MEETINGS AND CONFERENCES

MEETING ON ULTRASONIC FLAW DETECTION AND GENERAL PROBLEMS OF ULTRASONICS

An expanded meeting on questions of ultrasonic flaw detection and certain other problems of ultrasonics, convened by the Commission on Acoustics of the Academy of Sciences of the USSR, was held in Moscow. This meeting, which aroused great interest in scientific and technical circles, was attended by representatives of various ministries, factories, research and educational institutions, and laboratories.

The meeting heard 15 reports, of which 9 were devoted to ultrasonic flaw detection and 6 to other questions.

Of particular interest to those present at the meeting was the demonstration of various ultrasonic instruments proposed and developed by domestic laboratories. The operation of these instruments was shown on a variety of reference specimens and factory defective parts.

The high quality of inspection provided by the use of ultrasonic flaw detection and its advantage over the generally accepted selective method of inspection were convincingly demonstrated.

In lively debates on the reports, the participants in the meeting delivered a number of valuable communications on questions of the further development and introduction of ultrasonic flaw detection and other branches of ultrasonics into technology and scientific research.

Opening the meeting, the chairman of the Commission on Acoustics of the Academy of Sciences of the USSR, Corresponding Member of the Academy of Sciences of the USSR N. N. Andreev, pointed out that ultrasonic flaw detection, created in the USSR by the works of Professor S. Ya. Sokolov, only considerably later began to develop rapidly abroad. The broad introduction of acoustic flaw detection is of very great importance for industry and transport. Further, N. N. Andreev noted that at present in acoustics the questions of ultrasonics occupy almost first place—one of the most important branches of acoustics both in the USSR and abroad. Thus, ultrasonics is closely connected with hydroacoustics. Other fields of application of ultrasonics may be divided into three groups:

a) ultrasonic flaw detection, where ultrasound serves to detect defects of all kinds, to determine the dimensions of an article, etc.;

b) physical change in these or other constants of a substance with a small intensity of ultrasonic waves, which do not alter the conditions of the experiment or the substance itself (linear ultrasonics);

c) the application of intense ultrasound to create irreversible reactions and physical action on a substance.

N. N. Andreev cited some examples of such use of ultrasonic waves for crystallization in chemistry (S. Ya. Sokolov), for the coagulation of smokes, the precipitation of fogs, for the coagulation of colloids; in medicine—as a tool for diagnosis and medicinal action and for reverse biological action: the destruction of bacteria, the destruction of cells, etc.

In conclusion N. N. Andreev pointed out that in the near future one may expect a great development of industrial, medical, and other applications of ultrasound, and that the present conference should sum up the achievements obtained in some of these fields, as well as outline ways for their further development.

Professor S. Ya. Sokolov gave an extensive survey of “the present state of ultrasonic flaw detection,” convincingly showing that the Soviet Union holds first place in the field of research on ultrasound and ultrasonic flaw detection in particular. S. Ya. Sokolov pointed out the great importance of ultrasonic flaw-detection methods for industry, since with their aid it is possible to record immediately the smallest defects and flaws at a depth of up to several meters, whereas other physical methods of inspection are less effective. The speaker also noted that ultrasonic vibrations are characterized by very large accelerations, exceeding by several thousand times the acceleration produced by the earth’s gravitational force. These accelerations are the cause of the appearance of new effects, as yet almost unused in technology. In assessing the properties of quartz piezoelectric radiators, S. Ya. Sokolov indicated the causes that bring about distortion of a plane wave front: a) diffraction phenomena arising at the boundaries of the radiating plate; b) a nonuniform distribution of the values of the piezoelectric and elastic constants over the surface of the plate, and, as a consequence of this, a nonuniform distribution of the amplitude of the vibrations; c) a nonuniform orientation of the crystals. Because of this, in addition to sound waves propagating in the metal perpendicular to the irradiated surface, oblique wave beams arise; along with longitudinal vibrations, transverse vibrations are also excited, which confuse the picture of the pulses seen on the cathode-ray tube.

