Methods for determining hidden defects in structures flaw detection. Non-destructive testing and diagnostics - types of flaw detectors and their application. Other types and their principle of operation

Defectoscopy(from lat. defectus - defect, flaw and Greek skopeo - look) - a set of methods and means of non-destructive testing of materials and products to detect various defects in them. The latter include violations of the continuity or uniformity of the structure, corrosion damage zones, deviations in the chemical composition and dimensions, etc.

The most important methods of flaw detection are magnetic, electrical, eddy current, radio wave, thermal, optical, radiation, acoustic, penetrating substances. The best results are achieved with the complex use of different methods.

Magnetic, ultrasonic, and also X-ray flaw detection are used in cases where, during an external examination of a part, there is a suspicion of the presence of a hidden defect and when verification is provided for by the repair rules, in particular, when flaw detection devices are subject to verification according to the rules of Gosgortekhnadzor.

Magnetic flaw detection is based on registration of magnetic field distortions in places of defects. For indication use: magnetic powder or oil suspension of Fe 3 O 4 , the particles of which are deposited at the locations of defects (magnetic powder method); a magnetic tape (associated with a magnetic recording device) applied to the area under study and magnetized to varying degrees in defective and defect-free zones, which causes changes in current pulses recorded on the oscilloscope screen (magnetographic method); small-sized devices, which, when moving along the product at the site of a defect, indicate a distortion of the magnetic field (for example, a fluxgate metri). Magnetic flaw detection makes it possible to detect macrodefects (cracks, cavities, lack of penetration, delaminations) with a minimum size of > 0.1 mm at a depth of up to 10 mm in products made of ferri- and ferromagnetic materials (including in metal-filled plastics, metal-plastics, etc.) .

At electrical flaw detection fix the parameters of the electric field interacting with the object of control. The most common method that allows you to detect defects in dielectrics (diamond, quartz, mica, polystyrene, etc.) by changing the electrical capacitance when an object is introduced into it. Using the thermoelectric method, the EMF that occurs in a closed circuit is measured when the contact points of two dissimilar materials are heated. The method is used to determine the thickness of protective coatings, assess the quality of bimetallic materials, and sort products.

With the electrostatic method products made of dielectrics (porcelain, glass, plastics) or metals coated with dielectrics are placed in the field. Products using a spray gun are pollinated with highly dispersed chalk powder, the particles of which, due to friction against the ebonite tip of the spray gun, have a positive charge and, due to the difference in the dielectric constant of the intact and defective areas, accumulate at the edges of surface cracks.

The electropotential method is used to determine the depth (>> 5 mm) of cracks in electrically conductive materials by the distortion of the electric field when current flows around the defect.

Electrospark method, based on the occurrence of a discharge in places of discontinuity, allows you to control the quality of non-conductive (paint, enamel, etc.) coatings with a maximum thickness of 10 mm on metal parts. The voltage between the electrodes of the probe installed on the coating and the metal surface is about 40 kV.

Eddy current flaw detection is based on a change in the field of eddy currents in places of defects, which are induced in electrically conductive objects by an electromagnetic field (frequency range from 5 Hz to 10 MHz) of induction coils powered by alternating current. Used to detect surface (cracks, shells, hairs > 0.1 mm deep) and subsurface (depth 8-10 mm) defects, determination of chemical. composition and structural inhomogeneities of materials, measurement of coating thickness, etc.

With radio wave flaw detection there is an interaction (mainly reflection) with the object of control of radio waves 1-100 mm long, which are fixed by special devices - radio flaw detectors. The method makes it possible to detect defects with minimum sizes from 0.01 to 0.5 wavelengths, to control the chemical composition and structure of products, mainly from non-metallic materials. The method is especially widely used for non-contact control of conductive media.

Thermal flaw detection allows you to detect surface and internal defects in products made of heat-conducting materials by analyzing their temperature fields arising under the action of thermal radiation (wavelengths from 0.1 mm to 0.76 μm).

The most widely used is the so-called passive flaw detection(there is no external heating source), for example, a thermal imaging method based on scanning the surface of an object with a narrow optical beam, as well as a method of thermal paints, the color of which depends on the surface temperature of the product. During active flaw detection, products are heated by a plasma torch, an incandescent lamp, an optical quantum generator and the change in the thermal radiation transmitted through the object or reflected from it is measured.

