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Industrial Radiography Image forming techniques Digital radiography CR-image of a weld see acknowledgements* 1
Introduction to the overview of “Industrial Radiography” Image forming techniques The first issue of “Industrial Radiography” was published by Agfa in the sixties, for educational and promotional purposes. Some improved editions have been released since, providing information on the latest image forming radiographic techniques. The booklet has been published in a number of languages and has been very much in demand.
Contents 4.5 4.6 Artificial radioactive sources Advantages and disadvantages of artificial radioactive sources Properties of radioactive sources Activity (source strength) Specific activity Specific gamma-ray emission (k-factor) Half-life of a radioactive source 36 37 Introduction to the overview of “Industrial Radiography” Image forming techniques Preface 13 1. Introduction to industrial radiography 15 5. NDT equipment 39 2. 2.1 2.2 2.3 2.4 2.5 2.6 19 19 20 21 22 22 23 5.1 25 26 26 5.5 5.
10.7 Stopbath Fixing Final wash Drying in the drying cabinet Roller dryers Recommendations for the darkroom Silver recovery Automatic film processing NDT-U (universal) film processor NDT-E (economy) film processor Checking the developing process and film archiving properties PMC-strips to check the developing process Thiosulphate-test to check the film archival properties Storage of exposed films 95 11. Defect discernibility and image quality 97 11.1 97 11.4 11.
15. Film interpretation and reference radiographs 123 15.1 15.2 15.3 Film interpretation The film-interpreter Reference radiographs Weld inspection Casting radiography Examination of assembled objects 123 124 124 16. Digital Radiography (DR) 145 16.1 16.2 16.3 16.
17.4 17.5 17.6 CT for defect detection and sizing Effect of defect orientation 3D CT for sizing of defects in (welded) components Neutron radiography (neutrography) Compton backscatter technique 190 190 18. Special radiographic applications 193 18.1 18.2 18.3 193 194 195 18.7 18.
Preface To verify the quality of a product, samples are taken for examination or a non-destructive test (NDT) is carried out. In particular with fabricated (welded) assemblies, where a high degree of constructional skill is needed, it is necessary that non-destructive testing is carried out. Most NDT systems are designed to reveal defects, after which a decision is made as to whether the defect is significant from the point of view of operational safety and/or reliability.
1 Introduction to industrial radiography Image forming techniques source In industrial radiography, the usual procedure for producing a radiograph is to have a source of penetrating (ionising) radiation (X-rays or gamma-rays) on one side of the object to be examined and a detector of the radiation (the film) on the other side as shown in figure 1-1. The energy level of the radiation must be well chosen so that sufficient radiation is transmitted through the object onto the detector.
• By means of radiation detectors, e.g.: crystals, photodiodes or semiconductors in a linear array by which in a series of measurements an image is built up of a moving object. This method is applied in systems for luggage checks on airports. Assuming the grooves have sharp-machined edges, the images of the grooves could still be either sharp or blurred; this is the second factor: image blurring, called image unsharpness.
2 Basic properties of ionising radiation In 1895 the physicist Wilhelm Conrad Röntgen discovered a new kind of radiation, which he called X-rays. The rays were generated when high energy electrons were suddenly stopped by striking a metal target inside a vacuum tube – the X-ray tube. It was subsequently shown that X-rays are an electromagnetic radiation, just like light, heat and radiowaves. 2.
2.3 Gamma-rays (γ-rays) The radiation which is emitted by an X-ray tube is heterogeneous, that is, it contains X-rays of a number of wavelengths, in the form of a continuous spectrum with some superimposed spectrum lines. See fig. 1-2. Fig. 1-2. X-ray spectrum – intensity/wavelength distribution The small peaks are the characteristic radiation of the target material Radioactivity is the characteristic of certain elements to emit alpha (α), beta (β) or gamma (γ) rays or a combination thereof.
2.4 Main properties of X-rays and γ-rays 2.6 Absorption and scattering X-rays and γ-rays have the following properties in common: The reduction in radiation intensity on penetrating a material is determined by the following reactions : 1. invisibility; they cannot be perceived by the senses 2. they travel in straight lines and at the speed of light 3. they cannot be deflected by means of a lens or prism, although their path can be bent (diffracted) by a crystalline grid 4.
2.7 Penetrating power ejected electron The penetrating power of X-radiation increases with the energy (hardness). The relationship of energy and penetrating power is complex as a result of the various mechanisms that cause radiation absorption. When monochromatic ( homogeneous single wave length) radiation with an intensity Io passes through matter, the relative intensity reduction ΔI/Io is proportional to the thickness Δt.
Table 2-2 shows the average HVT-values for steel, table 3-2 shows the values for lead. 2.8 Filtering (hardening) All materials, for example a metal layer between the radiation source and the film, cause absorption and filtering. The position of the metal layer plays an important role in the effect it has. A metal layer in front of the object will “harden” the radiation because it filters out the soft radiation. The degree of hardening depends on the type and the thickness of the material.
