CN113405898B - Fiber grating monitoring composite insulator brittle fracture system and crack identification method - Google Patents
Fiber grating monitoring composite insulator brittle fracture system and crack identification method Download PDFInfo
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/02—Details
- G01N3/06—Special adaptations of indicating or recording means
- G01N3/068—Special adaptations of indicating or recording means with optical indicating or recording means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N17/00—Investigating resistance of materials to the weather, to corrosion, or to light
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/56—Investigating resistance to wear or abrasion
- G01N3/567—Investigating resistance to wear or abrasion by submitting the specimen to the action of a fluid or of a fluidised material, e.g. cavitation, jet abrasion
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- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract
The invention provides a fiber grating monitoring composite insulator brittle failure system, wherein a fiber composite insulator comprises a core rod, hardware fittings arranged at two ends of the core rod and at least two fibers arranged on the surface of the core rod, each fiber is connected with a fiber grating demodulator, and the fiber grating demodulators are connected with a computer through network cables; the optical fiber composite insulator is fixed on a horizontal tensile machine through hardware fittings at two ends, the acid container is used for providing stress corrosion conditions for the core rod, and an optical fiber Bragg grating used for acquiring wavelength offset to judge brittle fracture is arranged on each optical fiber relative to the acid container. And provides a corresponding identification method. Based on stress distribution analysis in the core rod crack development process and the fiber grating strain wavelength migration principle, a grating wavelength migration recognition mechanical model of axial section cracks is provided, the relation between crack development and grating wavelength migration is established, and the method can be used for recognizing cracks on the surface of the core rod in the composite insulator brittle fracture process.
Description
Technical Field
The invention relates to the technical field of on-line monitoring of power transmission and transformation equipment, in particular to a composite insulator brittle failure monitoring system and a crack identification method for a fiber grating.
Background
The composite insulator bears the insulating property and the mechanical supporting property in the power transmission line, and the brittle fracture (brittle failure) accident seriously damages the power system. The load is far lower than the normal breaking load during brittle fracture, and the breaking time is unpredictable, so that serious accidents such as line falling and even tower falling are often caused. The traditional composite insulator detection technology mainly detects the running state of the insulator of the power transmission line by regular inspection, field observation and detection methods of inspection personnel, the existing composite insulator detection means mainly comprise an infrared imaging method, an ultraviolet imaging method, an image method and the like, but for the brittle failure of the composite insulator, the traditional detection technologies are difficult to accurately and timely give an early warning and are sampling inspection.
Currently, the acoustic emission technology is used for monitoring the stress corrosion fracture process of the core rod, but the acoustic emission technology is only suitable for the E fiber, and the corrosion resistance of the ECR fiber is difficult to detect. In addition, 4MHz ultrasonic detection is used by Qinghua university for stress corrosion brittle failure process of core rod with 1mm small cracks on the surface, and ultrasonic echo energy is found to reflect the development process of cracks. However, the ultrasonic detection method cannot efficiently, accurately and intuitively find the brittle fracture of the composite insulator, and cannot achieve all-weather online monitoring of the running state of the composite insulator. The invention monitors the complete process of the brittle failure of the core rod by the fiber grating monitoring composite insulator brittle failure system, provides a grating wavelength deviation identification mechanical model of the axial section crack to identify the crack in the brittle failure process, can efficiently and accurately find the brittle failure of the composite insulator, and realizes the all-weather online monitoring of the running state of the composite insulator.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for identifying cracks in the brittle fracture process of a composite insulator based on the monitoring of a fiber grating technology. When the composite insulator starts to be brittle-broken, cracks appear on the surface of the core rod, so that the axial stress of the positions of the cracks is changed, and extrusion force appears at the relative positions of the sections of the cracks; under different degrees and numbers of cracks, the grating wavelength offset amounts of different positions on the surface of the core rod are different, so that the severity and the position of the crack in the brittle failure process of the composite insulator can be judged by observing and comparing the grating wavelength offset trend with the established grating wavelength offset identification mechanical model of the axial section crack, and the crack on the surface of the composite insulator can be identified. The method has the advantages of low detection cost and high detection precision, and can efficiently, accurately and visually identify and position the cracks in the brittle fracture process of the composite insulator core rod.
In order to achieve the purpose of the invention, the fiber grating monitoring composite insulator brittle fracture system provided by the invention comprises an acid container for containing corrosive media, a horizontal tensile machine, a fiber grating demodulator, a computer and a fiber composite insulator,
the optical fiber composite insulator comprises a core rod, hardware fittings arranged at two ends of the core rod and at least two optical fibers arranged on the surface of the core rod, wherein each optical fiber is connected with an optical fiber grating demodulator, and the optical fiber grating demodulators are connected with a computer through network cables;
the optical fiber composite insulator is fixed on a horizontal tensile machine through hardware fittings at two ends, the acid container is used for providing stress corrosion conditions for the core rod, and an optical fiber Bragg grating used for acquiring wavelength offset to judge brittle fracture is arranged on each optical fiber relative to the acid container.