Optical flaw detection is based on the interaction of the studied products with light radiation (wavelengths 0.4-0.76 μm). Control can be visual or with the help of light-sensitive devices; the minimum size of detected defects in the first case is 0.1-0.2 mm, in the second - tens of microns. In order to enlarge the image of the defect, projectors and microscopes are used. Surface roughness is checked with interferometers, incl. holographic, comparing the waves of coherent light beams reflected from the controlled and reference surfaces.

To detect surface defects (> 0.1 mm in size) in hard-to-reach places, endoscopes are used, which make it possible to transmit images over distances of up to several meters using special optical systems and fiber optics.

Radiation flaw detection provides for radioactive irradiation of objects with x-rays, a-, b- and g-rays, as well as neutrons. Radiation sources - X-ray machines, radioactive isotopes, linear accelerators, betatrons, microtrons. The radiation image of the defect is converted into a radiographic image (radiography), an electrical signal (radiometry) or a light image on the output screen of a radiation-optical transducer or device (radiation introscopy, radioscopy). Radiation computed tomography is being developed, which makes it possible to obtain its layer-by-layer image using a computer and scanning the surface of an object with focused X-rays. The method ensures the detection of defects with a sensitivity of 1.0-1.5% (the ratio of the length of the defect in the direction of transmission to the thickness of the part wall) in cast products and welded joints.

Acoustic flaw detection is based on changes under the influence of elastic vibration defects (frequency range from 50 Hz to 50 MHz) excited in metal products and dielectrics. There are ultrasonic (echo method, shadow, etc.) and actually acoustic (impedance, acoustic emission) methods. Ultrasonic methods are the most common. Among them, the most versatile is the echo method for analyzing the parameters of acoustic pulses reflected from surface and deep defects (reflecting surface area / 1 mm 2). With the so-called shadow method, the presence of a defect is judged by a decrease in the amplitude or a change in the phase of ultrasonic vibrations that envelope the defect. The resonance method is based on determining the natural resonant frequencies of elastic vibrations when they are excited in the product; used to detect corrosion damage or thinning of the walls of products with an error of about 1%. By changing the propagation velocity (bicycle-symmetric method) of elastic waves in places of discontinuity, the quality of multilayer metal structures is controlled. The impedance method is based on the measurement of the mechanical resistance (impedance) of products by a transducer that scans the surface and excites elastic vibrations of sound frequency in the product; this method reveals defects (with an area / 15 mm 2) of adhesive, soldered and other joints, between thin skin and stiffeners or fillers in multilayer structures. By analyzing the spectrum of vibrations excited in the product by impact, zones of broken connections between elements in multilayer glued structures of considerable thickness are detected (method of free vibrations).

The acoustic-emission method, based on the control of the characteristics of elastic waves that arise as a result of local rearrangement of the material structure during the formation and development of defects, makes it possible to determine their coordinates, parameters and growth rate, as well as the plastic deformation of the material; used to diagnose high pressure vessels, nuclear reactor vessels, pipelines, etc.

Compared to other methods, acoustic flaw detection is the most versatile and safe to use.

Defectoscopy by penetrating substances is divided into capillary and leak detection.

Capillary flaw detection(filling under the action of capillary forces of the cavities of defects with well-wetting liquids) is based on an artificial increase in the light and color contrast of the defective area relative to the undamaged one. The method is used to detect surface defects > 10 µm deep and > 1 µm wide on parts made of metals, plastics, and ceramics. The effect of detecting defects is enhanced by the use of substances that luminesce in UV rays (luminescent method) or mixtures of phosphors with dyes (color method). Leak detection is based on the penetration of gases or liquids through through defects and allows you to control the tightness of high or low pressure vessels, multilayer products, welds, etc.

With the help of gas tests, leaks or leaks are detected by determining the pressure drop (manometric method) created in products by a stream of air, nitrogen, helium, halogen or other gas, its relative content in the environment (mass spectrometric, halogen methods), change in thermal conductivity ( catharometric method), etc.; Based on these methods, the most highly sensitive leak detectors have been developed. During liquid tests, products are filled with liquid (water, kerosene, phosphor solution) and the degree of their tightness is determined by the appearance of drops and spots of liquid or luminous dots on the surface. Gas-liquid methods are based on creating an increase in gas pressure inside the product and immersing it in a liquid or smearing the leaks with soapy water; tightness is controlled by the release of gas bubbles or soap suds. The minimum size of a defect detected during leak detection is about 1 nm.

