In what units is ultrasound intensity measured? Ultrasound velocity measurement and ultrasonic equipment
The speed of propagation of ultrasound in concrete ranges from 2800 to 4800 m/s, depending on its structure and strength (Table 2.2.2).
Table 2.2.2
| Material | ρ, g/cm3 | v p p , m/s |
| Steel | 7.8 | |
| Duralumin | 2.7 | |
| Copper | 8.9 | |
| Plexiglas | 1.18 | |
| Glass | 3.2 | |
| Air | 1.29x10 -3 | |
| Water | 1.00 | |
| Transf oil | 0.895 | |
| Paraffin | 0.9 | |
| Rubber | 0.9 | |
| Granite | 2.7 | |
| Marble | 2.6 | |
| Concrete (more than 30 days) | 2.3-2.45 | 2800-4800 |
| Brick: | ||
| silicate | 1.6-2.5 | 1480-3000 |
| clay | 1.2-2.4 | 1320-2800 |
| Solution: | ||
| cement | 1.8-2.2 | 1930-3000 |
| lime | 1.5-2.1 | 1870-2300 |
Measuring such a speed in relatively small areas (on average 0.1-1 m) is a relatively complex technical problem that can only be solved with a high level of development of radio electronics. Of all existing methods measuring the speed of propagation of ultrasound, from the point of view of the possibility of their use for testing building materials, the following can be distinguished:
Acoustic interferometer method;
Traveling wave method;
Pulse method.
To measure the speed of ultrasound in concrete, the pulse method is most widely used. It is based on repeatedly sending short ultrasonic pulses into concrete with a repetition rate of 30-60 Hz and measuring the propagation time of these pulses at a certain distance, called the sounding base, i.e.
Therefore, in order to determine the speed of ultrasound, it is necessary to measure the distance traveled by the pulse (sounding base) and the time during which the ultrasound propagates from the point of emission to reception. The sounding base can be measured with any device with an accuracy of 0.1mm. The propagation time of ultrasound in most modern devices is measured by filling electronic gates with high-frequency (up to 10 MHz) counting pulses, the beginning of which corresponds to the moment of emission of the pulse, and the end to the moment of its arrival at the receiver. A simplified functional diagram of such a device is shown in Fig. 2.2.49.
The scheme works in the following way. The master oscillator 1 generates electrical pulses with a frequency of 30 to 50 Hz, depending on the design of the device, and starts a high-voltage generator 2, which generates short electrical pulses with an amplitude of 100 V. These pulses enter the emitter, in which, using the piezoelectric effect, they are converted into a pack ( from 5 to 15 pcs.) mechanical vibrations with a frequency of 60-100 kHz and are introduced through acoustic lubricant into the controlled product. At the same time, the electronic gates open, which are filled with counting pulses, and the scanning unit is triggered, and the electron beam begins to move across the screen of the cathode ray tube (CRT).
Rice. 2.2.49. Simplified functional diagram of an ultrasonic device:
1 - master oscillator; 2 - generator of high-voltage electrical pulses; 3 - ultrasonic pulse emitter; 4 - controlled product; 5 - receiver; 6 - amplifier; 7 - gate formation generator; 8 - counting pulse generator; 9 - scanner; 10 - indicator; 11 - processor; 12 - coefficient input block; 13 - digital value indicator t,V,R
The head wave of a pack of ultrasonic mechanical vibrations, having passed through a controlled product of length L, spending time t, enters the receiver 5, in which it is converted into a pack of electrical pulses.
The arriving packet of pulses is amplified in amplifier 6 and enters the vertical scanning unit for visual monitoring on the CRT screen, and the first pulse of this packet closes the gate, stopping the access of counting pulses. Thus, the electronic gates were open for counting pulses from the moment ultrasonic vibrations were emitted until they arrived at the receiver, i.e. time t. Next, the counter counts the number of counting pulses that filled the gate, and the result is displayed on indicator 13.
Some modern devices, such as Pulsar-1.1, have a processor and a coefficient input unit, with the help of which the analytical equation of the speed-strength relationship is solved, and the digital display displays time t, speed V and concrete strength R.
To measure the speed of propagation of ultrasound in concrete and other building materials, ultrasonic devices UKB-1M, UK-10P, UK-10PM, UK-10PMS, UK-12P, UV-90PTs, Beton-5 were mass-produced in the 80s, which well recommended.
In Fig. 2.2.50 shows a general view of the UK-10PMS device.
Rice. 2.2.50. Ultrasonic device UK-10PMS
Factors influencing the speed of propagation of ultrasound in concrete
All materials in nature can be divided into two large groups,” relatively homogeneous and with a large degree of heterogeneity or heterogeneity. Relatively homogeneous materials include materials such as glass, distilled water and other materials with a constant density under normal conditions and the absence of air inclusions. For them, the speed of propagation of ultrasound under normal conditions is almost constant. In heterogeneous materials, which include most building materials, including concrete, internal structure, the interaction of microparticles and large constituent elements is not constant both in volume and in time. Their structure includes micro- and macropores, cracks, which can be dry or filled with water.
The relative position of large and small particles is also variable. All this leads to the fact that the density and speed of propagation of ultrasound in them is inconsistent and fluctuates within wide limits. In table 2.2.2 shows the values of density ρ and ultrasound propagation speed V for some materials.
Next, we will consider how changes in such parameters of concrete as strength, composition and type of coarse aggregate, amount of cement, humidity, temperature and the presence of reinforcement affect the speed of propagation of ultrasound in concrete. This knowledge is necessary for an objective assessment of the possibility of monitoring the strength of concrete using the ultrasonic method, as well as for eliminating a number of errors in monitoring associated with changes in these factors
Effect of concrete strength
Experimental studies show that as the strength of concrete increases, the speed of ultrasound increases.
This is explained by the fact that the value of speed, as well as the value of strength, depends on the conditions of intrastructural connections.
As can be seen from the graph (Fig. 2.2.51), the “speed-strength” relationship for concrete of various compositions is not constant, which means that this relationship, in addition to strength, is influenced by other factors.
