Stress detection system and method based on Lamb wave zero group velocity mode

Abstract

The invention discloses a stress detection system and method based on a Lamb wave zero group velocity mode, and relates to the technical field of equipment detection. The stress detection system comprises a phased array control unit and a phased array probe connected with the phased array control unit; the phased array probe comprises a transmitting probe and a receiving probe, the transmitting probe comprises a plurality of array elements used for transmitting ultrasonic signals, and the receiving probe comprises a plurality of array elements used for receiving the ultrasonic signals; the phased array control unit controls a plurality of array elements of the transmitting probe, so that an ultrasonic signal transmitted by the transmitting probe is incident into the to-be-tested piece at a preset angle; the receiving probe is used for receiving an ultrasonic signal fed back by the to-be-tested piece and transmitting the received ultrasonic signal to the phased array control unit. According to the stress detection system and method, the phased array is used for controlling deflection of ultrasonic waves, excitation and receiving of a Lamb wave zero group velocity mode can be achieved, the stress detection efficiency and reliability are improved, and the detection cost is reduced.

Description

Technical Field

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

[0002] Aluminum alloy plates are widely used in aerospace, automobile, shipbuilding and other industries. In a typical service environment, due to temperature changes, electrochemical corrosion and external loads, aluminum plates may produce large residual stresses, which may induce fatigue cracks in the material and even lead to sudden failure. Therefore, it is of great significance to evaluate or monitor the residual stress of aluminum plates.

[0003] The ultrasonic method has become one of the most promising nondestructive testing methods for stress measurement due to its unique advantages, such as safety, high penetration, portability, and ease of on-site detection. Existing studies have shown that the effective excitation of the zero group velocity mode of the Lamb wave is difficult and has strict requirements on the ultrasonic incident angle. At present, the main solution is to design a wedge with a critical angle according to the Lamb wave group velocity of the material being tested, so as to achieve the excitation and reception of the zero group velocity mode of the Lamb wave. This method requires the design of wedges with different inclinations for different materials. For the case of anisotropy of sound velocity or time-varying sound velocity, multiple wedges not only have poor adaptability and increase detection costs, but also easily change the coupling state during the replacement of the wedges, which brings many inconveniences to the given quantity 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 stress using the traditional ultrasonic method.

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

[0006] Phased array control unit;

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

[0008] The phased array control unit is used to control the multiple array elements of the transmitting probe so that the ultrasonic signal emitted by the transmitting probe is incident on the inside of the test piece at a preset angle; the receiving probe is used to receive the ultrasonic signal fed back by the test piece and transmit the received ultrasonic signal 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, and used to control the transmitting probe to generate ultrasonic signals, and to receive and process ultrasonic signals received by the receiving probe;

[0011] A computer device is connected to the phased array board and is used to control the phased array board and receive the ultrasonic signal 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 test piece.

[0012] The stress detection method based on the Lamb wave zero group velocity mode provided by the present invention adopts the stress detection system based on the Lamb wave zero group velocity mode described above, and the method comprises the following steps:

[0013] S1, select the test piece and the test area on the test piece, and calculate the theoretical frequency and phase velocity of the zero group velocity mode of the excited Lamb wave;

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

[0015] 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;

[0016] 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;

[0017] 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: :

[0018]

[0019] In the formula, , = -0, k is the stress coefficient of the test piece.

[0020] Optionally, the step S1 specifically includes:

[0021] S11, obtaining the plate thickness of the test piece;

[0022] 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;

[0023] 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;

[0024] Among them, according to the thickness of the test piece, the dispersion curve of the test piece can be solved and obtained by the following equation:

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

[0026] ,

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

[0028] ,

[0029] In the formula, h is half of the thickness of the test piece, k is the wave number, p and q are propagation constants, and they satisfy the following formula:

[0030] ,

[0031] ,

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

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

[0034]

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

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

[0037]

[0038]

[0039] Wherein, 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, and J is the serial number of the ending array element in the aperture of the transmitting probe, wherein i, I, and J satisfy I≤i≤J≤n (i, I, J, and n are all positive integers, and n is the number of array elements in the aperture of the transmitting probe, and n≥2). is the delay time of the ith array element, is the array element spacing.

