A steel strand single-twist-pitch stress identification method based on guided wave peak time delay

Abstract

The application provides a steel strand single-twist stress identification method based on a wave peak time delay, and the steel strand stress is deduced from the calculation of a wave peak time delay index. The application can be applied to the identification of the existing stress of the steel strand, and the stress identification object is a seven-wire steel strand with a nominal diameter of 15.2 mm, and can be extended to other specifications of steel strands or general rod-shaped stress components. The steel strand stress identification distance can also be extended from the single-twist of the steel strand to the multi-twist. The wave peak time delay index used in the application has the characteristics of high sensitivity and high efficiency, and the optimal guided wave excitation frequency can be selected to improve the accuracy and robustness of the stress identification.

Description

Technical Field

[0001] This invention relates to the field of stress identification technology for single twist of steel strand, and in particular to a method for stress identification of single twist of steel strand based on guided wave peak delay. Background Technology

[0002] Prestressed concrete structures are widely used in industrial and civil buildings, especially in bridge structures, due to their advantages such as fully utilizing material properties, reducing structural weight, inhibiting crack development, and increasing structural span and stiffness. Steel strands are commonly used as load-bearing components in prestressed structures.

[0003] With long-term load application and environmental influences, the mechanical properties of steel strands in a structure will degrade, leading to wire corrosion and breakage, which in turn causes cracking of the protective layer and severe degradation of interfacial bonding performance. Furthermore, temperature changes, concrete creep or shrinkage, and steel strand creep can also cause variations in the stress level of the steel strands. Whether the tension in the steel strands increases or decreases, it will reduce the structural load-bearing capacity, ultimately leading to a major safety accident. Currently, the main methods for detecting the stress state of steel strands in existing structures include vibration frequency method, magnetoelastic method, strain gauge method, static equilibrium method, and ultrasonic guided wave method. The vibration frequency method is affected by factors such as sag, bending stiffness, and boundary conditions, resulting in significant uncertainty in measurement results. The magnetoelastic method requires a magnetostrictive sensor to pass through a magnetic ring, which is not feasible for testing steel strands inside existing structures. Strain testing methods include resistance strain gauge testing (pressure sensor method), vibrating wire pressure sensors, and fiber optic grating sensors; however, this method can only measure strain or stress changes and does not have the possibility of absolute stress detection, and these methods have poor durability. The static equilibrium method requires the application of transverse tension, which is difficult to apply when the stress level of the steel strand is high, resulting in low accuracy of the test results.

[0004] Ultrasonic guided wave stress identification based on guided wave acoustoelasticity has unique advantages in the field of non-destructive stress testing. However, most current detection indicators for ultrasonic guided wave stress identification are based on guided wave group velocity. Measuring guided wave group velocity requires stripping at least 1.5-2m of bare steel strand from the actual structure as the measurement gauge length. This is not only difficult to implement in actual testing, but also has a serious impact on the durability of the steel strand and the structural safety. If the measurement gauge length is to be shortened to about 20cm (this length is the single twist length of the steel strand), guided wave group velocity cannot continue to be used, and guided wave phase velocity must be used instead. Since guided waves inevitably disperse during propagation in the steel strand, guided wave phase velocity is difficult to measure directly, and other easily measurable identification indicators need to be constructed based on guided wave phase velocity. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for identifying stress in a single twist of a steel strand based on the wave crest delay. The method identifies the existing stress in the steel strand based on the wave crest delay index, and features a short measurement gauge length, high sensitivity of the delay index to stress changes, high signal strength ratio, and a good linear relationship between the delay index and stress changes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for identifying the stress of a single twist of steel strand based on the waveguide peak delay, comprising the following steps:

[0007] Step 1: Take m steel strand samples of the same type as those used in the actual engineering structure. For each steel strand sample, randomly select n segments, the length of which is the single-twist pitch or multi-twist pitch of the steel strand. Calibrate the stress sensitivity coefficient k of the peak delay at a certain excitation frequency and obtain the signal strength ratio index c and the linear fit goodness r. 2 Let k be the stress sensitivity coefficient of the j-th section of the i-th steel strand sample. ij Signal strength ratio index c ij and linear fit goodness At this point, 1≤i≤m, 1≤j≤n; the stress sensitivity coefficient k of the first section of the first steel strand sample. 11 Signal strength ratio index c 11 and linear fit goodness The first 1 in the subscript indicates the first steel strand, and the second 1 indicates the first section. The method for obtaining the indicators of other sections of other steel strand samples is the same.

