Ultrasonic longitudinal wave sound velocity detection method of material

Abstract

The invention discloses an ultrasonic longitudinal wave sound velocity detection method for a material, and the method comprises the steps: obtaining an original signal of the material through employing a liquid immersion ultrasonic pulse echo method; carrying out downsampling processing and wavelet transformation processing on the original signal, and positioning the time domain position of the primary surface reflection wave; determining reference transit time between the primary bottom wave and the secondary bottom wave according to the material thickness and the reference sound velocity; determining a first time domain position of a primary bottom wave and a second time domain position of a secondary bottom wave according to the reference transit time and the time domain position of the primary surface reflection wave; determining actual transit time between the primary bottom wave and the secondary bottom wave based on a digital correlation method, the first time domain position and the second time domain position; and determining the ultrasonic longitudinal wave velocity of the ultrasonic longitudinal wave in the material according to the actual transit time and the material thickness. The ultrasonic longitudinal wave sound velocity of large-size ultralow-expansion quartz glass or lithium-aluminum-silicon microcrystalline glass can be quickly, automatically and nondestructively detected.

Description

Technical Field

[0001] The present application relates to the technical field of nondestructive testing, and in particular to a method for detecting ultrasonic longitudinal wave velocity of a material. Background Art

[0002] Ultra-low Expansion Quartz Glass (ULE) and Li2O-Al2O3-SiO2 (LAS) glass-ceramics have become the preferred substrate materials for many large-aperture precision optical system reflectors at home and abroad because of their advantages of having a coefficient of thermal expansion (CTE) close to zero. However, in the preparation process of ULE, the uneven distribution of the TiO2 component inside it will cause the CTE of the prepared ULE to be unevenly distributed in micro-areas; similarly, in the preparation process of LAS glass-ceramics, the different numbers, types, and sizes of its crystals will cause the CTE of the prepared LAS glass-ceramics to be unevenly distributed in micro-areas. When this unevenness exceeds a certain range, the substrate formed by the ULE / LAS glass-ceramics will produce residual stress, resulting in reduced dimensional stability, which will ultimately affect the processing quality and imaging stability of the optical mirror. Therefore, it is necessary to quickly and accurately detect the CTE and uniformity of ULE / LAS glass-ceramics at different positions to select ULE / LAS glass-ceramics with excellent thermal expansion performance as the matrix material of the precision optical system reflector.

[0003] At present, the CTE detection methods of ULE / LAS series microcrystalline glass all require destructive sampling, and the measurement process is complicated and time-consuming. According to the relationship between ultrasonic longitudinal wave velocity, Young's modulus and CTE, the ultrasonic longitudinal wave velocity can characterize the thermal expansion coefficient of ULE / LAS series microcrystalline glass. Therefore, the ultrasonic detection method can be used to realize non-destructive detection of CTE of large-size ULE / LAS series microcrystalline glass. In order to realize fast and accurate detection of CTE of ULE / LAS series microcrystalline glass, it is necessary to efficiently obtain the ultrasonic longitudinal wave velocity of the ULE / LAS series microcrystalline glass.

[0004] At present, manual signal acquisition and calculation are often used to obtain the ultrasonic longitudinal wave velocity of ULE / LAS glass-ceramics. However, this process is time-consuming and is not conducive to full-aperture rapid scanning detection of the longitudinal wave velocity of large-size ULE / LAS glass-ceramics.

[0005] Based on this, there is an urgent need for a method that can automatically and quickly obtain the longitudinal wave velocity of the material, so as to realize rapid and automatic non-destructive testing of the ultrasonic longitudinal wave velocity of large-sized ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics. Summary of the invention

[0006] The purpose of the present application is to provide a method for detecting the ultrasonic longitudinal wave velocity of a material, so as to solve the problem that the ultrasonic longitudinal wave velocity of large-sized ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics cannot be rapidly and automatically non-destructively detected.

