Lecture No. 10
Flaw detection is a field of knowledge that covers the theory, methods and technical means of determining defects in the material of controlled objects, in particular in the material of machine parts and metal structure elements.
Flaw detection is an integral part of diagnosing the technical condition of equipment and its components. Work related to the identification of defects in the material of equipment elements is combined with repairs and maintenance or carried out independently during the period of technical inspection.
To identify hidden defects in structural materials, various non-destructive testing (flaw detection) methods are used.
It is known that defects in a metal cause changes in its physical characteristics: density, electrical conductivity, magnetic permeability, elastic and other properties. The study of these characteristics and the detection of defects with their help is the physical essence of non-destructive testing methods. These methods are based on the use of penetrating radiation of x-rays and gamma rays, magnetic and electromagnetic fields, vibrations, optical spectra, capillarity phenomena and others.
According to GOST 18353, non-destructive testing methods are classified by type: acoustic, magnetic, optical, penetrating substances, radiation, radio wave, thermal, electrical, electromagnetic. Each type is a conditional group of methods united by common physical characteristics.
The choice of the type of flaw detection depends on the material, design and size of the parts, the nature of the detected defects and the flaw detection conditions (in workshops or on a machine). The main qualitative indicators of flaw detection methods are sensitivity, resolution, and reliability of results. Sensitivity– smallest sizes of detected defects; resolution– the smallest distance between two adjacent minimum detectable defects, measured in units of length or the number of lines per 1 mm (mm -1). Reliability of results– the probability of missing defects or rejecting suitable parts.
Acoustic methods are based on recording the parameters of elastic vibrations excited in the object under study. These methods are widely used to control the thickness of parts, imperfections (cracks, porosity, cavities, etc.) and physical and mechanical properties (grain size, intergranular corrosion, depth of the hardened layer, etc.) of the material. The control is performed based on an analysis of the nature of the propagation of sound waves in the material of the part (amplitude, phase, speed, angle of refraction, resonance phenomena). The method is suitable for parts whose material is capable of elastically resisting shear deformations (metals, porcelain, plexiglass, some plastics).
Depending on the frequency, acoustic waves are divided into infrared - with a frequency of up to 20 Hz, sound (from 20 to 2∙10 4 Hz), ultrasonic (from 2∙10 4 to 10 9 Hz) and hypersonic (over 10 9 Hz). Ultrasonic flaw detectors operate with ultrasonic signals from 0.5 to 10 MHz.
The main disadvantages of ultrasonic methods include the need for a sufficiently high cleanliness of the surface of parts and the significant dependence of the quality of control on the qualifications of the flaw detector operator.
Magnetic methods are based on registration of magnetic scattering fields over defects or magnetic properties of the controlled object. They are used to detect surface and subsurface defects in parts of various shapes made of ferromagnetic materials.
In the magnetic particle method, magnetic powders (dry method) or their suspensions (wet method) are used to detect magnetic leakage flux. The developing material is applied to the surface of the product. Under the influence of a magnetic scattering field, powder particles are concentrated near the defect. The shape of its clusters corresponds to the outline of the defect.
The essence of the magnetographic method is to magnetize the product while simultaneously recording a magnetic field on a magnetic tape that covers the part, and then deciphering the information received.
The magnetic lines of force of the resulting field are directed along helical lines to the surface of the product, which makes it possible to detect defects of different directions.
After inspection, all parts, except defective ones, are demagnetized. Restoring non-demagnetized parts by mechanical processing can lead to damage to the working surfaces due to the attraction of chips. You should not demagnetize parts that are subjected to heating during restoration by welding, surfacing and other methods to a temperature of 600...700 o C.
The degree of demagnetization is controlled by showering the parts with steel powder. For well-demagnetized parts, the powder should not be retained on the surface. For the same purposes, devices equipped with fluxgate pole detectors are used.
To inspect parts using the magnetic particle method, stationary, portable and mobile flaw detectors are commercially produced. The latter include: current sources, devices for supplying current, magnetizing parts and for applying magnetic powder or suspension, electrical measuring equipment. Stationary devices are characterized by high power and performance. All types of magnetization can be carried out on them.
Eddy current methods are based on the analysis of the interaction of an external electromagnetic field with the electromagnetic field of eddy currents induced by an exciting coil in an electrically conductive object.
Eddy current methods make it possible to detect surface defects, including those under a layer of metal and non-metallic coatings, control the dimensions of coatings and parts (diameters of balls, pipes, wires, sheet thickness, etc.), determine the physical and mechanical properties of materials (hardness, structure, depth nitriding, etc.), measure vibrations and movements of parts during machine operation.
Flaw detection of parts radiation methods is based on recording the weakening of the intensity of radioactive radiation when passing through a controlled object. The most commonly used are X-ray and γ-inspection of parts and welds. The industry produces both mobile X-ray machines for work in workshops and portable ones for work in the field. Registration of radiation monitoring results is carried out visually (images on screens, including stereoscopic images), in the form of electrical signals, and recording on photographic film or plain paper (xeroradiography).
Advantages of radiation methods: high quality control, especially casting, welds, the state of closed cavities of machine elements; possibility of documentary confirmation of control results, which does not require additional decoding. Significant disadvantages are the complexity of the equipment and the organization of work related to ensuring the safe storage and use of radiation sources.
Radio wave methods are based on recording changes in electromagnetic oscillations interacting with the controlled object. In practice, ultra-high frequency (microwave) methods have become widespread in the wavelength range from 1 to 100 mm. The interaction of radio waves with an object is assessed by the nature of absorption, diffraction, reflection, refraction of the wave, interference processes, and resonance effects. These methods are used to control the quality and geometric parameters of products made of plastics, fiberglass, thermal protective and thermal insulation materials, as well as to measure vibration.
Thermal methods. In thermal methods, thermal energy propagating in an object, emitted by an object, and absorbed by an object is used as a diagnostic parameter. The temperature field of the surface of an object is a source of information about the characteristics of heat transfer processes, which, in turn, depend on the presence of internal and external defects, cooling of the object or part of it as a result of the outflow of a medium, etc.
