Non-destructive testing stress evaluation method based on the static component of non-linear ultrasonic Lamb waves

Abstract

The present invention discloses a non-destructive testing stress evaluation method based on the static component of non-linear ultrasonic Lamb waves, belonging to the technical field of stress detection, including: S1. Construct a Rayleigh-Lamb equation introducing the non-linear theory of elastic bodies, and then determine the Lamb wave dispersion curve of the plate to be detected, and determine the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve; S2. Excite the Lamb wave with the determined excitation frequency and excitation mode at one end of the stress detection area of the plate to be detected, and receive and extract the corresponding signal through the other end of the stress detection area; S3. Reverse the phase of the extracted signal, and re-excite and receive the signal with the reversed phase; S4. Process the received signal and calculate the ultrasonic non-linear coefficient as an evaluation parameter for the stress state of the plate to be detected. The fundamental wave frequency and mode of the present invention are more flexible in selection, can measure the relevant parameters of high-attenuation materials, and can effectively evaluate the early damage and stress state inside the materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of stress detection, and particularly relates to a non-destructive testing stress evaluation method based on the static component of non-linear ultrasonic Lamb waves. Background Art

[0002] Under long-term service conditions, the plate and shell structures will undergo varying degrees of deformation and damage under the influence of external forces, resulting in reduced reliability and even serious safety problems, causing immeasurable losses. Therefore, timely and accurately detecting the stress state in the plate is of great significance for ensuring the safety of related equipment.

[0003] Stress detection methods are mainly divided into three types according to the degree of damage to the test piece: full-destruction detection, semi-destruction detection, and non-destructive detection. The first two types include methods such as the contour method, crack compliance method, indentation method, and small hole method. Although these methods are technically mature and have high measurement accuracy, they need to damage the measured component during measurement, which is not conducive to in-service detection. Therefore, non-destructive detection has become the mainstream method for stress detection under working conditions, including X-ray diffraction method, magnetic measurement method, strain gauge detection method, ultrasonic detection method, etc. Among them, the X-ray diffraction method has the advantages of non-destructiveness, fast testing, high accuracy, and strong data repeatability, but its application range is relatively narrow and it is only applicable to the stress detection of crystal materials; the magnetic measurement method is simple to operate, low in cost, and fast in measurement speed, but the measurement process is easily interfered by the surrounding magnetic field environment, affecting the detection accuracy, and it can only detect magnetic materials; while the ultrasonic detection method is simple to operate, low in equipment cost, large in detection depth, not limited by the type and structure of materials, and high in detection efficiency, and is applicable to on-line, automatic, and remote stress monitoring of in-service equipment.

[0004] Currently, most ultrasonic stress detections are based on the linear ultrasonic technology of acoustoelastic theory, that is, by measuring the wave velocity changes under zero stress and specific stress states respectively, the acoustoelastic coefficient of the corresponding material is calibrated. Under the traditional linear ultrasonic technology, the ultrasonic wave velocity change caused by stress is very small. The wave velocity change caused by a stress change of 100 MPa is often only 1%, which cannot meet the high-precision requirements for stress measurement of some components. Therefore, for stress detection technology, non-linear ultrasonic technology with higher sensitivity to stress changes has gradually attracted the attention of researchers.

[0005] Nonlinear ultrasonic technology originates from the nonlinear effects induced by the interaction between ultrasonic waves and material nonlinearities (such as lattice distortion, microcracks), resulting in phenomena such as higher harmonics and static components. Current nonlinear ultrasonic testing mostly focuses on the second harmonic, that is, using the second harmonic to establish its nonlinear coefficient to quantitatively evaluate the stress magnitude. At the same time, ultrasonic Lamb waves have also been used for stress detection in plate structures. However, the presence of stress will affect the dispersion characteristics of Lamb waves, making its propagation characteristics more complex, resulting in the inevitable interference of the second harmonic signal of Lamb waves and making it difficult to distinguish. Summary of the Invention

[0006] Aiming at the above deficiencies in the prior art, the non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves provided by the present invention solves the problems that the existing ultrasonic stress detection technology is not sensitive to stress changes, the high-frequency ultrasonic waves in the plate attenuate quickly during the detection process, and the guided wave modes are numerous and difficult to distinguish and extract.

