Composite material impact damage evaluation method and device based on zero group velocity Lamb wave

Abstract

The invention discloses a composite material impact damage evaluation method and device based on zero group velocity Lamb wave, and the method comprises the steps: respectively placing air coupling ultrasonic transducers at two sides of any to-be-detected point of a detected composite material in a manner of being vertical to the surface of the composite material; the theoretical frequency of the zero group velocity lamb wave of the tested composite material is calculated, an excitation signal of the lamb wave with the theoretical frequency as the center frequency is excited through the air coupling ultrasonic transducer, and a zero group velocity lamb wave signal is received by the air coupling ultrasonic transducer on the other side of the tested composite material; and processing the received zero-group velocity lamb wave signal to obtain a frequency spectrum amplitude of the zero-group velocity lamb wave signal so as to evaluate the impact damage of the tested composite material.

Description

Technical Field

[0001] The present invention relates to the technical field of ultrasonic nondestructive testing, and in particular to a composite material impact damage evaluation method and device based on zero group velocity Lamb waves. Background Art

[0002] At present, compared with traditional materials, composite materials have advantages such as high specific strength, high specific modulus, high temperature resistance, corrosion resistance and designability, and are widely used in engineering fields such as aerospace, shipbuilding, and automobiles. Since the working environment of composite materials is relatively more complex and severe, and the loads they bear are larger, various defects are prone to occur inside the material structure. For example, composite materials are often subjected to various low-speed impact loads during service, which in turn cause internal damage such as matrix cracking, fiber breakage, and delamination. The interfacial voids caused by these damages cause the stiffness, strength, fatigue resistance and other properties of the composite materials to seriously degrade, posing a huge threat to engineering safety. Damage to composite materials is often formed inside the structure and is difficult to detect visually. Therefore, it is particularly important to effectively detect internal damage to composite materials.

[0003] Ultrasonic nondestructive testing (NDT) is a widely used inspection method in engineering. Leveraging the response characteristics of ultrasonic waves, it can measure the thickness of structures or detect internal damage without damaging the material being tested. Ultrasonic testing methods are widely used due to their high sensitivity, long propagation distance, ease of operation, and low cost. Lamb waves, as guided ultrasonic waves that propagate through plates, can be used for remote damage detection in plate-like structures. Lamb waves exhibit multimodal and dispersive characteristics. Among these modes, a special mode, the zero-group-velocity mode, exists. Its group velocity is zero, but its wavenumber and phase velocity are not zero. Its energy is confined to a local region of the waveguide and does not propagate far. Zero-group-velocity Lamb waves persist for a long time in the time domain and manifest as a sharp resonant peak in the spectrum, making them sensitive to changes in the material's condition. With their high sensitivity, high signal-to-noise ratio, and localized detection capabilities, zero-group-velocity Lamb waves can be used to assess plate thickness, damage, and other material conditions.

[0004] In recent years, air-coupled ultrasonic transducers, a new type of ultrasonic transducer using air as the acoustic coupling medium, have been widely researched and applied. Compared to traditional piezoelectric ultrasonic transducers, air-coupled ultrasonic transducers offer numerous advantages. For example, using air as a coupling agent allows for rapid online testing of the material being tested, avoiding corrosion and contamination of the material surface by the coupling agent. The absence of contact with the material being tested does not alter the material's boundary conditions, thereby affecting wave propagation within the material. Furthermore, the absence of contact with the material being tested or the coupling agent significantly extends the transducer's service life. Due to these advantages, air-coupled ultrasonic transducers are suitable for applications in material damage detection, medicine, food safety testing, and other fields.