The temperature field is monitored using thermometers, temperature indicators, pyrometers, radiometers, infrared microscopes, thermal imagers and other means.
Optical methods. Optical non-destructive testing is based on the analysis of the interaction of optical radiation with an object. To obtain information, the phenomena of interference, diffraction, polarization, refraction, reflection, absorption, light scattering are used, as well as changes in the characteristics of the object of study itself as a result of the effects of photoconductivity, luminescence, photoelasticity and others.
Defects detected by optical methods include discontinuities, delaminations, pores, cracks, inclusions of foreign bodies, changes in the structure of materials, corrosion cavities, deviation of the geometric shape from a given one, as well as internal stresses in the material.
Visual entroscopy allows you to detect defects on the surfaces of an object. Entroscopes (video borescopes) for internal examination of hard-to-reach areas of an object include a fiberglass probe, with which the researcher can penetrate inside the object, and a screen for visual observation of the surface, as well as a printer for video recording of the examined surface of the object. The use of optical quantum generators (lasers) makes it possible to expand the boundaries of traditional optical control methods and create fundamentally new methods of optical control: holographic, acousto-optical.
Capillary method flaw detection is based on the capillary penetration of indicator liquids into the cavities of surface and through discontinuities of an object, and registration of the resulting indicator traces visually or using a transducer (sensor).
Capillary methods are used to detect defects in parts of simple and complex shapes. These methods make it possible to detect defects of production, technological and operational origin: grinding cracks, thermal cracks, fatigue cracks, hairline cracks, sunsets, etc. Kerosene, colored, luminescent and radioactive liquids are used as penetrating substances, and the method of selectively filtered particles is also used.
When using colored liquids, the indicator pattern is colored, usually red, which stands out well against the white background of the developer - color flaw detection. When using luminescent liquids, the indicator pattern becomes clearly visible under the influence of ultraviolet rays - the luminescent method. Control of the nature of indicator patterns is carried out using a visual-optical method. In this case, the lines of the pattern are detected relatively easily, since they are tens of times wider and more contrasting than defects.
The simplest example of penetrant flaw detection is a kerosene test. The penetrating liquid is kerosene. The developer is chalk in the form of a dry powder or an aqueous suspension. Kerosene, seeping into the chalk layer, causes its darkening, which is detected in daylight.
The advantages of penetrant flaw detection are versatility in terms of shape and materials of parts, good clarity of results, simplicity and low cost of materials, high reliability and good sensitivity. In particular, the minimum dimensions of detectable cracks are: width 0.001 - 0.002 mm, depth 0.01 - 0.03 mm. Disadvantages: the ability to detect only surface defects, the long duration of the process (0.5 m - 1.5 hours) and labor intensity (the need for thorough cleaning), the toxicity of some penetrating liquids, insufficient reliability at subzero temperatures.
Cracks in parts can be detected using a kerosene test.
Kerosene has good wetting ability and penetrates deeply into through defects with a diameter of more than 0.1 mm. When controlling the quality of welds, kerosene is applied to one of the surfaces of the product, and an adsorbent coating (350...450 g of ground chalk suspension per 1 liter of water) is applied to the opposite surface. The presence of a through crack is determined by yellow stains of kerosene on the chalk coating.
Hydraulic and pneumatic testing methods are widely used to identify through pores and cracks.
With the hydraulic method, the internal cavity of the product is filled with working fluid (water), sealed, excess pressure is created with a pump and the part is kept for some time. The presence of a defect is determined visually by the appearance of water drops or sweating on the outer surface.
The pneumatic method for finding through defects is more sensitive than the hydraulic method, since air passes through the defect more easily than liquid. Compressed air is pumped into the internal cavity of the parts, and the outer surface is covered with a soap solution or the part is immersed in water. The presence of a defect is judged by the release of air bubbles. The air pressure pumped into the internal cavities depends on the design features of the parts and is usually equal to 0.05 - 0.1 MPa.
Non-destructive testing methods are not universal. Each of them can be used most effectively to detect specific defects. The choice of non-destructive testing method is determined by the specific requirements of practice and depends on the material, design of the object under study, the state of its surface, characteristics of defects to be detected, operating conditions of the object, control conditions and technical and economic indicators.
Surface and subsurface defects in ferromagnetic steels are detected by magnetizing the part and recording the stray field using magnetic methods. The same defects in products made from non-magnetic alloys, for example, heat-resistant, stainless, cannot be detected by magnetic methods. In this case, for example, the electromagnetic method is used. However, this method is also unsuitable for plastic products. In this case, the capillary method turns out to be effective. The ultrasonic method is ineffective in identifying internal defects in cast structures and alloys with a high degree of anisotropy. Such structures are monitored using X-rays or gamma rays.
Design (shape and dimensions) of parts also determines your
boron control method. If almost all methods can be used to control an object of a simple shape, then the use of methods to control objects of a complex shape is limited. Objects with a large number of grooves, grooves, ledges, and geometric transitions are difficult to control using methods such as magnetic, ultrasonic, and radiation. Large objects are monitored in parts, identifying the most dangerous areas.
Surface condition product, by which we mean its roughness and the presence of protective coatings and contaminants on it, significantly influences the choice of method and preparation of the surface for research. The rough rough surface excludes the use of capillary methods, the eddy current method, magnetic and ultrasonic methods in the contact version. Low roughness expands the capabilities of defetoscopy methods. Ultrasonic and capillary methods are used for surface roughness of no more than 2.5 microns, magnetic and eddy current methods - no more than 10 microns. Protective coatings do not allow the use of optical, magnetic and capillary methods. These methods can only be used after the coating has been removed. If such removal is impossible, radiation and ultrasound methods are used. Using the electromagnetic method, cracks are detected on parts with paint and other non-metallic coatings up to 0.5 mm thick and non-metallic non-magnetic coatings up to 0.2 mm thick.