3 Units and definitions 3.1 Units Until 1978 the “International Commission of Radiation Units and Measurements” (ICRU) used the conventional radiation units of roentgen (R), rad (rd), and curie (Ci). Since 1978 the ICRU has recommended the use of the international system units (SI) with special new units for radiation quantities; the Becquerel, Gray and Sievert. Table 1-3 shows the relationships of these new units to the older units.
3.2 Definitions Radioactivity The activity of a radioactive source of radiation (isotope) is equal to the number of disintegrations per second. The SI-unit is the Becquerel (Bq) and is equal to 1 disintegration per second. The Becquerel is too small a unit to be used in industrial radiography. Source strengths are, therefore, quoted in Giga Becquerel (GBq). 1 Curie = 37 GBq, see table 2-3.
4 Radiation sources 4.1 X-Ray tube The X-ray tube, see figure 1-4, consists of a glass (or ceramic) envelope containing a positive electrode (the anode) and a negative electrode (the cathode) evacuated to an ultra high vacuum [10 - 9 hPa (hectoPascal)]. The cathode comprises a filament that generates electrons. Under the effect of the electrical tension set up between the anode and the cathode (the tube voltage) the electrons from the cathode are attracted to the anode, which accelerates their speed.
Effective focal spot size The projections of the focal spot on a surface perpendicular to the axis of the beam of X-rays is termed the “ effective focal spot size” or “ focus size”, see figure 2-4. The effective focus size is one of the parameters in radiography, see section 11-1. The effective focus size, principally determining the sharpness in the radiographic image, has to be as small as possible in order to achieve maximum sharpness. The dimensions of the focus are governed by: 4.
4.4 Radioactive sources (isotopes) 4.6 Properties of radioactive sources Natural radioactive sources Activity (source strength) The elements from this group which have been used for the purposes of industrial radiography are radium and mesothorium. These give a very hard radiation, making them particularly suitable for examining very thick objects. The activity of a radioactive substance is given by the number of atoms of the substance which disintegrate per second.
1a-5 Bipolar tube 5 NDT equipment 5.1 X-ray equipment X-ray sets are generally divided in three voltage categories, namely: 1. Up to 320 kV, mainly for use on intermittent, ambulatory work. Tubes are generally of the unipolar alternating current type. Higher voltages are hardly possible with this type of equipment because of insulation problems. 2. Up to 450 kV, mainly for use on continuous, stationary or semi-ambulatory work, because of their dimensions, limited manageability and weight.
Special types of X-ray tubes Unipolar X-ray tubes with a long hollow anode, as shown in fig. 1c-5, are generally known as “rod anode tube” and can be inserted into pipes or vessels. These tubes produce an annular (panoramic) beam over 360º, so allowing a complete circumferential weld to be radiographed in one exposure. Figure 2-5 shows the conical anode of a (360º) panoramic tube, which allows a circumferential weld to be radiographed centrally, hence uniformly, from within.
The intensity of radiation is increased by double-phased rectifying and varying degrees of smoothing. At very low voltage ripple these sets are considered constant potential (CP) equipment. 6 7 7 In the latest types of tank sets the mains frequency is first converted to a high frequency alternating current and only then transformed upward, which makes it easier still to smooth the ripple.
5.4 Radioactive sources Table 1-5 shows various radioactive sources for industrial NDT. The most commonly used ones are Cobalt, Iridium and increasingly Selenium. Selenium is very attractive while it permits lighter containers than Iridium. Due to its average energy level it often is a good alternative for an X-ray tube, also attractive while no electricity is needed.
5.5 Source holders (capsules) All gamma-ray sources for radiography are supplied in hermetically sealed, corrosion resistant source holders (capsules), made out of monel, vanadium or titanium. The Atomic Energy Authority in the country of origin encapsulates the radioactive material. The supplier will supply the source with a certificate which indicates the type of source, its serial number, the activity at a certain date, and a disintegration graph.
Figure 14-5 shows an S-channel container with a flexible (metal) hose and cable in rolled up (transport) position. handle tungsten container Figure 15-5 shows a more recent (2006) S-channel Selenium75 container with operation hoses and pigtail. Selenium75 radio-isotope is becoming popular since new production (enrichment) methods resulted in a much better k-factor. Thus for a certain activity (source strength) Fig. 14-5. S-channel container with the flexible cable and deployment mechanism.
6 film defect Radiation images, filters and intensifying screens To influence the effects of radiation on an image, filters and intensifying screens are used to : negative (shadow) image on the film primary radiation • • filter / harden the radiation to influence contrast and/or to intensify the effect of radiation to improve contrast 6.1 Radiation images object radiation intensity after passing through the object Fig. 1-6.
Scattered radiation also occurs in radiographic examination of cylindrical objects, as shown in figure 3-6. When a metal plate, usually lead or copper, is placed between the tube window and the object, radiation “hardening” occurs leading to a lower image contrast. This may be counter-balanced by a metal filter placed immediately behind the object (i.e. between object and film).
