CN114858922B - Multilayer structure stress relaxation detection method based on ultrasonic wake wave - Google Patents

Abstract

The invention provides a multilayer structure stress relaxation detection method based on ultrasonic wake, which comprises the following steps: obtaining a multilayer structure sample to be detected, building a multilayer structure stress relaxation detection system, determining excitation signal parameters, obtaining ultrasonic signals in different stress states of the structure, taking the ultrasonic signals received in the maximum stress state as reference signals, extracting time shift characteristic parameters in stress states except the maximum stress state, combining the acoustic elastic effect and the stress strain relation, building a secondary relation model of the multilayer structure stress and the time shift characteristic parameters, and detecting stress relaxation of the multilayer structure. The invention has the advantages of very sensitive to small changes, convenient detection process, easy realization, low requirement on detection equipment, repeatable wake wave signals, good detection stability, realization of high-precision and high-sensitivity measurement of multi-layer structure stress changes, important engineering reference value, and more coincidence between the proposed quadratic relation model and experimental results compared with a general linear model.

Description

Stress relaxation detection method for multilayer structures based on ultrasonic coda wave

Technical Field

The invention belongs to the technical field of nondestructive detection of stress in multi-layer structures, and in particular to a method for detecting stress relaxation of multi-layer structures based on ultrasonic coda waves.

Background technique

As a typical structure of the shell of long-storage equipment, the multilayer structure needs to be in storage for a long time during its life cycle. Affected by natural factors (temperature, humidity, atmospheric pressure, mold, salt spray, etc.) and induced factors (electromagnetic radiation, static electricity, vibration, impact, etc.), the multilayer structure will age, degrade, and undergo stress relaxation reactions, resulting in continuous degradation of mechanical properties and even functional failure. The interlaminar stress of the multilayer structure is one of the key parameters for the assembly requirements of precision components and the assessment of their health status, and is of particular concern in engineering. Therefore, real-time monitoring of the performance degradation of the multilayer structure under storage, studying the changing laws of the stress state of the multilayer structure, and predicting the service life of the multilayer structure can further provide a theoretical basis for maintenance decisions of long-storage equipment, help improve economic benefits and service life, and also have great engineering value and strategic significance.

The current stress detection methods mainly include stress release method, neutron diffraction method, acoustic elastic method, electromechanical impedance method and nonlinear ultrasonic method. Among them, stress release method is a destructive detection method with low test accuracy and dependence on the test environment. Changes in the external environment can easily affect the measurement results. The detection principle of neutron diffraction method is based on Bragg diffraction, which is greatly disturbed by the surface state. The equipment is expensive and not portable, making it difficult to be applied on the engineering site. Acoustic elastic method is an offline detection method. The ultrasonic frequency used is very high, which requires high-precision measurement equipment, and requires the preparation of stress-free standard blocks and complex detection. Electromechanical impedance method combines piezoelectric sensors with the structure to be tested. By detecting the change of electrical impedance, the change of structural stress can be detected. It is only suitable for the detection of local dynamic characteristics, which is easily limited by the detection equipment and has poor detection robustness. Nonlinear ultrasonic method (high-order harmonic and sideband method) uses the interaction between ultrasonic waves and the nonlinear characteristics inside the structure to cause the generation of new frequency components. According to the change of the amplitude of high-order harmonics and sidebands in the ultrasonic signal spectrum, the change of stress can be detected, but the amplitude of harmonics and sidebands is very small, there are many sources of error, and it is easy to be masked by noise. Ultrasonic testing is an ideal online nondestructive testing method. However, traditional ultrasonic testing only focuses on the characteristics of direct waves, which is often not sensitive to the detection of changes in the microstructure of the medium. The tail wave formed by multiple scattering will sample the weak changes in the medium multiple times, so the tail wave interference method is very sensitive to small changes in the wave path. The tail wave refers to the tail part of the wave that has been strongly scattered multiple times. It was first used in earthquake engineering and has been mainly used in the field of concrete structure health monitoring in recent years. Therefore, considering the heterogeneity of multi-layer structures and the rough characteristics of the connection interface, when ultrasonic waves pass through, they will emit multiple scatterings to form tail waves that are very sensitive to small changes. With the help of tail wave interference technology, it is urgent and necessary to seek a multi-layer structure stress relaxation detection method based on ultrasonic tail waves to accurately obtain the stress size of multi-layer structures.

