Flaw Detection

an aggregate of methods of nondestructive testing of materials and products in order to detect defects. It includes the development of methods and apparatus (such as defectoscopes), the creation of testing procedures, and the processing of the data of defectoscopes.

Various defects—disruptions of the continuity or homogeneity of a material or deviation from the prescribed chemical composition, structure, or dimensions—occur in products as a result of the imperfect state of manufacturing processes or of operation under difficult conditions. Defects alter the physical properties of a material (such as density, electrical conductivity, and magnetic and elastic properties). The basis for existing methods of flaw detection is the investigation of the physical properties of materials upon exposure to X rays; infrared, ultraviolet, and gamma rays; radio waves; ultrasonic vibrations; and magnetic and electrostatic fields.

The simplest method of flaw detection is the visual method—by the naked eye or with the help of optical devices such as magnifying glasses. Special tubes with prisms and miniature illuminators (dioptric tubes) and television tubes are used to examine internal surfaces, deep cavities, and hard-to-reach places. Lasers are also used for testing—for example, the surface of a fine wire. Visual flaw detection allows only the detection of surface flaws (such as cracks or scabs) in metal products and internal flaws in products made of glass or plastics that are transparent to visible light. The minimum size of flaws detectable by the naked eye is 0.1-0.2 mm, or a few dozen microns when optical systems are used.

X-ray flaw detection is based on the absorption of X rays, which depends on the density of the medium and the atomic number of the elements that form the material of the medium. The presence of such defects as cracks, blisters, or inclusions of foreign material leads to a situation in which the rays passing through the material (Figure 1) are attenuated to a varying degree. By recording the distribution of the intensity of the transient rays it is possible to determine the presence and location of various discontinuities in the material.

Figure 1 Diagram of X-raying: (1) source of X radiation, (2) X-ray beam, (3) part being tested, (4) internal defect in the part, (5) X-ray image behind the part (invisible to the eye), (6) X-ray image recorder

The intensity of the X rays is recorded by several methods. Photographic methods are used to record the article on film. The visual method is based on observation of the image of an article on a fluorescent screen. This method is more efficient when electro-optical image converters are used. In the xerographic method, images are produced on metal plates that are coated with a layer of a substance whose surface has been given an electrostatic charge. Contrast pictures are taken on plates that can be used repeatedly. The ionization method is based on measurement of the intensity of the electromagnetic radiation by its ionizing effect (for example, on a gas). In this case the indicator may be placed at a considerable distance from the product, which makes possible the testing of products heated to high temperatures.

The sensitivity of the methods of X-ray flaw detection is determined by the ratio of the length of the defect in the direction of radioscopy to the thickness of the part in this cross section; it is 1-10 percent for various materials. The use of X-ray flaw detection is effective only for comparatively thin articles, since the penetrating power of X rays increases insignificantly with an increase in their energy. X-ray flaw detection is used to find blisters, deep cracks, and liquation inclusions in cast and welded steel products up to 80 mm thick and in products made of light alloys up to 250 mm thick. Industrial X-ray units with a radiant energy of 5-10 to 200-400 kilo electron volts (kev; 1 eV = 1.60210 x 10^-19 joules) have been created for this purpose. Products of great thickness (up to 500 mm) are irradiated with ultrahard electromagnetic radiation with an energy of dozens of MeV generated in a betatron.

Gamma-ray flaw detection has the same physical principles as X-ray flaw detection, but the radiation of gamma rays emitted by artificial radioactive isotopes of various metals (such as cobalt, iridium, or europium) is used. A radiant energy of several dozen kev to 1-2 MeV is used to irradiate thick parts. This method has significant advantages over X-ray flaw detection: the equipment is comparatively simple and the radiation source is compact, making it possible to examine hard-to-reach parts of products. Moreover, this method can be used when X-ray flaw detection would be difficult (for example, in field tests). In working with sources of X rays and gamma radiation, biological protection must be provided.

Radiographic inspection is based on the penetrating properties of radio waves in the centimeter and millimeter bands (microwaves) and makes it possible to detect flaws primarily on the surface of products that are made of nonmetallic materials. Because of the low penetrating power of microwaves, the radiographic inspection of metal products is limited. This method is used to find defects in sheet steel, steel bars, and steel wire during their manufacture and to measure their thickness or diameter, the thickness of dielectric coatings, and so on. Microwaves from a generator operating in continuous or pulse mode penetrate the product through electromagnetic horns and, after passing through an amplifier, are recorded by a receiver.

Infrared flaw detection uses infrared (heat) rays to detect inclusions that are opaque to visible light. A so-called infrared image of the defect is produced in the transient, reflected, or intrinsic radiation of the product under investigation. This method is used to test articles that heat up during operation: defective parts in the product change the heat flux. An infrared radiation flux is passed through the product and its distribution is recorded by a heat-sensitive receiver. Inhomogeneity of the structure of materials can also be investigated by the method of ultraviolet flaw detection.