The lead foil of the front screen is usually 0.02 to 0.15 mm thick. The front screen acts not only as an intensifier of the primary radiation, but also as an absorbing filter of the softer scatter, which enters in part at an oblique angle, see figure 2-6. The thickness of the back screen is not critical and is usually approx. 0.25 mm. The surface of lead screens is polished to allow as close a contact as possible with the surface of the film.
To achieve satisfactory radiographs with fluorometallic screens, they should be used in combination with the appropriate F-film type. When used correctly and under favourable conditions, exposure time can be reduced by a factor 5 to 10, compared with D7 film in combination with lead screens. This is not a constant factor because the energy level applied (radiation hardness) and ambient temperature also affects the extent of fluorescence.
a.layer of hardened gelatine b. emulsion layer c. substratum (bonding layer) 7 The X-ray film and its properties 7.1 Structure of the X-ray film An X-ray film, total thickness approx. 0.5 mm, is made up of seven layers, see figure 1-7 consisting of : d. cellulose triacetate or polyester base c. substratum b. emulsion layer a.layer of hardened gelatine A transparent cellulose triacetate or polyester base (d).
7.4 Characteristic curve (density curve) Development is the process by which a latent image is converted into a visible image. This result is obtained by selective reduction into black metallic silver of the silver halide crystals in the emulsion. These crystals carry traces of metallic silver and in doing so form the latent image. Several chemical substances can reduce the exposed silver halides to metallic silver: these are called “developing agents”.
Gradient of the density curve The density curve shows one of the most important characteristics of a film. The slope of the characteristic curve at any given point is equal to the slope of the tangent line at this point. This slope (a/b in figure 3-7), is called the “film gradient” GD, “film contrast” or the “film gamma”. D D A steeper gradient means an increase in density difference at equal radiation dose and so a greater contrast, resulting in better defect discernibility.
Effect of developing conditions on the density curve 7.5 Film speed (sensitivity) The characteristic curve of an X-ray film is not only determined by the emulsion characteristics but also by the way the film is developed. Parameters which can influence the characteristic curve are: developing time and its temperature, developer concentration and agitation. The effect of, for example, the developing time on speed (relative exposure factor), contrast and fog, has been made visible in figure 7-7.
image quality - better 8 Industrial X-ray films are produced by a limited number of manufacturers in an assortment for use with or without intensifying screens and filters. The selection of a particular film type not only depends on economics but in particular on the required, often prescribed, image quality. D2 8.1 The Agfa assortment of film types D3 The films produced by Agfa are exclusively marketed world-wide by GE Inspection Technologies.
Part of the Agfa film range with relative exposure factors and code classification has been listed in table 1-8 for various radiation intensities : Film type density Figure 3-8 shows graphs of relative exposure time versus density for the entire Agfa D-film range. For density 2, the difference between a D8 and a D2 film is a factor 14 (10(3.25-2.1) ), at 200 kV. Fig. 3-8. Density graphs of the Agfa film range D2 through to D8 with lead intensifying screens at 200 kV and automatic film development.
8.2 Film type selection Most procedures and codes of good practice for the performance of industrial radiography base the choice of type of film for a specific application on the EN or ASTM classification systems. For weld inspection, when one is attempting to detect small cracks, a film of class C2 or C3 would be specified. For the examination of castings or general radiography a film of class C4 or C5 would normally be used.
9 Exposure chart 9.1 Exposure chart parameters Codes for the inspection of welds and castings specify the maximum allowed radiation intensity, based on the type of material and the thickness of the object. Exposure charts are necessary to etablish the correct exposure value. A universal exposure graph or slide-rule can be used for radioactive sources, as these have a fixed natural radiation spectrum. The radiation spectrum of X-ray tubes varies with each tube, even if they are of the same type.
Type of X-ray equipment 9.2 Densitometer Among the factors to be taken into account are: the voltage (in kV), whether alternating or direct current, the limits of voltage adjustment and the current through the tube (in mA). It follows that the exposure chart is unique for a particular X-ray set. The radioactive source Radiation intensity and half-life-time of the source have to be taken into account.
Preliminary charts Before producing an exposure chart it is useful to first draw up preliminary charts, the so-called “density-thickness chart” for the voltage range of the specific X-ray set and a “kV- thickness chart”. The two preliminary charts are produced on the basis of the following data: 1. 2. 3. 4. 5. 6. 7. 8. X-ray set: tube voltage 60-200 kV, tube current 5-10 mA Filter: none Source-to-film distance: 80 cm Material: steel Intensifying screens: none Type of film: D7 Density: 2.
9.4 The exposure chart The exposure chart should be drawn on uni-directional logarithmic paper. The material thickness (in mm) is plotted on the horizontal linear axis and the exposure value (in mA.min) on the vertical logarithmic axis. For a given kilovoltage (for example 150 kV), we can, using the previously described intermediate kV-thickness chart, determine that for an exposure dose of 8 mA.min a density of 2 can be obtained at a thickness of 4.5 mm and for an exposure dose of 200 mA.
