Quick industrial X-ray testing without intermediate data carriers of information
2017-05-18 13:11
Guest contribution

Quick industrial X-ray testing without intermediate data carriers of information

Prof. V.A. TROITSKIY – The E.O.Paton Electric Welding Institute of the National Academy of Science of Ukraine

Flash-radiography (FR) is the radiography without intermediate data carriers (films and storage plates). FR produces a quick image. It provides for low cost of testing, capability of multi-angle real time monitoring of internal defects of the objects.

In film radiography if relative photometric density is more than 4, then the snapshots become virtually unreadable and they can be difficult to be digitized. Current film-free technologies do not have this disadvantage and, besides, provide for results in a digital form without special digitizing systems.

Digital information contains radiation images of internal defects, expands the flaw-detection possibilities and reduces testing cost. Flash-radiography is based on portable X-ray television, which is the observation of X-ray testing results on a monitor screen. The capability of examination of internal defects from different angles is provided.

Flash-radiography with digital solid-state transducers is the most perspective one with sensitivity up to 0.1% of thickness of examined metal at resolution exceeding 10 pairs of lines per mm. Application of small-size movable solid-state transducers opens new technological capabilities. They can be located and moved in the zones where positioning of film holders and storage plates is impossible. The new X-ray mini technology expands the application of NDT. The examples of practical application of Flash-radiography on the base of solid-state miniature transducers are represented.

Introduction

Radiation testing is a widespread type of non-destructive testing of quality, materials and parts. It can be used for parts from any materials, geometry and shape. Radiation methods are preferred in testing of quality of welded and brazed joints as well as in mastering of number of process solutions due to illustrative results. This method is also used for validation of other methods of non-destructive testing.

Significant qualitative changes took place in recent years expanding the possibilities of the radiation non-destructive testing, first of all due to appearance of new multi-element semiconductor radiation image detectors as well as intensive implementation of means for producing, processing and analysis of digital images, which are illustrative, easy for archiving and electronic transmission. Such detectors use electron means and transform ionizing irradiation, passed through object being examined and containing information about its internal defects, into an electric signals package. After that the signals are digitized, processed and used for formation of digital image of the object being examined. Digital image (DI) can be observed directly during inspection, i.e. in real time. Such a method of radiation testing without intermediate carriers of information is called Flash-radiography.  Virtually, it is portable X-ray television with electron record of information, which can be delivered to a customer, put in the Internet, archived and stored on memory cards without additional digitalizing and decoding.

A distinctive feature of the flash-radiography is absence of intermediate carriers of information, radiographic films, semiconductor (SC) store plates with photo-stimulated memory. Adjustment of mode in widespread technologies with intermediate carriers of information requires multiple exposures, highlighting, processing and expensive devices for digitization and reading of information. Therefore, absence of intermediate carriers of information (films, semiconductor plates) allows for increasing efficiency and significantly reducing cost of quality testing.

1. Methods for acquisition of digital imaging

Examination of the object internal defects with the help of portable X-ray television equipment having digital image processing provides for principle changes in technology of non-destructive radiation testing. Frequency of application of optical and radiation digital images (DI) has increased in recent time. Hardware and software complexes used for processing and digitization of X-ray films and providing digital images find more and more distribution. The digital images can also produced by means of storage plates instead of X-ray films. Methods and algorithms of DI processing are the same for all three variants of radiation testing (Figures 1-3). This is an important direction in current radiation flaw detection. Now digital images are typically produced by means of X-ray patterns digitization. Rarely, it is produced by processing of latent image being read from re-usable storage plates. The same result can be received from flash-radiography digital detectors without additional expenses related with intermediate information carriers.

The digital image produced by any of three indicated methods, shall have similar interpretation. The results of processing of radiography DI shall not be inferior to sensitivity and resolving power of the results of radiographic film received via film viewer. An image quality is evaluated using the reference specimen images. On DI they shall be similar to the reference specimen images of X-ray films examined using film viewer.

There are three technologies (see Figures 1,2,3) for receiving DI-results of radiation testing in electron form, but the principles of processing and further decoding of these images are the same.

Figure 1 shows a classical process for DI production by means of digitization of film X-ray patterns. This traditional technology is widespread in all branches of industry. It requires preparation of film cartridges and screens. Chemical treatment, film drying, reading of information on film viewer and digitization of the results with the help of corresponding computer complex follow up inspection. This technology is mainly used for compact archiving of NDT results in electron form and receiving receipt of additional information which cannot be obtained without digitization.

