Nondestructive testing stress evaluation method based on nonlinear ultrasonic Lamb wave static component

Abstract

The invention discloses a non-destructive testing stress evaluation method based on a nonlinear ultrasonic Lamb wave static component, and belongs to the technical field of stress detection, and the method comprises the following steps: S1, constructing a Rayleigh-Lamb equation introducing an elastomer nonlinear theory, and further determining a Lamb wave frequency dispersion curve of a to-be-detected plate, the excitation frequency and the excitation mode of the excitation source are determined according to the Lamb wave frequency dispersion curve; s2, exciting lamb waves for determining excitation frequency and excitation mode at one end of the stress detection area of the plate to be detected, and receiving and extracting corresponding signals through the other end of the stress detection area; s3, inverting the phase of the extracted signal, and exciting and receiving the inverted signal again; and S4, processing the received signal, and calculating an ultrasonic nonlinear coefficient as an evaluation parameter of the stress state of the plate to be detected. According to the invention, the selection of the fundamental wave frequency and mode is more flexible, the related parameters of the high-attenuation material can be measured, and the early damage and stress state in the material can be effectively evaluated.

Description

Technical Field

[0001] The invention belongs to the technical field of stress detection, and in particular relates to a non-destructive detection stress evaluation method based on a static component of a nonlinear ultrasonic Lamb wave. Background Art

[0002] Under the influence of external forces, plate and shell structures under long-term service conditions will produce different degrees of deformation and damage, reduce reliability, and even cause serious safety problems, resulting in immeasurable losses. Therefore, timely and accurate detection of the stress state in the plate is of great significance to ensure the safety of related equipment.

[0003] According to the degree of damage to the test piece, stress detection methods are mainly divided into three types: full destructive detection, semi-destructive detection and non-destructive detection. The first two types include contour method, crack flexibility method, indentation method, small hole method and other means. Although these methods are mature in technology and have high measurement accuracy, they need to destroy the measured component during measurement, which is not conducive to in-service detection. Therefore, non-destructive testing 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 and so on. Among them, X-ray diffraction method has the advantages of non-destructive, fast testing, high accuracy and strong data repeatability, but its application range is narrow and it is only suitable for stress detection of crystal materials; magnetic measurement method is simple to operate, low in cost and fast in measurement speed, but the measurement process is easily disturbed by the surrounding magnetic field environment, affecting the detection accuracy, and can only detect magnetic materials; 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. It is suitable for online, automatic and remote stress monitoring of in-service equipment.

[0004] Most of the current ultrasonic stress detection is based on linear ultrasonic technology of acoustic elasticity theory, that is, by measuring the wave velocity changes under zero stress and specific stress states, the corresponding material acoustic elastic coefficient is calibrated. Under traditional linear ultrasonic technology, the change in ultrasonic sound velocity 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 stress measurement requirements of some parts. Therefore, for stress detection technology, nonlinear ultrasonic technology with higher sensitivity to stress changes has gradually attracted the attention of researchers.

[0005] Nonlinear ultrasonic technology originates from the interaction between ultrasonic waves and material nonlinearity (such as lattice distortion and microcracks), which induces nonlinear effects and produces phenomena such as high-order harmonics and static components. Current nonlinear ultrasonic testing focuses on the second harmonic, that is, using the second harmonic to establish its nonlinear coefficient to quantitatively evaluate the stress. 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 the Lamb wave, making its propagation characteristics more complicated, which will inevitably interfere with the second harmonic signal of the Lamb wave and make it difficult to distinguish it. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the nondestructive 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 insensitive 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-mentioned invention object, 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 wave, comprising the following steps:

[0008] 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;

[0009] 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;

[0010] S3, inverting the phase of the extracted signal, and re-exciting and receiving the signal with the inverted phase;

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

[0012] Furthermore, in step S1, in the process of constructing the Rayleigh-Lamb equation introducing the nonlinear theory of elastic body, there are the following settings:

[0013] The continuity assumption of the medium in which ultrasound propagates;

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

[0015] The plate structure is an isotropically uniform elastomeric material and is an ideal elastomer without mechanical dissipation.

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

[0017]

[0018]

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025] 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.

[0026] Furthermore, 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:

[0027] 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.

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

[0029] S41, performing fast Fourier transform on each received signal to obtain a fundamental frequency domain amplitude;

[0030] S42, adding the positive and negative phase fundamental frequency waves received twice, and performing low-pass filtering to obtain a static component waveform;

[0031] S43, performing fast Fourier transform on the static component waveform to obtain the frequency domain amplitude of the static component;

[0032] S44. Calculate 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.

[0033] Furthermore, in step S44, the ultrasonic nonlinear coefficient for:

[0034]

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

[0036] Furthermore, in 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.

[0037] Compared with the existing plate stress detection technology, the advantages of using the static component of nonlinear ultrasonic Lamb waves to detect stress state in the present invention are:

[0038] (1) Accumulation does not require group velocity matching, and the selection of fundamental frequency and mode is more flexible;

[0039] (2) The carrier frequency of the static component can be regarded as 0. The attenuation of ultrasound is related to the frequency. Therefore, the small attenuation of the static component makes its propagation range wider and 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A flow chart of the nondestructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves provided by the present invention.

[0042] Figure 2 A time domain diagram of the received baseband Lamb wave signal provided by the present invention.

[0043] Figure 3 A schematic diagram of adding positive and negative phase fundamental frequency Lamb waves received by the present invention.

[0044] Figure 4The present invention provides a time domain diagram of a static component obtained by adding positive and negative phases and then low-pass filtering.