9.6 Relative exposure factors “Relative exposure factors” can be used to convert an exposure chart for one type of film to another film, although still for the same radiation energy. These factors are not constant for different radiation energies and should, therefore, be used with some caution. Some examples of relative exposure factors for Agfa films are shown in table 1-9.
Method and answer Method and answer The exposure chart (fig.7-9) shows that under the conditions mentioned above density D = 2 is obtained on D7-film through the 15 mm thick section, using an exposure of 10 mA.min, point A on the chart. The characteristic curve (fig. 9-9) shows that at the measured densities of 1.5 and 0.5 respectively, the corresponding logarithm of relative exposures are 2.15 and 1.65. The characteristic curve (fig.
and 10 Processing storage of X-ray films Film developing is the process by which a latent image, see section 7.2, is converted into a visible image. The crystals in the emulsion - carriers of the silver traces forming the latent image - are transformed into metallic silver by selective reduction as a result of which the visible image is created. The development procedure must be carried out carefully to achieve this and guarantee successful archiving over a longer period.
Another method is to place an unexposed film on the workbench and cover part of it up with a sheet of cardboard, which is then gradually removed so as to produce a series of different exposures. By developing the film in the usual way, it will then be possible to see how “safe” the light is, and how long a film can be exposed to it before it exceeds the maximum acceptable difference in density of 0.1. Darkroom layout The darkroom should preferably be divided into a dry side and a wet side.
Film agitation Final wash To prevent air bubbles from forming on the surface of the emulsion (which will cause spots on the finished radiographs), and to make sure that the developer penetrates all areas of the emulsion evenly, the films should be kept moving during their first 30 seconds in the developer. After that, it is recommended to move the film from time to time to prevent film faults such as lines or streaks.
10.3 Recommendations for the darkroom 10.5 Automatic film processing Cleaning of tanks NDT-U (universal) film processor Whenever the processing solution is renewed the tank must be cleaned, preferably with hot water and soap. If this proves inadequate, polyester tanks can be cleaned using a bleach solution (100-200 ml/litre of water), hydrochloric acid (10 ml/litre of water) or acetic acid (50 ml/litre of water).
NDT-E (economy) film processor In order to limit any detrimental effects on the environment, Agfa has developed the “Eco” (Ecology and economy) designated processors. Here, too, equipment and chemicals are carefully matched, thus complying with strict ecological requirements such as a maximum of 50 mg silver per square metre of processed film, for the disposal of rinse water. This figure for silver content is at least fifteen times lower than for conventional developing systems.
Unexposed area for the Thio-Test fog + base density Fig. 2-10. PMC-strip with an unexposed area for the Thio-Test Reference step for film sensitivity Reference step for film contrast A PMC-strip as shown in figure 2-10 has to be used whenever the chemicals in an automatic or manual processing system are replenished or changed. It is also advisable to use a PMC-strip regularly, but at least once a month, for a routine check of the development system.
discernibility 11 Defect and image quality anode Three factors govern the discernibility of defects in a radiograph: source 1. Geometrical effects: • Size of the source • Source-to-object distance • Defect-to-film distance 2. Film properties (governing image quality): • • • • (ffd) Graininess Contrast Fog Inherent unsharpness 3. Quality of radiation applied. 11.
Consequently, Ug can be reduced to any required value by increasing the source-to-film distance. However, in view of the inverse square law this distance cannot be increased without limitation, as extremely long exposure-times would result. The formula also indicates that geometric unsharpness assumes more and more importance as the distance between defect and film increases. In this situation the unsharp images of each of the two edges of the defect may overlap, as shown in example C.
Table 1-11 and figure 4-11 show experimentally determined values of inherent unsharpness for film exposed at various radiation energy levels. These values are based on the use of filters and thin lead intensifying screens; thicker screens produce slightly higher values. If no lead screens are used, Uf is 1.5 to 2 times smaller. Uf is influenced mainly by radiation intensity and the type of intensifying screens used; the type of film is hardly of any consequence.
11.3 Other considerations with regard to the source-to-film distance Inverse square law As explained in the previous section, the effect of Ug can be reduced by increasing the focus-tofilm distance F. One of the properties of electromagnetic radiation is that its intensity is inversely proportional to the square of the distance, better known as the “inverse square law”. Both X - and Gamma radiation follows that law.
11.4 Radiation hardness and film contrast 11.5 Summary of factors that influence image quality When radiation hardness increases, the half-value thickness (HVT) also increases. Tables 2-2 and 3-2 for steel and lead respectively show this in figures. The factors that influence image quality are: This is why in an object with different thicknesses, image contrast diminishes when radiation hardness increases. Figure 7-11 clearly illustrates this.
orientation, image distortion 12 Defect and useful film length 12.1 Defect detectability and image distortion source source On a radiograph, a three-dimensional object is presented in a two-dimensional plane (the film). The appearance of both the object and its defects depends on the orientation of radiation relative to the object. As shown in figure 1-12, the image of a gas cavity in a casting may be circular or elongated depending on beam orientation.