Figure 1

Fig. 1. Traditional scheme of radiographic testing with film and digitization of X-ray patterns: (1) cartridge with X-ray film; (2) processing of X-ray film; (3) image scanning; (4) digital image

Figure 2 gives a scheme of more perfect technology for digital image production based on storage plates, that is called CR. In comparison to previous scheme of DI producing, this technology provides for the possibility of multiple use of intermediate carrier of information (storage plate). This makes the process quicker, but does not reduce its price, since it requires qualified personnel, a lot of time for auxiliary operations and expensive readout equipment. Often the storage plates have their inherent defects. Eliminating the details of this method disadvantages, it is necessary to note an appearance of “sandwich” technology which allows exposing on film and storage plate simultaneously.

Figure 2

Fig. 2. Scheme of inspection using storage plate: (1) cartridge with store plate; (2) read of information from plates; (3) digital image

World manufacturers of film such as Agfa, Fuji, Kodak etc, kept the way of film replacement with semiconductor multiuse storage plates. Different equipment was developed for this technology realizing. The E. O. Paton Electric Welding Institute spent a lot of time on implementation of selenium plates and other intermediate carriers of information. All these technologies with re-usable carriers of information did not gain ground because of two reasons, i.e. due to expensive equipment and necessity of highly skilled personnel.

Figure 3 shows a scheme of X-ray technology (flash-radiography) based on fluoroscopic and solid detectors. This is the quickest and cheapest method to produce digital image in e-form, which does not require processing and reading equipment and corresponding additional time.

Figure 3

Fig. 3. Quick X-ray inspection scheme without intermediate carriers of information: (1) solid flash-transducer; (2) digital image

Both types of radiation testing without X-ray films (Figures 2, 3) can provide better results, than that of digitized image produced with the help of X-ray film.

2. The quality of digital images obtained by different methods

It is known that the higher optical density and the more exposure provide more information exposed film contains. Therefore, a good scanner is necessary for digitization of high density films to collect all the data available on the film. Widespread reading devices and cheap scanners cannot provide high quality of digitization of X-ray images, if their relative optical density is above three. All the attempts to receive satisfactory DI from the denser films have not been successful. Thus, satisfactory DI in the film variant (see Figure 1) is possible, if optical film density lies only in 1.5-2.5 range. At such values the digitizer noises do not introduce irreversible distortions in DI. Experience of digitization of film images with 3-3.2 order density has already shown unsatisfactory results. Fine information is difficult for displaying. For example, images of small pores of less than 0.2 mm diameter and cracks with small opening are lost. Therefore, film digitization has significant limitations. Part of the defects, detected with the help of film viewer, is not found on DI. This is a significant disadvantage of traditional film radiography, which is virtually impossible to eliminate in real production.

Technologies without film on schemes of Figure 2 and 3 do not have this disadvantage; they differ by large dynamic range that expands the possibilities of non-destructive testing. Analysis of DI by technological schemes of Figures 2 and 3 verified that a detectability of small pores, cracks and different inclusions in the welded joints exceeds information about them on the film. Technology of Figure 3 based on solid or optoelectronic transducers are particularly perspective. It provides for the possibility after DI computer processing to obtain up to 0.1% sensitivity and examine moving object. The defect detectability is increased due to the fact that moving small images are better distinguished by human eye, than that in static form. It is possible to change the inspection direction if intermediate carriers of information are absent during inspection on scheme of Figure 3.

DI received for three technologies, shown on Figures 1,2,3 is easily archived and webcasted. Time consumption and cost of information being received using presented technological schemes approximately refer correspondingly as 10:5:1 and 5:20:1.  Film radiography on Figure 1 offers large number of procedures, which sometimes is repeated several times to get the satisfactory results. There are no such procedures during FR. Film radiography is approximately 10 times longer than FR (Figure 3) to receive the same result. When using the storage plates less auxiliary procedures are needed to obtain the same information of object. Therefore the time spent is correlated approximately as 10:5:1.

As for the cost, the ratio of 5:20:1 means that during X-ray technologies on Fig. 1,2 the equipment for information reading, highly qualified specialists as well as repeated exposures should be used to receive the same results as at FR.

The technologies represented on the Fig. 1 and 3 do not need in expensive maintenance. Certainly the numbers 5:20:1 depend on many factors, including the level of life in given country.

