Defectoscopy of machine parts. Methods of flaw detection When flaw detection of products from what material is used

Parts flaw detection methods

Visual control and measurements of parts do not allow detecting sufficiently small or hidden defects located under the surface, but they can be detected by non-destructive testing methods (defectoscopy).

Rice. 1. Methods of flaw detection

Rice. 2. Schemes of endoscopes: a - straight, b - cranked

Non-destructive testing of parts has recently become widespread in the production of machines and much less so in their operation. The widespread introduction in ports of the most effective and at the same time quite simple and cheap control methods is associated with the need to create a control service provided with well-trained and technically competent personnel and the necessary diagnostic equipment.

When choosing one or another control method, one should proceed from the fact that there is no universal method, and, consequently, the possibilities of methods are limited to the search for defects defined by the nature and location. Knowing the nature of wear that affects the possible location or type of defect, as well as a sufficient variety of control methods, allows you to make the necessary choice. On fig. 41 shows a diagram of the most promising non-destructive testing methods for port conditions.

The optical method allows, without disassembling the structure, to control the condition of the surfaces of parts in closed and hard-to-reach places. The method is based on a circular or side view of the controlled area with autonomous illumination and an image magnification from 0.5 to 150. Control devices, called endoscopes, allow you to transmit an image at a distance of up to 7 m. Endoscopes consist of a housing in which an illuminator is placed, a screen for protection against illumination, prism or mirror attachment, optical system, eyepiece and deflecting prisms. To inspect part 6, a window in the body is provided. Endoscopes make it possible to detect scratches, cracks, corrosion damage and other defects with sizes up to 0.03-0.08 mm in parts with an inner diameter of 5-100 mm or more.

Rice. 3. Scheme of the capillary method

Rice. 4. The nature of defects in the capillary method of control

The capillary method is based on the capillary penetration of liquid into cracks and the contrast of the materials used. The method makes it possible to detect open cracks of welding, thermal, grinding, fatigue and other origin with an opening size of e more than 0.001 mm, a depth h - 0.01 mm and a length L - 0.1 mm, as well as porosity and other similar defects.

The method is as follows: an indicator liquid is applied to the surface of the part, which, under the action of capillary forces, fills the cavities on the surface. The surface is thoroughly wiped and covered with a developing composition. The indicator liquid from the cavity of the defect is adsorbed into the developing composition, forming an indicator trace, the width of which is much greater than the open cracks e. The trace image contrast is ensured due to the brightness of the color of the indicator liquid (color method) or its ability to luminesce when irradiated with ultraviolet rays (luminescent method). Control technology includes surface preparation (cleaning, degreasing), application of indicator and developing compositions, and inspection of the part.

When examining the surface, the resulting pattern of traces is analyzed, identifying their species. So, cracks of any origin, hairlines, lack of penetration appear in the form of clear solid or broken lines of various configurations (Fig. 44, a); cracking of the material - in the form of a group of separate short lines or a grid (4, b, c); pores, fatigue chipping and erosion, damage - in the form of individual points or stars.

The most difficult thing in the analysis is to distinguish real defects from imaginary ones - scratches, crumpled burrs, chips of the oxide film. For these purposes, additional features are used, such as the location of the pattern, the direction of the lines of the pattern relative to the axis of the part and the acting loads, the configuration and branching of the lines, the similarity of the pattern with other areas of the surface that are different in the acting loads.

Rice. 5. Scheme of the acoustic method

Rice. 6. Block diagram of an ultrasonic flaw detector

The acoustic method is based on the ability of sound waves to be reflected from the boundaries of the material density. Falling on the surface of the part, wave Ф is partially reflected from its surface, and partially propagates into the material (Fig. 5). In this case, the amount of reflected energy is the greater, the higher the difference between the acoustic impedances of media I and II. If medium I is air, and II is metal, all the energy supplied will be reflected.

The use of a normal or inclined finder depends on the intended location of the defect. The search for a defect is carried out by the echo method or by the shadow method, when 2 separate seekers are used - emitting and receiving, located on different sides of the part. In this case, the absence of a signal at the receiving finder indicates the presence of an obstacle (defect) in the path of wave propagation.

To determine the extent of the defect, the finder is moved along the surface of the part.

