Anisotropic material ultrasonic stress coefficient calibration method

Abstract

According to the anisotropic material ultrasonic stress coefficient calibration method, the ultrasonic pulse flight time of a to-be-calibrated test piece without stress and the ultrasonic pulse flight time corresponding to different stress values of the to-be-calibrated test piece in a set detection direction and a corresponding stress direction are obtained through a tensile experiment; obtaining sound time differences corresponding to different stress values in a set detection direction and a corresponding stress direction, and performing least square method linear fitting according to a relation curve of the stress values and the sound time differences in the detection direction and the corresponding stress direction to obtain a fitting straight slope value; changing the set detection direction and the stress direction corresponding to the detection direction for multiple times, repeating the experiment to obtain a plurality of fitting straight slope values, and obtaining a characteristic ultrasonic stress coefficient of the to-be-calibrated test piece according to the obtained fitting straight slope values; and the ultrasonic stress coefficient mathematical model of the to-be-calibrated test piece is further obtained, and the method is suitable for stress detection of an alloy material and a composite material plane structure with weak anisotropy.

Description

Technical Field

[0001] The present invention relates to the field of stress detection of planar structures of anisotropic materials, and particularly to a method for calibrating ultrasonic stress coefficients of anisotropic materials. Background Art

[0002] With the continuous popularization and wide application of new materials, anisotropic materials with excellent performance are gradually used in industrial equipment fields such as aviation, new energy equipment, and automobiles. Stress widely exists in mechanical components. At present, the measurement of structural stress of anisotropic materials is roughly divided into two methods: destructive testing and non-destructive testing. The non-destructive testing method does not affect the mechanical properties of materials and structures, and has been widely used for the reliability verification of components and the structural safety assessment.

[0003] Among non-destructive testing methods, ultrasonic non-destructive testing has strong reliability and wide applicability. Among them, the stress detection technology of the critical refraction longitudinal wave acoustic time difference method has matured. Based on the strong sensitivity of wave velocity to stress, it is widely used in the field of stress detection. However, for the stress detection results of some anisotropic materials and isotropic materials, it is difficult to evaluate and verify the accuracy; or only the acoustoelastic coefficients in some main directions can be measured, and the stress in any other direction cannot be detected; in addition, there is no unified calibration standard for the calibration method of the ultrasonic stress coefficient of new materials, resulting in the inability of ultrasonic stress detection technology to be quickly used in the detection of new processes and new materials. Summary of the Invention

[0004] To solve the problem in the prior art that the stress in any direction cannot be detected for some anisotropic materials and isotropic materials, a method for calibrating ultrasonic stress coefficients of anisotropic materials is proposed;

[0005] A method for calibrating ultrasonic stress coefficients of anisotropic materials includes:

[0006] Step 1: Set the detection direction, and measure the ultrasonic pulse flight time in the detection direction of the specimen to be calibrated without stress through an ultrasonic acoustic time measurement device;

[0007] Step 2: Set the stress direction corresponding to the detection direction, and measure the ultrasonic pulse flight times corresponding to different stress values of the specimen to be calibrated in the set detection direction and the corresponding stress direction through an ultrasonic acoustic time measurement device; Subtract the ultrasonic pulse flight times corresponding to different stress values measured from the ultrasonic pulse flight time in the detection direction of the specimen to be calibrated without stress to obtain the acoustic time differences corresponding to different stress values in the set detection direction and the corresponding stress direction, and obtain the relationship curve between the stress values and the acoustic time differences in the set detection direction and the corresponding stress direction;

[0008] Step 3: Perform least-squares linear fitting on the relationship curve between the stress values and the acoustic time differences in the obtained detection direction and the corresponding stress direction to obtain the slope value of the fitting straight line;

[0009] Step 4: Change the set detection direction and the corresponding stress direction multiple times, and repeat Steps 1 to 3 according to the changed detection direction and the corresponding stress direction to obtain multiple slope values of the fitting straight lines. Obtain the characteristic ultrasonic stress coefficient of the specimen to be calibrated based on all the obtained slope values of the fitting straight lines;

[0010] Step 5: Obtain the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated based on the obtained characteristic ultrasonic stress coefficient of the specimen to be calibrated.

[0011] Beneficial effects

[0012] An ultrasonic stress coefficient calibration method for anisotropic materials in this application obtains the ultrasonic pulse flight time in the detection direction of the specimen to be calibrated without stress and the ultrasonic pulse flight times corresponding to different stress values under the set detection direction and the corresponding stress direction of the specimen to be calibrated through a tensile experiment, and takes the difference between the two to obtain the acoustic time differences corresponding to different stress values in the set detection direction and the corresponding stress direction, and obtains the relationship curve between the stress values and the acoustic time differences in the set detection direction and the corresponding stress direction; perform least-squares linear fitting on the relationship curve between the stress values and the acoustic time differences in the obtained detection direction and the corresponding stress direction to obtain the slope value of the fitting straight line; change the set detection direction and the corresponding stress direction multiple times, and repeat the experiment according to the changed detection direction and the corresponding stress direction to obtain multiple slope values of the fitting straight lines. Obtain the characteristic ultrasonic stress coefficient of the specimen to be calibrated based on the obtained multiple slope values of the fitting straight lines; and further obtain the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated.

