CN116295987B - High-spatial-resolution stress dynamic measurement method based on air-coupled ultrasound - Google Patents
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- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
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Abstract
The invention discloses a high-spatial-resolution stress dynamic measurement method based on air-coupled ultrasound. When the distance between the receiving empty coupling transducer and the empty coupling transducer in the injection and the receiving of the sample to be detected is L, recording the motion position of the sample to be detected in one period of periodic motion and the receiving echo signal F (x) of the current position; comparing F (x) with the stress-free state to obtain acoustic time difference, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L propagation distance at each motion position; when the distance is L+DeltaL, recording the motion position of the sample to be measured in one period of periodic motion and the received echo signal F at the current position 1 (x) The method comprises the steps of carrying out a first treatment on the surface of the Will F 1 (x) Obtaining acoustic time difference compared with the stress-free state, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L+delta L propagation distance at each movement position; the stress value of the corresponding DeltaL position under each movement position is the difference between the two positions. The method is used for solving the problem of the influence of the couplant on measurement due to the contradictory relation between the spatial resolution and the measurement precision in the traditional ultrasonic stress measurement.
Description
Technical Field
The invention belongs to the technical field of ultrasonic detection, and particularly relates to a high-spatial-resolution stress dynamic measurement method based on air coupling ultrasound.
Background
High-end smart equipment components or structures always operate under complex stress conditions, including tensile stress, compressive stress, and combinations thereof. The presence of stress has a significant impact on the fatigue life, safety in use, and equipment maintenance of high-end smart equipment components or structures. Therefore, the research of the nondestructive testing method capable of effectively detecting the stress of the high-end intelligent equipment parts or the structure is significant for timely finding out stress concentration of the test piece and taking countermeasures, prolonging the service life of the test piece and reducing social and economic losses.
Up to now, many nondestructive stress measurement methods have been developed by international researchers in various countries. Diffraction, one of the main methods, mainly focuses on stress varying on the atomic or grain scale, and uses a high energy beam (such as X-rays, electrons, or neutrons) to detect the change in bragg scattering angle, and elastic strain has been determined. However, diffraction methods either require a stringent test environment or the test equipment or process is very complex and time consuming. Furthermore, the magnetoacoustic method based on the barkhausen effect is limited to ferromagnetic materials and has a limited range of applications. In recent years, ultrasonic methods using piezoelectric transducers have been developed for detecting and identifying stress states in complex structures.
Compared with other nondestructive detection methods, ultrasonic stress detection has the potential of low cost and high efficiency, and has good prospect in meeting the field measurement, dynamic measurement, in-service measurement and other aspects of stress measurement in industrial application. Essentially, ultrasonic stress measurement is based on the sonoelastic effect by measuring the change in ultrasonic velocity before and after the change in stress state. In general, critical refraction longitudinal waves (LCR) are widely used for stress measurement because they are more sensitive to stress than other ultrasonic waves. Ultrasonic testing is classified into contact and non-contact. The contact ultrasonic detection technology needs to use liquid as an acoustic coupling agent between an ultrasonic transducer and a sample to be detected so as to reduce the loss of ultrasonic wave propagation in air. The coupling agent increases the influence of human factors on the result, and on the other hand, the requirements of industrial automatic production and quality control are hardly met, so that the application range of ultrasonic detection is limited. The non-contact nondestructive detection does not need a coupling agent, the detection process is simple and convenient, the detection result can avoid the influence of artificial coupling factors, and the method is one of the main development directions of the rapid nondestructive detection technology. For the stress detection of high-end intelligent equipment components or structures in an operating state, a non-contact ultrasonic stress detection mode can only be adopted.
With the development of micromachining technology and the development of polymer material technology, the manufacture of the air-coupled ultrasonic transducer with high efficiency and high sensitivity makes a great breakthrough, and the development of the amplifier with low noise and high gain and the development of computer signal processing technology make the air-coupled ultrasonic nondestructive testing technology have great progress and obtain better application results in the field of nondestructive testing.