The use of ultrasonic testing is most effective for detecting fatigue and welding cracks in the metal structures of cranes, grabs, etc.

The magnetic method is based on the registration of stray magnetic fields formed over defects located in the path of the magnetic flux Fm. The strength of the stray field depends on the orientation of the defect in the magnetic flux and its location relative to the surface. In this regard, when testing by the magnetic method, defects in products made of ferromagnetic materials are reliably detected, which are in the nature of discontinuities that come to the surface or are located at a depth of not more than 1 mm.

The method is one of the simplest and most common, it allows you to control welds and parts of a wide variety of shapes and sizes.

The magnetic powder method has found the greatest application, in which a magnetized part is poured with a ferromagnetic suspension to visualize the stray field. Iron powder, which is in suspension in a mixture of kerosene, oil and water, settles on the surface of the part at the exit points of the stray field. Moreover, the width of the powder layer can be tens of times greater than the size of the crack opening, due to which a well-distinguishable relief trace of the defect is formed.

Rice. 7. Scheme for determining the location of defects

Rice. 8. Scheme of the formation of a stray magnetic field

Rice. 9. Schemes of magnetization in the magnetic powder method: 1 - controlled part: 2 - magnetizing device

The part is cleaned before inspection to ensure electrical contact and reduce the effect of non-magnetic coatings. Control is carried out in an applied magnetic field (in the process of magnetization), if the part is made of a low-magnetic material (StZ, steel 10, 20), of complex shape, defects are located deeper than 0.01 mm from the surface, or there is a protective non-magnetic coating of the same thickness (for example, chrome). In other cases, residual magnetization of the part can be used. The latter method is more convenient, as it allows you to break down control operations.

For magnetization (Fig. 9), the part is placed in the field of an electromagnet (Fig. 9, a), in the field of a solenoid (Fig. 9, b), and also in a circular way: or a current is passed through the entire part (Fig. 9, c) or in its individual sections using special clamping electrical contacts (Fig. 9, d). After the end of the control, the part is demagnetized. To do this, it is placed in an alternating magnetic field and gradually removed from it, or the magnetic field strength is gradually reduced to zero.

After the powder has settled, the part is inspected. All types of cracks are identified as clear branched solid or broken lines. However, it should be borne in mind that imaginary defects can also be detected, since a stray field can be formed when a magnetized part comes into contact with another ferromagnetic object, in places where the section of the part is sharply narrowed, along the boundaries of welds, and in a number of other cases.

The electromagnetic method is based on the use and measurement of the characteristics of eddy currents excited on the surface of the part when approaching (when moving along) the sensor-inductor coil. Depending on the size of the approach, the speed of movement and a number of other factors, a different interaction of the magnetic fields of the sensor and eddy currents is used. The result of this interaction underlies the determination of the physico-mechanical properties and chemical composition of the material, the quality of heat treatment, as well as the thickness of chrome, paint, ceramic, plastic and other types of non-conductive coatings.

Due to its simplicity, the thicknesses of coatings applied again or remaining as a result of wear can be determined widely under operational conditions. The control consists in setting the thickness gauge to the lower and upper limits of measurement according to the reference plates available in the kit and measuring the unknown coating thickness on the scale of the instrument after installing the sensor on the controlled area of ​​the surface. The choice of the required type of thickness gauge depends on the range of measured thicknesses within 0.003-10 mm with an error for most of them ± 2% of the measured value.

Rice. 10. Scheme of X-ray control method

The radiation method is based on the property of hard radiation to pass through materials of various densities, including aluminum and steel. The value of the attenuation of radiation, and therefore; and the degree of darkening of the X-ray film behind the part in the path of the rays depend on the thickness of the material. Pores, shells, cracks, etc. reduce it and are detected on the film in the form of more illuminated (dark) dots, spots or lines. Depending on the source of γ-radiation, the X-ray method and γ-control are distinguished.

The main element of the X-ray unit is the X-ray tube, the diagram of which is shown in fig. 10. Electrodes are placed in a glass bulb: a cathode and an anode, to which a voltage of over 100 kV is supplied from a transformer. In addition, a voltage of 4–12 V is supplied to the cathode from a step-down transformer to ensure that the helix filament is heated up to 3000–3500 °C. In this case, due to thermionic emission, electrons fly out of it, which, under the action of an electric potential on the electrodes, move at high speed to the anode through the focusing and calibrating devices. Collision with the anode leads to their absorption and emission of y-rays, which emerge in a narrow beam through a special window. Due to the high heating of the anode, a special cooling system is provided.