The speaker reported that, with the aid of a quartz plate 6 mm in diameter, he had succeeded in exciting vibrations with a frequency of the order of \(10^9\) cps in another quartz plate, and noted that obtaining very high frequencies is extremely important because the success of the development of ultrasonic microscopy depends on it. The speaker investigated the propagation of ultrasonic waves of high frequency in crystals and found that, for example, quartz is characterized by anisotropy not only with respect to the elastic moduli, but also with respect to the attenuation coefficients. Further, at oblique incidence of sound the author observed sound pulses multiply reflected from the boundaries of quartz; moreover, judging by the intervals between them, not all of them can be explained by the propagation of longitudinal and transverse waves in the quartz medium.

In assessing the possibilities of modern ultrasonic flaw detectors, the speaker noted that they make it possible not only to register the presence of a defect, but also to determine, with the accuracy required for practice, the depth at which it lies. However, one of the substantial shortcomings is at present the limited application of flaw detectors only to smooth surfaces of articles. In order to avoid treating the metal surface, various measures are taken to cover it with various kinds of pastes, putties, the construction of special probes, etc. One of the most effective methods is to lower the frequency to a value at which the size of the inhomogeneities on the metal surface becomes considerably smaller than the wavelength.

The speaker considered possible methods of obtaining visible images applicable to ultrasonic microscopy:

a) by means of the scanning beam of an electron-beam tube by the method known in television; b) according to schemes used in electron microscopes, in which the photocathode is replaced by a piezoelectric quartz plate; c) at the interface between a liquid and a gas. S. Ya. Sokolov noted that the resolving power of an ultrasonic microscope, like that of an optical and an electron microscope, depends on the wavelength of the radiation and on the quality of the focusing system.

In conclusion, photographs of various images obtained with the aid of one of the instruments constructed by the author were demonstrated. Photographs were shown of the process of dissolving crystals of tartaric acid in a liquid (indistinguishable by eye because of their color), photographs of the sound field of vibrating quartz plates, of defects in metallic foil, and others.

The report also gave examples of the use of ultrasound for studying the course of physicochemical processes and of the use of ultrasound for modulating light beams.

In the report by D. S. Shraiber, “On the Development of the Ultrasonic Inspection Method,” it was noted that ultrasonic flaw detection makes it possible to inspect products and semi-finished pieces of considerable cross sections and to reveal defects inaccessible to other nondestructive testing methods. The need has arisen to broaden the field of flaw detection by inspecting thin-walled products, detecting defects occurring at small depth, etc. The author emphasized that the successful application of the ultrasonic method and the interpretation of the results obtained are possible only under conditions of careful training of operators and the combined use of various flaw-detection methods that complement one another. The report noted that the ultrasonic method makes it possible successfully to detect discontinuities in metal, cavities, delaminations, coarse-grained structure and, in particular, coarse grain size (the latter by loss of the reflected pulse due to significant scattering of vibrations).

Recently the “refracted-beam method” has been gaining ground; in this method ultrasonic vibrations are introduced into the metal with the aid of probes of special design at a considerable angle. This method permits quite reliable inspection of welded seams, but only under the condition that signals caused by the propagation of longitudinal and transverse waves are confidently recognized.

D. S. Shraiber pointed out that there is no unambiguous relation between the height of the peak of the pulse reflected from a defect and the size of the defect, since the height of the peak depends on the shape of the defect and on whether a pulse reflected by such a defect is focused or scattered. It is therefore incorrect to judge a defect only by the amplitude of the pulse. Accumulated statistical material has made it possible to establish in this field certain regularities that are used in practice. The speaker described improvements introduced by him and his co-workers into the design of the pulsed flaw detector and search heads, which made it possible to expand significantly the range of application of the pulsed flaw detector and to reduce the “dead zone.” The speaker also described the design of an instrument for inspecting thin-walled products and for one-sided determination of their thickness. This instrument—an ultrasonic resonance flaw detector, based on recording standing elastic waves formed in the metal under certain conditions—makes it possible to measure with high accuracy the thicknesses of pipes and sheets, to detect delaminations, etc.