For FR the time and the cost were taken to be 1. Two other techniques (Figures 1, 2) take more time 10:5:1 and cost 5:20:1. The exposure at the dentist or fluoroscopy in the hospital is performed for a few seconds, and the picture costs few cents. While the similar results based on the technologies shown on the Fig. 1 and Fig. 2 are significantly longer and much more expensive.

In a short time, detection of the internal defect corrosion damages with the help of portable flash-radiography equipment would become mandatory for all oil-and-gas auxiliary pipelines, which virtually have no control at present time, since X-ray film testing is expensive and ultrasonic one testing is low efficiency.

Figure 4 provides for structural schemes of radiation testing image production in electron form on three described technologies (see Figures 1-3). Procedures of these technologies differ in a stage of digital image production, and DI processing is the same for all three schemes. Therefore, expenses for realizing these procedures and equipment for DI receiving are is also different.

A general disadvantage of the first two technologies with intermediate carriers of information (see Figures 1 and 2) is necessity of re-inspections, some times multiple inspections for determination of optimum values of anode voltage, exposure time, focal distance as well as auxiliary procedures with intermediate carriers of information. Usually, an operator, when working with new unknown objects, needs to find the correct inspection mode and procedure for intermediate carrier of information. Typically, it is performed be means of selection, multiple exposures, i.e. repeat of all preparatory operations before inspection.

The most important advantage of the technology, presented on Figure 3, is possibility to observe image changes on the screen during inspection. This is the way for determination for the optimum modes. Besides, there is a possibility of multi-angle examination of image of internal defect.

Technologies based on small, few square centimeters, solid digital electron transducers are of specific interest. They do not have limitations related with cartridges, screens, and storage plates. Mobile transducers can move freely over the object surface. Such possibilities are included in diagnostics of widely used on practice [8] large customs objects which can be of unlimited size. Testing of such objects with the help of intermediate carriers of information (films, storage plates) is virtually unreal [8]. Miniature solid transducers can be imbedded in structures of different shape. Images from separate small transducers are joined in general image of object of complex form.

Figure 4

Figure 4. Structural scheme of process procedures for getting the results of radiation testing in electronic form with film (see Figure 1), computer (see Figure 2) and flash-radiography (see Figure 3)

Flash-radiography allows varying all main parameters (focus distance, exposure, anode voltage and current) and observing the changes in the image on display screen in real time mode. This significantly reduces the time and consumables. Besides, artifacts from films, screens, storage plates, cartridges in the technologies with intermediate carriers of information are difficult to remove. In the case with real time image, i.e. on technology shown on Figure 3, with possibility of variation of testing mode parameters, the artifacts are easy to detect and further remove. There are algorithms of electron images operation. They provide for accumulation and extraction of separate fragments in DI.

3. Equipment for flash-radiography

The USA, Japan, Russia and other countries carry out intensive works on improvement of solid electron transducers, mobile X-ray television flaw detectors, which replace ultrasonic equipment thanks to better detection capabilities. In time, this tendency will also come in other countries. Therefore, it is necessary to study process capabilities of flash-radiography. A lot of companies manufacture different scintillation panels. Significant part of such devices is described in work [3]. The E. O. Paton Electric Welding Institute cooperates with Hamamatsu Photonics company (Japan). Figure 5 shows two principles of design of solid detectors of this company, and Table 1 provides characteristics of some of them.

Figure 5

Fig. 5. Design variants of flat flaw detectors of Hamamatsu Photonics company: (a) design in which image from screen to sensor is transferred by fiber-optic plate; (b) design with direct positioning of scintillation screen over sensor (CCD-matrix)

Table 1. Characteristics of scintillator panels Csl (Tl) of Hamamatsu company

Panel

Panel type

Size, mm

Effective area, mm

Substrate thickness, mm

Scintillator thickness, mm

Light relative output, %

Contrast transfer function, lp/mm

FOS

J6673

50´10

47´7

3

150

70

10

J6673-01

150

40

J6677

50´50

47´47

3

150

70

10

J6677-01

150

40

ACS

J8734

50´50

48´48

0,5

150

125

10

J8734-01

150

150

J8977

468´468

440´440

2

600

250

3

ALS

J8978

50´50

48´48

1

150

70

10

J9857

468´468

440´440

1

600

150

3

The following designations are taken in the Table 1: FOS - Fiber Optic Plate with Scintillator; ACS - Amorphous-Carbon Plate with Scintillator; ALS - Aluminum Plate with Scintillator. Light output and contrast transfer function (CTF) were measured with the help of CCD-matrix at 60kV voltage on X-ray tube. Aluminum filter of 1 mm thickness was used.

