EVALUATION OF GRAY
INTENSITY VALUE FOR
RELIABLE DIGITIZATION OF
DIGITAL RADIOGRAPHY IN
DEFECT DETECTION
P.Chitra1
ECE Department,
Sathyabama university,chennai 119
chitraperumal@gmail.com,
B.Sheela Rani2
Proffesor,Vice Principal, Sathyabama university,chennai 119
B.Venkatraman3
Head,Quality Assurance Division, Indira Gandhi Centre for Atomic Research,
Kalpakkam
Baldev Raj4
Distinguished Scientist and Director Indira Gandhi Centre for Atomic Research,
Kalpakkam.
Abstract:-Radiography is one of the oldest NDT technique used for evaluation of weld defects in metal. Radiographic defects are classified based on the shape, location, orientation, depth, width etc. Once a radiograph of a weld is taken, the radiographer examines the same for identifying the defects and quantitatively evaluating the same based on codes and specifications. In recent years digital imaging has superseded conventional imaging, which has led to a profound change in interpretation of radiographic images. A change in the Gray values of the digital image (edges) indicates the flaws on the radiographic image. This paper discusses the Gray level distribution of a test piece radiograph. The test piece considered is a stainless steel metal plate on which a square hole is drilled with different area and depth. The plate is radiographed using X-ray and gamma rays (IR 192).
Keywords:-Gray value, Digital radiography, Radiographic techniques, Image processing
1.Introduction
Gray at other end. A film that shows very light and very dark areas has high contrast or a short gray scale as there are few shades of gray from one extreme to the other. A radiograph that has many shades of gray is referred to as one with low contrast or long gray scale.
The successful use of radiography depends on the ability of the radiation source, be it x-ray or gamma, to provide sufficient radiation to penetrate the weld and to produce an image of acceptable contrast and definition on the processed radiographic film, using an acceptable and economic time [1]. The difference between x-ray and gamma-ray radiographs is that while x-rays and gamma-rays are fundamentally the same type of electromagnetic radiation, the quality of radiation produced is based on their origin and their differing energy spectra [2].
This paper concentrates on the quality of the radiographic image of a test piece radiographed using different sources X-ray and Gamma rays such as iridium 192.The Gray level distribution of the digital radiographic film is analyzed. To investigate the effect of parameters on the image, a radiographic film of X-ray and gamma rays such as iridium 195, selenium 75 is digitized using standard digitizer.
The paper is organized as follows.The section 2 reviews with materials and methods, Section 3 deals with X-ray radiography. The section 4 deals with Gamma source Radiography. Section 6 discusses the experimental results on the data acquired, and Section 6 concludes the paper.
2. Methods and Materials
A stainless steel metal of 3.5 mm thickness is taken .In the metal a square notch of different length and breadth is drilled, each notch is made in such a way that all of different depth. The plate is subjected to X-ray and Gamma ray radiography. For gamma ray iridium 192 radioactive source is used for experimental set up. The radiation is exposed on D4 film. Two type of Image Quality Indicator (IQI) are placed on the source side of the material they are the placard, or hole-type and the wire IQI. IQI comes in a variety of material types so that one with radiation absorption characteristics similar to the material being radiographed can be used. After radiographed the film is digitized using Array Corporation's 2905 X-Ray film digitizer from Indira Gandhi Center for Atomic Research, Kalpakkam. It is a Laser film digitizer. The radiographic film was digitized in two different DPI (100,300) and in two file format (BMP, TIFF). In the digital image from the Gray level distribution the penetration of the radioactive source is calculated. The length and breadth of the notch is calculated from the digital image using MATLAB software and the results is compared with manual calculation instruments. The instrument used for manual calculation is Non contact measuring machine (HAWK SYSTEM 7).
3. X-ray Radiography
X-ray is one of the powerful sources used for the evaluation of radiographic weld material. X-ray sources generate a continuous range of energies up to a maximum. And that is dependent on the operating kilo voltage (kV), while gamma-ray sources produce fixed line spectra at specific photon Energies [1].The test piece with square notch of varying depth and area is subjected to X-ray radiography. The specification of it is tabulated in table 1
Table 1.Specification for x-ray Radiography
Specification of the sources X-Ray
Voltage 250 Kv
Focus size 2.6 mm
Current 5 mA
Source to film distance(SFD) 700mm
Source size Ø2.7mmX2.1mm HT
Film type D4
Intensifying screen lead foil type Front size 0.1mm Back screen 0.1mm
Film size used 8” x 6”
4. Gamma source radiography
Weld inspection of pipe can be done using Gamma source. The Gamma ray sources have less weight equipment and low cost compared to X-ray radiography. Gamma equipment does not need a power supply and are very useful for mobile inspection and access in space restricted areas. The most commonly used gamma ray sources are Iridium 192(Ir 192) and Selenium 75 (Se 75)[2].
The test piece with square notch of varying depth and area is subjected to gamma radiography. The gamma sources considered for analysis is Ir-192. The specification of the radiographic technique is tabulated in the table 2. Test measurement on the test piece is conducted to find the effect of intensity value in two types of radiography, dimension variation on the digital image and manual calculation.
5. Data Analysis and Results
In the test piece under consideration the length, breadth, depth is manually calculated using Non contact measuring machine (HAWK SYSTEM 7). The result of this calculation is shown in table 4. .In the digitized radiographic image the length, breadth, depth is calculated using MATLAB software. The result is obtained in terms of pixel which is converted into equivalent millimeter. Obtained result is tabulated in table3. Gray value distribution for the radiographic images of X-ray and gamma rays were found by drawing a line profile length wise and breadth wise. The result obtained is shown in the Fig (2)-(5). From the graph it was found that the line
profile curve for X-ray is very smoother than gamma rays radiographic image. This shows that X-ray radiographic image is less affected by noise than gamma rays radiographic images. Then length and breadth of the square notches is shown in the line profile. Surface plot for the radiographic images is obtained and the result is shown in the figure (6)-(9). The depth of the square notch is shown in the surface plot.
