Stress Detection System and Method Based on Lamb Wave Zero-Group-Velocity Mode

Abstract

The present invention discloses a stress detection system and method based on the zero group velocity mode of Lamb waves, which relates to the technical field of equipment detection; the stress detection system includes a phased array control unit and a phased array probe connected to the phased array control unit; the phased array probe includes a transmitting probe and a receiving probe, the transmitting probe includes a plurality of array elements for transmitting ultrasonic signals, and the receiving probe includes a plurality of array elements for receiving ultrasonic signals; the phased array control unit controls the plurality of array elements of the transmitting probe to make the ultrasonic signals transmitted by the transmitting probe incident on the interior of the specimen to be tested at a preset angle; the receiving probe is used to receive the ultrasonic signals fed back by the specimen to be tested and transmit the received ultrasonic signals to the phased array control unit. The stress detection system and method of the present invention utilize the phased array to control the deflection of ultrasonic waves, can realize the excitation and reception of the zero group velocity mode of Lamb waves, improve the stress detection efficiency and reliability, and reduce the detection cost.

Description

Technical Field

[0001] The present invention relates to the technical field of equipment detection, and particularly to a stress detection system and method based on the zero group velocity mode of Lamb waves. Background Art

[0002] Aluminum alloy plates are widely used in industries such as aerospace, automotive, and shipbuilding. In typical service environments, due to temperature changes, electrochemical corrosion, and external loads, large residual stresses may be generated in the aluminum plates, which can induce fatigue cracks in the materials and even lead to sudden failures. Therefore, it is of great significance to evaluate or monitor the residual stresses in aluminum plates.

[0003] The ultrasonic method has become one of the most promising non-destructive testing methods for stress measurement due to its unique advantages, such as safety, high penetrability, portability, and convenience for on-site detection. Existing research shows that it is difficult to effectively excite the zero group velocity mode of Lamb waves, and strict requirements are imposed on the ultrasonic incident angle. Currently, the main solution is to design a wedge with an inclination angle equal to the critical angle according to the Lamb wave group velocity of the material to be measured, so as to achieve the excitation and reception of the zero group velocity mode of Lamb waves. This method requires designing wedges with different inclination angles for different materials. For the case of anisotropic sound velocity or sound velocity varying with time, multiple wedges not only have poor adaptability, increase the detection cost, but also easily change the coupling state during the process of replacing wedges, bringing many inconveniences to quantitative detection and characterization, and cannot meet the urgent needs of high-end equipment manufacturing and service. Summary of the Invention

[0004] The present invention provides a stress detection system and method based on the zero group velocity mode of Lamb waves, aiming to solve the problem of low detection efficiency caused by the need to design wedges with different inclination angles when detecting material stresses by the traditional ultrasonic method.

[0005] The stress detection system based on the zero group velocity mode of Lamb waves provided by the present invention includes:

[0006] A phased array control unit;

[0007] A phased array probe, connected to the phased array control unit; the phased array probe includes a transmitting probe and a receiving probe, the transmitting probe includes a plurality of array elements for transmitting ultrasonic signals, and the receiving probe includes a plurality of array elements for receiving ultrasonic signals;

[0008] The phased array control unit is used to control the plurality of array elements of the transmitting probe so that the ultrasonic signals transmitted by the transmitting probe are incident on the interior of the test piece to be tested at a preset angle; the receiving probe is used to receive the ultrasonic signals fed back by the test piece to be tested and transmit the received ultrasonic signals to the phased array control unit.

[0009] Optionally, the phased array control unit includes:

[0010] A phased array board, connected to the transmitting probe and the receiving probe, for controlling the transmitting probe to generate ultrasonic signals, and receiving and processing the ultrasonic signals received by the receiving probe;

[0011] A computer device, connected to the phased array board, for controlling the phased array board and receiving the ultrasonic signals processed by the phased array board; the computer device includes a memory and a processor, the memory stores a phased array ultrasonic detection program, and the processor executes the phased array ultrasonic detection program to obtain stress detection data of the specimen to be tested.

[0012] The stress detection method based on the Lamb wave zero group velocity mode provided by the present invention uses the above-mentioned stress detection system based on the Lamb wave zero group velocity mode, and the method includes the following steps:

[0013] S1. Select the specimen to be tested and the area to be tested on the specimen to be tested, and calculate the theoretical frequency and phase velocity of exciting the Lamb wave zero group velocity mode;

[0014] S2. Determine the incident angle of the Lamb wave zero group velocity mode according to the phase velocity ;

[0015] S3. Select a wedge block, and determine the phased array ultrasonic incident deflection angle according to the inclination angle of the wedge block and the incident angle , where ; and determine the delay rule of the phased array ultrasonic waves according to the phased array ultrasonic incident deflection angle ; S4. According to the theoretical frequency and the delay rule, perform ultrasonic stress detection on the specimen to be tested through the stress detection system based on the Lamb wave zero group velocity mode;

[0016] S5. Obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested and the center frequency f2 of the Lamb wave zero group velocity mode of the specimen with zero stress, and calculate the stress value of the specimen to be tested according to the following formula

[0017] : :

[0018]

[0019] In the formula, , = -0, and k is the stress coefficient of the specimen to be tested.