[0008] Step 2: Change the frequency of the narrowband pulse signal and repeat Step 1 to obtain the stress sensitivity coefficient, signal strength ratio and linear fit goodness at several preset frequencies;

[0009] Step 3: By comparing the values ​​of these three indicators, select the optimal frequency from several preset frequencies;

[0010] Step 4: Repeat step 1, but change the tensile stress of the steel strand sample to A and the number of sections to N, where N is selected according to the actual situation; measure the relative time coordinates of the third wave peak in all sections of all steel strand samples on the signal acquisition device and take the average value, which is set as S.

[0011] Step 5: Select sections 1 to P of the steel strand to be tested, where P is selected according to the actual situation; measure the relative time coordinates of the third peak of all sections displayed on the signal acquisition device and take the average value, which is Q.

[0012] Step 6: Based on SQ= (AB), the existing prestress of the steel strand is obtained as B = A - (SQ) / , This represents the mean value of the stress sensitivity coefficient. If the third peak is not used and other peak numbers are used for identification, the method is the same as above.

[0013] In a preferred embodiment, the stress sensitivity coefficient k ij Signal strength ratio index c ij and linear fit goodness The steps to obtain it are as follows:

[0014] (1) Select the excitation signal for guided wave stress identification, with a frequency of f1;

[0015] (2) Control the temperature of the test site to be consistent with the temperature of the measurement site, then introduce a pulse signal on one side of the j-th section of the i-th steel strand sample and receive the signal at the other end;

[0016] (3) This step is repeated w-1 times to obtain a two-dimensional sequence of stress and relative time coordinates (1400, t1), (p2, t2), ..., (p w , t w ), of which 1400 <p 2< p 3< ...< p w ;

[0017] (4) Fit a straight line based on the above sequence. The slope of the straight line is the stress sensitivity coefficient k. ij The absolute value of the amplitude of the waveguide received signal is c. out The absolute value of the amplitude c of the guided wave transmitted signal in Then the signal frequency f is obtained. i Under the condition of the signal strength ratio index c of the j-th section of the i-th steel strand sample ij =c out / c in ;

[0018] (5) Similarly, we can obtain k 12 ~k 1n ... k m1 ~ k mn , ~ ... ~ c 12 ~c 1n ... c m1 ~c mn ;

[0019] (6) For all k, R 2 Taking the average of the values ​​of c and c, we obtain the excitation frequency f. i At that time, the average value of all sections of all steel strand samples was taken in order to eliminate the influence of random errors to the greatest extent.

[0020] In a preferred embodiment, step 3 specifically includes:

[0021] (1) Set a threshold for the stress sensitivity coefficient, and screen out the frequency range corresponding to the stress sensitivity coefficient higher than this threshold;

[0022] (2) Set a threshold for the signal strength ratio, and screen out the frequency range corresponding to the signal strength ratio higher than this threshold;

[0023] (3) Set a threshold for the goodness of linear fit, and screen out the frequency range corresponding to the goodness of linear fit higher than this threshold;

[0024] (4) The intersection of the above three regions, that is, f2 < f < f1, is the interval where the optimal frequency is located. Select the preset frequency falling within this interval as the best excitation frequency. If there is more than one best excitation frequency, any one can be selected; the sensitivity coefficient corresponding to this frequency is the sensitivity coefficient of the steel strand sample.

[0025] In a preferred embodiment, for the steel strand under any stress state, there is:

[0026] (1)

[0027] σ —— The current stress of the steel strand (unit: MPa);

[0028] σ0 —— The reference stress of the steel strand (unit: MPa);

[0029] v —— The phase velocity of the guided wave signal corresponding to the stress σ of the steel strand (unit: m / s);

[0030] v0 —— The phase velocity corresponding to the reference stress σ0 of the steel strand (unit: m / s);

[0031] a —— The sensitivity coefficient of the guided wave signal phase velocity to stress (unit: m·s -1 / MPa);

[0032] Equation (1) is transformed into:

[0033] (2)

[0034] The photoelastic effect is a weak effect. Therefore, the difference between v and v0 is very small, while the difference between σ and σ0 is relatively large. It can be seen that a is a small quantity compared with v0 / (σ - σ0).

