CONTROL OF THE MACROSTRUCTURE OF POLYMERIC ELECTRICAL INSULATING MATERIALS USING X-RAY RADIATION

V. A. VOROB'EV, G. V. TITOV, L. G. KRASIN

(Presented by the scientific seminar of the Research Institute of High Voltages)

The real macrostructure of solids presupposes the existence of a large number of macroinhomogeneities of various origin. The nature, dimensions, and locations of such inhomogeneities are determined by the specific features of manufacture of the products, their material, the presence of impurities, etc.

The operating conditions of products made of electrical insulating materials impose stringent requirements on their macrostructure, since the presence of cracks and foreign inclusions in a dielectric reduces its electric strength [1, 2].

When inspecting the homogeneity of electrical insulating materials, it is important not only to determine the presence of an inclusion, but also to determine its shape and type. Such inspection can be carried out only by nondestructive methods, since no selective inspection involving destruction provides assurance that the remaining products meet the requirements imposed on them.

One of the best-developed and most widely used methods of nondestructive testing at the present time is radiation flaw detection [3, 4].

The special features of conducting radiation studies of the quality of polymeric materials are determined by their low density, as well as by the possibility that, as a result of irradiation, products of a chemical composition different from that of the base material may arise [5]. Absorption of the energy of electromagnetic radiation in the irradiated material affects the initiation of chemical reactions (radiolysis, destruction, or cross-linking of molecules), and the formation, as a result of these reactions, of products of a new chemical composition may be the cause of false signals concerning defects in the macrostructure of the material.

It was of interest to determine the influence of irradiation on the reliability of the information obtained about the quality of the macrostructure. For this purpose, reference specimens made of polystyrene, polyethylene, and fluoroplastic were inspected without preliminary irradiation and with preliminary irradiation of these specimens with an exposure dose of \(10^3\) roentgens. With detector sensitivity to a change in material thickness of no more than 0.5%, we did not detect any influence of radiation damage on the homogeneity of the macrostructure of Teflon and polystyrene.

Fig. 1. X-ray image of a polyethylene feed-through insulator. The light spots visible in the image are voids formed during casting.

Fig. 2. X-ray image of a polyethylene feed-through insulator. In the image, steel shavings that entered the material when it was poured into the mold are visible as dark spots.

Before carrying out the experimental study of the detectability of defects in polyethylene, fluoroplastic-4, and polystyrene, we evaluated the influence of certain radiation parameters on the size of the smallest detectable defect. Increasing the radiation energy within the range from 30 to 70 keV, at which radiography was performed, leads to a decrease in the values of the radiation attenuation coefficients for both materials under inspection. The magnitude of secondary radiation, which substantially affects defect detectability, decreases when the irradiation field is restricted. In the inspection of thin-walled products made of light materials, the minimum size of the detectable defect is limited by the film’s intrinsic unsharpness factor. The condition of the surface of the material under inspection is also of great importance.

The quality of the image of the material macrostructure on X-ray film was assessed using reference defects. The reference defects were made in the form of drilled holes of various diameters, and also in the form of rectangular grooves of various cross sections. The image of the reference defects on the radiographs makes it possible to evaluate the suitability of the selected radiographic conditions.

When radiographing products made of Teflon and polystyrene with a thickness of up to 100 mm, the optimum tube voltage selected was 35 kilovolts. The resulting detectability of defects is no worse than 0.8%. Defects in the form of air cavities with sharply defined edges are recorded reliably.

For products with a thickness of up to 250–300 mm, the tube voltage must be raised to 60–70 keV. Although this leads to an increase in the Compton-scattering cross section of the primary quanta and to a decrease in the attenuation coefficient, defects in the form of drilled holes 0.9–1 mm in diameter and 1.4–1.7 mm deep are visible on the radiograph with sufficient clarity.

Figure 1 shows an X-ray photograph of a feed-through insulator made of polyethylene. During casting of this insulator in the mold, voids formed in the material, which are clearly visible in the photograph.

Figure 2 shows a photograph of a similar part. In the form of dark spots, the photograph shows particles of metal shavings that entered the polyethylene during casting of the insulator in the mold.

Studies performed on the quality of various parts of high-voltage equipment made of polystyrene, polyethylene, and fluoroplastic by radiography with X-ray radiation, as well as subsequent cutting of the radiographed specimens, showed that this method of inspection ensures reliable detection of defects in the macrostructure of polymeric electrical insulating materials that pose a danger during their operation.

LITERATURE

1. A. A. Vorob’ev, E. K. Zavadovskaya. Electrical Strength of Solid Dielectrics. Moscow, GITL, 1956.

2. B. V. Budalin, A. A. Vorob’ev. The Effect of Radiation on Ionic Structures. Gosatomizdat, Moscow, 1962.

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

4. Industrial Radiography. Translated from English, edited by A. S. Shtans and V. I. Sinitsyn. Gosatomizdat, Moscow, 1960.

5. Charlesby. Nuclear Radiation and Polymers. Moscow, Foreign Literature Publishing House, 1960.