A steel strand axial stress monitoring method based on ultrasonic guided wave characteristics

Abstract

The application relates to a steel strand axial stress monitoring method based on ultrasonic guided wave characteristics, which comprises the following steps: S1, establishing a torsional coordinate system based on the spiral structure of the steel strand; calculating the phase velocity and energy velocity curve of the straight steel wire to analyze the frequency dispersion characteristics, and determining the monitoring mode and excitation frequency range; S2, extracting and analyzing the wave structure of the longitudinal mode and the bending mode of the straight steel wire; S3, calculating the frequency-stress-energy velocity curve of the straight steel wire under the action of different stresses; S4, calculating the energy velocity sensitive curve of the straight steel wire under the action of stress; S5, combining the results of S1, S2, S3 and S4, determining the appropriate mode and the corresponding excitation frequency as the monitoring mode and the excitation frequency of the steel strand axial stress monitoring; S6, designing the arrangement mode of the guided wave sensor according to the structure of the steel strand and the vibration mode diagram of the optimal mode; S7, obtaining the energy velocity and energy velocity change trend, frequency and other relationships of the guided wave mode through the receiving and processing of the sensor guided wave signal, and obtaining the fitting curve according to the relationship between the energy velocity change amount of the most suitable monitoring mode and the acoustic elastic constant under the most suitable excitation frequency, so that the steel strand axial stress monitoring is finally realized.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, and in particular to a method for monitoring axial stress in steel strands based on ultrasonic guiding characteristics. Background Technology

[0002] Steel strands are widely used in many industries, especially in civil engineering structures as commonly used tensile components. Their applications include cable-stayed bridge cables, suspension bridge cables, and prestressed steel bars in prestressed concrete, ensuring high mechanical strength and a long service life. However, due to factors such as tensioning process, material properties, and environmental conditions, prestress loss can occur in steel strands, significantly reducing their lifespan and even causing breakage, leading to serious accidents. Therefore, there is an urgent need to develop a simple, efficient, and accurate method for monitoring the stress state of steel strands during service, providing a supporting basis for gradually improving bridge monitoring systems.

[0003] Ultrasonic guided waves have advantages such as low attenuation along the propagation path, long propagation distance, and fast detection speed. However, due to the complex geometric helical structure of the steel strand and the unique coupling contact between its wires, the propagation modes become more complex. As a tension member, the third-order acoustoelastic constant of the steel strand undergoes a slight change under stress. Previous studies have neglected this, leading to discrepancies between research results and experimental results. Summary of the Invention

[0004] This invention aims to propose a method for monitoring axial stress in steel strands based on the characteristics of ultrasonic guided waves, addressing any of the aforementioned problems. It combines ultrasonic guided wave acoustoelastic theory with the proportional boundary finite element method. Under different axial forces, it utilizes the acoustoelastic effect to vary with frequency and mode. Based on the wave structure and modes at different frequencies, the most suitable ultrasonic guided wave mode and excitation frequency for stress detection are selected. Finally, a fitting curve is obtained based on the relationship between the group velocity change and the acoustoelastic constant, ultimately achieving the monitoring of axial stress in the steel strand.

[0005] Specifically, the present invention provides a method for monitoring axial stress in steel strands based on ultrasonic guiding characteristics, the method comprising the following steps:

[0006] S1. Based on the helical structure of steel strand, establish a torsional coordinate system; calculate the phase velocity and energy velocity curves of the straight steel wire to analyze its dispersion characteristics and determine the monitoring mode and excitation frequency range;

[0007] S2. Extract and analyze the wave structure of the longitudinal and bending modes of the straight steel wire;

[0008] S3. Calculate the frequency-stress-energy velocity curve of a straight steel wire under different stresses;

[0009] S4. Calculate the energy velocity sensitivity curve of a straight steel wire under stress.

[0010] S5. Combining the results of S1, S2, S3, and S4, determine the appropriate mode and corresponding excitation frequency as the monitoring mode and excitation frequency for axial stress monitoring of steel strands;

[0011] S6. Design the placement of the waveguide sensor based on the structure of the steel strand and the mode shape diagram of the optimal mode.

[0012] S7. By receiving and processing the guided wave signal from the sensor, the energy velocity and energy velocity change trend of the guided wave mode, as well as the frequency-stress-energy velocity curve, are obtained. Based on the relationship between the energy velocity change of the most suitable monitoring mode and the acoustoelastic constant at the most suitable excitation frequency, a fitting curve can be obtained, ultimately realizing the monitoring of the axial stress of the steel strand.