[0020] Optionally, the step S1 specifically includes:

[0021] S11. Obtain the plate thickness of the specimen to be tested;

[0022] S12. Obtain the dispersion curve of the specimen to be tested according to the plate thickness of the specimen to be tested, where the dispersion curve includes a group velocity - frequency - thickness product curve and a phase velocity - frequency - thickness product curve;

[0023] S13. Determine the frequency - thickness product corresponding to zero group velocity according to the group velocity - frequency - thickness product curve, and determine the theoretical frequency of the Lamb wave zero - group - velocity mode; determine the phase velocity of the Lamb wave zero - group - velocity mode according to the phase velocity - frequency - thickness product curve;

[0024] Among them, according to the plate thickness of the specimen to be tested, the dispersion curve of the specimen to be tested can be solved and obtained through the following equation:

[0025] The symmetric mode (S) of elastic waves in a thin plate with free boundaries satisfies the following formula:

[0026] ,

[0027] The antisymmetric mode (A) satisfies the following formula:

[0028] ,

[0029] In the formula, h is half of the plate thickness of the specimen to be tested, k is the wave number, p and q are the propagation constants respectively, and satisfy the following formula:

[0030] ,

[0031] ,

[0032] In the formula, is the angular frequency, and are the longitudinal wave velocity and the transverse wave velocity in the plate respectively.

[0033] Optionally, the calculation formula for the incident angle of the Lamb wave zero - group - velocity mode is:

[0034]

[0035] In the formula, is the sound velocity in the wedge, is the phase velocity of the Lamb wave zero - group - velocity mode.

[0036] Optionally, in the step S3, the calculation formula of the delay rule is:

[0037]

[0038]

[0039] In the formula, i is the serial number of any element within the aperture of the transmitting probe, I is the serial number of the starting element within the aperture of the transmitting probe, J is the serial number of the ending element within the aperture of the transmitting probe, where i, I, J satisfy I ≤ i ≤ J ≤ n (i, I, J, and n are all positive integers, n is the number of elements within the aperture of the transmitting probe, and n ≥ 2). is the delay time of the i-th element. is the element pitch.

[0040] Optionally, step S5 includes:

[0041] S51, recording the A-scan signal of the specimen to be tested or the zero-stress specimen;

[0042] S52, performing a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the specimen to be tested to obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested;

[0043] Or, performing a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the zero-stress specimen to obtain the center frequency f2 of the Lamb wave zero group velocity mode of the zero-stress specimen.

[0044] Optionally, in step S5, the stress coefficient k of the specimen to be tested is determined through the following steps:

[0045] Applying stress to the specimen to be tested and obtaining the center frequency f1 of the Lamb wave zero group velocity mode corresponding to the specimen to be tested under different stresses;

[0046] Using the least squares method to fit the stress data and the data of the corresponding center frequency f1 of the Lamb wave zero group velocity mode;

[0047] Performing thickness compensation on the slope of the fitted straight line to obtain the stress coefficient k of the specimen to be tested.

[0048] Optionally, the steps for performing thickness compensation on the slope of the fitted straight line specifically include:

[0049] Solving for the radial contraction amount of the specimen to be tested under axial tension ;

[0050] Based on the frequency-thickness product corresponding to zero group velocity, determining the actual value of the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested, and determining the actual slope of the fitted straight line, where the actual slope is the stress coefficient k of the specimen to be tested;

[0051] Among them, solving for the radial contraction amount of the specimen to be tested under axial tension is calculated through the following formula:

[0052]

[0053] In the formula, is the tensile stress, is the plate thickness, is the Poisson's ratio, and E is the Young's modulus.

[0054] Optionally, the method further includes the following steps:

[0055] Performing stress detection on multiple regions of the test piece to obtain the stress values of multiple detection points of the test piece;

[0056] According to the stress values of multiple detection points of the test piece, obtaining the stress distribution diagram of the test piece.

[0057] It can be seen from the above technical solutions that the present invention has the following beneficial effects:

[0058] Utilizing the phased array principle, that is, by arranging the probe array elements according to a certain rule to transmit / receive ultrasonic signals and controlling the deflection of ultrasonic waves with an appropriate delay rule, the ultrasonic waves are incident on the interior of the test piece at the critical angle, and then the excitation and reception of the Lamb wave zero group velocity mode are realized, which is applicable to the measurement of in-plane stress of plate-like structures; in the case of stress detection of different materials and changes in sound velocity, compared with the prior art, the present invention does not need to replace wedges with different inclination angles, avoiding problems such as cumbersome steps and easy change of the coupling state caused by replacing wedges, significantly improving the stress detection efficiency and reliability of the test piece, and reducing the detection cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0060] Figure 1 It is a schematic structural diagram of an embodiment of the stress detection system based on the Lamb wave zero group velocity mode of the present invention;

[0061] Figure 2 is Figure 1 the schematic structural diagram of the wedge block assembly in

[0062] Figure 3 It is a flow block diagram of the stress detection method based on the Lamb wave zero group velocity mode of the present invention;

[0063] Figure 4 It is one of the dispersion curve diagrams (group velocity - frequency-thickness product) of an embodiment of the stress detection method based on the Lamb wave zero group velocity mode of the present invention;

[0064] Figure 5 is Figure 4 The second dispersion curve diagram (phase velocity - frequency-thickness product) of the corresponding embodiment;

[0065] Figure 6 is Figure 4 The element number - delay time diagram of the corresponding embodiment when the aperture element number of the transmitting probe is 32 and the group velocity setting value is 9080 m / s;

[0066] Figure 7 is Figure 4 The A-scan signal diagram corresponding to the receiving probe elements in the corresponding embodiment;

[0067] Figure 8 is Figure 4 The stress coefficient fitting diagram of the corresponding embodiment;

[0068] Figure 9 is Figure 7 The frequency domain diagram of the S1-ZGV mode corresponding to the A-scan signal in;

[0069] Figure 10 is Figure 4 The specimen plan view of the corresponding embodiment;

[0070] Figure 11 is Figure 4 The plane scanning schematic diagram of the corresponding embodiment;

[0071] Figure 12 is Figure 4 The stress distribution diagram of the corresponding embodiment.

[0072] Explanation of the attached figure labels:

[0073] 1. Phased array control unit; 2. Transmitting probe; 3. Receiving probe; 4. Wedge block assembly; 101. Computer device; 102. Phased array board; 401. First wedge block; 402. Second wedge block; 403. Sound absorption layer. Specific implementation manners

[0074] To make the invention objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0075] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working process of the system described above can refer to the corresponding process in the method embodiment and will not be repeated here.