[0035] In a preferred embodiment, it is assumed that the guided wave is emitted from one end of the strand length and received from the other end, the wave propagation time is t, the propagation distance is l, the wave phase velocity at zero stress is v0, and the corresponding propagation time is t0. At this time, l = v0t0 = vt, that is, the change in the strand length caused by stress change is ignored. Thus, we have:

[0036] (3)

[0037] (4)

[0038] (5)

[0039] Since a is a small quantity compared to v0 / (σ-σ0), a can be ignored in the denominator of equation (5), and equation (5) can be transformed into:

[0040] (6)

[0041] In the formula, t-t0 is the peak delay index;

[0042] a, t0, and l are all constants, therefore -at0 2 / l is also a constant, assuming k = -at0 2 The specific value of / l.k can be obtained through experimental calibration;

[0043] Let ∆σ = σ - σ0 and ∆t = t - t0, then equation (6) transforms into:

[0044] (7)

[0045] Where k is called the sensitivity coefficient of the wave crest delay force;

[0046] In actual measurement, by measuring the value of the peak delay index ∆t=t-t0, the change in stress under this state compared to the reference stress state can be obtained as ∆σ=σ-σ0.

[0047] Different guided wave excitation frequencies (k values) result in different crest delay stress sensitivities at different frequencies; the optimal excitation frequency should be k, c, and R. 2 Each value corresponds to the frequency of the larger value within its respective numerical range;

[0048] Suppose that for any steel strand, we need to measure its existing prestress σ; take n steel strand samples of the same type, and according to the selected optimal excitation frequency f, measure the relative time coordinates t1, t2, t3...t of the wave peak at the single twist length of the n steel strand samples under the initial stress σ0. n Take their average By changing the stress state of the steel strands and measuring the time delay of different steel strands under different stress levels, the stress sensitivity coefficients of n steel strand samples are obtained as k1, k2, k3...k n Take their average Then, for the object being tested, using the optimal excitation frequency f, the relative time coordinate t of the peak displayed on the signal acquisition device at the single twist length is obtained, thus obtaining the peak delay t_t of the current stress state compared to the initial stress state. The existing prestress σ of the steel strand can be calculated by equation (7).

[0049] Compared with the prior art, the present invention has the following beneficial effects: Based on the waveguide phase velocity, the present invention constructs a waveguide peak time delay index that can detect the stress of steel strands within the gauge length under single-twist gauge length conditions. This is of great significance for stress detection of existing steel strands in the field of civil engineering, as well as structural safety assessment and subsequent reinforcement based on the detection results. Attached Figure Description

[0050] Figure 1 This is a schematic illustration of the sensitivity index of the peak delay to the stress in a preferred embodiment of the present invention.

[0051] Figure 2 This is a schematic illustration of the signal strength ratio index in a preferred embodiment of the present invention.

[0052] Figure 3 This is a schematic illustration of the goodness-of-fit index between peak delay and stress in a preferred embodiment of the present invention.

[0053] Figure 4 The time domain diagram (a) and frequency domain diagram (b) of the five-peak wave at 100kHz are preferred embodiments of the present invention.

[0054] Figure 5 This is a schematic illustration of the wave peak identification of the guided wave received signal and the transmitted signal, and the concepts of the first, second, third... wave peaks, in a preferred embodiment of the present invention.

[0055] Figure 6 This is a schematic illustration of the preferred excitation frequency of the guided wave in a preferred embodiment of the present invention. Detailed Implementation

[0056] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0057] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0058] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application; as used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise; furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0059] refer to Figures 1 to 6 According to the guided wave acoustoelasticity theory, for a steel strand under any stress state, we have:

[0060] (1)

[0061] σ — Current stress in the steel strand (unit: MPa);

[0062] σ0 — Reference stress of steel strand (unit: MPa);

[0063] v — Phase velocity of the guided wave signal corresponding to the stress σ in the steel strand (unit: m / s);

[0064] v0 — Phase velocity corresponding to the reference stress σ0 of the steel strand (unit: m / s);

[0065] a——Sensitivity coefficient of the waveguide signal phase velocity to force (unit: m·s) -1 / MPa);

[0066] Equation (1) is transformed into:

[0067] (2)

[0068] The acoustic elastic effect is a weak effect, so the difference between v and v0 is very small, while the difference between σ and σ0 is relatively large. Therefore, a is a small quantity compared to v0 / (σ-σ0).