[0013] This method has the advantages of accuracy, reliability, and strong applicability. It is widely applicable to the stress detection of plane structures of alloy materials and composite materials with weak anisotropy, greatly improving the accuracy, reliability, and consistency of ultrasonic stress detection results. It can be widely used for non-destructive testing and reliability assessment of plane stresses in alloy plates and composite material laminates in the fields of aerospace, power equipment manufacturing, new energy equipment manufacturing, etc. Description of the drawings

[0014] Figure 1 It is a flow chart of an ultrasonic stress coefficient calibration method for anisotropic materials in a specific embodiment of this application;

[0015] Figure 2 It is a schematic diagram of the stress direction, detection direction, specimen main direction, and material main direction in an ultrasonic stress coefficient calibration method for anisotropic materials in a specific embodiment of this application;

[0016] Figure 3 Structural diagram of the ultrasonic time measurement device in the specific embodiment of the present application;

[0017] Figure 4 Partial schematic diagram of the ultrasonic time measurement device in the specific embodiment of the present application;

[0018] Figure 5 Relationship curve between stress value and time difference of ultrasonic wave in the specific embodiment of the present application;

[0019] Figure 6 Mathematical model of ultrasonic stress coefficient in the embodiment of the present application;

[0020] Figure 7 Comparison result of calibration result and verification result of ultrasonic stress coefficient in the embodiment of the present application. Specific embodiment

[0021] Specific Embodiment 1: The following will combine the attached drawings in the embodiments of the present invention Figure 1 to Figure 7 to clearly and completely describe the technical solutions in the embodiments of the present invention:

[0022] Step 1: Set the detection direction, and measure the ultrasonic pulse flight time in the detection direction of the specimen to be calibrated without stress through the ultrasonic time measurement device;

[0023] Step 2: Set the stress direction corresponding to the detection direction, and measure the ultrasonic pulse flight time corresponding to different stress values of the specimen to be calibrated in the set detection direction and the corresponding stress direction through the ultrasonic time measurement device; Subtract the ultrasonic pulse flight time corresponding to different stress values measured from the ultrasonic pulse flight time in the detection direction of the specimen to be calibrated without stress to obtain the time difference of ultrasonic wave corresponding to different stress values in the set detection direction and the corresponding stress direction, and obtain the relationship curve between the stress value and the time difference of ultrasonic wave in the set detection direction and the corresponding stress direction;

[0024] Step 3: Perform least squares linear fitting on the relationship curve between the stress value and the time difference of ultrasonic wave in the obtained detection direction and the corresponding stress direction to obtain the slope value of the fitting straight line;

[0025] Step 4: Change the set detection direction and the stress direction corresponding to the detection direction multiple times, and repeat Steps 1 to 3 according to the changed detection direction and the corresponding stress direction to obtain multiple slope values of the fitting straight line, and obtain the characteristic ultrasonic stress coefficient of the specimen to be calibrated according to all the obtained slope values of the fitting straight line;

[0026] Step 5: Obtain the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated according to the obtained characteristic ultrasonic stress coefficient of the specimen to be calibrated.

[0027] Specifically, such asFigure 2 As shown, X is the fiber direction of the specimen, X0 is the main direction of the specimen to be calibrated. The main direction is the direction in which the mechanical properties of the material are the greatest. The Y0 direction is perpendicular to the X0 direction, and X i is the detection direction, and σ i is the stress direction; the angle between the detection direction and the main direction of the specimen to be calibrated is ω, the angle between the stress direction and the main direction of the specimen to be calibrated is θ, and the angle between the fiber direction of the specimen and the main direction of the specimen to be calibrated is Φ;

[0028] Set the detection direction, and measure the ultrasonic pulse flight time t0 in the detection direction of the specimen to be calibrated without stress through the ultrasonic time measurement device. The sound path is L;

[0029] As Figure 3 shown, conduct a tensile experiment. For the specimen to be calibrated 2, use the tensile machine 1 to induce a stress with an angle of θ with the main direction of the specimen to be calibrated in the specimen to be calibrated 2. The angle between the detection direction and the main direction of the specimen to be calibrated is ω. Measure the ultrasonic pulse flight times corresponding to different stress values of the specimen to be calibrated under the set detection direction and the corresponding stress direction through the ultrasonic time measurement device;

[0030] Subtract the ultrasonic pulse flight times corresponding to different stress values measured from the ultrasonic pulse flight time t0 of the specimen to be calibrated without stress to obtain the time differences of sound corresponding to different stress values under the set detection direction and the corresponding stress direction, and obtain the relationship curve between the stress value and the time difference of sound under the set detection direction and the corresponding stress direction; perform a least-squares linear fit on the relationship curve between the stress value and the time difference of sound obtained under the detection direction and the corresponding stress direction to obtain the slope value of the fitted straight line;

[0031] Change the detection direction multiple times, use the tensile machine 1 to induce the stress direction corresponding to the detection direction in the specimen to be calibrated 2, repeat the above steps to obtain multiple slope values of the fitted straight lines, and obtain the characteristic ultrasonic stress coefficient of the specimen to be calibrated according to the multiple slope values of the fitted straight lines obtained; obtain the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated according to the characteristic ultrasonic stress coefficient of the specimen to be calibrated obtained.