[0007] To achieve the above objectives, this application provides the following solutions:

[0008] The present application provides a method for detecting ultrasonic longitudinal wave velocity of a material, comprising:

[0009] The original signal of the material is obtained by using a liquid immersion ultrasonic pulse echo method; wherein the original signal is a time domain signal of a primary bottom wave and a time domain signal of a secondary bottom wave; the primary bottom wave represents the wave reflected back from the bottom surface of the material by the ultrasonic longitudinal wave for the first time; the secondary bottom wave represents the wave reflected back from the bottom surface of the material for the second time; the material comprises ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics;

[0010] The original signal is subjected to downsampling processing and wavelet transformation processing to locate the time domain position of a surface reflection wave; the time domain position represents the peak value and valley value of the reflection wave; the surface reflection wave represents the first reflection of the ultrasonic longitudinal wave from the surface of the material;

[0011] Determine a reference transit time between the primary bottom wave and the secondary bottom wave according to the material thickness and the reference sound velocity; wherein the transit time represents the interval time between the primary bottom wave and the secondary bottom wave;

[0012] Determining a first time domain position of the primary bottom wave and a second time domain position of the secondary bottom wave according to the reference transit time and the time domain position of the primary surface reflection wave;

[0013] Determining an actual transit time between the primary bottom wave and the secondary bottom wave based on a digital correlation method, the first time domain position, and the second time domain position;

[0014] The ultrasonic longitudinal wave velocity of the ultrasonic longitudinal wave in the material is determined according to the actual transit time and the material thickness.

[0015] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0016] The present application provides a method for detecting the ultrasonic longitudinal wave velocity of a material, which uses a liquid immersion ultrasonic pulse echo method to obtain the original signal of ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics, performs downsampling processing and wavelet transformation processing on the original signal, and realizes automatic and rapid positioning of the time domain position of the primary surface reflection wave. Among them, because the interval time between the primary bottom wave and the secondary bottom wave is the time for the ultrasonic longitudinal wave to travel from the material surface to the material bottom surface and then to the material surface, the reference transit time between the primary bottom wave and the secondary bottom wave can be determined by the material thickness and the reference sound velocity. Furthermore, based on the reference transit time and the time domain position of the primary surface reflection wave, the first time domain position of the primary bottom wave and the second time domain position of the secondary bottom wave can be determined, and then the actual transit time is calculated according to the digital correlation method. Finally, combined with the known material thickness, the ultrasonic longitudinal wave velocity of the material is automatically and quickly obtained, and the ultrasonic longitudinal wave velocity is usually used to characterize the thermal expansion coefficient of ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics. The present application can quickly and automatically non-destructively detect the ultrasonic longitudinal wave velocity of large-size ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics, and greatly improve the efficiency of non-destructive detection of the thermal expansion coefficient of ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 A schematic diagram of a method for detecting the ultrasonic longitudinal wave velocity of a material provided in an embodiment of the present application;

[0019] Figure 2 Schematic diagram of the structure of the ultrasonic longitudinal wave speed measurement system using the liquid immersion ultrasonic pulse echo method provided in the embodiments of the present application. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0021] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0022] like Figure 1As shown, the present application provides a method for detecting ultrasonic longitudinal wave velocity of a material, comprising:

[0023] Step 101: using a liquid immersion ultrasonic pulse echo method to obtain an original signal of a material; wherein the original signal is a time domain signal of a primary bottom wave and a time domain signal of a secondary bottom wave; the primary bottom wave represents a wave reflected from the bottom surface of the material by an ultrasonic longitudinal wave for the first time; the secondary bottom wave represents a wave reflected from the bottom surface of the material for the second time; the material comprises ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics.

[0024] Reference Figure 2 , using the liquid immersion ultrasonic pulse echo method, the ultrasonic pulse transceiver 11 of the detection data control acquisition hardware part 1 controls the ultrasonic probe 2 to vertically transmit ultrasonic longitudinal waves to the sample 4 placed in the groove 3, wherein there is a coupling agent 5 in the groove 3, and the ultrasonic probe 2 is partially located in the coupling agent 5, and the sample 4 is completely immersed in the coupling agent, and the ultrasonic probe 2 is used to receive the ultrasonic echo time domain signal of the sample 4, and the ultrasonic echo time domain signal is analog-to-digital converted to obtain a digitized echo signal, and the amplitude threshold of the initial pulse is given. The acquisition situation is that the initial pulse is located at the leftmost ultrasonic echo time domain signal for processing and calculation, that is, the initial pulse is the first pulse in the signal sequence corresponding to the original signal, that is, the starting point.

[0025] Furthermore, the initial pulse is used as the starting point for intercepting the effective time domain signal, the length of the intercepted effective time domain signal is given, and finally the effective time domain signal containing the primary bottom wave and the secondary bottom wave in the ultrasonic echo time domain signal is automatically intercepted as the original signal, and signal processing is further realized based on the original signal.

