Variable temperature stress detection method based on transverse wave birefringence sound-to-time ratio
By measuring the acoustic time ratio of transverse wave birefringence and introducing a temperature correction coefficient, the measurement error problem of the transverse wave method under temperature changes is solved, realizing high-precision stress detection, which is suitable for on-site and online detection of various materials and component types.
Patent Information
- Application Number
- CN202511328163.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-17
- Publication Date
- 2026-01-02
AI Technical Summary
The existing transverse wave birefringence method cannot effectively correct for temperature changes when measuring ambient temperature changes or when there is a temperature gradient in the measured object, resulting in significant measurement errors.
By measuring the transverse wave birefringence time-to-sound ratio and combining it with a temperature correction coefficient, a linear fitting method is used to establish the relationship between stress and the time-to-sound ratio, correct the transverse wave propagation time, and achieve temperature compensation for stress detection.
It improves the accuracy and consistency of stress measurement, reduces reliance on expensive temperature control equipment, facilitates on-site and online testing, is applicable to a variety of materials and component types, and has good versatility and engineering adaptability.
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Figure CN121253294A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of nondestructive testing, in particular to a variable-temperature stress detection method based on shear wave birefringence acoustic time ratio. BACKGROUND
[0002] Accurate measurement of residual stress and service stress in materials is of great significance in structural safety assessment, fatigue life prediction, manufacturing quality control and maintenance decision. Traditional mechanical methods usually require destructive sampling or complex mechanical calculations, which are difficult to realize online and nondestructive detection. Therefore, due to its high sensitivity, fast response, and online detection, ultrasonic-based nondestructive testing technology has become one of the main methods for measuring stress in engineering applications.
[0003] Shear wave birefringence is that the incident shear wave will split into two mutually perpendicular polarized shear wave components in the metal or stress state, and there is a difference in propagation speed or phase between the two. Existing works use the speed difference, phase difference or time difference of the two polarized components to infer the stress direction and amplitude. Such methods can provide direction-sensitive information and have been widely used in the field of electromagnetic ultrasonic stress detection.
[0004] However, the propagation speed of ultrasonic waves in solids is highly sensitive to temperature. Most existing acoustic time / wave speed methods directly rely on absolute wave speed or absolute propagation time, so when the measurement environment temperature changes or the measured piece has a temperature gradient, significant errors will occur. However, there is currently no temperature correction for shear wave birefringence method. SUMMARY
[0005] The purpose of the present application is to provide a variable-temperature stress detection method based on shear wave birefringence acoustic time ratio, which can solve the problems raised in the background art.
[0006] To solve the above technical problems, the technical scheme adopted by the present application is:
[0007] A variable-temperature stress detection method based on shear wave birefringence acoustic time ratio, comprising the following steps:
[0008] Step 1: At a calibration temperature (A ℃), collect the shear wave waveforms of the sample under different axial tensile stress conditions in the parallel polarization direction and the perpendicular polarization direction; take the first echo signal as the reference waveform, calculate the time difference between the second echo and the first echo, that is, the propagation time t || and the propagation time t ⊥ of the perpendicular polarization direction.