[0005] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention

[0006] In response to the deficiencies in the prior art, the present invention aims to provide a composite material impact damage evaluation method and device based on zero-group-velocity Lamb waves. Based on the nondestructive testing technology of air-coupled ultrasonic transducers and zero-group-velocity Lamb waves, the damage state of the composite material is characterized by changes in the amplitude of the zero-group-velocity Lamb wave spectrum, and non-contact nondestructive testing can be performed with high sensitivity.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a composite material impact damage evaluation method based on zero group velocity Lamb waves, comprising:

[0009] Step 1: For any test point of the composite material to be tested, place an air-coupled ultrasonic transducer on both sides of the composite material surface perpendicular to the composite material surface;

[0010] Step 2: Calculate the theoretical frequency of the zero-group-velocity Lamb wave of the composite material being tested, wherein an excitation signal of the Lamb wave with the theoretical frequency as the center frequency is excited by an air-coupled ultrasonic transducer, and the zero-group-velocity Lamb wave signal is received by an air-coupled ultrasonic transducer on the other side of the composite material being tested;

[0011] Step 3: Process the received zero group velocity Lamb wave signal to obtain the spectrum amplitude of the zero group velocity Lamb wave signal to evaluate the impact damage of the composite material being tested.

[0012] In the method described above, in step 2, the excitation signal includes a Hanning window weighted signal, the center frequency of which is the zero group velocity Lamb wave theoretical frequency f of the composite material being tested. th .

[0013] In the method described, in step 2, the Lamb wave dispersion curve fk relationship diagram of the composite material under test is calculated to obtain its zero group velocity point, where the zero group velocity point is a point in the dispersion curve fk relationship diagram where the slope does not exist, where f is the frequency and k is the wave number.

[0014] In the method described above, in step 3, the zero group velocity Lamb wave frequency at the intact state detection point is recorded as f ZGV , extract its spectrum amplitude I0, and extract the zero group velocity Lamb wave frequency f at the intact state detection point ZGVThe spectrum amplitude I1 of the spectrum signal containing the impact damage detection point is obtained, and the difference ΔI between the spectrum amplitude I0 and the spectrum amplitude I1 is calculated to quantitatively characterize the impact damage degree of the tested composite material.

[0015] In the method described, in step 3, the collected zero group velocity Lamb wave signal is processed by a computer, the zero group velocity Lamb wave signal is low-pass filtered, and fast Fourier transform is performed on the signal to obtain the corresponding spectrum signal to evaluate the impact damage of the composite material being tested.

[0016] In the method described, in step 3, the relative position of the air-coupled ultrasonic transducer and the composite material to be tested is changed to obtain zero-group velocity Lamb wave signals at different detection points, and then the zero-group velocity Lamb wave spectrum signals at different detection points in the composite material to be tested are compared to evaluate the impact damage degree of different detection points of the composite material to be tested; and the impact damage degree of different composite materials to be tested is evaluated by replacing different composite materials to be tested.

[0017] A device for implementing the method includes:

[0018] The testing platform fixes the composite material to be tested.

[0019] an air-coupled ultrasonic excitation transducer fixed to one side of the composite material to be measured to adjustably emit an excitation signal to generate a zero-group-velocity Lamb wave signal through the composite material to be measured,

[0020] an air-coupled ultrasonic receiving transducer fixed on the other side of the composite material to be tested relative to the air-coupled ultrasonic excitation transducer to receive the zero-group-velocity Lamb wave signal;

[0021] A computer is connected to the air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer, and the computer adjusts the excitation signal emitted by the air-coupled ultrasonic excitation transducer and generates a spectrum amplitude based on the zero group velocity Lamb wave signal to evaluate the impact damage of the tested composite material.

[0022] In the device, the composite materials to be tested include carbon fiber resin-based composite materials, glass fiber resin-based composite materials, boron fiber metal-based composite materials and carbon fiber ceramic-based composite materials.

[0023] In the device, the air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer are respectively fixed at fixed distances on both sides of the composite material to be tested, perpendicular to the plate surface of the composite material to be tested and targeting the same point to be tested. During the detection process, the relative distance between the air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer and the composite material to be tested remains unchanged.