[0076] The terms "first", "second", "third", "fourth", etc. (if any) in the specification and drawings of the present application are used to distinguish similar objects and do not necessarily describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily limit to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0077] It should be understood that in the present application, "at least one (item)" means one or more, and "a plurality" means two or more. "And / or" is used to describe the association relationship of associated objects and indicates that there can be three relationships. For example, "A and / or B" can mean: only A exists, only B exists, and both A and B exist at the same time. Among them, A and B can be singular or plural. The character " / " generally means that the associated objects before and after are in an "or" relationship. "At least one (one)" or its similar expression below refers to any combination of these items, including any combination of single item (one) or plural items (ones). For example, at least one (one) of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0078] In several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is only a logical function division, and there can be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed coupling or direct coupling or communication connection to each other can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be in electrical, mechanical or other forms.

[0079] In addition, in each embodiment of the present application, each functional unit can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of a software functional unit.

[0080] Lamb waves refer to guided waves formed by ultrasonic waves after multiple reflections and superpositions on the upper and lower surfaces of a thin plate, resulting in complex mutual interference and geometric diffusion. It is an acoustic guided wave with a thickness of the same order of magnitude as the wavelength of the exciting acoustic wave. Under certain specific frequency-thickness product (frequency-thickness product) conditions, the group velocity of Lamb waves will approach zero (i.e., the zero group velocity mode, zero group velocity, abbreviated as ZGV). In this case, the propagation speed of the wave packet is very slow, so it is easier to observe the scattering phenomenon of waves caused by defects, and it can be used for the residual stress detection of plate-like structural materials.

[0081] Lamb waves have multimodal and dispersive propagation characteristics. When performing stress detection on plate-like structural materials, it is usually necessary to design a wedge block with a critical angle according to the material to be measured, so as to achieve the excitation and reception of the zero group velocity mode of Lamb waves. This method requires designing wedge blocks with different inclination angles for different materials. For the case of acoustic velocity anisotropy or acoustic velocity varying with the specimen, replacing the wedge block not only has poor adaptability and increases the detection cost, but also easily changes the coupling state during the replacement process, bringing many inconveniences to quantitative detection and characterization.

[0082] The present invention proposes a stress detection system and method based on the zero group velocity mode of Lamb waves, aiming to solve the above problems.

[0083] Refer to Figure 1 , in an embodiment of the present invention, the stress detection system based on the zero group velocity mode of Lamb waves includes:

[0084] A phased array control unit 1;

[0085] A phased array probe, connected to the phased array control unit 1; the phased array probe includes a transmitting probe 2 and a receiving probe 3. The transmitting probe 2 includes a plurality of array elements for transmitting ultrasonic signals, and the receiving probe 3 includes a plurality of array elements for receiving ultrasonic signals;

[0086] The phased array control unit 1 is used to control a plurality of array elements of the transmitting probe 2 so that the ultrasonic signals transmitted by the transmitting probe 2 are incident on the interior of the specimen to be tested at a preset angle; the receiving probe 3 is used to receive the ultrasonic signals fed back by the specimen to be tested and transmit the received ultrasonic signals to the phased array control unit 1.

[0087] In this embodiment, both the transmitting probe 2 and the receiving probe 3 are phased array probes; a phased array probe is an array composed of multiple independent small piezoelectric wafers (that is, the multiple elements for transmitting ultrasonic signals included in the transmitting probe 2 are small piezoelectric wafers, and the multiple elements for receiving ultrasonic signals included in the receiving probe 3 are small piezoelectric wafers). These piezoelectric wafers can independently excite or receive ultrasonic waves; the transmitting probe elements are arranged in a linear array within the aperture of the transmitting probe, and the receiving probe elements are arranged in a linear array within the aperture of the receiving probe; after setting the theoretical frequency and delay law of the incident ultrasonic wave, the phased array control unit controls the excitation sequence and time of each transmitting probe element, so that the ultrasonic waves emitted by each transmitting probe element interfere and superpose in space, forming a predetermined sound beam shape and direction, and the deflection and focusing of the incident ultrasonic sound beam can be realized.

[0088] In this embodiment, the phased array control unit 1 is used to generate precise timing control signals to independently control the elements of any transmitting probe 2, ensuring that the ultrasonic sound beam can be emitted in accordance with a predetermined direction and focusing state; the phased array control unit 1 is also used to process the ultrasonic detection signals received by the receiving probe 3 and further calculate and analyze to obtain the stress detection data of the specimen to be tested.

[0089] In this embodiment, when performing stress detection on the specimen to be tested, it is usually also necessary to use a wedge block to introduce the ultrasonic waves emitted by the transmitting probe into the specimen to be tested; the purpose of using the wedge block is to change the angle at which the ultrasonic waves enter the specimen to be tested, and to avoid probe wear or other forms of damage, and can reduce the acoustic wave scattering effect caused by the unevenness or roughness of the surface of the specimen to be tested, thereby improving the accuracy and reliability of the detection results; correspondingly, another wedge block with the same inclination angle is used to introduce the ultrasonic detection signal after passing through the area to be detected into the receiving probe; in this embodiment, only a wedge block with a single angle needs to be used for operation. When replacing specimens to be tested with different materials or when the acoustic velocity situation inside the material changes (the acoustic velocity is related to factors such as the elastic modulus and density of the material), there is no need to replace wedge blocks with different angles.

[0090] In this embodiment, the specimen to be tested is usually a plate-like structural material, including metal plates such as aluminum, iron, copper, and various composite materials; the specimen to be tested can be a material to be used after production or a material in a service environment.

