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Industrial RadiographyImage forming techniquesIssued by GE Inspection Technologies©2006 General Electric Company. All Rights Reserved. We reserve the

1716• By means of radiation detectors, e.g.: crystals, photodiodes or semiconductors in alinear array by which in a series of measurements an image is

196 197When using the elliptical exposure technique, the images of the weld on the source sideand on the film side are shown separately, next to each

18.5 Determination the depth position and diameter of reinforcement steel in concrete Similar to the method for determination of the depth position o

201200Exposure timeObviously different exposure times are required for gas filled or liquid filled pipelines.Below are a few examples.For gas filled p

203This positioning device is used to stop the crawler at the correct X-ray tube position (withina few millimetres) with regard to the weld plane in o

205Crawler- and control technologies for on-and offshore application are almost thesame. The choice of film can be different.For onshore application t

19Radiation hazards, measuring- and recording instruments 19.1 The effects of radiation on the human bodyThe human body is constantly exposed to natu

209208As there is considered to be no totally safe lower limit below which no damage would besustained, the “ALARA” concept is being promoted. ALARA (

211210Personal protection equipmentPendosismeter (PDM)The PDM consists of a quartz fibre electrometer and a simple optic lens system housed in afounta

213212DistanceSince radiation is subjected to the inverse square law, its intensity is reduced with the incre-ase in distance to the square.Absorbing

21521420Standards, literature/references, acknowledgements and appendices European norms (EN-standards)Ever since the introduction of industrial radio

2Basic properties of ionising radiation19In 1895 the physicist Wilhelm Conrad Röntgen discovered a new kind of radiation, whichhe called X-rays. The

217216Literature and references 1. Industrial Radiology: Theory and Practice (English)R. Halmshaw. Applied Science Publishers Ltd. London and New

219218Fig. 5-11. Nomogram for minimum source-to-film distance Fmin according to EN 1435 criteria. See chapter 11Fig. 4-12. Graph for the minimum n

221220diameter in mmwall thicknessFig. 13-16. Relative image quality and speed of the various radiographic systems. See chapter 16Fig. 7-18. Areas o

2.2 X-raysThe radiation which is emitted by an X-raytube is heterogeneous, that is, it contains X-rays of a number of wavelengths, in theform of a con

2.4 Main properties of X-rays and γ-raysX-rays and γ-rays have the following properties in common:1. invisibility; they cannot be perceived by the sen

2.7 Penetrating power The penetrating power of X-radiation increases with the energy (hardness). The relationship of energy and penetrating power is c

Table 2-2 shows the average HVT-values for steel, table 3-2 shows the values for lead.For a heterogeneous beam the HVT is not constant; the second HVT

3.1 UnitsUntil 1978 the “International Commission of Radiation Units and Measurements” (ICRU)used the conventional radiation units of roentgen (R), r

31303.2 DefinitionsRadioactivityThe activity of a radioactive source of radiation (isotope) is equal to the number of disinte-grations per second. Th

4Radiation sources334.1 X-Ray tubeThe X-ray tube, see figure 1-4, consists of a glass (or ceramic) envelope containing a positiveelectrode (the anode)

4.3 Tube voltage and tube currentThe voltage across the X-ray tube determines the energy spectrum and so the hardness of theradiation, see figure 3-4.

Industrial RadiographyImage forming techniques1Digital radiography CR-image of a weldsee

4.6 Properties of radioactive sourcesActivity (source strength)The activity of a radioactive substance is given by the number of atoms of the substanc

5NDT equipment395.1 X-ray equipmentX-ray sets are generally divided in three voltage categories, namely:1. Up to 320 kV, mainly for use on intermitten

415.2 High voltage generatorsConventional (trans)portable X-ray equipment for use up to approximately 300 kV are pro-vided with step-up HT transforme

43The linear accelerator (linac)The energy levels mostly used for linacs (linear accelerators) are 4 MeV and 8 MeV. Linearaccelerators can be construc

5.4 Radioactive sources Table 1-5 shows various radioactive sources for industrial NDT. The most commonly usedones are Cobalt, Iridium and increasingl

Also greatly depleted uranium (with the highest radiation absorption) is used for shielding,resulting in very compact exposure containers. A disadvant

Figure 14-5 shows an S-chan-nel container with a flexible(metal) hose and cable in rol-led up (transport) position.Figure 15-5 shows a morerecent (200

51506Radiation images, filters and intensifying screensTo influence the effects of radiation on an image, filters and intensifying screens are used to

536.2 Radiation filtersWhen a metal plate, usually lead or copper, is placed between the tube window and theobject, radiation “hardening” occurs lead

55Steel and copper screensFor high-energy radiation, lead is not the best material for intensifying screens. WithCobalt60 gamma-rays, copper or steel

The first issue of “Industrial Radiography” was published by Agfa in the sixties, for educational and promotional purposes. Some improved editions ha

On balance for on-stream inspection, the relative time saving is much smaller; usually nomore than a factor 2 for an F6-film (at Ir192 and Co60) inste

59587The X-ray film and its properties7.1 Structure of the X-ray filmAn X-ray film, total thickness approx. 0.5 mm, is made up of seven layers, see fi

617.4 Characteristic curve (density curve)The characteristic or density curve indicates the relationship between increasing exposuresand resulting den

A steeper gradient means an increase indensity difference at equal radiation doseand so a greater contrast, resulting in betterdefect discernibility.