Rice. 2.2.51. The relationship between ultrasound speed V and strength R c for concrete of various compositions
Unfortunately, some factors affect ultrasonic speed more than strength, which is one of the serious disadvantages of the ultrasonic method.
If we take concrete of a constant composition, and change the strength by adopting a different W/C, then the influence of other factors will be constant, and the speed of ultrasound will change only from the strength of the concrete. IN in this case the speed-strength relationship will become more defined (Fig. 2.2.52).
Rice. 2.2.52. The speed-strength relationship for a constant concrete composition, obtained at the reinforced concrete plant No. 1 in Samara
Influence of the type and grade of cement
Comparing the results of testing concrete using ordinary Portland cement and other cements, we can conclude that the mineralogical composition has little effect on the speed-strength relationship. The main influence is exerted by the content of tricalcium silicate and the fineness of cement grinding. A more important factor influencing the speed-strength relationship is the consumption of cement per 1 m 3 of concrete, i.e. its dosage. With an increase in the amount of cement in concrete, the speed of ultrasound increases more slowly than mechanical strength concrete.
This is explained by the fact that ultrasound, when passing through concrete, propagates both through the coarse aggregate and through the mortar part connecting the aggregate granules, and its speed largely depends on the speed of propagation in the coarse aggregate. However, the strength of concrete mainly depends on the strength of the mortar component. The effect of the amount of cement on the strength of concrete and the speed of ultrasound is shown in Fig. 2.2.53.
Rice. 2.2.53. Effect of cement dosage on dependence
"speed-strength"
1- 400 kg/m3; 2 - 350 kg/m3; 3 - 300 kg/m 3 ; 4 - 250 kg/m3; 5 - 200 kg/m 3
Effect of water-cement ratio
As the W/C decreases, the density and strength of concrete increase, and the ultrasound speed increases accordingly. With increasing W/C, an inverse relationship is observed. Consequently, a change in W/C does not introduce significant deviations into the established speed-strength relationship. Therefore, when constructing calibration graphs to change the strength of concrete, it is recommended to use different W/C.
Influence of speciesAnd amount of coarse aggregate
The type and amount of coarse aggregate have a significant impact on the change in the speed-strength relationship. The speed of ultrasound in aggregate, especially in such as quartz, basalt, hard limestone, granite, is much higher than the speed of its propagation in concrete.
The type and amount of coarse aggregate also affect the strength of concrete. It is generally accepted that the stronger the aggregate, the higher the strength of the concrete. But sometimes you have to deal with a phenomenon where the use of less durable crushed stone, but with a rough surface, allows you to obtain concrete with a higher Re value than when using durable gravel, but with a smooth surface
With a slight change in the consumption of crushed stone, the strength of concrete changes slightly. At the same time, such a change in the amount of coarse aggregate has a great influence on the speed of ultrasound.
As the concrete becomes saturated with crushed stone, the ultrasonic speed increases. The type and amount of coarse aggregate influence the speed-strength relationship more than other factors (Fig. 2.2.54 – 2.2.56)
Rice. 2.2.54. The influence of the presence of coarse aggregate on the speed-strength relationship:
1 - cement stone; 2 - concrete with aggregate up to 30 mm in size
Rice. 2.2.55. Speed-strength relationship for concrete with different aggregate sizes: 1-1 mm; 2-3 mm; 3-7 mm; 4-30 mm
Rice. 2.2.56. Velocity-strength relationship for concrete with filler from:
1-sandstone; 2-limestone; 3-granite; 4-basalt
It is clear from the graphs that an increase in the amount of crushed stone per unit volume of concrete or an increase in the speed of ultrasound in it leads to an increase in the speed of ultrasound in concrete more intensely than strength.
Effect of humidity and temperature
The moisture content of concrete has an ambiguous effect on its strength and ultrasound speed. With increasing moisture content of concrete, the compressive strength decreases due to changes in intercrystalline bonds, but the speed of ultrasound increases as air pores and microcracks are filled with water, A speed in water is greater than in air.
The temperature of concrete in the range of 5-40 ° C has virtually no effect on strength and speed, but increasing the temperature of hardened concrete beyond the specified range leads to a decrease in its strength and speed due to an increase in internal microcracks.
At negative temperatures, the speed of ultrasound increases due to the transformation of unbound water into ice. Therefore, it is not recommended to determine the strength of concrete using the ultrasonic method at subzero temperatures.
Propagation of ultrasound in concrete
Concrete in its structure is a heterogeneous material, which includes a mortar part and coarse aggregate. The mortar part, in turn, is a hardened cement stone with the inclusion of particles of quartz sand.
Depending on the purpose of concrete and its strength characteristics, the ratio between cement, sand, crushed stone and water varies. In addition to ensuring strength, the composition of concrete depends on the manufacturing technology of reinforced concrete products. For example, with cassette production technology, greater plasticity of the concrete mixture is required, which is achieved by increased consumption of cement and water. In this case, the mortar portion of the concrete increases.
In the case of bench technology, especially with immediate stripping, rigid mixtures with reduced cement consumption are used.
The relative volume of coarse aggregate in this case increases. Consequently, with the same strength characteristics of concrete, its composition can vary within wide limits. The structure formation of concrete is influenced by the manufacturing technology of products: the quality of mixing of the concrete mixture, its transportation, compaction, thermal and moisture treatment during hardening. It follows from this that the properties of hardened concrete are influenced by a large number of factors, and the influence is ambiguous and random in nature. This explains the high degree of heterogeneity of concrete both in composition and in its properties. Heterogeneity and various properties concrete are also reflected in its acoustic characteristics.
At present, despite numerous attempts, a unified scheme and theory of ultrasound propagation through concrete has not yet been developed, which is explained by ) first of all, by the presence of the above numerous factors, which have different effects on the strength and acoustic properties of concrete. This situation is aggravated by the fact that a general theory of the propagation of ultrasonic vibrations through a material with a high degree of inhomogeneity has not yet been developed. This is the only reason why the speed of ultrasound in concrete is determined for a homogeneous material by the formula
where L is the path traveled by ultrasound, m (base);
t is the time spent traveling this path, μs.