[0069] Assuming the guided wave is emitted from one end of the strand length and received from the other, the wave propagation time is t, the propagation distance is l, the wave phase velocity at zero stress is v0, and the corresponding propagation time is t0 (at this time, l=v0t0=vt, i.e., the change in strand length caused by stress change is ignored), then we have:

[0070] (3)

[0071] (4)

[0072] (5)

[0073] Since a is a small quantity compared to v0 / (σ-σ0), a can be ignored in the denominator of equation (5), and equation (5) can be transformed into:

[0074] (6)

[0075] In the formula, t-t0 is the peak delay index.

[0076] Because guided waves inevitably disperse within the steel strand, the values ​​of v, v0, t, and t0 are difficult to measure accurately. However, the physical meaning of t-t0 is the movement of the guided wave crest (i.e., phase point) on the guided wave signal acquisition instrument before and after stress change. Since the abscissa of the guided wave signal on the waveform acquisition device is the relative time coordinate of the wave crest displayed on the signal acquisition device, the magnitude of the wave crest delay can be determined by measuring the change in the relative time coordinate. For a steel strand with fixed material parameters and a certain guided wave excitation frequency, a, t0, and l are all constant values, therefore -at0 2 / l is also a constant, assuming k = -at0 2 The specific value of / l.k can be obtained through experimental calibration.

[0077] Let ∆σ = σ - σ0 and ∆t = t - t0, then equation (6) transforms into:

[0078] (7)

[0079] Where k is called the sensitivity coefficient of the force corresponding to the peak delay.

[0080] This means that if the value of k corresponding to a certain steel strand is determined in advance through testing, the change in stress under this state compared to the reference stress state can be obtained by measuring the value of the peak delay index ∆t=t-t0 during actual measurement.

[0081] For a given steel strand, different guided wave excitation frequencies have different values ​​of k, meaning that the sensitivity of wave crest delay to stress varies at different frequencies. In actual testing, a guided wave excitation frequency with a larger k value is required, but other factors need to be considered when selecting the frequency. To this end, this invention proposes for the first time a method for selecting the optimal guided wave excitation frequency range for steel strands by comprehensively considering three indicators: the sensitivity of wave crest delay to stress changes, the signal strength ratio of the guided wave received signal and the guided wave transmitted signal, and the goodness of fit between wave crest delay and stress. The sensitivity of wave crest delay refers to the value of the sensitivity coefficient k. Even for the same steel strand, different guided wave excitation frequencies correspond to different k values, and obviously, the larger the k value, the better. The sensitivity index of wave crest delay is defined as shown in equation (7), as attached. Figure 1 As shown by the linear fitting line w between the peak delay and stress change, the greater the slope of line w, the higher the sensitivity of the peak delay to stress. The signal strength ratio index c is attached. Figure 2 absolute value of the amplitude c of the middle-guided wave received signal out The absolute value of the excitation signal amplitude c in The ratio of the wave crest delay to the stress varies with the guided wave excitation frequency, and the value of c also varies. The goodness of fit between the wave crest delay and the stress is attached. Figure 3 The coefficient of determination R of the fitted straight line of time delay and stress change shown is... 2 , Figure 3 Which peak in the middle corresponds to Figure 2 Peak number in R 2 The larger the value, the better the linear fit between the peak delay and stress change. Different guided wave excitation frequencies correspond to different R values. 2 Therefore, the optimal excitation frequencies should be k, c, and R. 2 Each value corresponds to the larger value within its respective numerical range.

[0082] Furthermore, assuming that for any steel strand, we need to measure its existing prestress σ (given that its initial prestress is σ0), we can take n steel strand samples of the same type. Based on the optimal excitation frequency f selected earlier, we can measure the relative time coordinates t1, t2, t3...t of the wave peaks at the single twist length of the n steel strand samples under the initial stress σ0 condition on the signal acquisition device. n Take their average By changing the stress state of the steel strands, the time delay of different steel strands under different stress levels can be measured, thereby obtaining the stress sensitivity coefficients of n steel strand samples, namely k1, k2, k3...k n Take their average Then, for the object being tested, using the optimal excitation frequency f, the relative time coordinate t of the peak displayed on the signal acquisition device at the single twist length is obtained, thus obtaining the peak delay t_t of the current stress state compared to the initial stress state. The existing prestress σ of the steel strand can be calculated using equation (7).