657.5 Film speed (sensitivity)In radiography the relationship between exposure (in C/kg) and resulting density is com-monly referred to as film speed.

8Film types and storage of films67Industrial X-ray films are produced by a limited number of manufacturers in an assortmentfor use with or without int

Part of the Agfa film range with relative exposure factors and code classification has beenlisted in table 1-8 for various radiation intensities :Note

71708.2 Film type selectionMost procedures and codes of good practice for the performance of industrial radiographybase the choice of type of film fo

9Exposure chart739.1 Exposure chart parametersCodes for the inspection of welds and castings specify the maximum allowed radiationintensity, based on

759.2 DensitometerA densitometer is used to accurately measure the photographic (optical) density at any spoton a radiographic film. For most types of

Introduction to the overview of “Industrial Radiography”Image forming techniquesPreface 131. Introduction to industrial radiography 15 2. Basi

77The “density-thickness (preliminary) charts” as described, provide the data needed to prepare the final exposure chart. In order to eliminate any in

79Therefore, an exposure chart for each individual X-ray set should be drawn up. This is anexcellent way to become familiar with the equipment, while

9.6 Relative exposure factors“Relative exposure factors” can be used to convert an exposure chart for one type of filmto another film, although still

83Method and answerThe characteristic curve (fig. 9-9) shows that at the measured densities of 1.5 and 0.5respectively, the corresponding logarithm of

10Processing and storage of X-ray films85Film developing is the process by which a latent image, see section 7.2, is converted intoa visible image. Th

87DeveloperDevelopment fog, graininess and contrast are dependent on the type of developer,which is preferably made up to suit the film used.If a conc

89Final washThe final wash is intended to remove the residual fixer and the soluble silver com-pounds left behind in the emulsion, which if not flushe

9110.5 Automatic film processingNDT-U (universal) film processorOver the last few years there has been a vast increase in the use of automatic process

9310.6 Checking the development process and film archiving propertiesBesides exposure technique, many aspects influence the quality of the final radi

The unexposed area on the PMC-strip shown in figure 2-10, apart from providing areference for fog and base density, also allows for the Thio-Test to b

7StopbathFixingFinal washDrying in the drying cabinetRoller dryers10.3 Recommendations for the darkroom 9010.4 Silver recovery 9010.5 Automatic film p

Three factors govern the discernibility of defects in a radiograph:1. Geometrical effects:• Size of the source• Source-to-object distance• Defect-to-f

99In this situation the unsharp images of each of the two edges of the defect may overlap,as shown in example C. The result is that image C not only

11.2 Selection of source-to-film distance Preceding paragraphs of this chapter described the effects of geometric unsharpness andthe possibility to in

Examples :15 mm steel: 100 + 15 x 8 = 220 kV12 mm aluminium: 50 + 12 x 2 = 74 kV10 mm plastics: 20 + 10 x 0.2 = 22 kVIn the range 200-400 kV, only a

10511.5 Summary of factors that influence image qualityThe factors that influence image quality are:1. Contrast2. Unsharpness3. Graininess1 Contrast d

12Defect orientation, image distortion and useful film length10712.1 Defect detectability and image distortion On a radiograph, a three-dimensional ob

109The number of radiographs necessary for 100 % examination of a circumferential weldcan, through calculation, also be obtained from the codes. When

11113.1 Factors influencing image qualityWith regard to image quality, the term frequently used is “sensitivity”. Sensitivity determines the extent to

113The image quality of a radiograph is, for example, defined as the number of the thinnestwire still visible, and is generally said to have “image q

115ASTM 1025 IQI’sThe plaques have markingsshowing their thickness inthousandths of an inch. Ea chplaque has three holes of dia-meters 1T, 2T and 4T.

916.7 Resolution number of bits 163Bit depthLateral resolution16.8 Comparison of film, CR- and DR methods 16416.9 Impact and status of CR- and DR sta

117116AFNOR IQI’sThe AFNOR-type IQI’s originate inFrance. They consist of metal stepwedges of the same material as theobject to be examined. The thic

14Film exposure and handling errors119Before a particular difference in density in a radiograph is attributed to a defect in theobject examined, it mu

1217. screen(s) in poor condition8. foreign bodies (for example metal particles ) between film and screen during exposure9. small, clear, hollow spots

15Film interpretation and reference radiographs12315.1 Film interpretationThe common term for film interpretation is film viewing. Film viewing in fac

12512415.2 The film-interpreterApart from the requirements regarding “viewing conditions” and “viewing equipment”the film-interpreter (film viewer) sh

127126External concavity or insufficient fill.The weld density is darker than the density of the pieces weldedand extending across the full width of t

129128Internal concavity (suck back).An elongated irregular darker density with fuzzy edges, in thecentre of the width of the weld image.Burn through.