[0013] In one implementation, the formula for the torsional coordinate system is as follows:

[0014] φ=R(s)+xN(s)+yB(s)

[0015] Where φ is any vector in the Cartesian coordinate system, and its expression in the Cartesian coordinate system is φ = Xe. X +Ye Y +Ze Z ;

[0016]

[0017]

[0018]

[0019] In the formula: s represents the outer...

[0020] In one implementation, the monitoring mode and excitation frequency range are determined by solving the phase velocity and energy velocity dispersion curves of the steel wire and analyzing the dispersion.

[0021] In one embodiment, the wave structures of the longitudinal and bending modes of the extracted steel wire are obtained by extraction using the acoustoelastic proportional boundary finite element method.

[0022] In one implementation, the frequency-stress-energy velocity curves of the steel wire under different stresses are obtained based on the acoustoelastic proportional boundary finite element method.

[0023] In one embodiment, the method for calculating the energy velocity sensitivity curve of the steel wire under stress is based on the acoustoelastic proportional boundary finite element method for calculating the energy velocity sensitivity curve of the steel wire under 0.2UTS (UTS is the ultimate tensile stress).

[0024] The solution of the present invention has the following effects.

[0025] 1) This invention can perform non-destructive monitoring of steel strands in service, and is simple to operate with high accuracy.

[0026] 2) This invention takes into account the change in the third elastic constant of the material under stress, and the selected excitation frequency and detection mode have high sensitivity, which can achieve high accuracy in stress detection. Attached Figure Description

[0027] Figure 1 This is a cross-sectional view of the steel strand of the present invention;

[0028] Figure 2 This is a discrete model of the cross-section of the central steel wire in the steel strand of the present invention;

[0029] Figure 3 The velocity curve for a straight steel wire phase;

[0030] Figure 4 The energy velocity curve of a straight steel wire;

[0031] Figure 5 The structure diagram of the L(0,1) wave at 400kHz is shown.

[0032] Figure 6 The structure diagram of the L(0,1) wave at 600kHz is shown.

[0033] Figure 7 The structure diagram of the F(1,1) wave at 400kHz is shown.

[0034] Figure 8 The structure diagram of the F(1,1) wave at 600kHz is shown.

[0035] Figure 9 The graph shows the energy velocity variation of the longitudinal mode L(0,1) under different stresses as a function of frequency.

[0036] Figure 10 The energy-velocity-stress sensitivity curve is 0.2 UTS.

[0037] Figure 11 Layout diagram of steel strand sensor;

[0038] Figure 12 This is the wavenumber-frequency curve of the steel strand;

[0039] Figure 13 The energy velocity curve of the steel strand;

[0040] Figure 14 The relative change in group velocity and axial stress

[0041] In the picture: Detailed Implementation

[0042] To make the technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Some technical terms and expressions used herein have the same meaning as understood by those skilled in the art to which this application pertains.

[0043] A method for monitoring axial stress in steel strands based on ultrasonic guiding characteristics includes the following steps:

[0044] S1. Based on the helical structure of steel strand, establish a torsional coordinate system, calculate the phase velocity and energy velocity curves of the straight steel wire, analyze its dispersion characteristics, and determine the monitoring mode and excitation frequency range.

[0045] S2. Extract and analyze the wave structure of the longitudinal and bending modes of the steel wire;

[0046] S3. Calculate the frequency-stress-energy velocity curve of the steel wire under different stresses;

[0047] S4. Calculate the energy velocity sensitivity curve of the steel wire under stress.

[0048] S5. Combining the results of S1, S2, S3, and S4, determine the appropriate mode and corresponding excitation frequency as the monitoring mode and excitation frequency for axial stress monitoring of steel strands;

[0049] S6. Design the placement of the waveguide sensor based on the structure of the steel strand and the mode shape diagram of the optimal mode.

[0050] S7. By receiving and processing the guided wave signal from the sensor, the energy velocity and energy velocity change trend of the guided wave mode, as well as the frequency-stress-energy velocity curve, are obtained. Based on the relationship between the energy velocity change of the most suitable monitoring mode and the acoustoelastic constant at the most suitable excitation frequency, a fitting curve can be obtained, ultimately realizing the monitoring of the axial stress of the steel strand.

[0051] Using the solution of this application, the present invention enables non-destructive monitoring of in-service steel strands, with simple operation and high accuracy. It avoids problems such as prestress loss, lifespan reduction, and even breakage that occur in steel strands in existing technologies.