Magnetic flaw detection is based on the study of distortions of the magnetic field that occur at the sites of defects in products made of ferromagnetic materials. A magnetic powder (ferrous or ferric oxide) or its suspension in oil with a particulate dispersion of 5-10 microns may serve as the indicator. Upon magnetization of the product, the powder precipitates at locations of defects (the magnetic powder method). The scattering field may be recorded on a magnetic tape that is applied to the section of the magnetized product being studied (the magnetographic method). Small sensors (ferroprobes) that, as they move over the article, show changes in the current impulse at the place of a defect that are recorded on an oscilloscope screen (the ferroprobe method) are also used.

The sensitivity of the method of magnetic flaw detection depends on the magnetic characteristics of the materials, the indicators used, and the types of magnetization of the product. The magnetic powder method can be used to detect cracks and other flaws to a depth of up to 2 mm; the magnetographic method is used primarily to test welded seams of pipes up to 10-12 mm thick and to detect fine cracks and poor penetration. The ferroprobe method is best suited for detecting defects at a depth of up to 10 mm and, in some cases, up to 20 mm in articles of regular shape. This method makes possible the complete automation of testing and rejection. The magnetization of the articles is accomplished by magnetic defectoscopes, which create magnetic fields of sufficient intensity. After testing, the articles are thoroughly demagnetized.

The methods of magnetic flaw detection are used to study the structure of materials (magnetic structurometry) and to measure thickness (magnetic thickness gauging). Magnetic structurometry is based on the determination of the basic magnetic characteristics of a material (coercive force, induction, residual magnetism, and magnetic permeability), which generally depend on the structural condition of an alloy subjected to various types of thermal treatment. Magnetic structurometry is used to determine structural components of an alloy that are present in it in small quantities and that differ significantly in their magnetic characteristics from the base of the alloy, as well as to measure such qualities as the depth of case hardening and surface hardening. Magnetic thickness gauging is based on measurement of the force of attraction of a permanent magnet or an electromagnet to the surface of a product made of a ferromagnetic material to which a layer of nonmagnetic coating has been applied; it makes possible the determination of the thickness of the coating.

Inductive (eddy-current) flaw detection is based on the excitation of eddy currents by the variable magnetic field of the defectoscope sensor. Eddy currents create their own field, which is opposite in sign to the exciting field. As a result of the interaction of these fields, the total resistance of the sensor coil changes, and the change is recorded by the indicator. The readings of the indicator depend on the electrical conductivity and magnetic permeability of the metal, the size of the article, and changes in electric conductivity caused by structural heterogeneities or to disruptions in the continuity of the metal.

The sensors of eddy-current defectoscopes are made in the form of induction coils within which the products are placed (pass-through sensors) or that are attached to the product (attachable sensors). The use of eddy-current flaw detection makes possible the automation of quality control of wire, bars, pipes, and sections that move at high speeds during their manufacture, as well as the continuous measurement of dimensions. Eddy-current defectoscopes may be used to monitor the quality of heat treatment, to estimate the contamination of highly conductive metals (such as copper or aluminum), to determine the depth of layers of chemical and heat treatment to an accuracy of up to 3 percent, to grade certain materials, to measure the electric conductivity of nonferromagnetic materials to an accuracy of up to 1 percent, and to detect surface cracks several microns deep and extending several tenths of a millimeter.

Thermoelectric flaw detection is based on the measurement of the electromotive or thermoelectromotive force that arises in a closed circuit when the point of contact of two different materials is heated. If one of these materials is taken as the standard, for a given difference in temperatures between the hot and cold contacts the magnitude and sign of the thermoelectromotive force will be determined by the chemical composition of the second material. This method is usually used in cases when it is necessary to determine the grade of the material from which an intermediate product or structural member, as well as finished structures, is made.

Triboelectric flaw detection is based on the measurement of the electromotive force that arises upon friction of materials of different natures. By measuring the potential difference between standard materials and the materials being tested, it is possible to distinguish the types of certain alloys.

Electrostatic flaw detection is based on the use of an electrostatic field, in which the article is placed. In order to observe surface cracks in articles made of nonconducting materials (porcelain, glass, and plastics), as well as of metals coated with these materials, the article is covered with a fine chalk powder from a pulverizer with an ebonite tip (the powder method). In this way the particles of chalk receive a positive charge. As a result of the inhomogeneity of the electrostatic field, the particles of chalk accumulate along the edges of the cracks. This method is also used to test articles made of insulation materials. Before spraying them with particles, it is necessary to wet them with an ionogenic liquid.

Ultrasonic flaw detection is based on the use of elastic oscillations, mainly in the ultrasonic frequency range. Disruption of the continuity or homogeneity of the medium affects the propagation of the elastic waves in the article or the conditions of oscillation of the article. The main ultrasonic methods are the echo, shadow, resonance, and velocimetric methods (ultrasonic methods proper) and the impedance method and method of free oscillations (acoustic methods).

