RU2520950C1 - Ultrasonic surface wave converter and method for manufacture thereof - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: ultrasonic surface wave converter comprises piezoelectric elements which can be connected to an ultrasonic flaw detector, an acoustic insulator, a housing, wherein the piezoelectric elements of the converter, which are polarised on the thickness, are mounted on one face perpendicular to the working surface, lie apart from each other by a distance greater than the spatial duration of a probing pulse (L>τc), are acoustically insulated from each other and are electrically connected together or separately with clock or synchronous connection.

EFFECT: designing a surface wave converter which enables to inspect metallic and non-metallic articles, having high efficiency and accuracy of determining coordinates and location of defects, as well as high resolution, having high inspection sensitivity, enabling adaptation when inspecting articles with a complex profile and limited access, which enables to detect differently oriented defects.

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Description

The group of inventions relates to the field of non-destructive testing and can be used for ultrasonic inspection of metals and nonmetals in mechanical engineering, aerospace, shipbuilding, etc.

Known ultrasonic transducer of surface waves, comprising a housing, connecting wires, a prism of plexiglass, with a piezo plate glued on it [Aleshin NP, Bely V.E., Vopilkin A.Kh. and others. Methods of acoustic control of metals. M., Engineering, 1989, p. 145]. The surface wave of such a converter is formed due to the transformation of elastic (longitudinal) waves propagating in a prism from plexiglas at the plexiglass / metal interface. The condition for the transformation of a longitudinal wave into a surface wave is: β> β2 _cr , where β is the angle of the prism, β2 _cr is the 2nd critical angle.

The disadvantages of such a converter include significant overall dimensions, the inability to adapt to control products with limited access and complex configuration, inaccuracies and difficulties in determining the coordinates and location of defects, it is also impossible to control non-metals, which reduces the area of its use.

Closest to the claimed group of inventions is a transducer with a matching element of metal, comb structure from the side of the working surface of the transducer [Viktorov I.A. "The physical basis for the application of Rayleigh and Lamb ultrasonic waves in technology." M., publishing house "Science", 1965, p.14]. An X-slice quartz plate is located on top of a matching element made of metal and in parallel with a controlled surface. The step or period of the comb structure of the matching element is chosen equal to λ _R / 2, where λ _R is the spatial period of the set of normal perturbations on the surface of a solid.

The disadvantages of such a converter are:

- low intensity of surface waves due to losses due to counterpropagating, damping, multidirectional surface waves under the surface of the comb;

- the presence in the controlled medium, simultaneously with the surface wave, of an intense longitudinal wave, which is "spurious," making it difficult to detect defects and determine their coordinates, especially when monitoring thin-walled products against the background of multiple re-reflections of the longitudinal wave in the controlled product;

- a large duration of the echo signal, worsening the resolution condition of the Converter;

- the inability to adapt such a converter to control parts of a complex profile and limited access;

- significant complexity and inaccuracies in determining the coordinates of defects on the surface of products, the same as that of prismatic transducers.

The technical result of the group of inventions is the creation of a surface wave transducer that provides control of metal and non-metal products, with improved performance and accuracy of determining the coordinates and location of defects, as well as high resolution, high control sensitivity, the ability to adapt when monitoring products of complex profile and limited access providing the identification of misoriented defects.

The specified technical result is ensured by the fact that in an ultrasonic transducer of surface waves containing piezoelectric elements that can be connected to an ultrasonic flaw detector, an acoustic insulator, a housing, it is new that the piezoelectric transducers of the transducer are polarized in thickness, mounted face perpendicular to the working surface, located at a distance from each other of each other, longer than the spatial duration of the probe pulse (L> τ · s _s ), are acoustically isolated from each other and electrically connected They are shared or separately with clock or synchronous inclusion. In the method of manufacturing an ultrasonic transducer of surface waves, it is new that the piezoelectric elements of the transducer are mounted in pairs, perpendicular to the working surface, mutually perpendicular to each other, are electrically connected in pairs according to a separate circuit, with a clock or synchronous inclusion, are acoustically isolated, and the emission of elastic waves into a controlled medium is carried out directly or through a matching layer (protector).