N. N. Baryshnikov spoke on “Ultrasonic Inspection of Steel Forgings,” used to determine the quality of individual forgings

without violating their integrity. The commonly accepted method for determining the quality of such shells by selectively making macrosections is, apart from the loss of metal, far from perfect. Having substantiated, by experimental data, the need to apply the ultrasonic method of inspection, the author indicated the significant advantages of this method over other physical methods: simplicity, low cost, and the fact that it makes it possible to detect defects of very small thickness in comparison with the thickness of the article itself.

The speaker noted that, according to his observations, various defects (cracks, porosity, slag inclusions) form defect pulses of different shape, and therefore, with a certain amount of skill, one can judge the nature of the defect from the shape of this pulse. The contact method of sounding currently used has its drawbacks. The black, rough surface of the article reduces the sensitivity of the method; therefore mechanical treatment of the surface is necessary. Defects located close to the surface are not detected, since the pulses reflected from these defects are superimposed on the initial ones.

V. S. Sokolov reported on “Certain works in the field of ultrasonic flaw detection.” The speaker and his colleagues developed a small-sized pulsed ultrasonic flaw detector.

A method has been investigated and developed for inspecting articles (lined with rubber) using lower ultrasonic frequencies than those used in ultrasonic flaw detectors. This method made it possible to record distinctly delaminations and air bubbles. For better acoustic contact, special liquid-rubber heads were fitted onto the piezoelectric plates. The speaker also indicated that a substantial reduction in ultrasonic frequency compared with the frequency range now used in ultrasonic flaw detection makes it possible to inspect coarse-grained metals, while liquid rubber heads make it possible to inspect articles with a rough surface. This may prove effective in a number of cases where only large defects need to be noted.

V. S. Sokolov further reported that he and his colleagues had built instruments for unilateral determination of the thickness of an article, in which the dependence of the mechanical reaction of the side of the article opposite the piezoelectric plate on the thickness of the article is used. These instruments are also suitable for measuring the degree of corrosion of the internal surfaces of pipes and boilers, etc.

The speaker described a method for obtaining a visible image of a defect, in which the article under investigation is sounded by an oblique beam of ultrasonic waves. Behind the article is placed an acoustic focusing lens, in whose focal plane there is a cassette filled with a suspension of aluminum powder in xylol or kerosene. The wall of this cassette on the side of the article is sound-permeable, and on the outer side is transparent to light. When an ultrasonic field is imposed, the aluminum particles are oriented across the direction of wave propagation, as a result of which their illumination changes (when the cassette is illuminated from outside by side light). If there are defects in the article, they are sources of secondary sound waves which, being focused by the lens, give an image of the defects in the form of light spots against a gray background. In order to increase the contrast of ultrasonic images obtained in this way, the aluminum suspension, at the suggestion of V. S. Sokolov and M. B. Kardash, is preliminarily oriented in the direction of propagation of the sound waves by means of an external electrostatic field. The speaker demonstrated a number of images obtained with a transfer model of such a device, making it possible not to immerse the article under test in the liquid.

Corresponding Member of the Academy of Sciences of the USSR B. M. Vul reported on “piezoelements made of barium titanate.” Investigations by the author and his collaborators were the first to establish the special properties of barium titanate that distinguish it among other ceramic materials: the presence in it of spontaneous polarization and deformation (in the absence of applied external mechanical stresses) at temperatures below the Curie point (about \(+120^\circ\)). Above the Curie point barium titanate has a cubic lattice of the perovskite type; below the Curie point and approximately down to \(0^\circ\) the elementary crystal cells have a tetragonal form. The change in the dimensions of the elementary cells accompanying the phase transition at the Curie point may be regarded as a deformation associated with spontaneous polarization. In polycrystalline specimens of barium titanate the crystals are arranged chaotically and are divided into domains with different directions of spontaneous polarization within them. Therefore such specimens, like any polycrystalline specimen of piezoelectric crystals, do not exhibit a piezoelectric effect. When an external electric field is applied, the spontaneous polarization is oriented along the direction of the field. This process is analogous to the well-known phenomenon of magnetization of ferromagnets in an external magnetic field. In this case barium titanate exhibits piezoelectric properties, which are retained after the external electric field is switched off. The direct piezoelectric effect is explained by the fact that an external mechanical action changes the established deformation and, together with it, correspondingly changes the polarization, which is revealed in the form of free electric charges on the electrodes of the piezoelement. An external electric action changes the established value of the polarization and, together with the deformation of the elementary crystal cells associated with it, which manifests itself as the inverse piezoelectric effect in a change in the dimensions of the piezoelement.