[0040] Optionally, step S5 includes:

[0041] S51, recording the A-scan signal of the test piece or the zero-stress test piece;

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

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

[0044] Optionally, in step S5, the stress coefficient k of the test piece is determined by the following steps:

[0045] Apply stress to the test piece to obtain the Lamb wave zero group velocity mode center frequency f1 corresponding to the test piece under different stresses;

[0046] The stress data and the corresponding Lamb wave zero group velocity mode center frequency f1 data are fitted using the least squares method;

[0047] The slope of the fitted straight line is compensated for thickness to obtain the stress coefficient k of the test piece.

[0048] Optionally, the step of performing thickness compensation on the slope of the fitted straight line specifically includes:

[0049] Determine the radial shrinkage of the test piece under axial tension ;

[0050] According to the frequency-thickness product corresponding to zero group velocity, the actual value of the Lamb wave zero group velocity modal center frequency f1 of the test piece is determined, and the actual slope of the fitted straight line is determined, and the actual slope is the stress coefficient k of the test piece;

[0051] Among them, the radial shrinkage of the test piece under axial tension is solved Calculated by the following formula:

[0052]

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

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

[0055] Perform stress detection on multiple areas of the test piece to obtain stress values ​​at multiple detection points of the test piece;

[0056] A stress distribution diagram of the test piece is obtained according to stress values ​​of multiple detection points of the test piece.

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

[0058] By utilizing the phased array principle, that is, by transmitting / receiving ultrasonic signals through probe array elements arranged in a certain pattern and applying appropriate delay rules to control the deflection of ultrasonic waves, ultrasonic waves are incident into the interior of the test piece at a critical angle, and then the excitation and reception of the Lamb wave zero group velocity mode are realized, which is suitable for measuring stress in the plane of a plate-like structure. Compared with the prior art, in the case of stress detection of different materials and changes in sound velocity, the present invention does not need to replace wedges with different inclination angles, thus avoiding the problems of cumbersome steps and easy changes in coupling states caused by replacing wedges, significantly improving the efficiency and reliability of stress detection of the test piece, and reducing the detection cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

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

[0061] Figure 2 for Figure 1 A schematic diagram of the structure of the middle wedge assembly;

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

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

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

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

[0066] Figure 7 for Figure 4 A scan signal diagram corresponding to the receiving probe array element in the corresponding embodiment;

[0067] Figure 8 for Figure 4 Stress coefficient fitting diagram corresponding to the embodiment;

[0068] Fig. 9 for Figure 7 Frequency domain diagram of the S1-ZGV mode corresponding to the A-scan signal;

[0069] Fig.10 for Figure 4 Plan view of the test piece corresponding to the embodiment;

[0070] Fig.11 for Figure 4 Schematic diagram of plane scanning corresponding to the embodiment;

[0071] Fig.12 for Figure 4 Stress distribution diagram of the corresponding embodiment.

[0072] Description of Figure Numbers:

[0073] 1. Phased array control unit; 2. Transmitter probe; 3. Receiving probe; 4. Wedge assembly; 101. Computer equipment; 102. Phased array board; 401. First wedge; 402. Second wedge; 403. Sound absorbing layer. DETAILED DESCRIPTION

[0074] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within 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 are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0077] It should be understood that in this application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least 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 the 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 only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as 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 mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

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

[0080] Lamb wave refers to the guided wave formed by ultrasonic waves after multiple reflections and superpositions on the upper and lower surfaces in a thin plate, complex mutual interference and geometric diffusion. It is a kind of acoustic guided wave whose thickness is of the same order of magnitude as the wavelength of the exciting sound wave. Under certain specific frequency-thickness product (frequency-thickness product) conditions, the group velocity of the Lamb wave will approach zero (i.e., 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 wave scattering phenomenon caused by defects, and it can be used for residual stress detection of plate-like structural materials.

[0081] Lamb waves have multi-modal and dispersive propagation characteristics. When performing stress testing on plate-like structural materials, it is usually necessary to design a wedge with a critical angle according to the material being tested, so as to achieve the excitation and reception of the zero group velocity mode of the Lamb wave. This method requires the design of wedges with different inclinations for different materials. For the case of anisotropy of sound velocity or the change of sound velocity with the test piece, replacing the wedge not only has poor adaptability and increases the detection cost, but also easily changes the coupling state during the replacement process, which brings many inconveniences to the given quantity 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] See also Figure 1 In one embodiment of the present invention, a stress detection system based on a Lamb wave zero group velocity mode includes:

[0084] Phased array control unit 1;