Let us consider in more detail the scheme of propagation of pulsed ultrasound through concrete as through a heterogeneous material. But first, we will limit the area in which our reasoning will be valid by considering the most common composition of the concrete mixture, consisting of cement, in reinforced concrete factories and construction sites, river sand, coarse aggregate and water. In this case, we will assume that the strength of coarse aggregate is higher than the strength of concrete. This is true when using limestone, marble, granite, dolomite and other rocks with a strength of about 40 MPa as coarse aggregates. Let us conventionally assume that hardened concrete consists of two components: a relatively homogeneous mortar part with density ρ and speed V and coarse aggregate with ρ and V.
Taking into account the noted assumptions and limitations, hardened concrete can be considered as a solid medium with acoustic impedance:
Let's consider a diagram of the propagation of the head ultrasonic wave from emitter 1 to receiver 2 through hardened concrete of thickness L (Fig. 2.2.57).
Rice. 2.2.57. Scheme of propagation of the head ultrasonic wave
in concrete:
1 - emitter; 2 - receiver; 3 - contact layer; 4 - wave propagation in granules; 5 - wave propagation in the solution part
The head ultrasonic wave from the emitter 1 first hits the contact layer 3 located between the radiating surface and the concrete. In order for an ultrasonic wave to pass through the contact layer, it must be filled with a conductive liquid or lubricant, which is most often used as technical petroleum jelly. Having passed through the contact layer (during time t 0), the ultrasonic wave is partially reflected in the opposite direction, and the rest will enter the concrete. The thinner the contact layer compared to the wavelength, the less of the wave will be reflected.
Having entered the thickness of the concrete, the head wave will begin to propagate in the mortar part of the concrete over an area corresponding to the diameter of the emitter. After traveling a certain distance Δ l 1, after time Δ t 1 head wave in a certain area will encounter one or more granules of coarse aggregate, will be partially reflected from them, and the majority will enter the granules and begin to propagate into them. Between the granules, the wave will continue to propagate through the solution part.
Taking into account the accepted condition that the ultrasound speed in the coarse aggregate material is greater than in the mortar part, the distance d is equal to the average value of the diameter of the crushed stone, the wave that propagated through the granules at a speed V 2 will pass first, and the wave passing through the mortar part will be delayed .
Having passed through the first granules of coarse aggregate, the wave approaches the interface with the mortar part, is partially reflected, and partially enters it. In this case, the granules through which the head wave passed can later be considered as elementary spherical sources of ultrasonic wave radiation into the mortar part of the concrete, to which Huygens’ principle can be applied.
Walking through the solution minimum distance between neighboring granules, the head wave will enter them and begin to propagate along them, turning them into the next elementary sources. Thus, after time t, having passed through the entire thickness of concrete L and the second contact layer 3, the head wave will enter receiver 2, where it will be converted into an electrical signal.
From the considered diagram it follows that the head wave from the emitter 1 to the receiver 2 propagates along a path passing through the granules of coarse aggregate and the mortar part connecting these granules, and this path is determined from the condition of the minimum elapsed time t.
Hence the time t is
where is the time spent passing the solution part connecting the granules;
Time taken to pass through the granules. The path L traveled by ultrasound is equal to
where: - the total path traveled by the head wave through the solution part;
The total path traveled by the head wave through the granules.
The total distance L that the head wave will travel may be greater than the geometric distance between the emitter and the receiver, since the wave travels along the path of maximum speed, and not along the minimum geometric distance.
The time spent by ultrasound passing through the contact layers must be subtracted from the total measured time.
The waves that follow the head wave also propagate along the path of maximum speed, but during their movement they will encounter reflected waves from the interface between the coarse aggregate granules and the mortar part. If the diameter of the granules turns out to be equal to the wavelength or half of it, then an acoustic resonance may occur inside the granule. The effect of interference and resonance can be observed by spectral analysis of a packet of ultrasonic waves passing through concrete with different aggregate sizes.
The scheme of propagation of the head wave of pulsed ultrasound discussed above is valid only for concrete with the properties indicated at the beginning of the section, i.e. the mechanical strength and speed of propagation of ultrasound in the material from which the coarse aggregate granules are obtained exceed the strength and speed in the mortar part of the concrete. Most concretes used in reinforced concrete factories and construction sites that use crushed stone from limestone, marble, and granite have these properties. For expanded clay concrete, foam concrete, and concrete with tuff filler, the ultrasonic propagation pattern may be different.
The validity of the considered scheme is confirmed by experiments. So, from Fig. 2.2.54 it can be seen that when a certain amount of crushed stone is added to the cement part, the ultrasonic speed increases with a slight increase (and sometimes decrease) in the strength of concrete.
In Fig. 2.2.56 it is noticeable that with an increase in the speed of ultrasound in the coarse aggregate material, its speed in concrete increases.
The increase in speed in concrete with larger aggregate (Fig. 2.2.55) is also explained by this scheme, since with increasing diameter the path of ultrasound through the aggregate material lengthens.
The proposed scheme for the propagation of ultrasound will allow us to objectively evaluate the capabilities of the ultrasonic method in flaw detection and monitoring the strength of concrete.
1. The speed of propagation of ultrasound depends on the temperature and pressure in the pipeline. The speed of ultrasound at various values of water temperature and atmospheric pressure is given in Table D.1.
Table E.1
Alexandrov A.A., Trakhtengerts M.S. Thermophysical properties of water at atmospheric pressure. M. Standards Publishing House, 1977, 100 p. (State Standard Reference Data Service. Ser. Monographs).