The spontaneous deformation is, in a first approximation, directly proportional to the square of the spontaneous polarization. However, if the polarization of the external field is considerably smaller than the spontaneous polarization and, consequently, the deformations caused from outside are small in comparison with the spontaneous deformation, the piezoelectric effect is linear, i.e., there is direct proportionality between the changes in the magnitude of the polarization and the deformation caused by external actions. B. M. Vul gave some quantitative data characterizing the piezoelectric properties of barium titanate, from which it follows that barium titanate is an effective piezoelectric. This makes it possible to use it successfully for various purposes in ultrasonics and acoustics in general. The technology for producing barium titanate is extremely simple, and specimens of any shape and thickness can be obtained.

A. S. Matveev gave a brief survey of certain works in the field of ultrasonic flaw detection. The speaker indicated that work on ultrasonic flaw detection had as its aim the development of apparatus for sounding articles by ultrasonic vibrations propagating perpendicular to the surface of the article. But since practice showed that in this way it was impossible to detect vertical cracks and that in a number of cases this method proved untenable, later work was begun on creating prismatic probes and on using, for flaw detection, transverse waves as well, propagating in the articles at an oblique angle to the surface.

In the speaker’s opinion, a modern ultrasonic flaw detector should make it possible to detect these defects at depth, beginning with the surface layer and down to hundreds of millimeters and more; to permit inspection of sheet products, in particular boiler drums and parts for them, without special machining, and also to inspect welded seams without removal of the reinforcement. The speaker believes that, in addition to the ultrasonic frequencies used in most existing flaw detectors, it is expedient—

accordingly, for the inspection of steel castings it is advisable to choose an additional, comparatively low frequency.

In the report by Yu. V. Bogoslovsky, “Ultrasonic flaw detector,” it was stated that the purpose of inspecting steel manufactured parts, flat and cylindrical welded seams, riveted joints, boiler drums, and steel castings is to reveal internal defects in the thickness of the product—slag inclusions, gas bubbles, flakes, delaminations, looseness, and also cracks, especially those situated normal to the plane of sounding. Yu. V. Bogoslovsky described various designs of probes developed by them: flat open and closed probes for operation in a single-probe and a two-probe arrangement, prismatic probes that make it possible to introduce ultrasonic vibrations at an angle to the surface of the article being tested and thereby ensure the inspection of welded seams, vertical cracks, etc.; probes with rubber cushions that make it possible to inspect unmachined surfaces. In the new model of the flaw detector there is also a depth gauge. The speaker pointed out that the introduction of these and certain other improvements into the new model of the flaw detector made it possible to apply it successfully in various cases.

The report by M. R. Gubanova concerned the “ultrasonic method for inspecting disks.” Pointing out that these disks operate under conditions of high speeds and stresses and that they must not contain any metal defects whatever, the speaker noted that the greatest danger is represented by cracks of radial direction, the occurrence of which is possible during casting, heat treatment of blanks, and also in the process of fitting the disk onto the shaft. In the opinion of the speaker, the existing mechanical methods of inspection—macroscopic etching of the surface to reveal flakes and cracks, etc.—cannot provide complete inspection, since they do not allow defects to be revealed, in particular radial cracks that do not come out to the surface of the disk. The application of ultrasonic flaw-detection methods made it possible to solve the problem of disk inspection. After prismatic probes began to be used for this purpose, false pulses disappeared that were due to propagation in a solid medium of two types of waves, longitudinal and transverse, and it became possible to detect invisible radial cracks in disks.