The scores of companies in the USA, Japan and Europe produce solid digital transducers virtually for any problems of radiation testing. Figure 6 shows the process of examination of pipeline corrosion damage with the help of solid transducer of DRP 2020 NDT type [9].

Figure 6

Fig. 6. Examination of corrosion damage of pipeline with the help of DRP 2020 NDT

Technical characteristics of digital radiation transducer DRP2020NDT
Size (thickness x width x length), cm 2.2 ´ 29.5 ´ 36
Weight, kg 3.7
Power source 110 – 240 В, 215 W
Temperature range, °С 10 – 40
Connection to computer USB 2.0, LAN
Safety class IP 65
Active operating zone 204.8 x 204.8
Digital capacity, bit 14 or 16

 

Image quality [2, 10] is catheterized by specific indices. There are some them:

  • basic spatial resolution SRb is measured with the help of Duplex IQI (EN 462-5), and it equals half of registered sharpness or effective pixel size;
  • spatial resolution is determined by distance of neighboring resolvable elements on the image;
  • spatial frequency is the value reverse to distance of neighboring resolvable elements on the image being measured in lp per millimeter;
  • fuzziness of the image has a multi-factor origin due to geometry and projector conditions, detector fuzziness;
  • Signal-to-noise ratio (SNR) depends on exposure and quality of radiation track. This relationship increases as square root of area of operating pixels;
  • Contrast-to-noise ratio (CNR) depends on relationship of signal-to-noise ratio and coefficient of absorption of object material.

Dynamic range shall be taken into account in comparison of possibilities of separate methods of radiation testing. These are thicknesses of the object available for tolerable analysis on one image. Large dynamic range provides for significant advantages for the technologies presented on Figures 2 and 3 in comparison with film radiography. Usually, a large dynamic range is achieved due to exposure dose that in film systems is limited by relative optical density of 3-4. Further, they become unreadable at larger film densities. In the case of digital detection systems H&D (without intermediate carriers of information) “exposure”, i.e. information storage, has no limitations due to computer technologies. At that the signal-to-noise ratio (SNR) rises as square root of the dose. It is equivalent to exposure time or amount of averaged images. Thus, SNR, being equal to several thousands and high quality of DI is achieved. In practice these processes are limited by contrast sensitivity 0.1% that corresponds to SNR of 1000 order.

Therefore, it is obvious that radiation testing without intermediate carriers of information (storage plates, films etc.) and with elements of scanning and possibility of change of direction of object irradiation is the thing of the future.

Digital processing of images (see Figure 4) is accompanied by reporting procedures. They include operations on evaluation of DI suitability, measurement of gray intensity, optical density and determination of sensitivity. Gray digital scale is usually 16 bites [6], has thousands of tones and DI histogram shall be approximately in the middle of this scale in order to prevent under- or overexposure. Central positioning of the histogram provides for the possibility of higher quality digital processing, i.e. allows scaling of gray intensity. Calibration on size is also used. It allows measuring the defects and performing other procedures not typical for traditional film radiography and ultrasonic testing.

Great success of radiation transducers based on shuffle bars with detectors is to be noted. They find application in customs. All the attempts to use such traducers for welded joint testing have been unsuccessful yet. These systems are continuously improved [11].

4. X-ray mini technology

Inspection X-ray system can be developed based on mini R-transducers (Figure 5). At that, the X-ray transducer is moved over the object surface as it takes place in ultrasonic testing.

The solid-body transducers allow eliminating exposure of large areas and checking only small zones, where interval defects are expected. Such a mobile flash-radiography was used (Figure 7) for examination of testing bench with critical bolts used for joining of power reactors, where internal defects can’t be found by other methods. Mini R-transducers are recommended for the objects similar to shown in Figure 7. Such a variant of flash-radiography is called “X-ray mini” technology. We realize it using any solid-stare transducers including shown in Figure 5.  Mobility of the R-transducer as well as R-emitter (isotope, ceramic tube) is used in X-ray mini technology realizing. Mini-detectors which are ten times smaller than large-panel ones (Figure 6) can easily realize tangential inspection  [12] of pipes and stop valves in heat and nuclear engineering. X-ray mini technology should find wide application in monitoring of technical condition of aircraft, lifting and other dangerous equipment. Mobility of the R-transducer as well as emitting source is expanding the capabilities of NDT. Thus, a success of X-ray mini technology is in its software. Each object has robotics-realized individual programs. It is performed on the customer’s request depending on technological processes using X-ray mini technology. The E. O. Paton Electric Welding Institute manufactures scanners for X-ray mini and releases corresponding software. X-ray mini testing can have complete or partial automation.

