Surface stress detection method of carbon fiber composite plate based on ultrasonic critical refraction longitudinal wave

Abstract

The present application provides a method for detecting surface stress of a carbon fiber composite plate based on ultrasonic critical refracted longitudinal waves, which adopts a stress detection system composed of an industrial computer, a pulse excitation card, a variable angle wedge, an ultrasonic transducer, and a data acquisition card. The method includes: emitting ultrasonic waves to the carbon fiber composite plate to obtain the ultrasonic incident angle when the critical refracted longitudinal wave is excited on the surface of the carbon fiber composite plate; fixing the variable angle wedge and the ultrasonic transducer to the area to be detected of the carbon fiber composite plate according to the ultrasonic incident angle; determining a stress detection model according to the material and layup of the carbon fiber composite plate; detecting the area to be detected of the carbon fiber composite plate by using the stress detection system, and calculating the stress state of the area to be detected according to the propagation sound time of the ultrasonic wave and the stress detection model.

Description

Technical Field

[0001] The present application relates to the technical field of ultrasonic nondestructive testing and structural health monitoring, and in particular to a method for detecting surface stress of carbon fiber composite plates based on ultrasonic critical refraction longitudinal waves. Background Art

[0002] In ultrasonic nondestructive testing, the critically refracted longitudinal wave (LCR wave) method has many applications, including material stress detection based on the principle of acoustic elasticity and material damage detection based on the principle of ultrasonic nonlinearity. Research on stress detection using the critically refracted longitudinal wave method started early and is usually used to detect welding stress and track residual stress. In the field of metal materials, the technology of using critically refracted longitudinal waves for stress measurement is relatively mature and has many applications in stress detection of bolts, building steel frame structures, and high-speed rail components. Its detection system has the advantages of portability, high accuracy, and fast response, and is an important means of structural health monitoring. In recent years, the feasibility of applying ultrasonic critically refracted longitudinal wave detection method to more materials and occasions has been proven.

[0003] Carbon fiber composites, with their high strength, high stiffness-to-weight ratio, and corrosion resistance, are currently one of the most widely used materials in civil infrastructure and are crucial for repairing and reinforcing existing structures. Stress testing of composite materials using the critical refraction longitudinal wave method is also feasible, as the relationship between ultrasonic propagation time and stress is opposite to that of metals. Because composite materials can be produced using a variety of different layup processes, each resulting in distinct structural characteristics and exhibiting strong anisotropy, using the ultrasonic critical refraction longitudinal wave method to test stress in composite materials requires further research into structural characteristics and the dynamics of acoustic time differences. Summary of the Invention

[0004] In view of this, the present application proposes a method for detecting the surface stress of carbon fiber composite plates based on ultrasonic critical refraction longitudinal waves. Based on the critical refraction longitudinal waves excited on the surface of the composite material, the critical refraction longitudinal wave method can be used to perform stress detection along a specific direction on carbon fiber composite plates with different layup methods.

[0005] The present application provides a method for detecting surface stress of a carbon fiber composite plate based on ultrasonic critical refraction longitudinal waves, using a stress detection system consisting of an industrial computer, a pulse excitation card, a variable angle wedge, an ultrasonic transducer, and a data acquisition card. The method includes:

[0006] By emitting ultrasonic waves toward the carbon fiber composite plate, an ultrasonic incident angle when a critical refracted longitudinal wave is excited on the surface of the carbon fiber composite plate is obtained;

[0007] According to the ultrasonic incident angle, the variable angle wedge and the ultrasonic transducer are fixed to the area to be detected of the carbon fiber composite plate;

[0008] Determining a stress detection model according to the material and layup of the carbon fiber composite plate;

[0009] The stress detection system is used to detect the area to be detected of the carbon fiber composite plate, and the stress state of the area to be detected is calculated according to the propagation time of the ultrasonic wave and the stress detection model.