[0007] In order to achieve the above invention purpose, the technical solution adopted by the present invention is: a non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves, including the following steps:

[0008] S1. Construct the Rayleigh-Lamb equation introducing the nonlinear theory of elastic bodies, and then determine the Lamb wave dispersion curve under the thickness condition of the plate to be detected, and determine the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve;

[0009] S2. Excite Lamb waves with the determined excitation frequency and excitation mode at one end of the stress detection area of the plate to be detected, and receive and extract the corresponding signals at the other end of the stress detection area;

[0010] S3. Reverse the phase of the extracted signal, and re-excite and receive the signal with the reversed phase;

[0011] S4. Process the received signal and calculate the ultrasonic nonlinear coefficient as an evaluation parameter for the stress state of the plate to be detected.

[0012] Further, in step S1, in the process of constructing the Rayleigh-Lamb equation introducing the nonlinear theory of elastic bodies, the following settings exist:

[0013] Continuity assumption of the ultrasonic propagation medium;

[0014] Small perturbations of Lamb waves can be superimposed on the finite deformation of the object under static stress;

[0015] The plate structure is an isotropic elastic body material and an ideal elastic body without mechanical dissipation.

[0016] Further, the Rayleigh-Lamb equation under load is:

[0017]

[0018]

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025] Wherein, and represent the variable functions of the Lamb wave number of, represents the Lamb wave number, represents the strains in the x, y, and z directions caused by the applied stress, represents the applied stress in different directions, , respectively represent and the new coefficients generated by the function under the stress condition compared with the stress-free condition, and represent the Lame constants of the material, represents the density, represents the third-order elastic constant of the material, represents half of the plate thickness, represents the angular frequency.

[0026] Furthermore, in the step S1, the method for determining the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve is:

[0027] Select the Lamb wave mode and frequency with a group velocity difference less than the preset threshold at 0 MHz of the Lamb wave S0 mode as the excitation frequency and excitation mode of the excitation source, and the group velocity of the selected excitation mode is greater than the group velocities of other modes at the selected excitation frequency.

[0028] Furthermore, the step S4 includes the following sub-steps:

[0029] S41. Perform a fast Fourier transform on each received signal to obtain the fundamental wave frequency domain amplitude;

[0030] S42. Add the forward and reverse fundamental frequency waves received twice and perform low-pass filtering to obtain the static component waveform;

[0031] S43. Perform a fast Fourier transform on the static component waveform to obtain the static component frequency domain amplitude;

[0032] S44. Calculate the ultrasonic nonlinear coefficient based on the fundamental wave frequency domain amplitude and the static component frequency domain amplitude, and use it as an evaluation parameter for the stress state of the plate to be detected.

[0033] Further, in step S44, the ultrasonic nonlinear coefficient is:

[0034]

[0035] In the formula, represents the fundamental wave frequency domain amplitude, represents the static component frequency domain amplitude.

[0036] Further, in step S4, the ultrasonic nonlinear coefficient increases monotonically with the increase of the stress on the plate, and the stress state of the plate to be detected is quantitatively evaluated by obtaining the change of the ultrasonic nonlinear coefficient under the stress state of the plate to be detected.

[0037] Compared with the existing plate stress detection technology, the advantage of using the static component of nonlinear ultrasonic Lamb waves for stress state detection in the present invention is that:

[0038] (1) Cumulative group velocity matching is not required, and the selection of the fundamental wave frequency and mode is more flexible;

[0039] (2) The carrier frequency of the static component can be regarded as 0. Since the attenuation of ultrasonic waves is related to the frequency, the characteristic of small attenuation of the static component enables its propagation range to be wider, and it can measure the relevant parameters of high-attenuation materials;

[0040] (3) The static component is highly sensitive to the nonlinear characteristics inside the material and can effectively evaluate the early damage and stress state inside the material. Description of the Drawings

[0041] Figure 1 is a flow chart of the non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves provided by the present invention.