Defects have different origins and differ in type, size, location, and orientation relative to the metal fiber. When choosing a control method, you should study the nature of possible defects. By location, defects can be internal, located at a depth of more than 1 mm, subsurface (at a depth of up to 1 mm) and superficial. To detect internal defects in steel products, radiation and ultrasonic methods are most often used. If the products have a relatively small thickness, and the defects to be detected are quite large, then it is better to use radiation methods. If the thickness of the product in the direction of transmission is more than 100-150 mm or it is necessary to detect internal defects in it in the form of cracks or thin delaminations, then it is not advisable to use radiation methods, since the rays do not penetrate to such a depth and their direction is perpendicular to the direction of the cracks. In this case, ultrasonic testing is most appropriate.
Flaw detection is a modern diagnostic method that allows you to identify defects in welding and internal structures of materials without destroying them. This diagnostic method is used to check the quality of welds and to determine the strength of metal elements. Let's talk in more detail about various flaw detection methods.
When performing welding work, it is not always possible to ensure a high-quality connection, which leads to a deterioration in the strength of the metal elements made. To determine the presence of such defects, special equipment is used that can detect deviations in the structure or composition of the material being tested. Flaw detection examines the physical properties of materials by exposing them to infrared and x-ray radiation, radio waves and ultrasonic vibrations. Such research can be carried out both visually and using special optical instruments. Modern equipment allows us to determine the slightest deviations in the physical structure of the material and identify even microscopic defects that can affect the strength of the connection.
It should be said that various flaw detection methods have their advantages and disadvantages. It is important to correctly select the optimal technology for each specific welded joint, which will ensure maximum accuracy in determining existing defects in metal alloys and welds.
In recent years, ultrasonic flaw detection technology has become most widespread, which is versatile in use and allows you to accurately determine existing structural inhomogeneities. Let us note the compactness of the equipment for ultrasonic flaw detection, the simplicity of the work performed and the productivity of such diagnostics. Currently, there are special installations for ultrasonic flaw detection, which make it possible to detect defects with an area of one square millimeter.
With the help of such multifunctional modern equipment, it is possible to determine not only existing damage and defects, but also to control the thickness of the material down to several millimeters of thickness. This allows us to significantly expand the scope of use of such equipment for flaw detection, the functionality of which has expanded significantly in recent years.
The use of such research in the production process and subsequent monitoring of metal welded products in use makes it possible to reduce time and money spent on quality control of manufactured materials and to most accurately determine the condition of various metal parts during their operation.
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Flaw detection is a modern method of testing and diagnostics. This is a highly effective tool for identifying defects in various materials. The method is based on the varying degree of absorption of X-rays by matter. The level of absorption depends on the density of the material and the atomic number of the elements included in its composition. Flaw detection is used in various fields of human activity: to detect cracks in forged machine parts, when examining the quality of steel, welds, and welding. This method is widely used to check the freshness of vegetable and fruit crops.
Flaw detection is a unifying name for several methods of non-destructive testing of materials, elements and products. They make it possible to detect cracks, deviations in the chemical composition, foreign objects, swelling, porosity, violation of homogeneity, specified dimensions and other defects. Buying equipment for flaw detection on the ASK-ROENTGEN website is convenient and simple. Such devices are in demand among enterprises that produce a variety of products. Flaw detection includes many methods:
There are many flaw detection techniques. They all serve one purpose - identifying defects. Using flaw detection, the structure of materials is examined and thickness is measured. E` use in production processes allows you to obtain a tangible economic effect. Flaw detection allows you to save metal. It helps prevent the destruction of structures, increasing durability and reliability.
Quality control of production and construction must be carried out at every stage. Sometimes it is necessary to check the operation of an object during operation. A device that helps carry out this kind of examination using a non-destructive method is called a flaw detector. There are a huge number of types of flaw detectors. They differ in operating principle and purpose. Learn the most popular flaw detection methods and useful recommendations for choosing a device so that you do not make a mistake when choosing and quickly master the work.
Depending on the purpose of flaw detection and the area of its application, the method of identifying damage and defects, on which the work of a particular flaw detector is based, changes radically.
Eddy current type device
Flaw detection is an activity aimed at identifying all possible deviations from the design and standards during production or operation of the facility. Flaw detection helps to detect a malfunction long before it makes itself felt. In this way, it is possible to prevent mechanical breakdowns, structural destruction and industrial accidents.
A flaw detector is a device designed to check and identify defects on the surface or in the body of various products. Defects can be very diverse. Some devices are needed to detect traces of corrosion, others to search for cavities, thinning, size discrepancies and other physical and mechanical defects, and still others can determine defects at the level of molecular structure - find changes in the structure of the body, its chemical composition.
Flaw detector with electronic display
The flaw detector belongs to the class of devices under the general name “non-destructive testing means”. During the production process, products are often subject to various checks. Some parts are tested in laboratories, where their strength margin and ability to withstand all kinds of loads and influences are determined. The disadvantage of this technique is that it is carried out selectively and does not guarantee 100% quality of all products.
Pipeline diagnostics
Non-destructive testing, which includes testing with a flaw detector, allows you to assess the condition of a specific product or structural element on site and without testing. The tool is indispensable in the following industries:
Non-destructive testing in aircraft manufacturing
A flaw detector is used to check the quality of the connection (this is especially important for welding high-pressure pipelines), the condition of the structure in construction (metal, reinforced concrete), the degree of wear of the mechanism, and the presence of damage to the part. In almost all industries where it is important to monitor the condition and compliance with standards of solid elements, various flaw detectors are used.
Depending on the testing method, the following types of flaw detectors are distinguished:
Ultrasonic flaw detector control panel
It is difficult to compare them; they are so different in structure, operation and even appearance that they are united only by their purpose. It is impossible to single out one of the devices and confidently say that it is the best, universal and will replace all the others. Therefore, when choosing, it is important not to make rash decisions and not buy the first model you come across.