A controlled steel part 8 with a thickness of up to 120-160 mm is installed in the path of the radiation flux, and behind it is a metal cassette with x-ray film. The exposure time, depending on the radiation power and the thickness of the part, ranges from several minutes to 1 hour. X-ray installations are stationary or mobile.

Rice. 11. Diagram of a flaw detector

Control installations - y-flaw detectors - are made portable. They are mobile, much (5-10 times) lighter than X-rays, easy to use and allow you to control steel planes up to 200 mm thick. The flaw detector (Fig. 11) consists of a protective steel case, a lead shell, a radioisotope radiation source and a shutter that blocks the beam exit channel in the non-working position of the flaw detector. The main characteristics of the radiation source are its activity and half-life, which determines the time during which the number of radioactive atoms will decrease by 2 times. Of the more than 60 isotopes produced by the industry, cobalt-60, cesium-137, iridium-192 and some others are used for control purposes.

Since γ-defectoscopes are always potentially dangerous, they are stored in concrete nests in closed and sealed rooms. Flaw detectors are recharged by specialists.

Particular attention should be paid to safety measures during radiation monitoring, it is obligatory to fence off the work area and put on duty for the time of monitoring or carry out monitoring in special rooms.

TO category: - Port handling vehicles

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 defecting devices that 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-layers, 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 a layered 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;

Order missing components online and wait for them to arrive, or buy 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.

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 screen of oscilloscope 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. 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.

1. Defectoscopy is a set of physical methods that allow to control the quality of materials, semi-finished products, parts and components of vehicles without destroying them. Methods of flaw detection make it possible to evaluate the quality of each individual part and carry out their complete (100%) control.

The task of flaw detection, along with the detection of defects such as cracks and other discontinuities, is to control the dimensions of individual parts (usually with one-sided access), as well as to detect leaks in specified areas. Flaw detection is one of the methods to ensure the safe operation of vehicles; the scope and choice of the type of flaw detection depend on the conditions of its operation.

2. Methods of flaw detection are based on the use of penetrating radiation (electromagnetic, acoustic, radioactive), the interaction of electric and magnetic fields with materials, as well as the phenomena of capillarity, light and color contrast. In the areas where defects are located in the material, due to changes in the structural and physical characteristics of the material, the conditions for its interaction with the indicated radiations, physical fields, as well as with substances applied to the surface of the controlled part or introduced into its cavity, change. By registering these changes with the help of appropriate equipment, it is possible to judge the presence of defects that represent a violation of the integrity of the material or the uniformity of its composition and structure, determine their coordinates and estimate the dimensions. With sufficiently high accuracy, it is also possible to measure the thickness of the walls of hollow parts and protective and other coatings applied to the products.

In the modern practice of the automotive industry and automotive service, the following methods of flaw detection of materials, semi-finished products, parts and assemblies have found application.

Optical methods- these are methods carried out visually (to detect surface cracks and other defects larger than 0.1 ... 0.2 mm) or using optical devices - endoscopes (Fig. 1), which allow detecting similar defects larger than 30 ... surfaces and hard to reach areas. Optical methods usually precede other methods and are used to control all parts of aircraft structures at all stages of manufacture and operation.

Rice. 1.

Examination with an endoscope is used, for example, to search for cracks on the inside of the side members of car frames.

radiation methods, using X-ray, gamma and other (for example, electrons) penetrating radiation of various energies, obtained using X-ray machines, radioactive isotopes and other sources, make it possible to detect internal defects larger than 1 ... 10% of the thickness of the translucent section in products with a thickness (for steel) up to 100 mm (when using X-ray equipment) and up to 500 mm (when using fast electrons). Radiation methods are used to control cast, welded and other parts of aircraft structures made of metallic and non-metallic materials, as well as to control defects in the assembly of various assemblies (Fig. 2).

Rice. 2.

In the automotive industry, radiation flaw detection is used to control the quality of liners and pistons.