[0024] The device further includes:

[0025] A signal excitation receiver is connected to the computer to excite an ultrasonic signal, the center frequency of which is the theoretical frequency f of the zero group velocity Lamb wave of the composite material being tested. th , the zero group velocity point is the point with no slope in the fk relationship diagram of the dispersion curve of the tested composite material, where f is the frequency and k is the wave number;

[0026] The attenuator is connected to the output end of the signal excitation receiver and the air-coupled ultrasonic excitation transducer to improve the impedance matching of the received ultrasonic signal and then introduce it into the air-coupled ultrasonic excitation transducer. The air-coupled ultrasonic excitation transducer introduces the ultrasonic signal into the composite material under test through the air, generates a zero group velocity Lamb wave signal inside the composite material, and is received by the air-coupled ultrasonic receiving transducer on the other side.

[0027] A preamplifier is connected to the air-coupled ultrasonic receiving transducer and the receiving end of the signal excitation receiver, amplifies the received zero-group-velocity Lamb wave signal and then introduces it into the signal excitation receiver;

[0028] The oscilloscope is connected to the signal excitation receiver and the computer, and averages the zero group velocity Lamb wave signal multiple times before importing it into the computer.

[0029] Beneficial effects

[0030] A composite material impact damage assessment method based on zero-group-velocity Lamb waves utilizes an air-coupled ultrasonic transducer to detect material damage. During testing, the air-coupled ultrasonic transducer maintains contact with the composite material being tested, preventing surface corrosion and contamination by coupling agents. This method also maintains the material's boundary conditions and thus does not affect ultrasonic propagation within the composite material, ensuring accurate and stable signals. The device boasts a simple structure, real-time convenience, and an effective, highly applicable method, enabling localized testing of the material being tested. While impact damage to composite plates generally leaves the surface unchanged, internal damage such as voids, microcracks, matrix cracking, fiber breakage, and delamination may occur, resulting in a reduction in material stiffness. These damages are typically small and difficult to detect within the material. Zero-group-velocity Lamb waves are highly sensitive to microdamage and can effectively detect impact damage to composite materials. The present invention assesses the material's damage state by comparing changes in the zero-group-velocity Lamb wave signal spectrum amplitude within the composite material. The smaller the zero-group-velocity Lamb wave signal spectrum amplitude, the greater the degree of material damage. This method is a linear ultrasonic nondestructive testing method. Compared with traditional linear ultrasonic detection methods, it has higher sensitivity, and compared with nonlinear ultrasonic detection methods, it has the advantages of high signal-to-noise ratio and easy signal extraction.

[0031] The above description is only an overview of the technical solution of the present invention. In order to make the technical means of the present invention clearer and easier to understand, so that those skilled in the art can implement it according to the contents of the specification, and to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described below by way of example. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments described in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0033] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are intended only to illustrate preferred embodiments and are not to be construed as limiting the present invention. It should be understood that the drawings described below are merely examples of the present invention, and that those skilled in the art will be able to derive other drawings from these drawings without inventive effort. Throughout the drawings, identical reference numerals are used to denote identical components.

[0034] In the attached figure:

[0035] Figure 1 It is a structural schematic diagram of the nondestructive testing device of the present invention;

[0036] Figure 2 is a graph showing the f frequency-k wave number dispersion of the composite material under test in one embodiment;

[0037] Figure 3 is a time domain waveform of an excitation signal applied in one embodiment;

[0038] Figure 4 is an amplitude-frequency curve of an excitation signal applied in one embodiment;

[0039] Figure 5 is a zero group velocity Lamb wave signal of the composite material under test in an intact state measured in one embodiment;

[0040] Figure 6 is a zero group velocity Lamb wave signal of the composite material under test after impact damage measured in one embodiment;

[0041] Figure 7 It is aimed at Figure 5 and Figure 6 The amplitude-frequency curve obtained by fast Fourier transform of the zero group velocity Lamb wave signal in .

[0042] The present invention will be further explained below with reference to the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0043] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are 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 making creative efforts shall fall within the scope of protection of the present invention.

[0044] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are also within the scope of protection of the present invention.