[0085] A phased array probe is 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 multiple array elements of the transmitting probe 2 so that the ultrasonic signal emitted by the transmitting probe 2 is incident on the inside of the test piece at a preset angle; the receiving probe 3 is used to receive the ultrasonic signal fed back by the test piece and transmit the received ultrasonic signal 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; the phased array probe is an array composed of multiple independent small piezoelectric chips (that is, the multiple array elements for transmitting ultrasonic signals included in the transmitting probe 2 are small piezoelectric chips, and the multiple array elements for receiving ultrasonic signals included in the receiving probe 3 are small piezoelectric chips), these piezoelectric chips can independently excite or receive ultrasonic waves; the transmitting probe array elements are arranged in a linear array within the aperture of the transmitting probe, and the receiving probe array 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 excitation sequence and time of each transmitting probe array element are controlled by the phased array control unit, so that the ultrasonic waves emitted by each transmitting probe array element interfere and overlap in space to form a predetermined sound beam shape and direction, which can realize the deflection and focusing of the incident ultrasonic sound beam.

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

[0089] In this embodiment, when stress testing is performed on the test piece, it is usually necessary to use a wedge to introduce the ultrasonic wave emitted by the transmitting probe into the test piece; the purpose of using the wedge is to change the angle at which the ultrasonic wave enters the test piece and to avoid wear or other forms of damage to the probe, and to reduce the sound wave scattering effect caused by the uneven or rough surface of the test piece, thereby improving the accuracy and reliability of the test result; accordingly, another wedge 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 with a single angle is required to work, and when the test piece is replaced with a different material or the sound velocity inside the material changes (the sound velocity is related to factors such as the elastic modulus and density of the material), there is no need to replace wedges with different angles.

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

[0091] In this embodiment, Lamb wave S1-ZGV mode (S1-zero group velocity mode) is used to realize stress detection of the test piece. Lamb waves can be divided into symmetric mode (S) and antisymmetric mode (A) according to their vibration form and propagation characteristics. S1 mode is one of the basic modes in the symmetric mode. S1 mode has a relatively low phase velocity, which enables it to cover a longer distance when propagating in a thin plate and can provide higher resolution, which is conducive to discovering 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 phased array ultrasonic signal transmitting-receiving detection system. When stress detection is performed on the test piece, the phased array control unit 1 transmits a control signal to each array element of the transmitting probe 2. The ultrasonic signals emitted by the array elements of the transmitting probe 2 are interfered and superimposed to form an incident sound beam. Through a certain delay law, 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 formation of a Lamb wave zero group velocity mode in the test piece, and the ultrasonic signal fed back by the test piece is received by the receiving probe 3, thereby realizing stress detection of the test piece.

[0093] Compared with the prior art, the embodiments of the present invention can directly change the delay law 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 sound velocity changes, a single angle wedge can be used without replacing the wedge, thereby avoiding the problems of cumbersome steps and easy change of coupling state caused by replacing the wedge, significantly improving the efficiency and reliability of stress detection of the test piece and reducing the detection cost.

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

[0095] The phased array board 102 is connected to the transmitting probe 2 and the receiving probe 3, and is used to control the transmitting probe 2 to generate ultrasonic signals, and to receive and process ultrasonic signals received by the receiving probe 3;

[0096] The computer device 101 is connected to the phased array board 102, and is used to control the phased array board 102 and receive the ultrasonic signal 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 array elements in the transmitting probe 2 and the receiving probe 3 to achieve dynamic control of the sound beam; the phased array board 102 generates a high-frequency pulse signal for exciting the transmitting probe array element at the transmitting end, and provides precise timing control for each independent transmitting probe array element to ensure that the ultrasonic sound beam can be emitted in a predetermined direction and focusing state; at the receiving end, the ultrasonic signal fed back from the test piece is received through the receiving probe, and the received signal is amplified, filtered and digitized, and 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 the passive electronic scanning array or the technical architecture of the active electronic scanning array according to actual needs, 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 more of PCI / PCIe, USB, Ethernet / GigE, PXI / PXIe and other interfaces.

[0099] In one implementation, the phased array board 102 may be 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, which is suitable for the detection of complex structures, such as pipeline detection in the aerospace, automotive, shipbuilding, oil and gas industries, and key component detection in nuclear power facilities. Due to the large number of channels, it is usually equipped with high-speed data acquisition and processing capabilities, which can handle multiple detection tasks at the same time, and can improve detection speed and efficiency.