[0026] Among them, sample 4 is the material in this application.

[0027] In practical applications, based on the ultrasonic echo time domain signal of the material, the process of automatically intercepting the effective time domain signal containing the primary bottom wave and the secondary bottom wave as the original signal is further introduced in detail:

[0028] (1) Prepare materials.

[0029] ULE / LAS series microcrystalline glass is generally used as a material. The shape of the material can be cylindrical. Based on this, preferably, the material preparation method may include: the thickness of multiple materials can be the same or different, the thickness can be 50mm, and the diameter of multiple materials can be the same or different, provided that it is larger than the wafer diameter of the ultrasonic probe 2, and the diameter can be 25mm. Among them, in order to avoid scattering and attenuation of ultrasonic longitudinal waves on the surface of the material, it is necessary to ensure that the upper and lower end faces of the material are parallel to each other. In this embodiment, the cylindrical sample 4 obtained by cutting is finely ground and polished to meet the flatness and parallelism requirements of 20μm.

[0030] (2) Obtain the ultrasonic echo time domain signal of sample 4.

[0031] Acquiring the ultrasonic echo time domain signal of the sample 4 may include: acquiring the ultrasonic echo time domain signal of the sample 4 using a liquid immersion ultrasonic pulse echo method. Figure 2 As shown, the device includes a three-axis automatic scanning detection platform 6, a detection data control and acquisition hardware part 1 and a detection data control and acquisition, real-time processing and display software part 7, wherein the three-axis motion control card 61 is used to control the x and y axes of the three-axis scanning mechanism 62, and the x and y axes are used to move the ultrasonic probe 2 to the central position vertically above the sample 4 in the groove 3. The groove 3 can be a glass groove, which contains a sufficient amount of coupling agent 5. The sample 4 is completely immersed in the coupling agent 5 of the groove 3, and the ultrasonic probe 2 is partially immersed in the coupling agent 5 of the groove 3. The z-axis is further moved to keep a certain distance between the ultrasonic probe 2 and the upper surface of the sample 4 to ensure that an obvious ultrasonic echo time domain signal can be received. At the same time, the ultrasonic probe 2 is adjusted to ensure that the axis of the ultrasonic probe 2 is perpendicular to the surface of the sample 4. At this time, the position of the ultrasonic probe 2 is fixed to achieve full liquid immersion coupling between the ultrasonic probe 2 and the sample 4.

[0032] Among them, water is preferably used as a coupling agent. Therefore, the ultrasonic probe can also be called an ultrasonic immersion probe, and the liquid immersion ultrasonic pulse echo method can also be called a full water immersion ultrasonic pulse echo method. The advantage of using water as a coupling agent is that it has good conductivity and coupling performance, and can effectively transmit ultrasonic signals and receive echo signals.

[0033] The ultrasonic pulse transceiver 11 of the detection data control and acquisition hardware part 1 is connected to the ultrasonic probe 2 to detect the sample 4. The sampling rate of the high-speed data acquisition card 12 is at least 1.25GHz. The high-speed data acquisition card 12 is connected to the ultrasonic pulse transceiver 11. The working process of the device is as follows: according to the size and material of the detected sample 4, the parameters of the ultrasonic pulse transceiver 11 are set based on the ultrasonic pulse instrument parameter control 13, and then the ultrasonic longitudinal wave is vertically transmitted to the sample 4 through the ultrasonic probe 2. The ultrasonic probe 2 receives the ultrasonic echo time domain signal reflected from the sample 4, and sends the ultrasonic echo time domain signal to the high-speed data acquisition card 12 through the ultrasonic pulse transceiver 11. With the help of the high-speed data acquisition card 12, the ultrasonic echo time domain signal received by the ultrasonic pulse transceiver 11 is quickly and accurately collected, thereby obtaining the ultrasonic echo time domain signal. The movement of the ultrasonic probe 2, the collection and real-time processing of the ultrasonic echo time domain signal and the display are all realized by the PC of the detection data control collection, real-time processing and display software part 7.

[0034] In some embodiments, a liquid immersion ultrasonic pulse echo method is used to obtain the original signal of the material, specifically including: using the liquid immersion ultrasonic pulse echo method to obtain the ultrasonic echo time domain signal of the material; according to the amplitude threshold of the initial pulse, the preset length and the time domain characteristics of the ultrasonic echo time domain signal, intercepting the ultrasonic echo time domain signal to obtain the original signal of the material; the initial pulse represents the first pulse in the signal sequence corresponding to the original signal.

