Radiographic Inspection of Complex-Configuration Products

V. A. Berdonosov, V. I. Gorbunov, G. V. Titov

In the practice of radiographic inspection, it is quite often necessary to radiograph products of complex configuration with a wide range of thickness variation in the direction of the radiation-beam axis. The main difficulty in radiographing such products lies in the need for selective inspection of portions of the product of different thicknesses; attempting to radiograph, in a single exposure, portions of a product with a large thickness variation leads to such a difference in the blackening densities of the radiograph that it becomes impossible to detect defects over its entire area. Consequently, defect detection over the whole radiograph is possible only when the difference in blackening densities of its individual regions, corresponding to a certain thickness variation of the inspected product, is permissible.

The permissible thickness variation of a product in regions radiographed in a single exposure depends on a rather large number of factors. These include the parameters of the absorber—its density, effective atomic number, and thickness—the radiation energy, and the contrast sensitivity of the detector. Using the law of attenuation of radiation intensity, it is easy to show that for absorbers having low density the permissible thickness variation $\Delta d$ in simultaneously radiographed regions considerably exceeds the values of $\Delta d$ for high-density materials (lead, tungsten). Similar calculations also lead to the conclusion that increasing the maximum energy of the radiation spectrum makes it possible to increase the values. As for the contrast sensitivity of the detector, as the latter increases the value of $\Delta d$ decreases [1, 2].

Since, during inspection, the parameters of the absorber cannot be changed, it is not possible to make use of changing $\Delta d$ by changing the properties of the absorber. Nor can the contrast sensitivity of the detector be reduced, since in order to improve the detectability of defects, the contrast sensitivity should, on the contrary, preferably be increased. Consequently, changing the radiation energy is the only acceptable means of changing the permissible thickness variation in a portion of the product exposed in a single exposure.

Let us note that the permissible thickness variation $\Delta d$ is determined by the permissible value of the difference in blackening densities $\Delta D$ of different regions of the radiograph corresponding to product regions of thickness $d$ and $d + \Delta d$; the larger the permissible values of $\Delta D$, the greater the value

\(\Delta d\). On the basis of psychological tests, we have adopted the assumption that the difference in optical densities on a radiograph at which defects are detected equally in different areas of the radiograph does not exceed 0.4 optical-density units. With a greater difference in blackening densities, defects are detected visually on the radiograph unequally; moreover, the best detectability occurs in areas of the radiograph with a density of 1.6–1.7 [1, 2, 3].

The experimental arrangement for determining the permissible differences in the thicknesses of sections of products radiographed in a single exposure is shown in Fig. 1. The bremsstrahlung generated by the betatron target (1), after passing through the collimator and through the absorber (4), acts on the emulsion of the X-ray film (5). On the side of the product facing the source, defectometers (2) are installed, and the distance between one of them and the absorber is additionally covered by a set of steel plates of various thicknesses (3), simulating thickness differences of the actual product.

Fig. 1. Experimental arrangement for determining permissible thickness differences:
1 — betatron; 2 — defectometers; 3 — thickness differences; 4 — absorber;
5 — detector.

Steel specimens 106 and 216 mm thick were investigated. As the radiation source, a B-30 betatron of the Tomsk Polytechnic Institute was used, with smooth adjustment of the radiation energy from 5 to 30 MeV; the radiation dose rate at a distance of 1 meter from the betatron target was 80–100 R/min with a stability of 1.5–2%. X-ray film of the RT-1 type with lead intensifying screens was used as the radiation detector. At maximum radiation-energy values of 22, 15, and 30 MeV, the thickness of the front screens was, respectively, 1.7, 1.5, and 2.0 mm. The thickness of the rear screens was 3.0 mm and remained unchanged during the experiments.

Figures 2 and 3 show the obtained dependence of the difference in blackening densities of radiograph areas corresponding to absorber thicknesses \(d\) and \(d+d\Delta\) on the magnitude of the thickness difference \(\Delta d\), mm. As follows from Figs. 2 and 3, with constant absorber and detector parameters the value is determined only by the maximum radiation energy. Thus, for a steel thickness of 106 mm, optical-density differences \(\Delta D=0.4\) correspond to permissible differences in the thicknesses of product sections exposed in a single exposure of, respectively, 28 mm, 24 mm, and 19 mm at maximum radiation-spectrum energies of 30, 22, and 15 MeV.

For an absorber thickness of 216 mm, the permissible thickness differences are 36 mm and 27 mm at \(E_{\max}\), respectively, 30, 22, and 15 MeV. With further increase in absorber thickness, the magnitude of the permissible thickness differences corresponding to a blackening-density difference of 0.4 mm increases monotonically.

Fig. 2. Dependence of the difference in optical densities of blackening on the thickness step at various maximum radiation energies of the betatron (steel absorber thickness 106 mm): 1 — \(E_{\max}=15\) MeV; 2 — \(E_{\max}=22\) MeV; 3 — \(E_{\max}=30\) MeV.

Fig. 3. Dependence of the difference in optical densities of blackening on the thickness step at various maximum radiation energies of the betatron (steel absorber thickness 216 mm): 1 — \(E_{\max}=15\) MeV; 2 — \(E_{\max}=22\) MeV; 3 — \(E_{\max}=30\) MeV.

The results obtained make it possible to draw certain practical conclusions concerning the methodology for radiographic inspection of products of complex configuration, the most important of which is the possibility of increasing the permissible thickness step by increasing the radiation energy.

REFERENCES

1. Electron Accelerators. Proceedings of the Fifth Interuniversity Conference on Electron Accelerators. Moscow, Atomizdat, 1966.

2. A. A. Vorob’ev, V. I. Gorbunov, V. A. Vorob’ev, G. V. Titov. Betatron Defectoscopy of Materials and Products. Moscow, Atomizdat, 1965.

3. V. S. Sokolov. Defectoscopy of Materials. Moscow–Leningrad, Gosenergoizdat, 1961.