The traditional stress measurement method based on acoustic moveout is the obtained average stress under the fixed propagation distance, and the spatial resolution of stress measurement directly depends on the magnitude of the propagation distance. The larger the propagation distance is, the smaller the influence of the direct wave detected on the same side of the air-coupled ultrasound on the useful LCR wave signal is, the more accurate the acoustic time difference measurement is, and the more accurate the stress measurement is; at the same time, however, the spatial resolution of the stress measurement is reduced. Meanwhile, the traditional stress measurement method cannot realize dynamic measurement of stress.
Disclosure of Invention
The invention provides a high spatial resolution stress dynamic measurement method based on air coupling ultrasound, which is used for solving the contradictory relation between spatial resolution and measurement precision in the traditional ultrasonic stress measurement and the problem of the influence of a coupling agent on the measurement.
The invention is realized by the following technical scheme:
the high-spatial resolution stress dynamic measurement method based on air coupling ultrasound specifically comprises exciting LCR waves in a sample (3) to be measured so as to determine the inclination angles of an excitation and receiving air coupling transducer which are excited to be sent by the air coupling ultrasonic transducer, and then assembling a measurement device according to the inclination angles;
when the distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) in the injection and the receiving of the sample (3) to be detected is L, recording the motion position of the sample (3) to be detected for one period of periodic motion and the received echo signal of the current position as F (x);
comparing F (x) with the stress-free state to obtain acoustic time difference, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L propagation distance at each motion position;
when the distance between the receiving space coupling transducer (2) and the space coupling transducer (1) between the injection and the receiving of the sample (3) to be measured is L+DeltaL, recording the motion position of the sample (3) to be measured for one period of periodic motion and the received echo signal at the current position as F 1 (x)；
Will F 1 (x) Obtaining the acoustic time difference compared with the stress-free state, and calculating according to the acoustic time differenceObtaining an average stress value of the corresponding L+delta L propagation distance at each motion position;
the stress value of the corresponding DeltaL position under each movement position is the difference between the two positions.
The measuring method comprises the steps that 1, a measuring device is assembled, namely, a hollow coupling transducer (1) is placed on one side of a to-be-measured sample (3) containing a stress concentration area according to a determined inclination angle;
the other end of the sample (3) to be tested is provided with a receiving empty coupling transducer (2), and the receiving empty coupling transducer (2) and the empty coupling transducer (1) are both arranged on one side of the sample (3) to be tested;
the distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) and the sample (3) to be measured is L A The method comprises the steps of determining the period of an excitation signal to be N, selecting the center frequency f of an empty coupled transducer, generating a sine pulse signal with the period of N, the Hanning window modulation and the frequency f by adopting an arbitrary signal function generator as the excitation signal, carrying out impedance matching through a 50 ohm load, applying the impedance matching to the excited empty coupled transducer (1) after a low-pass filter, and receiving echoes at receiving positions with the distance L by using a receiving empty coupled transducer (2).
According to the measuring method, the movement position of the sample (3) to be measured is recorded by a grating displacement sensor;
and generating a trigger signal at each movement position, and collecting a received echo signal at the current position by using a high-speed data collection board card.
The measuring method specifically includes the steps that when the sound is measured in the unstressed state, the unstressed test block is made to perform periodic motion for one period, the motion positions of the propagation distance L and the propagation distance L+delta L of the unstressed test block are recorded through the grating displacement sensor, trigger signals are generated at each motion position, the high-speed data acquisition board card is used for acquiring received echo signals at the current position and recorded as t 1,0 And t 2,0 。
The average stress value of the corresponding L propagation distance at each motion position is specifically obtained by comparing F (x) with the stress-free state to obtain the acoustic time difference and combining the acoustic time difference with the linear relation coefficient of the stress and the acoustic time difference.
According to the measuring method, the distance L+DeltaL between the receiving empty coupling transducer (2) and the empty coupling transducer (1) in the injection and the receiving of the sample (3) to be measured is specifically adjusted to keep the position of the exciting empty coupling transducer (1) unchanged, and the receiving empty coupling transducer (2) is moved to enable the distance L+DeltaL between the receiving empty coupling transducer and the receiving empty coupling transducer.