[0032] An ultrasonic stress coefficient calibration method for anisotropic materials further includes:

[0033] Step six: Set the first verification detection direction and the first verification stress direction corresponding to the first verification detection direction. The first verification detection direction is the same as the first verification stress direction, and the angle between the first verification detection direction and the main direction of the material to be calibrated is Obtain the first verification ultrasonic stress coefficient value under the first verification detection direction and the corresponding first verification stress direction according to the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated obtained in step five

[0034] Step 7: Set the second verification detection direction, and measure the ultrasonic pulse flight time in the second verification detection direction of the specimen to be calibrated when there is no stress through the ultrasonic time measurement device; set the second verification stress direction corresponding to the second verification detection direction, and the second verification detection direction is the same as the second verification stress direction. Measure the ultrasonic pulse flight time corresponding to different stress values of the specimen to be calibrated under the set second verification detection direction and the corresponding second verification stress direction through the ultrasonic time measurement device; subtract the measured ultrasonic pulse flight time corresponding to different stress values from the ultrasonic pulse flight time in the second verification detection direction of the calibration specimen when there is no stress to obtain the acoustic time difference corresponding to different stress values under the set second verification detection direction and the corresponding second verification stress direction, and obtain the relationship curve between the acoustic time difference and the stress value under the second verification detection direction and the corresponding second verification stress direction; perform least squares linear fitting on the obtained relationship curve between the acoustic time difference and the stress value under the second verification detection direction and the corresponding second verification stress direction to obtain the slope value of the second verification fitting straight line

[0035] Step 8: Subtract the obtained first verification ultrasonic stress coefficient value from the slope value of the second verification fitting straight line to obtain the deviation value, and judge whether the deviation value is greater than the set threshold. If so, return to Step 1; if not, end

[0036] Specifically, carry out the ultrasonic detection verification experiment, adopt the uniaxial prestress loading method, and obtain the slope value of the second verification fitting straight line

[0037] Furthermore, the stress value is taken in the range of 0 to σ e ; the stress value step size is in the range of σ e / 20 to σ e / 10; σ e is the elastic limit corresponding to the material to be calibrated and the stress direction

[0038] Specifically, the stress step size is selected according to the mechanical properties of the calibration material. Taking carbon fiber composite material as an example, the stress step size value in the fiber direction of the unidirectional ply of a 5-mm-thick laminate is 10 MPa, and the step size in the fiber direction of the cross-ply is 5 MPa

[0039] Furthermore, change the set detection direction and the corresponding stress direction multiple times, and repeat Steps 1 to 3 according to the changed detection direction and the corresponding stress direction to obtain multiple slope values of the fitting straight line, including:

[0040] Determine whether the specimen to be calibrated is an orthotropic material. If so, change the detection direction and the corresponding stress direction set N1 times; repeat steps 1 to 3 each time the detection direction and the corresponding stress direction are changed to obtain a fitting straight line slope value k θ,ω,i , until N1 fitting straight line slope values ​​are obtained; where N1≥4, N1 is an integer, i∈N1;

[0041] If the specimen to be calibrated is a transversely isotropic material, change the detection direction and the corresponding stress direction set N2 times, and repeat steps 1 to 3 each time the detection direction and the corresponding stress direction are changed to obtain a fitting straight line slope value k θ,ω,j , until N2 fitting straight line slope values ​​are obtained; where N2 ≥ 3, N2 is an integer, j∈N j ;

[0042] Furthermore, the characteristic ultrasonic stress coefficient of the specimen to be calibrated is obtained according to the slope values ​​of all fitted straight lines, including:

[0043] The specimen to be calibrated is an orthotropic material, and N1+1 fitting straight line slope values ​​are obtained, which are ;

[0044] The characteristic ultrasonic stress coefficients of the specimen to be calibrated include a first stress coefficient a, a second stress coefficient b, a third stress coefficient c and a fourth stress coefficient d;

[0045] k θ,ω,i =a+b(cos 2θ i +cos 2ω i )+c cos 2θ i cos 2ω i +d sin 2θ i sin 2ω i ;

[0046] Among them, k θ,ω,i Indicates the i-th detection direction X i and the corresponding stress direction σ i The slope of the fitted straight line of the specimen to be calibrated, θ i is the stress direction σ i Angle with the main direction of the material to be calibrated, ω i X is the detection direction i Angle with the main direction of the material to be calibrated;

[0047] The obtained N1+1 fitting straight line slope values ​​are combined to obtain the values ​​of the first stress coefficient a, the second stress coefficient b, the third stress coefficient c and the fourth stress coefficient d.

[0048] Specifically, for orthotropic materials, there are at least 4 independent stress coefficient equations. The combination of different stress directions and detection directions will result in the same data results, and such results should be avoided. The characteristic ultrasonic stress coefficients a, b, c, and d of the calibration material are calculated. According to the characteristic ultrasonic stress coefficient, a relationship diagram between any direction in the plane and the ultrasonic stress coefficient is drawn to obtain the linear relationship between the stress in any direction and the acoustic time difference, that is, the mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated;

[0049] Furthermore, the characteristic ultrasonic stress coefficient of the specimen to be calibrated is obtained according to the slope values ​​of all fitted straight lines, including:

[0050] The specimen to be calibrated is a transversely isotropic material, and N2+1 slope values ​​of the fitting straight line are obtained, which are

[0051] The characteristic ultrasonic stress coefficients of the specimen to be calibrated include a first stress coefficient a, a third stress coefficient c and a fourth stress coefficient d;

[0052] k θ,ω,j =a+c cos 2θ j cos 2ω j +d sin 2θ j sin 2ω j ;

[0053] Among them, k θ,ω,j Indicates the jth detection direction X j and the corresponding stress direction σ j The slope of the fitted straight line of the specimen to be calibrated, θ j is the stress direction σ j Angle with the main direction of the material to be calibrated, ω j X is the detection direction j Angle with the main direction of the material to be calibrated;

[0054] The obtained N2+1 fitting straight line slope values ​​are combined to obtain the values ​​of the first stress coefficient a, the third stress coefficient c and the fourth stress coefficient d.