Summary of the invention

In view of the defects in the above-mentioned prior art, the present invention proposes a stress relaxation detection method for a multilayer structure based on ultrasonic tail waves. The method includes obtaining a multilayer structure sample to be detected, building a stress relaxation detection system for a multilayer structure and determining the excitation signal parameters, obtaining ultrasonic signals under different stress states of the structure, taking the ultrasonic signal received under the maximum stress state as a reference signal, extracting the time-shift characteristic parameters under stress states other than the maximum stress state, combining the acoustic elastic effect and the stress-strain relationship, and establishing a quadratic relationship model between the stress and time-shift characteristic parameters of the multilayer structure to detect stress relaxation of the multilayer structure. The present invention is very sensitive to small changes, the detection process is convenient, easy to implement, and the requirements for the detection equipment are not high. The tail wave signal is repeatable and the detection stability is good. It can realize high-precision and high-sensitivity measurement of stress changes in multilayer structures, and has important engineering reference value. Compared with the general linear model, the proposed quadratic relationship model is more consistent with the experimental results.

The present invention provides a method for detecting stress relaxation of a multilayer structure based on ultrasonic coda waves, which comprises the following steps:

S1. Obtain a multi-layer structure sample to be tested, build a multi-layer structure stress relaxation detection system and determine the excitation signal parameters;

S2. Acquiring ultrasonic signals under different stress states of the structure: regulating the stress state of the multilayer structure, emitting excitation signals under different stress states, and collecting ultrasonic signals;

S3, taking the ultrasonic signal received under the maximum stress state σ ₀ as the reference signal, extracting the time-shift characteristic parameters under stress states other than the maximum stress state: selecting the coda signal with a time window of [tT, t+T] in the ultrasonic signal for signal processing, taking the received signal under the maximum stress state as the reference signal, and using the moving window cross-correlation method to extract the time-shift characteristic parameters under stress states other than the maximum stress state;

S4. Establishing a quadratic relationship model between multilayer structure stress and time-shift characteristic parameters;

S41. According to the acoustic elastic effect, when subjected to uniaxial stress σ, the velocity c _L of the longitudinal wave propagating along the direction of the applied uniaxial stress σ is expressed as:

Where c _L represents the longitudinal wave velocity parallel to the uniaxial stress σ; A represents the acoustic elastic constant that depends on the first-order Lame coefficient and the second-order Murnaghan coefficient; represents the longitudinal wave velocity without uniaxial stress;

S42, calculating the first coda wave arrival time t _C (σ ₁ ) and the second coda wave arrival time t _C (σ ₀ ) under the first stress state σ ₁ and the second maximum stress state σ ₀ by means of the third-order nonlinear elastic stress-strain relationship;

S43, respectively expand the first coda wave arrival time t _C (σ ₁ ) and the second coda wave arrival time t _C (σ ₀ ) in a second-order Taylor series at the second maximum stress state σ ₀ , and make a difference to obtain the travel time delay τ under stress variation:

Where, ξ ₁ ,ξ ₂ represent the second-order Taylor series expansion errors; k represents the proportionality constant between the first coda wave propagation path l _c1 and the direct wave propagation path 2l ₁ ; E represents the second-order elastic constant; E ₁ represents the third-order elastic constant; l ₀ represents the original length under no stress; represents the first constant that depends on the second maximum stress state σ ₀ , the acoustic elastic constant A, the third-order elastic constant E ₁ , and the second-order elastic constant E; /> represents the second constant that depends on the second maximum stress state σ ₀ , the acoustoelastic constant A, the third-order elastic constant E ₁ , and the second-order elastic constant E;

Formula (6) is the quadratic relationship model between the stress and time-shift characteristic parameters of the multilayer structure;

S5. Detect stress relaxation of multi-layer structures: Use the multi-layer structure stress relaxation detection system to obtain ultrasonic signals under unknown stress states, perform signal processing to extract time-shift characteristic parameters, and use the constructed quadratic relationship model between the multi-layer structure stress and the time-shift characteristic parameters to calculate the stress magnitude of the multi-layer structure.