The number of radiographs necessary for 100 % examination of a circumferential weld can, through calculation, also be obtained from the codes. When large numbers of similar welds are involved, this is an important figure, because too many radiographs would be uneconomical and too few would lead to insufficient quality of the examination. The minimum number of radiographs required for various pipe diameters and wall thicknesses at varying source positions can be derived from the graph in figure 4-12.
13 Image quality 13.1 Factors influencing image quality With regard to image quality, the term frequently used is “sensitivity”. Sensitivity determines the extent to which a radiograph is able to clearly show (anomaly) details of a certain size. Sensitivity in this sense must not be confused with the sensitivity or “speed” of the film. (see section 7.5).
The last factor, graininess, depends on : • • • • • the thickness of the emulsion layer the concentration of silver crystals in the emulsion (silver/gelatine ratio) the size of the silver crystals the radiation energy used the developing process employed The radiation energy level is the only factor that can be influenced by the radiographer; the other factors are determined by the film making process. 13.
Table 1-13 shows the wire combinations for the four IQI’s according to EN 462-01. The diameters of the wires are given in table 2-13. IQI 1 EN 6 EN 10 EN 13 EN Wire numbers 1 to 7 inclusive 6 to 12 inclusive 10 to 16 inclusive 13 to 19 inclusive Wire diameter from/to (mm) 3.2 to 0.80 inclusive 1 to 0.25 inclusive 0.40 to 0.10 inclusive 0.2 to 0.05 inclusive Table 1-13. Wire IQIs according to EN 462-01.
AFNOR IQI’s The AFNOR-type IQI’s originate in France. They consist of metal step wedges of the same material as the object to be examined. The thickness of the steps increases in arithmetical progression. Each step has one or more holes with a diameter equal to the thickness of that step. There are various models of step wedges. The most common types Fig. 4-13. French AFNOR IQIs are rectangular with square steps measuring 15x15 mm and hexagonal with triangular steps measuring 14mm. See figure 4-13.
exposure 14 Film and handling errors Before a particular difference in density in a radiograph is attributed to a defect in the object examined, it must be sure that it is not the result of incorrect handling- or processing of the film. It is, therefore, essential to be able to recognise such faults when examining the film in order to prevent their recurrence.
Grey fog (local or overall) 1. 2. 3. 4. 5. 6. 7. 8. 9. unsuitable dark room safelighting excessive exposure to safelight (i.e. too long or too close) film accidentally exposed to X-ray or Gamma-ray or to white light heavy scatter film out-of-date or stored under unsuitable conditions (ground fog) extreme under-exposure compensated by excessive developing exhausted or wrongly mixed developer film cassette with film exposed to heat (e.g. sunlight, heat from radiators etc.
interpretation 15 Film and reference radiographs 15.1 Film interpretation The common term for film interpretation is film viewing. Film viewing in fact means the evaluation of the image quality of a radiograph for compliance with the code requirements and the interpretation of details of any possible defect visible on the film. For this purpose, the film is placed in front of an illuminated screen of appropriate brightness/luminance.
15.2 The film-interpreter Apart from the requirements regarding “viewing conditions” and “viewing equipment” the film-interpreter (film viewer) shall have thorough knowledge of the manufacturing process of the object being examined and of any defects it may contain. The type of defects that may occur in castings, obviously, differs from those in welded constructions. Different welding processes have their own characteristic defects which the film interpreter must know to be able to interpret the radiograph.
External concavity or insufficient fill. The weld density is darker than the density of the pieces welded and extending across the full width of the weld. Excessive penetration. A lighter density in the centre of the width of the weld image, either extended along the weld or in isolated circular drops. 126 External undercut. An irregular darker density along the edge of the weld image. The density will always be darker than the density of the pieces being welded. Internal (root) undercut.
Internal concavity (suck back). An elongated irregular darker density with fuzzy edges, in the centre of the width of the weld image. Burn through. Localized darker density with fuzzy edges in the centre of the width of the weld image. It may be wider than the width of the root pass image 128 Incomplete - or Lack of Penetration (LoP) A darker density band, with very straight parallel edges, in the center of the width of the weld image. Interpass slag inclusions.
Elongated slag lines (wagon tracks). Elongated parallel or single darker density lines, irregular in width and slightly winding lengthwise. Lack of side wall fusion (LOF). Elongated parallel, or single, darker density lines sometimes with darker density spots dispersed along the LOF-lines which are very straight in the lengthwise direction and not winding like elongated slag lines 130 Interpass cold lap Small spots of darker densities, some with slightly elongated tails in the welding direction.
Cluster porosity. Rounded or slightly elongated darker density spots in clusters with the clusters randomly spaced. Root pass aligned porosity. Rounded and elongated darker density spots that may be connected, in a straight line in the centre of the width of the weld image. 132 Transverse crack Feathery, twisting lines of darker density running across the width of the weld image.
Casting radiography For the interpretation of X-ray films of castings, thorough knowledge of the specific manufacturing process is required. The type of defects in castings varies for the different types of materials and casting processes. Figures 15-1 and 15-2 show X-rays of complex castings. These radiographs were made to check the overall shape and possible presence of casting defects.