Radio wave methods are based on changes in intensities, shifts in time or phase, and other parameters of electromagnetic waves in the centimeter and millimeter ranges when they propagate in products made of dielectric materials (rubber, plastics, and others). At a depth of 15...20 mm, it is possible to detect delaminations with an area of ​​more than 1 cm 2 .

In the automotive industry, the radio wave method measures the thickness of dielectric coatings

Thermal methods- these are methods that use infrared (thermal) radiation of a heated part to detect the inhomogeneity of its structure (discontinuity in multilayer products, in welded and soldered joints). The sensitivity of modern equipment (thermal imagers, Fig. 3) makes it possible to register a temperature difference on the surface of a controlled part of less than 1 °C.

Rice. 3.

In the automotive industry, thermal methods are used to control the quality of welds, for example, when welding air brake reservoirs.

Magnetic methods are based on the analysis of stray magnetic fields arising in the areas of location of surface and subsurface defects in magnetized parts made of ferromagnetic materials. Under optimal conditions, when the defect is located perpendicular to the direction of the magnetizing field, rather thin defects can be detected, for example, grinding cracks (in steel) with a depth of 25 µm and an opening of 2 µm. Magnetic methods can also measure, with an error not exceeding 1...10 µm, the thickness of protective (non-magnetic) coatings deposited on a part made of ferromagnetic material (Fig. 4).

In the automotive industry and automotive service, magnetic flaw detection is used to control the quality of grinding of critical parts, for example, crankshaft journals.

Acoustic (ultrasonic) methods- these are methods that use elastic waves of a wide frequency range (0.5 ... 25 MHz), introduced into the controlled part at different angles. Propagating in the material of the part, elastic waves attenuate to varying degrees, and when they encounter defects, they are reflected, refracted, and scattered. Analyzing the parameters (intensity, direction, and others) of transmitted and (or) reflected waves, one can judge the presence of surface and internal defects of various orientations larger than 0.5 ... 2 mm 2 . Control can be carried out with one-way access.

Rice. 4.

It is also possible to measure the thickness of hollow products with an error of no more than 0.05 mm (limitations are the significant curvature of the surface of the part and the strong attenuation of ultrasonic waves in the material). Acoustic methods (at low frequencies) can detect delaminations with an area of ​​more than 20 ... 30 mm 2 in glued and brazed structures with metal and non-metal fillers (including honeycomb), in laminated plastics, as well as in clad sheets and pipes. Using the so-called acoustic emission method, it is possible to detect developing (i.e., the most dangerous) cracks in the loaded elements of automotive units, selecting them from less dangerous, non-developing defects detected by other methods (Fig. 5). In this case, the control zones are formed using a different arrangement of sensors on the structure. Wire gauges are installed in the control zone so that their direction does not coincide with the direction of fatigue crack development.

Rice. 5.

Eddy current (electroinductive) methods are based on the interaction of eddy current fields, excited by a flaw detector sensor in a product made of electrically conductive material, with the field of the same sensor. These flaw detection methods allow in the automotive industry to detect discontinuities (cracks with a length of more than 1 ... 2 mm and a depth of more than 0.1 ... 0.2 mm, films, non-metallic inclusions), measure the thickness of protective coatings on metal, judge the inhomogeneities of the chemical composition and structure material, internal stresses. Equipment for testing by eddy current methods is highly productive and allows you to automate sorting.

Electrical Methods based on the use of mainly weak direct currents and electrostatic fields; they make it possible to detect surface and subsurface defects in products made of metallic and non-metallic materials and to distinguish between some grades of alloys. flaw detection technological product production

Capillary methods are based on the phenomenon of capillarity, that is, on the ability of certain substances to penetrate into small cracks. Treatment with such substances increases the color and light contrast of the part of the product containing surface cracks relative to the undamaged surface surrounding this part. These methods make it possible to detect surface cracks with an opening of more than 0.01 mm, a depth of 0.03 mm and a length of 0.5 mm in parts made of non-porous materials, including complex-shaped parts, when the use of other methods is difficult or excluded (Fig. .6).

Rice. 6.

In the automotive industry, capillary methods are used to control the quality of welds, for example in the manufacture of tanks. The above methods of flaw detection individually are not universal, and therefore the most critical parts are usually checked using several methods, although this leads to additional time. To improve the reliability of inspection results and labor productivity, automated systems are being introduced, including the use of computers to control inspection and process information received from flaw detector sensors.

