean component can be inspected by an operator in adequate light with no equipment, it can be that easy. Frequently, the operator requires using optical equipment to aid the inspection, which can range from a hand-held magnifier lens to a flexible fibre-scope, or remote video systems.
An experienced operator, under optimum conditions, can be able to detect even small tight cracks. Repeatability is, however, a problem. If conditions are not optimized, the same operator can miss the same crack on the same component on a repeat inspection. This is why, optical aids are frequently used to give the operator the best chance of finding the fault condition as often as possible. Inspection is required to take place in a clean, comfortable environment with adequate lighting.
Attention is to be paid to safety, working position and atmospheric conditions. Inspection requires considerable concentration by the operator. Lighting is very important and can greatly affect the results. Natural daylight is the best type of light to perform visual inspection. Artificial light can also be used for visual inspection, however, the operator is to make sure that the correct light level stated in the specification or procedure being used.
The component is to be clean and free from protective coatings, for example dirt or paint can obscure the surface conditions being sought. It is of great importance that the operator has sufficient training and experience before performing visual inspection. The operator is also to have good eyesight. As it is known, the eye is a wonderfully sophisticated instrument but it does not see everything. It is designed to focus light onto the retina, convert the light to nerve impulses and send them to the brain. The brain then processes this information and forms the images which are seen. This leads us to perception, which is the difference between physical reality and the view the inspectors think they see. Different inspectors interpret the incoming information from the eye differently, so they all see the same physical scene slightly differently.
The Muller-Lyer illusion (Fig 1) shows the difference between perception and reality. The shafts of the two arrows are the same length but appear to be different. The difference of perception between two inspectors depends upon training and experience and the mental and physical state of the observers at the time the observation is made. Perception can be affected by fatigue and health. Fatigue reduces the efficiency and visual ability of the observer. These problems lead to inaccurate interpretation of physical data. An ideal inspection is the one in which all of the factors namely training, experience, lighting, and environmental conditions are optimized.
Fig 1 Visual and optical inspection techniques
Broadly speaking, visual inspection is divided into two types of viewing techniques. The first technique is the direct viewing. In this type of viewing of an object, the object is in operator’s immediate presence. This can be unaided or by using an equipment. The second is remote viewing. In this, the viewing of the object is done not in the operator’s immediate presence. This is done by using special equipment.
Visual inspection can be applied successfully to virtually anything. It can be used to locate many different types of surface condition, from discontinuities, such as corrosion or cracks, to the mottle effect of painted surfaces. An experienced heat treatment operator can even estimate the temperature of a component from its visual appearance once it has been heated to incandescence, such as dull cherry red steel is at around 550 deg C.
An operator is frequently required to locate small discontinuities. This can be very difficult with the naked eye, so optical aids are needed. The some of the most common optical aids are (i) hand-held magnifying lenses (normally from 1.5× magnification upto 10× magnification), (ii) measuring magnifiers which incorporate a measuring scale to enable the surface condition to be measured such as anglepoise magnifier which has upto 10× magnification and frequently has a circular fluorescent tube built in to provide uniform illumination, (iii) microscopes which are of various types and come in a wide variety of magnification ranges, (iv) rigid borescopes which are excellent piece of equipments for inspecting the inside of tubes or pipes (Fig1), (v) a similar device called endoscope (Fig 1) which is more flexible than borescopes due to the use of fibre optics for both the light guide and the image guide, and (vi) to improve image quality, the optical systems of borescopes can be replaced by a miniature video camera, which can contain an image tube, which uses an electron beam to scan a photo-conductive target, known as the light sensor, or alternatively which can contain a solid-state imaging device, such as a charge-coupled device or a charge-injected device.
Liquid penetrant testing
The basic principle of liquid penetrant testing (Fig 2) is that when a very low viscosity (highly fluid) liquid (the penetrant) is applied to the surface of a part, it penetrates into fissures and voids open to the surface. Once the excess penetrant is removed, the penetrant trapped in those voids flow back out, creating an indication. Penetrant testing can be performed on magnetic and non-magnetic materials, but does not work well on porous materials.
In order of decreasing sensitivity and decreasing cost, the liquid penetrant processes can be listed as (i) post emulsifiable fluorescent dye penetrant, (ii) solvent removable fluorescent dye penetrant, (iii) water washable fluorescent dye penetrant, (iv) post emulsifiable visible dye penetrant, (v) solvent removable visible dye penetrant, and (vi) water washable visible dye penetrant.