[0052] In one implementation, the formula for the torsional coordinate system is as follows:

[0053] φ=R(s)+xN(s)+yB(s)

[0054] Where φ is any vector in the Cartesian coordinate system, and its expression in the Cartesian coordinate system is φ = Xe. X +Ye Y +Ze Z ;

[0055]

[0056]

[0057]

[0058] In the formula: s is the curve length on the helix containing a point on the axis of the outer steel wire, (e x ,e y ,e z ) are Cartesian orthonormal basis vectors. L and L represent the length of the helical curve and its corresponding pitch, respectively, and θ is the helical angle. In Frenet-Serret, N(s), B(s), and T(s) are the unit tangent vector, normal vector, and binormal vector, respectively.

[0059] The derivation principle of the above formula is as follows:

[0060] The spiral centerline curve can be described as

[0061]

[0062] This approach fully considers the changes in the material's third-order elastic constants under stress, and the selected excitation frequency and detection mode have high sensitivity, enabling high accuracy in stress detection.

[0063] In one implementation, the monitoring mode and excitation frequency range are determined by solving the phase velocity and energy velocity dispersion curves of the steel wire and analyzing the dispersion.

[0064] In a preferred embodiment, the phase velocity and energy velocity dispersion curves are obtained using the following formula:

[0065]

[0066]

[0067] In the formula: ω and k are the frequency and wave number, respectively. The average of the cross-sectional kinetic energy and time in the propagation direction, It is the sum of kinetic energy and potential energy;

[0068] The derivation of the formula is as follows:

[0069] Based on the established curvilinear coordinate system φ, its corresponding nonorthogonal covariant and inverse covariant bases can be obtained. The covariant and inverse covariant bases are the foundation for calculating the stress and strain of waveguides in the Frenet-Serret coordinate system, and can be expressed as follows:

[0070] g1 = N(s), g2 = B(s)

[0071] g3=-τyN(s)+τxB(s)+(1+κx)T(s)

[0072] According to the definition of tensor g ij =g i ·g j The fundamental covariance metric matrix of the coordinate system is:

[0073]

[0074] In the case of small deformation in steel strands, based on the tensor transformation relationship mentioned above, the strain-displacement relationship in the proportional boundary finite element method can be rewritten in tensor form:

[0075]

[0076] In the formula: J is a Jacobian matrix, with b0, b1, b2, b... s For the differential operator, y ,η x ,ξ This represents the local coordinates of the node coordinates.

[0077] Assuming the steel strand is an isotropic elastic material, the stress-strain relationship is: σ=Dε

[0078] The application of Hamilton's variational principle in wave control equations is

[0079] ∫ V δε T σdV-ω 2 ∫ V δu T ρudV=0

[0080] Since steel strands are tension members, they are generally subjected to axial stress, and their deformation is basically uniform and axial. Furthermore, since the material is a Murnaghan hyperelastic, based on the Murnaghan theoretical model, the medium under axial stress is equivalent to isotropic, and its equivalent elastic modulus is:

[0081]

[0082] D is a symmetric matrix, where l, m, and n are Murnaghan's third-order elastic constants, and λ and μ are Murnaghan coefficients. σ represents stress.

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091]

[0092] Finally, based on the principle of acoustoelasticity and Hamilton's principle, the wave equation of the acoustoelastic proportional boundary finite element method is derived as follows:

[0093]

[0094] The above equation is the dispersion equation of a helical waveguide, which can be transformed into a polynomial eigenvalue problem with two variables (k, ω). For a given ω, it can be easily found that k and -k are eigenvalues ​​of the dispersion equation of the helical waveguide due to the symmetry of M, E1, and E3; The asymmetry represents positive and negative waves, respectively.

[0095] To facilitate solving the above equation, we can utilize the properties of the stiffness matrix derived from higher-order spectral elements, which simplifies the final problem to a generalized linear characteristic system:

[0096]

[0097]

[0098]

[0099]

[0100] In the formula: Let N be the nodal displacement and force vector, respectively; N be the interpolation function; ρ be the material density; D be the stiffness matrix; and J be the Jacobian matrix.

[0101] By solving the above equation, we can obtain the expressions for phase velocity and energy velocity:

[0102]

[0103]

[0104]

[0105]

[0106] In the formula: Im represents the imaginary part, k and w are the frequency and wavenumber, respectively. Let J be the displacement vector of the eigenvectors, and J be the Jacobian matrix.