Fig. 15-3. Shrinkage (worm-hole cavities) in a (high heat conductive) copper casting Fig. 15-2. Radiograph of an aluminium precision casting. Exposure on D2 film at 75 kV/5 mA/3.
Fig. 15-4. Filamentary shrinkage in an aluminium alloy casting Fig. 15-6. Shrinkage cavities in a bronze casting Fig. 15-7. Gas-holes and porosity in an aluminium alloy casting Fig. 15-5.
Fig. 15-8. Hot cracks (hot tears) Fig. 15-9. Stress cracks (cold tears) Fig. 15-10.
Examination of assembled objects In addition to radiography for detection of defects in welds and castings, it can also be applied to check for proper assembly of finished objects as figures 15-12 and 15-13 illustrate Fig. 15-12. Radiograph of transistors Exposure on D2 film with 27 μm lead screens at 100 kV/5 mA/2 min film-focus distance 70 cm. Fig.15-11. Radiograph of 25mm thick aluminium-copper alloy casting with gas porosity Exposure on D7 film at 140 kV/5 mA, film-focus distance 100 cm Fig. 15-13.
16 Digital Radiography (DR) 16.1 Introduction to DR As in other NDT methods, the introduction of microprocessors and computers has brought about significant changes to radiographic examination. Chapter 17 describes a number of systems such as Computed Tomography (CT), radioscopy and X-ray microscopy that have been made possible by newly developed technology which involves rapid digital processing of vast quantities of data.
The major parameters to compare film to digital radiography are spatial resolution, contrast sensitivity and optical density range.
Because scanners vary widely in resolution, dynamic range, and ability to scan dense films, evaluation is required to ensure that adequate scanning fidelity is achieved. Depending on selected resolution many Megabytes are needed to store a single film, see paragraph 16.12. Archiving of a digitised film, identical to CR- and DR images, is usually done on an optical mass storage facility e.g.: CD-ROM, DVD etc. For uniform application of film digitisation norm EN 14096 has been issued. 16.
Bare CR plates are nearly as pliable as film. They can be packed in paper or vinyl cassettes either with or without lead screens. These packages are still pliable. Technically the plates can be used many times (up to 1000 x), provided they are handled with utmost care while their surface despite a protective coating is very sensitive to touching and dirt. A single scratch can make the plate unsuitable for further use.
Exposure time and noise 16.5 Genuine Digital Radiography (DR) In addition to the wide dynamic range the dose sensitivity (speed) of CR plates is five to ten times higher, compare point A and B in figure 8-16 at a density of 2 (see also figure 27-16). This allows for shorter exposure times or weaker sources, reducing the unsafe radiation area. Unfortunately, if a source with lower energy is chosen this will result in reduction of the image quality.
Linear detectors 2D detectors Linear detector arrays (LDAs) based on CMOS technology, as shown in figure 12-16, are commonly used in applications where a mechanical means provides a relative motion between the object being inspected and the X-ray beam. LDAs can be made in virtually any length. In practice active lengths are available up to over 1 metre and energy ranges from 100 kV to several MeV.
16.5.3 Flat panel and flat bed detector systems There are different types, sizes and suppliers of true 2D flat panel detectors. A variety of flat panel systems exists with a wide range of pixel sizes and resolutions. More and smaller pixels and a high Fill Factor increase the resolution of a panel. As an indirect sensor material amorphous silicon is in wide use. As direct sensors CCD’s (Charge Coupled Devices) and CMOS (Complementary Metal Oxide Semiconductors) are also applied.
To determine the quality of a digital image, existing codes require two different IQIs in analogy to radioscopy. One wire- or plaque IQI for contrast, and one duplex wire IQI for the determination of spatial resolution (unsharpness). Peak Signal amplitude Linear scale • Some flat panel DR detectors are also subject to some memory effect, in jargon called “ghosting”. This is due to hysteresis of the scintillation layer after exposure.
16.6.3 Indicators of image quality - MTF and DQE Factors influencing image quality In the process of making a radiograph three factors influence the ultimate image quality: 1. exposure conditions 2. detector performance/efficiency 3. performance of the processing equipment to form an image To enable quantification of the quality of digital radiographs and the hardware used to create them, two notions are in use: MTF and DQE.
DQE (Defective Quantum Efficiency) BEST Remark: MTF and DQE are used to characterise detectors and systems. Some users may find these scientific notions rather abstract and hard to understand. While very useful in selecting a detector for a particular application, in practice they do not replace the duplex IQI as final indicator of image quality for CR- and DR applications.
The graph also shows that the speed is much higher to achieve the same image quality of D-type films. Depending on the required image quality a time saving of at least a factor 20 (D against E) and roughly 200 (F against E) can be achieved, however with poorer quality. The range for true real-time (real instant) images shows that exposures can be made with extremely low dose but at cost of image quality. Lateral resolution Lateral resolution is determined by pixel size.