[0055] Specifically, the propagation speed of ultrasound in solids has an approximately linear relationship with the stress in the solids, and the ultrasonic stress coefficient is a certain value in isotropic materials. For anisotropic materials, since the ultrasonic propagation law is affected by the mechanical properties of the material and is anisotropic, and coupled with the material stress characteristics, it is difficult to obtain the ultrasonic stress coefficient of the anisotropic material, which ultimately leads to the inability of ultrasonic stress detection to accurately measure the stress in the structure.

[0056] The core of the above problems lies in that the ultrasonic stress coefficient of anisotropic materials is uncertain, there is no accurate and reliable method to achieve rapid and accurate calibration of the ultrasonic stress coefficient, and there is a lack of a mathematical model for the ultrasonic stress coefficient. Therefore, this method provides a general calibration method for the ultrasonic stress coefficients of orthotropic and transversely isotropic materials, and uses tensile experiments to verify and evaluate the calibration coefficients and the mathematical model of the ultrasonic stress coefficient.

[0057] The change value of the body wave velocity in an orthotropic medium is approximately linearly related to the stress value. However, since the mechanical properties are more complex than those of isotropic materials, the following formula is introduced.

[0058]

[0059] Where is the plane sound velocity matrix, is the sound velocity change matrix, [σ] is the stress tensor, is the stress coefficient matrix, which is similar to the stiffness matrix of orthotropic materials.

[0060]

[0061] Performing matrix operations and principal value solutions on the sound velocity change matrix to obtain the sound velocity change ΔV in any detection direction ω ω The relationship with the plane principal stresses σ1 and σ2 is

[0062] ΔV ω =(A + B)(1 + cos 2ω)(σ1 + σ2)+(B + C)(1 + cos 2ω)cos 2θ(σ1 - σ2)+D sin 2θsin 2ω(σ1 - σ2)

[0063] Where the parameters A, B, C, and D are respectively

[0064]

[0065] Since the sound path L of the variable angle acoustic wedge used in ultrasonic testing is fixed and the measured result is a time value (in ns), the sound velocity change is equivalent to the time difference Δt ω ,

[0066] Δt ω =R0(A + B cos 2ω)(σ1 + σ2)+R0(B cos 2θ + C cos 2θcos 2ω + D sin 2θsin 2ω)(σ1 - σ2);

[0067] Where v0 is the wave velocity of the critical refracted longitudinal wave in the specimen under the stress-free state.

[0068] The characteristic ultrasonic stress coefficients include a first stress coefficient a, a second stress coefficient b, a third stress coefficient c, and a fourth stress coefficient d, (with the unit of ns / MPa);

[0069]

[0070] It is obtained that Δt = (a + b cos 2ω)(σ1 + σ2) + (b cos 2ω + c cos 2θ cos 2ω + d sin 2θ sin 2ω)(σ1 - σ2); for transversely isotropic materials, the second stress coefficient b = 0, and its simplified result is

[0071] Δt = (a + b cos 2ω)(σ1 + σ2) + (c cos 2θ cos 2ω + d sin 2θ sin 2ω)(σ1 - σ2); σ1 and σ2 are the principal stresses of the structure.

[0072] Specifically, for transversely isotropic materials, there are at least 3 mutually independent stress coefficient equations. Combinations of different stress directions and detection directions will result in the same data results, and such results should be avoided. Calculate the characteristic ultrasonic stress coefficients a, c, and d of the calibrated material. According to the characteristic ultrasonic stress coefficients, draw a relationship diagram between any direction in the plane and the ultrasonic stress coefficients, and obtain the linear relationship between the stress in any direction and the time difference of sound, that is, the mathematical model of the ultrasonic stress coefficients of the specimen to be calibrated.

[0073] Furthermore, the ultrasonic time measurement device includes a tensile machine 1, ultrasonic transducers, a variable-angle acoustic wedge 4, and an ultrasonic time measurement instrument 5. The ultrasonic transducers include an ultrasonic excitation transducer 301 and an ultrasonic receiving transducer 302;

[0074] The tensile machine 1 is used to stretch the specimen to be calibrated according to the set stress direction and different stress values;

[0075] The ultrasonic time measurement instrument 5 is used to generate ultrasonic signals and transmit the ultrasonic signals to the ultrasonic excitation transducer 301. The ultrasonic excitation transducer 301 is threadedly connected to the variable-angle acoustic wedge 4. The ultrasonic excitation transducer 301 transmits the ultrasonic signals to the variable-angle acoustic wedge 4. The variable-angle acoustic wedge 4 transmits the ultrasonic signals to the specimen to be calibrated stretched by the tensile machine according to the set incident angle of the ultrasonic signals. The ultrasonic signals propagate in the specimen to be calibrated stretched by the tensile machine. The ultrasonic receiving transducer 302 is threadedly connected to the variable-angle acoustic wedge. The variable-angle acoustic wedge 4 is also used to receive the ultrasonic signals transmitted in the specimen to be calibrated stretched by the tensile machine and transmit the received ultrasonic signals transmitted in the specimen to be calibrated stretched by the tensile machine to the ultrasonic receiving transducer 302. The ultrasonic receiving transducer 302 is used to transmit the received ultrasonic signals transmitted in the specimen to be calibrated stretched by the tensile machine to the ultrasonic time measurement instrument 5;

[0076] The ultrasonic time measurement instrument 5 obtains the ultrasonic pulse flight times corresponding to different stress values based on the generated ultrasonic signal and the ultrasonic signal received during the transmission in the material to be calibrated stretched by the stretching machine.

[0077] The ultrasonic time measurement instrument includes an ultrasonic signal excitation end 501, an ultrasonic signal receiving end 502, and a processor.

