CN115452893A - Method for detecting structural damage of multi-core cable by coupling type electromechanical impedance - Google Patents

Abstract

The invention discloses a method for detecting structural damage of a multi-core cable by coupling type electromechanical impedance. The method comprises the following steps: designing a coupling structural member according to the characteristics and the material attributes of the geometric mechanism of the multi-core cable; arranging and installing coupling structural members in a ring array in a pairwise parallel mode in the same circumferential plane on the surface of the multi-core cable; extracting dynamic frequency impedance signals at two ends of the coupling structure according to a voltage-current method, and collecting impedance signals to an admittance spectrogram in a frequency sweeping manner; acquiring impedance spectrum data of a healthy state and impedance spectrum data of a state to be detected; in response to the curve offset phase, judging that the multi-core cable is damaged; and calculating a damage index RMSD, and determining the damage degree of the multi-core cable according to the damage index RMSD value. The invention can avoid signal mixing and signal loss when the signal is transmitted in a complex structural member, solves the limitation that the electromechanical impedance technology cannot be used for a multi-core cable, and provides a new method for nondestructive testing of the multi-core cable.

Description

A method for detecting structural damage of multi-core cables by coupling electromechanical impedance

technical field

The invention relates to a method for detecting structural damage of a multi-core cable by coupling electromechanical impedance.

Background technique

Nowadays, in the process of industrial development, the application scenarios of multi-core cables are extremely common, such as power systems, transportation, industrial sites, construction and other industries, and their importance as a core component is beyond doubt. However, due to the complex and changeable working environment of multi-core cables, they are easily affected by corrosion fatigue, aging broken wires, mechanical damage, cracks and other damages during service. Once a fault occurs, it will have different effects and effects on the health of the engineering structure. The loss is huge and irreparable. Therefore, it is particularly important to be able to accurately identify and repair faults before they occur. Using structural health detection technology to detect damage to multi-core cables during engineering structures in service is an excellent flaw detection method.

Commonly used non-destructive testing methods for multi-core cables include magnetostrictive method and ultrasonic guided wave method; magnetostrictive method is effective in detecting broken wires, but its detection methods are complicated and the detection cost is high; ultrasonic guided wave detection method Damage detection at a certain distance has been realized on multi-core cables, but due to the large influence of the multi-wire structure of multi-core cables, it is easy to cause guided wave waveforms to mix between adjacent cables during the propagation process, and damage signals are easily submerged in the In the echo signal; compared with the ultrasonic guided wave detection method of long-channel detection, the low-frequency electromechanical impedance technology organically combines the characteristics of piezoelectric materials with wave theory, and the mechanical impedance of the structure is coupled with the electrical impedance of the piezoelectric sensor. When damage occurs, the mechanical impedance changes, which is reflected in the coupled electromechanical impedance. It is sensitive to local micro-defects and is an excellent detection method. Electromechanical impedance technology has achieved excellent detection results in complex structures, but this method is affected by the shape of the cable, and the piezoelectric sensor cannot be placed, which is greatly limited in the application of cable detection, and it has not yet been realized in multi-core Detection applications on cable structures.

Contents of the invention

Based on the above background, the present invention intends to design a coupling device as an intermediate coupling component, so that the piezoelectric sensor, the cable structure to be tested and the coupling structure are coupled into a whole, and it is also sensitive to local micro-defects in the cable. If the impedance value of any unit changes, the overall impedance value will change, thereby realizing the application of the electromechanical impedance method in detecting structural damage of multi-core cables.

The purpose of the present invention is to design a coupling structure to solve the limitation of traditional electromechanical impedance application, so as to realize the application of coupled electromechanical impedance method in multi-core cables. Design the coupling structure, on the basis of increasing the contact area with the cable, so that the piezoelectric sensor works with the largest contact area to obtain the maximum mechanical vibration, thus changing the current situation that the electromechanical impedance method cannot be used to detect the limitations of cable damage , providing a new method for non-destructive testing of cables.