Below, with reference to graphic materials, the construction and description of some of the possible types of transducers described in the claims are given.

An ultrasonic transducer of surface waves (Figs. 1-5) comprises: connecting wires 1, a damper or acoustic insulator 2 and 3, a housing 4, piezoelectric elements 5. The transducer may also contain a protector 6. The piezoelectric transducers of the transducer, polarized in thickness, can be connected by wires 1 with a flaw detector (position not indicated). The piezoelectric elements 5 are installed when they are mounted on the face, perpendicular to the working surface of the transducer A. The piezoelectric elements 5 can be in the form of plates (Figs. 1-3) mounted in the housing parallel to each other (Fig. 1) or at an angle to each other (Fig. 2 ) The piezoelectric elements 5 can have a round, for example radial, shape and are located perpendicular to each other (figure 4), or a cylindrical shape (figure 5) and are located coaxially relative to each other.

The Converter may contain (Fig.3) four piezoelectric plates forming two pairs, in each of which the plates are parallel to each other, and the plates of different pairs are located perpendicular to each other. This arrangement of piezoelectric plates forms a closed rectangle, the sides of which are the piezoelectric plates themselves, and the area inside the plates is a control zone.

In all cases, the piezoelectric elements are placed in the housing at a distance relative to each other, greater than the spatial duration of the probe pulse - L> τ · c _s .

Assembly (manufacturing) of the Converter is as follows.

The piezoelectric elements 5, polarized in thickness, with soldered contact wires, are mounted on the face of the piezoelectric element, perpendicular to the working surface of the transducer A and rigidly connected (for example, with glue or resin) to the acoustic insulator 2 and 3. The piezoelectric elements 5 can be mounted on the insulator parallel to each other to each other, at an angle to each other, perpendicular to each other, coaxially to each other or in pairs, mutually perpendicular to each other, that is, as shown in FIGS. 1-5.

Porous or cold pressed polymers, for example, expanded polystyrene, opaque to ultrasonic vibrations, epoxy resin with absorbing powder fillers, etc. can also be used as an acoustic insulator. In this case, the thickness of the insulator should be such as to ensure, firstly, the resolution of two echo pulses (probe and transmitted surface waves), and secondly, the absorption of a direct longitudinal wave through the insulator. To fulfill the first condition, it is necessary to know the duration τ of the probe pulse and the propagation velocity of the surface wave in the controlled material c _s . Thus, the resolution condition for two echo pulses is: L> τ · c _s , where L is the thickness of the acoustic insulator used. Usually these parameters are always known, or they can be directly measured using measuring equipment. To fulfill the second condition, it is sufficient to select a material with a large attenuation of ultrasonic vibrations; Styrofoam and pressed polystyrene correspond to this condition.

Next, the insulator, with the piezoelectric elements 5 and conductors rigidly fixed on it, is inserted into the body 4, so that the piezoelectric elements are at a certain distance from the walls of the body.In this case, the piezoelectric elements with the insulator must directly contact the surface through a thin layer of glue (for example, epoxy resin). After the glue hardens, the casing is filled with epoxy resin with a powder filler (for example, iron or lead minium) and a hardener to a certain level at which the acoustic insulator with piezoelectric elements is completely covered by the resin, but the soldered conductors are taken out of the resin level and fixed.

After completion of the polymerization of the epoxy resin (to speed up the process, it is carried out with heating the resin to a temperature of about 40 ° C) contact conductors are soldered to the connectors. As a result of the operations, the design of an ultrasonic transducer is obtained, which, when connected to an ultrasonic flaw detector, can operate in various control modes: separately-combined, combined, individually or jointly. In this case, the piezoelectric elements 5 of the transducer are in contact during operation with the surface of the controlled material directly or through a thin layer of matching medium (tread) 6.