[0091] In this embodiment, the stress detection of the specimen to be tested is realized by using the Lamb wave S1-ZGV mode (S1-zero group velocity mode); according to its vibration mode and propagation characteristics, Lamb waves can be divided into symmetric modes (S) and antisymmetric modes (A), and the S1 mode is one of the basic modes in the symmetric modes; the S1 mode has a relatively low phase velocity, which enables it to cover a longer distance when propagating in thin plates and can provide a higher resolution, which is beneficial to detecting subtle defects.

[0092] In an embodiment of the present invention, the phased array control unit 1, the transmitting probe 2, and the receiving probe 3 together constitute a transmitting-receiving detection system for phased array ultrasonic signals. When performing stress detection on a test piece, the phased array control unit 1 transmits a control signal to each element of the transmitting probe 2. The ultrasonic signals emitted by the elements of the transmitting probe 2 are interfered and superimposed to form an incident sound beam. By a certain delay rule, the incident sound beam can be emitted in a predetermined direction and focusing state. The predetermined incident sound beam direction is set to excite the zero group velocity mode of Lamb wave in the test piece, and the ultrasonic signals feedback from the test piece are received by the receiving probe 3, thereby realizing the stress detection of the test piece.

[0093] Compared with the prior art, in the embodiment of the present invention, the delay rule can be directly changed through the phased array control unit, thereby changing the incident angle of the ultrasonic wave entering the test piece. When performing stress detection on different materials or in the case of sound velocity change, a single-angle wedge can be used without replacing the wedge, avoiding problems such as cumbersome steps and easy change of the coupling state caused by replacing the wedge, significantly improving the stress detection efficiency and reliability of the test piece, and reducing the detection cost.

[0094] Based on the above embodiment, further, refer to Figure 1 , in some embodiments, the phased array control unit 1 includes:

[0095] A phased array board 102, connected to the transmitting probe 2 and the receiving probe 3, for controlling the transmitting probe 2 to generate ultrasonic signals, and receiving and processing the ultrasonic signals received by the receiving probe 3;

[0096] A computer device 101, connected to the phased array board 102, for controlling the phased array board 102 and receiving the ultrasonic signals processed by the phased array board 102; the computer device 101 includes a memory and a processor. The memory stores a phased array ultrasonic detection program, and the processor executes the phased array ultrasonic detection program to obtain stress detection data of the test piece.

[0097] In this embodiment, the phased array board 102 includes functions such as signal generation, reception, processing, and data acquisition, and can control the elements in the transmitting probe 2 and the receiving probe 3 to realize the dynamic control of the sound beam; the phased array board 102 generates high-frequency pulse signals for exciting the elements of the transmitting probe at the transmitting end, and provides precise timing control for each independent transmitting probe element to ensure that the ultrasonic sound beam can be emitted in a predetermined direction and focusing state; at the receiving end, the ultrasonic signals feedback from the test piece are received by the receiving probe, and the received signals are amplified, filtered, digitized, etc., and then the processed data is sent to the computer device for further calculation and analysis.

[0098] In this embodiment, the phased array board 102 can select the technical architecture of a passive electronic scanning array or an active electronic scanning array according to actual requirements, and can also select different interface types to adapt to different application requirements such as fixed, portable, remote connection, and distributed. The interface types include, but are not limited to, one or several of the interfaces such as PCI / PCIe, USB, Ethernet / GigE, and PXI / PXIe.

[0099] In one embodiment, the phased array board 102 can select a 256-channel phased array board.

[0100] The 256-channel phased array board has the ability to simultaneously control and process 256 independent signal channels, and is suitable for the detection of complex structures, such as pipeline detection in the aerospace, automotive, shipbuilding, and oil and gas industries, and the detection of key components in nuclear facilities; due to the large number of channels, it is usually equipped with high-speed data acquisition and processing capabilities, can simultaneously process multiple detection tasks, and can improve the detection speed and efficiency.

[0101] In this embodiment, the computer device 101 at least includes a memory and a processor. The memory stores a phased array ultrasonic detection program, and the processor executes the phased array ultrasonic detection program to obtain stress detection data of the specimen to be tested; the phased array ultrasonic detection program is used to simulate and analyze the dispersion characteristics of the zero group velocity mode of Lamb waves, and accordingly formulate a phased array delay rule, and is also used to calculate and analyze the ultrasonic signal received and processed by the phased array board to obtain the stress detection data of the specimen to be tested. The computer device 101 may also include a display module for visualizing and human-computer interaction of simulation analysis, calculation analysis, and signal acquisition.

[0102] It should be noted that when using the stress detection system based on the zero group velocity mode of Lamb waves provided in the above embodiment to detect the stress of the specimen to be tested, a wedge block assembly is usually required; based on the actual application scenario of the above embodiment, the present invention also designs a wedge block assembly.

[0103] Refer to Figure 2 , in some embodiments, the wedge block group 4 includes: a first wedge block 401, a second wedge block 402, and an acoustic absorption layer 403. The acoustic absorption layer 403 is disposed between the first wedge block 401 and the second wedge block 402 and is used to absorb ultrasonic waves.

[0104] When using the stress detection system based on the zero group velocity mode of Lamb waves to detect the stress of the specimen to be tested, the transmitting probe 2 abuts against the first wedge block 401, and the receiving probe 3 abuts against the second wedge block 402.

[0105] In these embodiments, the first wedge 401 is used to guide the ultrasonic beam emitted by the transmitting probe into the test piece, and the second wedge 402 is used to guide the ultrasonic signal passing through the detection area to be received by the receiving probe.

[0106] The first wedge 401 and the second wedge 402 can be designed as a separated type or an integral type; an acoustic absorption layer 403 is provided between the first wedge 401 and the second wedge 402 to ensure that there is no interference between the transmitted and received ultrasonic signals.

