CN110726774A - Measuring method and measuring device for ultrasonic attenuation system - Google Patents

Measuring method and measuring device for ultrasonic attenuation system Download PDF

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CN110726774A
CN110726774A CN201911077021.2A CN201911077021A CN110726774A CN 110726774 A CN110726774 A CN 110726774A CN 201911077021 A CN201911077021 A CN 201911077021A CN 110726774 A CN110726774 A CN 110726774A
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ultrasonic
test block
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attenuation coefficient
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CN110726774B (en
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张书增
张广栋
戴昭杰
李雄兵
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Central South Univ
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Abstract

The invention discloses a measuring method and a measuring device of an ultrasonic attenuation coefficient, wherein the measuring method comprises the following steps: transmitting an ultrasonic signal to a test block immersed in water by using an ultrasonic water immersion probe with the surface immersed in water, wherein the ultrasonic signal is an ultrasonic narrow-band pulse sine wave signal; receiving an echo signal of the ultrasonic signal reflected by the test block by using the ultrasonic water immersion probe; and calculating the ultrasonic attenuation coefficient of the test block according to the frequencies of the echo signal and the ultrasonic signal. According to the measuring method, the ultrasonic narrow pulse sine wave signal with the corresponding frequency value is transmitted to the test block through the ultrasonic water immersion probe, and the ultrasonic attenuation coefficient of the test block under the frequency is measured, so that the measured ultrasonic attenuation coefficient is more accurate.

Description

Measuring method and measuring device for ultrasonic attenuation system
Technical Field
The invention relates to the technical field of ultrasound, in particular to a method and a device for measuring an ultrasonic attenuation coefficient.
Background
The attenuation coefficient is one of important parameters of material characteristics in ultrasonic application, so that the method is widely applied to quantitative nondestructive evaluation of microstructures such as material grain size, porosity and fatigue. The ultrasonic frequency domain attenuation coefficient curve can truly reflect the change of the microstructure in the material and can be used for accurately predicting the service state of the material. The accurate measurement of the frequency domain attenuation coefficient curve is crucial to the quantitative evaluation of the microstructure inside the material, so that it is necessary to provide a method for accurately measuring the frequency domain attenuation coefficient curve of the material.
Ultrasonic attenuation includes ultrasonic beam diffuse attenuation, material internal scattering attenuation, and medium absorption attenuation. The attenuation coefficient measured by practical experiments is the sum of two parts of internal scattering of the material and absorption attenuation of the medium. In most engineering applications, a convenient method is to measure the attenuation coefficient of a material with a contact type straight probe. Treiber et al showed that when the attenuation coefficient was measured with a contact probe, the result was a large error if the partial reflection coefficient at the interface of the ultrasound probe and the material was not considered. Most engineering application studies therefore only use the attenuation coefficient to roughly characterize the change in microstructure within the material, without regard to the accuracy of its value.
The other effective method for measuring the frequency domain attenuation coefficient of the ultrasonic longitudinal wave is an ultrasonic water immersion pulse echo method, and the method utilizes the ratio of primary bottom waves and secondary bottom waves or surface waves and the primary bottom waves returned by ultrasonic waves through a material interface to calculate, and obtains the frequency domain attenuation coefficient in the effective bandwidth of the probe after the diffraction correction of the ultrasonic waves in the material. Miguel et al obtained an attenuation coefficient calculation method suitable for a hydrophobic material by using an ultrasonic water immersion pulse method and correcting the reflection coefficient of ultrasonic waves passing through a material interface for the hydrophobic material. However, the above methods all use a single probe broadband pulse to transmit signals, and the frequency bandwidth of the pulse signals is limited and difficult to determine, so that an accurate relation curve between the attenuation coefficient and the frequency cannot be obtained.
Another key problem when measuring attenuation coefficients for wideband ultrasonic pulse signals is that the frequency spectrum of the signal shifts as the wideband pulse propagates through the attenuating medium, and secondly that the attenuation coefficient values are not accurately measured when the frequency values are far from the effective frequency band of the probe. This may cause deviations in the curve relationship between attenuation coefficient and frequency, thereby affecting the attenuation coefficient measurement.
Disclosure of Invention
The present invention is directed to solving, at least to some extent, one of the technical problems in the related art. Therefore, an object of the present invention is to provide a method for measuring an ultrasonic attenuation coefficient, so as to improve the accuracy of measuring the ultrasonic attenuation coefficient.
Another object of the present invention is to provide an apparatus for measuring ultrasonic attenuation coefficient.
In order to achieve the purpose, the invention provides a method for measuring an ultrasonic attenuation coefficient, which comprises the following steps: transmitting an ultrasonic signal to a test block immersed in water by using an ultrasonic water immersion probe with the surface immersed in water, wherein the ultrasonic signal is an ultrasonic narrow-band pulse sine wave signal; receiving an echo signal of the ultrasonic signal reflected by the test block by using the ultrasonic water immersion probe; and calculating the ultrasonic attenuation coefficient of the test block according to the frequencies of the echo signal and the ultrasonic signal.