[0045] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0046] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0047] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0048] In the present invention, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood broadly. For example, they may refer to fixed connection, detachable connection, or integration; they may refer to direct connection or indirect connection through an intermediate medium; they may refer to internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0049] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0050] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings, and the accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0051] In one embodiment, if Figures 1 to 7 As shown, the present disclosure provides a composite material impact damage evaluation method based on zero group velocity Lamb wave, comprising the following steps:

[0052] Step 1: For any point to be tested on the composite material 5, place an air-coupled ultrasonic transducer on both sides of the composite material surface perpendicular to the composite material surface;

[0053] Step 2: Calculate the theoretical frequency of the zero-group-velocity Lamb wave of the composite material 5 under test, wherein an excitation signal of the Lamb wave with the theoretical frequency as the center frequency is excited by an air-coupled ultrasonic transducer, and the zero-group-velocity Lamb wave signal is received by the air-coupled ultrasonic transducer on the other side of the composite material 5 under test;

[0054] Step 3: Process the received zero group velocity Lamb wave signal to obtain the spectrum amplitude of the zero group velocity Lamb wave signal to evaluate the impact damage of the composite material 5 under test.

[0055] In a preferred embodiment of the method, in step 2, the excitation signal comprises a Hanning window weighted signal, the center frequency of which is the theoretical frequency f of the zero group velocity Lamb wave of the composite material 5 under test. th .

[0056] In a preferred embodiment of the method, in step 2, the Lamb wave dispersion curve fk relationship diagram of the composite material 5 under test is calculated to obtain its zero group velocity point, where the zero group velocity point is a point where the slope does not exist in the dispersion curve fk relationship diagram, where f is the frequency and k is the wave number.

[0057] In a preferred embodiment of the method, in step 3, the zero group velocity Lamb wave frequency at the intact state detection point is recorded as f ZGV , extract its spectrum amplitude I0, and extract the zero group velocity Lamb wave frequency f at the intact state detection point ZGV The spectrum amplitude I1 of the spectrum signal containing the impact damage detection point is obtained, and the difference ΔI between the spectrum amplitude I0 and the spectrum amplitude I1 is calculated to quantitatively characterize the impact damage degree of the tested composite material 5.

[0058] In a preferred embodiment of the method, in step 3, the collected zero group velocity Lamb wave signal is processed by the computer 1, the zero group velocity Lamb wave signal is low-pass filtered, and fast Fourier transform is performed to obtain the corresponding spectrum signal to evaluate the impact damage of the composite material 5 being tested.

[0059] In a preferred embodiment of the method, in step 3, the relative position of the air-coupled ultrasonic transducer and the composite material 5 to be tested is changed to obtain zero group velocity Lamb wave signals at different detection points, and then the zero group velocity Lamb wave spectrum signals at different detection points in the composite material 5 to be tested are compared to evaluate the degree of impact damage at different detection points of the composite material 5 to be tested; and the degree of impact damage of different composite materials 5 to be tested is evaluated by replacing different composite materials 5 to be tested.

[0060] A device for implementing the method includes:

[0061] The testing platform 6 fixes the tested composite material 5.

[0062] An air-coupled ultrasonic excitation transducer 4 is fixed to one side of the composite material 5 to adjustably emit an excitation signal to generate a zero-group-velocity Lamb wave signal through the composite material 5.

[0063] The air-coupled ultrasonic receiving transducer 7 is fixed on the other side of the composite material 5 to be tested relative to the air-coupled ultrasonic excitation transducer 4 to receive the zero group velocity Lamb wave signal.

[0064] A computer 1 is connected to the air-coupled ultrasonic excitation transducer 4 and the air-coupled ultrasonic receiving transducer 7. The computer 1 adjusts the excitation signal emitted by the air-coupled ultrasonic excitation transducer 4 and generates a spectrum amplitude based on the zero group velocity Lamb wave signal to evaluate the impact damage of the composite material 5 under test.