V. V. Rakhmanov reported on the “ultrasonic method of inspecting riveted seams.” Pointing out that brittle fracture of metal, explained by the phenomenon of alkaline corrosion of highly stressed metal, is often encountered near riveted and rolled seats, the author noted that these cracks are usually detected only when they have developed considerably, when they emerge at the surface. The method used until recently for detecting cracks—etching a ground surface—and the magnetic-powder method of flaw detection require the seams to be opened and therefore permit only selective inspection to be carried out. For the ultrasonic method of inspecting these cracks, a prismatic probe was used, eliminating the effect of interference produced by partial reflection of the pulse from the surface of the prism and of the sheet. This made it possible to detect even small cracks.

N. V. Khimchenko shared experience in “ultrasonic flaw detection of apparatus parts.” A characteristic feature of the developed technique is inspection of parts in assembled apparatus. Since these parts, while differing from one another in size, have almost the same shape, it proved possible to obtain typical oscillograms characterizing sound parts. Using these “standard pictures,” defects could be detected from additional pulses appearing on the oscillograms. In some cases, strong attenuation of ultrasonic vibrations was observed, explained—as further investigation showed—by the coarse-grained structure of the steel.

defective parts. Defects were also discovered in one large part, and by systematically sounding individual sections and from several sides, the shape and dimensions of the zones in which defects occurred were determined. The speaker indicated that the results of ultrasonic investigation are insufficiently precise for judging the nature of the detected defects, and that, for a final judgment, it is also necessary to use data from gamma-ray or X-ray analysis.

The report by Yu. N. Shtremer concerned “the use of ultrasound for detecting defects in rails.” The ultrasonic method that has been developed makes it possible to introduce ultrasonic pulses into a rail and to detect typical defects. Characterizing the form obtained for the reflected pulses of ultrasonic waves as a function of the type of defect, the author noted a curious fact. It turns out that if there is a strictly vertical thin crack in the rail (directed parallel to the beam of ultrasonic waves), then it leads to the complete disappearance of the reflected pulse. In order to clarify the essence of this phenomenon, the author placed a plate of mica or metal in the path of ultrasonic waves propagating in a liquid. As the plate was brought closer to the axis of the beam, the intensity of the reflected pulse decreased, and when the plate was located exactly on the axis of the beam, the intensity became equal to zero. Replacement of a visual inspection of joint connections by ultrasonic inspection increases the safety of railway traffic and produces a considerable economic effect.

L. M. Brekhovskikh reported on “the scattering of ultrasound on uneven surfaces.” Pointing out that this problem is important not only for ultrasonics, but also for acoustics in general, and likewise for optics, the author recalled that up to the present time only one approximate solution has been known, given by Rayleigh for the very particular case of waves incident perpendicularly on a surface whose roughness is small both in comparison with the wavelength of sound and with the pitch of the waviness. The present communication relates to the initial stage of a theoretical consideration of this problem, namely: the case of the incidence of a plane wave on a strongly rough surface. Considering the field at any point in space as the sum of waves radiated by the entire scattering surface, and making the Kirchhoff approximation that the field on the rough surface itself is determined by the laws of geometrical optics, L. M. Brekhovskikh calculated the resulting integral expression for a spherical wave, representing it as a sum of solutions for plane waves.

The author illustrated the general solution thus obtained for the case of an elementary waviness and demonstrated diagrams of the directivity of the reflected sound for various ratios of the amplitude of the waviness to the pitch and for various angles of incidence, showing thereby that the Rayleigh solution can be obtained as a special case of the solution found. The speaker noted that the limits of applicability of the theory developed are restricted only by the condition that the Kirchhoff assumption be valid, including the requirement that the wavelength be small in comparison with the radius of curvature of the waviness.

In the case of a partly shaded rough surface, a shadowing function should be introduced into the calculation; at the same time, the transition zone between the illuminated region and the shadow region can also be taken into account.