According to the method for measuring the ultrasonic attenuation coefficient, the ultrasonic water immersion probe is used for transmitting the ultrasonic narrow pulse sine wave signal with the corresponding frequency value to the test block, the ultrasonic attenuation coefficient of the test block under the frequency is measured, and the measured ultrasonic attenuation coefficient is more accurate.
As an example, the echo signal includes a reflection signal of the upper surface of the test block and a reflection signal of the bottom surface of the test block, wherein the ultrasonic attenuation coefficient of the test block is calculated by the following formula:
Figure BDA0002262799290000021
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2D ') is a reflected signal from the bottom surface of the test block, D' is a propagation distance of the ultrasonic signal between the test block and water, D1(f,2d)For a diffraction correction term, D, of the ultrasonic signal as it propagates through the water2(f,2d') is a diffraction correction term, T, of the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal when passing through the water interface21Is the transmission coefficient, R, of the ultrasonic signal passing through the interface of the test block21The reflection coefficient of the ultrasonic signal passing through the interface of the test block is shown.
As an example, the surface of the ultrasonic water immersion probe is 20mm away from the upper surface of the test block, the test block is of a cuboid structure, and the upper surface area and the lower surface area of the test block are both 50mm multiplied by 50mm2And the thickness of the test block is 20 mm.
As an example, the ultrasonic water immersion probe has a center frequency of 1MHz, 2.25MHz, or 5 MHz.
As one example, a plurality of ultrasonic signals of different frequencies are transmitted using one or more ultrasonic water immersion probes of different center frequencies, the method further comprising: calculating to obtain a plurality of ultrasonic attenuation coefficients according to a plurality of different frequencies and corresponding echo signals; and fitting the plurality of ultrasonic attenuation coefficients and the corresponding frequencies thereof to obtain an ultrasonic attenuation coefficient-frequency curve.
In order to achieve the above object, the present invention provides an apparatus for measuring an ultrasonic attenuation coefficient, comprising: the ultrasonic water immersion probe comprises an ultrasonic water immersion probe, a current probe, an amplifier, a function generator, an oscilloscope and an upper computer, wherein the surface of the ultrasonic water immersion probe is immersed in water, the current probe is respectively and electrically connected with the ultrasonic water immersion probe, the amplifier and the oscilloscope, the function generator is electrically connected with the amplifier, and the upper computer is electrically connected with the oscilloscope; the ultrasonic water immersion probe transmits acquired echo signals reflected by the test block to the oscilloscope through the current probe for displaying on the oscilloscope, and the upper computer is used for acquiring the echo signals displayed by the oscilloscope and calculating the ultrasonic attenuation coefficient of the test block according to the echo signals and the frequency of the ultrasonic signals.
According to the device for measuring the ultrasonic attenuation coefficient, the function generator is used for driving the ultrasonic water immersion probe to emit the ultrasonic narrow pulse sine wave signal with the corresponding frequency value to the test block in a curved mode, the ultrasonic attenuation coefficient of the test block under the frequency is measured, and the measured ultrasonic attenuation coefficient is more accurate.
As an example, the echo signal includes a reflection signal of the upper surface of the test block and a reflection signal of the bottom surface of the test block, and the upper computer calculates the ultrasonic attenuation coefficient of the test block by the following formula:
Figure BDA0002262799290000031
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2D ') is a reflected signal from the bottom surface of the test block, D' is a propagation distance of the ultrasonic signal between the test block and water, D1(f,2D) is a diffraction correction term for the ultrasonic signal as it propagates through the water, D2(f,2d') is a diffraction correction term, T, of the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal when passing through the water interface21Is the transmission coefficient, R, of the ultrasonic signal passing through the interface of the test block21The reflection coefficient of the ultrasonic signal passing through the interface of the test block is shown.
As an example, the surface of the ultrasonic water immersion probe is 20mm away from the upper surface of the test block, the test block is of a cuboid structure, and the upper surface area and the lower surface area of the test block are both 50mm multiplied by 50mm2And the thickness of the test block is 20 mm.
As an example, the ultrasonic water immersion probe has a center frequency of 1MHz, 2.25MHz, or 5 MHz.