The method of luminescent flaw detection requires the use of a luminescent flaw detector or portable mercury-quartz devices such as LUM-1, LUM-2, etc. The method is based on the introduction of a luminescent substance into the cavity of defects, followed by irradiation of the surface of the part with ultraviolet rays. Under their influence, defects become visible due to the luminescence of the substance. The method makes it possible to detect surface defects with a width of at least 0.02 mm in parts of any geometric shape.

The sequence of operations for luminescent flaw detection:

Cleaning the surface from contaminants;

Application of a penetrating luminescent composition;

Application of developing powder;

Inspection of the part in ultraviolet rays.

You can use luminescent: kerosene - 55-75%, vaseline oil - 15-20%; benzene or gasoline - 10-20%; emulsifier - OP-7 - 2-3 g / l; defectol green-golden - 0.2 g / l. Developing powders - magnesium carbonate, talc or silica gel.

List of defects.

After carrying out a detailed fault detection, a defective statement is drawn up. The defective statement indicates the nature of damage or wear of parts, the amount of necessary repairs, indicating newly manufactured parts; all work related to the overhaul (disassembly, transportation, washing, etc.) and the work that completes the repair (preparation, scraping, assembly, strength test, testing, commissioning) are also indicated.

Fault and repair cards are one of the main technical documents for repair. They contain instructions for defecting parts. The cards are arranged in ascending order of the numbering of assembly units and parts or according to the constructive sequence of assembly units.

In the upper left corner of the map, a sketch of a part or a tenological process is placed. Overall dimensions are put down on the sketch, the profiles of gear teeth, splines, splined and key grooves, fists, etc. are shown separately. Numbers of positions and places of control are taken out from the dimensional arrow and are arranged in ascending order clockwise or from left to right.

In the upper right corner of the map, data with drawings characterizing the part is given.

The following order of map construction is adopted:

The position numbers of the defects indicated on the sketch are put down. Defects of the part not indicated on the sketch are applied first of all without putting down positions;

Possible defects of the part, which are formed during the operation of the machine, are entered according to the technological sequence of their control. First, defects determined visually are canceled, and then defects determined by measurements;

Methods and means of defect control are indicated;

Nominal dimensions are affixed with indication of tolerances in accordance with the drawings of the manufacturer;

Permissible dimensions are affixed with an accuracy of 0.01 mm when pairing this part with a new one;

Permissible dimensions are affixed, but in conjunction with the part that was in operation;

Repair procedure.

1. This procedure establishes and explains the features of non-warranty and warranty repairs of equipment. Hereinafter in the text, the Master is the person who performs the repair and bears the associated costs, and the Customer is the person who hands over the equipment for repair and pays for this repair.

2. Delivery of equipment to the territory of the Master, as well as the return of equipment from repair by mutual agreement of the Master and the Customer can be carried out either by the Master, or by the Customer, or by another person authorized by the Customer. In case of delivery of equipment by the Master, this delivery is subject to payment as a transport cost (departure of the Master) according to the price list valid at the time of departure. Payment is subject to both the departure for the delivery of equipment for repair, and the departure for the return of equipment from repair.

3. When transferring the equipment for repair, the customer agrees that the equipment is accepted without disassembly and troubleshooting. The Customer agrees that all malfunctions discovered by the Master during the technical inspection of the equipment occurred before the transfer of the equipment to the Master. The Customer agrees that the Master may detect other malfunctions not indicated by the Customer when transferring the equipment for repair.

4. The customer assumes the risk of partial loss of consumer properties of the repaired equipment, which may occur after repair. The master during the repair tries to prevent the loss of consumer properties and, if possible, minimizes the risk of such losses.

5. Equipment repair works are carried out only after the estimated repair cost has been agreed with the Customer. If the Customer refuses to repair, the cost of work on diagnosing the malfunction is subject to payment.

6. Repair can be of four categories of complexity:

7. During the repair, the Master may need to carry out indirect operations. These are operations that are not directly related to the performance of repair work, but without which the repair would be impossible or extremely difficult.