131Elongated slag lines (wagon tracks).Elongated parallel or single darker density lines, irregular inwidth and slightly winding lengthwise.Lack of si

133132Cluster porosity.Rounded or slightly elongated darker density spots in clusterswith the clusters randomly spaced.Root pass aligned porosity.Roun

135Casting radiographyFor the interpretation of X-ray films of castings, thorough knowledge of the specific manu-facturing process is required. The ty

1120.Standards, literature / references, acknowledgements and appendices 215European norms (EN-standards) Literature and references Acknowledgements A

137Fig. 15-3. Shrinkage (worm-hole cavities) in a (high heat conductive) copper casting136Fig. 15-2. Radiograph of an aluminium precision casting. E

139Fig. 15-6. Shrinkage cavities in a bronze castingFig. 15-7. Gas-holes and porosity in an aluminium alloy casting138Fig. 15-5. Micro shrinkage (l

141Fig. 15-10. Radiograph of an aluminium casting with coarse porosity Exposure on D7 film at 60 kV/5 mA/15 sec, film-focus distance 100 cm140Fig. 15

143Fig. 15-12. Radiograph of transistorsExposure on D2 film with 27 μm lead screens at 100 kV/5 mA/2 min film-focus distance 70 cm.Fig. 15-13. Rad

16Digital Radiography (DR)14516.1 Introduction to DRAs in other NDT methods, the introduction of microprocessors and computers hasbrought about signif

147Digitisation of these films provides an excellent alternative that also preventsdegrading. Special equipment has been developed for this purpose. C

Because scanners vary widely in resolution, dynamic range, and ability to scan densefilms, evaluation is required to ensure that adequate scanning fid

151150For desktop scanners the cassette can be opened, asshown in figure 6-16. CR plates can be exposed tosubdued light (< 10 lux: a candle creates

16.5 Genuine Digital Radiography (DR)One-step digital radiographyDigital radiography, DR for short, is also known as “direct” radiography to indicate

155154Linear detectorsLinear detector arrays (LDAs) based on CMOS tech-nology, as shown in figure 12-16, are commonlyused in applications where a mech

Preface13To verify the quality of a product, samples are taken for examination or a non-destructivetest (NDT) is carried out. In particular with fabr

157CMOS detectors and flat bed scannersFor some applications CMOS detectors are an alternitive for temperature controlledamorphous materials. CMOS has

15916.6.2 Determination of image qualityTo determine the quality of a digital image, existing codes require two different IQIs in analogy to radioscop

161Exposure parametersThe quality of a digital image is affected by a number of factors. The final image cannot bebetter than the quality of the X-ray

163162Remark: MTF and DQE are used to characterise detectors and systems. Some users mayfind these scientific notions rather abstract and hard to unde

165164Lateral resolutionLateral resolution is determined by pixel size. Firstof all the pixel size of the detector and secondly thepixel size of the d

167166Status of CR standardsFor CR, standard EN 14784 has been issued with EN 444, EN 584-1 and EN 462-5 in mindto achieve conformity with film radiog

169168Although the electronics needed for both methods, e.g. workstation, cost approximately thesame (and partly can be shared!), a flat panel detecto

171Useful life of plate and panelCR plates (by handling), like DR detectors (by radiation) have a finite useful life which hasto be included in an eco

173In addition algorithms have been developed for e.g. the comparison of parts of an image withconformance criteria, carrying out dimensional checks (

175174Figure 39-16 shows a detail of theselected pipe wall area with thereported results. This example of a valve with agreat variety of wall thicknes

15141Introduction to industrial radiographyImage forming techniquesIn industrial radiography, the usual procedure for producing a radiograph is to hav

177176The previous chapter (16) dealt with techniques that would be impossible without the aidof computers. These techniques share a common feature, w

179Two types of X-ray tubes exist:• The closed X-ray tube, a sealed evacuated glass tube containing all components to generate X-rays. No part in it

181System set-upFigure 3-17 shows the concept of a two-dimensional (2D) X-ray microscopy systemto inspectsmall components consisting of a micro- or na

Stationary real-time installationsDisplay monitor systems, as illustrated in figure 9-17, are almost exclusively used instationary set-ups for product

185This way an image is obtained of the pipe “horizon” with possible presence ofcorrosion (swelling or pitting). The image is presented real-time on a

187If too close to the X-ray source the geometry of the object can hamper full rotation as illustratedin this figure. The subject of CT is more elabor

189188In practice the following rule of thumb isapplied to the detection of planar defects-with a high probability: “a defect is detectable if the ang

191The scanner comprises an X-ray tube and a detector consisting of a number of elements asillustrated in figure 13-17. A collimator reduces the beam

193192X-ray image of an X-ray crawler in pipe at weld location18Special radiographic applicationsThere are many special applications of radiography in

19519418.2 Radiographs of objects of varying wall thicknessFor radiographs of an object with limited differences in wall thickness, it is common to ba