As an example, the function generator is configured to output a plurality of narrow pulse sine wave signals with different frequencies, and transmit an ultrasonic wave signal with a corresponding frequency by using an ultrasonic water immersion probe with a corresponding center frequency, wherein the upper computer is further configured to obtain echo signals corresponding to the ultrasonic wave signals with different frequencies, calculate a plurality of ultrasonic attenuation coefficients according to the plurality of different frequencies and the echo signals corresponding to the different frequencies, and perform fitting processing on the plurality of ultrasonic attenuation coefficients and the frequencies corresponding to the ultrasonic attenuation coefficients to obtain an ultrasonic attenuation coefficient-frequency curve.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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FIG. 1 is a schematic flow chart of a method of measuring an ultrasonic attenuation coefficient according to an embodiment of the present invention;
FIG. 2 is a schematic illustration of the propagation of an ultrasonic signal according to an example of the present invention;
fig. 3(a), fig. 3(b), and fig. 3(c) are time domain signal waveforms of ultrasonic signals with frequencies of 2.25MHz, 5MHz, and 10MHz, respectively, when propagating in an acrylic material;
FIG. 4(a), FIG. 4(b), and FIG. 4(c) are frequency domain signal waveforms of ultrasonic signals with frequencies of 2.25MHz, 5MHz, and 10MHz respectively propagating in an acrylic material;
FIG. 5 is a graphical representation of test results for one example of the present invention;
FIG. 6 is a graphical representation of test results for another example of the present invention;
fig. 7 is a schematic structural diagram of an ultrasonic attenuation coefficient measuring apparatus according to an embodiment of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
The ultrasonic attenuation coefficient measuring method and the ultrasonic attenuation coefficient measuring apparatus according to the embodiments of the present invention will be described below with reference to the drawings.
Example 1
Fig. 1 is a schematic flow chart of a method for measuring an ultrasonic attenuation coefficient according to an embodiment of the present invention.
As shown in fig. 1, the method for measuring the ultrasonic attenuation coefficient includes the following steps:
and S1, transmitting an ultrasonic signal to the test block immersed in the water by using the ultrasonic water immersion probe with the surface immersed in the water, wherein the ultrasonic signal is an ultrasonic narrow-band pulse sine wave signal.
And S2, receiving the echo signal of the ultrasonic signal reflected by the test block by using the ultrasonic water immersion probe.
And S3, calculating the ultrasonic attenuation coefficient of the test block according to the frequencies of the echo signal and the ultrasonic signal.
As an example, the echo signal includes a reflection signal of the upper surface of the test block and a reflection signal of the bottom surface of the test block, and the ultrasonic attenuation coefficient of the test block is calculated by the following formula (1):
Figure BDA0002262799290000041
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2D ') is a reflected signal from the bottom surface of the test block, D' is a propagation distance of the ultrasonic signal between the test block and water, and D1(f,2D) is a diffraction correction term when the ultrasonic signal propagates through water, D2(f,2d') is a diffraction correction term, T, for the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal passing through the water interface21Is the transmission coefficient, R, of an ultrasonic signal passing through an interface of a test block21Is the reflection coefficient of the ultrasonic signal passing through the interface of the test block.
Specifically, the ultrasonic attenuation coefficient of a solid material (namely a test block) is measured by using an ultrasonic water immersion probe. An ultrasonic narrow-band pulse sine wave signal is used as a transmitting source, ultrasonic longitudinal waves are transmitted in the measuring process as shown in figure 2, and an ultrasonic signal transmitted by an ultrasonic water immersion probe is reflected by the upper surface and the bottom surface of a tested block and is received by the ultrasonic water immersion probe.
Assuming that the thickness of the tested block is z, the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, namely the water depth distance is d. The amplitudes of the surface echo (i.e. the reflected signal of the upper surface of the test block) and the primary bottom wave signal (i.e. the reflected signal of the bottom surface of the test block) of the longitudinal wave received by the ultrasonic immersion probe can be expressed as:
S1(f,2d)＝S0(f,0)R12M1(f,2d)D1(f,2d) (2)
S2(f,2d')＝S0(f,0)T12R21T21M2(f,2d')D2(f,2d') (3)
in formulae (2) and (3), S0(f,0) is the initial amplitude of the ultrasonic signal transmitted by the ultrasonic water immersion probe;
in the formulae (2) and (3), R12And R21The reflection coefficient of the ultrasonic signal passing through the interface can be expressed as:
Figure BDA0002262799290000051
R21＝-R12(5)
wherein Z is1＝ρ1cp1For the acoustic impedance, Z, of the ultrasonic signal in the test block2＝ρ2cp2Acoustic impedance, p, of ultrasonic signals in water1And ρ2Density of water and test block, respectively, cp1And cp2The propagation velocities of the ultrasonic signal in water and in the test block, respectively.
In formulae (2) and (3), T12And T21The transmission coefficients of the ultrasonic signal passing through the interface between water and the test block can beExpressed as:
Figure BDA0002262799290000052
Figure BDA0002262799290000053
in formulae (2) and (3), M1(f,2d) and M2(f,2d') are respectively the attenuation coefficient of the ultrasonic signal in water after the echo is reflected by the surface of the test block and the attenuation coefficient of the primary bottom echo in water and the test block, and can be expressed as follows:
M1(f,2d)＝exp[-2αf(f)d](8)
M2(f,2d')＝exp[-2αf(f)d-αL(f)z](9)
wherein alpha isf(f) And alphaL(f) The attenuation coefficients of the ultrasonic signal in water and the test block are respectively, and the propagation distance d' of the ultrasonic signal in two layers of media is given by the formula (10)
Figure BDA0002262799290000061
In formulae (2) and (3), D1(f,2D) and D2(f,2d') are diffraction correction terms in the propagation path of the ultrasonic signal in water and in the test block, respectively, which can be expressed as:
Figure BDA0002262799290000062
Figure BDA0002262799290000063
wherein k is 2 pi f/cp1Representing the wave number of the ultrasonic signal in water, a is the radius of the ultrasonic water immersion probe, J0And J1Respectively, zero order and first order bessel functions.