The advantages of liquid penetrant testing are (i) relatively low cost, (ii) highly portable NDT technique, (iii) highly sensitive to fine, tight discontinuities, (iv) applicable to a variety of materials, and (v) large area inspection. The limitations of liquid penetrant technique are (i) test surface is to be free of all dirt, oil, grease, paint, and rust, etc., (ii) detects surface discontinuities only, (iii) cannot be used on porous and very rough surfaces, (iv) removal of all penetrant materials, following the test, is frequently needed, and (v) there is no easy method to produce permanent record.
In this technique, penetrants can be ‘visible’, meaning they can be seen in ambient light, or fluorescent, requiring the use of a ‘black’ light. The visible dye penetrant process is shown in Fig 2. When performing liquid penetrant testing, it is imperative that the surface being tested is clean and free of any foreign materials or liquids which can block the penetrant from entering voids or fissures open to the surface of the part. After applying the penetrant, it is allowed to remain on the surface for a specified period of time (penetrant dwell time), then the part is carefully cleaned to remove excess penetrant from the surface. When removing the penetrant, the operator is to be careful not to remove any penetrant which has flown into the voids. A light coating of developer is then be applied to the surface and a time is provided (developer dwell time) to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication. Following the prescribed developer dwell time, the part is inspected visually or with the aid of a black light for fluorescent penetrants. Majority of the developers are fine-grained, white talcum-like powders which provide a colour contrast to the penetrant being used.
Fig 2 Liquid penetrant testing technique
Solvent removable penetrants are those penetrants which need a solvent other than water to remove the excess penetrant. These penetrants are normally visible in nature, normally dyed a bright red colour which contrasts well against a white developer. The penetrant is normally sprayed or brushed onto the part, and then after the penetrant dwell time is over, the part is cleaned with a cloth dampened with penetrant cleaner after which the developer is applied. Following the developer dwell time the part is examined to detect any penetrant bleed-out showing through the developer.
Water-washable penetrants have an emulsifier included in the penetrant which allows the penetrant to be removed using a water spray. They are most frequently applied by dipping the part in a penetrant tank, but the penetrant can be applied to large parts by spraying or brushing. Once the part is fully covered with penetrant, the part is placed on a drain board for the penetrant dwell time, and then taken to a rinse station where it is washed with a course water spray to remove the excess penetrant. Once the excess penetrant has been removed, the part can be placed in a warm air dryer or in front of a gentle fan until the water has been removed. The part can then be placed in a dry developer tank and coated with developer, or allowed to sit for the remaining dwell time then inspected.
Post-emulsifiable penetrants are penetrants which do not have an emulsifier included in its chemical make-up like water-washable penetrants. Post-emulsifiable penetrants are applied in a similar manner, but prior to the water-washing step, emulsifier is applied to the surface for a prescribed period of time (emulsifier dwell time) to remove the excess penetrant. When the emulsifier dwell time has elapsed, the part is subjected to the same water wash and developing process used for water-washable penetrants. Emulsifiers can be lipophilic (oil-based) or hydrophilic (water-based).
Magnetic particle testing
Magnetic particle testing uses one or more magnetic fields to locate surface and near-surface discontinuities in ferro-magnetic materials. It is used to locate surface and slight sub-surface discontinuities or defects in ferro-magnetic materials. Such flaws present in a magnetized part cause a magnetic field, i.e. flux, to leave the part. If magnetic particles are applied to this surface, they are held in place by the flux leakage to give a visual indication. While several different methods of magnetic particle tests can be used, they all rely on this same general principle. Hence, any magnetic particle test is conducted by creating a magnetic field in a part and applying the magnetic particles to the test surface.
The magnetic field can be applied with a permanent magnet or an electro-magnet. When using an electro-magnet, the field is present only when the current is being applied. When the magnetic field encounters a discontinuity transverse to the direction of the magnetic field, the flux lines produce a magnetic flux leakage field of their own as shown in Fig 3. This can be seen, when very fine coloured ferro-magnetic particles (magnetic particles) are applied to the surface of the part the particles get drawn into the discontinuity, reducing the air gap and producing a visible indication on the surface of the part. The magnetic particles can be a dry powder or suspended in a liquid solution, and they can be coloured with a visible dye or a fluorescent dye that fluoresces under an ultraviolet (black) light.