[0083] Example:

[0084] Assume there is a 7-wire prestressed steel strand with a nominal diameter of 15.2 mm in the project. The existing prestress is unknown, but the initial stress is known to be A MPa. Let the existing prestress be BMPa. The steps for measuring B are as follows:

[0085] Step 1: Take m steel strand samples of the same type as those used in the actual engineering structure. For each steel strand sample, randomly select n sections (the length of each section is the single-twist pitch of the steel strand, or multiple-twist pitch is also acceptable). Calibrate the stress sensitivity coefficient k of the peak delay at a certain excitation frequency and obtain the signal strength ratio index c and the linear fit goodness r. 2 Let k be the stress sensitivity coefficient of the j-th section of the i-th steel strand sample. ij, signal strength ratio index c ij and goodness of linear fit , where 1 ≤ i ≤ m and 1 ≤ j ≤ n. Taking the stress sensitivity coefficient k 11 , signal strength ratio index c 11 and goodness of linear fit (the first subscript 1 represents the first steel strand, and the second 1 represents the first section) as an example, the method for obtaining the indexes of other steel strand samples in other sections is the same. k 11 , c 11 and are obtained as follows:

[0086] (1) Select the excitation signal for guided wave stress identification. Assume that the narrowband pulse signal shown in Figure 4 is used first, and its frequency is f1.

[0087] (2) Control the temperature of the test field to be the same as that of the measurement site. Tension the stress of the first steel strand sample to 1400 MPa (elastic limit of the steel strand), then introduce the pulse signal in (1) on one side of the first section of the first steel strand sample, and receive the signal at the other end. Assume that the relative time coordinate of the third peak of the guided wave reception signal shown in Figure 5 displayed on the signal acquisition device is t1, and the peaks of other serial numbers can also be used.

[0088] (3) Tension the stress of the first steel strand sample to p2 MPa (0 < p2 < 1400) in the test field. Assume that the relative time coordinate of the third peak of the signal displayed on the signal acquisition device is t2 at this time. This step is repeated w - 1 times to obtain a two-dimensional sequence of stress and relative time coordinates (1400, t1), (p2, t2), ……, (p w , t w ), where 1400 < p 2< p 3< …… < p w .

[0089] (4) A straight line can be fitted according to the above sequence, and the slope of the straight line is k 11 . The coefficient of determination of the fitted straight line is . Assume that the guided wave signal obtained by experiment at a certain excitation frequency in the first section of the first steel strand sample is as shown in Figure 2 . The absolute value of the amplitude of the guided wave reception signal is c out , and the absolute value of the amplitude of the guided wave transmission signal is c in . Then, the signal strength ratio index c 11 of the first section of the first steel strand sample under the signal frequency f1 can be obtained as c out / c in .

[0090] (5) Similarly, k 12 ~k 1n ,..., k m1 ~ k mn , ~ ,... ~ , c 12 ~c 1n ,... m1 ~c mn .

[0091] (6) Take the average of all k, R 2 and c values to obtain the stress sensitivity coefficient K f1 , signal strength ratio c f1 and linear fitting goodness of fit of the third peak delay of the strand sample of this type at the excitation frequency f1. Take the mean of all sections of all strand samples to minimize the influence of accidental errors.

[0092] Step 2: Change the frequency of the narrowband pulse signal and repeat Step 1 to obtain the stress sensitivity coefficients, signal strength ratios and linear fitting goodness of fit at several preset frequencies.

[0093] Step 3: Select the optimal frequency from several preset frequencies by comparing the values of these three indicators. The specific method is as follows:

[0094] (1) Set the stress sensitivity coefficient threshold, screen out the frequency range corresponding to the stress sensitivity coefficient higher than this threshold, and assume that the screening result f is f < f1 as shown in the appendix Figure 6 .