[0042] Figure 2 is a time domain diagram of the received fundamental Lamb wave signal provided by the present invention.

[0043] Figure 3 is a schematic diagram of adding the received forward and reverse fundamental Lamb waves provided by the present invention.

[0044] Figure 4The time-domain diagram of the static component obtained after adding the positive and negative phases and passing through a low-pass filter provided by the present invention.

[0045] Figure 5 The time-domain diagram of the static component under different stress states provided by the present invention.

[0046] Figure 6 The schematic diagram of the change of the ultrasonic nonlinear parameter under different stress states provided by the present invention. Specific embodiments

[0047] The specific embodiments of the present invention will be described below to facilitate the understanding of the present invention by those skilled in the art. It should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0048] In the embodiment of the present invention, a non-destructive testing stress evaluation method based on the static component of the nonlinear ultrasonic Lamb wave is provided. As Figure 1 shown, it includes the following steps:

[0049] S1. Construct the Rayleigh-Lamb equation introducing the elastomer nonlinear theory, and then determine the Lamb wave dispersion curve under the thickness condition of the plate to be detected, and determine the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve;

[0050] S2. Based on the determined stress of the plate to be detected, excite the Lamb wave with the determined excitation frequency and excitation mode at one end of the stress detection area of the plate to be detected, and receive and extract the corresponding signal at the other end of the stress detection area;

[0051] S3. Reverse the phase of the extracted signal, and re-excite and receive the signal with the reversed phase;

[0052] S4. Process the received signal and calculate the ultrasonic nonlinear coefficient as an evaluation parameter for the stress state of the plate to be detected.

[0053] Based on the acoustoelastic effect and the second-order perturbation theory, second-order body driving forces will be generated along the propagation path of ultrasonic Lamb waves propagating in a plate, including the surface body driving force responding on the upper surface and the body body driving force between the two surfaces. These two body driving forces are related to the second-order and third-order elastic constants of the material and contain different frequency components, such as static components and second-harmonic components. The zero-frequency body driving force and surface driving force stress generated along with the propagation of the fundamental wave will serve as the body driving source and surface driving source, respectively, and a series of zero-frequency ultrasonic guided wave modes will be excited along the propagation direction of the fundamental-frequency Lamb wave. They are superimposed on each other and finally constitute the static component sound field of the ultrasonic Lamb wave. Combining with the acoustoelastic effect, the generated static component will change its sound field and dispersion characteristics under the action of stress.

[0054] Based on this, the present invention proposes a non-destructive testing stress evaluation method based on the static component of non-linear ultrasonic Lamb waves. In this embodiment, the tested plate is a thin metal plate, and ultrasonic transducers that meet the testing requirements, such as high-frequency excitation and low-frequency receiving probes, are used to excite the target fundamental-frequency ultrasonic mode and receive the fundamental wave and static component signals.

[0055] According to the material parameters and stress magnitude, as well as the numerically calculated Lamb wave dispersion curve, using the group velocity matching condition of the Lamb wave fundamental wave and static component, the corresponding Lamb wave excitation mode and excitation frequency are selected, and based on this, the working frequency and its main structural parameters of the testing transducer are determined. Then, the specific fundamental-frequency Lamb wave and static component mode pair are focused and analyzed, and further the stress state detection result of the plate is obtained.

[0056] Specifically, in step S1 of the embodiment of the present invention, during the propagation of the Lamb wave, its propagation speed changes with the change of frequency, and this phenomenon is called the dispersion effect. The Rayleigh-Lamb equation can represent the dispersion of the Lamb wave. Assuming that the thin plate medium is infinite and the upper and lower boundaries of the thin plate are free, the equation expression is:

[0057] Symmetric mode:

[0058]

[0059] Asymmetric mode:

[0060]

[0061] In the formula, represents half of the plate thickness, represents the wave number, represents the angular frequency, and represent the longitudinal wave and transverse wave velocities.