[0101] In this embodiment, the computer device 101 includes at least 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; the phased array ultrasonic detection program is used to simulate and analyze the dispersion characteristics of the zero group velocity mode of the Lamb wave, and formulate the phased array delay law based on this, 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 test piece. The computer device 101 may also include a display module for simulation analysis, calculation analysis, and visualization and human-computer interaction of signal acquisition.

[0102] It should be noted that when the stress detection system based on the Lamb wave zero group velocity mode provided in the above embodiment is used to perform stress detection on the test piece, a wedge block assembly is usually required; based on the actual application scenarios of the above embodiment, the present invention also designs a wedge block assembly.

[0103] See also Figure 2 In some embodiments, the wedge block group 4 includes: a first wedge block 401, a second wedge block 402 and a sound absorbing layer 403, and the sound absorbing layer 403 is provided between the first wedge block 401 and the second wedge block 402 for absorbing ultrasonic waves.

[0104] When the stress detection system based on the Lamb wave zero group velocity mode is used to perform stress detection on the test piece, 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 block 401 is used to guide the ultrasonic sound beam emitted by the transmitting probe into the test piece, and the second wedge block 402 is used to guide the ultrasonic signal passing through the detection area to be received by the receiving probe.

[0106] The first wedge block 401 and the second wedge block 402 can be designed to be separate or integrated; a sound absorbing layer 403 is provided between the first wedge block 401 and the second wedge block 402 to ensure that the transmitting and receiving ultrasonic signals do not interfere with each other.

[0107] The material of the wedge block can be selected from organic glass, epoxy resin, polyurethane, hard rubber, etc. according to the requirements of sound velocity matching, compatibility with the test piece material, durability, etc.; the material of the sound absorbing 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, the heights of the centers of the transmitting probe and receiving probe array elements need to be determined based on the acoustic attenuation coefficient of the wedge material so that the sound beam reaching the surface of the test piece has sufficient signal strength, while ensuring that the focus of the sound beam is located at the optimal position to achieve the best detection effect.

[0109] Furthermore, when the stress detection system based on the Lamb wave zero group velocity mode is used to perform stress detection on the test piece, a coupling agent may be applied between the first wedge block 401 , the second wedge block 402 and the surface of the test piece.

[0110] In ultrasonic testing, a coupling agent is applied between the wedge and the surface of the material to be tested to eliminate air gaps, ensuring that ultrasonic waves can be effectively transmitted from the wedge to the material to be tested. It can also reduce the difference in acoustic impedance between the wedge and the material to be tested, thereby improving the transmission efficiency of sound waves. 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 test conditions and the test requirements, the coupling agent can be any of water-based, oil-based and solid coupling agents.

[0112] The present invention also proposes a stress detection method based on the Lamb wave zero group velocity mode. The method adopts the stress detection system based on the Lamb wave zero group velocity mode provided by the above embodiment.

[0113] See also Figure 3 In one embodiment of the present invention, a stress detection method based on the Lamb wave zero group velocity mode comprises the following steps:

[0114] S1, select the test piece and the test area on the test piece, and calculate the theoretical frequency and phase velocity of the zero group velocity mode of the excited Lamb wave;

[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 free boundaries satisfies the following equation:

[0128] , (2)

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

[0130] , (3)

[0131] In the formula, h is half of the thickness of the test piece, k is the wave number, p and q are propagation constants, and they satisfy the following formula:

[0132] , (4)

[0133] , (5)

[0134] In the formula, is the angular frequency, and are the longitudinal wave velocity and shear wave velocity in the plate, respectively.

[0135] After determining the thickness of the test piece, by solving the above equations (2)-(5), the dispersion curve (group velocity-frequency-thickness product curve and phase velocity-frequency-thickness product curve) of the test piece can be obtained. The frequency-thickness product corresponding to the group velocity being zero is determined according to the group velocity-frequency-thickness product curve, so the theoretical frequency of the corresponding Lamb wave zero group velocity mode is obtained; and the phase velocity corresponding to the frequency-thickness product is determined according to the phase velocity-frequency-thickness product curve, which is the phase velocity under the corresponding Lamb wave zero group velocity mode. .

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

[0137] , (6)

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

[0139] In step S3, after selecting the material and inclination of the wedge, the number of transmitting probe aperture array elements n is preliminarily selected according to the acoustic attenuation coefficient of the wedge, and the longitudinal wave sound velocity value in the test piece is given. , the speed of sound in the wedge Must meet < .