In the report by B. D. Tartakovsky, a survey was presented of the results of theoretical and experimental investigation of “sound-focusing lenses and transition layers.” The speaker characterized the features of sound-focusing lenses and gave the calculated data that take these features into account. Sound-focusing lenses, whether gaseous, liquid, or solid, may, in contrast to optical ones, have a refractive index either greater or less than unity. In solid sound-

... in such lenses, along with longitudinal waves, transverse waves also propagate, creating an additional focus. Since, because of the inequality of the wave resistances of the lens and the surrounding media, strong losses can occur at the boundaries, the use of acoustic transition layers, analogous to “antireflection” layers in optics, is very important.

It was noted that such layers make it possible to increase severalfold the passage of sound energy through the lens boundary, which significantly broadens the range of materials that can be used for making lenses. For a number of reasons, acoustic lenses are distinguished by a highly nonuniform amplitude along the wave front. Owing to the not very large ratios of lens diameters to sound wavelengths, the dimensions of the diffraction maxima, especially along the lens axis, are not very small compared with the focal length.

The report presented some data from an experimental study of lenses and layers, showing satisfactory agreement with calculation. In particular, the focusing properties of plastic focusing lenses proposed by the author were demonstrated.

B. B. Kudryavtsev, considering the “application of ultrasound for the purposes of investigating various physicochemical systems and processes,” indicated that the principal method of investigation is the measurement of the velocity of ultrasound and its attenuation during propagation in gases, liquids, and solids. The study of the dispersion of the velocity of ultrasound in polyatomic gases made it possible to estimate the delay in the distribution of energy among the different degrees of freedom of gas molecules and to calculate the probability of excitation of the vibrational state upon collision of gas molecules; the presence of one or several vibrational levels of the molecules; the relaxation time of gas molecules; and so forth. Until recently, the dispersion of sound had been explained only by relaxation processes. Recently A. S. Predvoditelev advanced the suggestion that, in a gas, during the propagation of sound, fluctuations of density and temperature arise, characterized by their own frequency, which can be estimated on the basis of acoustic measurements. Another approach to the study of the velocity of sound in a gas is based on the idea that, in a sufficiently rarefied gas, its molecules do not interact with one another except at the moments of collision of gas molecules, and therefore the velocity of sound should be the same as the velocity of motion of the molecules. This is confirmed by experiment and serves to determine, on the basis of measurements of the velocity of sound, the duration of molecular collisions.

Further, the author noted that a hydrodynamic treatment of the process of sound propagation in a gas makes it possible to determine the constants of the equation of state from acoustic measurements.

Proceeding to a consideration of work concerned with the velocity of sound in liquids, the speaker dwelt on liquids remarkable for their exceptionally low velocity of sound \((C = 400—500\ \text{m/sec})\). Such an anomalously low velocity of sound in liquids testifies to sharp differences in the potential interaction of the molecules of these liquids from ordinary ones \((C = 1000—2000\ \text{m/sec})\).

The report indicated that the empirical Rama-Rao rule (the cube root of the velocity of sound times the molecular volume of a liquid is a constant independent of temperature) is applicable to the overwhelming majority of liquids, and the importance of developing a method for determining molecular weights from data on the velocity of sound and the density of a liquid was emphasized. B. B. Kudryavtsev also noted that treating the molecular velocity of sound as a quantity additive with respect to the atomic increments of the bonds present in a given compound is fruitful for studying the structure of molecules on the basis of

acoustic measurements. In this way, for example, one can assess the branching of molecules. The observed (V. V. Tarasov, I. I. Mikhailov, and others) breaks and inflections in the curves of the dependence of the speed of sound on the composition of mixtures, when the composition of the mixtures is varied smoothly, are connected with various interactions of the molecules of the components of the mixture and, in some cases, with the “fragility” of water.

The study of the propagation of ultrasound in solutions has recently been used to determine the mass of solvated ions. This is connected with the fact that, during the propagation of sound, displacement of ions and solvate shells is observed, which leads to the appearance of an alternating electric potential. The precise method of measuring changes in the speed of sound in liquids, developed by S. Ya. Sokolov, makes it possible to apply ultrasound to the study of the course of chemical reactions, for example the polymerization of methacrylic ester. Ultrasound has also been applied to the study of the process of diffusion.