[0062] Regarding the problem of Lamb wave velocity change in a plate structure under static load, in the present invention, the nonlinear theory of elastic bodies is introduced. In the process of constructing the Rayleigh-Lamb equation incorporating the nonlinear theory of elastic bodies, the following assumptions are made:

[0063] The continuity assumption of the ultrasonic propagation medium;

[0064] The small perturbation of Lamb wave can be superimposed on the finite deformation of the object under static stress;

[0065] The plate structure body is a homogeneous elastic material in all directions and is an ideal elastic body without mechanical dissipation.

[0066] Among them, the continuity assumption is a mechanical model assumption established in continuum mechanics for facilitating mathematical analysis, which includes: the physical connection is continuously distributed in the occupied space, and the macroscopic physical quantities are continuous functions of space and time.

[0067] Thus, the Rayleigh-Lamb equation under load is obtained as follows:

[0068]

[0069] In the dispersion equation in the symmetric mode, the exponential reading takes the positive sign, and in the anti-symmetric mode, it takes the negative sign; where the wave number is defined as:

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076]

[0077] In the formula, and represent the variable functions of the Lamb wave number ; represents the Lamb wave number, represents the strains in the x, y, and z directions caused by the applied stress, represents the applied stress in different directions, 、 respectively represent and the new coefficients generated by the and represents the Lame constant of the material, represents the density, represents the third-order elastic constant of the material, represents half of the plate thickness, represents the angular frequency.

[0078] After determining the Lame constant and the third-order elastic constant of the material, as well as the magnitudes of the stresses in different directions, the corresponding values can be substituted into the expressions of each equation. Finally, by numerically solving the Rayleigh-Lamb equation, the frequency-velocity dispersion curve equation can be obtained, and then the Lamb wave dispersion curve can be obtained.

[0079] In step S1 of the embodiment of the present invention, the method for determining the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve is as follows:

[0080] Select the Lamb wave mode and frequency whose group velocity difference at 0 MHz of the Lamb wave S0 mode is less than the preset threshold as the excitation frequency and excitation mode of the excitation source, and the group velocity of the selected excitation mode is greater than the group velocities of other modes at the selected excitation frequency, thereby improving the accuracy of signal extraction when receiving signals.

[0081] In step S2 of this embodiment, a Lamb wave sine signal is excited at one end of the stress detection area of the plate to be detected, and the propagated signal is received at the other end, and the time-domain diagram of the received fundamental frequency Lamb wave signal is as Figure 2 shown.

[0082] Step S4 of this embodiment includes the following sub-steps:

[0083] S41. Perform a fast Fourier transform on the signal received each time to obtain the fundamental wave frequency domain amplitude;

[0084] S42. Add the fundamental frequency waves received in the positive and negative phases twice and perform low-pass filtering to obtain the static component waveform;

[0085] S43. Perform a fast Fourier transform on the static component waveform to obtain the static component frequency domain amplitude;

[0086] S44. Calculate the ultrasonic nonlinear coefficient according to the fundamental wave frequency domain amplitude and the static component frequency domain amplitude, and use it as an evaluation parameter for the stress state of the plate to be detected.

[0087] Among them, the ultrasonic nonlinear coefficient is:

[0088]

[0089] In the formula, represents the fundamental wave frequency domain amplitude, represents the static component frequency domain amplitude.

[0090] In step S42, the extracted signal is phase-inverted by 180°. Similarly, the propagated signal is received at the other end, and the amplitudes of the two received signals are added together. The schematic diagram of the addition of the forward and reverse fundamental Lamb waves is as shown in Figure 3 the figure, and the corresponding time-domain diagram of the static component obtained after low-pass filtering is as shown in Figure 4 the figure.