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

[0046] Figure 6 This is a schematic diagram of the change of ultrasonic nonlinear parameters under different stress states provided by the present invention. DETAILED DESCRIPTION

[0047] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

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

[0049] 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;

[0050] S2. Based on the determined stress of the plate to be detected, a Lamb wave with a determined excitation frequency and excitation mode is excited at one end of the stress detection area of ​​the plate to be detected, and a corresponding signal is received and extracted through the other end of the stress detection area;

[0051] S3, inverting the phase of the extracted signal, and re-exciting and receiving the signal with the inverted phase;

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

[0053] Based on the acoustoelastic effect and the second-order perturbation theory, the propagation path of the ultrasonic Lamb wave propagating in the plate will generate a second-order body driving force, including the surface body driving force responding on the upper surface and the body body driving force between the two surfaces. The two body driving forces are related to the second-order elastic constant and the third-order elastic constant of the material, and contain different frequency components, such as static components and double frequency 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, and excite a series of zero-frequency ultrasonic guided wave modes along the propagation direction of the fundamental frequency Lamb wave. They are superimposed on each other and eventually constitute the static component sound field of the ultrasonic Lamb wave. Combined with the acoustoelastic effect, the static component generated above 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 nonlinear ultrasonic Lamb waves. In this embodiment, the test plate is a metal thin plate, and an ultrasonic transducer that meets the detection requirements, such as a high-frequency excitation and a low-frequency receiving probe, is 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, the corresponding Lamb wave excitation mode and excitation frequency are selected using the group velocity matching conditions of the Lamb wave fundamental wave and the static component, and the operating frequency and main structural parameters of the detection transducer are determined accordingly. Then, the specific fundamental frequency Lamb wave and static component mode pairs are focused and analyzed to obtain the stress state detection results of the plate.

[0056] Specifically, in step S1 of the embodiment of the present invention, during the propagation of Lamb waves, the propagation speed thereof will change with the frequency, which is a phenomenon called dispersion effect. The Rayleigh-Lamb equation can represent the dispersion of Lamb waves. Assuming that the thin plate medium is infinite and the upper and lower boundaries of the thin plate are free, the equation is expressed as:

[0057] Symmetrical Mode:

[0058]

[0059] Asymmetric mode:

[0060]

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

[0062] For the problem of Lamb wave velocity variation in plate structure under static load, the present invention introduces the nonlinear theory of elastic body. In the process of constructing the Rayleigh-Lamb equation introducing the nonlinear theory of elastic body, there are the following settings:

[0063] The continuity assumption of the medium in which ultrasound propagates;

[0064] Small perturbations of Lamb waves can be superimposed on the finite deformation of an object under static stress;

[0065] The plate structure is an isotropically uniform elastomeric material and is an ideal elastomer without mechanical dissipation.

[0066] Among them, the continuity hypothesis is a mechanical model hypothesis created in continuous medium mechanics to facilitate mathematical analysis, which includes: objects are continuously distributed in the space they occupy, and macroscopic physical quantities are continuous functions of space and time.

[0067] It is thus obtained that the Rayleigh-Lamb equation under load is:

[0068]

[0069] The exponential reading in the dispersion equation in the symmetric mode takes a positive sign, and the exponential reading in the antisymmetric mode takes a negative sign; where the wave number is defined as:

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076]

[0077] 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.

[0078] Once the Lame constant and third-order elastic constant of the material, as well as the magnitude of the stress in different directions, are determined, 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 is obtained, and then the Lamb wave dispersion curve is 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:

[0080] A Lamb wave mode and frequency whose group velocity difference with the Lamb wave S0 mode at 0MHz 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, thereby improving the accuracy of signal extraction when receiving signals.

[0081] In step S2 of this embodiment, a Lamb wave sinusoidal 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 to obtain a time domain diagram of the received baseband Lamb wave signal as shown in FIG. Figure 2 shown.

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

[0083] S41, performing fast Fourier transform on each received signal to obtain a fundamental frequency domain amplitude;

[0084] S42, adding the positive and negative phase fundamental frequency waves received twice, and performing low-pass filtering to obtain a static component waveform;

[0085] S43, performing fast Fourier transform on the static component waveform to obtain the frequency domain amplitude of the static component;

[0086] S44. Calculate 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.

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

[0088]

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

[0090] In step S42, the extracted signal is phase-reversed by 180°, and the propagated signal is received at the other end. The amplitudes of the two received signals are added together to obtain the schematic diagram of the addition of positive and negative phase baseband Lamb waves. Figure 3 As shown, the corresponding static component time domain diagram obtained after low-pass filtering is as follows Figure 4 shown.

[0091] In the embodiment of the present invention, based on the above method, the stress state of the test piece is changed, gradually increasing from 0MPa to 200MPa, increasing by 50MPa each time, and repeating the steps of excitation signal-receiving signal-inverting signal-receiving signal-adding two-phase signals-obtaining the time domain waveform of the static component each time, and finally obtaining the following Figure 5 The comparison diagram of the static component time domain waveform under different stress conditions is shown in Figure 1, which corresponds to the changes of ultrasonic nonlinear parameters under different stress states. Figure 6 shown.

[0092] In the implementation of the present invention, the ultrasonic nonlinear coefficient It will increase monotonically with the increase of tensile stress on the plate. By obtaining the ultrasonic nonlinear coefficient of the same test piece under stress-free state, the ultrasonic nonlinear coefficient can be used The stress state is quantitatively evaluated by the change of

[0093] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

[0094] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations 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. Process the received signal and calculate the ultrasonic nonlinear coefficient as an evaluation parameter of the stress state of the plate to be detected.

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 2 is characterized in that: 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.

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 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.

5. 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: 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. Calculate 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.

6. The non-destructive testing stress evaluation method based on the static component of nonlinear ultrasonic Lamb waves according to claim 5 is characterized in that: 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.

7. 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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