[0078] The ultrasonic signal excitation end 501 is used to generate an ultrasonic signal and transmit the ultrasonic signal to the ultrasonic excitation transducer 301 and the processor.

[0079] The ultrasonic signal receiving end 502 is used to receive the ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine by the ultrasonic receiving transducer 302, and transmit the ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine to the processor.

[0080] The processor is used to obtain the ultrasonic pulse flight times corresponding to different stress values based on the ultrasonic signal generated by the ultrasonic signal excitation end 501 received and the ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine.

[0081] Specifically, the processor of the ultrasonic time-of-flight measuring instrument 5 further includes: a pulse signal transceiver card and an industrial computer. The pulse signal transceiver card serves as the core component for exciting and receiving ultrasonic waves (for example, the ZXUS-80SM type ultrasonic pulse transceiver card is selected). Its main functions include performing A / D and D / A conversions, converting the digital electrical signal at the transmitting end into an ultrasonic analog signal, and at the same time converting the ultrasonic analog signal obtained at the receiving end into a digital signal. The key performance parameters include the sampling frequency and resolution. According to the Nyquist sampling theorem, the sampling frequency of the signal should be twice the highest frequency of the ultrasonic signal. Its receiving bandwidth is 0.5 MHz - 30 MHz, the sampling frequency reaches 100 MS / s, and the sampling interval is 10 ns. The high-voltage power supply provides high voltage for the ultrasonic time-of-flight measuring instrument (5) to generate ultrasonic electrical signals with sufficient energy thresholds. The industrial computer contains ultrasonic signal control and processing software, and its functions include setting parameters such as the high and low levels, pulse period, and gain of the pulse signal transceiver card, and controlling signal amplification, filtering, etc. The signal is subjected to cross-correlation operation and piecewise linear interpolation processing to obtain and store detection data such as the time difference of sound. According to experience, since the time-of-flight change caused by stress reaches dozens of nanoseconds, the 100 MS / s sampling frequency cannot directly obtain effective sound signals for the operation and extraction of the time difference of sound. To improve the detection accuracy of the time difference of sound, densification processing is performed between limited data points. The ultrasonic time-of-flight measuring instrument 5 used in this method adopts the piecewise linear interpolation method, with 20 interpolation operations performed between each acquisition point, resulting in a time resolution of 0.5 ns and a time-of-flight measurement accuracy of 0.1 ns, meeting the requirements for the measurement resolution of the time difference of sound. The flight time of the ultrasonic pulse signal is obtained through cross-correlation calculation. The principle is that the ultrasonic signal control and processing software performs piecewise linear interpolation on the digital signals of the ultrasonic signal excitation end 501 and the ultrasonic signal receiving end 502, and performs cross-correlation calculation on the two obtained discrete waveforms to obtain the time point corresponding to the maximum value of the cross-correlation function, which is used as the time experienced by the ultrasonic pulse from the excitation end to the receiving end, that is, the flight time t of the ultrasonic pulse. The ultrasonic signal excitation end 501 and the ultrasonic signal receiving end 502 provide BNC interfaces for the ultrasonic transducers.

[0082] Specifically, the incident angle of the ultrasonic signal is denoted as θ CRω , and is calculated based on the longitudinal wave velocity V ω,0 in the detection direction, the longitudinal wave velocity V0 of the wedge material, and Snell's law.

[0083] Furthermore, the ultrasonic time-of-flight measuring instrument includes an ultrasonic signal excitation end 501, an ultrasonic signal receiving end 502, and a processor;

[0084] The ultrasonic signal excitation end 501 is used to generate ultrasonic signals and transmit the ultrasonic signals to the ultrasonic excitation transducer 301 and the processor;

[0085] The ultrasonic signal receiving end 502 is used to receive the ultrasonic signal transmitted by the ultrasonic receiving transducer 302 in the material to be calibrated being stretched by the stretching machine, and send the ultrasonic signal transmitted in the material to be calibrated being stretched by the stretching machine to the processor;

[0086] The processor is used to obtain the ultrasonic pulse flight time corresponding to different stress values according to the ultrasonic signal generated by the ultrasonic signal excitation end 501 and the ultrasonic signal transmitted in the material to be calibrated being stretched by the stretching machine.

[0087] Further, the variable angle wedge block 4 includes an incident slider 401 and a receiving slider 402. The incident slider 401 is threadedly connected to the ultrasonic excitation transducer 301, and the receiving slider 402 is threadedly connected to the ultrasonic receiving transducer 302;

[0088] The incident slider 401 is used to receive the ultrasonic signal transmitted by the ultrasonic excitation transducer 301 and transmit the ultrasonic signal to the material to be calibrated being stretched by the stretching machine;

[0089] The receiving slider 402 is used to receive the ultrasonic signal transmitted in the material to be calibrated being stretched by the stretching machine, and transmit the received ultrasonic signal transmitted in the material to be calibrated being stretched by the stretching machine to the ultrasonic receiving transducer 302.

[0090] Specifically, the variable angle wedge block further includes an angle plate 403, on which angle values are marked, facilitating the adjustment of the ultrasonic signal incident angle of the variable angle wedge block.

[0091] Further, the variable angle wedge block is made of polymethyl methacrylate material.

[0092] Further, the ultrasonic time measurement instrument further includes a temperature monitoring device 503, and the temperature detection device 03 is used to keep the temperature constant in Steps 1 to 8 of a method for calibrating the ultrasonic stress coefficient of an anisotropic material.