B. B. Kudryavtsev gave a brief description of methods for measuring the speed of sound propagation in a solid medium, dwelling especially on the impulse method, the diffraction method applied to transparent bodies, and the resonance method based on the excitation of longitudinal or shear vibrations in a composite rod consisting of quartz and the material under investigation. By the composite-rod method, for example, the change with temperature of the shear modulus of a crystal and the change in the crystal structure of certain bodies have been studied. By the impulse method, the dependence on frequency of the mechanical properties of rubber and polymers has been considered, and the influence of temperature, vulcanization, and the amount of fillers in rubber on the magnitude of the modulus of elasticity, etc., has been studied. In conclusion, the speaker touched upon recent work in the field of the action of sound on matter, pointing to a method of preparing emulsion sulfamide preparations with the aid of ultrasound and to a method for the artificial preparation of “alloys” of two metals that normally do not give a solid solution (under the influence of ultrasound a finely dispersed system is obtained, the crystallization of which gives an alloy not produced by ordinary means). Along with its dispersing action, according to the literature data, ultrasound has a coagulating action and is used for the precipitation of smoke and dust. The ultrasonic sirens used for this purpose radiate vibrational energy of up to 2 kW. Ultrasound accelerates the polymerization of styrene and of certain other substances. In this case, within a certain interval of time, a doubling of the yield of the finished product can be achieved in comparison with the control sample. Ultrasonic vibrations have been successfully used for the study of complex adsorption equilibria (K. V. Chmutov), for accelerating extraction, for changing the crystalline structure of metals, etc.

In his report “On the Biological Action of Ultrasonic Waves,” I. E. Elpiner dwelt on the application of ultrasonic waves in medical practice. Ultrasonic vibrations have proved effective in the treatment of a number of diseases: gastric ulcers, acute diseases of the joints and muscles, neuralgias, lesions of the vascular system, etc. One of the principal obstacles to the further development of this branch of physical therapy is that the technique of dosing ultrasonic waves has so far been very poorly developed. This question is closely connected with the problem of the safety of this physical therapeutic procedure. The speaker cited a number of facts (established in experiments on animals) showing that the differences between the so-called indifferent dose and the dose causing deep injury to the tissues being irradiated are extremely small. In the opinion of foreign authors, a criterion of overdosage in therapeutic practice may be the severe pains that appear at the sites of irradiation. There arise-

...moreover, pains, in the opinion of these researchers, should serve as a warning signal because they always precede organic lesions of the corresponding tissues. It is obvious that such a subjective method of dosing ultrasonic energy cannot be considered satisfactory. Other biological methods of ultrasonic dosimetry are also unsatisfactory and extremely complicated: measuring the amount of hydrogen ions in insonated tissues, etc. Further research in this direction is necessary.

The speaker devoted the second part of his report to the problem of the mechanism of action of ultrasonic waves, setting forth chiefly the results of his own investigations. It turned out that, in the field of ultrasonic waves, important biologically active substances undergo decomposition: proteins, enzymes, amino acids, protoporphyrins, nucleic acids, and others. Analysis of this phenomenon led to the concept of an indirect action of ultrasonic waves (in addition to mechanical action). In an insonated aqueous medium, water molecules are split with the formation of valence-unsaturated radicals and atomic hydrogen (OH, HO₂, H, etc.), distinguished by great reactivity. As is known, such is also the mechanism of action of ionizing radiation (α-, β-, and γ-rays). The difference consists in the fact that ionization of water molecules in the case of ultrasonic waves takes place not in the liquid phase but in cavitation gas bubbles. The collapse (annihilation) of a gas bubble promotes the dispersion of the products of the splitting of water molecules that have formed into the surrounding medium, where they interact with the substances dissolved in the liquid. Proceeding from these ideas, the speaker showed that chemical processes occurring in the field of ultrasonic waves can be inhibited by adding to the insonated solution certain foreign substances. The latter are active in this respect when, possessing a great chemical affinity for the products of the splitting of water molecules, they easily interact with them. In this way they block the reactive radicals formed in the aqueous medium under the influence of ultrasound. Thus, certain amino acids (leucine, methionine, etc.), which are readily destroyed in the field of ultrasonic waves, retard the process of decolorization of methylene blue. In the presence of tryptophan (an amino acid), the oxidation of iodine in an insonated solution is inhibited. Leucine and methionine retard the decomposition of purine and pyrimidine bases, which are constituents of nucleic acids and play an important role in the vital activity of cells, etc. The phenomenon of “protection,” or “competition,” discovered by the speaker opens up new possibilities in the application of ultrasonic waves for solving a number of biological problems.