[0091] In the embodiment of the present invention, based on the above method, the stress state of the test piece is changed, gradually increased from 0 MPa to 200 MPa, with an increase of 50 MPa each time. Each time, the steps of exciting the signal - receiving the signal - inverting the signal - receiving the signal - adding the two-phase signals - obtaining the time-domain waveform of the static component are repeated. Finally, the comparison diagram of the time-domain waveforms of the static components under different stress conditions is obtained as shown in Figure 5 the figure, and the change of the ultrasonic nonlinear parameter corresponding to different stress states is as shown in Figure 6 the figure.

[0092] In the implementation of the present invention, the ultrasonic nonlinear coefficient will monotonically increase with the increase of the tensile stress on the plate. By obtaining the ultrasonic nonlinear coefficient of the same test piece in the stress-free state, the change of the ultrasonic nonlinear coefficient can be used to quantitatively evaluate the stress state.

[0093] Specific embodiments are applied in the present invention to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

[0094] Those of ordinary skill in the art will realize that the embodiments described here are for helping readers understand the principle of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various specific deformations and combinations that do not deviate from the essence of the present invention according to the technical revelations disclosed in the present invention, and these deformations and combinations are still within the protection scope of the present invention.

Claims

1. A non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves, characterized in that: The following steps are involved: S1. Construct the Rayleigh-Lamb equation which introduces the nonlinear theory of elastic body, and then determine the Lamb wave dispersion curve under the condition of the thickness of the plate to be tested, and determine the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve; S2, exciting a Lamb wave with a determined excitation frequency and excitation mode at one end of the stress detection area of ​​the plate to be detected, and receiving and extracting a corresponding signal through the other end of the stress detection area; S3, inverting the phase of the extracted signal, and re-exciting and receiving the signal with the inverted phase; S4, processing the received signal and calculating the ultrasonic nonlinear coefficient as an evaluation parameter of the stress state of the plate to be detected; The Rayleigh-Lamb equation under load is: ； ； ； ； ； ； ； ； In the formula, and Indicates the Lamb wave number Variable function, represents the Lamb wave number, represents the strain in the x, y, and z directions caused by the applied stress, represents the external stress in different directions, , Respectively and The new coefficients of the function under stress conditions compared to the stress-free conditions, and represents the Lame constant of the material, represents density, represents the third-order elastic constant of the material, represents half the plate thickness, represents the angular frequency; The step S4 comprises the following sub-steps: S41, performing fast Fourier transform on each received signal to obtain a fundamental frequency domain amplitude; S42, adding the positive and negative phase fundamental frequency waves received twice, and performing low-pass filtering to obtain a static component waveform; S43, performing fast Fourier transform on the static component waveform to obtain the frequency domain amplitude of the static component; S44, calculating the ultrasonic nonlinear coefficient according to the fundamental wave frequency domain amplitude and the static component frequency domain amplitude as a stress state evaluation parameter of the plate to be tested; In step S44, the ultrasonic nonlinear coefficient for: ； In the formula, represents the fundamental frequency domain amplitude, Represents the frequency domain amplitude of the static component.

2. The non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves according to claim 1 is characterized in that: In step S1, in the process of constructing the Rayleigh-Lamb equation that introduces the nonlinear theory of elastic body, there are the following settings: The continuity assumption of the medium in which ultrasound propagates; Small perturbations of Lamb waves can be superimposed on the finite deformation of an object under static stress; The plate structure is an isotropically uniform elastomeric material and is an ideal elastomer without mechanical dissipation.

3. The non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves according to claim 1 is characterized in that: In step S1, the method for determining the excitation frequency and excitation mode of the excitation source according to the Lamb wave dispersion curve is: A Lamb wave mode and frequency whose group velocity difference with the Lamb wave S0 mode at 0 MHz is less than a preset threshold is selected as the excitation frequency and excitation mode of the excitation source, and the group velocity of the selected excitation mode is greater than the group velocity of other modes at the selected excitation frequency.

4. The non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves according to claim 1 is characterized in that: In the step S4, the ultrasonic nonlinear coefficient increases monotonically as the stress on the plate increases, and the stress state of the plate to be detected is quantitatively evaluated by obtaining the change of the ultrasonic nonlinear coefficient under the stress state of the plate to be detected.

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