These are operations such as:

Internet search for diagrams, manuals, service instructions, datasheets for components, products and blocks;

Obtaining confidential information necessary for repair from manufacturers of microelectronic products and components;

Drawing up schematic diagrams, maintaining electronic libraries and databases;

Manufacture or purchase of special devices, tools and installations for repairs;

Development of service programs and utilities or searching for them on the Internet;

Ordering missing components online and waiting for them to arrive or buying them in stores.

Indirect operations in no way relate to the relationship between the Master and the Customer and are not paid by the Customer. This is a purely internal matter of the Master, which is paid for by the Master. In relation to the Customer, indirect operations lead only to additional delays in the execution of repairs.

8. The cost of blocks, parts and assemblies replaced in the repaired equipment is paid by the Customer and is included in the repair calculation. The cost of consumables (special fluxes and other chemicals, wires, etc.) is included in the cost of repair work and is not paid separately.

9. Replaced during the repair, defective parts, assemblies and blocks are issued to the Customer at his request. For the storage of these parts, assemblies and blocks, the Master is responsible for one day after the issuance of the repaired equipment to the Customer. After a day, defective parts, assemblies and blocks are disposed of.

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 welding seams and to determine the strength of metal elements. Let's talk in more detail about the various methods of flaw detection.

Why is it necessary to carry out such a diagnosis

When performing welding work, it is not always possible to provide 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 under study. Flaw detection investigates the physical properties of materials by influencing them with infrared and X-rays, radio waves and ultrasonic vibrations. Such a study can be carried out both visually and with the help of special optical instruments. Modern equipment makes it possible to determine the slightest deviations in the physical structure of the material and to detect even microscopic defects that can affect the strength of the connection.

Defectoscopy control methods

• Photographic is a common way to identify condition defects when shooting on film or digital media, then zoom in and check for possible defects. It should be said that this diagnostic method was widespread earlier, but today it is gradually being replaced by modern flaw detection technologies.
• Infrared technology makes it possible to detect welding defects that are invisible during visual inspection. This technology involves the use of special infrared radiation, which in turn provides a qualitative definition of microcracks, blisters and discontinuities.
• The magnetic diagnostic method allows you to detect cracks by detecting magnetic field distortion. This technology has become widespread in recent years, due to its efficiency and ease of use.
• Ultrasonic flaw detection allows to determine the presence of internal welding defects, therefore these technologies are widely used in metallurgical production, mechanical engineering and construction.
• The imperance method of diagnostics measures the mechanical resistance of products, on the basis of which internal defects, deviations in the chemical composition, the presence of porosity and violation of uniformity are detected.

An effective method of ultrasonic flaw detection

It should be said that various methods of flaw detection 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 the existing defects in metal alloys and welds.

In recent years, the most widespread ultrasonic flaw detection technology, which is versatile in use and allows you to accurately determine the existing structure inhomogeneities. We 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 allow you 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 the existing damages and defects, but also to control the thickness of the material up to several millimeters of thickness. This allows you to significantly expand the scope of use of such equipment for flaw detection, the functionality of which has significantly expanded in recent years.

The use of such a study in the production process and the subsequent monitoring of the metal welded products in operation makes it possible to reduce the time and money spent on quality control of manufactured materials and to determine the condition of various metal parts during their operation as accurately as possible.

Non-destructive control methods make it possible to check the quality of forgings and parts (for the absence of external and internal defects) without violating their integrity and can be used in continuous control. Such control methods include X-ray and gamma flaw detection, as well as ultrasonic, magnetic, capillary and other types of flaw detection.

X-ray flaw detection

X-ray flaw detection is based on the ability of X-ray radiation to pass through the thickness of the material and be absorbed by the latter to varying degrees, depending on its density. Radiation, the source of which is an X-ray tube, is directed through a controlled forging onto a sensitive photographic plate or a luminous screen. If there is a defect in the forging (for example, a crack), the radiation passing through it is absorbed weaker, and the film is illuminated more strongly. By adjusting the intensity of x-ray radiation, an image is obtained in the form of an even light background in defect-free places of the forging and a distinctive dark area at the location of the defect.

The X-ray units produced by the industry make it possible to scan steel forgings up to 120 mm thick, and light alloy forgings up to 250 mm thick.