[0010] From the above, the present application provides a method for detecting the surface stress of a carbon fiber composite plate based on ultrasonic critical refracted longitudinal waves. By conducting an ultrasonic incident test on the carbon fiber composite plate, the ultrasonic incident angle when the critical refracted longitudinal wave is excited on the surface of the carbon fiber composite plate is obtained. The fixed angle of the variable angle wedge is adjusted according to the ultrasonic incident angle to fix the variable angle wedge and the ultrasonic transducer to the area to be detected of the carbon fiber composite plate. Then, through the stress detection system of the present application, an ultrasonic transducer at one end is used to transmit an ultrasonic wave to the area to be detected, and an ultrasonic transducer at the other end is used to receive the transmitted waveform. The stress state of the area to be detected can be calculated based on the propagation time of the ultrasonic wave and the stress detection model of the carbon fiber composite plate. By using the stress detection method of the present application, it is possible to use the critical refracted longitudinal wave method to perform stress detection along a specific direction on carbon fiber composite plates with different layup methods based on the critical refracted longitudinal wave excited on the surface of the composite material.

[0011] Optionally, fixing the variable angle wedge and the ultrasonic transducer to the area to be detected of the carbon fiber composite plate includes:

[0012] Two variable-angle wedges and two ultrasonic transducers are respectively fixed to the area to be detected of the carbon fiber composite plate. One group of ultrasonic transducers and variable-angle wedges receives the pulse wave from the pulse excitation card and transmits ultrasonic waves to the carbon fiber composite plate according to the ultrasonic incident angle. The other group of ultrasonic transducers and variable-angle wedges receives the waveform transmitted from the carbon fiber composite plate.

[0013] As described above, two variable-angle wedges and two ultrasonic transducers are respectively fixed to the area to be detected of the carbon fiber composite plate, wherein one group of ultrasonic transducers and variable-angle wedges receives the pulse wave from the pulse excitation card, and transmits ultrasonic waves to the carbon fiber composite plate according to the ultrasonic incident angle, and the other group of ultrasonic transducers and variable-angle wedges receive the waveform transmitted from the carbon fiber composite plate, thereby calculating the stress state of the area to be detected of the carbon fiber composite plate according to the propagation time of the ultrasonic wave and the stress detection model.

[0014] Optionally, also include:

[0015] The surfaces of the variable-angle wedge, the ultrasonic transducer, and the area to be detected of the carbon fiber composite plate are coupled using coupling agents.

[0016] From the above, in order to reduce the influence of air between the surfaces on ultrasonic wave propagation, it is necessary to use a coupling agent to couple the contacting surfaces of the variable angle wedge, ultrasonic transducer and carbon fiber composite plate.

[0017] Optionally, determining the stress detection model according to the material and ply of the carbon fiber composite plate includes:

[0018] According to the material and ply of the carbon fiber composite plate, a tensile test is performed on the carbon fiber composite plate to obtain ultrasonic propagation sound time under different stresses;

[0019] The stress detection model of the carbon fiber composite plate is determined according to the ultrasonic wave propagation sound under the different stresses.

[0020] From the above, by conducting a tensile test on the carbon fiber composite plate, the propagation time of the ultrasonic wave under different stresses can be obtained. According to the stress difference and the propagation time difference, the stress detection model of the carbon fiber composite plate can be fitted, wherein the stress and the propagation time are positively correlated.

[0021] Optionally, the variable angle wedge is fixed in such a manner that a propagation direction of the ultrasonic wave in the carbon fiber composite plate is parallel to a direction of one of the fibers in the carbon fiber composite plate.

[0022] From the above, when using this method for measurement, the propagation direction of the ultrasonic wave should be parallel to the direction of any fiber in the ply, and the measured stress value is the stress component in this direction.

[0023] Optionally, the thickness of the carbon fiber composite plate should be greater than 5 mm, and the surface of the plate should not be specially treated.