The average stress value of the corresponding L+DeltaL propagation distance at each motion position is specifically expressed as F 1 (x) And compared with the stress-free state, obtaining the acoustic time difference, and obtaining the average stress value of the corresponding L+delta L propagation distance at each motion position by combining the acoustic time difference with the linear relation coefficient of the stress and the acoustic time difference.
The measuring method, the LCR wave ultrasonic stress detection is specifically that by introducing a material constitutive model into an ultrasonic propagation dynamics equation based on a limited deformation theory, an acoustic elasticity equation is,
in delta IK As Kronecker function, ρ is the material density of the initial configuration, X J For the spatial position of the midpoint of the initial configuration, u K Is X K Displacement in direction C IJKL For Cathy stress fieldIs a spring constant of (2);
when a plane wave propagates in a solid material, the particle vibration can be expressed as,
u I ＝U I exp[jK(N J X J -Vt)] (2)
in U I For the polarization amplitude of the direction, N J The direction cosine of the ultrasonic wave propagation direction, K is the wave number of the ultrasonic wave, and V is the propagation speed of the ultrasonic wave;
the characteristic equation of the propagation of ultrasonic waves in a stress material is obtained by combining the formulas (1) and (2)
For isotropic materials, when the longitudinal elastic wave propagates in the uniaxial stress direction, the longitudinal velocity V and the stress magnitude σ can be expressed as
Where λ and μ are the Lame constants, and l and m are the Murraghan constants.
In the measuring method, if the incident angle of the ultrasonic wave is a first critical angle, LCR waves are generated, the first critical angle theta can be obtained according to Snell's law,
wherein V is A Is the ultrasonic wave propagation velocity in the air, V M Is the ultrasonic wave propagation speed in the measured material;
for receiving LCR waves, the receiving air-coupled transducer needs to be symmetrically arranged with the exciting air-coupled transducer, and the LCR wave speed V can be obtained according to the formula (4) M The relationship with the stress sigma is that,
wherein V is M0 The ultrasonic wave propagation speed of the material to be measured in the stress-free state is set;
for an ultrasonic propagation distance L in the measured material, the obtained acoustic time difference Deltat=t-t is deduced 0 The relationship with the stress sigma is that,
wherein t is 0 ＝L/V M0 For the time of flight TOF of the material under test in the stress-free state, t=L/V M The time of flight TOF under the stress state of the measured material is represented by a stress coefficient K, and the linear relation between the stress of the measured material and the acoustic time difference is obtained by experimental calibration in practice.
The beneficial effects of the invention are as follows:
according to the invention, the dynamic measurement of the high-end intelligent equipment component or structure is realized by using the hollow-coupled ultrasonic excitation LCR wave, the detection distance of the two hollow-coupled transducers is changed in different periods of the periodic movement of the high-end intelligent equipment component or structure, the contradictory relation between the spatial resolution and the measurement precision in the traditional ultrasonic stress measurement is overcome, and the stress measurement spatial resolution is improved while the measurement precision is ensured. Meanwhile, the influence of the couplant is eliminated, the flexibility of detection is enhanced, the detection efficiency is improved, and the high-spatial-resolution dynamic qualitative and quantitative characterization of stress is better realized.
The invention adopts air coupling LCR wave ultrasonic detection, and takes air as a transmission medium to replace a coupling agent in the traditional ultrasonic nondestructive detection in the detection process, so that the secondary pollution problem of a coupling material to a piece to be detected can be fundamentally avoided, the invention has the advantages of no contact, no invasion and no damage in the detection process, the service life of an air coupling transducer can be greatly prolonged, the air coupling LCR wave detection can realize on-line rapid detection, and the invention is suitable for the dynamic measurement of the stress of high-end intelligent equipment parts or structures. The invention uses the grating displacement sensor to record the position information of the periodic motion of the measured material and controls the high-speed acquisition board card to obtain the received echo signal of the current position. The detection distance of the two empty-coupled transducers is changed in different periods of the periodic motion of the high-end intelligent equipment component or structure, so that the contradictory relation between the spatial resolution and the stress resolution in the traditional ultrasonic stress measurement is overcome, and the spatial resolution of stress detection is improved. Meanwhile, the influence of the couplant is eliminated, the flexibility of detection is enhanced, the detection efficiency is improved, and the high-spatial-resolution dynamic qualitative and quantitative characterization of stress is better realized.