The most popular flaw detectors that can be used to carry out non-destructive testing are: ultrasonic (acoustic), magnetic and eddy current. They are compact, mobile and easy to operate and understand the principle. Others are not used as widely, but each firmly occupies its niche among other flaw detection tools.
Types of flaw detection
An acoustic flaw detector is a concept that combines non-destructive testing devices that are similar in general principle. Acoustic flaw detection is based on the properties of the sound wave. It is known from a school physics course that the basic parameters of a wave do not change when moving in a homogeneous medium. However, if a new medium appears on the path of the wave, its frequency and length change.
The higher the sound frequency, the more accurate the result, so ultrasonic waves are used from the entire range. An ultrasonic flaw detector emits sound waves that pass through the object being tested. If there are cavities, inclusions of other materials or other defects, the ultrasonic wave will definitely indicate them by changing the parameters.
All results must be logged
Ultrasonic flaw detectors operating on the principle of the echo method are the most common and affordable. An ultrasonic wave penetrates an object; if no defects are detected, no reflection occurs, and accordingly, the device does not pick up or record anything. If a reflection of the ultrasound occurs, this indicates the presence of a flaw. The ultrasound generator is also a receiver, which is very convenient and facilitates flaw detection.
Ultrasonic type mini model
The mirror method is similar to echo, but uses two devices - a receiver and a transmitter. The advantage of this method is that both devices are located on the same side of the object, which facilitates the installation, configuration and measurement process.
Separately, there are methods for analyzing ultrasound that has passed right through an object. The concept of “sound shadow” is used. If there is a defect inside the object, it contributes to a sharp attenuation of vibrations, that is, it creates a shadow. The shadow method of ultrasonic flaw detection is based on this principle, when the generator and vibration receiver are located on the same acoustic axis from different sides.
Ultrasonic testing
The disadvantages of such a device are that there are strict requirements for the size, configuration and even the degree of surface roughness of the element being tested, which makes the device not entirely universal.
French physicist Jean Foucault devoted more than one year to studying eddy currents (Foucault currents), which arise in conductors when an alternating magnetic field is created in close proximity to them. Based on the fact that if there is a defect in the body, these same eddy currents create their own - a secondary magnetic field, eddy current devices carry out flaw detection.
An eddy current flaw detector creates an initial alternating magnetic field, but a secondary field, which makes it possible to identify and analyze a defect in an object, arises as a result of electromagnetic induction. The flaw detector detects the secondary field, records its parameters and draws a conclusion about the type and quality of the defect.
The performance of this device is high, the check is carried out quite quickly. However, eddy currents can only arise in those materials that are conductors, so the scope of application of such a device is much narrower than its analogues.
The device causes eddy currents in the material
Another common flaw detection method is magnetic particle testing. It is used to evaluate welded joints, the quality of the protective layer, the reliability of pipelines, and so on. This method is especially appreciated for checking complex-shaped elements and areas that are difficult to reach with other instruments.
The operating principle of a magnetic flaw detector is based on the physical properties of ferromagnetic materials. They have the ability to be magnetized. Using permanent magnets or special devices that can create a longitudinal or circular magnetic field.
After exposing an area of an object to a magnet, a so-called reagent - magnetic powder - is applied to it using a dry or wet method. Under the influence of a magnetic field that arises as a result of magnetization, the powder is connected into chains, structured and forms a clear pattern on the surface in the form of curved lines.
Magnetization with a special device
This figure clearly demonstrates the operation of a magnetic field. Knowing its features and basic parameters, using a magnetic flaw detector you can determine where the defect is located. As a rule, a pronounced accumulation of powder is observed directly above the flaw (crack or cavity). To determine the characteristics of the defect, the resulting image is checked against a standard.
Magnetic powder in spray
Flaw detection methods are improved every year. New techniques appear, others are gradually becoming obsolete. Many flaw detectors have a rather highly specialized purpose and are used only in certain industries.
The operating principle of a fluxgate flaw detector is based on the assessment of impulses generated when the device moves along an object. It is used in metallurgy, in the production of rolled metal and in the diagnosis of welded joints.
A radiation flaw detector irradiates an object with x-rays, alpha, beta, gamma radiation or neutrons. As a result, a detailed snapshot of the element is obtained with all defects and inhomogeneities present. The method is expensive, but very informative.
A capillary flaw detector detects surface cracks and discontinuities as a result of exposure of the object to a special developing substance. The result is assessed visually. Penetrant flaw detection is used mostly in mechanical engineering, aviation, and shipbuilding.
In the energy industry, an electron-optical flaw detector is used to analyze the operation and identify imperfections of elements under high voltage. It is able to detect the slightest changes in corona and surface partial discharges, which makes it possible to evaluate the operation of equipment without stopping it - remotely.
Radiation flaw detection images
The main parameters that you should pay attention to when choosing a flaw detector of any type:
Magnetic particle device MD-M
Different models differ in measurement range. This means that some are able to detect defects of 1 micron, while the limit for others is 10 mm, for example. If in mechanical engineering microcracks in parts play a significant role, then for flaw detection in construction there is no point in buying an ultra-precise device.
Also, the manufacturer must indicate for what materials a particular flaw detector is intended, and what type of defects it should detect. There may be requirements for the nature of the surface of the element, the presence of a protective layer, the size and shape of the object.
The “performance” parameter refers to the scanning speed and the amount of work that can be performed per unit of time using a specific flaw detector. Thus, eddy current and fluxgate methods provide high speed, while the process of magnetization and processing of each individual section with a magnetic tool can take quite a long time.
An important detail is installation. When choosing a flaw detector model, it makes sense to think about how long and how difficult it will be to install it. Hand-held mobile devices that can be taken out of a bag at any time are preferable for on-duty flaw detection during production or installation. More complex and precise equipment requires time-consuming installation and setup.
The ultrasonic device requires adjustment before starting work.