[0107] The material of the wedge can be selected from plexiglass, epoxy resin, polyurethane, hard rubber, etc. according to requirements such as sound velocity matching, compatibility with the test piece material, durability, etc.; the material of the acoustic absorption layer 403 can be selected from rubber (such as soft rubber, sponge rubber), foam (such as polyurethane foam, polyethylene foam), fiber (such as glass fiber, rock wool), or composite materials, etc.

[0108] In these embodiments, it is necessary to determine the heights of the centers of the transmitting probe and the receiving probe elements according to the sound attenuation coefficient of the wedge material, so that the acoustic beam reaching the surface of the test piece to be tested has sufficient signal strength, and at the same time ensure that the focus of the acoustic beam is located at the optimal position to achieve the best detection effect.

[0109] Further, when using a stress detection system based on the zero group velocity mode of Lamb waves to detect the stress of the test piece to be tested, a coupling agent can be applied between the first wedge 401, the second wedge 402 and the surface of the test piece to be tested.

[0110] In ultrasonic testing, applying a coupling agent between the wedge and the surface of the material to be tested can be used to eliminate the air gap, ensure that the ultrasonic wave can effectively be transmitted from the wedge to the material to be tested, and can also reduce the acoustic impedance difference between the wedge and the material to be tested, thereby improving the transmission efficiency of the sound wave. A good coupling agent can also keep the contact surface stable and avoid signal instability caused by poor contact.

[0111] According to the physical properties of the wedge and the material to be tested, the detection conditions, and the detection requirements, the coupling agent can be selected from any one of water-based, oil-based, and solid coupling agents.

[0112] The present invention also proposes a stress detection method based on the zero group velocity mode of Lamb waves, and this method uses the stress detection system based on the zero group velocity mode of Lamb waves provided in the above embodiments.

[0113] Refer to Figure 3 , in an embodiment of the present invention, the stress detection method based on the zero group velocity mode of Lamb waves includes the following steps:

[0114] S1, select the test piece to be tested and the area to be tested on the test piece to be tested, and calculate the theoretical frequency and phase velocity of exciting the zero group velocity mode of Lamb waves;

[0115] S2, determining the incident angle of the Lamb wave zero group velocity mode according to the phase velocity ;

[0116] S3, select the wedge, according to the inclination angle of the wedge and the incident angle Determine the incident deflection angle of phased array ultrasound ,in ; and according to the phased array ultrasonic incident deflection angle Determine the delay law for phased array ultrasound;

[0117] S4, performing ultrasonic stress detection on the test piece by using the stress detection system based on the Lamb wave zero group velocity mode according to the theoretical frequency and the delay law;

[0118] S5, obtain the Lamb wave zero group velocity modal center frequency f1 of the test piece and the Lamb wave zero group velocity modal center frequency f2 of the zero stress test piece, and calculate the stress value of the test piece according to the following formula: :

[0119] , (1)

[0120] In the formula, , = -0, k is the stress coefficient of the test piece.

[0121] In this embodiment, the test piece is usually a plate-like structural material, so the Lamb wave zero group velocity mode is used to perform ultrasonic stress detection on it; in the mathematical model for analyzing the Lamb wave zero group velocity mode, the dispersion characteristics of elastic waves in the free boundary thin plate can be solved by the Rayleigh-Lamb equation.

[0122] Therefore, step S1 specifically includes:

[0123] S11, obtaining the plate thickness of the test piece;

[0124] S12, obtaining a dispersion curve of the test piece according to the plate thickness of the test piece, wherein the dispersion curve includes a group velocity-frequency-thickness product curve and a phase velocity-frequency-thickness product curve;

[0125] S13, determining the frequency-thickness product corresponding to the group velocity being zero according to the group velocity-frequency-thickness product curve, and determining the theoretical frequency of the corresponding Lamb wave zero group velocity mode; determining the phase velocity of the Lamb wave zero group velocity mode according to the phase velocity-frequency-thickness product curve;

[0126] Among them, according to the plate thickness of the test piece, the dispersion curve of the test piece can be solved and obtained by the following Rayleigh-Lamb equation:

[0127] The symmetric mode (S) of elastic waves in a thin plate with a free boundary satisfies the following equation:

[0128] , (2)

[0129] The antisymmetric mode (A) satisfies the following equation:

[0130] , (3)

[0131] where h is half of the thickness of the test specimen plate, k is the wave number, p and q are the propagation constants respectively, and satisfy the following equations:

[0132] , (4)

[0133] , (5)

[0134] where is the angular frequency, and are the longitudinal wave velocity and the transverse wave velocity in the plate respectively.

[0135] After determining the thickness of the test specimen plate, by solving the above equations (2)-(5), the dispersion curve of the test specimen (group velocity-frequency thickness product curve and phase velocity-frequency thickness product curve) can be obtained. According to the group velocity-frequency thickness product curve, the frequency thickness product corresponding to the zero group velocity is determined, and thus the theoretical frequency in the zero group velocity mode of the corresponding Lamb wave is obtained; and according to the phase velocity-frequency thickness product curve, the phase velocity corresponding to this frequency thickness product is determined, which is the phase velocity in the zero group velocity mode of the corresponding Lamb wave .

[0136] In step S2, the calculation formula for the incident angle of the zero group velocity mode of Lamb wave is:

[0137] , (6)

[0138] where is the sound velocity in the wedge, is the phase velocity of the zero group velocity mode of Lamb wave.

[0139] In step S3, after selecting the material and inclination angle of the wedge, according to the sound attenuation coefficient of the wedge, the number of aperture array elements n of the transmitting probe is initially selected, and the longitudinal wave sound velocity value in the test specimen is given. The sound velocity in the wedge must satisfy < .

[0140] According to the inclination angle of the wedge and the incident angle Determine the incident deflection angle of the phased array ultrasonic wave , where .