[0093] Specific embodiment: Set the material to be calibrated as T300 carbon fiber reinforced resin matrix material. When this material is hot-pressed and formed, since the fiber direction determines the mechanical properties and ultrasonic properties of the laminate, the carbon fiber laminate has strong anisotropy. However, due to certain regularities, the laminate structure has a certain symmetry and is a typical orthotropic material. In this embodiment, unidirectional layup is used as the calibration object for the ultrasonic stress coefficient;

[0094] Step 1: Set the detection direction, and measure the ultrasonic pulse flight time t0 of the specimen to be calibrated in the set detection direction without stress through the ultrasonic time measurement device;

[0095] Step 2: Design a dog-bone tensile specimen. When sampling, the angle Ф between the main direction of the specimen and the fiber direction of the specimen is 0°, 45°, and 90°. The angle θ between the stress direction and the main direction of the specimen is the same as Ф, and the angle ω between the detection direction and the main direction of the specimen is 0°, 45°, and 90°. Measure the ultrasonic pulse flight time corresponding to different stress values of the specimen to be calibrated under the set detection direction and the corresponding stress direction through an ultrasonic time measurement device. The parameters are shown in the following table:

[0096]

[0097]

[0098] The variable-angle wedge has a sound path L = 30 mm. The wedge material is polymethyl methacrylate (PMMA), and the ultrasonic pulse wave velocity V0 at a frequency of 2.5 MHz is 2700 m / s. Calculate the stress-free L CR wave velocity V ω,0 (m / s). According to Snell's law, θ CRω = arcsin(V0 / V ω,0 ) to calculate the ultrasonic incident angle. Use a tensile machine (1) to stretch the dog-bone specimen (2). During the experiment, ensure that the ambient temperature is 22°C. Measure the ultrasonic pulse flight time corresponding to different stress values of the specimen to be calibrated under the set detection direction and the corresponding stress direction through an ultrasonic time measurement device. Subtract the ultrasonic pulse flight time corresponding to different stress values measured from the ultrasonic pulse flight time of the specimen to be calibrated when stress-free to obtain the time difference Δt corresponding to different stress values under the set detection direction and the corresponding stress direction, and obtain the relationship curve between the stress value and the time difference under the set detection direction and the corresponding stress direction for each group of experiments, as Figure 5 shown;

[0099] Step 3: Perform a least-squares linear fit on the relationship curves between the stress values and the time differences under the detection directions and the corresponding stress directions of the four groups of experimental results to obtain the slopes of the fitted straight lines as k 0,0 = -0.78 ns / MPa, k 45,0 = -0.43 ns / MPa, k 45,45 = 1.08 ns / MPa, k 90,90 = 2.83 ns / MPa;

[0100] Step 4: Substitute k 0,0 = -0.78 ns / MPa, k 45,0 = -0.43 ns / MPa, k 45,45 = 2.83 ns / MPa, k 90,90 = 1.08 ns / MPa into the stress coefficient equation of the orthotropic material to obtain:

[0101] -0.78 = a + b(cos0 + cos0) + ccos0cos0 + dsin0sin0

[0102] -0.43 = a + b(cos90 + cos0) + ccos90cos0 + dsin90sin0

[0103] 1.08 = a + b(cos90 + cos90) + ccos90cos90 + dsin90sin90

[0104] 2.83 = a + b(cos180 + cos180) + ccos180cos180 + dsin180sin180

[0105] Obtain the calibration results of the four characteristic ultrasonic stress coefficients for the unidirectional ply: a = 0.48 ns / MPa, b = -0.90 ns / MPa, c = 0.55 ns / MPa, d = 0.60 ns / MPa;

[0106] Step 5: Obtain the mathematical model of the ultrasonic stress coefficient for the specimen to be calibrated based on the obtained characteristic ultrasonic stress coefficients of the specimen to be calibrated, k θ,ω = 0.48 + 0.90(cos2θ + cos2ω) + 0.55cos2θcos2ω + 0.60sin2θsin2ω;

[0107] Calculate the ultrasonic stress coefficient values in each direction The mathematical model of the ultrasonic stress coefficient when the ultrasonic detection direction ω = θ, as Figure 6 shown;

[0108] Step 6: Design and conduct a tensile experiment and an ultrasonic detection verification experiment. The main tensile directions of the specimens are 0°, 15°, 22.5°, 60°, 75° and 90°. After sampling, stretch and measure the acoustic time difference Δt in the direction. The parameters of the verification experiment scheme are as shown in the following table:

[0109]

[0110] Use a tensile machine to stretch the above six specimens to induce the generation of uniaxial stress in the direction, measure the acoustic time difference Δt of each stress value, and plot the acoustic time difference Δt - stress curve;

[0111] Step 6: Perform a least squares linear fitting on the acoustic time difference Δt - stress curve to obtain the slope of the fitting line (unit: ns / MPa). For the 1st verification experiment, k'0 = -0.78 ns / MPa, for the 2nd verification experiment, k' 15= -0.53 ns / MPa, k' for the 3rd verification test 22.55 = -0.20 ns / MPa, k' for the 4th verification test 60 = 1.87 ns / MPa, k' for the 5th verification test 75 = 2.57 ns / MPa, k' for the 6th verification test 90 = 2.82 ns / MPa;

[0112] Step 7: Compare the calibration values corresponding to the direction of the ultrasonic stress coefficient mathematical model and the stress coefficient values measured in the verification experiment As Figure 7 shown, the calibration results in the ultrasonic stress coefficient mathematical model are: k0 = -0.780 ns / MPa, k 15 = -0.525 ns / MPa, k 22.5 = -0.226 ns / MPa, k 60 = 1.967 ns / MPa, k 75 = 2.604 ns / MPa, k 90 = 2.83 ns / MPa. The maximum deviation between the verification result and the calibration result is 5%, which appears in the 60° direction, meeting the calibration result evaluation standard with a maximum deviation of 20%. The above results verify the accuracy of the calibration experiment and the ultrasonic stress coefficient mathematical model.