Either alternating current (AC) or direct current (DC) can be used to induce a magnetic field. The magnetic field created by AC due to the ‘skin effect’ is strongest at the surface of the test object. AC also provides greater particle mobility on the surface of the part allowing them to move about freely to locate areas of flux leakage, even though the surface of the part can be irregular. DC induces magnetic fields which have greater penetrating power and can be used to detect near surface discontinuities.
Majority of the field inspections are carried out using a yoke (Fig 3). An electric coil is wrapped around a central core, and when the current is applied, a magnetic field is generated which extends from the core down through the articulated legs into the part. This is known as longitudinal magnetization because the magnetic flux lines run from one leg to the other. When the legs are placed on a ferro-magnetic part and the yoke is energized, a magnetic field is introduced into the part. Since the flux lines run from one leg to the other, discontinuities oriented perpendicular to a line drawn between the legs can be found. For ensuring no indications are missed, the yoke is used once in the position as shown in the figure and then used again with the yoke turned 90 degree so no indications are missed. Because all of the electric current is contained in the yoke and only the magnetic field penetrates the part, this type of application is also known as indirect induction.
Fig 3 Magnetic particle testing
Poke units use direct induction, where the current runs through the part and a circular magnetic field is generated around the legs as shown in Fig 3. Since the magnetic field between the pokes is travelling perpendicular to a line drawn between the pokes, indications oriented parallel to a line drawn between the pokes can be found. As with the yoke, two inspections are done, the second with the pokes oriented 90 degree to the first application.
Electric coils are used to generate a longitudinal magnetic field. When energized, the current creates a magnetic field around the wires making up the coil so that the resulting flux lines are oriented through the coil. Because of the longitudinal field, indications in parts placed in a coil are oriented transverse to the longitudinal field.
Majority of horizontal wet bath machines (bench units) have both a coil and a set of heads through which electric current can be passed, generating a magnetic field. These machines use fluorescent magnetic particles in a liquid solution, hence the name ‘wet bath’. When testing a part between the heads, the part is placed between the heads, the moveable head is moved up so that the part being tested is held tightly between the heads, the part is wetted down with the bath solution containing the magnetic particles and the current is applied while the particle are flowing over the part. Since the current flow is from head to head and the magnetic field is oriented 90 degrees to the current, indications oriented parallel to a line between the heads are visible. This type of inspection is normally called a ‘head shot’.
When testing hollow parts such as pipes, tubes and fittings, a conductive circular bar can be placed between the heads with the part suspended on the bar (the central conductor) as shown in Fig 3. The part is then wetted down with the bath solution and the current is applied, travelling through the central conductor rather than through the part. The ID and OD of the part can then be inspected. As in the case of head shot, the magnetic field is perpendicular to the current flow, wrapping around the test piece, so indications running axially down the length of the part can be found using this technique.
The advantages of the magnetic particle testing are (i) is economical, (ii) is aid to visual testing, (iii) can be fixed or portable equipment, (iv) provides instant repeatable results, (v) effective inspection technique, and (vi) contrast or fluorescent consumables. The limitations of the magnetic particle testing are (i) parts being inspected are to be ferro-magnetic, (ii) high currents are needed, (iii) can detect only surface and slightly sub-surface flaws, (iv) parts need to be demagnetized, (v) parts are to be clean and relatively smooth, (vi) equipment can be bulky and heavy, (vi) power supply is normally needed, (vii) coating can mask indications, and (viii) material or part permeability can affect results.
Eddy current testing
Eddy currents are created through a process called electromagnetic induction. When AC is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the AC rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current is induced in this second conductor. These currents are influenced by the nature of the material such as voids, cracks, changes in grain size, as well as physical distance between coil and material. These currents form impedance on a second coil which is used to as a sensor. In practice a probe is placed on the surface of the part to be inspected, and electronic equipment monitors the eddy current in the work piece through the same probe. The sensing circuit is a part of the sending coil.
The main applications of the eddy current technique are for the detection of surface or sub-surface flaws. The technique is sensitive to the material conductivity, permeability and dimensions of the product. Eddy currents can be produced in any electrically conducting material which is subjected to an alternating magnetic field (typically 10 Hz to 10 MHz). The alternating magnetic field is normally generated by passing an alternating current through a coil. The coil can have many shapes and can have between 10 turns to 500 turns of wire. The magnitude of the eddy currents generated in the product is dependent on the conductivity, permeability and the set up geometry. Any change in the material or geometry can be detected by the excitation coil as a change in the coil impedance.