[0024] These and other aspects of the present application will become more apparent from the following description of the embodiment(s). BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic structural diagram of a stress detection system provided in an embodiment of the present application;

[0026] Figure 2 A schematic diagram of the structure of a variable angle wedge provided in an embodiment of the present application;

[0027] Figure 3 A schematic diagram of the ultrasonic wave propagation path provided in an embodiment of the present application;

[0028] Figure 4Flowchart of a method for detecting surface stress of a carbon fiber composite plate based on ultrasonic critical refraction longitudinal waves provided in an embodiment of the present application.

[0029] It should be understood that the sizes and shapes of the blocks in the above structural diagrams are for reference only and should not constitute an exclusive interpretation of the embodiments of this application. The relative positions and inclusion relationships between the blocks presented in the structural diagrams are only schematic representations of the structural relationships between the blocks, and do not limit the physical connection methods of the embodiments of this application. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings.

[0031] The embodiment of the present application provides a method for detecting surface stress of carbon fiber composite plates based on ultrasonic critical refraction longitudinal waves. Based on the critical refraction longitudinal waves excited on the surface of the composite material, the critical refraction longitudinal wave method is used to perform stress detection along a specific direction on carbon fiber composite plates with different layup methods.

[0032] like Figure 1 As shown, the surface stress detection method of carbon fiber composite plate based on ultrasonic critical refraction longitudinal wave provided in the present application adopts a stress detection system composed of an industrial computer 100, a pulse excitation card 200, a variable angle wedge block 300, an ultrasonic transducer 400, and a data acquisition card 500.

[0033] The industrial computer 100 may be a computer system that directly interacts with a person, and is directly connected to the pulse excitation card 200 and the data acquisition card 500. The industrial computer sends commands to the pulse excitation card 200 through the industrial computer, analyzes and displays the ultrasonic data recorded by the data acquisition card 500, and calculates and displays the stress state of the carbon fiber composite plate.

[0034] One end of the pulse excitation card 200 is connected to the industrial computer 100, and the other end is connected to the ultrasonic transducer 400. After receiving the command from the industrial computer 100, it sends a pulse wave to the ultrasonic transducer 400 to stimulate the ultrasonic transducer 400 to emit ultrasonic waves.

[0035] The data acquisition card 500 is connected to the ultrasonic transducer 400 responsible for receiving waveform information, collects the relevant electrical signals of the ultrasonic transducer 400, and sends them to the industrial computer 100;

[0036] like Figure 2As shown, the variable-angle wedge 300 is an auxiliary module between the ultrasonic transducer 400 and the carbon fiber composite sheet. It is used to fix the ultrasonic transducer 300, so that the ultrasonic transducer 300 forms a temperature-position relationship with the carbon fiber composite sheet, so that the ultrasonic waves emitted by it are transmitted to the surface of the carbon fiber composite sheet at a certain incident angle. The variable-angle wedge 300 has a coupling chamber, a coupling platform, a drainage groove, and a sound insulation groove structure. The coupling chamber is used to couple the ultrasonic transducer 400 to the variable-angle wedge 300 via a coupling agent, and the coupling platform is used to couple the variable-angle wedge 300 to the carbon fiber composite sheet via a coupling agent.

[0037] like Figure 3 As shown, the ultrasonic transducer 400 can convert input electrical power into ultrasonic waves before transmitting them, and can also convert received ultrasonic waves into electrical power. This system uses two ultrasonic transducers 400 and two variable-angle wedges 300. One set of ultrasonic transducers 400 and variable-angle wedges 300 receives pulse waves from the pulse excitation card 200 and transmits ultrasonic waves toward the carbon fiber composite sheet according to the ultrasonic incident angle. The other set of ultrasonic transducers 400 and variable-angle wedges 300 receives the waveforms transmitted from the carbon fiber composite sheet, converts them into electrical power signals, and sends them to the data acquisition card 500.