Gamma flaw detection

The control of forgings by gamma flaw detection is similar to the control by X-ray flaw detection. At a certain distance from the object under study, a source of gamma radiation is installed, for example, a capsule with radioactive cobalt-60, and on the opposite side of the object, a device for recording the radiation intensity. On the intensity indicator (photographic film), defective areas appear inside the workpiece or forging. The thickness of controlled blanks (forgings, parts) reaches 300 .. .500 mm.

In order to avoid irradiation when using X-ray and gamma-ray flaw detection as control methods, it is necessary to strictly observe safety requirements and be extremely careful.

Rice. 9.7. Installation for ultrasonic testing of metal: 1 - oscilloscope, 2, 3, 4 - light pulses, 5 - block, 6 - head, 7 - forging, 8 - defect

Ultrasonic flaw detection

Ultrasonic flaw detection is the most common testing method that allows you to check forgings with a thickness of up to 1 m. The installation for ultrasonic testing by the echo method (Fig. 9.7) consists of a search head 6 and block 5, which contains a generator of ultrasonic electrical oscillations (frequency over 20 kHz) and oscilloscope 1. Head 6 is a piezoelectric converter of electrical vibrations into mechanical ones.

With the help of a search head, a pulse of ultrasonic vibrations is directed to the investigated section of the forging 7, which will be reflected first from the surface of the forging, then (with some delay) from the defect 8 and even later from the bottom surface of the object. The reflected pulse (echo) causes the piezocrystal of the search head to vibrate, which converts mechanical vibrations into electrical ones.

The electrical signal is amplified in the receiver and recorded on the oscilloscope screen 1: the distance between pulses 2,3 and 4 determines the depth of the defect, and the shape of the curves determines the magnitude and nature of the latter.

Magnetic flaw detection

The most common type of magnetic flaw detection is the magnetic powder method used to test magnetic alloys of iron, nickel and cobalt. The steel part is magnetized with an electromagnet and then coated with a suspension of kerosene and magnetic powder. In places where there is a defect, magnetic powder particles accumulate, copying the shape and size of not only surface cracks, but also defects located at a depth of up to 6 mm.

The magnetic powder method makes it possible to detect large and very small defects with a width of 0.001 ... 0.03 and a depth of up to 0.01 ... 0.04 mm.

Capillary flaw detection is based on the property of liquids to fill the cavities of surface defects (cracks) under the action of capillary forces. The liquids used for testing either have the ability to luminesce under the action of ultraviolet radiation (luminescent flaw detection), or have a color that clearly stands out against the general background of the surface. For example, in fluorescent flaw detection, forgings are immersed in a solution of mineral oil in kerosene, washed, dried, and then dusted with magnesium oxide powder. If such a surface is examined with the naked eye under the light of a mercury lamp, bright white cracks are clearly visible against the background of the dark purple surface of the forging. The method allows to determine the presence of cracks with a width of 1 to 400 microns.

Lecture N 10

Flaw detection is a field of knowledge that covers the theory, methods and technical means for determining defects in the material of controlled objects, in particular in the material of machine parts and metal structures.

Flaw detection is an integral part of the diagnostics of the technical condition of equipment and its components. Works related to the detection of defects in the material of equipment elements are combined with repairs and maintenance or are performed independently during the technical inspection period.

To reveal hidden defects in structural materials, various methods of non-destructive testing (defectoscopy) 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 X-ray and gamma-ray radiation, magnetic and electromagnetic fields, vibrations, optical spectra, capillarity phenomena, and others.

According to GOST 18353, non-destructive testing methods are classified by types: 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 dimensions of the parts, the nature of the detected defects and the conditions of flaw detection (in workshops or on a machine). The main qualitative indicators of flaw detection methods are sensitivity, resolution, and reliability of results. Sensitivity– the 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 skipping defects or rejecting good 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, oversights (cracks, porosity, cavities, etc.) and physical and mechanical properties (granularity, intergranular corrosion, depth of the hardened layer, etc.) of the material. The control is performed on the basis of an analysis of the nature of the propagation of sound waves in the material of the part (amplitude, phase, velocity, angle of refraction, resonance phenomena). The method is suitable for parts whose material is able to elastically resist 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 work with ultrasonic testing from 0.5 to 10 MHz.

The main disadvantages of ultrasonic methods include the need for a sufficiently high surface cleanliness of parts and a significant dependence of the quality of control on the qualifications of the flaw detector operator.

