|NDT.net - May 2000, Vol. 5 No. 05|
|TABLE OF CONTENTS|
Good workmanship criteria for industrial radiography were developed during the last decades in all countries. The corresponding rules are defined in a system of radiographic standards. But these rules are not harmonized in the different countries, because of differences in the traditions and training contents. This leads to deviations in the understanding of requirements for quality and expense of testing. European countries were forced to spend extraordinary efforts for the harmonization of rules due to the process of European centralization. The traditional European schools of NDT had to agree on common standards which allow a harmonized procedure and mutual agreement of NDT quality and certificates. These standards are generally submitted to ISO on the basis of the Vienna agreement between ISO and CEN. This provokes a new process of international harmonization. Nevertheless, we still observe differences in the standards between USA, Japan, Europe and others, which complicates the international mutual recognition.
During the last decade, several joint committees have developed European standards on industrial radiography from the previous national standards. They include guidelines on the measurement of instrumental parameters, as well as minimum requirements for instruments, practice and evaluation.
|Vocabulary||EN 1330-3/ISO 5576||EN 1330-3/ISO 5576|
EN 584, EN 462,EN 25580
|General Rules||EN 444 ISO 5579||prEN 13068-3|
|Sectors(Inspection Rules)||EN 1435
||Sectors(Evaluation Rules)||[EN 25817 (Quality Levels)]
||EN 12062(General Rules)|
EN 12517(Acceptance levels)
|Table 1: Overview about basic standards for industrial radiology|
|Fig 1: Maximum energy for X-ray inspections up to 500keV|
|Fig 2: Wall Thickness Ranges Corresponding to EN444, EN1435 and ISO 5579|
The basic parts of European and international standards (see tab. 1) are the general rules (EN444 corresponding to ISO5579) for radiographic examinations and the standard for radiographic weld inspection (EN1435) and foundry inspection (prEN12681). These are accompanied by the vocabulary (EN1330-3=ISO5576) and a set of standards for equipment properties (See figure 4).
Considerable differences appear in the understanding about the required sharpness of radiographs. These differences can be found e.g. between the European and US-American system. Figure 3 shows the differences of the minimum required sharpness (maximum geometric unsharpness) for some selected international, European and US-standards.
|Fig 3: Comparison of limits for geometric unsharpness of different international standards. ISO 5579, EN444, EN 1435, prEN 12681 class A and B require the highest sharpness. Radioscopy (prEN 13068-3) and ASME V/2 allow relatively unsharp testing. The 1% and 2% lines shall support the interpretation of this figure. Due to the definitions, some ASTM-penetrameters (e.g.E1742-95) are suitable to check unsharpness in the frame of EPS down to 500 or 250 µm only.|
|Fig 4: Scheme of standards for measurement of equipment properties, classification and minimum requirements (pr: proposal).|
The image quality of the radiographs depends on the equipment properties. Two source parameters are of importance: the size of the radiator and the tube voltage. The corresponding standards are under final vote now. The most significant change is the new definition of the focal spot size or gamma source size, which now corresponds to the largest measured dimension (length or width). These values are normally significantly greater than those of IEC 336. This will influence the daily test practice. The standards for the measurement of the voltage of x-ray tubes are less critical concerning the influence on the daily NDT-work. They are important for in-service quality control and for a better comparison of manufacturer's parameters.
The standard EN584 defines 6 film system classes for Europe instead of the four classes in ISO 11699. ASTM (E1815-96) and JIS (K 7627-97) followed the same philosophy, but with a different definition of the system classes (table 2). The given parameter ranges for the film system classes are now defined for the complete system consisting of film type and development process in all standards. Peripherals: The IQIs and film illuminators (light boxes) are defined in the EN462 and EN25580 (ISO5580) respectively.
|System Class||Minimum gradient at||Minimum gradient-noise ratio at||Maximum granularity at|
|D=2 above Do||D=4 above Do||D=2 above Do||D=2 above Do|
|Table 2: Definition of film system classes|
The CEN standard project of subworking group 3 is entitled: "Fundamental parameters of radiography digitizers". Two drafts are proposed. Part 1 is: "Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control". Here, the standard reference film is taken over from ASTM with the required modification in the CEN-texts for the same film. Part 2 defines the minimum requirements to digitizer systems.
Film digitization is used for different reasons. The applications of image processing, quantitative analysis, and archiving are the most important tasks. While PC-based image scanners are available for paper images, transparencies and slides in a wide variety and at low prices, they are generally not suitable for X-ray film scanning. Industrial X-ray films are used with optical densities up to 4 and sometimes up to 5. Only a few available scanners are able to cover that density range. Furthermore, the NDT X-ray films are characterized by a high signal-to-noise ratio (SNR) which is defined in EN 584, ISO 11699, ASTM E1815-96and JIS 7627-97 by maximum granularity values related to an optical density of 2 (above fog). A suitable digitizer must not only be able to reach the density of 4 or 5, it also must not increase the image noise by its own detector noise.