[0038] like Figure 4 As shown, the method for detecting surface stress of carbon fiber composite plate based on ultrasonic critical refraction longitudinal wave provided by the present application includes the following steps:

[0039] S100: transmitting ultrasonic waves to the carbon fiber composite plate to obtain an ultrasonic incident angle when a critical refracted longitudinal wave is excited on the surface of the carbon fiber composite plate;

[0040] In this step, COMSOL can be used to simulate the propagation of ultrasound through an ultrasonic wedge (not the variable-angle wedge used in the actual experiment, but a replacement designed for the simulation) and a carbon fiber material. Ultrasonic waves are excited from the left wedge and, upon entering the carbon fiber material, excite refracted longitudinal waves, refracted shear waves, and head waves. The refracted longitudinal wave is refracted again into the right wedge and received by the sensor. This allows the optimal ultrasonic incident angle to be determined for exciting the critical refracted longitudinal wave on the surface of the carbon fiber composite sheet.

[0041] S200: fixing the variable angle wedge and the ultrasonic transducer to the area to be detected of the carbon fiber composite plate according to the ultrasonic incident angle;

[0042] The thickness of the carbon fiber composite sheet used in this embodiment should be greater than 5mm, and the surface of the sheet does not require special treatment. The carbon fiber composite sheet is placed on a horizontal flat surface, and two variable-angle ultrasonic wedges responsible for transmission and reception are placed on the surface of the sheet. An ultrasonic coupling agent is used to couple the material surface with the surface of the variable-angle ultrasonic wedge, and between the variable-angle ultrasonic wedge and the ultrasonic transducer. The edges of the two variable-angle ultrasonic wedges are aligned with the edges of the sheet to ensure that the centers of the wedges are collinear. Depending on the different fiber laying methods of the carbon fiber composite sheet, the wedges should be placed so that the propagation direction of the ultrasonic refracted wave is parallel to the fiber direction of one of the sheets.

[0043] S300: Determine a stress detection model according to the material and ply of the carbon fiber composite plate;

[0044] based on Figure 3 The propagation path of the ultrasonic wave is shown in FIG. 1 . The time taken for the ultrasonic wave to travel from the transmitting end to the receiving end can be expressed as:

[0045] t0=2×t w +t m0

[0046] Where t0 is the total acoustic time of ultrasonic wave propagation between two ultrasonic transducers measured by the detection system under no stress, t w is the propagation time of ultrasonic wave in the variable angle wedge, t m0 is the time it takes for the ultrasonic wave to propagate in the material.

[0047] When the material is under stress, the time it takes for the ultrasonic wave to travel from the transmitting end to the receiving end can be expressed as:

[0048] t1=2×t w +t m1

[0049] Where t1 is the total acoustic time of ultrasonic wave propagation between two ultrasonic transducers measured by the detection system under stress state, t w is the time it takes for the ultrasonic wave to propagate in the wedge. Since the wedge is not subjected to stress, this term remains unchanged. m1 is the time it takes for an ultrasonic wave to propagate in a material under stress.

[0050] The time difference Δt obtained during the detection is:

[0051] Δt=t1-t0=t m1 -t m0

[0052] Also available:

[0053]

[0054] Where v1 and v0 are the sound velocities in the stressed and unstressed states, respectively, and l is the ultrasonic path length in the material. In this method, since the material path length is fixed, the variable Δt can reflect the change in sound velocity.