The simplest coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a crack, for example, occurs in the product surface the eddy currents travel farther around the crack and this is detected by the impedance change (Fig 4). Coils can also be used in pairs, generally called a driven pair, and this arrangement can be used with the coils connected differentially. In this way the ‘lift off’ (distance of the probe from the surface) signals can be enhanced. Coils can also be used in a transformer type configuration where one coil winding is a primary and one (or two) coil windings are used for the secondaries. .
The detected eddy current signals contain amplitude and phase information which can be displayed on CRT (cathode ray tube) type displays normally non digital displays. Signals can be displayed as the actual, i.e. absolute signal, or with appropriate electronics, only a signal change is displayed. The best results are achieved where only one product parameter changes, e.g. the presence of a crack. In practice changes in eddy current signals are caused by differences in composition, hardness, texture, shape, conductivity, permeability and geometry. In some cases the effects of the crack can be hidden by changes in other parameters and unnecessary rejection can occur. However, the coils can be selected for configuration, size, and test frequency in order to enhance detection of cracks, conductivity, metal loss etc. as needed.
Fig 4 Eddy current testing
The depth to which the eddy currents penetrate a material can be changed by adjusting the test frequency, i.e. the higher is the frequency the lower is the penetration. However, the lower is the frequency the lower is the sensitivity to small defects. Larger coils are less sensitive to surface roughness and vice versa. The latest electronic units are able to operate a wide range of coil configurations in absolute or differential modes and at a wide range of frequencies. For surface testing for cracks in single or complex shaped components, coils with a single ferrite cored winding are normally used. The probe is placed on the component and ‘balanced’ by use of the electronic unit controls. As the probe is scanned across the surface of the component the cracks can be detected.
Where surfaces are to be scanned automatically the single coil windings are suitable only if the lift off distance is accurately maintained. Normally differential coil configurations are used with higher speed scanning systems where lift off effects, vibration effects, etc. can be cancelled out to an acceptable extent. Tubes, bar and wire can be inspected using an encircling coil and these normally have a coil configuration with one primary and two secondaries connected differentially.
Majority of eddy current electronics have a phase display and this gives an operator the ability to identify defect conditions. In many cases signals from cracks, lift off and other parameters can be clearly identified. Units are also available which can inspect a product simultaneously at two or more different test frequencies. These units allow specific unwanted effects to be electronically cancelled in order to give improved defect detection.
The eddy current test is purely electrical. The coil units do not need to contact the product surface and thus the technique can be easily automated. Most automated systems are for components of simple geometry where mechanical handling is simplified.
The advantages of eddy current testing are (i) suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals, (ii) information can be provided in simple terms frequently as go / no go with the phase display electronic units can be used to achieve much greater product information, (iii) extremely compact and portable units are available, (iv) no consumables (except probes which can sometimes be repaired), (v) flexibility in selection of probes and test frequencies to suit different applications, and (vi) suitable for total automation. The disadvantages of eddy current testing are (i) the wide range of parameters which affect the eddy current responses means that the signal from a desired material characteristic, e.g. a crack, can be masked by an unwanted parameter, e.g. hardness change, hence careful selection of probe and electronics are needed in some applications, and (ii) normally tests are restricted
The radiographic testing method is used for the detection of internal flaws in many different materials and configurations. X-rays, generated electrically, and gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes. The greater is the thickness, the greater is the absorption. Furthermore, the denser is the material the greater is the absorption. X-ray and gamma rays also have the property, like light, of partially converting silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and hence forming a latent image. This can be developed and fixed in a similar way to normal photographic film (Fig 5).
Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both x-ray and gamma ray radiography.
In x-ray radiography the penetrating power is determined by the number of volts applied to the x-ray tube. In case of steel, it is around 1,000 volts per inch thickness. In gamma ray radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus iridium 192 is used for 15 mm to 25 mm thick steel, and cesium 134 is used for 20 mm to 265 mm thick steel. In x-ray radiography the intensity, and hence the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is normally expressed in terms of milli-ampere minutes. With gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of iridium 192 is 74 days, and cesium 134 is 2.1 years.
The exposure factor is a product of the number of curies and time, normally expressed in curie hours. The time of exposure is to be increased as the isotope decays. When the exposure period becomes uneconomical the isotope is to be renewed. As the isotope is continuously emitting radiation it is to be housed in a container of depleted uranium or similar dense shielding material, whilst not exposed for protecting the environment and personnel.