2. When using a flow meter to measure the flow and volume of water in water and heat supply systems, the ultrasonic speed is determined according to the data in table. D.2 by linear interpolation method for temperature and pressure in accordance with the formula:
where c(t,P) is the speed of ultrasound in the liquid flowing through the pipeline, m/s;
c(t1) – table value of ultrasound speed at a temperature lower than measured, m/s;
c(t2) – table value of ultrasound speed at a temperature higher than measured, m/s;
c(P1) – table value of ultrasound speed at a pressure less than measured, m/s;
c(P2) – table value of ultrasound speed at a pressure greater than measured, m/s;
t – water temperature in the pipeline, ºС;
P – water pressure in the pipeline, MPa;
t1, t2 – table temperature values, ºС;
P1, P2 – table pressure values, MPa;
NOTE.
1. The values of c(t1) and c(t2) are determined according to the data in table. D.1. The values of c(P1) and c(P2) are determined according to the data in table. D 2. at a temperature closest to the water temperature in the pipeline.
2. Measurements of temperature and pressure of water in the pipeline must be carried out with an error of no more than ±0.5 ºС and ±0.5 MPa, respectively.
Table E.2
Continuation of Table E.2
Alexandrov A.A., Larkin D.K. Experimental determination of ultrasound speed in a wide range of temperatures and pressures. Journal "Thermal Power Engineering", No. 2, 1976, p. 75.
3. In the absence of tables of the dependence of ultrasonic speed on liquid temperature, the ultrasonic speed can be determined using the device shown in Fig. E.1. Immediately before measuring the ultrasonic speed, the body of the device (steel bracket) is immersed in the test liquid, and the thickness gauge is adjusted to measure the ultrasonic speed. Then an ultrasonic thickness gauge directly measures the ultrasonic speed.
To measure the speed of ultrasound in a liquid, it is also possible to use the US-12 IM device (ShchO 2.048.045 TO) or other types of thickness gauges.
Fig.D.1. A device for measuring the speed of ultrasound in liquid.
Ultrasound- elastic sound vibrations of high frequency. The human ear perceives elastic waves propagating in the medium with a frequency of approximately 16-20 kHz;
Higher frequency vibrations are ultrasound (beyond the audible limit). Typically, the ultrasonic range is considered to be the frequency range from 20,000 to a billion Hz. Sound vibrations with a higher frequency are called hypersound. In liquids and solids, sound vibrations can reach 1000 GHz
Although scientists have known about the existence of ultrasound for a long time, its practical use in science, technology and industry began relatively recently. Now ultrasound is widely used in various fields of physics, technology, chemistry and medicine.The frequency of ultra-high-frequency ultrasonic waves used in industry and biology lies in the range of the order of several MHz.
Focusing of such beams is usually carried out using special sonic lenses and mirrors. An ultrasonic beam with the necessary parameters can be obtained using an appropriate transducer. The most common ceramic transducers are barium titanite. In cases where the power of the ultrasonic beam is of primary importance, mechanical ultrasound sources are usually used. Initially, all ultrasonic waves were received mechanically (tuning forks, whistles, sirens).
In nature, ultrasound is found both as a component of many natural noises (in the noise of wind, waterfall, rain, in the noise of pebbles rolled by the sea surf, in the sounds accompanying thunderstorm discharges, etc.), and among the sounds of the animal world. Some animals use ultrasonic waves to detect obstacles and navigate in space. Ultrasound emitters can be divided into two large groups. The first includes emitter-generators; oscillations in them are excited due to the presence of obstacles in the path of a constant flow - a stream of gas or liquid. The second group of emitters are electroacoustic transducers; they convert already given fluctuations in electrical voltage or current into a mechanical vibration of a solid body, which emits in environment
acoustic waves. Examples of emitters: Galton whistle, liquid and ultrasonic whistle, siren.
Ultrasound propagation. Ultrasound propagation is the process of movement in space and time of disturbances that take place in.
sound wave A sound wave propagates through a substance in a gaseous, liquid or solid state , in the same direction in which the particles of this substance are displaced, that is, it causes deformation of the medium. Deformation consists in the fact that sequential discharge and compression of certain volumes of the medium occurs, and the distance between two adjacent areas corresponds to the length of the ultrasonic wave. The greater the specific acoustic impedance
environment, the greater the degree of compression and discharge of the environment for a given amplitude of oscillations.
The particles of the medium involved in the transfer of wave energy oscillate around their equilibrium position. The speed at which particles oscillate around the average equilibrium position is called oscillatory
speed.
When ultrasonic waves propagate, diffraction, interference and reflection phenomena are possible.
Diffraction (waves bending around obstacles) occurs when the ultrasonic wavelength is comparable (or greater) to the size of the obstacle in the path. If the obstacle is large compared to the acoustic wavelength, then there is no diffraction phenomenon.
When several ultrasonic waves move simultaneously in tissue at a certain point in the medium, a superposition of these waves can occur. This superposition of waves on each other is generally called interference. If, in the process of passing through a biological object, ultrasonic waves intersect, then at a certain point in the biological environment an increase or decrease in vibrations is observed. The result of interference will depend on the spatial relationship of the phases of ultrasonic vibrations at a given point in the medium. If ultrasonic waves reach a certain area of the medium in the same phases (in phase), then the particle displacements have the same signs and interference under such conditions helps to increase the amplitude of ultrasonic vibrations. If ultrasonic waves arrive at a specific area in antiphase, then the displacement of particles will be accompanied different signs, which leads to a decrease in the amplitude of ultrasonic vibrations.
Interference plays an important role in assessing phenomena occurring in tissues around the ultrasound emitter. Interference is especially important when ultrasonic waves propagate in opposite directions after being reflected from an obstacle.
Absorption of ultrasonic waves
If the medium in which ultrasound propagates has viscosity and thermal conductivity or there are other internal friction processes in it, then sound absorption occurs as the wave propagates, that is, as it moves away from the source, the amplitude of ultrasonic vibrations becomes smaller, as well as the energy that they carry. The medium in which ultrasound propagates interacts with the energy passing through it and absorbs part of it. The predominant part of the absorbed energy is converted into heat, the smaller part causes irreversible structural changes in the transmitting substance. Absorption is the result of friction of particles against each other; it is different in different media. Absorption also depends on the frequency of ultrasonic vibrations.