Further, each optical fiber is fixedly adhered to the surface of the core rod.
Furthermore, 3-5 fiber Bragg gratings are written in the range of the acid container on each optical fiber.
Further, the acid container is used for containing a nitric acid solution.
Furthermore, the number of the optical fibers is three, and the three optical fibers are uniformly distributed on the surface of the core rod along the central axis of the composite insulator in a circumferential direction of 120 degrees.
The invention also provides a method for monitoring and identifying by adopting the system.
A crack identification method in the brittle failure process of a composite insulator comprises the steps of soaking a core rod of the optical fiber composite insulator in an acid container, and performing a stress corrosion experiment under the action of a horizontal tensile machine;
monitoring the brittle failure process of the optical fiber composite insulator by an optical fiber Bragg grating arranged on the optical fiber to obtain the wavelength offset;
comparing the obtained wavelength shift trend with a grating wavelength shift recognition mechanical model of the axial section crack to obtain the position and the severity of the crack;
the grating wavelength migration recognition mechanical model comprises 7 stages, wherein when cracks on the surface of the core rod crack, fibers break, the stressed cross-sectional area is reduced, the external axial force of the cross section is basically unchanged, the stress at the cracks is increased, and the grating wavelength migration amount is increased, and is defined as stage I; when the cracks are tiny, the stress change at the symmetrical part of the central axis of the microcrack is small, the wavelength offset is basically unchanged, and the stage II is defined; when the cracks on the surface of the core rod develop to a certain depth, a large number of fibers are broken, the stressed area is reduced, the displacement is generated, so that the external axial force on the cross section is reduced, but the area reduction rate is smaller than the external axial force reduction rate, the stress at the cracks begins to be reduced, the wavelength offset is reduced, and the stage III is defined; meanwhile, the axial stress of the core rod begins to incline, the central axis of the crack generates extrusion stress at a symmetrical position, the larger the crack is, the deeper the core rod is stressed, the more the extrusion stress is, the smaller the comprehensive tensile stress under the external axial force of the cross section is, the smaller the wavelength offset is, and the phase IV is defined; when the crack develops to a certain depth, the stretching displacement of the core rod is increased, the axial force of the whole axial direction where the crack is located begins to be reduced, and the wavelength offset of the grating in the axial direction begins to be reduced, which is defined as a stage V; the comprehensive tensile stress under the axial force of the cross section is correspondingly reduced because the extrusion force is increased at the symmetrical position of the central axis of the crack, and the wavelength offset is continuously reduced, which is defined as a stage VI; and in the final stage of brittle fracture, the optical fiber Bragg grating is positioned in the region which is symmetrical to the central axis of the crack and is not influenced by the section of the axial section of the crack, because the axial stressed area of the cross section is reduced, the axial force of the cross section is also reduced, but the area reduction rate is greater than the force reduction rate, so that the stress is increased, and the wavelength offset is increased, which is defined as stage VII.
Further, the grating wavelength shift identification mechanical model is obtained based on stress distribution analysis in the core rod crack development process and a strain wavelength shift principle of the fiber Bragg grating.
Further, the relative positions of the fiber bragg grating and the surface crack of the core rod include four conditions: the fiber Bragg grating is respectively positioned on the crack or near the tip of the crack, a central axis symmetry point of the axial section of the crack, the axial vicinity of the core rod of the crack and an axial region which is axially symmetrical with the central axis of the crack and is not influenced by the axial section of the crack.
Further, when the fiber Bragg grating completely detects the development process of a crack, the wavelength offset is inverted V-shaped, namely stage I-stage III; however, when the fiber bragg grating is damaged on the way of increasing the wavelength shift, the wavelength shift amount shows a monotonous increase, i.e., only stage I.
Further, when a plurality of cracks at different relative positions on the surface of the core rod are detected by a certain fiber Bragg grating, the wavelength offset of the grating can occur in any one of the stages I-III-I, I-III-I-III, IV-VI-I and IV-VI-I-III.
Compared with the prior art, the invention at least has the following beneficial effects:
(1) The composite insulator brittle failure monitoring system monitors the brittle failure process of the composite insulator, the Bragg grating sensor adopts a passive sensing technology, a terminal power supply is not needed, and the composite insulator brittle failure monitoring system has the advantages of good insulativity, electromagnetic interference resistance, high sensitivity to stress and temperature and the like, and can accurately and stably monitor the axial stress change of the surface of the core rod.
(2) The method can accurately identify cracks generated in the brittle fracture process of the composite insulator, and the severity and the position of the cracks on the surface of the core rod are judged by observing the wavelength shift trend of the grating and comparing the wavelength shift trend with a grating wavelength shift identification mechanical model of the cracks on the axial section, so that the severity of the brittle fracture of the core rod is mastered, and the early warning of the brittle fracture of the core rod is realized.
(3) The method has the advantages of low detection cost and high detection precision, and can efficiently, accurately and visually identify and position the cracks of the composite insulator core rod in the brittle fracture process.