[0035] Specifically, the initial pulse is used as the starting point for intercepting the effective time domain signal, the length of intercepting the effective time domain signal is given, and finally the effective time domain signal containing the primary bottom wave and the secondary bottom wave in the ultrasonic echo time domain signal is automatically intercepted as the original signal, and signal processing is further realized based on the original signal.

[0036] The preset length is the length of the intercepted effective time domain signal, and the interception process is: according to the distance from the end face of the ultrasonic probe 2 to the upper surface of the sample 4 (i.e., the water distance), the effective time domain signal from the beginning of the initial pulse to the end of the secondary bottom wave is intercepted. Specifically, since the interval time between the initial pulse and the primary surface reflection wave is the time taken by the ultrasonic longitudinal wave to go back and forth twice the water distance, the length of the intercepted effective time domain signal is 1.5 times the length of the signal corresponding to the water distance, but it is not limited to 1.5 times, and can be changed according to actual conditions.

[0037] Among them, the data points in the length of the intercepted effective time domain signal eliminate invalid data points, thereby effectively reducing the time consumption of automatic calculation.

[0038] Step 102: downsample and wavelet transform the original signal to locate the time domain position of a surface reflection wave; the time domain position represents the peak and valley of the reflection wave; the surface reflection wave represents the first ultrasonic longitudinal wave reflected from the material surface.

[0039] In some embodiments, the original signal is subjected to downsampling processing and wavelet transformation processing to locate the time domain position of the primary surface reflection wave, specifically including: performing analog-to-digital conversion on the original signal to obtain a digitized echo signal; performing the downsampling processing on the digitized echo signal to obtain target original data; performing the wavelet transformation on the target original data to obtain a frequency domain signal; and determining the time domain position of the primary surface reflection wave based on the correspondence between the frequency domain signal and the time domain signal.

[0040] Specifically, the wavelet transform can be a Morlet wavelet transform. The original signal is collected by a high-speed data acquisition card 12, and a digital echo signal is obtained after analog-to-digital conversion. Therefore, under the premise of satisfying the Nyquist theorem, the original signal is sampled at a reduced acquisition rate, which neither causes distortion of the original signal nor reduces the time consumption of Morlet wavelet transform of the original signal; since the corresponding frequency domain signal is obtained after Morlet wavelet transform, the energy of the first surface reflection wave of the sample is the largest, and the first surface reflection wave is the wave reflected and returned by the ultrasonic wave on the surface of the sample for the first time, the frequency domain position of the first surface reflection wave can be determined by finding the position point with the largest energy, and the time domain position of the first surface reflection wave of the original signal can be quickly and accurately automatically located through the corresponding relationship between the frequency domain signal and the time domain signal.

[0041] Step 103: Determine a reference transit time between the primary bottom wave and the secondary bottom wave according to the material thickness and the reference sound velocity; wherein the transit time represents the interval time between the primary bottom wave and the secondary bottom wave.

[0042] Specifically, according to the measurement principle of the immersion ultrasonic pulse echo method, the interval time between the primary surface reflection wave and the primary bottom wave is equal to the interval time between the primary bottom wave and the secondary bottom wave, that is, the time of flight (TOF).

[0043] Further, according to Δt1=2d / c1, wherein d is the material thickness, c1 is the material reference sound velocity, and Δt1 is the reference TOF calculated using the material reference sound velocity.

[0044] The reference sound velocity of ULE is 5740m / s, and the reference sound velocity of LAS glass-ceramics is 6480m / s. Since the primary bottom wave refers to the wave reflected from the bottom surface of the sample for the first time, and the secondary bottom wave refers to the wave reflected from the bottom surface of the sample for the second time, the time domain position of the primary surface reflection wave of the original signal has been automatically located. If the reference TOF of the sample is known, the preliminary location of the time domain positions of the primary and secondary bottom waves can be quickly achieved.

[0045] It should be noted that the reference TOF is not the actual TOF of sample 4, but is obtained based on the reference sound velocity of the material, so as to achieve preliminary automatic positioning of the primary bottom wave and the secondary bottom wave. Since the thermal expansion coefficients of the samples are different, the sound velocities of the samples are also different, so the actual TOF needs to be obtained through steps 104 and 105.