The amount of absorption can be characterized by the absorption coefficient, which shows how the intensity of ultrasound changes in the irradiated medium. It increases with increasing frequency. The intensity of ultrasonic vibrations in the medium decreases exponentially. This process is caused by internal friction, thermal conductivity of the absorbing medium and its structure. It is roughly characterized by the size of the semi-absorbing layer, which shows at what depth the intensity of vibrations decreases by half (more precisely, by 2.718 times or by 63%). According to Pahlman, at a frequency of 0.8 MHz, the average values of the semi-absorbing layer for some tissues are as follows: adipose tissue - 6.8 cm; muscular - 3.6 cm; fat and muscle tissue together - 4.9 cm. With increasing ultrasound frequency, the size of the semi-absorbing layer decreases. So, at a frequency of 2.4 MHz, the intensity of ultrasound passing through fat and muscle tissue is halved at a depth of 1.5 cm.
In addition, abnormal absorption of the energy of ultrasonic vibrations in some frequency ranges is possible - this depends on the characteristics of the molecular structure of a given tissue. It is known that 2/3 of ultrasound energy is attenuated at the molecular level and 1/3 at the level of microscopic tissue structures.
Penetration depth of ultrasonic waves
Ultrasound penetration depth refers to the depth at which the intensity is reduced by half. This value is inversely proportional to absorption: the stronger the medium absorbs ultrasound, the shorter the distance at which the ultrasound intensity is attenuated by half.
Scattering of ultrasonic waves
If there are inhomogeneities in the medium, then sound scattering occurs, which can significantly change the simple propagation pattern of ultrasound and, ultimately, also cause the wave to attenuate in the original direction of propagation.
Refraction of ultrasonic waves
Since the acoustic resistance of human soft tissues is not much different from the resistance of water, it can be assumed that at the interface between the media (epidermis - dermis - fascia - muscle) refraction of ultrasonic waves will be observed.
Reflection of ultrasonic waves
Based on the phenomenon of reflection ultrasound diagnostics. Reflection occurs in the border areas of skin and fat, fat and muscle, muscle and bone. If ultrasound, while propagating, encounters an obstacle, then reflection occurs; if the obstacle is small, then the ultrasound seems to flow around it. Heterogeneities of the body do not cause significant deviations, since in comparison with the wavelength (2 mm) their sizes (0.1-0.2 mm) can be neglected. If ultrasound on its path encounters organs whose dimensions are larger than the wavelength, then refraction and reflection of the ultrasound occurs. The strongest reflection is observed at the boundaries of bone - surrounding tissue and tissue - air. Air has low density and almost complete reflection of ultrasound is observed. Reflection of ultrasonic waves is observed at the boundary of muscle - periosteum - bone, on the surface of hollow organs.
Traveling and standing ultrasonic waves
If, when ultrasonic waves propagate in a medium, they are not reflected, traveling waves are formed. As a result of energy losses, the oscillatory movements of the particles of the medium gradually attenuate, and the further the particles are located from the radiating surface, the smaller the amplitude of their oscillations. If, on the path of propagation of ultrasonic waves, there are tissues with different specific acoustic resistances, then, to one degree or another, the ultrasonic waves are reflected from the boundary interface. The superposition of incident and reflected ultrasonic waves can result in standing waves. For standing waves to occur, the distance from the emitter surface to the reflecting surface must be a multiple of half the wavelength.
Dmitry Levkin
Ultrasound- mechanical vibrations located above the frequency range audible to the human ear (usually 20 kHz). Ultrasonic vibrations travel in waveforms, similar to the propagation of light. However, unlike light waves, which can travel in a vacuum, ultrasound requires an elastic medium such as a gas, liquid or solid.
, (3)
For transverse waves it is determined by the formula
Sound dispersion- dependence of the phase speed of monochromatic sound waves on their frequency. The dispersion of the speed of sound can be due to both the physical properties of the medium and the presence of foreign inclusions in it and the presence of boundaries of the body in which the sound wave propagates.
Types of ultrasonic waves
Most ultrasound techniques use either longitudinal or shear waves. There are also other forms of ultrasound propagation, including surface waves and Lamb waves.
Longitudinal ultrasonic waves– waves, the direction of propagation of which coincides with the direction of displacements and velocities of particles of the medium.
Transverse ultrasonic waves– waves propagating in a direction perpendicular to the plane in which the directions of displacements and velocities of particles of the body lie, the same as shear waves.
Surface (Rayleigh) ultrasonic waves have elliptical particle motion and spread over the surface of the material. Their speed is approximately 90% of the speed of shear wave propagation, and their penetration into the material is equal to approximately one wavelength.
Lamb wave- an elastic wave propagating in a solid plate (layer) with free boundaries, in which the oscillatory displacement of particles occurs both in the direction of wave propagation and perpendicular to the plane of the plate. Lamb waves are one of the types of normal waves in an elastic waveguide - in a plate with free boundaries. Because these waves must satisfy not only the equations of the theory of elasticity, but also the boundary conditions on the surface of the plate; the pattern of motion in them and their properties are more complex than those of waves in unbounded solids.
Visualization of ultrasonic waves
For a plane sinusoidal traveling wave, the ultrasound intensity I is determined by the formula
, (5)
IN spherical traveling wave Ultrasound intensity is inversely proportional to the square of the distance from the source. IN standing wave I = 0, i.e., there is no flow of sound energy on average. Ultrasound intensity in harmonic plane traveling wave equal to the energy density of the sound wave multiplied by the speed of sound. The flow of sound energy is characterized by the so-called Umov vector- the vector of the energy flux density of the sound wave, which can be represented as the product of the ultrasound intensity and the wave normal vector, i.e., a unit vector perpendicular to the wave front. If the sound field is a superposition of harmonic waves of different frequencies, then for the vector medium density flow of sound energy, there is an additivity of components.
For emitters creating a plane wave, they speak of radiation intensity, meaning by this emitter power density, i.e. the radiated sound power per unit area of the radiating surface.
Sound intensity is measured in SI units in W/m2. In ultrasonic technology, the range of changes in ultrasound intensity is very large - from threshold values of ~ 10 -12 W/m2 to hundreds of kW/m2 at the focus of ultrasonic concentrators.