Further, the acquisition of the time-shift characteristic parameters in step S3 specifically includes the following steps:

S31. The calculation formula of the cross-correlation function R(t _s ) of the waveform in the time window before and after the disturbance is:

Where, u(·) represents the reference coda signal; represents the coda wave signal after disturbance under stress states other than the maximum stress state; T represents the time window length of the coda wave part; t represents the center position of the time window; _ts represents the travel time difference in the cross-correlation function; the cross-correlation function R( _ts ) of the waveforms in the time window before and after the disturbance represents the correlation degree of the two waves;

S32. Obtain the travel time delay τ when the cross-correlation function R(t _s ) takes the maximum value, as the time shift characteristic parameter under the desired stress state.

Preferably, the acquisition of the ultrasonic signal in step S2 specifically includes the following steps:

S21, the ultrasonic signal is generated by the signal generator, amplified by the power amplifier and then drives the transmitting transducer to work;

S22. Considering the effectiveness of ultrasonic propagation, the piezoelectric contact transducer is coupled to the multilayer structure through an ultrasonic coupling agent;

S23. The ultrasonic signal measured by the receiving transducer is collected by an oscilloscope and transmitted to a computer for signal processing.

Preferably, the step S42 specifically includes the following steps:

S421, under the first stress state σ ₁ , the first coda wave arrival time corresponding to the first coda wave propagation path l _c1 is:

in, represents the first coda wave velocity under the first stress state σ ₁ ; l ₁ represents the deformation length under the stress state σ ₁ ;

S422, under the second maximum stress state σ ₀ , the second coda wave arrival time corresponding to the second coda wave propagation path l _c2 is:

in, represents the second coda wave velocity under the second maximum stress state σ ₀ .

Preferably, the third-order nonlinear elastic stress-strain relationship in step S42 is:

Preferably, the excitation signal is a sinusoidal pulse train signal modulated by a 3-cycle Hanning window. In order to achieve the maximum response of the structure, the frequency of the excitation signal is set to the center frequency of the excitation sensor.

Preferably, in step S2, the stress state of the multilayer structure is controlled by a torque wrench, and ultrasonic signals under different stress states are collected.

Compared with the prior art, the technical effects of the present invention are:

1. The present invention designs a method for detecting stress relaxation of multi-layer structures based on ultrasonic coda waves. The method utilizes the characteristic that the coda waves formed by multiple scattering are sensitive to stress changes. The moving window cross-correlation method is used to extract the coda wave time-shift characteristic parameters related to the changes in the structural stress state, thereby achieving high-precision and high-sensitivity measurement of stress changes in multi-layer structures. By constructing a theoretical quadratic relationship model between the coda wave time-shift characteristic parameters and stress changes, a mechanism reference is provided for the subsequent fitting physical model. Compared with the general linear model, the proposed quadratic relationship model is more consistent with the experimental results and can be better applied to the detection of stress relaxation of multi-layer structures.

2. The present invention designs a multi-layer structure stress relaxation detection method based on ultrasonic coda waves, which is aimed at the typical multi-layer structure of the shell of long-storage equipment, and finally realizes the detection of stress relaxation of complex and critical multi-layer structures, and has high detection sensitivity and resolution, and has important engineering reference value; the method is very sensitive to small changes, the detection process is convenient, easy to implement, and has low requirements on detection equipment, and the coda wave signal is repeatable and the detection stability is good.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the present application will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings.

FIG1 is a flow chart of a method for detecting stress relaxation of a multilayer structure based on ultrasonic coda waves of the present invention;

FIG2 is a schematic diagram of the composition of a multilayer structure sample of the present invention;

3 is a schematic diagram of a multilayer structure stress relaxation detection system and a process flow in an embodiment of the present invention;

FIG4 is a comparison diagram of coda wave waveforms within a time window [150 μs, 250 μs] under different stresses measured in an embodiment of the present invention;

FIG. 5 is a comparison diagram of the fitting of the quadratic relationship model and the general linear model to the coda wave time shift characteristic parameters and stress in an embodiment of the present invention.