Beside the requirements for maximum density and SNR, X-ray films require a very high spatial resolution. The limiting structure for very low X-ray energies is the grain size of the photoactive silver based crystals, which is below 1 µm. This is particularly important for micro radiography. General NDT applications require X-ray energies between 50 and 12000 keV. In medicine, the application range is normally below 150 keV only. Due to this large energy range for NDT radiography, it was decided to reduce the requirements for spatial resolution to the unsharpness which is caused by interaction of high energy X-rays with the screen film system. Measured functions provide unsharpness values between 30 and 800 µm (Klasens criterion), depending on the energy and the screen film system. Based on these measurements, the following tables define the minimum requirements. Table 3 defines the minimum working range of the radiographic film digitization system. In this working range, the digitizer shall provide an optical density contrast sensitivity DDcs which is DDcs £ 0.02 O.D. The minimum digital resolution is given for all devices converting the digital value proportional to the optical density. If the digital value is converted proportional to the light intensity, the digital resolution must be increased by at least 2 additional bits.
|Class DS||Class DB||Class DA|
|density range* DR [OD]||0.5 - 4.5||0.5 - 4.0||0.5 - 3.5|
|digital resolution [bit]||³12||³ 10||³ 10|
|density contrast sensitivity DDCS [OD] within DR||£0.02||£0.02||£0.02|
|Table 3: Minimum density range of the radiographic digitisation system with a minimum density contrast sensitivity|
Table 4 specifies the minimum spatial resolution as a function of the X-ray energy.
|Energy||Class DC||Class DB||Class DA|
|[keV]||Pixel size |
|MTF 20 % |
|MTF 20 % |
|MTF 20 % |
|Table 4: Proposed minimum spatial resolution of film digitisation systems|
On the basis of the image quality of film radiography and the state of the art of digitizing systems, the committee has defined three quality classes; DA, DB and DC. The user may select the testing class based on the needs of the problem:
|DS -||the enhanced technique, which performs the digitisation with an insignificant reduction of signal-to-noise-ratio and spatial resolution,
Application field : digital archiving of films (digital storage)
|DB -||the enhanced technique, which permits some reduction of image quality,
Application field : digital analysis of films, films have to be archived,
|DA -||the basic technique, which permits some reduction of image quality and further reduced spatial resolution, meeting the needs of radiographs according to ISO 5579 and EN 444 class A above 5 mm wall thickness.|
Due to the required international harmonization, the standard reference film is taken over from ASTM for test and evaluation as well as for long term stability tests of digitizers.
Each radiographic film digitization system for NDT applications shall be identified with all working ranges of optical densities. It shall be classified corresponding to table 3 and the maximum MTF 20 % value, which can be performed by this system. So for instance, a digitization system of class DS 5 can be applied for archiving of radiographs taken with X-rays above 200 keV or gammarays and can be applied for all class DB and DA digitization tasks.
New standards (Draft: prEN13068/1-3) were proposed for radioscopic testing. The first two parts on equipment properties are in final voting now. The third one, "general principles", passed successfully the 6 month inquiry. It requires to carry out all measurements with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (figure 5). Tables 5 defines the minimum wire and duplex wire values, depending on the testing class (SA or SB) and the wall thickness for metallic materials. Another table is given for lightalloy materials. Advantages of real time testing are considered here.
|Fig 5: Image quality indicators. Left: Wire IQI for contrast measurement. Right: Duplex wire IQI for spatial resolution measurement. Both IQI's are required for radioscopy and Computed Radiography.|
The equipment is subdivided into three system classes SC1 - SC3, which depend on the test problem. Lower requirements for spatial resolution than in film radiography (EN 444, ISO 5579) are compensated by increased requirements for contrast.