Table 1 - Properties of some common materials
| Material | Density, kg/m 3 | Longitudinal wave speed, m/s | Shear wave speed, m/s | , 10 3 kg/(m 2 *s) |
| Acrylic | 1180 | 2670 | - | 3,15 |
| Air | 0,1 | 330 | - | 0,00033 |
| Aluminum | 2700 | 6320 | 3130 | 17,064 |
| Brass | 8100 | 4430 | 2120 | 35,883 |
| Copper | 8900 | 4700 | 2260 | 41,830 |
| Glass | 3600 | 4260 | 2560 | 15,336 |
| Nickel | 8800 | 5630 | 2960 | 49,544 |
| Polyamide (nylon) | 1100 | 2620 | 1080 | 2,882 |
| Steel (low alloy) | 7850 | 5940 | 3250 | 46,629 |
| Titanium | 4540 | 6230 | 3180 | 26,284 |
| Tungsten | 19100 | 5460 | 2620 | 104,286 |
| Water (293K) | 1000 | 1480 | - | 1,480 |
Ultrasound attenuation
One of the main characteristics of ultrasound is its attenuation. Ultrasound attenuation is a decrease in amplitude and, therefore, a sound wave as it propagates. Ultrasound attenuation occurs due to a number of reasons. The main ones are:
The first of these reasons is due to the fact that as a wave propagates from a point or spherical source, the energy emitted by the source is distributed over an ever-increasing surface of the wave front and, accordingly, the energy flow through a unit surface decreases, i.e. . For a spherical wave, the wave surface of which increases with distance r from the source as r 2, the amplitude of the wave decreases proportionally, and for a cylindrical wave - proportionally.
The attenuation coefficient is expressed either in decibels per meter (dB/m) or in decibels per meter (Np/m).
For a plane wave, the amplitude attenuation coefficient with distance is determined by the formula
, (6)
The attenuation coefficient versus time is determined
, (7)
The unit dB/m is also used to measure the coefficient, in this case
, (8)
Decibel (dB) is a logarithmic unit of measurement of the ratio of energies or powers in acoustics.
, (9)
- where A 1 is the amplitude of the first signal,
- A 2 – amplitude of the second signal
Then the relationship between the units of measurement (dB/m) and (1/m) will be:
Reflection of ultrasound from the interface
When a sound wave falls on the interface, part of the energy will be reflected into the first medium, and the rest of the energy will pass into the second medium. The relationship between the reflected energy and the energy passing into the second medium is determined by the wave impedances of the first and second medium. In the absence of sound speed dispersion characteristic impedance does not depend on the waveform and is expressed by the formula:
The reflection and transmission coefficients will be determined as follows
, (12)
, (13)
- where D is the sound pressure transmission coefficient
It is also worth noting that if the second medium is acoustically “softer”, i.e. Z 1 >Z 2, then upon reflection the phase of the wave changes by 180˚.
The coefficient of energy transmission from one medium to another is determined by the ratio of the intensity of the wave passing into the second medium to the intensity of the incident wave
, (14)
Interference and diffraction of ultrasonic waves
Sound interference- uneven spatial distribution of the amplitude of the resulting sound wave depending on the relationship between the phases of the waves that develop at one point or another in space. When harmonic waves of the same frequency are added, the resulting spatial distribution of amplitudes forms a time-independent interference pattern, which corresponds to a change in the phase difference of the component waves when moving from point to point. For two interfering waves, this pattern on a plane has the form of alternating bands of amplification and attenuation of the amplitude of a value characterizing the sound field (for example, sound pressure). For two plane waves, the stripes are rectilinear with an amplitude that varies across the stripes according to the change in the phase difference. An important special case of interference is the addition of a plane wave with its reflection from a plane boundary; in this case, a standing wave is formed with the planes of nodes and antinodes located parallel to the boundary.
Sound diffraction- deviation of sound behavior from the laws of geometric acoustics, due to the wave nature of sound. The result of sound diffraction is the divergence of ultrasonic beams when moving away from the emitter or after passing through a hole in the screen, the bending of sound waves into the shadow region behind obstacles large compared to the wavelength, the absence of a shadow behind obstacles small compared to the wavelength, etc. n. Sound fields created by diffraction of the original wave on obstacles placed in the medium, on inhomogeneities of the medium itself, as well as on irregularities and inhomogeneities of the boundaries of the medium, are called scattered fields. For objects on which sound diffraction occurs that are large compared to the wavelength, the degree of deviation from the geometric pattern depends on the value of the wave parameter
, (15)
- where D is the diameter of the object (for example, the diameter of an ultrasonic emitter or obstacle),
- r - distance of the observation point from this object
Ultrasound emitters
Ultrasound emitters- devices used to excite ultrasonic vibrations and waves in gaseous, liquid and solid media. Ultrasound emitters convert energy of some other type into energy.
The most widely used ultrasound emitters are electroacoustic transducers. In the vast majority of ultrasound emitters of this type, namely in piezoelectric transducers , magnetostrictive converters, electrodynamic emitters, electromagnetic and electrostatic emitters, Electric Energy is converted into vibrational energy of a solid body (radiating plate, rod, diaphragm, etc.), which emits acoustic waves into the environment. All of the listed converters are, as a rule, linear, and, therefore, the oscillations of the radiating system reproduce the exciting electrical signal in shape; Only at very large oscillation amplitudes near the upper limit of the dynamic range of the ultrasound emitter can nonlinear distortions occur.
Converters designed to emit monochromatic waves use the phenomenon resonance: they operate on one of the natural oscillations of a mechanical oscillatory system, to the frequency of which the generator of electrical oscillations, the exciting converter, is tuned. Electroacoustic transducers that do not have a solid-state radiating system are used relatively rarely as ultrasound emitters; these include, for example, ultrasound emitters based on an electrical discharge in a liquid or on the electrostriction of a liquid.