[0140] According to the inclination angle of the wedge and the incident angle obtained in step S2 Determine the incident deflection angle of phased array ultrasound ,in .

[0141] The calculation formula of the delay law is further obtained:

[0142] , (7)

[0143] , (8)

[0144] Wherein, 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, and J is the serial number of the ending array element in the aperture of the transmitting probe, wherein i, I, and J satisfy I≤i≤J≤n (i, I, J, and n are all positive integers, and n is the number of array elements in the aperture of the transmitting probe, and n≥2). is the delay time of the ith array element, is the array element spacing.

[0145] After the delay law is calculated based on step S3, the critical refracted longitudinal wave is excited and received by the stress detection system based on the zero group velocity mode of Lamb waves provided in the above embodiment, and the scanning signal is recorded in the phased array control unit.

[0146] Further, step S5 includes:

[0147] S51, recording the A-scan signal of the test piece or the zero-stress test piece;

[0148] S52, performing fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the test piece to obtain the center frequency f1 of the Lamb wave zero group velocity mode of the test piece;

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

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

[0151] After obtaining the Lamb wave zero group velocity modal center frequency f2 of the zero stress specimen and the Lamb wave zero group velocity modal center frequency f1 of the test piece, the stress value of the test piece is calculated. .

[0152] The stress coefficient k of the test piece is determined by the following steps:

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

[0154] The stress data and the corresponding Lamb wave zero group velocity mode center frequency f1 data are fitted using the least squares method;

[0155] The slope of the fitted straight line is compensated for thickness to obtain the stress coefficient k of the test piece.

[0156] The step of performing thickness compensation on the slope of the fitted straight line specifically includes:

[0157] Determine the radial shrinkage of the test piece under axial tension ;

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

[0159] Among them, the radial shrinkage of the test piece under axial tension is solved Calculated by the following formula:

[0160] , (9)

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

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

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

[0164] A stress distribution diagram of the test piece is obtained according to stress values ​​of multiple detection points of the test piece.

[0165] In order to explain in more detail the implementation process and beneficial effects of the stress detection method embodiment 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 of the present method, and those skilled in the art can make adjustments according to the detection object and detection scene in actual application.

[0166] Example 1

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

[0168] In this embodiment, the 6061 aluminum plate belongs to an aluminum-magnesium-silicon alloy, has good processing properties, is easy to weld, is often used to manufacture structural parts, aircraft parts, ship components, bicycle frames, etc., and is a very common aluminum alloy plate.

[0169] Step S1, select the test piece and the test area on the test piece, and calculate the theoretical frequency and phase velocity of the zero group velocity mode of the excited Lamb wave:

[0170] By solving equations (2)-(5), we can obtain the dispersion curve of 6061 aluminum plate with a thickness of 3 mm. Figure 4 (Group velocity-frequency thickness product curve) and Figure 5 (Phase velocity-frequency thickness product curve).

[0171] Depend on Figure 4 It can be seen that the interaction between the forward wave of the S1 mode and the backward wave of the S2 mode produces the S1-ZGV mode with zero group velocity. Due to the interaction between the forward wave and the backward wave, the acoustic wave energy is confined 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 0.959MHz.

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

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

[0174] The longitudinal wave speed of 6061 aluminum alloy is about 6300 m / s. According to formula (6), the incident angle corresponding to the speed of 6370 m / s is It is 15°, so it can be determined that the incident angle corresponding to 6061 aluminum alloy is about 15°.

[0175] according to < According to the requirements, plexiglass with a longitudinal wave speed of 2334m / s was selected as the material of the wedge.

[0176] Step S3, select a wedge block, and adjust the wedge block according to its inclination angle. 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:

[0177] Select 10° as the wedge inclination angle The selected wedge block is an integrated one with a sound-absorbing layer in the middle to ensure that the transmitted and received ultrasonic signals do not interfere with each other. The wedge block is placed on the surface of the 6061 aluminum alloy specimen, and the coupling between the wedge block and the specimen is stable with the help of engine oil; the phased array ultrasonic incident deflection angle θ is calculated: =15°-10°=5°.

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

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

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

[0181] Step S4: According to the obtained theoretical frequency and delay law, ultrasonic stress detection is performed on the test piece by using the stress detection system based on the Lamb wave zero group velocity mode provided by the above embodiment.