Defectoscopy this is a field of knowledge covering 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 structure elements.

Due to the imperfection of the manufacturing technology or as a result of operation in difficult conditions, various defects appear in the products - violations of the continuity or uniformity of the material, deviations from the specified chemical composition or structure, as well as from the specified dimensions. Defects change the physical properties of the material (density, electrical conductivity, magnetic, elastic properties, etc.). Based on existing methods Defectoscopy lies the study of the physical properties of materials when exposed to x-ray, infrared, ultraviolet and gamma rays, radio waves, ultrasonic vibrations, magnetic and electrostatic fields, etc.

The simplest method Defectoscopy is visual - with the naked eye or with the help of optical instruments (for example, a magnifying glass). To inspect internal surfaces, deep cavities and hard-to-reach places, special tubes with prisms and miniature illuminators (diopter tubes) and television tubes are used. Lasers are also used to control, for example, the quality of the surface of a thin wire, etc. Visual Defectoscopy allows you to detect only surface defects (cracks, films, etc.) in metal products and internal defects in glass products or plastics that are transparent to visible light. The minimum size of defects detected by the naked eye is 0.1-0.2 mm, and when using optical systems - tens of microns.

X-ray flaw detection is based on the absorption of x-rays, which depends on the density of the medium and the atomic number of the elements that form the material of the medium. The presence of defects such as cracks, cavities, or inclusions of foreign material leads to the fact that rays passing through the material ( rice. 1) are attenuated to varying degrees. By registering the intensity distribution of the transmitted rays, it is possible to determine the presence and location of various material inhomogeneities.

Rice. 1. Scheme of X-ray transillumination: 1 - X-ray source; 2 - X-ray beam; 3 - detail; 4 - internal defect in the part; 5 - X-ray image invisible to the eye behind the detail; 6 - x-ray image recorder.

Gamma flaw detection (radiation) has the same physical foundations as X-ray flaw detection, but the radiation of gamma rays emitted by artificial radioactive isotopes of various metals (cobalt, iridium, europium, etc.) is used. Radiation energy is used from several tens of keV to 1-2 MeV for transillumination of thick parts. This method has significant advantages over X-ray flaw detection: the equipment for gamma flaw detection is relatively simple, the radiation source is compact, which makes it possible to examine hard-to-reach parts of products. In addition, this method can be used when the use of X-ray flaw detection is difficult (for example, in the field). When working with sources of X-ray and gamma radiation, biological protection must be provided.

Radio flaw detection is based on the penetrating properties of radio waves in the centimeter and millimeter ranges (microradio waves), it allows detecting defects mainly on the surface of products, usually from non-metallic materials. Due to the low penetrating power of microradio waves, radiodefectoscopy of metal products is limited (see Skin effect). This method determines defects in steel sheets, bars, wires during their manufacture, and also measures their thickness or diameter, the thickness of dielectric coatings, etc. From a generator operating in a continuous or pulsed mode, microradio waves penetrate into the product through horn antennas and, having passed the received signal amplifier, are recorded by a receiving device.

infrared Defectoscopy uses infrared (thermal) rays (see infrared radiation) to detect inclusions that are opaque to visible light. The so-called infrared image of the defect is obtained in the transmitted, reflected or intrinsic radiation of the product under study. This method controls products that heat up during operation. Defective areas in the product change the heat flux. A stream of infrared radiation is passed through the product and its distribution is recorded by a heat-sensitive receiver. The heterogeneity of the structure of materials can also be studied by the ultraviolet method. Defectoscopy

Magnetic Defectoscopy is based on the study of magnetic field distortions that occur in places of defects in products made of ferromagnetic materials. The indicator can be a magnetic powder (ferrous oxide) or its suspension in oil with a particle size of 5-10 microns. When the product is magnetized, the powder settles at the location of defects (magnetic powder method). The stray field can be recorded on a magnetic tape, which is applied to the investigated area of ​​the magnetized product (magnetographic method). Small-sized sensors (flux probes) are also used, which, when moving along the product at the defect site, indicate changes in the current pulse recorded on the oscilloscope screen (flux probe method).