Drawings
Fig. 1 is a schematic structural view of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The high-spatial resolution stress dynamic measurement method based on air coupling ultrasound specifically comprises exciting LCR waves in a sample (3) to be measured so as to determine the inclination angles of an excitation and receiving air coupling transducer which are excited to be sent by the air coupling ultrasonic transducer, and then assembling a measurement device according to the inclination angles;
when the distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) in the injection and the receiving of the sample (3) to be detected is L, recording the motion position of the sample (3) to be detected for one period of periodic motion and the received echo signal of the current position as F (x);
comparing F (x) with the stress-free state to obtain acoustic time difference, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L propagation distance at each motion position;
when the distance between the receiving space coupling transducer (2) and the space coupling transducer (1) between the injection and the receiving of the sample (3) to be measured is L+DeltaL, recording the motion position of the sample (3) to be measured for one period of periodic motion and the received echo signal at the current position as F 1 (x)；
Will F 1 (x) Obtaining acoustic time difference compared with the stress-free state, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L+delta L propagation distance at each movement position;
the stress value of the corresponding DeltaL position under each movement position is the difference between the two positions. Thereby realizing the dynamic measurement of high spatial resolution stress of high-end intelligent equipment components or structures.
The measuring method comprises the steps that 1, a measuring device is assembled, namely LCR waves are excited in a sample (3) to be measured, and the inclination angle of an excitation and receiving air-coupled transducer (1) is determined by utilizing the sound speed of a measured material and the sound speed of air and combining with Snell's law;
placing the hollow coupling transducer (1) at a determined inclination angle on one side of a sample (3) to be tested containing a stress concentration area;
the other end of the sample (3) to be tested is provided with a receiving empty coupling transducer (2), and the receiving empty coupling transducer (2) and the empty coupling transducer (1) are both arranged on one side of the sample (3) to be tested;
the distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) and the sample (3) to be measured is L A In order to ensure that the sound beam has enough energy, the period of an excitation signal is determined to be N, the center frequency f of a proper empty coupled transducer is selected, a sine pulse signal with the period of N, the Hanning window modulation and the frequency f is generated by adopting an arbitrary signal function generator as the excitation signal, impedance matching is carried out through a 50 ohm load, the impedance matching is carried out after the impedance matching is carried out through a low-pass filter, the impedance matching is applied to the excitation empty coupled transducer (1), and an echo is received by a receiving empty coupled transducer (2) at a receiving position with the distance L.
According to the measuring method, the movement position of the sample (3) to be measured is recorded by a grating displacement sensor;
and generating a trigger signal at each movement position, and collecting a received echo signal at the current position by using a high-speed data collection board card.
The measuring method specifically includes the steps that when the sound is measured in the unstressed state, the unstressed test block is made to perform periodic motion for one period, the motion positions of the propagation distance L and the propagation distance L+delta L of the unstressed test block are recorded through the grating displacement sensor, trigger signals are generated at each motion position, the high-speed data acquisition board card is used for acquiring received echo signals at the current position and recorded as t 1,0 And t 2,0 。
The average stress value of the corresponding L propagation distance at each motion position is specifically obtained by comparing F (x) with the stress-free state to obtain the acoustic time difference and combining the acoustic time difference with the linear relation coefficient of the stress and the acoustic time difference.
According to the measuring method, the distance L+DeltaL between the receiving empty coupling transducer (2) and the empty coupling transducer (1) in the injection and the receiving of the sample (3) to be measured is specifically adjusted to keep the position of the exciting empty coupling transducer (1) unchanged, and the receiving empty coupling transducer (2) is moved to enable the distance L+DeltaL between the receiving empty coupling transducer and the receiving empty coupling transducer.
The average stress value of the corresponding L+DeltaL propagation distance at each motion position is specifically expressed as F 1 (x) And compared with the stress-free state, obtaining the acoustic time difference, and obtaining the average stress value of the corresponding L+delta L propagation distance at each motion position by combining the acoustic time difference with the linear relation coefficient of the stress and the acoustic time difference.