[0009] Step 2: According to the propagation times t || and t ⊥The acoustic time-to-sound ratio R is obtained; a linear fit is performed on the acoustic time-to-sound ratio R and the stress value σ to obtain the stress coefficient K and intercept b, thus obtaining the formula for calculating the stress σ of the sample at the calibration temperature (A℃):
[0010]
[0011] Step 3: Measure the transverse wave velocity inside the sample at temperatures from T to T0, and obtain the temperature-wave velocity coefficient ξ by linear fitting method;
[0012] Step 4: Measure the propagation speed of parallel polarized transverse waves inside the sample at the calibration temperature (A℃). Propagation speed of transverse waves in the vertical polarization direction The temperature correction coefficient α for the parallel polarization direction is obtained. || Temperature correction factor α in the vertical polarization direction ⊥ ;
[0013] Step 5: Correct the propagation time t of the parallel polarization direction transverse wave || The propagation time t of the transverse wave in the vertical polarization direction ⊥ Obtain the temperature difference ΔT between the current temperature and the calibrated temperature. Let v be the wave velocity of the transverse wave in the parallel polarization direction corresponding to the current temperature. ||前 The wave velocity of the transverse wave in the vertical polarization direction is v. ⊥前 Based on the temperature difference ΔT and the temperature correction factor α obtained in step 4. || α ⊥ The propagation time t′ of the transverse wave in the corrected parallel polarization direction is obtained. || and corrected propagation time t′ of transverse waves in the vertical polarization direction ⊥ ;
[0014] Step 6: Calculate the propagation time t′ of the corrected parallel polarization direction transverse wave obtained in Step 5. || Correction of transverse wave propagation time t′ in the vertical polarization direction ⊥ Substituting the formula for calculating the stress σ of the sample at the calibrated temperature (A℃) obtained in step 2, we obtain the formula for calculating the temperature-corrected stress of transverse wave birefringence based on the acoustic time ratio:
[0015]
[0016] Furthermore, in step 2, the formula for calculating the sound-time ratio R is:
[0017]
[0018] Further, in step 3, the specific method for measuring the transverse wave speed inside the sample at T to T0 is to perform gradient heating at P℃ (P>0) as a step size in the range of T to T0℃, and after each temperature is stable, the transverse wave signals in the parallel polarization and the vertical polarization directions are synchronously collected, the propagation time is extracted based on the cross-correlation algorithm, and the corresponding transverse wave speed v || and the transverse wave speed v ⊥ .
[0019] Further, the calculation formula of the temperature-wave speed coefficient ξ in step 3 is as follows:
[0020]
[0021] In the formula, v T is the corresponding propagation speed when the temperature is T℃; is the corresponding propagation speed when the temperature is T0; ξ is the temperature-wave speed coefficient; and ΔT is the temperature difference between the temperature T and the temperature T0.
[0022] Further, in step 4, the calculation formula of the temperature correction coefficient α || in the parallel polarization direction is as follows:
[0023]
[0024] The calculation formula of the temperature correction coefficient α ⊥ in the vertical polarization direction is as follows:
[0025]
[0026] Further, in step 5, the calculation formulas of the corrected transverse wave propagation time t′ || in the parallel polarization direction and the corrected transverse wave propagation time t′ ⊥ in the vertical polarization direction are as follows:
[0027]
[0028] In the formula, v ||前 and v ⊥前 are the corresponding transverse wave speeds in the parallel polarization direction and the vertical polarization direction at the current temperature, respectively.
[0029] Further, because the corrected transverse wave propagation time t′ || in the parallel polarization direction and the corrected transverse wave propagation time t′ ⊥ in the vertical polarization direction can be expressed as:
[0030]
[0031] The present application has at least the following advantages or beneficial effects:
[0032] 1. The present application provides a variable temperature stress detection method based on shear wave birefringence acoustic time ratio, which can obtain stress coefficient through constant temperature calibration, and determine temperature correction coefficient through temperature test. The calibration steps are clear and easy to promote and reuse in the laboratory or on site. The method is suitable for various conductive or non-conductive metal materials and common component forms, and can be used with different types of shear wave probes and array probes, and has good universality and engineering adaptability. The method can realize an error control of ± 3% in the high stress area at room temperature, and provides a high-precision solution for stress detection in complex environments.
[0033] 2. The detection method improves the measurement accuracy, uses the propagation time ratio (acoustic time ratio) of shear waves with different polarization directions as the stress characterization quantity, and establishes a linear relationship between the acoustic time ratio and the stress, which significantly reduces the measurement error caused by relying solely on absolute wave velocity, thereby improving the accuracy and consistency of stress measurement. The introduction of temperature correction coefficient or temperature compensation method effectively eliminates the influence of material wave velocity change with temperature on stress determination, so that reliable stress results can still be obtained within a wide temperature range. Field measurement no longer needs to be strictly constant temperature, which reduces the dependence on expensive temperature control equipment, and is convenient for field and online detection application. It can meet the needs of engineering detection for repeatability and reliability.