Since non-destructive testing can be carried out both indoors and outdoors, including in winter, check in advance whether the selected device can be operated at sub-zero temperatures. It is also necessary to find out whether it is permissible to perform diagnostics in an aggressive environment, if necessary.
Knowing how a flaw detector of one type or another works, you can easily decide on the main thing - the flaw detection method. An experienced consultant will help you decide on the model.
DEFECTOSCOPY(from Latin defectus - lack, flaw and Greek skopeo - examining, observing) - complex physical. methods and means of non-destructive quality control of materials, workpieces and products in order to detect defects in their structure. D. methods make it possible to more fully assess the quality of each product without destroying it and to carry out continuous control, which is especially important for responsible products. purposes for which selective destructive testing methods are insufficient.
Failure to comply with specified technical standards. parameters when processing complex chemical materials. and phase composition, exposure to aggressive environments and operating conditions. loads during storage of the product and during its operation can lead to the appearance of decomposition in the material of the product. type of defects - violations of continuity or homogeneity, deviations from a given chemical. composition, structure or dimensions that impair the performance characteristics of the product. Depending on the size of the defect in the area of its location, the physical parameters change. properties of the material - density, electrical conductivity, magnetic, elastic characteristics, etc.
D. methods are based on the analysis of distortions introduced by a defect into the physical components attached to the controlled product. fields divers. nature and the dependence of the resulting fields on the properties, structure and geometry of the product. Information about the resulting field allows one to judge the presence of a defect, its coordinates and size.
D. includes the development of non-destructive testing methods and equipment - flaw detectors, devices for testing, systems for processing and recording the information received. Optical, radiation, magnetic, acoustic, el-magnetic are used. (eddy current), electric and other methods.
Optical D. is based on direct. inspecting the surface of the product with the naked eye (visually) or using an optical lens. instruments (magnifying glass, microscope). To inspect the internal surfaces, deep cavities and hard-to-reach places use special. endoscopes are diopter tubes containing light guides made of fiber optics, equipped with miniature illuminators, prisms and lenses. Optical methods D. in the visible range, it is possible to detect only surface defects (cracks, films, etc.) in products made from materials that are opaque to visible light, as well as surface and internal defects. defects - in transparent ones. Min. the size of the defect detectable visually with the naked eye is 0.1-0.2 mm, when using optical. systems - tens of microns. To control the geometry of parts (for example, thread profile, surface roughness), projectors, profilometers and microinterferometers are used. New implementation of optical A method that can significantly increase its resolution is laser diffraction, which uses diffraction of a coherent laser beam with indication using photoelectronic devices. When automating optical The control method is used by television. image transmission.
Radiation radiation is based on the dependence of the absorption of penetrating radiation on the length of the path traveled by it in the material of the product, on the density of the material and the atomic number of the elements included in its composition. The presence of discontinuities in the product, foreign inclusions, changes in density and thickness leads to decomposition. weakening of rays in different its sections. By registering the intensity distribution of transmitted radiation, it is possible to obtain information about the internal structure of the product, including judging the presence, configuration and coordinates of defects. In this case, penetrating radiation of various types can be used. hardness: x-ray radiation with energies of 0.01-0.4 MeV; radiation received in linear (2-25 MeV) and cyclic. (betatron, microtron 4-45 MeV) accelerators or in an ampoule with -active radioisotopes (0.1-1 MeV); gamma radiation with energies of 0.08-1.2 MeV; neutron radiation with energies of 0.1-15 MeV.
Registration of the intensity of transmitted radiation is carried out separately. ways - photographic. method with obtaining an image of a transilluminated product on photographic film (film radiography), on reusable xeroradiographic. plate (electroradiography); visually, observing images of the transilluminated product on a fluorescent screen (radioscopy); using electron-optical converters (x-ray television); measuring the intensity of radiation special. indicators, the action of which is based on the ionization of gas by radiation (radiometry).
Sensitivity of radiation methods D. is determined by the ratio of the extent of a defect or zone having a different density in the direction of transmission to the thickness of the product in this section and for decomp. materials ranges from 1 to 10% of its thickness. Application of X-ray D. effective for products cf. thicknesses (steel up to ~80 mm, light alloys up to ~250 mm). Ultra-hard radiation with an energy of tens of MeV (betatron) makes it possible to illuminate steel products up to ~500 mm thick. Gamma-D. characterized by a greater compactness of the radiation source, which makes it possible to control hard-to-reach areas of products up to ~250 mm thick (steel), moreover, in conditions where X-ray. D. difficult. Neutron D. max. effective for testing thin products made from low-density materials. One of the new methods of X-ray control is calculating. tomography based on radiometric processing. information using a computer, obtained by repeatedly scanning products at different angles. In this case, it is possible to visualize layers of internal images. product structure. When working with sources of ionizing radiation, appropriate biol. protection.
Radio wave D. is based on changes in electromagnetic parameters. waves (amplitude, phase, direction of the polarization vector) of the centimeter and millimeter range when they propagate in products made of dielectric materials (plastics, rubber, paper).
The source of radiation (usually coherent, polarized) is a microwave generator (magnetron, klystron) of low power, feeding a waveguide or special. antenna (probe) transmitting radiation to the controlled product. The same antenna, when receiving reflected radiation, or a similar one, located on the opposite side of the product, when receiving transmitted radiation, supplies the received signal through an amplifier to the indicator. The sensitivity of the method allows you to detect delaminations with an area of 1 cm 2 in dielectrics at a depth of up to 15-20 mm, measure the moisture content of paper, bulk materials with an error of less than 1%, the thickness of metallic materials. sheet with an error of less than 0.1 mm, etc. It is possible to visualize the image of the controlled area on the screen (radio imager), fix it on photographic paper, as well as use holographic. ways to capture images.