Magnetic methods are based on the registration of magnetic stray fields over defects or the 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 the leakage magnetic flux. The developing material is applied to the surface of the product. Under the action 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 lies in the magnetization of the product while simultaneously recording the magnetic field on a magnetic tape that covers the part, and subsequent decoding of 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 control, all parts, except for defective ones, are demagnetized. Restoration of non-demagnetized parts by machining can lead to damage to the working surfaces due to the attraction of chips. It is not necessary to demagnetize parts that are subjected to heating by welding-surfacing and other methods to a temperature of 600 ... 700 ° C during restoration.

The degree of demagnetization is controlled by sprinkling parts with steel powder. On well demagnetized parts, the powder should not be held on the surface. For the same purposes, devices equipped with fluxgate pole detectors are used.

Stationary, portable and mobile flaw detectors are mass-produced for the control of parts by the magnetic particle method. 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 metallic 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 the operation of the machine.

Defectoscopy of parts radiation methods is based on registration of the weakening of the intensity of radioactive radiation when passing through a controlled object. The most commonly used X-ray and γ-control 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 the results of radiation monitoring is carried out visually (image on screens, including a stereoscopic image), in the form of electrical signals, fixation 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; the possibility of documentary confirmation of the results of control, which does not require additional interpretation. 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 registration of changes in electromagnetic oscillations interacting with a controlled object. In practice, microwave methods have become widespread in the wavelength range from 1 to 100 mm. The interaction of radio waves with an object is evaluated by the nature of absorption, diffraction, reflection, refraction of the wave, interference processes, resonance effects. These methods are used to control the quality and geometric parameters of products made of plastics, fiberglass, thermal protection and heat-insulating materials, as well as to measure vibration.

Thermal methods. In thermal methods, the thermal energy propagating in the object, emitted by the object, absorbed by the object is used as a diagnosed parameter. The temperature field of the surface of an object is a source of information about the features of heat transfer processes, which, in turn, depend on the presence of internal and external defects, cooling of the object or its part as a result of the outflow of the medium, etc.

The temperature field is controlled using thermometers, thermal 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, scattering of light, 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 are used.

The 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 the given one, as well as internal stresses in the material.

Visual entroscopy allows you to detect defects on the surfaces of the object. Entroscopes (video borescopes) for internal examination of hard-to-reach places 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 surface of the object under study. 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-optic.

capillary method flaw detection is based on capillary penetration of indicator liquids into the cavities of surface and through discontinuities of the 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, hairlines, 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. The nature of the indicator patterns is controlled by the visual-optical method. In this case, the lines of the pattern are detected relatively easily, since they are tens of times wider and more contrast than defects.

The simplest example of capillary flaw detection is a kerosene sample. The penetrating liquid is kerosene. The developer is chalk in the form of a dry powder or an aqueous suspension. Kerosene, seeping into the layer of chalk, causes it to darken, which is detected in daylight.

The advantages of capillary flaw detection are versatility in terms of shape and materials of parts, good visibility 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, a long process (0.5 m - 1.5 h) and laboriousness (the need for thorough cleaning), the toxicity of some penetrating liquids, insufficient reliability at low temperatures.

Cracks in parts can be detected using a kerosene test.

Kerosene has good wetting ability, 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 absorbent coating is applied to the opposite surface (350 ... 450 g of ground chalk suspension per 1 liter of water). The presence of a through crack is determined by yellow spots of kerosene on the chalk coating.

Hydraulic and pneumatic test methods are widely used to detect through pores and cracks.

With the hydraulic method, the internal cavity of the product is filled with a working fluid (water), sealed, an excess pressure is created by the pump and the part is kept for some time. The presence of a defect is established visually by the appearance of water drops or sweating of 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 soapy water or the part is immersed in water. The presence of a defect is judged by the release of air bubbles. The pressure of the air pumped into the internal cavities depends on the design features of the parts and is usually 0.05 - 0.1 MPa.

Non-destructive testing methods are not universal. Each of them can be used most effectively to detect certain defects. The choice of a non-destructive testing method is determined by the specific requirements of practice and depends on the material, the design of the object under study, the state of its surface, the characteristics of defects to be detected, the operating conditions of the object, the control conditions and technical and economic indicators.

