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Critical Comments about the German and French Uses of I.Q.I.S for Monitoring Radiographic Sensitivity
Universidad Central de Venezuela, Facultad de Ingenieria, Departamento de Física Aplicada, Los Chaguaramos, Caracas 1041, Venezuela.
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This paper presents a comparative study between several types of Image-Quality-Indicators, (I.Q.I.) to monitoring the industrial radiographic image. The study was made to different inclinations of the X-ray beam on the specimen under inspection. We used I.Q.I A.F.N.O.R -step-drilled hole type and DIN-wire penetrameters. According to Halmshaw for a steel plate 38 mm of thickness the value of sensitivity is S = 0,7% using steel D.I.N-wire penetrameter and, S = 0.2% using I.Q.I A.F.N.O.R. In this case, I.Q.I A.F.N.O.R is more sensitive than D.I.N-wire penetrameter. In our case, an experiment was designed to compare the use of the I.Q.I D.I.N and A.F.N.O.R for angles of incidence of the X-ray beam on the specimen under inspection. The experimental values obtained allow us to conclude that the radiographic image quality, also depends on the inclination of the incident X-ray beam on the I.Q.I. In addition, D.I.N-wire penetrameter offers bigger sensitivity than the I.Q.I A.F.N.O.R for this case.
The most important feature of an image quality indicator (I.Q.I) is:
In this paper we presents a comparative study between A.F.N.O.R-step-drilled hole penetrameter and D.I.N-wire penetrameter utilization for angle incidence of the X-ray beam on the specimen tested. This type of radiography is not usual, but there are situations were we cannot irradiate perpendicularly the specimens due to the geometry of the specimen and/or because of the way the equipment is build. Such tests are not known very much and that is why we will study them in this paper. In addition, we will mention that the sensitivity of an indicator does not give us any information about the minimum detectable defect due to the shape of the defect.
- to be sensitive to the changes of radiographic technique, inclusive of film viewing conditions, and
- to give reliable information about sensitivity [1-6].
The two basic formulae expressing thickness sensitivity are :
For the case where scattered radiation is assumed to be negligible, and,
for the most general case when a proportion of the radiation reaching the film is assumed to be scattered radiation generated within the specimen. In these equations, Dx is the minimum observable change in thickness in a total specimen x; DD is the minimum discernible density difference on the film; mi is the narrow-beam absorption coefficient for the radiation used; GD is the gradient of the film characteristic curve, at the film density D, employed; IS is the intensity of the scattered radiation reaching the film and is assumed to be uniform across the small thickness change Dx, and ID is the intensity of the direct radiation reaching the film at the same point.
In practice, the radiographic sensibility S is established through the I.Q.I using the mathematical relation,
being, Dx is the thickness of smallest visible element ( wire, hole, etc) of the I.Q.I.s on radiography, an x is the total thickness of the specimen. The relative sensibility does not give us any information about the minimum detectable defect by the radiographic method used. Today among the I.Q.I most used around the world we have:
- A.F.N.O.R - steps-drilled holes penetrameters. Essentially consist of steps in each of which there is a drilled hole of diameter equal to step thickness.
- D.I.N -wires penetrameters. These consist of a series of wires, each 5 cm long, of different thicknesses.
These two types of I.Q.I are the ones most used in Europe. In the present experimental study we will use the I.Q.I.s described.
According to Halmshaw (1966) , for a steel specimen with 38mm of thickness we have the following values for the sensibility: Steel I.Q.I D.I.N , S=0,7 %; and I.Q.I. A.F.N.O.R, S= 2%. In the other hand C. Brachet (1955), , established that the visibility of the defect is determined by the product between the deepness of the defect and the average geometric length i.e., the volume of the defect. From this point of view, the A.F.N.O.R is the best.
In this experimental study we will take in cosideration only D.I.N-wire penetrameter and A.F.N.O.R step-drilled holes penetrameter. The values of the sensibility, for all constant parameters (source, exposure, Ia, Va, etc) differ from and I.Q.I to another. They are also a function of the specimen thickness (Table 1).
|Sensibility S% |
|Sensibility S% |
|TABLE 1: |
We used specimens, I.Q.I D.I.N-wires penetrameter, and I.Q.I A.F.N.O.R-step-drilled hole penetrameter of steel (Fe) and aluminum (Al). The X-ray was an equipment Seifert Eresco, and, Helling industrial viewers calibrated densitometers was used for film density measurements and evaluation.
We irradiated right angles (90 degrees) and other angles of inclination on a series of films. The specimen tested is a circular ring and the beam incident of X-ray falls directly on the I.Q.I. The thickness of specimen is d=40mm, the film used was AGFA D4, the focal distance was fc =500mm, Ia =3,5 mA, Va =130 KV, exposition time t=3minutes. The following I.Q.I were used: D.I.N-wire penetrameter type 1/7 (3.2,2.5,2, 1.6,1.25,1,0.8); D.I.N wire penetrameter type 10/16 (0.4,0.32,0.25,0.2,0.16,0.13); A.F.N.O.R - step-drilled hole pemetramer type 6/12 (1,0.8,0.63,0.5,0.4,0.32,1.25). The experimental data obtained is plotting in figure 1, where the visibilities I.Q.I D.I.N of .5 mm and I.Q.I A.F.N.O.R of .4mm were compared.We observed that the D.I.N penetrameter allowed a specimen testing for angles from 45 degrees to 130 degrees (both sides of the axis of the incident X-ray beam). The angular range is smaller for the I.Q.I A.F.N.O.R and goes from 80 degrees to 100 degrees. This result is very interesting because we get hole diameters close to the value of unsharpness where the I.Q.I A.F.N.O.R does not work very well while I.Q.I D.I.N works very well.
Fig 1: ( Aluminium )
We irradiated 90 degree, and they were compared with the ones done on 25 degrees. The experimental conditions used were: focal distances fc =530mm and fc =760mm; steel specimen plate thickness is 6mm, the electrical current in the tube Ia = 4mA, voltage Va=180KV, the film used was AGFA G5 film. The following I.Q.Is were used: A.F.N.O.R-step-drilled-hole (6/12) penetrameter and D.I.N-wire (6/12) penetrameter. The criteria applied for sizing the minimum visible detail was: both last wire/ hole and I.Q.I sharply visible. Dependence of the visibility versus angle of inclination is presented in figure 2 .We observe that steel visibility of the I.Q.I A.F.N.O.R-holes decreases for the small holes (0.4) with the incidence angles faster than for the D.I.N-wires. This behavior increases with focal distance from 760mm to 530mm.
Fig 2(a): Fe,Fc=530mm
Fig 2(b): Fe,Fc=760mm
We can conclude from our experiment that I.Q.I D.I.N-wire penetrameter allow us to work at a greater angular range than I.Q.I A.F.N.O.R -step-drilled hole penetrameter. In this case D.I.N.- wires offers better results.
In addition, we can affirm from the present study that the radiographic quality of image also depends on the inclination of the incident X-ray beam on the I.Q.I s.
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