Status of CR standards For CR, standard EN 14784 has been issued with EN 444, EN 584-1 and EN 462-5 in mind to achieve conformity with film radiography. Part 1 of EN 14784 describes classification of systems and part 2 describes principles and applications (not including welds). Although a working group for the compilation of an EN standard for welds exists, the issue of such a standard would still take several years.
Although the electronics needed for both methods, e.g. workstation, cost approximately the same (and partly can be shared!), a flat panel detector (~ € 150,000 ) is roughly 200 times more expensive than a phosphor plate (~ € 750 ). Hence selection of a DR solution requires careful considerations with regard to return of investment (pay back period). Another aspect of paramount importance, which influences selection between CR and DR, is the availability (or lack) of industrial standards.
Automated/mechanised inspection Useful life of plate and panel The choice of DR flat panel detectors depends on the image quality required and the number of parts to be inspected to make it cost effective (return on investment). CR plates (by handling), like DR detectors (by radiation) have a finite useful life which has to be included in an economic evaluation. The working life of flat panel devices can range up to millions of images, dependent on application-specific details, see paragraph 16.5.3.
In addition algorithms have been developed for e.g. the comparison of parts of an image with conformance criteria, carrying out dimensional checks (sizing), for instance to measure remaining wall thickness (see figure 37-16). Versatility of the software Images can be adjusted and enhanced in many ways: brightness, contrast, sharpness, noise suppression (averaging), rotation, filtering, inversion, colouring, magnification, zoom-pan-scroll, etc.
Wall Thickness Result Nr. 1 2 Material Steel DA WDSoll WDIst 3,5 3,5 3,5 1,3 Figure 39-16 shows a detail of the selected pipe wall area with the reported results. This example of a valve with a great variety of wall thicknesses also shows one of the strengths of a digital exposure. If needed the same image can be used to study the thin and thick wall parts of the valve thanks to the large dynamic range contained in the image.
17 Special radiographic techniques The previous chapter (16) dealt with techniques that would be impossible without the aid of computers. These techniques share a common feature, whereby the processing, interpretation and storage of data is done by a central computer and monitor, also called the work station. In the current chapter (17) computers also play an everincreasing important role in some of the techniques discussed.
17.1.2 High resolution X-ray microscopy Magnification factors For a number of years magnification factors up to 25 were sufficient. The maximum magnification factor was determined by the smallest possible focal spot size. As illustrated in figure 1-17 larger magnification factors create unsharp images without providing more information. Moreover the intensity of the output is limited by the heat dissipation of the target anode. For some time this was a physical barrier.
System set-up Figure 3-17 shows the concept of a two-dimensional (2D) X-ray microscopy system to inspect small components consisting of a micro- or nanofocus X-ray tube, an X-Y-Z manipulator and detector. The manipulator can be joystick- or CNC-controlled. Full automation is possible. The geometric magnification can be controlled by the Z-axis. Closer to the tube results in a larger magnification factor.
Imaging systems for high resolution radiography Stationary real-time installations High-resolution X-ray inspection systems usually apply an image intensifier for presentation of results as shown in figure 7-17. This electro-optical device amplifies and converts the invisible X-ray shadow to visible light by means of a scintillation crystal and photo cathode.
• CCD-camera as a substitute for the relatively slow conversion screen • Photo array detector, minimal size per diode (pixel) approx. 100 microns to inspect slowly moving objects (airport luggage checks) • Flat panel detector consisting of millions of light-sensitive pixels. Although the image intensifier is still most commonly used, the flat panel detector is becoming more and more attractive.
Each individual detector element measures, during a short exposure period, the total absorption across a certain angular position of the object. This information including the coordinates is used to create a numerical reconstruction of the volumetric data. This process produces a huge data stream to be stored and simultaneously processed, in particular when an image of high resolution is required. A three dimensional representation (3D CT) of the radiographic image requires vast computing capacity.
In practice the following rule of thumb is applied to the detection of planar defectswith a high probability: DEFECT CONTRAST GOOD Open planar defects “a defect is detectable if the angle between the X-ray beam and the defect is approximately 10° or less”. The value of 10° is based on decades of practical experience but does not guarantee detection. The rule is visualised POOR in the graph of figure 15-17.
Such 3D CT systems are primarily intended for defect analysis. Scanning of a girth weld typically takes about one hour. Inspection and reconstruction of one cross section takes less than 10 minutes. An example of such a system is the so-called TomoCAR***, see “acknowledgments” at the end of this book. This system uses a CMOS line array and is capable to inspect (analyse) pipe diameters of up to 500 mm with a total irradiated thickness of up to 50 mm (2 x 25 mm).
18 Special radiographic applications There are many special applications of radiography in NDT. This chapter describes a limited number of different examples to illustrate this diversity. Apart from the use of radiation in image forming radiography, it is also used in, for instance, measuring instruments such as metal alloy analysing instruments (Positive Material Identification, PMI) and humidity detection in insulation of thermally insulated pipping.