|Testing Class SA||Testing Class SB|
|System Class||SC2||System Class||SC3|
|Wire no.||Duplex no.||Wire no.||Duplex no.|
|Wall thickness||Wall thickness|
|1.2 - 2.0 mm||W17||11D||W19||13D||-1.5 mm|
|2.0 - 3.5 mm||W16||10D||W18||12D||1.5 - 2.5 mm|
|3.5 - 5.0 mm||W15||9D||W17||11D||2.5 - 4.0 mm|
|5.0 - 7.0 mm||W14||8D||W16||10D||4.0 - 6.0 mm|
|7.0 - 10 mm||W13||7D||W15||9D||6.0 - 8.0 mm|
|10 - 15 mm||W12||7D||W14||9D||8.0 - 12 mm|
|15 - 25 mm||W11||7D||W13||9D||12 - 20 mm|
|25 - 32 mm||W10||7D||W12||9D||20 - 30 mm|
|32 - 40 mm||W9||7D||W11||9D||30 - 35 mm|
|40 - 55 mm||W8||7D||W10||9D||35 - 45 mm|
|55 - 85 mm||W7||6D||W9||9D||45 - 65 mm|
|Table 5: System performance for metallic materials testing class SA and SB|
A new proposal on "General Principles for Examination of Metallic Materials by Computed Radiography" has been developed in a committee of the German Society for NDT. Again, the concept of the standard is the definition of minimum requirements to ensure a certain spatial resolution and contrast which should be similar to the requirements of the EN444 (or ISO5579). In analogy to radioscopy, measurements are needed with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (figure 5). Table 6 defines the minimum wire and duplex wire values depending on the testing class (IPA or IPB), the wall thickness and radiation energy. The values were derived from the measured film unsharpness as function of energy and the geometric unsharpness corresponding to the wall thickness, on the basis of EN 444 and ISO 5579. The requirements for spatial resolutions are higher than those for radioscopy and are similar to film digitization.
|Radiation source||Wall thickness|
|Class IPA||Class IPB|
|Double wire |
Up £ 50 kV
|w < 4||40||> 133)||30||>> 134)|
|4 £ w||60||13||40||> 133)|
50 < Up £150 kV
|w < 4||60||13||30||>> 134)|
|4 £ w < 12||70||12||40||> 133)|
|w ³ 12||85||11||60||13|
150 < Up £ 250 kV
|w < 4||60||13||30||>> 134)|
|4 £ w < 12||70||12||40||> 133)|
|w ³ 12||85||11||60||13|
250 < Up £ 350 kV
|12 £ w < 50||110||10||70||12|
|w ³ 50||125||9||110||10|
350 < Up < 450 kV
|w < 50||125||9||85||11|
|w ³ 50||160||8||110||10|
|Yb 169,Tm 170||85||11||60||13|
|Se 75, Ir 192||w < 40||160||8||110||10|
|w ³ 40||200||7||125||9|
|X-ray Up > 1MeV||250||6||200||7|
|NOTE:||1) If magnification technique is used, duplex wire IQI-readout is required only
2) The given IQI-numbers indicate the readout value of the first unresolved wire pair corr. to EN 462-5.
3) The symbol ">13" requires the 13th wire pair to be resolved.
4) The ">>13" is not readable with the double wire IQI corr. To EN 462-5. Up - Tube voltage.
|Table 6: Required spatial system resolution in dependence on energy and wall thickness|
Phosphor imaging plate system classes are derived from EN584 (or ISO 11699, ASTM E1815-96, JIS K 7627-97) on the basis of the Signal-to-Noise Ratio (SNR) (see Table 8). Detailed guidelines are given on how to determine the exposure time to provide the specified SNR. Even with the same CR-phosphor imaging plate (IP) and scanner, different IP-classes can be obtained by using different exposure times, if the homogeneity of the phosphor layer is sufficient. Furthermore, a minimum lead filter thickness is specified to reduce the influence of scattered radiation (table 7), which is generally more intensive than for X-ray film.
|Screen for film||27 mm|
|Screen for IP's||100 mm|
|1 mm Pb|
0.5 mm Fe
|Specific contrast for|
IP's to film
|Table 7: Equivalent screen thickness for IP's(BAS III)|
|IP System classes|
|System class||Minimum |
|Table 8: IP-scanner system classes, depending on the minimum SNR|
Further activities for DIR-standardization are the "standard data format" and "Computed Tomography". The standard data format was the attempt to unify and simplify the manifold of available data formats for NDT technology. This is a very difficult task, because different data formats are already installed in a variety of equipment and used for different applications. The original intend to develop a standard was changed to the publication of a technical CEN report, consisting of two parts: part 1 "General format for NDE data", part 2 "...application programming interfaces" . The ASTM has opened a new project with a similar goal. The data format DICOM, which is widely used in medical applications, shall be modified to NDT-applications under the name DICONDE.
Computed Tomography (CT) is a method which is routinely used in medicine. Now it can be observed that CT becomes a more and more established method for NDT-applications. Therefore, corresponding standards were developed during the last fifty years by ASTM.
The following ASTM - Standards are valid:
|CT Examination||E 1570-95a|
|CT Examination of Castings||E 1814-96|
|Measurement of CT System Performance||E 1695-95|
|CT Imaging||E 1441-97|
|CT System Selection||E 1672-95|
|Calibrating and Measuring CT Density||E 1935-97|
|X-Ray Compton Scatter Tomography||E 1931-97|
The most of these standards were submitted to ISO.
|© NDT.net - email@example.com|||Top||