In order to achieve the above object, the present invention provides a method for coupling electromechanical impedance detection of multi-core cable structural damage, comprising the following steps:

(1) According to the geometric mechanism characteristics and material properties of the multi-core cable, design the coupling structure, wherein the coupling structure is made of metal and is in the shape of a strip. The upper surface of the coupling structure is equipped with a piezoelectric sensor, and the lower surface is used for Contact the detected multi-core cable with the largest contact area, so that the piezoelectric sensor, the detected multi-core cable and the coupling structure are coupled as a whole;

(2) In the same circumferential surface of the multi-core cable surface, arrange and install the coupling structural members in a circular array and in parallel;

(3) Obtain the impedance value signals at both ends of the piezoelectric sensor of the coupling structure according to the voltage-current method, and draw the admittance spectrum;

(4) Detect the response signal of the multi-core cable in the healthy state and the state to be detected, and repeatedly measure and obtain the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected;

(5) According to the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected, determine whether there is a curve offset phase in the impedance spectrum data of the state to be detected relative to the impedance spectrum data of the healthy state; in response to the presence of a curve offset phase, determine the multi-core wire the cable is damaged;

(6) According to the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected, the damage index RMSD is calculated, and the damage degree of the multi-core cable is determined according to the value of the damage index RMSD.

In some embodiments, the material of the coupling structural member is a metal material, which is used to obtain the maximum mechanical vibration and coupling transmission efficiency; the coupling structural member includes a strip-shaped structural body, and the two ends of the structural body in the length direction It is a fixed end, and groove structures are provided at the corresponding positions of the two length sides of the fixed end, which are used to fix the coupling structure and the multi-core cable through the groove of the fixed end through the cable tie;

A plurality of coupling structural members arranged in parallel in an annular array are fixed by the groove structure, and the lower surface and the upper surface of the coupling structural members are flat to facilitate the pasting of the piezoelectric sensor and the fixing of the coupling structural members.

In some embodiments, step (2) includes: arranging two coupling structures in parallel at the same circumferential surface position on the outer surface of the multi-core cable, and connecting the piezoelectric sensors on the coupling structures in series in sequence.

Furthermore, the number of coupling structural members is determined according to the overall diameter of the measured multi-core cable, and the size of the coupling structural members is within the same circumferential surface of adjacent cables and larger than the diameter of the piezoelectric sensor, which is convenient for pasting.

In some embodiments, step (3) includes:

The voltage and current method is used to build the experimental circuit. After the AC excitation signal is applied to the series voltage divider resistor, it is added to the piezoelectric sensor in series. When passing through the data acquisition card and the signal generator, the electrical impedance value at both ends of the piezoelectric sensor is measured by sweeping frequency. Plot real and imaginary impedance spectrum information:

压电传感器的端电压为

由于压电传感器本身具有电容特性，因此压电传感器的电压信号会产生一个幅度和相位的变化；设两路信号的相位差为θ，

的幅值分别为U_AM、U_BM，电导纳实部为G，电导纳虚部为B。Where the series divider resistance is R _m , and the excitation voltage in the measurement circuit is

The terminal voltage of the piezoelectric sensor is

Since the piezoelectric sensor itself has capacitive characteristics, the voltage signal of the piezoelectric sensor will produce a change in amplitude and phase; if the phase difference between the two signals is θ,

The amplitudes of U AM and U BM are respectively U _AM , U _BM , the real part of the electric admittance is G, and the imaginary part of the electric admittance is B.

In some embodiments, the amplitude of the AC excitation signal is determined based on the signal-to-noise ratio of the collected signal, and the number of frequency points is more than 10 points; the resistance value of the series voltage dividing resistor should be the same as the resistance value of the piezoelectric sensor or similar.

In some embodiments, step (4) includes:

On the healthy and undamaged multi-core cable, a coupling structure with a piezoelectric sensor is arranged, and the impedance spectrum data of the healthy state is obtained through repeated measurements;

On the multi-core cable in the state to be detected, a coupling structure with a piezoelectric sensor is arranged, and the original impedance spectrum data of the state to be detected is obtained by repeated measurements, and the original impedance spectrum data of the state to be detected is processed to eliminate the inconsistency of the overall The impedance spectrum data whose offset direction and offset phase size exceed 1/2 of the reasonable threshold range are obtained to obtain the final impedance spectrum data of the state to be detected.

In some embodiments, calculating the damage index RMSD includes:

Among them, x _i is the impedance of the piezoelectric sensor when the i-th measured structure is healthy, y _i is the impedance of the piezoelectric sensor when the i-th measured structure is damaged, N is the number of measurements, and the RMSD size corresponds to the difference between the impedance curves ;

The larger the RMSD value, the more severe the damage.

Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

Based on the designed coupling structure, the present invention realizes the damage detection of the multi-core cable by the coupled electromechanical impedance method, and provides a new method for multi-core cable damage detection; the present invention is based on the coupled electromechanical impedance method, and gets rid of The limitations of traditional detection methods, while avoiding the signal drowning and loss that occur when the signal propagates in complex structures, the quantification method is an excellent way to characterize the signal.

Description of drawings

A plan view of the coupling structure in the example of the present invention in Fig. 1;

Fig. 2 is a schematic diagram of the arrangement of coupling structural members in the example of the present invention;

Experimental flowchart in Fig. 3 example of the present invention;

In Fig. 4 example of the present invention, the schematic diagram of voltage-current method circuit schematic diagram;

Fig. 5 is a schematic diagram of impedance spectra of defects with different distances in the example of the present invention;

Figure 6 is a schematic diagram of the results of multiple damage impedance spectra in the example of the present invention;

Figure 7 results of different degrees of damage impedance spectrum in the examples of the present invention;

Fig. 8 is a schematic diagram of the change of RMSD value with the degree of damage in the example of the present invention;

Fig. 9 is a schematic diagram of the change of RMSD value with the number of damages in the example of the present invention;

Fig. 10 is a schematic diagram of the change of RMSD value with the damage distance in the example of the present invention.

detailed description

The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings.

In the description of the present invention, several means more than one, and multiple means more than two. Greater than, less than, exceeding, etc. are understood as not including the original number, and above, below, within, etc. are understood as including the original number. If the description of the first and second is only for the purpose of distinguishing the technical features, it cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features relation.

In the description of the present invention, reference to the terms "one embodiment," "some embodiments," "exemplary embodiments," "examples," "specific examples," or "some examples" is intended to mean that the embodiments are A specific feature, structure, material, or characteristic described by or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

A method for detecting structural damage of a multi-core cable by coupled electromechanical impedance, comprising the following steps:

(1) According to the geometric mechanism characteristics and material properties of the multi-core cable, design the coupling structure, wherein the coupling structure is made of metal and is in the shape of a strip. The upper surface of the coupling structure is equipped with a piezoelectric sensor, and the lower surface is used for Contact the detected multi-core cable with the largest contact area, so that the piezoelectric sensor, the detected multi-core cable and the coupling structure are coupled as a whole;

(2) In the same circumferential surface of the multi-core cable surface, arrange and install the coupling structural members in a circular array and in parallel;

(3) Obtain the impedance value signals at both ends of the piezoelectric sensor of the coupling structure according to the voltage-current method, and draw the admittance spectrum;

(4) Detect the response signal of the multi-core cable in the healthy state and the state to be detected, and repeatedly measure and obtain the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected;

(5) According to the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected, determine whether there is a curve offset phase in the impedance spectrum data of the state to be detected relative to the impedance spectrum data of the healthy state; in response to the presence of a curve offset phase, determine the multi-core wire the cable is damaged;

(6) According to the impedance spectrum data of the healthy state and the impedance spectrum data of the state to be detected, the damage index RMSD is calculated, and the damage degree of the multi-core cable is determined according to the value of the damage index RMSD.

The invention provides a method for coupling electromechanical impedance detection of structural damage of multi-core cables, the basic principle of which is: design a metal coupling structure, and the concave groove structure 1 at both ends can firmly fix the ring array and arrange it in parallel The coupling structural parts, the lower surface 3 and the upper surface 4 are flat to facilitate the pasting of the piezoelectric sensor and the fixing of the coupling structural parts; after the number of the required coupling structural parts is determined according to the diameter of the cable to be tested and the cable wire, they are arranged in parallel and circularly. Fasten the piezoelectric sensor, the coupling structure, and the cable tightly with cable ties.

During the detection, the voltage and current method is used to build the detection circuit, and the electrical impedance value at both ends of the piezoelectric sensor is measured by sweeping frequency through the data acquisition card and the signal generator, and the impedance spectrum is drawn; when any of the three resistance values changes, the measured When the impedance value changes, the impedance spectrum shifts, so as to judge whether the damage occurs according to the shift characteristic of the impedance spectrum, and determine the damage degree according to the quantitative index, this is the basic principle of the method of coupling electromechanical impedance detection method for multi-core cable structure damage.

