NDT.net • Mar 2005 • Vol. 10 No.3

Defect location and radiographic response

P.PETCULESCU
Deparment of Physics, Ovidius University of Constanta, Constanta, 900527, Romania

Corresponding Author Contact:
petculescu@univ-ovidius.ro


Abstract

The aim of present paper is to investigate the influence of defect-to-film distance (DFD) on the radiographic response. That is shown by radiographing the same defects placed at different distances from the radiographic film. Five cylindrical artificial defects were machined in the reference block. The irradiation was made with an X-ray source ERESCO 200 MF having F=1.5mm, source-to-film distance SFD=700mm at U= 170kV and I= 5mA. Two radiographic techniques were used: in the first one, the defects were orientated towards the film (DOF) and in the second one, the defects were orientated towards the source (DOS). For both techniques, the X-ray source was at normal incidence on the biggest defect (Fart= 4.96mm) and on the smallest defect (Fart=1.01mm). The blackness density D in the defects center and the background density Df (the reference block thickness) in the vecinity of the defects were measured. The paper gives three empirical relations: two of them are used to calculate the defect diameter if the defect diameter measured on the radiographic film is known and one is used to calculate the defect depth if the linear attenuation coefficient is known.

The differences between the values of D for defects with maximum (Fart=4.96mm) and minimum (Fart=1.01mm) diameter show always a constant value of 0.59 for all techniques and directions. This proves an independence of this blackness density differences on the techniques and directions used here. It is noticed that the values for the blackess density Df are identical for the two radiographic techniques 5DOF and 5DOS.

In this paper we have studied the influence of DFD on the blackness density and on the radiographic image noticing a constant variation for any defects orientations.

Introduction

In the radiographic technique, the defects detectability of materials depends on many factors which cooperates in the same time beginning with the radiation source characteristics , continuing with the exposure time, defect location and ending with the radiographic film and material characteristics [1,2,3].

The radiographic image is influenced by the radiation source or by the defect shape and size [4,5], by the material thickness and also by the defect-to-film distance. The influence of defect-to-film distance (DFD) on defect detection is shown in Fig.1 by radiographing the same defects placed at different distances from the radiographic film.


Figure 1

In Fig.1a are shown the defects orientated towards the film (DOF) and towards the radiation source (DOS) and in Fig.1b is shown the blackness density variation. It is noticed a decrease of the contrast and of the shadow zone and also a dispersion of the defect image with the increase of DFD. The study of the influence of DFD on the radiographic response it's very important in determining the corrosion in pipes. The evaluation of corrosion by radiographic technique has been studied a lot but a quantitative evaluation has not been achieved yet. The results obtained in this paper can be applyed in the evaluation of inside and outside corrosion in pipes.


Figure 2

Experimental set-up

The experiments were made on a steel reference block type A-234 in which there were machined five cylindrical artificial defects [6]. The reference block and defect sizes were measured with a Mitutoya digital caliber with a precision of 0.01 mm. NG1 and NG2 films produced in Romania having normal dimensions were used as detectors. Film processing was done automatically at 21°C according to the specifications and a densitometer was used to measure the blackness density from the defect and the reference block thickness in the fixed points on the radiographic film. For interpretation, the blackess density was not below 2.0 and the maximum density was 4.0 . The irradiation was made with an ERESCO 200 MF (F=1.5 mm) at a constant SFD (F=700 mm) at U=170 kV and I=5mA. Two radiographic techniques were used here: A) the reference block having the defects orientated towards the film (DOF); B) the reference block having the defects orientated towards the source (DOS). For both techniques, the X-ray source was at normal incidence on the biggest defect (Fart=4.96 mm), (5DOF) and (5DOS) positions in Fig.3a,b and on the smallest defect (Fart=1.01 mm), (1DOF) and (1DOS) positions in Fig.4a,b. The blackness density D in the defects center and the background density Df (the reference block thickness) in the vecinity of defects were measured. The read values of these densities, D and Df, for both discussed techniques are written under each defect in the Fig.3 and Fig.4.


Figure 3

Figure 4

Results-Discussions

Analyse of these values of the blackness density D with the defect diameter shows a proportional increase along with the defect for all used techniques (Fig.3,4). The differences between the values of D for defects with maximum (Fart=4.96mm) and minimum (Fart=1.01mm) diameter show always a constant value of 0.59 for all techniques and directions. This proves an independence of this blackness density differences on the techniques and directions used here.