Surface and subsurface defects in ferromagnetic steels are detected by magnetizing the part and fixing the stray field using magnetic methods. The same defects in products made of 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 is effective. The ultrasonic method is ineffective in detecting internal defects in cast structures and alloys with a high degree of anisotropy. Such structures are controlled using x-rays or gamma rays.

Design (shape and dimensions) of parts also causes you

control method boron. If almost all methods can be used to control an object of a simple shape, then the use of methods is limited to control objects of a complex shape. Objects with a large number of grooves, grooves, ledges, geometric transitions are difficult to control using methods such as magnetic, ultrasonic, radiation. Large-sized objects are controlled in parts, determining the zones of the most dangerous areas.

Surface condition products, by which we mean its roughness and the presence of protective coatings and contaminants on it, significantly affect the choice of method and the preparation of the surface for research. Rough rough surface excludes the use of capillary methods, eddy current method, magnetic and ultrasonic methods in the contact version. Small roughness expands the possibilities of defetoscopy methods. Ultrasonic and capillary methods are used when the surface roughness is not more than 2.5 microns, magnetic and eddy current methods are not more than 10 microns. Protective coatings do not allow the use of optical, magnetic and capillary methods. These methods can only be applied after the coating has been removed. If such removal is not possible, radiation and ultrasonic methods are used. The electromagnetic method detects cracks on parts with paintwork and other non-metallic coatings up to 0.5 mm thick and non-metallic non-magnetic coatings up to 0.2 mm.

Defects have a different origin and differ in type, size, location, orientation relative to the metal fiber. When choosing a control method, the nature of possible defects should be studied. By location, defects can be internal, occurring at a depth of more than 1 mm, subsurface (at a depth of up to 1 mm) and surface. To detect internal defects in steel products, radiation and ultrasonic methods are used more often. If the products have a relatively small thickness, and the defects to be detected are large enough, then it is better to use radiation methods. If the thickness of the product in the direction of transillumination is more than 100-150 mm or it is required 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 cracks. In this case, ultrasonic testing is most acceptable.

A flaw detector is an electronic device designed to detect hidden defects in solid products. The device allows diagnosing deviations from the norm without creating a load or destroying the object under study. It can be used to assess the uniformity of the structure of the product, the presence of indulgences on its surface as a result of corrosion, deviations in the chemical composition or the presence of microcracks.

Where is a flaw detector used?

Flaw detectors are used in mechanical engineering and construction. With their help, various components and assemblies, as well as workpieces, are checked. These devices are indispensable in the oil and gas industry and energy. With their help, pipes and tanks are checked for weak walls. This equipment allows you to detect defects, which excludes its use in the construction of critical facilities. With the help of flaw detectors, it is possible to control the reliability of welds, glue layer or solder density.

This equipment is produced in a portable and stationary version. Some models allow you to scan even those objects that move at high speed. Such devices are used to check pipes that are pulled through the scanned area. There are also large flaw detectors that are moved on a trolley along rails. These devices are used in construction and industrial production, in particular aircraft and ships. There are many types of flaw detectors adapted to certain operating conditions. In the metal processing industry, devices are used that can detect defects in heated metal blanks.

Flaw detector designs

To ensure the operation of the flaw detector, various physical phenomena are used, the nature of which differs significantly from each other. In this regard, there are many design features of these devices.

Among the most common flaw detectors that are mass-produced are:

• Acoustic.
• Magnetic particle.
• Eddy current.
• Ferroprobe.
• Electrospark.
• Thermoelectric.
• Radiation.
• infrared.
• Radio wave.
• Electron-optical.
• Capillary.

Each of these types of equipment has its own strengths and weaknesses. In this regard, they may be ideal for some purposes, but not suitable for others. To make the right choice of a flaw detector, it is important to first understand the principle of operation of each type.

Acoustic flaw detector

Also called pulsed or ultrasonic. It works on the principle of echo. A short ultrasonic pulse is sent to the product being tested, after which its vibrations are recorded. As a result, a defect map is displayed on the screen. This device is one of the most popular. It gives a very clear picture of the defects that are hidden on the surface. The advantages of such equipment include the fact that it works with different materials. There are many subspecies of acoustic flaw detectors, which also operate from ultrasonic waves.