Finally, this report also pointed to prospects with respect to the use of ultrasonic waves for diagnostic purposes.

N. N. Dolgopolov spoke on the “application of ultrasound in chemistry and technology.” Quoting the words of the great Russian scientist A. M. Butlerov, spoken by him at the ceremonial meeting of the Academy of Sciences on December 29, 1870: “...as always happens, success in a certain branch of science did not remain without influence on its movement in general: experiments lead to the discovery of facts, facts call forth theory, and theory poses new questions and makes necessary new experiments leading to further discoveries,” the speaker indicated that the development of ultraacoustics in the USSR has led to the creation of a new field of chemistry, which the author, by analogy with the generally accepted terms electrochemistry, photochemistry, and thermochemistry, proposes to call sonochemistry. The speaker noted that the action of ultrasound on the course of chemical reactions, on substances, and on the physicochemical state of substances is associated by a number of authors (Ya. I. Frenkel, G. L. Natanson, V. L. Levshin, and S. N. Rzhevkin) with the occurrence of local electri-

physical fields in bubbles and cavities that form during cavitation of a liquid. In the opinion of N. N. Dolgopolov, attention must be paid not only to the photochemical character of discharges in cavitation bubbles, but also to the fact that cavitation gives rise to local increases in pressure and temperature in a small number of degrees. The influence of cavitation is associated with the conversion of water into hydrogen peroxide, nitric and nitrous acids. Under the influence of ultrasound, decomposition occurs of halogen derivatives of hydrocarbons (carbon disulfide and carbon tetrachloride), decomposition of hydrocarbons of the aromatic series, decomposition of azides, and so on. The study of the action of ultrasound on aqueous solutions of organic compounds shows a substantial change in the concentration of hydrogen ions. The author noted that the action of ultrasound on reactions is not reduced only to the splitting of compounds or to oxidative reactions. Activation of molecules caused by cavitation and by the alternating action of ultrasound is also of great importance. An example of this is the sharp acceleration of hydrolysis and saponification reactions of fats (N. N. Dolgopolov and E. I. Gorlinskaya).

The application of ultrasound in physicochemical processes gives, in some cases, results exceptional in their effect: for example, the dispersion of sulfur, tin, copper, mercury, and paraffin in water (S. N. Rzhevkin and others); the production of stable emulsions, for example of camphor and camphor oil in alcohol, sulfidine preparations and albichthol in water; the dispersion of coals in fuel oils for the purpose of obtaining fuel suspensions—colloidal fuel, and so on. The speaker indicated that in a suspension subjected to the action of ultrasound, the temperature of the suspended particles is always higher than the average temperature of the entire mixture—a phenomenon common to the nonuniform heating of suspensions under high-frequency electromagnetic oscillations. The speaker named possible areas for the application of ultrasound in technology and, as examples, discussed laboratory work that had been carried out on accelerating the manifestation process, bleaching and fixing motion-picture film, and the process of washing wool under the influence of ultrasound (N. N. Dolgopolov, V. M. Fridman, E. I. Gorlinskaya, E. L. Ruban).

The meeting adopted a number of resolutions, in which it noted the high level of scientific work and technical developments in the field of ultrasonic flaw detection and the significant results achieved in the application of ultrasonic waves in physics, chemistry, and biology.

The meeting recognized the necessity of further comprehensive development of ultrasonic acoustics and recommended that the participants of the meeting expand still further the areas of its application in technology, medicine, and scientific research.

The meeting emphasized the importance of the promptest possible publication in print of the results achieved, and resolved to request that the Acoustics Commission of the Academy of Sciences of the USSR publish a collection of the proceedings of this meeting.

B. D. Tartakovskii