[0034] 3. The detection method uses acoustic time ratio as a relative quantity to improve sensitivity and anti-interference ability, which is conducive to suppressing the influence of system inherent bias and environmental noise on measurement results, thereby improving signal-to-noise ratio and measurement stability. BRIEF DESCRIPTION OF DRAWINGS
[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.
[0036] Figure 1 The flowchart of the detection method of the present application is shown in the figure;
[0037] Figure 2 The acoustic time ratio-load stress relationship provided by the present application is shown in the figure;
[0038] Figure 3 The relationship between the double polarization direction shear wave velocity and temperature provided by the present application is shown in the figure. DETAILED DESCRIPTION
[0039] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.
[0040] Referring to Figure 1 , the present application provides a variable temperature stress detection method based on shear wave birefringence acoustic time ratio, which can effectively overcome the influence of environmental temperature and meet the requirements of repeatability and reliability of engineering detection. The method specifically comprises the following steps:
[0041] Step 1: At a calibration temperature (A ℃), an axial tensile stress is applied to the sample for M times (M>1), the loading step is N MPa (N>0), after each load is stable, the shear wave waveforms of parallel polarization direction and vertical polarization direction are collected respectively using a piezoelectric ultrasonic shear wave probe or an electromagnetic acoustic transducer (EMAT) probe. Then, the time difference between the second echo and the first echo is calculated, that is, the propagation time t || of the parallel polarization direction and the propagation time t ⊥ of the vertical polarization direction, taking the first echo signal as the reference waveform. It should be noted that, since the yield strength of each sample is different, the maximum load of the tensile stress applied to the sample is determined by the yield strength of the sample, and the maximum load of the tensile stress does not exceed the yield strength. In order to ensure the accuracy of the subsequent calculation results, the number of times M of applying the tensile stress to the sample in the present application is preferably not less than 10 times.
[0042] Step 2: Referring to Figure 2 , the acoustic time ratio R is obtained according to the propagation time t || of the parallel polarization direction and the propagation time t ⊥ of the vertical polarization direction; the calculation formula of the acoustic time ratio R is:
[0043]
[0044] The stress coefficient K and the intercept b are obtained by linear fitting of the acoustic time ratio R and the stress value σ, and the calculation formula of the stress σ of the sample at the calibration temperature (A ℃) is obtained, and the specific calculation method is as follows:
[0045] When the propagation direction of the shear wave is perpendicular to the stress direction, the relationship between the shear wave velocity v ⊥ of the polarization direction perpendicular to the stress direction and the stress is:
[0046]
[0047] The propagation direction of the shear wave is perpendicular to the stress direction, and the shear wave velocity v|| The relationship with stress is:
[0048]
[0049] Where: ρ0 is the density of the sample under zero stress; v0 is the ultrasonic wave velocity under zero stress; v is the ultrasonic wave velocity under stress; σ is the stress inside the sample; μ, λ are the second-order elastic Lamé constants; m, n are the third-order elastic Murnaghan constants. ⊥ , v || are the wave velocities of the transverse wave in the perpendicular polarization direction and the parallel polarization direction respectively under constant temperature; σ is the stress inside the sample; μ, λ are the second-order elastic Lamé constants; m, n are the third-order elastic Murnaghan constants.
[0050] Simplify formula (1), (2):
[0051]
[0052] Where: A = 3λ + 2μ; A and K || are introduced for the purpose of simplifying the formula, and have no actual meaning.
[0053] From formula (1), (2), the ultrasonic wave velocity v0 under zero stress is:
[0054]
[0055] Introduce the dimensionless stress parameter Then formula (3), (4) can be simplified as:
[0056]
[0057] Comparing formula (6), (7), we get:
[0058]
[0059] Where: r is the wave velocity ratio,
[0060] Formula (8) can be obtained by rearrangement:
[0061]
[0062] Substitute to get the stress-velocity ratio relationship:
[0063]
[0064] The change in wave velocity due to stress can be ignored compared with the total wave velocity, i.e. r 2 ≈1, which can be linearized at r = 1 using Taylor expansion. Let δ = r - 1, (δ is a small quantity), then r 2 -1 ≈ 2δ. Similarly, for the denominator: K ⊥ -K|| r 2 ≈K ⊥ -K || (1+2δ)=(K ⊥ -K || )-2K || . Equation (10) can be approximately simplified as:
[0065]
[0066] Taylor expansion is performed on equation (11):
[0067]
[0068] Taylor expansion can be expanded to infinite terms, but since the subsequent values are small, they can be ignored, so the ellipsis is replaced in equation 12.