Magnetic particle flaw detector

It is used to control parts of various shapes. With it, you can scan welds and recesses obtained by drilling. An important drawback of the method is that it allows only superficial deviations to be checked. He will not be able to identify internal problems if they do not have an external outlet. To ensure the scanning of parts, a special powder is used, which is dispersed over the surface of the object and fills in the irregularities and cracks in it. After that, the magnetic field is scanned, which allows finding the place of the greatest accumulation of powder. This allows you to create a map of defects, since the powder does not linger on normal smooth surfaces, but clogs in irregularities.

The disadvantage of this method is that it requires the purchase of magnetic powder. It is a consumable, so it ends quickly and pours out in the role of dirt, which must be collected periodically.

Eddy current flaw detectors

They operate on the physical principle of eddy currents. This device excites eddy currents in the testing area, after which it analyzes the state of the object according to their behavior. This method is one of the most inaccurate. The crack control depth is up to 2 mm. In this regard, it is difficult to obtain an objective picture of the actual state of the measured surface.

Ferroprobe flaw detector

Generates current pulses that are sent to the surface under study. Based on their behavior, the existing defects are analyzed. This equipment is quite sensitive and can detect irregularities with a depth of 0.1 mm. This equipment controls the quality of cast parts, rolled metal products and welding joints.

Electrospark flaw detectors

They create an electrical discharge between their sensitive probe and the surface under study. The probe is a bundle of electrodes, which increases the study area. The discharges break through the air gap between the surfaces. As a result, a map of the object under study with marked damage is created. For examination by this method, it is necessary that the object of study be made of conductive material.

Thermoelectric flaw detector

It works on the physical principle of the electromotive force that occurs when the contact area between two different materials is heated. This equipment is one of the most expensive, since it requires the use of high quality materials that allow you to record minimal temperature changes between the standard and the surface under study.

Radiation

Objects are irradiated with x-rays and neutrons. They work on the same principle as the X-ray machine used in medicine. The result is a radiographic image or a bright image on the instrument screen. This equipment is unsafe for the operator because X-rays adversely affect health. The device allows for a really deep study of objects, but can not be used on all materials.

infrared

They send heat rays that bounce off the surface of an object and allow analysis of the deviation from the norm. A heat map is viewed on the device screen, where areas with defects have changed colors. This equipment allows you to detect defects, but does not give an accurate picture of their characteristics. It is difficult to determine the depth of the cracks, since only the outlines of the disturbed areas are considered.

radio wave

They generate radio waves that are sent to the subject of study. Because they fight off the object, it is possible to determine not only cracks or thickening, but also the diameter and even the thickness of the insulating coating. Such equipment is used to work with metals and other materials.

Electro-optical

They are used to control objects that are under high voltage. They are used by electricians. Such equipment allows not only to identify the places of fracture of the wires, but also the quality of the insulation.

Capillary flaw detection

They mean covering the surface under study with a special indicator substance that fills the existing microcracks. In those places where the thickness of the substance is greater, its color is more saturated in comparison with flat areas. These colors visually determine the recesses. This method does not involve the use of an electronic device, but only an indicator substance and a magnifying glass or microscope.

Criterias of choice

When choosing a flaw detector, you should pay attention to some key characteristics. First of all, you need to focus on the measurement range. Different models differ in sensitivity. The most accurate device is able to detect a defect, the depth of which is only 1 micron. For certain purposes, such sensitivity is really needed, but for others it is unnecessary. For example, if you need to find microcracks on the crankshaft or other rotating parts, then it is better to use precision equipment. If you need to analyze the state of the metal frame in construction, then such microcracks are not so important. Given the thickness of the body of reinforcement or beams, a small defect with a depth of 1 micron can in no way cause the metal to burst, especially if it is used for the purposes for which it is intended.

Also, when choosing a flaw detector, one should be guided by the materials for which it is intended. Some models can only work with metals, while others are universal. Also in relation to flaw detectors, an important concept is performance. It shows the scanning speed. The higher it is, the faster the state of the object can be assessed. If we focus on this indicator, then the undisputed leaders are eddy current and fluxgate equipment. If you use a magnetic particle device, then the duration of the diagnosis will take a long time, in addition, there will be a need to grind the powder.

Considering flaw detectors, it is worth giving preference to ultrasonic devices first of all. They do not harm the operator as radiation, and at the same time they give a completely sufficient idea of ​​the existing defects and the advisability of sending the part for culling.