Detailed ways

The present application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It should also be noted that, for ease of description, only the parts related to the relevant invention are shown in the accompanying drawings.

It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

FIG1 shows a method for detecting stress relaxation of a multilayer structure based on ultrasonic coda waves of the present invention, the method comprising the following steps:

S1. Obtain a multi-layer structure sample to be tested, build a multi-layer structure stress relaxation detection system and determine the excitation signal parameters.

The multi-layer structure specimen to be tested includes a multi-layer structure and a metal shell layer. The multi-layer structure is mainly composed of a stack of silicon foam material, simulated functional material and silicon foam material. The metal shell layer is composed of two upper and lower stainless steel plates connected by bolts.

The multilayer structure stress relaxation detection system comprises a signal generator, a high voltage power amplifier, an excitation transducer, a receiving transducer, an oscilloscope and a computer.

The excitation signal is a sine pulse train signal modulated by a 3-cycle Hanning window. In order to achieve the maximum response of the structure, the frequency of the excitation signal is set to the center frequency of the excitation sensor.

S2. Acquire ultrasonic signals under different stress states of the structure: regulate the stress state of the multilayer structure, emit excitation signals under different stress states, and collect ultrasonic signals.

The acquisition of ultrasonic signals specifically includes the following steps:

S21. The ultrasonic signal is generated by a signal generator, amplified by a power amplifier, and then drives the transmitting transducer to work.

S22. Considering the effectiveness of ultrasonic propagation, the piezoelectric contact transducer is coupled to the multilayer structure through an ultrasonic coupling agent.

S23. The ultrasonic signal measured by the receiving transducer is collected by an oscilloscope and transmitted to a computer for signal processing.

The stress state of the multilayer structure is controlled by a torque wrench, and ultrasonic signals under different stress states are collected.

S3. Taking the ultrasonic signal received under the maximum stress state σ ₀ as a reference signal, extracting the time-shift characteristic parameters under stress states other than the maximum stress state.

The coda signal with a time window of [t-T, t+T] in the ultrasonic signal is selected for signal processing. The received signal under the maximum stress state is taken as the reference signal, and the moving window cross-correlation method is used to extract the time-shift characteristic parameters under stress states except the maximum stress state.

The acquisition of time-shift characteristic parameters specifically includes the following steps:

S31. The calculation formula of the cross-correlation function R(t _s ) of the waveform in the time window before and after the disturbance is:

Where, u(·) represents the reference coda signal; represents the coda wave signal after disturbance under stress states other than the maximum stress state; T represents the time window length of the coda wave part; t represents the center position of the time window; _ts represents the travel time difference in the cross-correlation function. The cross-correlation function R( _ts ) of the waveforms in the time window before and after the disturbance represents the correlation degree of the two waves.

S32. Obtain the travel time delay τ when the cross-correlation function R(t _s ) takes the maximum value, as the time shift characteristic parameter under the desired stress state.

S4. Establish a quadratic relationship model between the stress and time-shift characteristic parameters of a multi-layer structure.

S41. According to the acoustic elastic effect, when subjected to uniaxial stress σ, the velocity c _L of the longitudinal wave propagating along the direction of the applied uniaxial stress σ is expressed as:

Where c _L represents the longitudinal wave velocity parallel to the uniaxial stress σ; A represents the acoustic elastic constant that depends on the first-order Lame coefficient and the second-order Murnaghan coefficient; represents the longitudinal wave velocity in the absence of uniaxial stress.

S42, using the third-order nonlinear elastic stress-strain relationship, calculate the first coda wave arrival time t _C (σ ₁ ) and the second coda wave arrival time t _C (σ ₀ ) under the first stress state σ ₁ and the second maximum stress state σ _0. The third-order nonlinear elastic stress-strain relationship is:

Where E represents the second-order elastic constant; _E1 represents the third-order elastic constant; _l1 represents the deformation length under stress state _σ1 ; _l0 represents the original length under no stress.