[0182] Step S5, obtaining 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 calculating the stress value of the test piece according to formula (1): .

[0183] In step S5, the receiving probe receives the ultrasonic detection signal and uploads it to the phased array ultrasonic detection software after processing through the phased array board. The signal is recorded as an A scan signal in the phased array ultrasonic detection software. The sampling frequency of the signal is selected to be 100 MHz. Figure 7 , Figure 7 The A-scan signal corresponding to the receiving array element is interpolated, and the time interval between two adjacent signal points is 1ns.

[0184] Furthermore, the stress coefficient of the test piece is obtained through a tensile stress experiment. The tensile stress can be applied to the sample by a universal testing machine, and the S1-ZGV mode is excited and the corresponding signal is collected through the above-mentioned stress detection system; within the tensile force range of 0~13kN, the ultrasonic signal will be collected once for every increase of 1kN; the experimental results within the tensile force range of 5~13kN are selected, and then the frequency of the resonant wave is analyzed.

[0185] When the applied stress changes from 48MPa to 153MPa, the modal frequency of the S1-ZGV test piece changes as follows: Figure 8 As shown in the figure, through linear regression analysis, it can be calculated that the stress coefficient between the S1-ZGV modal frequency shift and the applied stress is -3.4494 Hz / MPa, and the determination coefficient is 0.9948.

[0186] In this embodiment, the Poisson's ratio of the test piece is 0.331, and the Young's modulus E is 72 GPa. The radial shrinkage of the test piece can be calculated by formula (9): ; From the above steps, it can be seen that the frequency-thickness product of the test piece in the S1-ZGV mode is 2.877MHz×mm. According to the thickness change value (i.e. radial shrinkage ) can be solved to find the corresponding frequency change, and then refer to Figure 8 The test data is used to compensate for the frequency offset caused by the thickness change under each stress; after thickness compensation, the actual stress coefficient of the test piece can be calculated to be -214.1201Hz / MPa.

[0187] Further, according to the actual stress coefficient of the test piece obtained in the above steps, the stress value of the test piece is calculated. . See Fig. 9 , Fig. 9 To obtain the frequency domain diagram of the S1-ZGV mode corresponding to the receiving array element, the A-scan signal is selected for fast Fourier transform to obtain the frequency domain, and the center frequency of the signal is read. 1=0.9653MHz; the center frequency of the zero stress specimen is 2 = 0.95 MHz, so 0.0153MHz, calculated according to formula (1) 71.455MPa, which is the stress value of the 6061 aluminum plate to be tested 71.455MPa.

[0188] Further, see Fig.10 , Fig.10 The two-dimensional plane diagram of the test piece is shown in Figure 1. The phased array probe is moved to scan the surface of the test piece. 10 different test points are selected and the above stress test steps are repeated at the 10 different test points. The arrangement of the test points and the scanning direction refer to Fig.11 , get the stress values ​​corresponding to the 10 detection points , the stress distribution diagram of the test piece can be obtained, such as Fig.12 shown.

[0189] It can be seen from the above embodiments that the present invention proposes a stress detection system based on the Lamb wave zero group velocity mode, and proposes a corresponding detection method based on the system. The present invention utilizes the phased array principle, that is, by transmitting / receiving ultrasonic signals through probe array elements arranged in a certain pattern and controlling the deflection of ultrasonic waves with appropriate delay rules, so as to realize that ultrasonic waves are incident into the interior of the test piece at the critical angle required by the Lamb wave zero group velocity mode, and then stress detection is performed on the test piece through the excitation and reception of the Lamb wave zero group velocity mode. Compared with the prior art, when performing stress detection on different materials and under the condition of sound velocity changes, the present invention can realize the deflection of the ultrasonic incident angle through the phased array control unit and the phased array probe, without the need to replace wedges with different inclination angles, avoiding the cumbersome steps and easy change of coupling state caused by replacing wedges, significantly improving the efficiency and reliability of stress detection of test pieces, and reducing the detection cost, which is of great significance to the development of high-end equipment detection and characterization technology.

[0190] The embodiments described above 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 aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A stress detection system based on Lamb wave zero group velocity mode, characterized in that: include: Phased array control unit; A phased array probe connected to the phased array control unit; the phased array probe comprises a transmitting probe and a receiving probe, the transmitting probe comprises a plurality of array elements for transmitting ultrasonic signals, and the receiving probe comprises a plurality of array elements for receiving ultrasonic signals; The phased array control unit is used to control multiple array elements of the transmitting probe so that the ultrasonic signal emitted by the transmitting probe is incident on the inside of the test piece at a preset angle; the receiving probe is used to receive the ultrasonic signal fed back by the test piece and transmit the received ultrasonic signal to the phased array control unit.