[0141] Further obtain the calculation formula of the delay law:

[0142] , (7)

[0143] , (8)

[0144] In the formula, i is the serial number of any array element in the aperture of the transmitting probe, I is the serial number of the starting array element in the aperture of the transmitting probe, J is the serial number of the terminating array element in the aperture of the transmitting probe, where i, I, J satisfy I ≤ i ≤ J ≤ n (i, I, J, n are all positive integers, n is the number of array elements in the aperture of the transmitting probe, n ≥ 2), is the delay time of the i-th array element, is the array element pitch.

[0145] After calculating the delay law based on step S3, excite and receive the critically refracted longitudinal wave through the stress detection system based on the Lamb wave zero group velocity mode provided by the above embodiments, and record the scanning signal in the phased array control unit.

[0146] Further, step S5 includes:

[0147] S51, record the A-scan signal of the specimen to be tested or the zero-stress specimen;

[0148] S52, perform a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the specimen to be tested to obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested;

[0149] Or, perform a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the zero-stress specimen to obtain the center frequency f2 of the Lamb wave zero group velocity mode of the zero-stress specimen.

[0150] In an optional implementation manner, the sampling frequency of the scanning signal is not less than 50 MHz; and perform linear interpolation processing on the scanning signal received by the receiving probe to ensure that the sampling frequency after processing is not less than 500 MHz.

[0151] After obtaining the center frequency f2 of the Lamb wave zero group velocity mode of the zero-stress specimen and the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested respectively, calculate the stress value of the specimen to be tested .

[0152] Among them, the stress coefficient k of the specimen to be tested is determined by the following steps:

[0153] Apply stress to the test piece and obtain the center frequency f1 of the Lamb wave zero group velocity mode corresponding to the test piece under different stresses;

[0154] Use the least squares method to fit the stress data and the data of the center frequency f1 of the Lamb wave zero group velocity mode corresponding thereto;

[0155] Perform thickness compensation on the slope of the fitted straight line to obtain the stress coefficient k of the test piece.

[0156] Among them, the steps of performing thickness compensation on the slope of the fitted straight line specifically include:

[0157] Solve the radial contraction amount of the test piece under axial tension ;

[0158] According to the frequency-thickness product corresponding to zero group velocity, determine the actual value of the center frequency f1 of the Lamb wave zero group velocity mode of the test piece, and determine the actual slope of the fitted straight line, where the actual slope is the stress coefficient k of the test piece;

[0159] Among them, solving the radial contraction amount of the test piece under axial tension is calculated by the following formula:

[0160] , (9)

[0161] In the formula, is the tensile stress, is the plate thickness, is the Poisson's ratio, and E is the Young's modulus.

[0162] Based on the embodiments of the above stress detection method, in a further embodiment, the stress detection method further includes the following steps:

[0163] Perform stress detection on multiple regions of the test piece to obtain the stress values at multiple detection points of the test piece;

[0164] According to the stress values at multiple detection points of the test piece, obtain the stress distribution map of the test piece.

[0165] To more specifically illustrate the implementation process and beneficial effects of the embodiments of the stress detection method based on the Lamb wave zero group velocity mode proposed by the present invention, the present invention also proposes the following specific embodiments. It should be noted that the following embodiments are not limitations on this method, and those skilled in the art can adjust according to the detection object and detection scenario in actual applications.

[0166] Embodiment 1

[0167] Take a 6061 aluminum plate with a plate thickness of 3 mm as the test piece.

[0168] In this embodiment, the 6061 aluminum plate belongs to the aluminum-magnesium-silicon alloy system, has good processing performance and is easy to weld. It is commonly used in manufacturing structural parts, aircraft parts, ship components, bicycle frames, etc., and is a very common aluminum alloy plate.

[0169] Step S1: Select the test piece to be tested and the area to be tested on the test piece, and calculate the theoretical frequency and phase velocity of the exciting Lamb wave zero group velocity mode:

[0170] By solving equations (2)-(5), the dispersion curve of the 6061 aluminum plate with a thickness of 3 mm can be obtained. Refer to Figure 4 (group velocity-frequency thickness product curve) and Figure 5 (phase velocity-frequency thickness product curve).

[0171] From Figure 4 it can be seen that the forward wave of the S1 mode and the backward wave of the S2 mode interact to generate the S1-ZGV mode with a zero group velocity. Due to the interaction between the forward wave and the backward wave, the acoustic wave energy is restricted near the excitation point of the Lamb wave zero group velocity mode. The frequency thickness product corresponding to the S1-ZGV mode is , so its theoretical frequency is 0.959 MHz.

[0172] From Figure 5 it can be seen that when the frequency thickness product of the S1-ZGV mode is , its phase velocity is 9030 m / s.

[0173] Step S2: Determine the incident angle of the Lamb wave zero group velocity mode according to the phase velocity :

[0174] The longitudinal wave sound velocity of 6061 aluminum alloy is about 6300 m / s. It can be calculated from equation (6) that the incident angle corresponding to the sound velocity value of 6370 m / s is 15°. Therefore, it can be determined that the incident angle corresponding to 6061 aluminum alloy is about 15°.

[0175] According to < requirements, plexiglass with a longitudinal wave sound velocity of 2334 m / s is selected as the material for the wedge block.

[0176] Step S3: Select the wedge block, and determine the phased array ultrasonic incident deflection angle according to the inclination angle of the wedge block and the incident angle , where ; and determine the delay rule of the phased array ultrasonic waves according to the phased array ultrasonic incident deflection angle :

[0177] Select 10° as the inclination angle of the wedge block , the selected wedge is integrated and has an acoustic absorption layer in the middle to ensure that the ultrasonic signals transmitted and received do not interfere with each other. Place the whole wedge on the surface of the 6061 aluminum alloy specimen, and use machine oil to ensure stable coupling between the wedge and the specimen; calculate the incident deflection angle θ of the phased array ultrasonic wave: = 15° - 10° = 5°.