S421, under the first stress state σ ₁ , the first coda wave arrival time corresponding to the first coda wave propagation path l _c1 is:

in, represents the first coda wave velocity under the first stress state σ ₁ ; k represents the proportionality constant between the first coda wave propagation path l _c1 and the direct wave propagation path 2l ₁ .

S422, under the second maximum stress state σ ₀ , the second coda wave arrival time corresponding to the second coda wave propagation path l _c2 is:

in, represents the second coda wave velocity under the second maximum stress state σ ₀ .

S43, respectively expand the first coda wave arrival time t _C (σ ₁ ) and the second coda wave arrival time t _C (σ ₀ ) in a second-order Taylor series at the second maximum stress state σ ₀ , and make a difference to obtain the travel time delay τ under stress variation:

Among them, ξ ₁ ,ξ ₂ represent the second-order Taylor series expansion errors; represents the first constant that depends on the second maximum stress state σ ₀ , the acoustic elastic constant A, the third-order elastic constant E ₁ , and the second-order elastic constant E; /> represents the second constant that depends on the second maximum stress state σ ₀ , the acoustoelastic constant A, the third-order elastic constant E ₁ , and the second-order elastic constant E.

Formula (6) is the quadratic relationship model between the stress and time-shift characteristic parameters of the multilayer structure.

The quadratic relationship model between the multilayer structure stress and the time-shift characteristic parameter established in step S4 is a theoretical model derived on the basis of the standard specimen, and has no coupling relationship with the parameters in the aforementioned steps S1 to S3 in terms of computational derivation. Based on the quadratic relationship model, step S5 is executed to obtain the corresponding multilayer structure stress through the time-shift characteristic parameter extracted in step S3.

S5. Detect stress relaxation of multi-layer structures: Use the multi-layer structure stress relaxation detection system to obtain ultrasonic signals under unknown stress states, perform signal processing to extract time-shift characteristic parameters, and use the constructed quadratic relationship model between the multi-layer structure stress and the time-shift characteristic parameters to calculate the stress magnitude of the multi-layer structure.

The present invention will be further described in detail below in conjunction with a specific multi-layer structure stress relaxation detection case.

S1. Obtain a multi-layer structure sample to be tested. As shown in FIG2 , the multi-layer structure is mainly composed of silicon foam material. -Simulation of functional materials/> -Silicon foam/> In addition, the metal shell layer is composed of two stainless steel plates, upper and lower. The structure is connected by M6 bolts. Figure 3 shows the stress relaxation detection system of the multilayer structure. The detection system includes a signal generator (Tektronix, AFG 31022), a high-voltage power amplifier (Aigtek, ATA-4012), an excitation transducer (OLYMPUS, V1011, with a center frequency of 100KHz), a receiving transducer (OLYMPUS, V101-RB, with a center frequency of 500KHz), an oscilloscope (Tektronix, MDO3104) and a computer. The excitation signal is a sine pulse train signal modulated by a 3-cycle Hanning window. In order to achieve the maximum response of the structure, its frequency is set to the center frequency of the excitation sensor, 100KHz.

S2. The ultrasonic signal is generated by the signal generator, amplified by the power amplifier, and then driven to work by the transmitting transducer; the piezoelectric contact transducer is coupled to the multilayer structure through the ultrasonic coupling agent (Symeda CG-88) to ensure that the ultrasonic wave can be effectively transmitted; the receiving signal measured by the receiving transducer is collected by the oscilloscope and transmitted to the computer for signal processing. The stress state of the multilayer structure is controlled by the torque wrench (from 10N·m to 2N·m), and the ultrasonic signals under different stresses are collected.

Figure 3 and Figure 4 are the coda wave signals with a time window of [150μs, 250μs] under different stresses. It can be seen that the coda wave signals under different stress states show different travel time delays. The received signal under the maximum stress state (10N·m) is used as the reference signal, and the moving window cross-correlation method is used to extract the time-shift characteristic parameters under other stress states.