Fig 5 Schematics of radiographic testing
To produce an x-ray or gamma ray radiograph, the film package ((enclosed in a light tight cassette and comprising film and intensifying screens, the latter being required to reduce the exposure time) is placed close to the surface of the subject. The source of radiation is positioned on the other side of the subject some distance away, so that the radiation passes through the subject and on to the film. After the exposure period the film is removed, processed, dried, and then viewed by transmitted light on a special viewer. Different radiographic and photographic accessories are necessary, including such items as radiation monitors, film markers, image quality indicators, dark-room equipment, etc. As far as the last is concerned there are many degrees of sophistication, including fully automatic processing units. These accessories are the same for both x-ray and gamma radiography systems. Also needed are such consumable items as radiographic film and processing chemicals
Recent developments in radiography permit ‘real time’ diagnosis. Such techniques as computerized tomography yield much important information, though these methods can be suitable for only investigative purposes and not generally employed in production quality control.
Industrial radiography involves exposing a test object to penetrating radiation so that the radiation passes through the object being inspected and a recording medium placed against the opposite side of that object. For thinner or less dense materials such as aluminum, electrically generated x-radiations (x-rays) are normally used, and for thicker or denser materials, gamma radiation is generally used. Gamma radiation is given off by decaying radioactive materials, with the two most commonly used sources of gamma radiation being Iridium-192 (Ir-192) and Cobalt-60 (Co-60). Ir-192 is normally used for steel upto 15 mm to 25 mm, depending on the Curie strength of the source, and Co-60 is normally used for thicker materials due to its greater penetrating ability. The recording media can be industrial x-ray film or one of several types of digital radiation detectors. With both, the radiation passing through the test object exposes the media, causing an end effect of having darker areas where more radiation has passed through the part and lighter areas where less radiation has penetrated. If there is a void or defect in the part, more radiation passes through, causing a darker image on the film or detector.
Film radiography uses a film made up of a thin transparent plastic coated with a fine layer of silver bromide on one or both sides of the plastic. When exposed to radiation these crystals undergo a reaction which allows them, when developed, to convert to black metallic silver. This silver is then ‘fixed’ to the plastic during the developing process, and when dried, becomes a finished radiographic film. To be a usable film, the area of interest on the film is to be within a certain density (darkness) range and is to show enough contrast and sensitivity so that discontinuities of interest can be seen. These items are a function of the strength of the radiation, the distance of the source from the film and the thickness of the part being inspected. If any of these parameters are not met, another exposure (is to be made for that area of the part.
Computed radiography is a transitional technology between film and direct digital radiography. This technique uses a reusable, flexible, photo-stimulated phosphor plate which is loaded into a cassette and is exposed in a manner similar to traditional film radiography. The cassette is then placed in a laser reader where it is scanned and translated into a digital image, which take from one to five minutes. The image can then be uploaded to a computer or other electronic media for interpretation and storage. Computed tomography uses a computer to reconstruct an image of a cross sectional plane of an object as opposed to a conventional radiograph. The computed tomography image is developed from multiple views taken at different viewing angles which are reconstructed using a computer. With traditional radiography, the position of internal discontinuities cannot be accurately determined without making exposures from several angles to locate the item by triangulation. With computed tomography, the computer triangulates using every point in the plane as viewed from many different directions.
Digital radiography digitizes the radiation which passes through an object directly into an image which can be displayed on a computer monitor. The three principle technologies used in direct digital imaging are amorphous silicon, charge coupled devices, and complementary metal oxide semi-conductors. These images are available for viewing and analysis in seconds compared to the time needed to scan in computed radiography images. The increased processing speed is a result of the unique construction of the pixels; an arrangement which also allows a superior resolution than is found in computed radiography and most film applications.
The advantages of radiographic testing include (i) is useful on wide variety of materials, (ii) can be used for checking internal mal-structure, misassembly or misalignment, (iii) provides permanent record, and (iv) devices for checking the quality of radiograph are available. Some of the limitations of this method are (i) access to both sides of the object is needed, (ii) cannot detect planar defects readily, (iii) thickness range which can be inspected is limited, (iv) sensitivity of inspection decreases with thickness of the test object, (v) considerable skill is needed for interpretation of the radiographs, (vi) depth of defect is not indicated readily, and (vii) x-rays and gamma rays are hazardous to human health.