[0069] Since Equation (12) can be approximated as:
[0070]
[0071] In addition, due to the anisotropy of the sample, the wave speed in different polarization directions will be slightly different, so an anisotropy correction factor b is introduced. Replace A, δ, K ⊥ and K || in equation (13) to get the final relationship:
[0072]
[0073] Assume the stress coefficient Equation (14) can be finally simplified as:
[0074]
[0075] In actual engineering detection, the temperature of the sample is the same as the calibration temperature, so the propagation times t || and t ⊥ of parallel and vertical polarization directions are directly measured, and the measured data are substituted into equation (15) to obtain the stress value σ.
[0076] Step 3: Measure the shear wave speed inside the sample at temperatures T to T0. In the range of T to T0℃, perform gradient heating with P℃ (P>0) as the step size. After each temperature is stable, simultaneously collect the shear wave signals in parallel and vertical polarization directions, extract the acoustic time difference based on the cross-correlation algorithm, and calculate the corresponding parallel polarization direction shear wave speed v || and the vertical polarization direction shear wave speed v ⊥According to the temperature difference ΔT and the obtained multiple sets of shear wave velocity data, the relationship between the shear wave velocity of the two polarization directions and the temperature can be obtained by a linear fitting method, please refer to Figure 3 The temperature-wave velocity coefficient ξ can be obtained by the following formula:
[0077]
[0078] In the formula: v T is the propagation velocity corresponding to the temperature T, which is applicable to the parallel and vertical direction shear waves; is the propagation velocity corresponding to the temperature T0; ξ is the temperature-wave velocity coefficient; and ΔT is the temperature difference between T and T0.
[0079] It should be noted that the wave velocity of the vertical polarization direction and the parallel polarization direction has the same change trend with the temperature, that is, the slopes of the two fitting straight lines are the same, so the same temperature-wave velocity coefficient can be used.
[0080] In this step, the shear wave velocity v(v || , v ⊥ ) is measured by the propagation distance of the ultrasonic wave divided by the propagation time. The propagation time t of the ultrasonic wave is obtained by the experiment. Taking a sample with a thickness of D as an example, D*2 is the propagation distance of the ultrasonic wave. The speed measurement method and measurement principle of different polarization directions are consistent. The calculation formula is as follows:
[0081]
[0082] Preferably, when measuring the wave velocity of the shear wave, a high-precision temperature sensor can be integrated on the basis of the stress testing system. The temperature measurement probe is closely attached to the surface of the sample, and the sample is coated with thermal insulation material outside to isolate the thermal interference of the heating medium, so as to accurately measure the temperature of the sample.
[0083] Step 4: Measure the propagation velocity of the parallel polarization direction shear wave inside the sample at the calibration temperature (A℃) and the propagation velocity of the vertical polarization direction shear wave Obtain the parallel polarization direction temperature correction coefficient α || and the vertical polarization direction temperature correction coefficient α ⊥ .
[0084] The calculation formula of the parallel polarization direction temperature correction coefficient α || is as follows:
[0085]
[0086] The calculation formula of the vertical polarization direction temperature correction coefficient α ⊥ is as follows:
[0087]
[0088] Step 5: Correct the propagation time t of the parallel polarization direction transverse wave || The propagation time t of the transverse wave in the vertical polarization direction ⊥ The specific correction process is as follows: Obtain the temperature difference ΔT between the current temperature and the calibration temperature, and let v be the wave velocity of the transverse wave in the parallel polarization direction corresponding to the current temperature. ||前 The wave velocity of the transverse wave in the vertical polarization direction is v. ⊥前 Based on the temperature difference ΔT obtained in step 3 and the temperature correction coefficient α obtained in step 4. || α ⊥ The propagation time t′ of the transverse wave in the corrected parallel polarization direction is obtained. || and corrected propagation time t′ of transverse waves in the vertical polarization direction ⊥ Then we have:
[0089]
[0090] In the formula v ||前 v ⊥前 These represent the transverse wave velocities in the parallel polarization direction and the perpendicular polarization direction, respectively, at the current temperature; t || v ||前 Indicates sample thickness; v ||前 +ξΔT represents the wave velocity at the calibration temperature.