S4. With the help of the quadratic relationship model derived from the theory, the experimental results are fitted to establish a quadratic relationship model of the stress and time-shift characteristic parameters of the multilayer structure as shown in Figure 5. The results show that the goodness of fit of the quadratic relationship model R ² =0.9815, and the degree of agreement between the experimental results and the fitting results of the quadratic relationship model is very high, which proves the effectiveness of the method proposed in the present invention. Moreover, compared with the general linear model (linear model goodness of fit R ² =0.9255), the proposed quadratic relationship model has a better fitting effect and can better describe the actual situation. The detection method based on ultrasonic coda waves proposed in the present invention has a resolution of up to 0.2N·m, which is much higher than other stress detection methods, and can identify smaller stress relaxations in multilayer structures. Therefore, the method of the present invention can be more consistent with engineering practice and more suitable for monitoring stress relaxation of multilayer structures.

S5. Use the multi-layer structure stress relaxation detection system to obtain ultrasonic signals under unknown stress states, perform signal processing to extract time-shift characteristic parameters, and use the constructed quadratic relationship model between the multi-layer structure stress and the time-shift characteristic parameters to calculate the stress magnitude of the multi-layer structure.

The present invention takes into account the roughness and unevenness of the contact surface of the multi-layer structure, introduces a tail wave that is more sensitive to small changes, and proposes a high-resolution stress relaxation detection method based on ultrasonic tail waves. The experimental results show that with the change of torque, the tail wave formed by multiple scattering will produce obvious time offset, and the degree of time offset will show a quadratic increase trend as the torque decreases. Compared with the general linear model, the fitting results of the proposed quadratic relationship model are more consistent with the experimental results, and the method of the present invention can better detect the stress relaxation of the multi-layer structure. At the same time, the detection resolution of the multi-layer structure stress relaxation detection method based on ultrasonic tail waves proposed by the present invention is as high as 0.2N·m, that is, it is very sensitive to small stress relaxation, so it can effectively monitor the early stress relaxation of the multi-layer structure.

The present invention designs a multi-layer structure stress relaxation detection method based on ultrasonic coda waves. The method utilizes the characteristic that the coda waves formed by multiple scattering are sensitive to stress changes. The moving window cross-correlation method is used to extract the coda wave time-shift characteristic parameters related to the structural stress state changes, so as to achieve high-precision and high-sensitivity measurement of the stress changes of the multi-layer structure. By constructing a theoretical quadratic relationship model between the coda wave time-shift characteristic parameters and the stress changes, a mechanism reference is provided for the subsequent fitting physical model. Compared with the general linear model, the proposed quadratic relationship model is more consistent with the experimental results and can be better applied to the detection of stress relaxation of the multi-layer structure. For the typical multi-layer structure of the long-storage equipment shell, the detection of the complex and critical multi-layer structure stress relaxation is finally achieved, and the detection sensitivity and resolution are high, which has important engineering reference value. The method is very sensitive to small changes, the detection process is convenient, easy to implement, and the requirements for the detection equipment are not high. The coda wave signal is repeatable and the detection stability is good.

Finally, it should be noted that the above embodiments are only intended to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims (7)

1. The multilayer structure stress relaxation detection method based on ultrasonic wake wave is characterized by comprising the following steps of:

s1, acquiring a multilayer structure sample to be detected, building a multilayer structure stress relaxation detection system and determining excitation signal parameters;

S2, acquiring ultrasonic signals under different stress states of the structure: regulating and controlling the stress state of the multilayer structure, transmitting excitation signals under different stress states, and collecting ultrasonic signals;

S3, extracting time shift characteristic parameters under stress states except the maximum stress state by taking the ultrasonic signals received under the maximum stress state sigma ₀ as reference signals: selecting wake wave signals with time windows of [ T-T, t+T ] in ultrasonic signals for signal processing, taking a received signal in a maximum stress state as a reference signal, and extracting time shift characteristic parameters in stress states except the maximum stress state by adopting a moving window cross-correlation method;

s4, establishing a secondary relation model of the multilayer structure stress and the time shift characteristic parameter;