Figure 8 shows Flash-radiography in X-ray mini variant. It was used for testing 1,5 kilometers of main pipeline consisting of 4 pipes of 18 mm diameter at oxygen plant. The program is designed in such a way that it is possible to examine simultaneously all pipes or each separately. It can be done only using a small-size solid-state transducer, positioning of which is determined by an operator. Similar radiation testing with intermediate carries of information requires much more money and time than FR.

Figure 7

Figure 7. Test bench for bolts of nuclear reactors: (a) total view; (b) plunger; (c) body; (d)  bolt being tested; (,2,3) cracks in the plunger and body appearing in testing of high-strength bolts; (4) one; (5) two; (6) three-section solid-state radiation transducers; (7) radiation source (isotope, R – tube).

Figure 8

Figure 8. Structure scheme of the portable monocrystalline DI-detector for X-ray television system

Figure 9

Figure 9. Testing of main pipeline at oxygen plant
(a) setting up of X-ray apparatus; (b) monitoring of X-ray mini image

5. Conclusions

  1. Flash-radiography with digital solid transducers is the most perspective technology. It can provide sensitivity of up to 0,1% of thickness of inspected metal at resolution, exceeding 10 lp/mm. Besides, this technology is compatible film radiography, i.e. can be carried on the same X-ray equipment. All branches of industry need in quick and cheap FR.
  2. Application of small-size movable solid transducers opens new technological capabilities. Solid transducers can be set and moved in the zones where positioning of cartridges with films and storage plate is virtually impossible. Digital solid transducer reveals new process capabilities for non-destructive testing, being not available for other physical methods. X-ray mini technology is an expanding application of NDT in industry.
  3. The R-transducer and R-emitter in X-ray mini technology should move on agreed trajectories with recording at each exposure: time, coordinates, energy, and distance to object, orientation in relation to each other.
  4. Current equipment allows producing the R-detectors and R-emitters of very small size, therefore, X-ray mini technology expands the capabilities of NDT as for inspection of the objects of any complex geometry, and require automation of radiation testing process.
References

[1] Troitskiy V.A. Flash-radiography// NDT Territory, 2013. No.4. P. 44-49.

[2] Mayorov A.A. Digital technologies in radiation testing //In the world of non-destructive testing. 2009. No.3. P. 5-12

[3] Troitskiy V.A., Mikhaylov S. R., Pastovenskii R. A., Shilo D. S. Current systems of non-destructive radiation testing // Technical diagnostics and non-destructive testing, 2015. No.1. P. 23-35. 

[4] Stepanov A.V., Lozhkova D. S., Kosarina E. S. Computer radiography of results of practical investigations on possibility of replacement of film technologies. M.: VIAM, 2010.

[5] Grudskiy A.Ya., Velichko V. Ya. Digitization of radiographic images is not very simple// In the world of non-destructive testing. 2011. No.4. P. 74-76.

[6] Tsvetkova N.K., Novitskaya K. A., Kologov A. V., Smirnov V. G. Peculiarities of application digital radiography complexes in non-destructive testing of body production // Machine-building technologies. 2014. No.7. P. 47-50.

[7] Varlamov A.N. Experience of operation of digital radiography complex under field conditions // In the world of non-destructive testing. 2014. No.3 (63). P. 25-28.

[8] Kokkoori S., Wrobel N., Hohendorf S. et al. Mobile High-energy X-ray Radiography for NDT of Cargo Containers // Materials Evaluation. 2015. V. 73. N 2. P. 175 – 185.

[9]  Duerr NDT GmbH and Co. KG. URL: http://duerr-ndt.com. Panels NDT.net.

[10] Zscherpel U., Ewert U., Bavendiek K. Possibilities and Limits of Digital Industrial Radiology: The new high contrast sensitivity technique. Examples and system theoretical analysis. Lyon, 2007.

[11] Yatsenko S. Ya., Kokorovets Yu. Ya., lozenko A. P. et. al. X-ray television systems “Polyscan”// Technical diagnostocs and non-destructive testing

[12] Troitskiy V.A Non-destructive testing of multilayer welded structures// Insight. 1997. Vol. 39. No. 9

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