The measuring method, the LCR wave ultrasonic stress detection is specifically that by introducing a material constitutive model into an ultrasonic propagation dynamics equation based on a limited deformation theory, an acoustic elasticity equation is,
in delta IK As Kronecker function, ρ is the material density of the initial configuration, X J For the spatial position of the midpoint of the initial configuration, u K Is X K Displacement in direction C IJKL For Cathy stress fieldIs a spring constant of (2);
when a plane wave propagates in a solid material, the particle vibration can be expressed as,
u I ＝U I exp[jK(N J X J -Vt)] (2)
in U I For the polarization amplitude of the direction, N J The direction cosine of the ultrasonic wave propagation direction, K is the wave number of the ultrasonic wave, and V is the propagation speed of the ultrasonic wave;
the characteristic equation of the propagation of ultrasonic waves in a stress material is obtained by combining the formulas (1) and (2)
For isotropic materials, when the longitudinal elastic wave propagates in the uniaxial stress direction, the longitudinal velocity V and the stress magnitude σ can be expressed as
Where λ and μ are the Lame constants, and l and m are the Murraghan constants.
In the measuring method, if the incident angle of the ultrasonic wave is a first critical angle, LCR waves are generated, the first critical angle theta can be obtained according to Snell's law,
wherein V is A Is the ultrasonic wave propagation velocity in the air, V M Is the ultrasonic wave propagation speed in the measured material;
for receiving LCR waves, the receiving space-coupling transducer needs to be symmetrically arranged with the exciting space-coupling transducer, as shown in fig. 1, and the LCR wave speed V can be obtained according to the formula (4) M The relationship with the stress sigma is that,
wherein V is M0 The ultrasonic wave propagation speed of the material to be measured in the stress-free state is set;
for an ultrasonic wave propagation distance L in the measured material, the obtained acoustic time difference Deltat=t-t 0 The relationship with the stress sigma is that,
wherein t is 0 ＝L/V M0 For the time of flight TOF of the material under test in the stress-free state, t=L/V M The time of flight TOF under the stress state of the measured material is represented by a stress coefficient K, and the linear relation between the stress of the measured material and the acoustic time difference is obtained by experimental calibration in practice.
Claims (7)
1. The high-spatial-resolution stress dynamic measurement method based on air-coupled ultrasound is characterized in that LCR waves are excited in a sample (3) to be measured so as to determine excitation sent by an excitation air-coupled ultrasonic transducer and an inclination angle of a receiving air-coupled transducer, and then a measurement device is assembled according to the inclination angle;
when the distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) in the injection and the receiving of the sample (3) to be detected is L, recording the motion position of the sample (3) to be detected for one period of periodic motion and the received echo signal of the current position as F (x);
comparing F (x) with the stress-free state to obtain acoustic time difference, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L propagation distance at each motion position;
when the distance between the receiving space coupling transducer (2) and the space coupling transducer (1) between the injection and the receiving of the sample (3) to be measured is L+DeltaL, recording the motion position of the sample (3) to be measured for one period of periodic motion and the received echo signal at the current position as F 1 (x)；
Will F 1 (x) Obtaining acoustic time difference compared with the stress-free state, and calculating according to the acoustic time difference to obtain an average stress value of the corresponding L+delta L propagation distance at each movement position;
the stress value of the corresponding delta L position under each movement position is the difference between the two;
the LCR wave ultrasonic stress detection is specifically carried out by introducing a material constitutive model into an ultrasonic propagation dynamics equation based on a limited deformation theory, and the acoustic elasticity equation is,
in delta IK As Kronecker function, ρ is the material density of the initial configuration, X J For the spatial position of the midpoint of the initial configuration, u K Is X K Displacement in direction C IJKL For Cathy stress fieldIs a spring constant of (2);
when a plane wave propagates in a solid material, the particle vibration can be expressed as,
u I ＝U I exp[jK(N J X J -Vt)] (2)
in U I For the polarization amplitude of the direction, N J The direction cosine of the ultrasonic wave propagation direction, K is the wave number of the ultrasonic wave, and V is the propagation speed of the ultrasonic wave;
the characteristic equation of the propagation of ultrasonic waves in a stress material is obtained by combining the formulas (1) and (2)