Ultrasonic technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials. The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection, this frequency is normally in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals with little reduction. The velocity at which these waves propagate is related to the Young’s Modulus for the material and is characteristic of the material. For example the velocity in steel is 5,900 metres per second, and in water 1,400 metres per second.
Ultrasonic energy is considerably reduced in air, and a beam propagated through a solid, on reaching an interface (e.g. a defect, or intended hole, or the back wall) between that material and air reflects a considerable amount of energy in the direction equal to the angle of incidence. For contact testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid (referred to as ‘couplant’), usually oil, water or grease, is applied to the surface. The crystal does not oscillate continuously but in short pulses, between each of which it is quiescent.
Piezo electric materials not only convert electrical pulses to mechanical oscillations, but also transduce mechanical oscillations into electrical pulses. Hence, there is not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe.
The standard method of presenting information in ultrasonic testing is by means of a cathode ray tube, in which horizontal movement of the spot from left to right represents time elapsed. The principle is not greatly different in digitized instruments that have a LCD (liquid crystal display) flat screen. The rate at which the spot moves is such that it gives the appearance of a horizontal line on the screen. The system is synchronized electronically so that at the instant the probe receives its electrical pulse the spot begins to traverse the screen. An upward deflection (peak) of the line on the screen is an indication of this occurrence. This peak is normally termed the initial pulse.
Whilst the base line is perfectly level the crystal is quiescent. Any peaks to the right of the initial pulse indicate that the crystal has received an incoming pulse reflected from one or more interfaces in the material. Since the spot moves at a very even speed across the tube face, and the pulse of ultrasonic waves moves at a very even velocity through the material, it is possible to calibrate the horizontal line on the screen in terms of absolute measurement. The use of a calibration block, which produces a reflection from the back wall a known distance away from the crystal together with variable controls on the flaw detector, allows the screen to be calibrated in units of distance, and hence determination of origins of returned pulses obtained from a test piece.
It is hence possible not only to discover a defect between the surface and the back wall, but also to measure its distance below the surface. It is important that the equipment is properly calibrated and, since it is in itself not able to discriminate between intended boundaries of the object under test and unintended discontinuities, the operator is required to identify the origin of each peak. Further as the pulses form a beam it is also possible to determine the plan position of a flaw. The height of the peak (echo) is roughly proportional to the area of the reflector, though there is on all instruments a control, which can reduce or increase the size of an indication – variable sensitivity in fact. Not only is part of the beam reflected at a material / air interface but also at any junction where there is a velocity change, for example steel / slag interface in a weld.
Probing all faces of a test piece not only discovers the three-dimensional defect and measures its depth, but can also determine its size. Two-dimensional (planar) defects can also be found but, unlike radiography, it is best that the incident beam impinges on the defect as near to right angles to the plane as possible. To achieve this some probes introduce the beam at an angle to the surface. In this manner longitudinal defects in tubes (inner or outer surface) are detected.
Interpretation of the indications on the screen requires a certain amount of skill, particularly when testing with hand held probes. The technique is, however, admirably suited to automatic testing of regular shapes by means of a monitor – an electronic device which fits into the main equipment to provide an electrical signal when an echo occurs in a particular position on the trace. The trigger level of this signal is variable and it can be made to operate a variety of mechanical gates and flaw warnings. Furthermore, improvements in computer technology allow test data and results to be displayed and out-putted in a wide variety of formats.
Fig 6 Ultrasonic testing
Modern ultrasonic flaw detectors are fully solid state and can be battery powered, and are robustly built to withstand site conditions. Since the velocity of sound in any material is characteristic of that material, it follows that some materials can be identified by the determination of the velocity. This can be applied, for example in spheroidal graphite cast irons to determine the percentage of graphite nodularity.
When the velocity is constant, as it is in a wide range of steels, the time taken for the pulse to travel through the material is proportional to its thickness. Hence, with a properly calibrated instrument, it is possible to measure thickness from one side with accuracy in hundredths of a millimeter. This technique is now in very common use. A development of the standard flaw detector is the digital wall thickness gauge. This operates on similar principles but gives an indication, in LED (light emitting diode) or LCD 9liquid crystal display) numerics, of thickness in absolute terms of millimetres. These equipments are easy to use but need prudence in their application.