Characteristics of the ultrasound emitter
The main characteristics of ultrasound emitters include their frequency spectrum, emitted sound power, radiation directivity. In the case of monofrequency radiation, the main characteristics are operating frequency ultrasound emitter and its frequency band, the boundaries of which are determined by a drop in radiated power by half compared to its value at the frequency of maximum radiation. For resonant electroacoustic transducers, the operating frequency is natural frequency f 0 converter, and The width of the lineΔf is determined by its quality factor Q.
Ultrasound emitters (electroacoustic transducers) are characterized by sensitivity, electroacoustic efficiency and their own electrical impedance.
Ultrasound emitter sensitivity- the ratio of sound pressure at the maximum directional characteristic at a certain distance from the emitter (most often at a distance of 1 m) to the electrical voltage across it or to the current flowing in it. This characteristic applies to ultrasonic emitters used in systems sound alarm, in sonar and other similar devices. For emitters for technological purposes, used, for example, for ultrasonic cleaning, coagulation, influence on chemical processes, the main characteristic is power. Along with the total radiated power, estimated in W, ultrasound emitters are characterized by specific power, i.e., the average power per unit area of the emitting surface, or the average radiation intensity in the near field, estimated in W/m2.
The efficiency of electroacoustic transducers emitting acoustic energy into the sounded environment is characterized by their magnitude electroacoustic efficiency, which is the ratio of emitted acoustic power to expended electrical power. In acoustoelectronics, to evaluate the efficiency of ultrasound emitters, the so-called electrical loss coefficient is used, equal to the ratio (in dB) of electrical power to acoustic power. The efficiency of ultrasonic tools used in ultrasonic welding, machining and the like is characterized by the so-called efficiency coefficient, which is the ratio of the square of the amplitude of the oscillatory displacement at the working end of the concentrator to the electrical power consumed by the transducer. Sometimes the effective electromechanical coupling coefficient is used to characterize energy conversion in ultrasound emitters.
Emitter sound field
The sound field of the transducer is divided into two zones: near zone and far zone. Near zone this is the area directly in front of the transducer where the amplitude of the echo passes through a series of maxima and minima. The near zone ends at the last maximum, which is located at a distance N from the converter. It is known that the location of the last maximum is the natural focus of the transducer. Far zone This is the area beyond N, where the sound field pressure gradually decreases to zero.
The position of the last maximum N on the acoustic axis, in turn, depends on the diameter and wavelength and for a circular disk emitter is expressed by the formula
, (17)
However, since D is usually much larger, the equation can be simplified to the form
The characteristics of the sound field are determined by the design of the ultrasonic transducer. Consequently, the propagation of sound in the area under study and the sensitivity of the sensor depend on its shape.
Ultrasound Applications
The diverse applications of ultrasound, in which its various features are used, can be divided into three areas. is associated with obtaining information through ultrasonic waves, - with active influence on matter, and - with the processing and transmission of signals (the directions are listed in the order of their historical formation). For each specific application, ultrasound of a certain frequency range is used.
Oscillations and waves. Oscillations are the repeated repetition of identical or close to identical processes. The process of propagation of vibrations in a medium is called wave. The line indicating the direction of propagation of the wave is called a ray, and the boundary that defines the oscillating particles from particles of the medium that have not yet begun to oscillate is called the wave front.
The time during which a complete cycle of oscillations occurs is called period T and is measured in seconds. The value ƒ = 1 / T, showing how many times per second the oscillation is repeated, is called frequency and is measured in s -1.
The quantity ω, showing the number of complete revolutions of a point around a circle in 2T s, is called the circular frequency ω = 2 π / T = 2 π ƒ and is measured in radians per second (rad/s).
The wave phase is a parameter that shows how much of the period has passed since the start of the last cycle of oscillations.
Wavelength λ is the minimum distance between two points oscillating in the same phase. Wavelength is related to frequency ƒ and speed with the relation: λ = c/ƒ. A plane wave propagating along the horizontal X axis is described by the formula:
u = U cos (ω t - kh) ,
where k = 2 π /λ. - wave number; U is the amplitude of oscillations.
From the formula it is clear that the value of u changes periodically in time and space.
The displacement of particles from the equilibrium position u and the acoustic pressure p are used as quantities that change during oscillations.
In ultrasonic (US) flaw detection, vibrations with a frequency of 0.5...15 MHz (longitudinal wavelength in steel 0.4...12 mm) and a displacement amplitude of 10 -11 ...10 -4 mm (arising in steel at a frequency of 2 MHz acoustic stresses are 10... 10 8 Pa).
The intensity of wave I is equal to I = р 2 /(2ρс),
where ρ is the density of the medium in which the wave propagates.
The intensity of the waves used for control is very low (~10 -5 W/m 2). During flaw detection, it is not the intensity, but the amplitude of waves A that is recorded. Usually, the attenuation of the amplitude A" relative to the amplitude of the oscillations A o (probing pulse) excited in the product is measured, i.e. the ratio A "/A o. For this, logarithmic units of decibels (dB) are used, i.e. A" / A o = 20 Ig A" / A o.
Types of waves. Depending on the direction of particle oscillations relative to the beam, several types of waves are distinguished.
A longitudinal wave is a wave in which the oscillatory motion of individual particles occurs in the same direction in which the wave propagates (Fig. 1).
A longitudinal wave is characterized by the fact that the medium alternates between areas of compression and rarefaction, or high and low pressure, or high and low density. Therefore, they are also called pressure, density or compression waves. Longitudinal can spread in solids, liquids, and gases.
Rice. 1. Vibration of medium particles v in a longitudinal wave.
Shear (transverse) is a wave in which individual particles oscillate in a direction perpendicular to the direction of propagation of the wave. In this case, the distance between the individual vibration planes remains unchanged (Fig. 2).
Rice. 2. Oscillation of medium particles v in a transverse wave.
Longitudinal and transverse waves, collectively called “body waves,” can exist in an unlimited medium. These are most widely used for ultrasonic flaw detection.