[0113] Finally, the ultrasonic stress coefficient and the critical refraction longitudinal wave stress detection model of the T300 carbon fiber unidirectional ply laminate are accurately obtained using this method.

[0114] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. An ultrasonic stress coefficient calibration method for anisotropic materials, characterized in that: include: Step 1: Set the detection direction, and measure the ultrasonic pulse flight time in the detection direction when the specimen to be calibrated is stress-free by using an ultrasonic acoustic time measurement device; Step 2: Set the stress direction corresponding to the detection direction, and measure the ultrasonic pulse flight time corresponding to different stress values ​​of the test piece to be calibrated in the set detection direction and the corresponding stress direction by using an ultrasonic acoustic time measurement device; subtract the ultrasonic pulse flight time corresponding to the measured different stress values ​​from the ultrasonic pulse flight time in the detection direction when the test piece to be calibrated is stress-free to obtain the acoustic time difference corresponding to different stress values ​​in the set detection direction and the corresponding stress direction, and obtain the relationship curve between the stress value and the acoustic time difference in the set detection direction and the corresponding stress direction; Step 3: Perform a least squares linear fit on the relationship curve between the stress value and the acoustic time difference in the obtained detection direction and the corresponding stress direction to obtain the slope value of the fitting line; Step 4: changing the set detection direction and the stress direction corresponding to the detection direction for multiple times, and repeating steps 1 to 3 according to the changed detection direction and the corresponding stress direction, to obtain multiple fitting straight line slope values, and obtaining the characteristic ultrasonic stress coefficient of the specimen to be calibrated according to all the obtained fitting straight line slope values; Step 5: Obtain a mathematical model of the ultrasonic stress coefficient of the specimen to be calibrated according to the obtained characteristic ultrasonic stress coefficient of the specimen to be calibrated.

2. The ultrasonic stress coefficient calibration method for an anisotropic material according to claim 1, characterized in that: Also includes: Step Six: Set the first verification detection direction and the first verification stress direction corresponding to the first verification detection direction. The first verification detection direction is the same as the first verification stress direction, and the angle between the first verification detection direction and the main direction of the material to be calibrated is Obtain the first verification ultrasonic stress coefficient value in the first verification detection direction and the corresponding first verification stress direction according to the ultrasonic stress coefficient mathematical model of the specimen to be calibrated obtained in Step Five Step 7: Set a second verification detection direction, and measure the ultrasonic pulse flight time in the second verification detection direction when the specimen to be calibrated is stress-free by using an ultrasonic acoustic time measurement device; set a second verification stress direction corresponding to the second verification detection direction, the second verification detection direction is the same as the second verification stress direction, and measure the ultrasonic pulse flight time corresponding to different stress values ​​of the specimen to be calibrated under the set second verification detection direction and the corresponding second verification stress direction by using an ultrasonic acoustic time measurement device; subtract the measured ultrasonic pulse flight time corresponding to different stress values ​​from the ultrasonic pulse flight time of the calibration specimen in the second verification detection direction when stress-free to obtain the acoustic time difference corresponding to different stress values ​​under the set second verification detection direction and the corresponding second verification stress direction, and obtain the relationship curve between the acoustic time difference and the stress value under the second verification detection direction and the corresponding second verification stress direction; Perform least squares linear fitting on the relationship curve between the acoustic time difference and the stress value under the obtained second verification detection direction and the corresponding second verification stress direction to obtain the slope value of the second verification fitting straight line Step 8: Take the difference between the obtained first verified ultrasonic stress coefficient value and the second verified fitting straight line slope value to obtain a deviation value, and determine whether the deviation value is greater than a set threshold. If so, return to Step 1; if not, end.

3. The method for calibrating ultrasonic stress coefficient of anisotropic material according to claim 1, characterized in that: The stress value is taken within the range of 0 to σ e ; the stress value step size is within the range of σ e / 20 to σ e / 10; σ e is the elastic limit corresponding to the material to be calibrated and the stress direction.

4. A method for calibrating the ultrasonic stress coefficient of an anisotropic material according to claim 1, characterized in that: The set detection direction and the stress direction corresponding to the detection direction are changed multiple times, and steps 1 to 3 are repeated according to the changed detection direction and the corresponding stress direction to obtain multiple fitting straight line slope values, including: Determine whether the specimen to be calibrated is an orthotropic material. If so, change the set detection direction and the corresponding stress direction N1 times; repeat steps one to three once for each change in the detection direction and the corresponding stress direction to obtain a fitting straight line slope value k θ,ω,i , until N1 fitting straight line slope values are obtained; where N1 ≥ 4, N1 is an integer, and i ∈ N1; If the specimen to be calibrated is a transversely isotropic material, change the detection direction and the corresponding stress direction set N2 times, and repeat steps 1 to 3 once for each change of the detection direction and the corresponding stress direction to obtain a fitting straight line slope value k θ,ω,j , until N2 fitting straight line slope values are obtained; where N2≥3, N2 is an integer, and j∈N j .