Thermal (infrared) D. is based on the dependence of the body surface temperature in both stationary and non-stationary fields on the presence of a defect and heterogeneity of the body structure. In this case, IR radiation is used in the low temperature range. The temperature distribution on the surface of the controlled product, arising in transmitted, reflected or self-radiation, is an IR image of a given area of the product. By scanning the surface with a radiation receiver sensitive to IR rays (a thermistor or pyroelectric), on the screen of the device (thermal imager) you can observe the entire cut-off or color image, the temperature distribution across sections, or, finally, select a section. isotherms. The sensitivity of thermal imagers allows recording a temperature difference of less than 1 o C on the surface of a product. The sensitivity of the method depends on the size ratio d defect or heterogeneity to depth l its occurrence is approximately as ( d/l) 2, as well as on the thermal conductivity of the product material (inversely proportional relationship). Using the thermal method, it is possible to control products that heat up (cool) during operation.
Magnetic D. can only be used for ferromagnetic products. alloys and is sold in two versions. The first is based on the analysis of magnetic parameters. stray fields arising in the zones of location of surface and subsurface defects in magnetized products, the second - on the dependence of magnetic. properties of materials from their structure and chemistry. composition.
When testing using the first method, the product is magnetized using electromagnets, solenoids, by passing current through the product or a rod passed through a hole in the product, or by inducing a current in the product. For magnetization, constant, alternating and pulsed magnetic fields are used. Optim. control conditions are created when the defect is oriented perpendicular to the direction of the magnetizing field. For magnetically hard materials, control is carried out in the field of residual magnetization, for magnetically soft materials - in the applied field.
Magnetic indicator the defect field can serve as a magnetic field. powder, e.g. Highly dispersed magnetite (magnetic powder method), coloring (to control products with a dark surface) or fluorescent (to increase sensitivity) components are sometimes added to rum. After sprinkling or pouring a suspension of a magnetized product, powder particles settle on the edges of defects and are observed visually. The sensitivity of this method is high - cracks with a depth of ~25 µm and an opening of -2 µm are detected.
With magnetographic In this method, the indicator is a magnet. the tape, the edges, is pressed against the product and is magnetized along with it. Rejection is carried out based on the results of analysis of the magnetic recording. tape. The sensitivity of the method to surface defects is the same as that of the powder method, and to deep defects it is higher - at a depth of up to 20-25 mm, defects with a depth of 10-15% of the thickness are detected.
Passive induction converters can be used as an indicator of the defect field. Product moving with relative. at a speed of up to 5 m/s or more, after passing through the magnetizing device, it passes through the converter, inducing a signal in its coils containing information about the parameters of the defect. This method is effective for monitoring metal during the rolling process, as well as for monitoring railway rails.
The fluxgate indication method uses active transducers - fluxgates, in which coils are wound on a thin permalloy core: exciting, the field of the cut interacts with the field of the defect, and measuring, by the emf of the cut the strength of the field of the defect or the gradient of this field is judged. The fluxgate indicator allows you to detect defects with a length (in depth) of ~10% of the product thickness in products of simple shape, moving at a speed of up to 3 m/s, at a depth of up to 10 mm. To indicate the defect field, converters based on Hall effect and magnetoresistive. After testing using magnetic magnetic resonance methods, the product must be thoroughly demagnetized.
The second group of magnetic methods. D. serves to control the structural state, thermal regimes. processing, mechanical properties of the material. So, coercive force carbon and low alloy. steel is correlated with carbon content and therefore hardness, magnetic permeability- with the content of a ferrite component (oc-phase), the maximum content of the cut is limited due to the deterioration of mechanical properties. and technological properties of the material. Specialist. devices (ferritometers, a-phase meters, coercimeters, magnetic analyzers) using the relationship between magnetic. characteristics and other properties of the material, also allow you to practically solve magnetic problems. D.
Magnetic methods D. are also used to measure the thickness of protective coatings on ferromagnetic products. materials. Devices for these purposes are based either on ponderomotive action - in this case, the force of attraction (separation) of the DC is measured. magnet or electromagnet from the surface of the product to which it is pressed, or by measuring the magnetic tension. fields (using Hall sensors, fluxgates) in the magnetic circuit of an electromagnet installed on this surface. Thickness gauges allow measurements in a wide range of coating thicknesses (up to hundreds of microns) with an error not exceeding 1-10 microns.
Acoustic(ultrasonic) D. uses elastic waves (longitudinal, shear, surface, normal, bending) of a wide frequency range (mainly ultrasonic range), emitted in a continuous or pulsed mode and introduced into the product using piezoelectric. (less often - el-magnetoacoustic) converter excited by an el-magnetic generator. hesitation. Propagating in the material of the product, elastic waves attenuate into decomposition. degrees, and when they encounter defects (violations of continuity or homogeneity of the material), they are reflected, refracted and scattered, while changing their amplitude, phase and other parameters. They are accepted by the same or separately. converter and, after appropriate processing, the signal is supplied to an indicator or recording device. There are several acoustic options D., which can be used in various combinations.
The echo method is an ultrasonic location in a solid medium; this is the most universal and widespread method. Pulses of an ultrasonic frequency of 0.5-15 MHz are introduced into the controlled product and the intensity and time of arrival of echo signals reflected from the surfaces of the product and from defects are recorded. Control using the echo method is carried out with one-sided access to the product by scanning its surface with a finder at a given speed and step at optimal. US input angle. The method is highly sensitive and is limited by structural noise. In optimal conditions, defects of several sizes can be detected. tenths of mm. The disadvantage of the echo method is the presence of an uncontrolled dead zone near the surface, the extent of the cut (depth) is determined by Ch. arr. the duration of the emitted pulse and is usually 2-8 mm. The echo method effectively controls ingots, shaped castings, and metallurgical materials. semi-finished products, welded, glued, soldered, riveted joints and other structural elements during manufacturing, storage and operation. Superficial and internal are detected. defects in workpieces and products shapes and dimensions made of metals and non-metallic. materials, zones of violation of crystalline homogeneity. structure and corrosion damage to metal. products. The thickness of the product can be measured with high accuracy with one-sided access to it. A variant of the echo method using Lamb waves, which have a full-flowing nature of distribution, allows for control of long-length sheet semi-finished products with high productivity; The limitation is the requirement for constant thickness of the controlled semi-finished product. Control using Rayleigh waves allows you to identify surface and near-surface defects; The limitation is the requirement for high surface smoothness.