The manufactured ultrasonic transducer operates as follows.

When connected to an ultrasonic flaw detector in a separately combined mode (Figs. 1, 2), one of the piezoelectric elements connected to the generator performs elastic oscillations in a pulsed mode at a resonant frequency, the other connected to the receiver operates in standby mode. In this case, only one probe pulse is observed on the screen of the flaw detector. When the transducer is installed on the surface of the controlled material (metal / non-metal) moistened with water (oil), the piezoelectric element connected to the probe pulse generator emits an elastic surface wave into the medium, which is received by the face by another receiving piezoelectric element operating in the standby mode. If there is no defect in the installation location of the transducer, then an echo pulse of the transmitted surface wave will appear on the screen of the flaw detector at some distance from the probing signal. If there is a defect (surface / subsurface) in the material under the transducer surface, then the echo pulse of the transmitted surface wave will weaken by a certain amount depending on the size of the defect and its depth. On the flaw detector screen, the echo pulse will decrease or disappear completely, which will be registered as a defect (implementation of the shadow control method). In this case, the location of the defect is localized in the zone of the contact surface of the transducer, between the two piezoelectric elements, and is actually determined only by the duration of the probe pulse and the selected distance between the piezoelectric elements, while the minimum possible distance is selected from the resolution condition of two pulses, one of which is probing. A surface wave in a controlled medium is formed due to alternating shear deformations directed in different directions from a piezoelectric element oscillating with a given frequency and amplitude, polarized in thickness, oriented parallel to the input surface. Particles of the medium perceive pulsed shear deformations transmitted directly or through the tread from the piezoelectric elements, and form a running elastic surface wave with a speed equal to c _s . In this case, a surface wave is formed without transformation of one type of wave into another, as occurs, for example, in a prism from plexiglass or in a matching “comb”, and therefore, in this case there are no losses due to transformation. The emission of elastic surface waves from piezoelectric elements mounted on a controlled surface occurs in both directions, which in justified cases increases the performance of the control. In converters operating according to a separate circuit, the second wave becomes “spurious" and get rid of it by applying acoustic insulation.

It should also be noted that in the proposed design of the transducer when exciting surface waves there is no “parasitic" longitudinal wave, as this happens in the prototype, which complicates the process of decoding waveforms / defectograms, especially when controlling thin-walled products. The principle of formation of surface waves laid down in the design allows control, for example, of a thin foil several microns thick, as well as the delamination of thin coatings. However, it should be borne in mind that a surface wave will occur only in solid materials, where shear deformations of the medium are possible. These can be metals and their alloys, as well as non-metals - composite materials, polymers: polycarbonates, organic glass, polyamides, etc. Thus, this converter, unlike analogs, simultaneously allows monitoring of non-metals.

When only one piezoelectric element is connected to the flaw detector, in the combined mode (the second piezoelectric element is disconnected), the transducer emits elastic vibrations in both directions, thereby increasing the monitoring performance. However, this makes it difficult to decrypt and identify defects. In each case, the developers of the control methodology evaluate and choose the control mode (connecting piezoelectric plates) that provides the necessary reliability and control performance.

When two piezoelectric elements are connected in the combined control mode, productivity doubles, however, an undesirable effect of oscillations of one piezoelectric element on another is possible. To prevent this, the piezoelectric elements are included in different measures: in the first measure of the repetition rate, the first works, in the second - the second, in the third measure - again the first, in the fourth - the second, etc. Or each piezoelectric element is connected to different independent channels. This is possible with a two-channel flaw detector. The operation of such a converter is similar to that described above, only an indication of defects occurs on each of the independent channels.