[0178] Furthermore, calculate the delay law of the phased array ultrasonic transmitting probe according to Equation (7) and Equation (8), that is, determine the delay time for each element of the transmitting probe to emit ultrasonic signals; for example, the delay time t6 of the 6th element is calculated to be 186.7 ns.

[0179] Set the number of aperture elements n of the transmitting probe to 32, and select 6300 m / s as the longitudinal wave sound velocity in the specimen , the delay times of each transmitting probe element are referred to Figure 6 , Figure 6 are the delay times corresponding to different element numbers when the number of aperture elements of the phased array ultrasonic transmitting probe is 32 and the set value of the phase velocity is 9080 m / s.

[0180] The delay of the receiving probe is opposite to the delay time of the transmitting probe elements. Select the number of aperture elements to be 32, and adjust the instrument gain to 20 dB. At this time, the amplitude of the received signal is 90%, and the signal-to-noise ratio is 45 dB, meeting the requirements that the amplitude of the received signal is not less than 50% of the full screen and the signal-to-noise ratio is not less than 12 dB.

[0181] Step S4, according to the obtained theoretical frequency and delay law, use the stress detection system based on the Lamb wave zero group velocity mode provided by the foregoing embodiments to perform ultrasonic stress detection on the specimen to be tested.

[0182] Step S5, obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested, and the center frequency f2 of the Lamb wave zero group velocity mode of the zero-stress specimen, and calculate the stress value of the specimen to be tested according to Equation (1) .

[0183] In step S5, the receiving probe receives the ultrasonic detection signal and uploads it to the phased array ultrasonic detection software after being processed by the phased array board card. It is recorded as an A-scan signal in the phased array ultrasonic detection software. The sampling frequency of this signal is selected as 100 MHz, referred to Figure 7 , Figure 7 is the A-scan signal corresponding to the receiving element; perform interpolation processing on the A-scan signal corresponding to the receiving probe element, and the time interval between adjacent two points of the signal is 1 ns.

[0184] Furthermore, the stress coefficient of the specimen to be tested is obtained through a tensile stress experiment. The tensile stress can be applied to the specimen by a universal testing machine. The S1-ZGV mode is excited by the above stress detection system and the corresponding signals are collected. In the tensile force range of 0 to 13 kN, the ultrasonic signal is collected once for every 1 kN increase. The experimental results in the tensile force range of 5 to 13 kN are selected, and then the frequency of the resonance wave is analyzed.

[0185] When the applied stress changes from 48 MPa to 153 MPa, the frequency change of the S1-ZGV mode of the specimen to be tested is as Figure 8 shown. Through linear regression analysis, the stress coefficient between the frequency shift of the S1-ZGV mode and the applied stress can be calculated to be -3.4494 Hz / MPa, and the coefficient of determination is 0.9948.

[0186] In this embodiment, the Poisson's ratio of the specimen to be tested is 0.331, and the Young's modulus E is 72 GPa. The radial contraction of the specimen can be calculated through Equation (9) ; From the above steps, it can be known that the frequency-thickness product of the specimen to be tested in the S1-ZGV mode is 2.877 MHz×mm. According to the thickness change value (i.e., the radial contraction ), the corresponding frequency change can be solved, and then referring to Figure 8 the test data, the frequency shift caused by the thickness change under each stress is compensated; after thickness compensation, the actual stress coefficient of the specimen to be tested can be calculated to be -214.1201 Hz / MPa.

[0187] Furthermore, according to the actual stress coefficient of the specimen to be tested obtained in the above steps, the stress value of the specimen to be tested is calculated . Refer to Figure 9 , Figure 9 which is the frequency domain diagram of the S1-ZGV mode corresponding to the receiving array element. The selected A-scan signal is subjected to a fast Fourier transform to obtain the frequency domain, and the center frequency of the signal is read 1 = 0.9653 MHz; the center frequency of the zero-stress specimen is 2 = 0.95 MHz. Therefore 0.0153 MHz. According to the calculation through Equation (1), it can be known that 71.455 MPa, that is, the stress value of the 6061 aluminum alloy plate of the specimen to be tested is 71.455 MPa.

[0188] Furthermore, refer to Figure 10 , Figure 10 which is the two-dimensional plan view of the specimen to be tested. The phased array probe is moved to scan the surface of the specimen to be tested. Ten different detection points are selected, and the above stress detection steps are repeated at the ten different detection points. The arrangement and scanning direction of the detection points refer toFigure 11 , obtain the stress values corresponding to 10 detection points , and then the stress distribution diagram of the specimen to be tested can be obtained, as Figure 12 shown.

[0189] As can be seen from the above embodiments, the present invention proposes a stress detection system based on the zero group velocity mode of Lamb wave, and a corresponding detection method is proposed based on this system. The present invention utilizes the phased array principle, that is, ultrasonic signals are transmitted / received by the probe array elements arranged according to a certain rule and the deflection of ultrasonic waves is controlled by an appropriate delay rule, so that the ultrasonic waves are incident into the specimen at the critical angle required by the zero group velocity mode of Lamb wave. Then, the stress of the specimen to be tested is detected by the excitation and reception of the zero group velocity mode of Lamb wave. Compared with the prior art, when the present invention performs stress detection under different materials and changes in sound velocity, the deflection of the incident angle of ultrasonic waves can be realized through the phased array control unit and the phased array probe, without replacing wedges with different inclination angles, avoiding problems such as cumbersome steps and easy change of the coupling state caused by replacing wedges, significantly improving the stress detection efficiency and reliability of the specimen, reducing the detection cost, and having important significance for the development of high-end equipment detection and characterization technology.