[0107] In one implementation, the wave structure of the longitudinal and bending modes of the extracted steel wire is obtained by the acoustoelastic proportional boundary finite element method, specifically as follows:

[0108] The wave structure diagram, also known as the displacement field distribution of the waveguide cross section during wave propagation, is generally described by the displacement field distribution of the cross section where the extreme displacement of a particle vibration point within a wavelength is located. The particle displacement function at any point can be represented by shape functions and nodal displacement functions; therefore, the element displacement is expressed as:

[0109]

[0110] In the formula: the subscripts i = n, b, t correspond to N, B, T respectively.

[0111] By combining equations (1) and (2), the displacement vectors of the characteristic vectors at each frequency can be obtained. After normalization, the wave structure diagram of the guided wave at that frequency can be obtained. The mode with the largest axial displacement is selected as the monitoring stress mode.

[0112] In one implementation scheme, the frequency-stress-energy velocity curves of the steel wire under different stresses are obtained based on the acoustoelastic proportional boundary finite element method. The specific method is as follows:

[0113] According to the acoustoelastic theory, the stiffness matrix D will change under different stresses. By substituting the stiffness matrix D under different stresses into the waveguide equation for solution, and using the energy velocity formula, the energy velocity curves under different stresses can be obtained and plotted on the same graph.

[0114]

[0115] In one implementation scheme, the method for calculating the energy velocity sensitivity curve of the steel wire under stress is as follows:

[0116]

[0117] In the formula: f represents frequency, σ represents stress, and V e (f,σ), V e(f,0) represents the energy velocity under stress and without stress, respectively.

[0118] In one implementation scheme, the method for determining the appropriate mode and corresponding excitation frequency as the monitoring mode and excitation frequency for axial stress monitoring of steel strands is as follows:

[0119] 1. First, the phase velocity and energy velocity dispersion curves of the straight steel wire are obtained by solving the waveguide equation, and its dispersion characteristics are analyzed. Then, according to the stress monitoring mode, a mode with low dispersion should be selected, and the energy velocity of this mode should have a significant difference from the energy velocity of other modes at the same frequency, which is convenient for the identification and analysis of its waveform signal. Thus, the frequency range of the stress monitoring mode and excitation frequency can be preliminarily determined. The most suitable monitoring mode should be a mode sensitive to stress. According to domestic and foreign research, the axial displacement of the wave structure is significantly sensitive to stress monitoring. Therefore, the wave structure within the frequency range of the excitation frequency in step 1 is extracted, the axial displacement in the wave structure is analyzed, and finally, the mode with the largest axial displacement is selected as the monitoring mode. 2. The monitoring mode has been determined in step 1, and the range of excitation frequencies has been preliminarily determined. The most suitable excitation frequency is determined next. The frequency-stress-energy velocity curve and the energy velocity sensitivity curve are solved using the acoustoelastic proportional boundary finite element method. The frequency at which the energy velocity changes the most due to the influence of stress, and the position of the maximum energy velocity sensitivity, is analyzed, which is the most suitable excitation frequency.

[0120] In summary, the optimal excitation frequency and corresponding mode can be determined.

[0121] In one implementation scheme, the method for designing the placement of the waveguide sensor based on the structure of the steel strand and the mode shape diagram of the optimal modes is as follows:

[0122] Since the selected monitoring mode is the longitudinal mode L(0,1), waveguide sensors are arranged according to the center wire of the steel strand. Only this mode can be excited, reducing the interference of other modes and facilitating the identification and analysis of its waveform signal.

[0123] In one implementation, the methods for obtaining the energy velocity and energy velocity variation trend of the guided wave mode, and the frequency-stress-energy velocity curve are as follows:

[0124] After solving equation (1), the guided wave energy velocity curve is obtained by the following equation:

[0125]

[0126] Assuming the steel strand is subjected to axial stresses of 0.2 UTS, 0.4 UTS, 0.6 UTS, and 0.8 UTS respectively, its stiffness matrix D will change. Substituting the stiffness matrix D under different stresses into the waveguide equation for solution, the guided wave energy velocity under different axial stresses can be obtained, yielding the frequency-stress-energy velocity curve. Simultaneously, the trend of guided wave energy velocity variation can be calculated using the following formula:

[0127] ΔV e =V e (σ)-V e

[0128] In the formula: V e (σ), V e These represent the energy velocities under stress and in a free state, respectively.

[0129] In one implementation, the method for monitoring the stress of the steel strand using the stress change of the steel strand and the energy velocity change of the guided wave is as follows:

[0130] The optimal monitoring mode energy velocity of the guided wave under stress and without stress was determined using the acoustoelastic proportional boundary finite element method. The energy velocity variation was calculated, and a fitting curve was obtained, ultimately enabling the monitoring of the axial stress of the steel strand.