[0065] In a preferred embodiment of the device, the composite material 5 to be tested includes a carbon fiber resin-based composite material, a glass fiber resin-based composite material, a boron fiber metal-based composite material and a carbon fiber ceramic-based composite material.

[0066] In a preferred embodiment of the device, the air-coupled ultrasonic excitation transducer 4 and the air-coupled ultrasonic receiving transducer 7 are respectively fixed at a fixed distance on both sides of the composite material 5 to be tested, and are perpendicular to the plate surface of the composite material 5 to be tested and are aimed at the same point to be tested. During the detection process, the relative distance between the air-coupled ultrasonic excitation transducer 4 and the air-coupled ultrasonic receiving transducer 7 and the composite material 5 to be tested always remains unchanged.

[0067] In a preferred embodiment of the device, the device further comprises:

[0068] The signal excitation receiver 2 is connected to the computer 1 to excite an ultrasonic signal, the center frequency of which is the theoretical frequency f of the zero group velocity Lamb wave of the composite material 5 being tested. th , the zero group velocity point is the point without slope in the fk relationship diagram of the dispersion curve of the tested composite material 5, where f is the frequency and k is the wave number;

[0069] The attenuator 3 is connected to the output end of the signal excitation receiver 2 and the air-coupled ultrasonic excitation transducer 4 to improve the impedance matching of the received ultrasonic signal and then introduce it into the air-coupled ultrasonic excitation transducer 4. The air-coupled ultrasonic excitation transducer 4 introduces the ultrasonic signal into the composite material 5 under test through the air, generates a zero group velocity Lamb wave signal inside the composite material, and is received by the air-coupled ultrasonic receiving transducer 7 on the other side.

[0070] a preamplifier 8 connected to the air-coupled ultrasonic receiving transducer 7 and the receiving end of the signal excitation receiver 2, amplifying the received zero-group-velocity Lamb wave signal and then introducing it into the signal excitation receiver 2;

[0071] The oscilloscope 9 is connected to the signal excitation receiver 2 and the computer 1 , and averages the zero group velocity Lamb wave signal multiple times before importing it into the computer 1 .

[0072] The device uses an air-coupled ultrasonic transducer to excite zero-group velocity Lamb waves in the composite material 5 under test, and evaluates the degree of impact damage of the composite material 5 under test by the relative magnitude of the zero-group velocity Lamb wave spectrum amplitudes at different test points of the composite material 5 under test; the smaller the spectrum amplitude I1 of the zero-group velocity Lamb wave spectrum signal at the test point containing impact damage, that is, the larger the difference ΔI between the zero-group velocity Lamb wave spectrum amplitudes between the damaged and intact test points, the greater the degree of damage at the test point containing impact damage in the composite material 5 under test, and the magnitude of ΔI is positively correlated with the degree of material damage.

[0073] Reference Figure 1The embodiment of the present invention discloses a composite material impact damage evaluation device based on zero group velocity Lamb wave, which mainly includes: a computer 1, a signal excitation receiver 2, an attenuator 3, an air-coupled ultrasonic excitation transducer 4, a detection platform 6, an air-coupled ultrasonic receiving transducer 7, a preamplifier 8, and an oscilloscope 9.

[0074] The detection platform 6 is used to fix the composite material 5 to be tested. The air-coupled ultrasonic excitation transducer 4 and the air-coupled ultrasonic receiving transducer 7 are respectively fixed on both sides of the composite material 5 to be tested. In this embodiment, the composite material to be tested is a carbon fiber resin-based composite material. During the detection process, the relative distance between the composite material to be tested 5 and the air-coupled ultrasonic excitation transducer 4 and the air-coupled ultrasonic receiving transducer 7 remains unchanged, and the two transducers are perpendicular to the board surface for the same test point, maintain a certain distance, and do not touch.