[0190] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A stress detection method based on the zero group velocity mode of Lamb waves, characterized in that, The method uses a stress detection system based on the Lamb wave zero group velocity mode. The stress detection system based on the Lamb wave zero group velocity mode includes: A phased array control unit; A phased array probe, connected to the phased array control unit; the phased array probe includes a transmitting probe and a receiving probe. The transmitting probe includes multiple elements for transmitting ultrasonic signals, and the receiving probe includes multiple elements for receiving ultrasonic signals; The phased array control unit is used to control multiple elements of the transmitting probe so that the ultrasonic signals transmitted by the transmitting probe are incident on the interior of the specimen to be tested at a preset angle; the receiving probe is used to receive the ultrasonic signals fed back by the specimen to be tested and transmit the received ultrasonic signals to the phased array control unit; The method includes the following steps: S1. Select the specimen to be tested and the area to be tested on the specimen to be tested, and calculate the theoretical frequency and phase velocity for exciting the Lamb wave zero group velocity mode; S2. Determine the incident angle of the Lamb wave zero group velocity mode according to the phase velocity ; S3. Select a wedge block and determine the incident deflection angle of the phased array ultrasound according to the inclination angle of the wedge block and the incident angle wherein ; and determine the delay law of the phased array ultrasonic waves according to the incident deflection angle of the phased array ultrasound ​​ S4. According to the theoretical frequency and the delay law, perform ultrasonic stress detection on the specimen to be tested through the stress detection system based on the Lamb wave zero group velocity mode; S5. Obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested and the center frequency f2 of the Lamb wave zero group velocity mode of the specimen with zero stress, and calculate the stress value of the specimen to be tested according to the following formula :[[]]END]] ， In the formula, , = -0, where k is the stress coefficient of the specimen to be tested; Among them, the step S5 includes: S51. Record the A-scan signal of the specimen to be tested or the zero-stress specimen; S52. Perform a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the specimen to be tested to obtain the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested; Or, perform a fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the zero-stress specimen to obtain the center frequency f2 of the Lamb wave zero group velocity mode of the zero-stress specimen; The stress coefficient k of the specimen to be tested is determined through the following steps: Apply stress to the specimen to be tested and obtain the center frequency f1 of the Lamb wave zero group velocity mode corresponding to the specimen to be tested under different stresses; Use the least squares method to fit the stress data and the data of the corresponding center frequency f1 of the Lamb wave zero group velocity mode; Perform thickness compensation on the slope of the fitted straight line to obtain the stress coefficient k of the specimen to be tested; The step of performing thickness compensation on the slope of the fitted straight line specifically includes: Solve for the radial contraction of the test piece under axial tension ; According to the frequency-thickness product corresponding to the zero group velocity, determine the actual value of the center frequency f1 of the Lamb wave zero group velocity mode of the specimen to be tested, and determine the actual slope of the fitted straight line. The actual slope is the stress coefficient k of the specimen to be tested; Among them, the radial contraction amount of the specimen to be tested under axial tension is solved It is calculated by the following formula: ， In the formula, is the tensile stress, is the plate thickness, is the Poisson's ratio, and E is the Young's modulus.

2. The method according to claim 1, characterized in that The phased array control unit includes: A phased array board, connected to the transmitting probe and the receiving probe, for controlling the transmitting probe to generate ultrasonic signals, and receiving and processing the ultrasonic signals received by the receiving probe; A computer device, connected to the phased array board, for controlling the phased array board and receiving the ultrasonic signals processed by the phased array board; the computer device includes a memory and a processor. The memory stores a phased array ultrasonic detection program, and the processor executes the phased array ultrasonic detection program to obtain the stress detection data of the specimen to be tested.

3. The method according to claim 1, wherein The step S1 specifically includes: S11. Obtain the plate thickness of the specimen to be tested; S12. Obtain the dispersion curve of the specimen to be tested according to the plate thickness of the specimen to be tested. The dispersion curve includes a group velocity-frequency thickness product curve and a phase velocity-frequency thickness product curve; S13. Determine the frequency-thickness product corresponding to zero group velocity based on the group velocity-frequency thickness product curve, and determine the theoretical frequency of the Lamb wave zero group velocity mode; determine the phase velocity of the Lamb wave zero group velocity mode based on the phase velocity-frequency thickness product curve. Among them, according to the plate thickness of the specimen to be tested, the dispersion curve of the specimen to be tested can be solved and obtained through the following equation: The symmetric mode (S) of elastic waves in a thin plate with a free boundary satisfies the following formula: ， The antisymmetric mode (A) satisfies the following formula: ， In the formula, h is half of the plate thickness of the specimen to be tested, k is the wave number, p and q are the propagation constants respectively, and satisfy the following formula: ， ， In the formula, is the angular frequency, and are the longitudinal wave velocity and the transverse wave velocity in the plate, respectively.

4. The method according to claim 1, characterized in that, In the step S2, the incident angle of the zero group velocity mode of Lamb wave is calculated by the formula: ， In the formula, is the sound velocity in the wedge, is the phase velocity of the zero group velocity mode of Lamb wave.

5. The method according to claim 1, wherein In the step S3, the calculation formula of the delay rule is: ， ， Wherein, i is the serial number of any array element within the aperture of the transmitting probe, I is the serial number of the starting array element within the aperture of the transmitting probe, J is the serial number of the terminating array element within the aperture of the transmitting probe, where i, I, J satisfy I ≤ i ≤ J ≤ n (i, I, J, n are all positive integers, n is the number of array elements within the aperture of the transmitting probe, n ≥ 2). is the delay time of the i-th array element. is the element pitch.

6. The method according to claim 1, characterized in that, It further includes the following steps: Perform stress detection on multiple regions of the specimen to be tested to obtain the stress values at multiple detection points of the specimen to be tested; Obtain the stress distribution map of the specimen to be tested according to the stress values at multiple detection points of the specimen to be tested.

Images