[0046] Step 104: Determine a first time domain position of the primary bottom wave and a second time domain position of the secondary bottom wave according to the reference transit time and the time domain position of the primary surface reflection wave.

[0047] In some embodiments, determining the first time domain position of the primary bottom wave and the second time domain position of the secondary bottom wave based on the reference transit time and the time domain position of the primary surface reflection wave specifically includes: determining the first time domain position based on the reference transit time and the time domain position of the primary surface reflection wave; determining the second time domain position based on the first time domain position and the reference transit time.

[0048] Step 105: Determine the actual transit time between the primary bottom wave and the secondary bottom wave based on the digital correlation method, the first time domain position and the second time domain position.

[0049] In some embodiments, based on the digital correlation method, the first time domain position and the second time domain position, the actual transit time between the primary bottom wave and the secondary bottom wave is determined, specifically including: according to the first time domain position, extracting the first zero-crossing position, the first peak and the first valley of the primary bottom wave, respectively recorded as the first zero-crossing position, the first peak and the first valley; determining the first waveform data according to the first zero-crossing position, the first peak and the first valley; according to the second time domain position, extracting the second zero-crossing position, the second peak and the second valley of the secondary bottom wave, respectively recorded as the second zero-crossing position, the second peak and the second valley; determining the second waveform data according to the second zero-crossing position, the second peak and the second valley; performing digital correlation calculation on the first waveform data and the second waveform data to generate the actual transit time between the primary bottom wave and the secondary bottom wave.

[0050] Among them, the interval time between the primary surface reflection wave and the primary bottom wave is the same as the interval time between the primary bottom wave and the secondary bottom wave, both of which are TOF. Since the primary bottom wave and the secondary bottom wave have extremely high similarity, the digital correlation method is used to perform correlation calculation on the waveform data of the primary bottom wave and the waveform data of the secondary bottom wave to obtain TOF. The digital correlation method usually refers to the use of correlation technology in digital signal processing to analyze the time or space relationship between signals. The digital correlation method is an existing mature technology and will not be described in detail here.

[0051] Specifically, the time domain signals of the primary bottom wave and the secondary bottom wave are automatically extracted. According to the characteristics of the primary bottom wave and the secondary bottom wave time domain signals having the same phase of the waveform and the alternating positive and negative half-cycles of the waveform, the peak and valley values ​​(peak and valley values ​​are time domain positions) of the primary bottom wave and the secondary bottom wave time domain signals are automatically and accurately located. The peak value is the value with the maximum amplitude of the positive half-cycle waveform, and the valley value is the value with the minimum amplitude of the negative half-cycle waveform. The first zero-crossing position (i.e., the first zero-crossing position and the second zero-crossing position) of the primary bottom wave and the secondary bottom wave time domain signals are further searched to achieve the accurate extraction of the first waveform data B1 of the primary bottom wave and the second waveform data B2 of the secondary bottom wave. B1 and B2 are calculated by digital correlation method to obtain TOF. Among them, peak detection can help determine the main position of the reflected wave, and the zero-crossing position can provide the boundary of the reflected wave.

[0052] It should be noted that the TOF obtained at this time is the actual TOF of sample 4.

[0053] Step 106: Determine the ultrasonic longitudinal wave velocity of the ultrasonic longitudinal wave in the material according to the actual transit time and the material thickness.

[0054] In some embodiments, the ultrasonic longitudinal wave velocity is:

[0055] c L =2d / Δt;

[0056] Among them, c L is the ultrasonic longitudinal wave speed; d is the material thickness; Δt is the actual transit time.

[0057] This application is based on analyzing the propagation law of ultrasound in materials and the characteristics of echo signals. By adopting the liquid immersion ultrasonic pulse echo method to obtain the ultrasonic echo time domain signal of the material, the ultrasonic echo signal is processed to realize rapid and automatic calculation of the ultrasonic longitudinal wave velocity of the ULE / LAS series microcrystalline glass to be tested. When measuring the thermal expansion coefficient of the ULE / LAS series microcrystalline glass to be tested, after the material thickness and the reference sound velocity are known, the ultrasonic longitudinal wave velocity of the ULE / LAS series microcrystalline glass to be tested can be automatically determined, thereby effectively improving the efficiency of rapid and automatic non-destructive testing of CTE of large-size ULE / LAS series microcrystalline glass.