[0075] The signal excitation receiver 2 excites an ultrasonic signal, the center frequency of which is the theoretical frequency f of the zero group velocity Lamb wave of the composite material 5 being tested. th The zero group velocity point is the point with no slope in the fk relationship diagram of the dispersion curve of the composite material 5 under test. Its frequency f th is the theoretical frequency of the zero group velocity Lamb wave, where f is the frequency and k is the wave number.

[0076] The attenuator 3 receives the signal excited by the signal excitation receiver 2 , improves the impedance matching, and increases the signal-to-noise ratio, and then introduces the signal into the air-coupled ultrasonic excitation transducer 4 .

[0077] The air-coupled ultrasonic excitation transducer 4 introduces the signal into the composite material 5 under test through the air, generates a zero group velocity Lamb wave inside the composite material 5, and is received by the air-coupled ultrasonic receiving transducer 7 on the other side.

[0078] The preamplifier 8 receives the zero-group-velocity Lamb wave signal introduced by the air-coupled ultrasonic receiving transducer 7 and amplifies it, introduces it into the signal excitation receiver 2, and further introduces it into the oscilloscope 9.

[0079] The oscilloscope 9 displays the amplified zero-group-velocity Lamb wave signal, and after multiple averaging, inputs it into the computer 1 for signal processing and analysis.

[0080] The present invention also discloses a composite material impact damage evaluation method based on zero group velocity Lamb wave, which excites zero group velocity Lamb wave at each detection point of the composite material to be tested by an air-coupled ultrasonic transducer, receives and stores the zero group velocity Lamb wave signal, performs low-pass filtering and fast Fourier transform processing on the acquired zero group velocity Lamb wave signal, and measures the zero group velocity Lamb wave frequency of the intact detection point to be f ZGV , extract f respectively ZGVThe spectral amplitudes I0 and I1 of the zero-group velocity Lamb wave spectrum signals of the intact state and the impact damage detection point at the frequency are compared to evaluate the impact damage degree of the composite material under test. The smaller the value of I1, that is, the larger the difference ΔI between I0 and I1, the greater the impact damage degree of the composite material containing the impact damage detection point. Based on this method, by adjusting the relative position of the transducer and the composite material under test at different detection points, while keeping other conditions unchanged, the zero-group velocity Lamb wave signals at different detection points are obtained to achieve damage evaluation of different areas of the composite material under test; or by replacing the composite material under test, keeping other conditions unchanged, damage evaluation of different composite materials can be achieved.

[0081] This method is mainly based on the fact that after a composite material plate is damaged by impact, it may produce internal damage such as matrix cracking, fiber breakage, and delamination that are difficult to detect visually, resulting in stiffness reduction of the material. The zero-group velocity Lamb wave is very sensitive to micro-damage of the material. Compared with the intact state, the zero-group velocity Lamb wave at the damage detection point decays faster and has a smaller amplitude. In addition, the amplitude change ΔI of the zero-group velocity Lamb wave spectrum is positively correlated with the degree of damage, which can effectively detect impact damage of composite materials.

[0082] In conjunction with this embodiment, the detection method of the device of the present invention is further described in detail:

[0083] 1) The composite material 5 to be tested is fixed horizontally on the testing platform 6. An air-coupled ultrasonic excitation transducer 4 and an air-coupled ultrasonic receiving transducer 7 are fixed on either side of the composite material 5, close to and perpendicular to the surface of the test platform. They are facing the same test point on the composite material 5, maintaining a fixed distance and not in contact. In this embodiment, the composite material 5 to be tested is a carbon fiber resin-based composite material with dimensions of 500mm x 500mm. The zero-group velocity Lamb wave signal is detected and analyzed at the same test point before and after damage. The test point is the exact center area of the composite material 5 to be tested.