[0095] (2) Set the signal strength ratio threshold, screen out the frequency range corresponding to the signal strength ratio higher than this threshold, and assume that the screening result f is f2 < f < f3 or f < f4 as shown in the appendix Figure 6 .

[0096] (3) Set the linear fitting goodness of fit threshold, screen out the frequency range corresponding to the linear fitting goodness of fit higher than this threshold, and assume that the screening result f is f5 < f < f6 as shown in the appendix Figure 6 .

[0097] (4) The intersection of the above three regions, that is, f2 < f < f1, is the interval where the optimal frequency is located. The preset frequency falling within this interval can be taken as the optimal excitation frequency. If there is more than one optimal excitation frequency, any one can be selected. The sensitivity coefficient corresponding to this frequency is the sensitivity coefficient of the strand sample.

[0098] Step 4: Repeat Step 1, but change the tensile stress of the steel strand sample to A and the number of sections to N, where N can be selected according to the actual situation. Measure the relative time coordinates of the third peak in all sections of all steel strand samples as displayed on the signal acquisition device and take the average value, which is set as S.

[0099] Step 5: Select sections 1 to P of the steel strand to be tested, where P can be selected according to the actual situation. Measure the relative time coordinates of the third peak of all sections displayed on the signal acquisition device and take the average value, which is Q.

[0100] Step 6: From equation (7), we have SQ = (AB), from which we can obtain the existing prestress of the steel strand B = A - (SQ) / If the third peak is not used and other numbered peaks are used for identification, the method is the same as above.

Claims

1. A method for identifying the stress of a single twist of steel strand based on the waveguide peak delay, characterized in that... Includes the following steps: Step 1: Take m steel strand samples of the same type as those used in the actual engineering structure. For each steel strand sample, randomly select n segments, the length of which is the single-twist pitch or multi-twist pitch of the steel strand. Calibrate the stress sensitivity coefficient k of the peak delay at a certain excitation frequency and obtain the signal strength ratio index c and the linear fit goodness r. 2 ; set up The stress sensitivity coefficient k of the j-th section of the i-th steel strand sample ij Signal strength ratio index c ij and linear fit goodness At this point, 1≤i≤m, 1≤j≤n; the stress sensitivity coefficient k of the first section of the first steel strand sample. 11 Signal strength ratio index c 11 and linear fit goodness The first 1 in the subscript indicates the first steel strand, and the second 1 indicates the first section. The method for obtaining the indicators of other sections of other steel strand samples is the same. Step 2: Change the frequency of the narrowband pulse signal and repeat Step 1 to obtain the stress sensitivity coefficient, signal strength ratio and linear fit goodness at several preset frequencies; Step 3: By comparing the values ​​of these three indicators, select the optimal frequency from several preset frequencies; Step 4: Repeat step 1, but change the tensile stress of the steel strand sample to A and the number of sections to N, where N is selected according to the actual situation; measure the relative time coordinates of the third wave peak in all sections of all steel strand samples on the signal acquisition device and take the average value, which is set as S. Step 5: Select sections 1 to P of the steel strand to be tested, where P is selected according to the actual situation; measure the relative time coordinates of the third peak of all sections displayed on the signal acquisition device and take the average value, which is Q. Step 6: Based on SQ= (AB), the existing prestress of the steel strand is obtained as B = A - (SQ) / , This represents the mean value of the stress sensitivity coefficient. If the third peak is not used and other peak numbers are used for identification, the method is the same as above.

2. The method for identifying single-twist stress of steel strand based on guided wave crest time delay according to claim 1, characterized in that, Stress sensitivity coefficient k ij Signal strength ratio index c ij and linear fit goodness The steps to obtain it are as follows: (1) Select the excitation signal for guided wave stress identification, with a frequency of f1; (2) Control the temperature of the test site to be consistent with the temperature of the measurement site, then introduce a pulse signal on one side of the j-th section of the i-th steel strand sample and receive the signal at the other end; (3) This step is repeated w-1 times to obtain a two-dimensional sequence of stress and relative time coordinates (1400, t1), (p2, t2), ..., (p w , t w ), of which 1400 <p 2< p 3< ...< p w ; (4) Fit a straight line based on the above sequence. The slope of the straight line is the stress sensitivity coefficient k. ij The absolute value of the amplitude of the waveguide received signal is c. out The absolute value of the amplitude c of the guided wave transmitted signal in Then the signal frequency f is obtained. i Under the condition of the signal strength ratio index c of the j-th section of the i-th steel strand sample ij =c out / c in ; (5) Similarly, we can obtain k 12 ~k 1n ... k m1 ~ k mn , ~ ... ~ c 12 ~c 1n ... c m1 ~c mn ; (6) For all k, R 2 Taking the average of the values ​​of c and c, we obtain the excitation frequency f. i At that time, the average value of all sections of all steel strand samples was taken in order to eliminate the influence of random errors to the greatest extent.