And dividing the formula (3) by the formula (2) and substituting the corresponding variables to obtain an attenuation coefficient expression of the ultrasonic signal in the test block, namely the formula (1).
Therefore, for an ultrasonic water immersion probe with a given center frequency, as long as a narrow pulse signal with a single frequency f is transmitted in the bandwidth of the probe, the attenuation coefficient alpha at the corresponding frequency can be calculated by the formula (1)L(f,2z)。
As an example, the surface of the ultrasonic water immersion probe is 20mm away from the upper surface of the test block, the test block adopts a cuboid structure, and the upper surface area and the lower surface area of the test block are both 50mm multiplied by 50mm2The thickness of the test block was 20 mm. Therefore, the parallelism of high standards is ensured on the upper surface and the lower surface of the test block, and the surfaces are polished, so that errors caused by measurement can be reduced or prevented.
Optionally, the center frequency of the ultrasonic water immersion probe is 1MHz, 2.25MHz, or 5 MHz.
As an example, one or more ultrasonic water immersion probes with different center frequencies may be used to transmit ultrasonic signals of a plurality of different frequencies, wherein the test method of the embodiment of the present invention further includes: calculating to obtain a plurality of ultrasonic attenuation coefficients according to a plurality of different frequencies and corresponding echo signals; and fitting the plurality of ultrasonic attenuation coefficients and the corresponding frequencies thereof to obtain an ultrasonic attenuation coefficient-frequency curve.
Specifically, several ultrasonic water immersion probes with different center frequencies can be simultaneously selected to repeat the processes of the steps S1-S3 to obtain a group of ultrasonic attenuation coefficients alphaL(f,2z) and a corresponding group of ultrasonic signal frequencies f, and performing curve fitting on all the numerical points to obtain an accurate frequency domain attenuation curve (namely, ultrasonic attenuation coefficient-frequency (alpha))L-f) curve).
For example, five ultrasonic water immersion probes with different central frequencies can be selected to measure the ultrasonic attenuation coefficient and realize the measurement of an ultrasonic attenuation coefficient-frequency curve, wherein the central frequencies are 1MHz, 2.25MHz, 5MHz, 7.5MHz and 10MHz respectively, and the signal frequencies emitted by the probes with different central frequencies are shown in tables 1-5.
TABLE 1
Figure BDA0002262799290000071
TABLE 2
Figure BDA0002262799290000072
TABLE 3
Figure BDA0002262799290000073
TABLE 4
TABLE 5
Figure BDA0002262799290000075
Fig. 3(a), 3(b) and 3(b) show time domain signal waveforms of ultrasonic signals with frequencies of 2.25MHz, 5MHz and 10MHz respectively propagating in an acrylic material. The result shows that the method has high signal-to-noise ratio when measuring the ultrasonic attenuation coefficient, and the average speed of the ultrasonic signal propagating in the acrylic material can be calculated to be c 2720m/s from the time domain signal diagram.
The frequency domain characteristics of the ultrasonic signal obtained by extracting the surface echo and the primary bottom wave signal with a "hanning window" and then performing fast fourier transform mapping are shown in fig. 4(a), 4(b), and 4 (c). It can be seen from the figure that the signal center frequencies of the ultrasonic surface echo and the primary bottom wave are stabilized at the transmitting frequency, and the phenomenon that the center frequency is shifted to a low frequency like the broadband pulse signal does not occur. Therefore, the narrow-band pulse provided by the invention is more stable in the calculation of the ultrasonic attenuation coefficient than the wide-band pulse, and the obtained result is more accurate.
By calculating the frequency characteristic of each transmitting frequency point, the ultrasonic attenuation coefficient at the corresponding frequency can be calculated by using the formula (1) and the value at the frequency is extracted, and the attenuation coefficient measurement result is shown in fig. 5.
Referring to fig. 5, the attenuation values of the narrow-band pulses with different frequencies show a certain rule, and the fitting expression of the attenuation coefficient in the frequency domain is alpha obtained by averaging the three test results and performing optimal curve fitting through the originL(f,2z) ═ 10.2 f. At the same time, the narrowband pulse and the broadband pulse test results are compared. Because the broadband pulse signal has a certain frequency band range, the attenuation coefficient characteristic in a certain range can be measured; but when the frequency is beyond the effective range, the measurement result of the attenuation coefficient is disordered; because it is difficult to determine the effective frequency range, the results obtained by fitting curves using different frequency bands will vary greatly. Compared with a local attenuation curve of a broadband ultrasonic pulse, the method for testing the ultrasonic attenuation coefficient by using the narrowband ultrasonic pulse provided by the invention has the advantage that the accuracy of the attenuation value at each frequency is improved, so that the finally-fitted ultrasonic frequency domain attenuation-frequency curve is more accurate.