[0084] 2) The theoretical frequency f of the zero group velocity of the composite material 5 under test is obtained by theoretical calculation th The dispersion curve in this embodiment is as follows: Figure 2 As shown, where f th =705.4kHz;

[0085] 3) The computer controls the signal to stimulate the receiver 2, and excites the center frequency at f th The ultrasonic excitation signal is a Hanning window weighted signal with a period number of n. Figure 3 and Figure 4 As shown, the center frequency is 700kHz and the number of cycles n=30;

[0086] 4) After passing through the attenuator 3 to improve impedance matching and increase the signal-to-noise ratio, the ultrasonic signal reaches the air-coupled ultrasonic excitation transducer 4, enters the composite material under test 5 through air coupling, propagates inside the composite material, generates a zero-group-velocity Lamb wave signal, and is received by the air-coupled ultrasonic receiving transducer 7 on the other side;

[0087] 5) The zero-group velocity Lamb wave signal is amplified by the air-coupled ultrasonic receiving transducer 7 through the preamplifier 8 and then enters the signal excitation receiver 2. The signal is then input into the oscilloscope 9. The zero-group velocity Lamb wave signal is observed by the oscilloscope 9 and averaged multiple times before being input into the computer 1 to obtain a more stable and reliable signal. In this embodiment, the number of averages is 128.

[0088] 6) The zero group velocity Lamb wave signal is processed by a 900 kHz low-pass filter through the computer 1 to obtain the zero group velocity Lamb wave signal of the tested composite material 5 in a good state as shown in FIG. Figure 5 As shown in , it can be seen that the zero group velocity Lamb wave signal will exist for a long time;

[0089] 7) The zero group velocity Lamb wave signal is processed by fast Fourier transform using MATLAB software. In order to make the zero group velocity Lamb wave signal more obvious, a Hanning window is applied to the zero group velocity Lamb wave region (200μs-380μs) of the time domain signal. The amplitude-frequency curve results are shown as follows: Figure 7 Shown by the solid line.

[0090] 8) A low-speed impact is applied to the same test point of the composite material 5 under test. After the impact, there is no obvious damage to the surface visible to the naked eye. Other test conditions remain unchanged. The above operation is repeated for the test piece with impact damage to obtain the corresponding zero-group-velocity Lamb wave signal result, such as Figure 6 and Figure 7 Indicated by the dashed line. In this embodiment, the impact energy is about 25J;

[0091] 9) Compare the zero group velocity Lamb wave signal results of the tested composite material 5 before and after damage. Figure 7 As shown, in this embodiment, when the composite material 5 is in good condition, the zero group velocity Lamb wave frequency f at the detection point is ZGV =689kHz, extract f ZGV The spectrum amplitudes I0 and I1 of the same test point of the tested composite material 5 before and after damage at the frequency are compared. It can be found that the spectrum amplitude I1 after damage is much smaller than the spectrum amplitude I0 before damage; where ΔI = I0-I1, the larger ΔI is, the greater the degree of damage to the material, and the size of ΔI can be used to evaluate the degree of impact damage to the material.

[0092] In summary, the present invention is a new technology for evaluating impact damage of composite materials based on air-coupled ultrasonic transducers and zero-group velocity Lamb waves. The difference ΔI between the zero-group velocity Lamb wave spectrum amplitudes in two states, whether or not it contains damage, is used to evaluate the degree of damage to the material. The main principle of the present invention is that after a composite material is damaged by impact, invisible micro-damage will be generated inside the material. The zero-group velocity Lamb wave is very sensitive to early micro-damage inside the material. The greater the degree of damage, the greater the attenuation of the zero-group velocity Lamb wave and the smaller the amplitude, thereby effectively evaluating the degree of impact damage to the composite material. The present invention has strong applicability, and has the advantages of non-contact, local detection, and high sensitivity. It can quickly and effectively evaluate the degree of impact damage to composite materials.

[0093] Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments and application fields. The above-mentioned specific embodiments are merely illustrative and instructive, and are not restrictive. A person skilled in the art, guided by this specification and without departing from the scope of protection of the claims of the present invention, may also devise various forms, all of which fall within the scope of protection of the present invention.