Table.3 Length and Breadth of square defect image taken at 150kv,5 mA with source to film distance 700mm on a D4 film using MATLAB
1 2 3 4 5 6 7
A L 9.2495 9.2495 8.9604 9.5385 9.2495 9.5385 10.406
B 8.9604 8.9604 9.2495 9.2495 9.5385 9.5385 10.117
B L 7.5152 7.5152 7.8042 7.5152 7.8042 8.0933 8.0933
B 7.8042 7.5152 7.5152 7.5152 7.8042 8.0933 7.8042
C L 5.7809 5.4919 5.7809 5.7809 5.6013 5.7809 5.7912
B 5.4919 5.7809 5.7809 6.07 6.07 5.7809 6.359
Table2. Specification for Gamma ray Radiography
Specification of the sources Ir-192
Radiographic technique Single Wall Single Image(SWSI)
Source to film distance(SFD) 700mm
curies 2ci
Size Ø2.7mmX2.1mm HT
Exposure time 25 min
Film type D4
Intensifying screen lead foil type Front size 0.1mm Back screen 0.1mm
Table.4. Length and Breadth of square defect image taken at 150kv,5 ma with source to film distance 700mm on a D4 film using Non contacting measuring device
1 AVG 2 AVG 3 AVG 4 AVG 5 AVG 6 AVG 7 AVG A L 9.764
9.735 9.931
9.843 9.758 9.871 9.843 9.903 9.828 9.935 9.999 9.995 10.001 10.001
9.807 9.756 9.983 9.904 10.042 9.991 10.000
B 9.740 9.755 9.838 9.851 9.940 9.997 9.866 9.879 9.959 10.060 9.966 9.848 10.010 10.055
9.771 9.865 10.054 9.892 10.161 9.730 10.100
B L 7.916 7.865 7.976 7.925 7.917 7.866 8.001 7.997 8.100 7.998 8.091 8.101 8.001 8.012
7.815 7.874 7.812 7.993 7.987 8.112 8.023
B 7.870 7.907 7.971 8.005 7.962 7.952 8.002 8.027 8.005 8.048 8.095 8.096 8.034 8.105
7.944 8.040 7.941 8.053 8.091 8.092 8.176
C L 5.876 5.888 6.000 5.963 6.009 6.016 5.926 5.954 6.118 6.152 6.169 6.093 6.163 6.079
5.900 5.927 6.042 5.982 6.187 6.017 5.996
B 5.884 5.931 5.953 5.963 6.032 6.008 6.072 6.063 6.115 6.073 6.133 6.161 6.144 6.039
5.979 5.972 5.985 6.053 6.032 6.189 5.935
D L 3.971 3.989 3.927 3.951 3.947 3.988 4.061 4.045 4.080 4.040 4.095 4.101 3.964 3.977
4.008 3.974 4.029 4.029 4.000 4.107 3.990
B 3.924 3.964 3.932 3.949 4.001 3.996 4.082 4.065 4.085 4.052 4.113 4.086 3.929 3.964
4.004 3.966 3.992 4.049 4.019 4.059 3.999
E L 1.795 1.810 1.891 1.906 1.961 1.956 1.985 1.933 1.988 1.994 2.039 2.037 2.039 2.023
1.825 1.921 1.952 1.881 2.000 2.035 2.007
B 1.823 1.821 1.933 1.906 1.859 1.880 2.026 2.009 1.993 1.998 2.022 2.042 1.979 1.980
The Gray value intensity for X-ray source is higher than that of the digitized gamma ray source radiography. In the surface plot it was found that there are maximum peeks for the Xray radiographic film. In X axis direction of the test piece there are seven square notches they are oriented in such a way that there area is equal but the depth is different, so there is seven peeks in the surface plot of varying height along the x axis direction for the film radiographed using X-ray. In Y axis direction of the test piece there are five square notches they are oriented in such a way that there depth is equal but the area is different, so there are five peeks in the surface plot of same height along the y axis direction for the film radiographed using X-ray. In case of gamma rays radiography the depth variation is not much observed. The peak Gray level distribution in the surface plot is also less compared with X-ray radiography.
The results are obtained using MATLAB software for calculation of area and it is found that the result is close to the manual calculation. Only few calculations are obtained because the digital radiographic images do not have good contrast.
Fig.3.Gray value distribution along the Y axis direction for the film radiographed using Ir-192
Fig.2.Gray value distribution along the X axis direction for the film radiographed using Ir-192
Fig.6. Surface plot along the y axis for the film radiographed using X-ray source
Fig.7. Surface plot along the X axis direction for the film radiographed using X-ray source
Fig. 8.Surface plot along the y axis for the film radiographed using Ir-192 source
Fig.9. Surface plot along the x axis for the film radiographed using Ir-192 source
Fig.5 .Gray value distribution along Y axis direction for the film radiographed using X-ray source
From the result it was found that X-ray radiographic images gives good gray level distribution with reduced noise level compared with gamma rays radiography. Even though gamma rays are more satisfactory than X-rays for examining objects of varying thickness, where as X-rays provide better results for welded plates of very small thickness. X-ray method is much more rapid than gamma-ray method, because while taking radiography X-ray method requires seconds or minutes instead of hours.
Acknowledgement
The authors wish to thank the scientists of Indira Gandhi Centre for Atomic Research, Government of India for the technical support provided by them. The author also wishes to thank the scientist Mr.N.Vasudevan, Radiological safety officer and all staff members of Scaanray metallurgical services Chennai for the technical support in taking radiography
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