To further illustrate the effectiveness of the proposed method of the present invention, the same test procedure was performed on brass, another metallic material, and the final ultrasound attenuation coefficient results are shown in fig. 6.
Referring to fig. 6, according to the measured numerical result, an expression of the relationship between the ultrasonic attenuation coefficient and the frequency, which is obtained by performing the best curve fitting, is αL(f,2z)＝0.788f2. Meanwhile, the obtained result is compared with the broadband pulse measurement result, and the result shows that when the probes with the central frequencies of 5MHz and 10MHz are used, the frequency domain attenuation is regular only near the central frequency and cannot fit the result on the whole frequency domain, and when the 5MHz probe is selected and the frequency is far away from the central frequency, the ultrasonic attenuation coefficient has a negative value, which is not in accordance with the actual situation. In actual measurement, the center frequency of the acquired signal is not constant at 5MHz and 10MHz, which brings large errors to the measurement result.
In summary, in the method for testing the ultrasonic attenuation coefficient according to the embodiment of the present invention, the ultrasonic water immersion probe transmits the ultrasonic narrow pulse sine wave signal with the corresponding frequency value to the test block, the ultrasonic attenuation coefficient of the test block at the frequency is measured, and then the optimal curve fitting is performed on the entire frequency interval through the multi-point attenuation values. Therefore, the ultrasonic narrow-band pulse sine wave signal is stable, the energy is concentrated at the transmitting frequency, and the measured ultrasonic attenuation coefficient is more accurate; and an ultrasonic attenuation coefficient curve on the whole frequency domain can be obtained by measuring the ultrasonic attenuation coefficients of a plurality of points.
Example 2
Fig. 7 is a schematic structural diagram of an ultrasonic attenuation coefficient measuring apparatus according to an embodiment of the present invention.
As shown in fig. 7, the apparatus for measuring an ultrasonic attenuation coefficient includes: the ultrasonic water immersion probe comprises an ultrasonic water immersion probe 10, a current probe 20, an amplifier 30, a function generator 40, an oscilloscope 50 and an upper computer 60, wherein the surface of the ultrasonic water immersion probe 10 is immersed in water, the current probe 20 is respectively and electrically connected with the ultrasonic water immersion probe 10, the amplifier 30 and the oscilloscope 50, the function generator 40 is electrically connected with the amplifier 30, and the upper computer 60 is electrically connected with the oscilloscope 50.
The narrow pulse sine wave signal output by the function generator 40 is amplified by the amplifier 30, and then the ultrasonic water immersion probe 10 is driven to transmit an ultrasonic signal to the test block immersed in water, the ultrasonic water immersion probe 10 transmits an echo signal reflected by the collected test block to the oscilloscope 50 through the current probe 20 to be displayed on the oscilloscope 50, and the upper computer 60 is used for acquiring the echo signal displayed on the oscilloscope 50 and calculating the ultrasonic attenuation coefficient of the test block according to the frequency of the echo signal and the ultrasonic signal.
As an example, the echo signal includes a reflection signal of the upper surface of the test block and a reflection signal of the bottom surface of the test block, and the upper computer 60 may calculate the ultrasonic attenuation coefficient of the test block by the following formula (1):
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2d ') is a reflection signal of the bottom surface of the test block, d' isPropagation distance of ultrasonic signal in test block and water, D1(f,2D) is a diffraction correction term when the ultrasonic signal propagates through water, D2(f,2d') is a diffraction correction term, T, for the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal passing through the water interface21Is the transmission coefficient, R, of an ultrasonic signal passing through an interface of a test block21Is the reflection coefficient of the ultrasonic signal passing through the interface of the test block.
Specifically, the present invention utilizes an ultrasonic water immersion probe 10 to measure the ultrasonic attenuation coefficient of a solid material (i.e., a test block). An ultrasonic narrow-band pulse sine wave signal is used as a transmitting source, ultrasonic longitudinal waves are transmitted in the measuring process as shown in figure 2, and an ultrasonic signal transmitted by an ultrasonic water immersion probe is reflected by the upper surface and the bottom surface of a tested block and is received by the ultrasonic water immersion probe.