[0131]

[0132] In the formula: V e (σ,f) represents the energy velocity under stress, V e0 (f) represents the energy velocity in the free state, and σ represents the stress.

[0133] The present invention will now be described in conjunction with specific embodiments.

[0134] Example 1:

[0135] Steel strand parameters

[0136]

[0137] Lame' and Murnaghan constants of steel strand

[0138]

[0139] The central steel wire of the structure is meshed using high-order spectral element sections, such as... Figure 2 As shown, the other six steel wires adopt the same division method. The number of grid divisions should be guaranteed, otherwise it will affect the accuracy of subsequent stress monitoring. Under stress-free conditions, the central steel wire and the outer steel wire are in point contact. When subjected to axial stress, the contact between the steel wires is Hertzian contact.

[0140] The phase velocity and energy velocity dispersion curves of a 5mm diameter steel wire under stress-free conditions were solved using the aforementioned acoustic-elastic proportional boundary finite element method, as shown below. Figure 3 and 4 As shown.

[0141] The dispersion curves reveal that the number of guided wave modes increases dramatically with increasing frequency, and the degree of dispersion varies. Only the T(0,1) mode is non-dispersive, while the F(1,1) mode exhibits significant dispersion. Non-dispersiveness is evident in the low-frequency range, and the energy velocity within the 0-500kHz range is far greater than other modes. This implies that the guided wave mode can be detected first in later monitoring and that mode separation is significantly simplified.

[0142] Since steel wire is a tension member, it mainly generates axial displacement. Therefore, it is necessary to select a mode with high sensitivity to circumferential defects, large axial displacement, stress sensitivity, and small attenuation.

[0143] The above analysis shows that only the L(0,1) mode and the F(1,1) mode have obvious non-dispersion properties. Therefore, the longitudinal mode L(0,1) wave structure and the bending mode F(1,1) wave structure at 400kHz and 600kHz are extracted by the acoustoelastic proportional boundary finite element method. Figure 5 The structure diagram of the L(0,1) wave at 400kHz is shown. Figure 6 The structure diagram of the L(0,1) wave at 600kHz is shown. Figure 7 The structure diagram of the F(1,1) wave at 400kHz is shown. Figure 8 The structure diagram of the F(1,1) wave at 600 kHz is shown. Based on the wave structure diagram... Figure 5 — Figure 8 It can be seen that the L(0,1) mode mainly exhibits axial displacement with relatively small radial displacement, while the F(1,1) mode mainly exhibits radial displacement with weak axial displacement, which is not conducive to the detection of axial stress. Therefore, L(0,1) is selected as the monitoring mode.

[0144] Based on the optimal detection mode determined above, the frequency-stress-energy velocity curve and energy velocity sensitivity curve of the longitudinal mode L(0,1) are solved using the acoustoelastic proportional boundary finite element method. Figure 9 The graph shows the energy velocity variation versus frequency for the longitudinal mode L(0,1) under different stresses. Figure 10 The curves show the energy-velocity-stress sensitivity of the longitudinal mode L(0,1) under axial stress of 0.2UTS. Figure 9 This indicates that the guided wave energy velocity increases under axial stress, showing an initial increase followed by a decrease within the 1000kHz range, reaching a peak near 580kHz. Figure 10The results show that the energy velocity sensitivity of the longitudinal mode L(0,1) under 0.2 UTS axial stress initially increases and then decreases with increasing frequency, reaching a maximum near 580 kHz. This indicates that the longitudinal mode L(0,1) is most sensitive to stress at this frequency. Through analysis of... Figure 9 and Figure 10 Analysis revealed that the suitable mode for ultrasonic guided wave for stress monitoring is L(0,1), with an excitation frequency between 400kHz and 600kHz. Ultimately, an excitation frequency of 500kHz was selected.

[0145] Simultaneously, based on the most suitable frequency and mode for monitoring the steel strand selected above, the arrangement of the guided wave sensors for the steel strand is as follows: Figure 11 As shown, the middle rectangle is the sensor.