In some embodiments, there is provided a method for detecting structural damage of a multi-core cable by coupled electromechanical impedance, comprising the following steps:

(1) According to the geometric mechanism characteristics and material properties of the multi-core cable, the material of the coupling structure is determined to be a metal material, the shape of a long strip, and the groove 1 is designed with a semicircular depression; the area 2 is the fixed end, and the cable tie is parallel to the bottom of the groove. Pass through the groove 1 to fix the structural member and the multi-core cable at the fixed end (both ends); 3. The bottom surface of the structural member contacts the cable with the largest area; 4. Paste the piezoelectric sensor on the upper surface of the structural member.

The coupling structure is made of metal and is in the shape of a strip. The upper surface 4 of the coupling structure is equipped with a piezoelectric sensor, and the lower surface 3 is used to contact the detected multi-core cable with the largest contact area, so that the piezoelectric sensor , The detected multi-core cable and the coupling structure are coupled as a whole;

The material of the coupling structure is metal, which is used to obtain the maximum mechanical vibration and coupling transmission efficiency; the coupling structure includes a strip-shaped structural body, and the two ends of the structural body in the length direction are fixed ends 2. There is a groove structure 1 at the corresponding position of the two length sides of the end, which is used to fix the coupling structure and the multi-core cable through the groove of the fixed end through the cable tie;

A plurality of coupling structural members arranged in parallel in an annular array are fixed by the groove structure, and the lower surface 3 and the upper surface 4 of the coupling structural members are flat to facilitate pasting of piezoelectric sensors and fixing of the coupling structural members.

It is convenient for processing and manufacturing. The coupling structure is made of ferrous material. The cable to be tested is composed of a single cable helically wound, with a total diameter of 55mm and a single cable diameter of 7mm, so its shape is designed as a long strip (see figure 1), the thickness is 2mm, the length is 35mm, and the width is 10mm. The two ends are designed with semicircular depressions as the groove structure 1, the radius is 2mm, and the distance from the wide side is 5mm. There is one place at each end, and the cable tie passes through the groove structure 1 to fix the coupling structure and the multi-core cable at the fixed end 2; the coupling structure is 25mm long and 10mm wide, and the lower surface 3 contacts the cable with the largest area; the upper surface Paste the piezoelectric sensor at 4 places, and the size of the piezoelectric sensor is 8mm.

(2) In the same circumferential surface of the multi-core cable surface, arrange and install the coupling structural members in a circular array and in parallel;

To further explain, at the same circumferential surface position on the outer surface of the cable, the coupling structural members are arranged in parallel and in series, as shown in Figure 2. The coupling structural members are arranged in parallel and in series, which can detect all cables, and the overall coupling impedance of any cable is defective. The value will change, and any defect of the cable structure can be captured. During the detection, in order to facilitate the experimental operation, a single coupling structure is arranged to detect the cable structure with circumferential defects. The design size of the coupling structure is determined according to the diameter of the cable and the piezoelectric sensor. The size of the coupling structure should be in the same plane as the adjacent cable and should be larger than the diameter of the piezoelectric sensor, which is easy to paste. After the piezoelectric sensor is pasted on the upper surface 4 , use a cable tie to firmly arrange on the surface of the cable.

(3) According to the voltage-current method, the impedance value signals at both ends of the piezoelectric sensor of the coupling structure are obtained, and the admittance spectrum is drawn;

压电传感器的端电压为

压电传感器的阻抗为

由于压电传感器本身具有电容特性，因此压电传感器的电压信号会产生一个幅度和相位的变化。假设两路信号的相位差为θ，

的幅值分别为U_AM、U_BM，电导纳实部为G，电导纳虚部为B，因此：The voltage-current method is used to build the experimental circuit, as shown in Figure 3 and Figure 4. In formula 1, the frequency-controllable AC signal source is AC, the series voltage dividing resistor is R _m , and the excitation voltage in the measurement circuit is

The terminal voltage of the piezoelectric sensor is

The impedance of the piezoelectric sensor is

Since the piezoelectric sensor itself has a capacitive characteristic, the voltage signal of the piezoelectric sensor will produce a change in amplitude and phase. Suppose the phase difference between the two signals is θ,

The amplitudes of are respectively U _AM and U _BM , the real part of the electric admittance is G, and the imaginary part of the electric admittance is B, so:

According to formula (1), the real part and imaginary part of the electric admittance of the piezoelectric sensor at the current frequency are derived, that is, the electrical impedance information of the piezoelectric sensor. Therefore, within a certain frequency range, the change of the damage can be preliminarily judged according to the change of the real part information of the electric admittance of the multi-core cable before and after the damage occurs;

The amplitude of the sinusoidal AC excitation signal added to the series voltage divider resistor is a 9V signal, and the matched series voltage divider resistance is 10KΩ. After the AC excitation signal is applied to the voltage divider resistor, it is connected to the piezoelectric sensor in series, and passed The data acquisition card and the signal generator measure the electrical impedance value at both ends of the piezoelectric sensor in a frequency sweep mode, draw the real and imaginary part impedance spectrum information according to formula 1, and repeatedly measure and save the impedance spectrum.

(4) Detect the response signals of cables in different states, and repeatedly measure and save the impedance spectrum;

Step 1: On a single damaged cable, arrange a coupling structure with a piezoelectric sensor at a distance from the damage, measure and save the impedance spectrum information repeatedly, move the coupling structure, and measure and save the impedance spectrum information repeatedly;

In the second step, on multiple damaged cables, a coupling structure with a piezoelectric sensor is arranged at a distance from the damage, and the measurement is repeated several times to save the impedance spectrum information;

In the third part, on cables with single damage but different damage degrees, a coupling structure with a piezoelectric sensor is arranged at a fixed distance from the damage, and the impedance spectrum information is saved through repeated measurements.

(5) Compare the impedance spectrum curves of the coupling structural parts in different states, and intuitively find the deviation law of the impedance spectrum curve according to the curve difference, and judge the damage degree, damage amount, and damage distance of the cable;

In the actual detection, there are errors. For the curve offset rule and offset phase size, those that do not conform to the overall offset direction and the offset phase size exceeds 1/2 of the reasonable threshold range, all are discarded, and the impedance spectrum information that meets the requirements is retained. For further comparison and index quantification, it can be seen from Figures 5-7 that, compared with the impedance spectrum in a healthy state, when the impedance spectrum shifts as a whole, and the phase shift is within 1/2, it indicates that the detected cable structure exists damage.

(6) Further analysis and research were carried out to extract the characteristic parameters of cumulative damage, and construct a damage recognition index to quantitatively characterize the damage.

The root mean square deviation is selected as the damage index, and the root mean square deviation is to characterize the difference between the two curves by calculating the sum of the relative differences of each point under the structural health state and the damage state. The mathematical calculation expression is as follows:

In formula (2), x _i is the impedance of the piezoelectric sensor when the i-th measured structure is healthy, y _i is the impedance of the piezoelectric sensor when the i-th measured structure is damaged, N is the number of measurements, and the RMSD size corresponds to the impedance curve the size of the difference. Therefore, the greater the RMSD, the deeper the cumulative damage of the structure. The impedance value information obtained from multiple measurements is calibrated with the RMSD index. Obviously, the more severe the damage, the greater the RMSD value.

The embodiments of the present invention have been described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific embodiments, which are only illustrative, rather than restrictive, and those of ordinary skill in the art will Under the enlightenment of the present invention, many forms can also be made without departing from the gist of the present invention and the protection scope of the claims, and these all belong to the protection of the present invention.

Claims (8)

1. A method for detecting structural damage of a multi-core cable by coupled electromechanical impedance is characterized by comprising the following steps:

(1) Designing a coupling structural member according to the geometric mechanism characteristics and the material attributes of the multi-core cable, wherein the coupling structural member is made of metal and is in a strip shape, a piezoelectric sensor is arranged on the upper surface of the coupling structural member, and the lower surface of the coupling structural member is used for being in contact with the multi-core cable to be detected in a maximum contact area mode, so that the piezoelectric sensor, the multi-core cable to be detected and the coupling structural member are coupled into a whole;

(2) Arranging and installing coupling structural members in a circular array and pairwise parallel mode in the same circumferential plane on the surface of the multi-core cable;

(3) Obtaining impedance value signals at two ends of a piezoelectric sensor of the coupling structural member according to a voltage-current method, and drawing an admittance spectrogram;

(4) Detecting response signals of the multi-core cable in a healthy state and a state to be detected, and repeatedly measuring for multiple times to obtain impedance spectrum data in the healthy state and impedance spectrum data in the state to be detected;

(5) Judging whether curve offset phase exists in the impedance spectrum data to be detected relative to the impedance spectrum data of the health state or not according to the impedance spectrum data of the health state and the impedance spectrum data of the state to be detected; in response to the curve offset phase, judging that the multi-core cable is damaged;

(6) And calculating a damage index RMSD according to the impedance spectrum data of the health state and the impedance spectrum data of the state to be detected, and determining the damage degree of the multi-core cable according to the damage index RMSD value.