Assuming that the thickness of the test block is z, the distance from the surface of the ultrasonic water immersion probe 10 to the upper surface of the test block, i.e. the water depth distance is d. The amplitudes of the surface echo (i.e. the reflected signal of the upper surface of the test block) and the primary bottom wave signal (i.e. the reflected signal of the bottom surface of the test block) of the longitudinal wave received by the ultrasonic water immersion probe 10 can be expressed as:
S1(f,2d)＝S0(f,0)R12M1(f,2d)D1(f,2d) (2)
S2(f,2d')＝S0(f,0)T12R21T21M2(f,2d')D2(f,2d') (3)
in formulae (2) and (3), S0(f,0) is the initial amplitude of the ultrasonic signal emitted by the ultrasonic water immersion probe 10;
in the formulae (2) and (3), R12And R21The reflection coefficient of the ultrasonic signal passing through the interface can be expressed as:
Figure BDA0002262799290000092
R21＝-R12(5)
wherein Z is1＝ρ1cp1For the acoustic impedance, Z, of the ultrasonic signal in the test block2＝ρ2cp2Acoustic impedance, p, of ultrasonic signals in water1And ρ2Density of water and test block, respectively, cp1And cp2The propagation velocities of the ultrasonic signal in water and in the test block, respectively.
In formulae (2) and (3), T12And T21The transmission coefficients of the ultrasonic signal passing through the water and test block interface can be expressed as:
Figure BDA0002262799290000101
Figure BDA0002262799290000102
in formulae (2) and (3), M1(f,2d) and M2(f,2d') are respectively the attenuation coefficient of the ultrasonic signal in water after the echo is reflected by the surface of the test block and the attenuation coefficient of the primary bottom echo in water and the test block, and can be expressed as follows:
M1(f,2d)＝exp[-2αf(f)d](8)
M2(f,2d')＝exp[-2αf(f)d-αL(f)z](9)
wherein alpha isf(f) And alphaL(f) The attenuation coefficients of the ultrasonic signal in water and the test block are respectively, and the propagation distance d' of the ultrasonic signal in two layers of media is given by the formula (10)
In formulae (2) and (3), D1(f,2D) and D2(f,2d') are diffraction correction terms in the propagation path of the ultrasonic signal in water and in the test block, respectively, which can be expressed as:
Figure BDA0002262799290000104
Figure BDA0002262799290000105
wherein k is 2 pi f/cp1Representing the wave number of the ultrasonic signal in water, a is the radius of the ultrasonic water immersion probe, J0And J1Respectively, zero order and first order bessel functions.
And dividing the formula (3) by the formula (2) and substituting the corresponding variables to obtain an attenuation coefficient expression of the ultrasonic signal in the test block, namely the formula (1).
Therefore, for an ultrasonic water immersion probe 10 with a given center frequency, as long as a narrow pulse signal with a single frequency f is transmitted within the bandwidth thereof, the attenuation coefficient α at the corresponding frequency can be calculated by equation (1)L(f,2z)。
As an example, the surface of the ultrasonic water immersion probe 10 is 20mm away from the upper surface of the test block, the test block adopts a cuboid structure, and the upper surface area and the lower surface area of the test block are both 50mm multiplied by 50mm2The thickness of the test block was 20 mm.
Optionally, the center frequency of the ultrasonic water immersion probe 10 is 1MHz, 2.25MHz, or 5 MHz.
As an example, the function generator 40 is configured to output a plurality of narrow pulse sine wave signals with different frequencies, and transmit an ultrasonic wave signal with a corresponding frequency by using the ultrasonic water immersion probe 10 with a corresponding center frequency, wherein the upper computer 60 is further configured to obtain echo signals corresponding to the ultrasonic wave signals with different frequencies, calculate a plurality of ultrasonic attenuation coefficients according to the plurality of different frequencies and the echo signals corresponding thereto, and perform fitting processing on the plurality of ultrasonic attenuation coefficients and the frequencies corresponding thereto to obtain an ultrasonic attenuation coefficient-frequency curve.
For example, the ultrasonic attenuation coefficient may be measured and the measurement of the ultrasonic attenuation coefficient-frequency curve may be implemented using the apparatus shown in fig. 7. Five ultrasonic water immersion probes with different central frequencies are selected, and the central frequencies are 1MHz, 2.25MHz, 5MHz, 7.5MHz and 10MHz respectively. Narrow pulse sine wave signals of 15 periods are transmitted through a function generator 40, and the signals are amplified through an amplifier 30 and then drive an ultrasonic water immersion probe 10; the current probe 20 is directly electrically connected with the amplifier 30 and the ultrasonic water immersion probe 10 for receiving the reflected echo signals collected by the ultrasonic water immersion probe 10. The frequencies of the signals transmitted by the probe at the different center frequencies are shown in tables 1-5. Ultrasonic signals emitted by the ultrasonic water immersion probe 10 are reflected by the test block and displayed on the oscilloscope 50, and are finally stored in the upper computer 60 (such as a computer) for subsequent processing.
Wherein, referring to fig. 7, the ultrasonic water immersion probe 10 can be fixed on the water tank 1 by the 6-degree-of-freedom motion control 3, the distance from the surface of the ultrasonic water immersion probe 10 to the upper surface of the test block, namely the water depth distance, is adjusted to be 20mm, wherein, the cushion block 2 can be arranged below the test block, so as to better adjust the water depth distance. The upper and lower surface areas of the test block are 50X 50mm2The influence of the edge effect on the measurement is avoided, and the thickness is designed to be 20 mm. Therefore, the parallelism of high standards is ensured on the upper surface and the lower surface of the test block, and the surfaces are polished, so that errors caused by measurement can be reduced or prevented.