The two most commonly used types of sound waves used in industrial inspections are the compression (longitudinal) wave and the shear (transverse) wave (Fig 6). Compression waves cause the atoms in a part to vibrate back and forth parallel to the sound direction and shear waves cause the atoms to vibrate perpendicularly (from side to side) to the direction of the sound. Shear waves travel at around half the speed of longitudinal waves. Sound is introduced into the part using an ultrasonic transducer (probe) which converts electrical impulses from the ultrasonic testing machine into sound waves, then converts returning sound back into electric impulses which can be displayed as a visual representation on a digital or LCD screen. If the machine is properly calibrated, the operator can determine the distance from the transducer to the reflector, and in many cases, an experienced operator can determine the type of discontinuity which caused the reflector. Because ultrasound does not travel through air (the atoms in air molecules are too far apart to transmit ultrasound), a liquid or gel called ‘couplant’ is used between the face of the transducer and the surface of the part to allow the sound to be transmitted into the part.
Straight beam inspection uses longitudinal waves to interrogate the test piece as shown at the right. If the sound hits an internal reflector, the sound from that reflector reflects to the transducer faster than the sound coming back from the back-wall of the part due to the shorter distance from the transducer. This results in a screen display. Digital thickness testers use the same process, but the output is shown as a digital numeric readout rather than a screen presentation.
Angle beam inspection uses the same type of transducer but it is mounted on an angled wedge (also called a probe) that is designed to transmit the sound beam into the part at a known angle. The most commonly used inspection angles are 45 degrees, 60 degrees, and 70 degrees, with the angle being calculated up from a line drawn through the thickness of the part (not the part surface). A 60 degree probe is shown in Fig 6. If the frequency and wedge angle is not specified by the governing code or specification, it is upto the operator to select a combination which adequately inspects the part being tested. In angle beam inspections, the transducer and wedge combination (also referred to as a probe) is moved back and forth towards the weld so that the sound beam passes through the full volume of the weld. As with straight beam inspections, reflectors aligned more or less perpendicular to the sound beam sends sound back to the transducer and are displayed on the screen.
Immersion Testing is a technique where the part is immersed in a tank of water with the water being used as the coupling medium to allow the sound beam to travel between the transducer and the part. The ultrasonic testing machine is mounted on a movable platform (a bridge) on the side of the tank so it can travel down the length of the tank. The transducer is swivel-mounted on at the bottom of a waterproof tube which can be raised, lowered and moved across the tank. The bridge and tube movement permits the transducer to be moved on the X-, Y- and Z-axes. All directions of travel are gear driven so the transducer can be moved in accurate increments in all directions, and the swivel allows the transducer to be oriented so the sound beam enters the part at the required angle. Round test parts are frequently mounted on powered rollers so that the part can be rotated as the transducer travels down its length, allowing the full circumference to be tested. Multiple transducers can be used at the same time so that multiple scans can be performed.
Through transmission inspections are performed using two transducers, one on each side of the part (Fig 6). The transmitting transducer sends sound through the part and the receiving transducer receives the sound. Reflectors in the part cause a reduction in the amount of sound reaching the receiver so that the screen presentation shows a signal with lower amplitude (screen height).
Phased array inspections are done using a probe with multiple elements which can be individually activated. By varying the time when each element is activated, the resulting sound beam can be steered, and the resulting data can be combined to form a visual image representing a slice through the part being inspected.
Time of flight diffraction uses two transducers located on opposite sides of a weld with the transducers set at a specified distance from each other. One transducer transmits sound waves and the other transducer acting as a receiver. Unlike other angle beam inspections, the transducers are not manipulated back and forth towards the weld, but travel along the length of the weld with the transducers remaining at the same distance from the weld. Two sound waves are generated, one travelling along the part surface between the transducers, and the other travelling down through the weld at an angle then back up to the receiver. When a crack is encountered, some of the sound is diffracted from the tips of the crack, generating a low strength sound wave which can be picked up by the receiving unit. By amplifying and running these signals through a computer, defect size and location can be determined with much greater accuracy than by conventional ultrasonic testing methods.
The advantages of ultrasonic flaw detection include (i) thickness and lengths upto 10 meter can be tested, (ii) position, size and type of defect can be determined, (iii) instant test results, (iv) portable, (v) extremely sensitive if required, (vi) capable of being fully automated, (vi) access to only one side necessary, and (vii) no consumables needed. The disadvantages of ultrasonic flaw detection include (i) no permanent record available unless one of the more sophisticated test results and data collection systems is used, (ii) the operator can decide whether the test piece is defective or not whilst the test is in progress, (iii) indications need interpretation, (iv) considerable degree of skill