[0055] According to the acoustic elasticity theorem, when an isotropic solid material is subjected to stress in one direction, the relationship between the ultrasonic longitudinal wave propagating along the stress direction and the stress is basically linear, which can be expressed by derivation and simplification as follows:

[0056] Δσ=K·Δt

[0057] Carbon fiber composites are anisotropic materials, and the ultrasonic phenomena caused by internal stress differ somewhat from the classical acoustoelasticity principle. Experiments have shown that the variation in ultrasonic velocity caused by internal stress in carbon fiber composites is similar to that described in the acoustoelasticity theorem. To ensure the accuracy of the detection model, this method, based on the classical formula, designs the following stress calculation model for composite materials:

[0058]

[0059] The following explains this model: stress σ is a function of time difference Δt. However, during the calibration process, Δt data is directly obtained under known stress states, so the model uses σ as a variable to fit the stress difference Δt. Experimental findings show that the variation of acoustic time difference at equal stress intervals is more pronounced depending on the type of composite material being measured. Therefore, let f(σ) = Δt(σ) - Δt(σ - dσ). This function actually represents the difference in time differences between two adjacent stress recording points. This function reflects the variation in the slope of the fitting function Δt(σ). Therefore, integrating f(σ) yields the stress calculation formula Δt(σ). Here, we call f(σ) the adjacent difference fitting function and Δt(σ) the acoustic time difference fitting function.

[0060] Depending on the different layup methods of composite materials, f(σ) has different forms and should be fitted with different functions. We will now discuss the two cases of unidirectional composite plates, orthogonal layups, and other layups separately:

[0061] (1) Stress calculation model of one-way plate composite materials

[0062] Compared with other complex layup methods, the ultrasonic parameters of unidirectional laminated plates change stably, and the acoustic time difference changes more evenly with stress. However, as σ increases, the acoustic time difference between adjacent stress intervals still tends to decrease. Therefore, a linear fitting method is used to fit f(σ):

[0063] f(σ)=a·σ+b

[0064] (2) Stress calculation models for orthogonal and other laminated composite materials

[0065] Unlike single-ply lamination, as the ply structure becomes more complex, the nonlinearity of acoustic transit time is more pronounced at relatively low stresses. As stress increases, f(σ) gradually becomes linear. This curve has a multi-short feature, so a piecewise function is used to fit f(σ):

[0066]

[0067] Where σ0 is the linear turning point of the fitting function, which is set manually. When σ>σ0, the m and n parameters are determined according to the linear law of adjacent differences; when σ≤σ0, the a, b, and c parameters are determined based on the continuity and smoothness of the two-segment function.

[0068] In some embodiments, the wedge sound path used in the calibration experiment in this solution does not have to be the same as the wedge sound path used in the actual test. Let the sound path in the calibration experiment be l and the sound path in the test be l1. In this case, the distance factor is defined as m, and

[0069]

[0070] At this time, the stress calculation model is replaced by:

[0071]

[0072] In this embodiment, the setting of the distance factor increases the flexibility of the detection area.

[0073] S400: Detecting the to-be-detected area of ​​the carbon fiber composite plate by the stress detection system, and calculating the stress state of the to-be-detected area according to the propagation time of the ultrasonic wave and the stress detection model.

[0074] According to the aforementioned stress detection system, an industrial computer sends a command to the pulse excitation card. The pulse excitation card, in response to the command, sends a pulse wave to the ultrasonic transducer, stimulating the ultrasonic transducer to emit ultrasonic waves. The ultrasonic transducer and the variable-angle wedge then transmit the ultrasonic waves at a fixed angle of incidence toward the area to be detected on the carbon fiber composite sheet. After propagating through the sheet for a period of time, the ultrasonic waves are received by the ultrasonic transducer at the other end, converted into electrical power signals, and sent to the data acquisition card. The data acquisition card then sends the collected electrical signals to the industrial computer. Based on the propagation time of the ultrasonic waves and the aforementioned stress detection model, the industrial computer can calculate the stress state of the area to be detected on the carbon fiber composite sheet.

[0075] It should be noted that the embodiments described in this application are only a part of the embodiments of this application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the above detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the application for protection, but merely represents selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application.