TABLE 1
Figure BDA0002262799290000111
TABLE 2
Figure BDA0002262799290000112
TABLE 3
Figure BDA0002262799290000113
TABLE 4
TABLE 5
Figure BDA0002262799290000115
Figure BDA0002262799290000121
Fig. 3(a), 3(b) and 3(b) show time domain signal waveforms of ultrasonic signals with frequencies of 2.25MHz, 5MHz and 10MHz respectively propagating in an acrylic material. The result shows that the method has high signal-to-noise ratio when measuring the ultrasonic attenuation coefficient, and the average speed of the ultrasonic signal propagating in the acrylic material can be calculated to be c 2720m/s from the time domain signal diagram.
The frequency domain characteristics of the ultrasonic signal obtained by extracting the surface echo and the primary bottom wave signal with a "hanning window" and then performing fast fourier transform mapping are shown in fig. 4(a), 4(b), and 4 (c). It can be seen from the figure that the signal center frequencies of the ultrasonic surface echo and the primary bottom wave are stabilized at the transmitting frequency, and the phenomenon that the center frequency is shifted to a low frequency like the broadband pulse signal does not occur. Therefore, the narrow-band pulse provided by the invention is more stable in the calculation of the ultrasonic attenuation coefficient than the wide-band pulse, and the obtained result is more accurate.
By calculating the frequency characteristic of each transmitting frequency point, the ultrasonic attenuation coefficient at the corresponding frequency can be calculated by using the formula (1) and the value at the frequency is extracted, and the attenuation coefficient measurement result is shown in fig. 5.
Referring to fig. 5, the attenuation values of the narrow-band pulses with different frequencies show a certain rule, and the fitting expression of the attenuation coefficient in the frequency domain is alpha obtained by averaging the three test results and performing optimal curve fitting through the originL(f,2z) ═ 10.2 f. At the same time, the narrowband pulse and the broadband pulse test results are compared. Because the broadband pulse signal has a certain frequency band range, the attenuation coefficient characteristic in a certain range can be measured; but when the frequency is beyond the effective range, the measurement result of the attenuation coefficient is disordered; because it is difficult to determine the effective frequency range, the results obtained by fitting curves using different frequency bands will vary greatly. Compared with the local attenuation curve of the broadband ultrasonic pulse, the method for testing the ultrasonic attenuation coefficient by using the narrowband ultrasonic pulse provided by the invention has the advantages that the accuracy of the attenuation value at each frequency is improved, and thus the finally fitted ultrasonic frequency domain attenuation-frequencyThe rate curve will be more accurate.
To further illustrate the effectiveness of the proposed method of the present invention, the same test procedure was performed on brass, another metallic material, and the final ultrasound attenuation coefficient results are shown in fig. 6.
Referring to fig. 6, according to the measured numerical result, an expression of the relationship between the ultrasonic attenuation coefficient and the frequency, which is obtained by performing the best curve fitting, is αL(f,2z)＝0.788f2. Meanwhile, the obtained result is compared with the broadband pulse measurement result, and the result shows that when the probes with the central frequencies of 5MHz and 10MHz are used, the frequency domain attenuation is regular only near the central frequency and cannot fit the result on the whole frequency domain, and when the 5MHz probe is selected and the frequency is far away from the central frequency, the ultrasonic attenuation coefficient has a negative value, which is not in accordance with the actual situation. In actual measurement, the center frequency of the acquired signal is not constant at 5MHz and 10MHz, which brings large errors to the measurement result.
In summary, in the device for testing the ultrasonic attenuation coefficient according to the embodiment of the present invention, the function generator drives the ultrasonic water immersion probe to emit the ultrasonic narrow pulse sine wave signal with the corresponding frequency value to obtain the ultrasonic attenuation coefficient at the frequency, and the optimal curve fitting can be performed on the whole frequency interval through the multi-point attenuation value. Therefore, the ultrasonic narrow-band pulse sine wave signal is stable, the energy is concentrated at the transmitting frequency, and the measured ultrasonic attenuation coefficient is more accurate; and an ultrasonic attenuation coefficient curve on the whole frequency domain can be obtained by measuring the ultrasonic attenuation coefficients of a plurality of points.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.

Claims (10)

1. A method for measuring an ultrasonic attenuation coefficient is characterized by comprising the following steps:
transmitting an ultrasonic signal to a test block immersed in water by using an ultrasonic water immersion probe with the surface immersed in water, wherein the ultrasonic signal is an ultrasonic narrow-band pulse sine wave signal;
receiving an echo signal of the ultrasonic signal reflected by the test block by using the ultrasonic water immersion probe;
and calculating the ultrasonic attenuation coefficient of the test block according to the frequencies of the echo signal and the ultrasonic signal.