3. The method for identifying single-twist stress of steel strand based on guided wave crest time delay according to claim 1, characterized in that, Step 3 specifically includes: (1) Set a threshold for the stress sensitivity coefficient and filter out the frequency range corresponding to the stress sensitivity coefficients that are higher than the threshold; (2) Set a signal strength ratio threshold and filter out the frequency ranges corresponding to signal strength ratios higher than the threshold; (3) Set a goodness-of-fit threshold and filter out the frequency range corresponding to goodness-of-fit values ​​that are higher than the threshold; (4) The intersection of the above three regions, that is, f2 < f < f1, is the interval where the optimal frequency lies. The preset frequency falling within this interval is taken as the best excitation frequency. If there is more than one best excitation frequency, any one can be selected; the sensitivity coefficient corresponding to this frequency is the sensitivity coefficient of the steel strand sample.

4. The method for identifying single-twist stress of steel strand based on guided wave crest time delay according to claim 1, characterized in that, For a steel strand under any stress state, we have: (1) σ — Current stress in the steel strand, in MPa; σ0—Reference stress of steel strand, in MPa; v—The phase velocity of the guided wave signal corresponding to the stress σ in the steel strand, in m / s; v0—Phase velocity corresponding to the reference stress σ0 of the steel strand, in m / s; a — Sensitivity coefficient of the waveguide signal phase velocity to force, in m·s -1 / MPa; Equation (1) is transformed into: (2) The acoustic elastic effect is a weak effect, so the difference between v and v0 is very small, while the difference between σ and σ0 is relatively large. Therefore, a is a small quantity compared to v0 / (σ-σ0).

5. The method for identifying the single-twist stress of steel strand based on guided wave crest time delay according to claim 4, characterized in that, Assuming the guided wave is emitted from one end of the strand length and received from the other, with a propagation time of t and a propagation distance of l, and a wave phase velocity of v0 at zero stress, corresponding to a propagation time of t0, then l = v0t0 = vt. Ignoring the change in strand length caused by stress variation, we have: (3) (4) (5) Since a is a small quantity compared to v0 / (σ-σ0), a is ignored in the denominator of equation (5), and equation (5) is transformed into: (6) In the formula, t-t0 is the peak delay index; a, t0, and l are all constants, therefore -at0 2 / l is also a constant, assuming k = -at0 2 The specific value of / l;k can be obtained through experimental calibration; Let ∆σ = σ - σ0 and ∆t = t - t0, then equation (6) transforms into: (7) Where k is called the sensitivity coefficient of the wave crest delay force; In actual measurement, by measuring the value of the peak delay index ∆t=t-t0, the change in stress under this state compared to the reference stress state can be obtained as ∆σ=σ-σ0. Different guided wave excitation frequencies (k values) result in different crest delay stress sensitivities at different frequencies; the optimal excitation frequency should be k, c, and R. 2 Each value corresponds to the frequency of the larger value within its respective numerical range; Suppose that for any steel strand, we need to measure its existing prestress σ; take n steel strand samples of the same type, and according to the selected optimal excitation frequency f, measure the relative time coordinates t1, t2, t3...t of the wave peak at the single twist length of the n steel strand samples under the initial stress σ0. n Take their average ; By changing the stress state of the steel strands, the time delay of different steel strands under different stress levels was measured, and the stress sensitivity coefficients of the n steel strand samples were obtained as k1, k2, k3...k n Take their average Then, for the object being tested, using the optimal excitation frequency f, the relative time coordinate t of the peak displayed on the signal acquisition device under the single twist length is obtained, and the peak delay t- of the current stress state compared to the initial stress state is obtained. The existing prestress σ of the steel strand can be calculated by equation (7).

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