S41, as obtained according to the sonoelastic effect, when the uniaxial stress σ is applied, the longitudinal wave velocity c _L propagating along the direction of applying the uniaxial stress σ is expressed as:

Wherein c _L represents a longitudinal wave velocity parallel to the uniaxial stress σ direction; a represents the acoustic elastic constant depending on the first-order Lame coefficient and the second-order Murnaghan coefficient; representing longitudinal wave velocity without uniaxial stress;

S42, calculating a first wake arrival time t _C(σ₁) and a second wake arrival time t _C(σ₀ under a first stress state sigma ₁ and a second maximum stress state sigma ₀ by means of a third-order nonlinear elastic stress-strain relation;

S43, respectively expanding the second order taylor series at the second maximum stress state σ ₀ by the first wake arrival time t _C(σ₁) and the second wake arrival time t _C(σ₀), and obtaining the travel time delay τ under the stress change by the difference:

Wherein ζ ₁,ξ₂ represents a second order taylor series expansion error; k represents a proportionality constant of the first wake propagation path l _c1 and the direct wave propagation path 2l ₁; e represents a second-order elastic constant; e ₁ represents a third-order elastic constant; l ₀ denotes the original length under no stress; Representing a first constant that depends on a second maximum stress state σ ₀, an acoustic elastic constant a, a third-order elastic constant E ₁, a second-order elastic constant E; /(I) Representing a second constant that depends on the second maximum stress state σ ₀, the acoustic elastic constant a, the third-order elastic constant E ₁, the second-order elastic constant E;

The formula (6) is a secondary relation model of the multilayer structure stress and the time shift characteristic parameter;

S5, detecting stress relaxation of the multilayer structure: and acquiring ultrasonic signals in an unknown stress state by using a stress relaxation detection system of the multilayer structure, performing signal processing to extract time shifting characteristic parameters, and obtaining the stress of the multilayer structure by using a constructed quadratic relation model of the stress of the multilayer structure and the time shifting characteristic parameters.

2. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: the step S3 of obtaining the time shift characteristic parameter specifically includes the following steps:

S31, a calculation formula of a cross-correlation function R (t _s) of waveforms in a time window before and after disturbance is as follows:

wherein u (·) represents the reference wake signal; Representing wake signals after disturbance in stress states other than the maximum stress state; t represents the time window length of the wake segment; t represents the center position of the time window; t _s denotes the travel time difference in the cross-correlation function; the cross-correlation function R (t _s) of waveforms in the time window before and after disturbance represents the correlation degree of two rows of waves;

S32, obtaining the travel time delay tau when the cross-correlation function R (t _s) takes the maximum value as a time shift characteristic parameter in the state of the required stress.

3. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: the collecting of the ultrasonic signal in the step S2 specifically includes the following steps:

S21, ultrasonic signals are generated by a signal generator and are amplified by a power amplifier to drive a transmitting transducer to work;

S22, taking the effectiveness of ultrasonic propagation into consideration, coupling the piezoelectric contact transducer to the multilayer structure through an ultrasonic couplant;

S23, ultrasonic signals measured by the receiving transducer are collected through an oscilloscope and transmitted to a computer for signal processing.

4. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: the step S42 specifically includes the following steps:

S421, under a first stress state sigma ₁, a first wake arrival time corresponding to a first wake propagation path l _c1 is:

Wherein, Representing a first wake wave velocity at a first stress state σ ₁; l ₁ denotes the deformation length under stress state σ ₁;

S422, under the second maximum stress state σ ₀, the second wake arrival time corresponding to the second wake propagation path l _c2 is:

Wherein, Representing the second wake wave velocity at the second maximum stress state σ ₀.

5. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: the third-order nonlinear elastic stress-strain relationship in step S42 is:

6. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: the excitation signal is a sine pulse train signal modulated by a 3-period hanning window, and the frequency of the excitation signal is set to be the center frequency of the excitation sensor in order to enable the structure to reach maximum response.

7. The ultrasonic wake-based multilayer structure stress relaxation detection method of claim 1, wherein: in the step S2, the stress state of the multilayer structure is controlled by a torque wrench, and ultrasonic signals under different stress states are collected.