Electroinductive (eddy current) Defectoscopy is based on the excitation of eddy currents by an alternating magnetic field of the flaw detector sensor. Eddy currents create their own field, opposite in sign to the exciting one. As a result of the interaction of these fields, the impedance of the sensor coil changes, which is indicated by the indicator. The indicator readings depend on the electrical conductivity and magnetic permeability of the metal, the dimensions of the product, as well as changes in electrical conductivity due to structural inhomogeneities or discontinuities in the metal.

thermoelectric Defectoscopy is based on the measurement of the electromotive force (thermopower) that occurs in a closed circuit when the contact point of two dissimilar materials is heated. If one of these materials is taken as a standard, then for a given temperature difference between hot and cold contacts, the value and sign of the thermoelectric power will be determined by the chemical composition of the second material. This method is usually used in cases where it is required to determine the grade of material that makes up a semi-finished product or structural element (including in a finished structure).

electrostatic Defectoscopy is based on the use of an electrostatic field in which the product is placed. To detect surface cracks in products made of non-conductive materials (porcelain, glass, plastics), as well as from metals coated with the same materials, the product is dusted with fine chalk powder from a spray gun with an ebonite tip (powder method). In this case, the chalk particles receive a positive charge. As a result of the inhomogeneity of the electrostatic field, chalk particles accumulate at the edges of cracks. This method is also used to control products made of insulating materials. Before pollination, they must be moistened with an ionic liquid.

Rice. Fig. 5. Block diagram of an ultrasonic echo flaw detector: 1 - electrical pulse generator; 2 - piezoelectric transducer (search head); 3 - receiving-amplifying path; 4 - timer; 5 - sweep generator; 6 - cathode ray tube; H - initial signal; D - bottom echo signal; DF - echo signal from a defect.

Ultrasonic Defectoscopy is based on the use of elastic vibrations (see Elastic waves), mainly in the ultrasonic frequency range. Violations of the continuity or homogeneity of the medium affect the propagation of elastic waves in the product or the vibration mode of the product. Main methods: echo method, shadow, resonant, velosymmetric (actually ultrasonic methods), impedance and free vibration method (acoustic methods). (Fig. 5)

The resonance method is based on determining the natural resonant frequencies of elastic vibrations (with a frequency of 1-10 MHz) when they are excited in the product. This method measures the wall thickness of metal and some non-metal products. With the possibility of measuring on one side, the measurement accuracy is about 1%. In addition, this method can identify zones of corrosion damage. Resonance flaw detectors carry out manual control and automated control with recording instrument readings.

The velocimetric method of echo flaw detection is based on measuring changes in the velocity of propagation of elastic waves in the area of ​​defects in multilayer structures, and is used to detect areas of debonding between metal layers.

The impedance method is based on measuring the mechanical resistance (impedance) of a product with a sensor that scans the surface and excites elastic vibrations of sound frequency in the product. This method can detect defects in adhesive, soldered, and other joints, between thin skin and stiffeners or fillers in multilayer structures. Detected defects with an area of ​​15 mm 2 or more are marked by a signaling device and can be recorded automatically.

The method of free oscillations (see. Natural oscillations) is based on the analysis of the spectrum of free oscillations of a controlled product excited by an impact; is used to detect areas of broken connections between elements in multilayer glued structures of considerable thickness from metallic and non-metallic materials.

Ultrasonic Defectoscopy, which uses several variable parameters (frequency range, wave types, radiation modes, contact methods, etc.), is one of the most versatile non-destructive testing methods.

capillary Defectoscopy is based on an artificial increase in the light and color contrast of the defective area relative to the undamaged one. Capillary methods Defectoscopy make it possible to detect with the naked eye thin surface cracks and other material discontinuities that form during the manufacture and operation of machine parts. The cavities of surface cracks are filled with special indicator substances (penetrants), which penetrate into them under the action of capillary forces. For the so-called luminescent method, penetrants are based on phosphors (kerosene, noriol, etc.). A thin powder of a white developer (magnesium oxide, talc, etc.), which has sorption properties, is applied to the surface cleaned of excess penetrant, due to which the penetrant particles are removed from the crack cavity to the surface, outline the crack contours and glow brightly in ultraviolet rays. With the so-called color control method, penetrants are based on kerosene with the addition of benzene, turpentine and special dyes (for example, red paint). To control products with a dark surface, a magnetic powder colored with phosphors (magnetoluminescent method) is used, which facilitates the observation of fine cracks.