[0076] The words "first, second, third, etc." or module A, module B, module C and other similar terms in the specification and claims are only used to distinguish similar objects and do not represent a specific ordering of the objects. It is understandable that the specific order or sequence can be interchanged where permitted so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0077] In the above description, the numbers representing the steps involved do not necessarily mean that the steps must be executed. Intermediate steps may also be included or replaced by other steps. If permitted, the order of the previous and next steps may be interchanged or executed simultaneously.

[0078] The term "comprising" as used in the specification and claims should not be construed as limiting to what is listed thereafter; it does not exclude other elements or steps. Thus, it should be interpreted as specifying the presence of the features, integers, steps, or components mentioned, but not excluding the presence or addition of one or more other features, integers, steps, or components, or groups thereof. Thus, the expression "a device comprising means A and B" should not be limited to a device consisting solely of components A and B.

[0079] The term "one embodiment" or "an embodiment" mentioned in this specification means that the specific features, structures, or characteristics described in conjunction with the embodiment are included in at least one embodiment of the present application. Therefore, the phrases "in one embodiment" or "in an embodiment" appearing in various places in this specification do not necessarily refer to the same embodiment, but may refer to the same embodiment. In addition, in the various embodiments of the present application, unless otherwise specified or there is a logical conflict, the terms and / or descriptions between different embodiments are consistent and can be referenced to each other. The technical features in different embodiments can be combined to form new embodiments based on their inherent logical relationships.

[0080] Note that the above are only preferred embodiments of the present application and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present application has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A method for detecting surface stress of carbon fiber composite sheet based on ultrasonic critical refraction longitudinal wave, characterized in that: A stress detection system consisting of an industrial computer, a pulse excitation card, a variable angle wedge, an ultrasonic transducer, and a data acquisition card is used, and the method includes: By emitting ultrasonic waves toward the carbon fiber composite plate, an ultrasonic incident angle when a critical refracted longitudinal wave is excited on the surface of the carbon fiber composite plate is obtained; According to the ultrasonic incident angle, the variable angle wedge and the ultrasonic transducer are fixed to the area to be detected of the carbon fiber composite plate; According to the material and ply of the carbon fiber composite plate, a tensile test is performed on the carbon fiber composite plate to obtain ultrasonic propagation sound times under different stresses; based on the ultrasonic propagation sound times under different stresses, a stress detection model of the carbon fiber composite plate is determined, and the stress detection model includes: Wherein, σ is the stress to be detected, Δt is the time difference of the ultrasonic wave passing through the area to be detected, f(σ) is the adjacent difference fitting function, Δt(σ) is the acoustic time difference fitting function, and the function f(σ) reflects the variation law of the slope of the function Δt(σ). The stress detection model is obtained by integrating f(σ); The stress detection system is used to detect the area to be detected of the carbon fiber composite plate, and the stress state of the area to be detected is calculated according to the propagation time of the ultrasonic wave and the stress detection model.

2. The method according to claim 1, characterized in that The step of fixing the variable angle wedge and the ultrasonic transducer to the area to be detected of the carbon fiber composite plate comprises: Two variable-angle wedges and two ultrasonic transducers are respectively fixed to the area to be detected of the carbon fiber composite plate. One group of ultrasonic transducers and variable-angle wedges receives the pulse wave from the pulse excitation card and transmits ultrasonic waves to the carbon fiber composite plate according to the ultrasonic incident angle. The other group of ultrasonic transducers and variable-angle wedges receives the waveform transmitted from the carbon fiber composite plate.

3. The method according to claim 1 or 2, characterized in that Also includes: The surfaces of the variable-angle wedge, the ultrasonic transducer, and the area to be detected of the carbon fiber composite plate are coupled using coupling agents.

4. The method according to claim 1, wherein The variable angle wedge is fixed in such a manner that a propagation direction of ultrasonic waves in the carbon fiber composite plate is parallel to a direction of one of the fibers in the carbon fiber composite plate.

5. The method according to claim 1, characterized in that The thickness of the carbon fiber composite plate is greater than 5 mm, and the surface of the plate has no special treatment.

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