[0146] The wavenumber diagram and energy velocity diagram of a seven-wire steel strand were calculated using the acoustoelastic-scaled boundary finite element method, as follows: Figure 12-13 As shown, Figure 12-13 This indicates that compared to a single high-strength straight steel wire, the interaction between steel strands introduces many new modes, resulting in more complex conduction characteristics. Furthermore, the L(0,1), T(0,1), and F(1,1) modes all exhibit frequency cutoff at low frequencies. As the frequency increases, the bending mode shows branching, with the branching becoming more pronounced at higher frequencies. Therefore, this frequency region should be avoided as much as possible when testing steel strands. It is noteworthy that a frequency dip, also known as a "frequency trap," appears on the group velocity curve.

[0147] Finally, the fitted curve of energy velocity change versus stress can be obtained according to the following formula:

[0148]

[0149] In the formula: V e (σ,f) represents the energy velocity under stress, V e0 (f) represents the energy velocity in the free state, and σ represents the stress.

[0150] Assuming the steel strand is subjected to stresses of 0.2 UTS, 0.4 UTS, 0.6 UTS, and 0.8 UTS, the energy velocity variation of the optimal excitation frequency monitoring mode is calculated under different axial stresses, specifically the energy velocity variation of the longitudinal mode L(0,1) at 500 kHz. Finally, the fitted curve relationship between the energy velocity variation of the longitudinal mode L(0,1) at 500 kHz and the stress under different longitudinal stresses of the steel strand is obtained, as shown in the figure. Figure 14 As shown.

[0151] The calculation results show that the guided wave acoustoelastic theory is excellent for monitoring the axial stress of steel strands, which fully demonstrates the correctness and accuracy of the guided wave acoustoelastic theory.

[0152] It should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for monitoring axial stress in steel strands based on ultrasonic guiding characteristics, characterized in that: Includes the following steps: S1. Based on the helical structure of steel strand, establish a torsional coordinate system; calculate the phase velocity and energy velocity curves of the straight steel wire to analyze its dispersion characteristics and determine the monitoring mode and excitation frequency range; S2. Extract and analyze the wave structure of the longitudinal and bending modes of the straight steel wire; S3. Calculate the frequency-stress-energy velocity curve of a straight steel wire under different stresses; S4. Calculate the energy velocity sensitivity curve of a straight steel wire under stress. S5. Combining the results of S1, S2, S3, and S4, determine the appropriate mode and corresponding excitation frequency as the monitoring mode and excitation frequency for axial stress monitoring of steel strands; S6. Design the placement of the waveguide sensor based on the structure of the steel strand and the mode shape diagram of the optimal mode. S7. By receiving and processing the guided wave signal from the sensor, the energy velocity and energy velocity change trend of the guided wave mode, as well as the frequency-stress-energy velocity curve, are obtained. Based on the relationship between the energy velocity change of the most suitable monitoring mode and the acoustoelastic constant at the most suitable excitation frequency, a fitting curve can be obtained, ultimately realizing the monitoring of the axial stress of the steel strand.

2. The method for monitoring axial stress of steel strand based on ultrasonic guiding characteristics according to claim 1, characterized in that: The formula for the torsional coordinate system is as follows: in, φ Therefore, any vector in the Cartesian coordinate system is expressed as follows: ； 3. The method for monitoring axial stress in steel strands based on ultrasonic guiding characteristics according to claim 1, characterized in that: By solving the phase velocity and energy velocity dispersion curves of the steel wire and analyzing the dispersion, the monitoring mode and excitation frequency range can be determined.

4. The method for monitoring axial stress of steel strand based on ultrasonic guiding characteristics according to claim 1, characterized in that: The wave structures of the longitudinal and bending modes of the extracted straight steel wire were obtained by the acoustoelastic proportional boundary finite element method.

5. The method for monitoring axial stress of steel strand based on ultrasonic guiding characteristics according to claim 1, characterized in that: The frequency-stress-energy velocity curves of the straight steel wire under different stresses were obtained based on the acoustoelastic proportional boundary finite element method.

6. The method for monitoring axial stress of steel strand based on ultrasonic guiding characteristics according to claim 1, characterized in that: The method for calculating the energy velocity sensitivity curve of a straight steel wire under stress is based on the acoustoelastic proportional boundary finite element method for the longitudinal mode L(0,1) of the steel wire under 0.2UTS.

7. The method for monitoring axial stress of steel strand based on ultrasonic guiding characteristics according to claim 1, characterized in that: The method for monitoring the axial force of steel strands is based on the acoustoelastic proportional boundary finite element method to calculate the energy velocity change value of the most suitable monitoring mode of the steel strand under stress. According to the functional relationship between the acoustoelastic constant and the energy velocity change value, a fitting curve can be obtained, and finally the axial stress of the steel strand can be monitored.

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