2. The method of measuring an ultrasonic attenuation coefficient according to claim 1, wherein the echo signal includes a reflection signal of an upper surface of the test block and a reflection signal of a bottom surface of the test block, wherein the ultrasonic attenuation coefficient of the test block is calculated by the following formula:
Figure FDA0002262799280000011
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2D ') is a reflected signal from the bottom surface of the test block, D' is a propagation distance of the ultrasonic signal between the test block and water, D1(f,2D) is a diffraction correction term for the ultrasonic signal as it propagates through the water, D2(f,2d') is a diffraction correction term, T, of the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal when passing through the water interface21Is the transmission coefficient, R, of the ultrasonic signal passing through the interface of the test block21The reflection coefficient of the ultrasonic signal passing through the interface of the test block is shown.
3. The method for measuring the ultrasonic attenuation coefficient according to claim 1, wherein the surface of the ultrasonic water immersion probe is 20mm away from the upper surface of the test block, the test block is of a cuboid structure, and the measurement is carried outThe upper and lower surface areas of the test block are 50X 50mm2And the thickness of the test block is 20 mm.
4. The method of measuring an ultrasonic attenuation coefficient of claim 1, wherein the center frequency of the ultrasonic water immersion probe is 1MHz, 2.25MHz, or 5 MHz.
5. The method of measuring ultrasonic attenuation coefficient according to claim 4, wherein a plurality of ultrasonic signals of different frequencies are transmitted by one or more ultrasonic water immersion probes of different center frequencies, the method further comprising:
calculating to obtain a plurality of ultrasonic attenuation coefficients according to a plurality of different frequencies and corresponding echo signals;
and fitting the plurality of ultrasonic attenuation coefficients and the corresponding frequencies thereof to obtain an ultrasonic attenuation coefficient-frequency curve.
6. An ultrasonic attenuation coefficient measuring apparatus, comprising: the ultrasonic water immersion probe comprises an ultrasonic water immersion probe, a current probe, an amplifier, a function generator, an oscilloscope and an upper computer, wherein the surface of the ultrasonic water immersion probe is immersed in water, the current probe is respectively and electrically connected with the ultrasonic water immersion probe, the amplifier and the oscilloscope, the function generator is electrically connected with the amplifier, and the upper computer is electrically connected with the oscilloscope;
the ultrasonic water immersion probe transmits acquired echo signals reflected by the test block to the oscilloscope through the current probe for displaying on the oscilloscope, and the upper computer is used for acquiring the echo signals displayed by the oscilloscope and calculating the ultrasonic attenuation coefficient of the test block according to the echo signals and the frequency of the ultrasonic signals.
7. The apparatus of claim 6, wherein the echo signal includes a reflection signal of an upper surface of the test block and a reflection signal of a bottom surface of the test block, and the upper computer calculates the ultrasonic attenuation coefficient of the test block by the following formula:
Figure FDA0002262799280000021
wherein alpha isL(f,2z) is the ultrasonic attenuation coefficient of the test block, f is the frequency of the ultrasonic signal, z is the thickness of the test block, d is the distance from the surface of the ultrasonic water immersion probe to the upper surface of the test block, S1(f,2d) is a reflected signal of the upper surface of the test block, S2(f,2D ') is a reflected signal from the bottom surface of the test block, D' is a propagation distance of the ultrasonic signal between the test block and water, D1(f,2D) is a diffraction correction term for the ultrasonic signal as it propagates through the water, D2(f,2d') is a diffraction correction term, T, of the ultrasonic signal as it propagates through the test block12Is the transmission coefficient, T, of the ultrasonic signal when passing through the water interface21Is the transmission coefficient, R, of the ultrasonic signal passing through the interface of the test block21The reflection coefficient of the ultrasonic signal passing through the interface of the test block is shown.
8. The ultrasonic attenuation coefficient measuring device of claim 6, wherein the surface of the ultrasonic water immersion probe is 20mm away from the upper surface of the test block, the test block is of a cuboid structure, and the upper surface area and the lower surface area of the test block are both 50 x 50mm2And the thickness of the test block is 20 mm.
9. The apparatus for measuring ultrasonic attenuation coefficient of claim 6, wherein the center frequency of the ultrasonic water immersion probe is 1MHz, 2.25MHz or 5 MHz.
10. The apparatus for measuring ultrasonic attenuation coefficient according to claim 9, wherein the function generator is configured to output a plurality of narrow pulse sine wave signals with different frequencies, and transmit ultrasonic signals with corresponding frequencies by using an ultrasonic water immersion probe with corresponding center frequency, wherein the upper computer is further configured to obtain echo signals corresponding to the ultrasonic signals with different frequencies, calculate a plurality of ultrasonic attenuation coefficients according to the plurality of different frequencies and the corresponding echo signals, and perform fitting processing on the plurality of ultrasonic attenuation coefficients and the corresponding frequencies thereof to obtain an ultrasonic attenuation coefficient-frequency curve.
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