2. The method for detecting the structural damage of the multi-core cable through the coupled electromechanical impedance according to claim 1, wherein the coupling structural member is made of metal and is used for obtaining the maximum mechanical vibration and coupling transmission efficiency; the coupling structural part comprises a strip-shaped structural body, two ends of the structural body in the length direction are fixed ends, and groove structures are arranged at the corresponding positions of the two length edges of the fixed ends and used for fixing the coupling structural part and the multi-core cable through a binding belt through the grooves of the fixed ends;

through a plurality of coupling structure spare of fixed annular array parallel arrangement of groove structure, the surfacing of the lower surface and the upper surface of coupling structure spare is convenient for piezoelectric sensor to paste and the fixed of coupling structure spare.

3. The method for detecting structural damage of a multi-core cable by coupled electromechanical impedance according to claim 1, wherein the step (2) comprises: and the piezoelectric sensors on the coupling structural members are sequentially connected in series.

4. The method for detecting structural damage of the multi-core cable through coupled electromechanical impedance according to claim 1 or 3, wherein the number of the coupling structural members is determined according to the overall diameter of the multi-core cable to be detected, and the size of the coupling structural members is larger than the diameter of the piezoelectric sensor in the same circumferential plane of the adjacent cable wires, so that the coupling structural members are convenient to adhere.

5. The method for detecting structural damage of a multi-core cable by using coupled electromechanical impedance as claimed in claim 1, wherein the step (3) comprises:

an experimental circuit is built by a voltage-current method, an alternating current excitation signal is applied to a series divider resistor and then is serially connected to a piezoelectric sensor, electrical impedance values at two ends of the piezoelectric sensor are measured in a frequency sweeping mode through a data acquisition card and a signal generator, and real part and imaginary part impedance spectrum information is drawn:

wherein the series voltage-dividing resistor is R _m The excitation voltage in the measuring circuit is

Terminal voltage of piezoelectric sensor is

Because the piezoelectric sensor has capacitance characteristics, the voltage signal of the piezoelectric sensor generates a change of amplitude and phase; the phase difference of the two paths of signals is set as theta,

are respectively U _AM 、U _BM The real part of the electrical susceptance is G, and the imaginary part of the electrical susceptance is B.

6. The method for detecting the structural damage of the multi-core cable through the coupled electromechanical impedance as claimed in claim 1, wherein the amplitude of the alternating current excitation signal is determined according to the signal-to-noise ratio of the collected signal, and the frequency is more than 10 points in the number of wave points; the resistance of the series divider resistor should be the same as or similar to the resistance of the piezoelectric sensor.

7. The method for detecting structural damage of a multi-core cable by using coupled electromechanical impedance as claimed in claim 1, wherein the step (4) comprises:

arranging a coupling structural member with a piezoelectric sensor on the healthy and nondestructive multi-core cable, and repeatedly measuring for many times to obtain impedance spectrum data in a healthy state;

and arranging a coupling structural member with a piezoelectric sensor on the multi-core cable to be detected, repeatedly measuring for multiple times to obtain original impedance spectrum data of the state to be detected, processing the original impedance spectrum data of the state to be detected, and eliminating impedance spectrum data which do not accord with the whole offset direction and have the offset phase position exceeding a reasonable threshold range by 1/2 to obtain final impedance spectrum data of the state to be detected.

8. The method for detecting structural damage of a multi-core cable according to claim 1, wherein calculating a damage index RMSD comprises:

wherein x is _i Is the impedance of the piezoelectric sensor at the ith time when the structure to be measured is healthy, y _i The resistance of the piezoelectric sensor is the ith time when the measured structure is damaged, N is the measuring time, and the RMSD size corresponds to the difference between the resistance curves;

the larger the RMSD value, the more severe the damage.

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