[0091] Step 6: Use the corrected parallel polarization direction transverse wave propagation time t′ obtained in Step 5 || Correction of transverse wave propagation time t′ in the vertical polarization direction ⊥ The parallel polarization direction temperature correction coefficient α obtained in step 4 || Temperature correction factor α in the vertical polarization direction ⊥ Simplify formulas (17) and (18):
[0092] because Simplifying equations (17) and (18), we get:
[0093]
[0094] Then, substituting equations (19) and (20) into the formula for calculating the stress σ of the sample at the calibrated temperature (A℃) obtained in step 2, we can obtain the formula for calculating the temperature-corrected stress of transverse wave birefringence based on the acoustic time ratio:
[0095]
[0096] Measuring the propagation time t of a transverse wave in a parallel polarization direction within the sample || Propagation time t in the direction of perpendicular polarization ⊥And substitute the sound time ratio based on step 6 to get the stress value corresponding to the stress value of the sample at different temperatures.
[0097] Example 1
[0098] In order to quantitatively evaluate the measurement accuracy of the method, under constant temperature (27.5±0.5℃) conditions, 5 step stresses are applied to the 6061 aluminum alloy test piece, and the ultrasonic stress measurement system is used to measure repeatedly 3 times at each stress stage, and the average stress value is calculated based on formula (15). The measurement results are shown in Table 1.
[0099] Table 1 Stress detection results at T=27.5℃
[0100]
[0101] From the above table, the stress measurement error is less than 10%, and it decreases monotonously with the increase of stress load, from 8.65% in the low stress area (15MPa) to 2.5% in the high stress area (90MPa), and tends to be stable. The transverse wave sound time ratio method proposed in the present application has high detection precision, which meets the requirements of engineering detection on repeatability and reliability. The method can realize the error control of ±3% in the high stress area under room temperature environment, and provides a high-precision solution for stress detection in complex environment.
[0102] Example 2
[0103] In order to verify the effectiveness of the temperature correction method, aluminum alloy tensile test samples were tested under constant temperature conditions of 23.5℃ and 33.0℃(0-90MPa, step 10MPa) respectively, and the double-polarized transverse wave sound time data were collected synchronously. Based on the correction formula, the stress value is calculated, and the experimental results are shown in Table 2 and Table 3. And compared with the test results without temperature correction.
[0104] Table 2 Stress detection results at T=23.5℃(ΔT=-4℃)
[0105]
[0106] Table 3 Stress detection results at T=33.0℃(ΔT=5.5℃)
[0107]
[0108]
[0109] It can be seen from Tables 2 and 3 that the measurement error without temperature correction is 12.40% to 23.97%, and the detection accuracy of stress after temperature correction is significantly improved, the measurement error range is 2.29% to 10.82%, the measurement accuracy is significantly improved compared with the uncorrected method, and the test error in the high stress area (50-90MPa) is reduced to <3.5%. The results verify the effectiveness of the temperature correction model under the variable temperature environment. The experimental results show that the method proposed in the application can effectively overcome the influence of environmental temperature and meet the high-precision engineering detection demand in the complex temperature environment.