[0058] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0059] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A method for detecting ultrasonic longitudinal wave velocity of a material, characterized in that: include: The original signal of the material is obtained by using a liquid immersion ultrasonic pulse echo method; wherein the original signal is a time domain signal of a primary bottom wave and a time domain signal of a secondary bottom wave; the primary bottom wave represents the wave reflected back from the bottom surface of the material by the ultrasonic longitudinal wave for the first time; the secondary bottom wave represents the wave reflected back from the bottom surface of the material for the second time; the material comprises ultra-low expansion quartz glass or lithium aluminum silicon glass-ceramics; The original signal is subjected to downsampling processing and wavelet transformation processing to locate the time domain position of a surface reflection wave; the time domain position represents the peak value and valley value of the reflection wave; the surface reflection wave represents the first reflection of the ultrasonic longitudinal wave from the surface of the material; Determine a reference transit time between the primary bottom wave and the secondary bottom wave according to the material thickness and the reference sound velocity; wherein the transit time represents the interval time between the primary bottom wave and the secondary bottom wave; Determining a first time domain position of the primary bottom wave and a second time domain position of the secondary bottom wave according to the reference transit time and the time domain position of the primary surface reflection wave; Determining an actual transit time between the primary bottom wave and the secondary bottom wave based on a digital correlation method, the first time domain position, and the second time domain position; The ultrasonic longitudinal wave velocity of the ultrasonic longitudinal wave in the material is determined according to the actual transit time and the material thickness.

2. The method for detecting the ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: The liquid immersion ultrasonic pulse echo method is used to obtain the original signal of the material, including: Using a liquid immersion ultrasonic pulse echo method, obtaining an ultrasonic echo time domain signal of the material; According to the amplitude threshold of the initial pulse, the preset length and the time domain characteristics of the ultrasonic echo time domain signal, the ultrasonic echo time domain signal is intercepted to obtain the original signal of the material; the initial pulse represents the first pulse in the signal sequence corresponding to the original signal.

3. The method for detecting the ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: The original signal is subjected to downsampling processing and wavelet transformation processing to locate the time domain position of the primary surface reflection wave, specifically including: Performing analog-to-digital conversion on the original signal to obtain a digitized echo signal; Performing the downsampling process on the digitized echo signal to obtain target original data; Performing the wavelet transform on the target original data to obtain a frequency domain signal; The time domain position of the primary surface reflection wave is determined according to the corresponding relationship between the frequency domain signal and the time domain signal.

4. The method for detecting the ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: According to the material thickness and the reference sound velocity, the reference transit time between the primary bottom wave and the secondary bottom wave is determined, including: Determining the propagation time of the ultrasonic longitudinal wave in the material according to the material thickness and the reference sound velocity; The propagation time is taken as the transit time of reference between the primary bottom wave and the secondary bottom wave.

5. The method for detecting the ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: Determining a first time domain position of a primary bottom wave and a second time domain position of a secondary bottom wave according to the reference transit time and the time domain position of the primary surface reflection wave specifically comprises: Determining the first time domain position according to the reference transit time and the time domain position of the primary surface reflection wave; The second time domain position is determined according to the first time domain position and the reference flight time.

6. The method for detecting the ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: Determining an actual transit time between a primary bottom wave and a secondary bottom wave based on a digital correlation method, the first time domain position, and the second time domain position, specifically includes: According to the first time domain position, extract the first zero crossing position, the first peak value and the first valley value of the primary bottom wave, which are recorded as the first zero crossing position, the first peak value and the first valley value respectively; Determine first waveform data according to the first zero-crossing point position, the first peak value, and the first valley value; According to the second time domain position, extract the first zero crossing position, the first peak value and the first valley value of the secondary bottom wave, which are recorded as the second zero crossing position, the second peak value and the second valley value respectively; Determine second waveform data according to the second zero-crossing point position, the second peak value and the second valley value; The first waveform data and the second waveform data are calculated by digital correlation method to generate the actual transit time between the primary bottom wave and the secondary bottom wave.

7. The method for detecting ultrasonic longitudinal wave velocity of a material according to claim 1, characterized in that: The ultrasonic longitudinal wave velocity is: c L =2d / Δt; Among them, c L is the ultrasonic longitudinal wave speed; d is the material thickness; Δt is the actual transit time.

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