By applying the radiographic technique 5DOF, the values for D are bigger for all five defects compared with the ones obtained by 5DOS technique, with values from 0.05 to 0.09. It is noticed that the values for the blackess density Df are identical for the two radiographic techniques 5DOF and 5DOS. In the case of techniques 1DOF and 1DOS, the values for D using 1DOF technique are bigger then the ones obtained by the 1DOS technique, with values from 0.04 to 0.18.

From the experimental data obtained in this paper, there were found two empirical relations between the artificial defect diameter Fart and the defect diameter Fcalc for the X-ray unit ERESCO-200 at F=700mm, which are

In Fig. 5 are shown the ratio Fart/Fcalc graphs for the two applied radiographic techniques, where Fart- are the artificial defects diameters machined on the reference block (see Fig.2) and Fcalc- are the same defects diameters calculated after the empirical relations (3) and (4).


Figure 5

We noticed in Fig.5 that the value of the ratio Fart/ Fcalc =>1 with the increase of the artificial defect diameter which proves the valability of the empirical relations (3) and (4).

Knowing the values for the two blackness densities D and Df for the two radiographic techniques (DOF and DOS) (see Fig. 3 and 4) we calculated the defects depth hcalc after the relation:

(5)
where µ- is the linear attenuation coefficient. The graphs of the ratio hart/hcalc vs. artificial defects diameters for the two radiographic techniques (DOF and DOS) are shown in Fig.6a and Fig.6b


Figure 6a

In Fig.6a we noticed that the calculation of the defect depth hcalc after the 5DOF technique it is closer to 1 when hart=hcalc , especially towards bigger defect diameters.

In Fig.6b when 1DOF and 1DOS techniques are used, it is shown that the equality hart=hcalc it is true in the 1DOF technique for bigger defect diameters. Both Fig.5 and Fig.6 prove that the relations 3,4,5 can be used in order to calculate the defect diameter and depth in a reference block when the blackness density and defect diameter on the radiographic film are known.


Figure 6b

The values of hart are written on the reference block (see Fig.2).

Conclusions

In this paper we presented the influence of DFD on the radiographic response by using two techniques, DOF and DOS. For both techniques the X-ray source was at normal incidence on the biggest defect (Fart=4.96mm) and on the smallest defect (Fart=1.01mm). For the irradiation we have used the ERESCO 200MF X-ray source having F=1.5mm, SFD=700mm at U=170kV and I=5mA. The graphs from Figs.5 and 6 prove the valability of our empirical relations.

The experiments were made on a reference block machined with five artificial defects having different diameters. The following conclusions are drawn:

  • at ERESCO 200 source one noticed that for ratio F/ Fart =1.5/3=0.5 it was obtained theoretical an image dilatation from 3mm to 3.04mm (an increase of 1.3%); for ratio F/ Fart=0.3 the image dilatation is of 2%.
  • the dilatation and the contraction of the image are noticed very clearly on the showed film for the smallest (Fart=1.01mm) and the biggest (Fart=4.96mm) defects;
  • there were given two empirical relations in order to calculate the defect diameter(3),(4) and one empirical relation in order to calculate the defect depth(5).

  • there was discussed and analysed the influence of defect location regarding the radiographic film on D and on Df by applying the radiographic techniques DOF and DOS (a constant value of 0.59 appears).
  • our research focused to find out the best location in which the radiographic response to be made: on the smallest defect (Fart=1.01mm) or on the biggest one (Fart=4.96mm) in the DOF technique.

In this paper we have studied the influence of DFD on the blackness density and on the radiographic image noticing a constant variation for any defects orientations.

References

  1. ASME: Fundamental of Radiography, Materials Evaluation July, 1978, pg.245-248;
  2. R.Halmshaw: Radiographic Methods- Image Quality, Insight vol.42, nr.3 2000, pg.199-201;
  3. R.Halmshaw: Radiographic Methods Standards for Radiography, Insight vol.42, nr.2, 2000, pg.103-105;
  4. O. Yokota, Y. Ishii: Crack Detectability by Radiography, J.NDI 27(1) 1978 pg.239-244;
  5. R.Halmshaw: Defect Size Measurement by Radiography, British Journal of NDT, sept. 1979,pg.245-248;
  6. P. Petculescu: Influence of the exposure geometry on the radiographic image, Ovidius University Annals of Mechanical Engineering, vol.IV, Tom.I, 2002
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