The shadow method involves introducing ultrasound from one side of the product, and receiving it from the opposite side. The presence of a defect is judged by a decrease in amplitude in the zone of the sound shadow formed behind the defect, or by a change in the phase or time of reception of the signal enveloping the defect (time version of the method). With one-sided access to the product, a mirror version of the shadow method is used, in which the indicator of a defect is a decrease in the signal reflected from the bottom of the product. The shadow method is inferior in sensitivity to the echo method, but its advantage is the absence of a dead zone.
The resonance method is used in Chap. arr. to measure the thickness of the product. By exciting ultrasonic vibrations in the local volume of the wall of the product, they are modulated in frequency within 2-3 octaves, and from the values of the resonant frequencies (when an integer number of half-waves fits along the wall thickness) the thickness of the wall of the product is determined with an error of approx. 1%. When vibrations are excited throughout the entire volume of the product (integrated version of the method), one can also judge by the change in the resonant frequency the presence of defects or changes in the elastic characteristics of the material of the product.
The free vibration method (integral version) is based on shock excitation of elastic vibrations in a controlled product (for example, a striking LF vibrator) and subsequent measurement using a mechanical piezoelectric element. vibrations, by changes in the spectrum of which the presence of a defect is judged. The method is successfully used to control the quality of gluing low-quality materials (textolite, plywood, etc.) to each other and to metal. sheathing.
The impedance method is based on measuring local mechanical strength. resistance (impedance) of the controlled product. The impedance flaw detector sensor, operating at a frequency of 1.0-8.0 kHz, being pressed to the surface of the product, reacts to the reaction force of the product at the pressing point. The method allows you to determine delaminations with an area of 20-30 mm 2 in glued and soldered structures with metal. and non-metallic. filling, in laminates, as well as in clad sheets and pipes.
The velocimetric method is based on changing the speed of propagation of bending waves in a plate depending on the thickness of the plate or on the presence of delaminations inside a multilayer glued structure. The method is implemented at low frequencies (20-70 kHz) and makes it possible to detect delaminations with an area of 2-15 cm 2 (depending on depth), located at a depth of up to 25 mm in products made of laminated plastics.
Acoustic-topographical The method is based on the observation of vibration modes, including “Chladni figures,” using finely dispersed powder when excitation of bending vibrations with a modulated (within 30-200 kHz) frequency in a controlled product. Powder particles, moving from surface areas oscillating with max. amplitude, to the areas where this amplitude is minimal, the contours of the defect are outlined. The method is effective for testing products such as multilayer sheets and panels and allows you to detect defects with a length of 1 - 1.5 mm.
Acoustic method emission (related to passive methods) is based on the analysis of signals characterizing stress waves emitted when cracks appear and develop in a product during its mechanical process. or thermal loading. The signals are received piezoelectrically. finders located on the surface of the products. The amplitude, intensity and other parameters of the signals contain information about the initiation and development of fatigue cracks, stress corrosion and phase transformations in the material of structural elements, etc. types, welds, pressure vessels, etc. Acoustic method. emissions allows you to detect developing ones, i.e. most. dangerous defects and separate them from defects detected by other methods, non-developing ones, less dangerous for the further operation of the product. The sensitivity of this method when using special measures to protect the receiving device from the effects of external noise interference are quite high and make it possible to detect cracks at the beginning. stages of their development, long before the product’s service life is exhausted.
Promising directions for the development of acoustics. control methods are sound vision, including acoustic. holography, acoustic tomography.
Eddy current(electroinductive) D. is based on recording electrical changes. parameters of the eddy current flaw detector sensor (impedance of its coil or emf), caused by the interaction of the field of eddy currents excited by this sensor in a product made of electrically conductive material with the field of the sensor itself. The resulting field contains information about changes in electrical conductivity and magnetic field. permeability due to the presence of structural inhomogeneities or discontinuities in the metal, as well as the shape and size (thickness) of the product or coating.
Sensors of eddy current flaw detectors are made in the form of inductance coils placed inside the controlled product or surrounding it (pass-through sensor) or applied to the product (applied sensor). In screen-type sensors (pass-through and overhead), the controlled product is located between the coils. Eddy current testing does not require mechanical contact of the sensor with the product, which allows monitoring at high speeds. movements (up to 50 m/s). Eddy current flaw detectors are divided into traces. basic groups: 1) devices for detecting discontinuities with pass-through or clamp-on sensors operating in a wide frequency range - from 200 Hz to tens of MHz (increasing the frequency increases sensitivity to the length of cracks, since small-sized sensors can be used). This allows you to identify cracks, non-metallic films. inclusions and other defects with a length of 1-2 mm at a depth of 0.1-0.2 mm (with a surface-mounted sensor) or with a length of 1 mm at a depth of 1-5% of the diameter of the product (with a pass-through sensor). 2) Devices for controlling dimensions - thickness gauges, with the help of which the thickness of decomposition is measured. coatings applied to the base from decomposition. materials. Determination of the thickness of non-conductive coatings on electrically conductive substrates, which is essentially a measurement of the gap, is carried out at frequencies up to 10 MHz with an error within 1-15% of the measured value.
To determine the thickness of electrically conductive galvanic. or cladding. coatings on an electrically conductive base, eddy current thickness gauges are used, in which special ones are implemented. schemes for suppressing the influence of changes in beats. electrical conductivity of the base material and changes in the gap size.
Eddy current thickness gauges are used to measure the wall thickness of pipes and non-ferromagnetic cylinders. materials, as well as sheets and foils. Measuring range 0.03-10 mm, error 0.6-2%.