[0110] The above merely illustrates the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims
1. A method for detecting variable-temperature stress based on the transverse wave birefringence acoustic time ratio, characterized in that, Includes the following steps: Step 1: At the calibration temperature (A℃), collect transverse wave waveforms of the sample under different axial tensile stresses in the parallel polarization direction and the perpendicular polarization direction; calculate the propagation time t in the parallel polarization direction. || Propagation time t in the vertical polarization direction ⊥ ; Step 2: Based on the propagation time t obtained in Step 1 || and t ⊥ The acoustic time-to-sound ratio R is obtained; a linear fit is performed on the acoustic time-to-sound ratio R and the stress value σ to obtain the stress coefficient K and intercept b, and the formula for calculating the stress σ of the sample at the calibration temperature (A℃) is obtained: Step 3: Measure the transverse wave velocity inside the sample at temperatures from T to T0℃, and obtain the temperature-wave velocity coefficient ξ using a linear fitting method; Step 4: Measure the propagation speed of parallel polarized transverse waves inside the sample at the calibration temperature (A℃). Propagation speed of transverse waves in the vertical polarization direction The temperature correction coefficient α for the parallel polarization direction is obtained. || Temperature correction factor α in the vertical polarization direction ⊥ ; Step 5: Correct the propagation time t of the parallel polarization direction transverse wave || The propagation time t of the transverse wave in the vertical polarization direction ⊥ Obtain the temperature difference ΔT between the current temperature and the calibrated temperature. Let v be the wave velocity of the transverse wave in the parallel polarization direction corresponding to the current temperature. ||前 The wave velocity of the transverse wave in the vertical polarization direction is v. ⊥前 Based on the temperature difference ΔT and the temperature correction factor α obtained in step 4. || α ⊥ The propagation time t′ of the transverse wave in the corrected parallel polarization direction is obtained. || and corrected propagation time t′ of transverse waves in the vertical polarization direction ⊥ ; Step 6: Calculate the propagation time t′ of the corrected parallel polarization direction transverse wave obtained in Step 5. || Correction of transverse wave propagation time t′ in the vertical polarization direction ⊥ Substituting the formula for calculating the stress σ of the sample at the calibrated temperature (A℃) obtained in step 2, we obtain the formula for calculating the temperature-corrected stress of transverse wave birefringence based on the acoustic time ratio:
2. The method for detecting variable-temperature stress based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, In step 2, the formula for calculating the sound-time ratio R is:
3. The variable-temperature stress detection method based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, In step 3, the specific method for measuring the transverse wave velocity inside the sample at temperatures from T to T0 is as follows: within the range of T to T0℃, a gradient temperature increase is performed with a step size of P℃ (P > 0). After each temperature level stabilizes, transverse wave signals in both parallel and perpendicular polarization directions are simultaneously acquired. The propagation time is extracted based on a cross-correlation algorithm, and the corresponding transverse wave velocity v in the parallel polarization direction is calculated by combining it with the acoustic path length. || transverse wave velocity v in the vertical polarization direction ⊥ .
4. The variable-temperature stress detection method based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, The formula for calculating the temperature-wave velocity coefficient ξ in step 3 is: In the formula v T This represents the propagation speed at a temperature of T℃. Let ξ be the propagation speed at temperature T0; ξ be the temperature-wave velocity coefficient; and ΔT be the temperature difference between temperature T and temperature T0.
5. The method for detecting variable-temperature stress based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, In step 4, the temperature correction coefficient α for the parallel polarization direction || The calculation formula is: Temperature correction factor α in vertical polarization direction ⊥ The calculation formula is:
6. The variable-temperature stress detection method based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, In step 5, the propagation time t′ of the transverse wave in the parallel polarization direction is corrected. || and corrected propagation time t′ of transverse waves in the vertical polarization direction ⊥ The calculation formulas are as follows: In the formula v ||前 v ⊥前 These represent the transverse wave velocities in the parallel polarization direction and the perpendicular polarization direction, respectively, at the current temperature.
7. The variable-temperature stress detection method based on transverse wave birefringence acoustic time ratio according to claim 6, characterized in that, because Then correct the propagation time t′ of the transverse wave in the parallel polarization direction. || and corrected propagation time t′ of transverse waves in the vertical polarization direction ⊥ It can be represented as:
8. The method for detecting variable-temperature stress based on transverse wave birefringence acoustic time ratio according to claim 1, characterized in that, In step 1, the propagation time t in the parallel polarization direction || Propagation time t in the vertical polarization direction ⊥ It is obtained by calculating the time difference between the second echo and the first echo, using the first echo signal as the reference waveform.