2. The stress detection system based on Lamb wave zero group velocity mode according to claim 1, characterized in that: The phased array control unit comprises: A phased array board connected to the transmitting probe and the receiving probe, and used to control the transmitting probe to generate ultrasonic signals, and to receive and process ultrasonic signals received by the receiving probe; A computer device is connected to the phased array board and is used to control the phased array board and receive the ultrasonic signal 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 test piece.

3. A stress detection method based on Lamb wave zero group velocity mode, characterized in that: The method adopts the stress detection system based on the Lamb wave zero group velocity mode according to claim 1 or 2, and the method comprises the following steps: S1, select the test piece and the test area on the test piece, and calculate the theoretical frequency and phase velocity of the zero group velocity mode of the excited Lamb wave; S2, determining the incident angle of the Lamb wave zero group velocity mode according to the phase velocity ; 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; 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; 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: : In the formula, , = -0, k is the stress coefficient of the test piece.

4. The method according to claim 3, characterized in that The step S1 specifically includes: S11, obtaining the plate thickness of the test piece; 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; 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; Among them, according to the thickness of the test piece, the dispersion curve of the test piece can be solved and obtained by the following equation: The symmetric mode (S) of elastic waves in a thin plate with free boundaries satisfies the following equation: ， The antisymmetric mode (A) satisfies the following equation: ， In the formula, h is half of the thickness of the test piece, k is the wave number, p and q are propagation constants, and they satisfy the following formula: ， ， In the formula, is the angular frequency, and are the longitudinal wave velocity and shear wave velocity in the plate, respectively.

5. The method according to claim 3, characterized in that In step S2, the incident angle of the Lamb wave zero group velocity mode is The calculation formula is: In the formula, is the speed of sound in the wedge, is the phase velocity of the zero group velocity mode of the Lamb wave.

6. The method according to claim 3, characterized in that In step S3, the calculation formula of the delay rule is: Where 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, and J is the serial number of the ending array element in the aperture of the transmitting probe, where i, I, and J satisfy I≤i≤J≤n (i, I, J, and n are all positive integers, n is the number of array elements in the aperture of the transmitting probe, and n≥2). is the delay time of the ith array element, is the array element spacing.

7. The method according to claim 3, characterized in that The step S5 comprises: S51, recording the A-scan signal of the test piece or the zero-stress test piece; S52, performing fast Fourier transform on the Lamb wave zero group velocity mode in the A-scan signal of the test piece to obtain the center frequency f1 of the Lamb wave zero group velocity mode of the test piece; Alternatively, the Lamb wave zero group velocity mode in the A-scan signal of the zero stress specimen is subjected to a fast Fourier transform to obtain the Lamb wave zero group velocity mode center frequency f2 of the zero stress specimen.

8. The method according to claim 7, characterized in that In step S5, the stress coefficient k of the test piece is determined by the following steps: Apply stress to the test piece to obtain the Lamb wave zero group velocity mode center frequency f1 corresponding to the test piece under different stresses; The stress data and the corresponding Lamb wave zero group velocity mode center frequency f1 data are fitted using the least squares method; The slope of the fitted straight line is compensated for thickness to obtain the stress coefficient k of the test piece.

9. The method according to claim 8, characterized in that The steps of performing thickness compensation on the slope of the fitted straight line specifically include: Determine the radial shrinkage of the test piece under axial tension ; According to the frequency-thickness product corresponding to zero group velocity, the actual value of the Lamb wave zero group velocity modal center frequency f1 of the test piece is determined, and the actual slope of the fitted straight line is determined, and the actual slope is the stress coefficient k of the test piece; Among them, the radial shrinkage of the test piece under axial tension is solved Calculated by the following formula: In the formula, is the tensile stress, is the plate thickness, is Poisson's ratio, and E is Young's modulus.

10. The method according to claim 3, characterized in that The following steps are also included: Performing stress detection on multiple areas of the test piece to obtain stress values ​​at multiple detection points of the test piece; A stress distribution diagram of the test piece is obtained according to stress values ​​of multiple detection points of the test piece.

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