Sound wave propagation speed c is the speed of propagation of a certain state in a material medium (for example, compression or rarefaction for a longitudinal wave). The speed of sound is different for different types of waves, and for transverse and longitudinal waves it is a characteristic of the medium that does not depend on the parameters of the ultrasonic wave.The speed of propagation of a longitudinal wave in an unbounded solid body is determined by the expression
where E is Young's modulus, defined as the ratio between the magnitude of the tensile force applied to a certain rod and the resulting deformation; v - Poisson's ratio, which is the ratio of the change in the width of the rod to the change in its length, if the rod is stretched along its length; ρ is the density of the material.
The velocity of a shear wave in an unbounded solid is expressed as follows:
Since in metals v ≈ 0.3, there is a relationship between the longitudinal and transverse waves
c t ≈ 0.55 s l.
Surface waves(Rayleigh waves) are elastic waves that propagate along the free (or lightly loaded) boundary of a solid body and quickly decay with depth. A surface wave is a combination of longitudinal and transverse waves. Particles in a surface wave perform oscillatory motion along an elliptical trajectory (Fig. 3). The major axis of the ellipse is perpendicular to the boundary.
Since the longitudinal component included in the surface wave decays with depth faster than the transverse component, the elongation of the ellipse changes with depth.
The surface wave has a speed of s = (0.87 + 1.12v) / (1+v)
For metals with s ≈ 0.93c t ≈ 0.51 c l.
Depending on the geometric shape of the front, the following types of waves are distinguished:
- spherical - a sound wave at a short distance from a point source of sound;
- cylindrical - a sound wave at a short distance from the sound source, which is a long cylinder of small diameter;
- flat - it can be emitted by an endlessly oscillating plane.
The pressure in a spherical or plane sound wave is determined by the relation:
where v is the magnitude of the oscillatory speed.
The quantity ρс = z is called acoustic resistance or acoustic impedance.
Rice. 3. Oscillation of medium particles v in a surface wave.
If the acoustic impedance is large, then the medium is called hard, but if the impedance is small, it is called soft (air, water).
Normal (waves in plates), are called elastic waves propagating in a solid plate (layer) with free or lightly loaded boundaries.
Normal waves come in two polarizations: vertical and horizontal. Of the two types of waves, Lamb waves are the most widely used in practice - normal waves with vertical polarization. They arise due to resonance during the interaction of the incident wave with repeatedly reflected waves inside the plate.
To understand the physical essence of waves in plates, let us consider the issue of the formation of normal waves in a liquid layer (Fig. 4).
Rice. 4. On the issue of the emergence of normal will in the liquid layer.
Let a plane wave of thickness h fall from the outside at an angle β. Line AD shows the front of the incident wave. As a result of refraction at the boundary, a wave with a CB front appears in the layer, propagating at an angle α and undergoing multiple reflections in the layer.
At a certain angle of incidence β, the wave reflected from the lower surface is in phase with the direct wave coming from the upper surface. This is the condition for the occurrence of normal waves. The angle a at which this phenomenon occurs can be found from the formula
h cos α = n λ 2 / 2
Here n is an integer; λ 2 - wavelength in the layer.
For a solid layer, the essence of the phenomenon (resonance of body waves during oblique incidence) is preserved. However, the conditions for the formation of normal waves are very complicated due to the presence of longitudinal and transverse waves in the plate. Various types waves that exist at different values of n are called modes of normal waves. Ultrasonic waves with odd values n are called symmetrical, since the movement of particles in them is symmetrical relative to the axis of the plate. Waves with even values of n are called antisymmetric(Fig. 5).
Rice. 5. Oscillation of medium particles v in a normal wave.
Head waves. In real conditions of ultrasonic testing with an inclined transducer, the ultrasonic wave front of the radiating piezoelectric element has a non-planar shape. From the emitter, the axis of which is oriented at the first critical angle to the interface, longitudinal waves with angles slightly smaller and slightly larger than the first critical angle also fall on the boundary. In this case, a number of types of ultrasonic waves are excited in the steel.
A non-uniform longitudinal surface wave propagates along the surface (Fig. 6). This wave, consisting of surface and volumetric components, is also called leaky or creeping. Particles in this wave move along trajectories in the form of ellipses, close to circles. The phase velocity of the leaking wave c in slightly exceeds the speed of the longitudinal wave (for steel with b = 1.04 c l).
These waves exist at a depth approximately equal to the wavelength and decay rapidly as they propagate: the wave amplitude decays 2.7 times faster at a distance of 1.75λ. along the surface. The weakening is due to the fact that at each point of the interface, transverse waves are generated at an angle α t2 equal to the third critical angle, called lateral waves. This angle is determined from the relation
sin α t2 = (c t2 - c l2)
for steel α t2 = 33.5°.
Rice. 6. Acoustic field of the head wave transducer: PEP - piezoelectric transducer.
In addition to the leaking wave, the head wave is also excited, which is widely used in the practice of ultrasonic testing. The head wave is a longitudinal-subsurface wave excited when an ultrasonic beam is incident on the interface at an angle close to the first critical one. The speed of this wave is equal to the speed of the longitudinal wave. The head wave reaches its amplitude value under the surface along the beam with an input angle of 78°.
Rice. 7. Amplitude of head wave reflection depending on the depth of flat-bottomed holes.
The head wave, like the leaky wave, generates lateral transverse ultrasonic waves at the third critical angle to the interface. Simultaneously with the excitation of the longitudinal surface wave, a reverse longitudinal surface wave is formed - the propagation of an elastic disturbance in the direction opposite to the direct radiation. Its amplitude is ~100 times less than the amplitude of the direct wave.
The head wave is insensitive to surface irregularities and responds only to defects lying under the surface. The attenuation of the amplitude of a longitudinal-subsurface wave along a beam of any direction occurs as in an ordinary volumetric longitudinal wave, i.e. proportional to l/r, where r is the distance along the beam.
In Fig. Figure 7 shows the change in the amplitude of the echo signal from flat-bottomed holes located at different depths. Sensitivity to defects near the surface is close to zero. The maximum amplitude at a distance of 20 mm is achieved for flat-bottomed holes located at a depth of 6 mm.
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