5. A method for calibrating the ultrasonic stress coefficient of an anisotropic material according to claim 4, characterized in that: The characteristic ultrasonic stress coefficient of the specimen to be calibrated is obtained according to the slope values ​​of all fitted straight lines, including: The specimen to be calibrated is an orthotropic material, and N1 + 1 fitting straight line slope values are obtained, which are respectively The characteristic ultrasonic stress coefficients of the specimen to be calibrated include a first stress coefficient a, a second stress coefficient b, a third stress coefficient c and a fourth stress coefficient d; k θ,ω,i = a + b(cos2θ i + cos2ω i ) + c cos2θ i cos2ω i + d sin2θ i sin2ω i ; where k θ,ω,i represents the slope value of the fitting straight line of the specimen to be calibrated under the i-th detection direction X i and the corresponding stress direction σ i , θ i is the angle between the stress direction σ i and the main direction of the material to be calibrated, ω i is the angle between the detection direction X i and the main direction of the material to be calibrated; The obtained N1+1 fitting straight line slope values ​​are combined to obtain the values ​​of the first stress coefficient a, the second stress coefficient b, the third stress coefficient c and the fourth stress coefficient d.

6. The ultrasonic stress coefficient calibration method for an anisotropic material according to claim 4, characterized in that: The characteristic ultrasonic stress coefficient of the specimen to be calibrated is obtained according to the slope values ​​of all fitted straight lines, including: The specimen to be calibrated is a transversely isotropic material, and N2+1 fitting straight-line slope values are obtained, which are respectively The characteristic ultrasonic stress coefficients of the specimen to be calibrated include a first stress coefficient a, a third stress coefficient c and a fourth stress coefficient d; k θ,ω,j = a + c cos 2θ j cos 2ω j + d sin 2θ j sin 2ω j ; where k θ,ω,j represents the fitting straight line slope value of the specimen to be calibrated under the j-th detection direction X j and the corresponding stress direction σ j θ j is the angle between the stress direction σ j and the main direction of the material to be calibrated, ω j is the angle between the detection direction X j and the main direction of the material to be calibrated; The obtained N2+1 fitting straight line slope values ​​are combined to obtain the values ​​of the first stress coefficient a, the third stress coefficient c and the fourth stress coefficient d.

7. The method for calibrating ultrasonic stress coefficient of anisotropic material according to claim 1, characterized in that: The ultrasonic acoustic time measuring device comprises a stretching machine, an ultrasonic transducer, a variable angle acoustic wedge and an ultrasonic acoustic time measuring instrument, wherein the ultrasonic transducer comprises an ultrasonic excitation transducer and an ultrasonic receiving transducer; The stretching machine is used to stretch the specimen to be calibrated according to the set stress direction and different stress values; The ultrasonic acoustic time measuring instrument is used to generate an ultrasonic signal and transmit the ultrasonic signal to an ultrasonic excitation transducer. The ultrasonic excitation transducer is threadedly connected to a variable angle acoustic wedge. The ultrasonic excitation transducer transmits the ultrasonic signal to the variable angle acoustic wedge. The variable angle acoustic wedge transmits the ultrasonic signal to the material to be calibrated stretched by the stretching machine according to the set ultrasonic signal incident angle. The ultrasonic signal propagates in the material to be calibrated stretched by the stretching machine. The ultrasonic receiving transducer is threadedly connected to the variable angle acoustic wedge. The variable angle acoustic wedge is also used to receive the ultrasonic signal transmitted from the material to be calibrated stretched by the stretching machine, and transmit the received ultrasonic signal transmitted from the material to be calibrated stretched by the stretching machine to the ultrasonic receiving transducer. The ultrasonic receiving transducer is used to transmit the received ultrasonic signal transmitted from the material to be calibrated stretched by the stretching machine to the ultrasonic acoustic time measuring instrument. The ultrasonic acoustic time measuring instrument obtains the ultrasonic pulse flight time corresponding to different stress values ​​according to the generated ultrasonic signal and the received ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine.

8. A method for calibrating the ultrasonic stress coefficient of an anisotropic material according to claim 7, characterized in that: The ultrasonic acoustic time measuring instrument comprises an ultrasonic signal exciting end, an ultrasonic signal receiving end and a processor; The ultrasonic acoustic time measuring instrument comprises an ultrasonic signal exciting end, an ultrasonic signal receiving end and a processor; The ultrasonic signal excitation end is used to generate an ultrasonic signal and transmit the ultrasonic signal to the ultrasonic excitation transducer (301) and the processor; The ultrasonic signal receiving end is used to receive the ultrasonic signal transmitted by the ultrasonic receiving transducer and transmitted in the material to be calibrated stretched by the stretching machine, and send the ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine to the processor; The processor is used to obtain the ultrasonic pulse flight time corresponding to different stress values ​​according to the ultrasonic signal generated by the received ultrasonic signal excitation end and the ultrasonic signal transmitted in the material to be calibrated stretched by the stretching machine.

9. The ultrasonic stress coefficient calibration method for an anisotropic material according to claim 8, characterized in that: The variable angle acoustic wedge comprises an incident slider and a receiving slider, wherein the incident slider is threadedly connected to the ultrasonic excitation transducer, and the receiving slider is threadedly connected to the ultrasonic receiving transducer; The incident slider is used to receive the ultrasonic signal sent by the ultrasonic excitation transducer and transmit the ultrasonic signal to the material to be calibrated stretched by the stretching machine; The receiving slider is used to receive the ultrasonic signal transmitted from the material to be calibrated stretched by the stretching machine, and transmit the received ultrasonic signal transmitted from the material to be calibrated stretched by the stretching machine to the ultrasonic receiving transducer.

10. A method for calibrating the ultrasonic stress coefficient of an anisotropic material according to claim 7, characterized in that: The variable angle acoustic wedge is made of polymethyl methacrylate.

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