For isotropic materials, when the longitudinal elastic wave propagates in the uniaxial stress direction, the longitudinal velocity V and the stress magnitude σ can be expressed as
Wherein λ and μ are Lame constants, and l and m are Murraghan constants;
if the incident angle of the ultrasonic wave is a first critical angle, which can be obtained according to Snell's law,
wherein V is A Is the ultrasonic wave propagation velocity in the air, V M Is the ultrasonic wave propagation speed in the measured material;
for receiving LCR waves, the receiving air-coupled transducer needs to be symmetrically arranged with the exciting air-coupled transducer, and the LCR wave speed V can be obtained according to the formula (4) M The relationship with the stress sigma is that,
wherein V is M0 The ultrasonic wave propagation speed of the material to be measured in the stress-free state is set;
for an ultrasonic propagation distance L in the measured material, the obtained acoustic time difference Deltat=t-t is deduced 0 The relationship with the stress sigma is that,
wherein t is 0 ＝L/V M0 For the time of flight TOF of the material under test in the stress-free state, t=L/V M The time of flight TOF under the stress state of the measured material is represented by a stress coefficient K, and the linear relation between the stress of the measured material and the acoustic time difference is obtained by experimental calibration in practice.
2. The measuring method according to claim 1, characterized in that the assembled measuring device is embodied by placing the hollow-coupled transducer (1) at a determined inclination on one side of a test specimen (3) containing a stress concentration zone;
the other end of the sample (3) to be tested is provided with a receiving empty coupling transducer (2), and the receiving empty coupling transducer (2) and the empty coupling transducer (1) are both arranged on one side of the sample (3) to be tested;
the saidThe distance between the receiving empty coupling transducer (2) and the empty coupling transducer (1) and the sample (3) to be tested is L A The method comprises the steps of determining the period of an excitation signal to be N, selecting the center frequency f of an empty coupled transducer, generating a sine pulse signal with the period of N, the Hanning window modulation and the frequency f by adopting an arbitrary signal function generator as the excitation signal, carrying out impedance matching through a 50 ohm load, applying the impedance matching to the excited empty coupled transducer (1) after a low-pass filter, and receiving echoes at receiving positions with the distance L by using a receiving empty coupled transducer (2).
3. The measuring method according to claim 1, characterized in that the movement position of the test sample (3) is recorded by a grating displacement sensor;
and generating a trigger signal at each movement position, and collecting a received echo signal at the current position by using a high-speed data collection board card.
4. The method of claim 3, wherein the measuring of the sound in the unstressed state is performed by periodically moving the unstressed test block for one period, recording the moving positions of the propagation distance L and the propagation distance L+DeltaL of the unstressed test block by the grating displacement sensor, generating trigger signals at each moving position, and collecting the received echo signals at the current position by the high-speed data collecting board card, wherein the received echo signals are recorded as t 1,0 And t 2,0 。
5. The method of measuring according to claim 4, wherein the average stress value of the corresponding L propagation distance at each motion position is specifically that the acoustic moveout is obtained by comparing F (x) with the stress-free state, and the average stress value of the corresponding L propagation distance at each motion position is obtained by combining the acoustic moveout with a linear relation coefficient of stress and acoustic moveout.
6. A measuring method according to claim 3, characterized in that the distance between the receiving empty-coupled transducer (2) and the empty-coupled transducer (1) between the injection and the reception of the sample (3) to be measured is specifically adjusted to be l+Δl, the position of the exciting empty-coupled transducer (1) is kept unchanged, and the receiving empty-coupled transducer (2) is moved so that the distance therebetween becomes l+Δl.
7. The method according to claim 6, wherein the average stress value of the propagation distance of L+ΔL at each of the movement positions is represented by F 1 (x) And compared with the stress-free state, obtaining the acoustic time difference, and obtaining the average stress value of the corresponding L+delta L propagation distance at each motion position by combining the acoustic time difference with the linear relation coefficient of the stress and the acoustic time difference.
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