18.2 Radiographs of objects of varying wall thickness 18.3 Radiography of welds in small diameter pipes For radiographs of an object with limited differences in wall thickness, it is common to base exposure time on the average thickness to obtain the required film density of at least 2. It is possible that parts of the film are either under- or over-exposed if there are great differences in wall thickness.
When using the elliptical exposure technique, the images of the weld on the source side and on the film side are shown separately, next to each other. The distance between two weld images has to be approximately one weld width. This requires a certain amount of source offset (O), relative to the perpendicular through the weld. The offset can be calculated with the following formula : O= 1.2 . w .
The actual pipe wall thickness (t) is equal to the image on film (tf ) multiplied by the correction factor (see fig. 5-18). Determination the depth position and diameter of reinforcement steel in concrete Similar to the method for determination of the depth position of a defect in metals is the determination of the depth position (cover) of reinforcement steel in concrete. Subsequently, the true diameter of the reinforcing bar (D) can be calculated. Correction factor = d / (H-d).
Exposure time Obviously different exposure times are required for gas filled or liquid filled pipelines. Below are a few examples. Notes: • In the most commonly used insulation materials absorption is negligible. • The long exposure times cause over-irradiation at the edge of the pipe. As a result the pipe wall shows up ‘thinner’.
18.8 Radiography of welds in large diameter pipes Positioning device Receiver Wrapped film To create an on- or offshore pipeline individual pipes (length usually 12 m) or pipe sections (double or multiple joints) are welded together by a circumferential weld, a so-called girth weld. Onshore production rates can be far beyond 100 welds per day dependent on pipe diameter and terrain conditions.
Control electronics Battery pack Isotope container Fig. 13-18 Small diameter gamma-ray crawler Positioning devices using different physical methods are used to stop the crawler, and thus the source of radiation, at the correct position (at the weld plane). These positioning devices have to transmit or receive a position signal through the (often thick) pipe wall.
hazards, measuring19 Radiation and recording instruments 19.1 The effects of radiation on the human body The human body is constantly exposed to natural radiation (e.g. from space, the soil and buildings), also known as background radiation. All ionising radiation, whether electromagnetic (gamma-γ) or corpuscular (particles in the form of alpha-α or beta-β), and neutrons, are harmful to the human body. The unit “absorbed dose” (D) defines the effect of radiation on various substances.
19.3 The effects of exposure to radiation The understanding of the effect that exposure to radiation has on human beings has grown over the past 50 years and has led to a substantial reduction of the maximum permissible dose. There are two categories of biological effects that an overdose of radiation can cause: somatic and genetic effects. Somatic effects are the physical effects. A reduction in the number of white blood cells is an example of a somatic effect.
19.6 Radiation measurement and recording instruments Personal protection equipment From what has been said before, it follows that establishing the presence of ionising radiation and measuring its level is of paramount importance. Since ionising radiation cannot be detected by the senses, detectors and measuring equipment are used. There are various instruments with which the radiographer can measure or register radiation. Pendosismeter (PDM) The most common measuring instruments are: 1.
Film dose meter (film badge) Distance The film badge consists of two pieces of X-ray film contained, with filters, in a special holder. At the end of a specified period, the films are developed and the density measured. The radiation dose received by the wearer can then be determined by consulting the density/exposure curves, and the type of radiation received can be established by checking the densities behind the filters. Film dose Fig. 4-19.
literature/references, 20 Standards, acknowledgements and appendices European norms (EN-standards) Ever since the introduction of industrial radiography, there has been a growing need for standardisation of examination techniques and procedures. At first, these standards had mainly a national character, e.g. ASTM and ASME, DIN, AFNOR, BS, JIS etc, but as a result of industrial globalisation the need for international standards grew.
Literature and references Appendices: tables and graphs 1. Industrial Radiology: Theory and Practice (English) R. Halmshaw. Applied Science Publishers Ltd. London and New Jersey, 1982. 2. Niet-destructief onderzoek. ISBN 90-407-1147-X (Dutch) W.J.P. Vink. Delftse Universitaire Pers. 3. Die Röntgenprüfung, Band 7 ISBN 3-934225-07-8 (German) The X-ray Inspection Volume 7 ISBN 3-934255-22-1 (English translation) Both compiled by Dr.Ing. M. Purschke. Castell-Verlag GmbH 4.
Fig.5-11. Nomogram for minimum source-to-film distance fmin according to EN 1435-criteria. Catagory A - less critical applications (general techniques) Catagory B - techniques with high requirements of detail discernability distance (b) distance fmin for catagory A distance fmin for catagory B f min = minimum distance source to source-side object (mm) s = source size (mm) b = distance source-side object to film (mm) Operating range not being used. Fig. 4-12.
wall thickness Relative image quality DQE Faster F True real-time Radioscopy DR-Panels CR-Plates RCF-Films C Better D7 (coarse grain) A D D-Films B E D2 (fine grain) diameter in mm Relative dose Fig. 13-16. Relative image quality and speed of the various radiographic systems. See chapter 16 220 Fig. 7-18. Areas of application for selection of source, screen and filter in on-stream radiography. See chapter 18.