The most versatile echo method is based on sending short pulses of ultrasonic oscillations into the article (Figure 2) and recording the intensity and return time of the echo signals reflected from defects. To test the article, the sensor of the echo defectoscope scans its surface. The method makes it possible to observe surface and internal defects with various orientations. Industrial installations for testing various types of articles have been created. The echo signals may be observed on the screen of an oscilloscope or may be recorded by a recording device. In the latter case the reliability and objectivity of the estimate, as well as the productivity and reproducibility of the testing, are increased. The sensitivity of the echo method is very high: under optimal testing conditions at a frequency of 2-4 megahertz (MHz) it is possible to observe defects whose reflecting surface is about 1 sq mm.

Figure 2 Block diagram of an ultrasonic echo defectoscope: (1) electrical pulse generator, (2) piezoelectric converter (selector head), (3) reception-amplification circuit, (4) timer, (5) scanning generator, (6) cathode tube. / is the initial signal, 8 is the bottom re-turn, and DF is a blip from the defect.

In the shadow method, ultrasonic oscillations that encounter a defect are reflected in the opposite direction. The presence of a defect may be judged by the reduction in energy of the ultrasonic oscillations or by the change in the phase of the oscillations that are passing around the defect. The method is widely used for testing weld seams and rails.

The resonance method is based on the determination of the intrinsic resonance frequencies of elastic oscillations with a frequency of 1-10 MHz excited inside the article. The thickness of the walls of metallic and some nonmetallic articles are measured in this way. When measurements can be made only from one side, the measurements are accurate to about 1 percent. In addition, it is possible through this method to discover zones of corrosion damage. Resonance defecto-scopes are used for both manual testing and automated testing (when the readings of the device are recorded).

The velocimetric method of echo flaw detection is based on the measurement of the change in speed of propagation of elastic waves in the zone where defects are located in multilayered structures; it is used to find areas of disruption of the cohesion between metal layers.

The impedance method is based on the measurement of the mechanical drag (impedance) of the article by a sensor that scans its surface and excites in it elastic oscillations of sonic frequency. This method makes it possible to find defects in laminated and soldered joints and between thin sheathing and stiffening members or fillers in multilayered structures. Defects with an area of 15 sq mm and more are marked by a signaling device and may be recorded automatically.

The method of free oscillations is based on the analysis of the free oscillation spectrum of the article being tested when it has been excited by an impact; it is used to find areas of breakdown of the joints between elements in multilayered laminated structures of considerable thickness made of metallic and nonmetallic materials.

Ultrasonic flaw detection, which uses several variable parameters (frequency range, type of waves, radiation conditions, and means of making contact), is one of the most versatile methods of nondestructive testing.

Capillary flaw detection is based on the artificial enhancement of the light and color contrast of the defective portion as compared to the undamaged portion. Methods of capillary flaw detection make it possible to find with the naked eye fine surface cracks and other discontinuities of the material that form during the manufacture and use of machine parts. The cavities of surface cracks are filled with special indicator substances (penetrants), which impregnate them under the influence of capillary forces. For the so-called luminescent method, the penetrants are based on phosphors (kerosine, noriol, and others). A developer in the form of a fine white powder (magnesium oxide or talc) having sorption properties is applied to the surface, which has been cleansed of excess penetrant; because of the powder’s sorption properties, particles of the penetrant are pulled from the cavities of the cracks, showing the contours of the cracks and glowing brightly under ultraviolet light. In the so-called color method of testing, the penetrants are made on a kerosine base, with the addition of benzene, turpentine, and special dyes (for example, red paint). To test an article with a dark surface, magnetic powder dyed with phosphors (the magnetoluminescent method) is used, which makes it easier to see fine cracks.

The sensitivity of the capillary flaw detection methods makes it possible to see surface cracks with an opening of less than 0.02 mm. However, the use of these methods is limited because of the high toxicity of the penetrants and developers.

Flaw detection is an equal and integral link in technological processes that makes possible an increase in the reliability of products. However, the methods of flaw detection are not absolute, since a number of random factors influence the results of the tests. It is possible to speak of the absence of flaws in an article only to a degree of certainty. The reliability of the tests is enhanced by automation, the perfection of methods, and the rational combination of several methods. The usefulness of articles is determined by the quality standards, which are formulated during the design stage and the development of the production technology. The quality standards are different for various types of articles, for articles of the same type that are operating under different conditions, and even for various areas of the same article if they are subject to different mechanical, thermal, or chemical influences.

The use of flaw detection in the production process and during the use of articles has a great economic effect in terms of a decrease in the time spent in processing articles with internal defects, as well as in terms of the consumption of metal. In addition, flaw detection plays a significant role in the prevention of damage to structures, thus increasing their reliability and service life.