Claims

1. A composite material impact damage evaluation method based on zero group velocity Lamb wave, characterized in that: It includes the following steps: Step 1: For any test point of the composite material to be tested, place an air-coupled ultrasonic transducer on both sides of the composite material surface perpendicular to the composite material surface; Step 2: Calculate the theoretical frequency of the zero-group-velocity Lamb wave of the composite material being tested, wherein an excitation signal of the Lamb wave with the theoretical frequency as the center frequency is excited by an air-coupled ultrasonic transducer, and the zero-group-velocity Lamb wave signal is received by an air-coupled ultrasonic transducer on the other side of the composite material being tested; Step 3: Process the received zero group velocity Lamb wave signal to obtain the spectrum amplitude of the zero group velocity Lamb wave signal to evaluate the impact damage of the composite material being tested.

2. The method according to claim 1, characterized in that Preferably, in step 2, the excitation signal comprises a Hanning window weighted signal, the center frequency of which is the zero group velocity Lamb wave theoretical frequency f of the composite material being tested. th .

3. The method according to claim 1, characterized in that In step 2, the Lamb wave dispersion curve fk relationship diagram of the tested composite material is calculated to obtain its zero group velocity point. The zero group velocity point is the point where the slope of the dispersion curve fk relationship diagram does not exist, where f is the frequency and k is the wave number.

4. The method according to claim 1, wherein In step 3, the collected zero-group-velocity Lamb wave signal is processed by a computer, low-pass filtered, and fast Fourier transform is performed on the zero-group-velocity Lamb wave signal to obtain the corresponding spectrum signal to evaluate the impact damage of the composite material under test.

5. A device for implementing the method according to any one of claims 1 to 4, characterized in that: It includes, The testing platform fixes the composite material to be tested. an air-coupled ultrasonic excitation transducer fixed to one side of the composite material to be measured to adjustably emit an excitation signal to generate a zero-group-velocity Lamb wave signal through the composite material to be measured, an air-coupled ultrasonic receiving transducer fixed on the other side of the composite material to be tested relative to the air-coupled ultrasonic excitation transducer to receive the zero-group-velocity Lamb wave signal; A computer is connected to the air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer, and the computer adjusts the excitation signal emitted by the air-coupled ultrasonic excitation transducer and generates a spectrum amplitude based on the zero group velocity Lamb wave signal to evaluate the impact damage of the tested composite material.

6. The device according to claim 5, characterized in that The composite materials to be tested include carbon fiber resin-based composite materials, glass fiber resin-based composite materials, boron fiber metal-based composite materials and carbon fiber ceramic-based composite materials.

7. The device according to claim 5, characterized in that The air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer are respectively fixed at fixed distances on both sides of the composite material to be tested, perpendicular to the plate surface of the composite material to be tested and aimed at the same point to be tested. During the detection process, the relative distance between the air-coupled ultrasonic excitation transducer and the air-coupled ultrasonic receiving transducer and the composite material to be tested always remains unchanged.

8. The device according to claim 5, characterized in that Also includes, A signal excitation receiver is connected to the computer to excite an ultrasonic signal, the center frequency of which is the theoretical frequency f of the zero group velocity Lamb wave of the composite material being tested. th , the zero group velocity point is the point with no slope in the fk relationship diagram of the dispersion curve of the tested composite material, where f is the frequency and k is the wave number; The attenuator is connected to the output end of the signal excitation receiver and the air-coupled ultrasonic excitation transducer to improve the impedance matching of the received ultrasonic signal and then introduce it into the air-coupled ultrasonic excitation transducer. The air-coupled ultrasonic excitation transducer introduces the ultrasonic signal into the composite material under test through the air, generates a zero group velocity Lamb wave signal inside the composite material, and is received by the air-coupled ultrasonic receiving transducer on the other side. A preamplifier is connected to the air-coupled ultrasonic receiving transducer and the receiving end of the signal excitation receiver, amplifies the received zero-group-velocity Lamb wave signal and then introduces it into the signal excitation receiver; The oscilloscope is connected to the signal excitation receiver and the computer, and averages the zero group velocity Lamb wave signal multiple times before importing it into the computer.

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