For operation of the transducer with two pairs of piezoelectric plates (Fig. 3), they are connected in pairs, for example, according to a separate circuit with a clock, synchronous or channel, independent inclusion. Clock inclusion - in the first cycle of the repetition rate, the first pair of piezoelectric elements is excited and received, in the second - respectively the second pair, in the third cycle - again the first couple, in the fourth - the second and so on throughout the control cycle. Synchronous inclusion - both pairs of piezoelectric elements work simultaneously. Channel, independent inclusion - the operation of each pair of piezoelectric elements in the transducer is carried out through two independent flaw detector channels, with independent indication of information on defects in separate or combined mode. It is also possible to include two adjacent piezoelectric plates to the probe pulse generator for operation in the radiation mode, and two other adjacent ones in the reception mode. There are other options for the inclusion of piezoelectric plates, not considered in this description. When you turn on the transducer in one of the described modes (for example, separate mode with clock switching), only one pulse is observed at the beginning of the sweep - probing (when the transducer is not loaded on the acoustic medium). When the transducer is installed on the surface of the controlled product moistened with the contact medium, two echo pulses will appear on the scan at a distance determined by the delay value. When the transducer scans over the surface of the product with the specified step and control path when the defect (medium discontinuity) enters the control region under the transducer, the amplitude of two echo pulses decreases to a certain value, which will be detected by the flaw detector indicator. And regardless of the orientation of the material defect, it will be detected by the corresponding pair of piezoelectric plates of the transducer, with respect to which it is optimally oriented, from the point of view of detectability. Those. with this arrangement of piezoelectric elements, a shadow control method is implemented with the identification of defects in the controlled product located in an arbitrary place and in any orientation, regardless of material (metal / non-metal).

The Converter shown in figure 4, is equipped with piezoelectric elements in the form of a part of the cylinder 5 (a special case of the piezoelectric element is a half-cylinder).

A characteristic feature of a transducer with radius-shaped piezoelectric elements is that if a flat-shaped piezoelectric element emits a flat front of a surface wave into the medium on both opposite sides, then when using piezoelectric elements of this shape due to their curvature, a converging elastic wave front is formed on the one hand (with the focusing effect) ) and the focal zone at a distance proportional to the radius of curvature, the diverging front of the elastic surface wave, non-directional (spherical lna). It is advantageous to use both of these phenomena in control in the following cases: the focusing effect is important from the point of view of increasing sensitivity and resolution, and the effect of the formation of an undirected (spherical) wave increases the performance of the control and the detection of misoriented defects of the controlled material.

The use of a cylindrical piezoelectric element in the transducer (Fig. 5) is most appropriate for identifying misoriented defects that fall into the annular sounding zone between two coaxially located piezoelectric elements. When connecting the transducer to an ultrasonic flaw detector, the transducer works the same as the transducer in Fig. 1, the difference is in the form of piezoelectric elements, therefore, it is not practical to describe its operation. Its advantage is that, due to the shape of the cylinder, it forms an undirected surface wave in a controlled medium, which ensures the detection of misoriented material defects.

Information sources

1. Aleshin N.P., Bely V.E., Vopilkin A.Kh. and other Methods of acoustic control of metals. M., Mechanical Engineering, 1989, p. 145.

2. Viktorov I.A. Physical fundamentals of the application of Rayleigh and Lamb ultrasonic waves in technology. M., publishing house "Science", 1965, p. 14.

Claims (2)

1. An ultrasonic transducer of surface waves containing piezoelectric elements that can be connected to an ultrasonic flaw detector, an acoustic insulator, a housing, characterized in that the transducer piezoelectric elements polarized in thickness are mounted on one of the faces perpendicular to the working surface, located at a distance from each other, greater spatial duration of the probe pulse (L> τc), acoustically isolated from each other and electrically connected together or separately with the clock or sync inclusion. 2. A method of manufacturing an ultrasonic transducer of surface waves, characterized in that the piezoelectric elements of the transducer are mounted in pairs on one of the faces perpendicular to the working surface, the piezoelectric elements of each pair are parallel to each other, and the pairs themselves are arranged perpendicular to each other, electrically connected in pairs according to a separate clock circuit or by synchronous inclusion, acoustically isolate, and the emission of elastic waves in a controlled environment is carried out directly or through matching layer (protector).

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