3) Eddy current structure meters allow, by analyzing the beat values. electrical conductivity and magnetic permeability, as well as parameters of higher voltage harmonics, judge the chemical. composition, structural state of the material, internal size. stress, sort products by material grade, thermal quality. processing, etc. It is possible to identify zones of structural heterogeneity, fatigue zones, estimate the depth of decarbonized layers, thermal layers. and chemical-thermal. processing, etc. For this, depending on the specific purpose of the device, either high-intensity LF fields, or low-intensity HF fields, or dual- and multi-frequency fields are used. In structure meters, to increase the amount of information taken from the sensor, as a rule, they are used multi-frequency fields and spectral analysis of the signal is carried out. Instruments for monitoring ferromagnetic materials operate in the low-frequency range (50 Hz-10 kHz), to control non-ferromagnetic materials - in the high-frequency range (10 kHz-10 mHz), which is due to the dependence of the skin effect on the magnetic value. permeability.
Electrical D. is based on the use of weak DC. currents and electric static. fields and is carried out by electric contact, thermoelectric, triboelectric. and el-static. methods. The electronic contact method makes it possible to detect surface and subsurface defects by changes in electrical resistance on the surface of the product in the area where this defect is located. With the help of special contacts located at a distance of 10-12 mm from one another and tightly pressed to the surface of the product, current is supplied, and on another pair of contacts located on the current line, a voltage proportional to the resistance in the area between them is measured. A change in resistance indicates a violation of the homogeneity of the material structure or the presence of a crack. The measurement error is 5-10%, which is due to the instability of the current and measurement resistance. contacts.
Thermoelectric The method is based on measuring the thermoelectromotive force (TEMF) generated in a closed circuit when the contact point between two dissimilar metals is heated. If one of these metals is taken as a standard, then for a given temperature difference between the hot and cold contacts, the value and sign of the thermoelectric force will be determined by the properties of the second metal. Using this method, you can determine the grade of metal from which a workpiece or structural element is made, if the number of possible options is small (2-3 grades).
Triboelectric The method is based on measuring triboEMF that occurs when dissimilar metals rub against each other. By measuring the potential difference between the reference and test metals, it is possible to distinguish between brands of certain alloys. Change in chem. alloy composition within the limits permitted by technical standards. conditions, leads to scattering of thermo- and triboelectric readings. devices. Therefore, both of these methods can be used only in cases of sharp differences in the properties of the alloys being sorted.
El-static method is based on the use of ponderomotive forces el-static. fields in which the product is placed. To detect surface cracks in metal coatings. Its products are pollinated with fine chalk powder from a spray bottle with an ebonite tip. Chalk particles, when rubbed against ebonite, become positively charged due to triboelectricity. effect and settle on the edges of cracks, since near the latter there is heterogeneity of el-static. fields expressed at most. noticeable. If the product is made of non-electrically conductive materials, then it is pre-wetted with an ionogenic penetrant and after removing its excess from the surface of the product, a charge is powdered. chalk particles, which are attracted by the liquid filling the crack cavity. In this case, it is possible to detect cracks that do not extend to the surface being inspected.
Capillary D. is based on the arts. increasing the color and light contrast of the area of the product containing surface cracks relative to the surrounding surface. Implemented ch. arr. luminescent and color methods, allowing to detect cracks, detection of which with the naked eye is impossible due to their small size, and the use of optical devices are ineffective due to insufficient image contrast and small field of view at the required magnifications.
To detect a crack, its cavity is filled with a penetrant - an indicator liquid based on phosphors or dyes, which penetrates into the cavity under the action of capillary forces. After this, the surface of the product is cleaned of excess penetrant, and the indicator liquid is extracted from the crack cavity using a developer (sorbent) in the form of a powder or suspension, and the product is examined in a darkened room under UV light (luminescent method). The luminescence of the indicator solution absorbed by the sorbent gives a clear picture of the location of cracks with a min. opening 0.01 mm, depth 0.03 mm and length 0.5 mm. With the color method, no shading is required. A penetrant containing a dye additive (usually bright red), after filling the crack cavity and cleaning the surface of its excess, diffuses into a white developing varnish applied in a thin layer to the surface of the product, clearly outlining the cracks. The sensitivity of both methods is approximately the same.
The advantage of capillary D. is its versatility and uniformity of technology for various parts. shapes, sizes and materials; disadvantage is the use of materials that are highly toxic, explosive and fire hazardous, which imposes special safety requirements.
The meaning of D. D. methods are used in various ways. areas of the national economy, helping to improve the technology of manufacturing products, improving their quality, extending service life and preventing accidents. Certain methods (chiefly acoustic) allow for periodic control of products during their operation, assess the damageability of the material, which is especially important for predicting the residual life of critical products. In this regard, the requirements for the reliability of information obtained when using data methods, as well as for control performance, are constantly increasing. Because metrological The characteristics of flaw detectors are low and their readings are influenced by many random factors; assessment of inspection results can only be probabilistic. Along with the development of new methods of D., main. direction of improving existing ones - automation of control, use of multi-parameter methods, use of computers for processing the received information, improvement of metrological. characteristics of the equipment in order to increase the reliability and performance of control, the use of internal visualization methods. structure and defects of the product.
Lit.: Schreiber D.S., Ultrasonic flaw detection, M., 1965; Non-destructive testing. (Handbook), ed. D. McMaster, trans. from English, book. 1-2, M.-L., 1965; Falkevich A. S., Khusanov M. X., Magnetographic testing of welded joints, M., 1966; Dorofeev A.L., Electroinductive (induction) flaw detection, M., 1967; Rumyantsev S.V., Radiation defectoscopy, 2nd ed., M., 1974; Instruments for non-destructive testing of materials and products, ed. V.V. Klyueva, [vol. 1-2], M., 1976; Non-destructive testing of metals and products, ed. G. S. Samoilovich, M., 1976. D. S. Schreiber.