Drawings
FIG. 1 is a schematic diagram of the operation steps of a fiber grating crack identification model method for evaluating a composite insulator brittle fracture process according to the present embodiment;
FIG. 2 is a schematic diagram of a composite insulator brittle failure monitoring system for fiber bragg grating in this embodiment;
FIG. 3 is a schematic diagram showing the relative positions of the fiber grating and the crack in the present embodiment;
fig. 4 is a graph of a mechanical model for identifying grating wavelength shift of axial section cracks in this embodiment, (a) a graph of stress distribution of small cracks, medium cracks and deep cracks in the crack development process, and (b) a graph of evolution of grating wavelength shift along with crack time.
The device comprises a 1-acid container, a 2-horizontal tensile machine, a 21-hydraulic system, a 22-console, a 3-fiber grating demodulator, a 4-computer, a 5-fiber composite insulator, a 51-mandrel, a 52-fiber, a 6-crack, a 61-small crack, a 62-middle crack, a 63-deep crack, a 7-fiber Bragg grating, a 71-crack or a crack tip vicinity, a 72-crack central axis symmetry point, a 73-crack axial vicinity and a 74-crack central axis symmetry axial direction and a region which is not influenced by the crack axial section.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 2, the fiber grating monitoring composite insulator brittle fracture system provided by the invention includes an acid container 1, a horizontal tensile machine 2, a fiber grating demodulator 3, a computer 4 and a fiber composite insulator 5.
In one embodiment of the present invention, the horizontal pulling force machine 2 comprises a clamp, a hydraulic system 21 and a control console 22, and the hydraulic system 21 is controlled by the control console 22, so as to control the magnitude of the pulling force provided to the mandrel 51.
In the invention, the optical fiber composite insulator 5 comprises a core rod 51, hardware fittings arranged at two ends of the core rod 51 and at least two optical fibers 52 arranged on the surface of the core rod 51, each optical fiber 52 is connected with the fiber grating demodulator 3, and the fiber grating demodulator 3 is connected with the computer 4 through a network cable; the optical fiber composite insulator 5 is fixed on a clamp of the horizontal tensile machine 2 through hardware fittings at two ends so as to fix the optical fiber composite insulator 5 on the horizontal tensile machine 2, the acid container 1 is used for providing a stress corrosion condition for the mandrel 51, and an optical fiber Bragg grating 7 used for acquiring wavelength offset to judge brittle fracture is arranged on each optical fiber 52 corresponding to the acid container 1. The wavelength of the fiber grating is demodulated through the fiber demodulator 3, and the data is transmitted to the computer 4; the computer 4 is used for collecting and storing data.
In one embodiment of the invention, the mandrel 51 is unsheathed on its surface.
In one embodiment of the invention, the acid container 1 contains a nitric acid solution, and provides an etching medium in the process of simulating the brittle fracture of the core rod.
In order to effectively monitor the three-dimensional axial stress change and the physical state of the surface of the mandrel 51 in the composite insulator brittle fracture process, at least two optical fibers 52 are adhered to the surface of the mandrel 51 of the composite insulator, and a plurality of fiber bragg gratings 7 for acquiring wavelength offset to judge brittle fracture are engraved in the range of an acid container on each optical fiber 52, specifically, in one embodiment of the invention, three optical fibers 52 are arranged and respectively defined as a 1# optical fiber, a 2# optical fiber and a 3# optical fiber, the 1# optical fiber, the 2# optical fiber and the 3# optical fiber are adhered and fixed on different positions of the surface of the mandrel 51 through optical fiber adhesives, the axial stress change of the surface of the mandrel 51 is observed through acquiring the wavelength offset of the fiber bragg gratings 7, and then the crack severity and the crack location in the composite insulator brittle fracture process are judged by comparing the grating wavelength offset recognition mechanical model with the grating wavelength offset of the axial section cracks according to the wavelength offset trends measured by the fiber bragg gratings 7 arranged on different positions of the surface of the mandrel 51.
In one embodiment of the invention, the three optical fibers are uniformly distributed in the range of the acid container on the surface of the core rod 51 along the central axis of the composite insulator in the circumferential direction of 120 degrees, and the uniform arrangement can more intuitively judge and position the fault position.
The invention also provides a monitoring method adopting the system.
Referring to fig. 1, a method for identifying cracks in a composite insulator brittle failure process includes the following steps:
step 1: and (4) building the fiber bragg grating monitoring composite insulator brittle failure system.
Step 2: and (3) performing a stress corrosion test to simulate the brittle fracture of the composite insulator, and monitoring the brittle fracture process of the composite insulator by using the fiber Bragg grating 7.
In this step, the portion of the mandrel 51 corresponding to the fiber bragg grating 7 is immersed in the corrosive medium in the acid container 1, and a stress is provided to the mandrel 51 by the horizontal tensile machine 2, so as to perform a stress corrosion experiment.
And 3, step 3: the change of the axial stress causes the wavelength shift, and the fiber bragg grating 7 arranged on the optical fiber 52 monitors the brittle failure process of the optical fiber composite insulator 5 and acquires the wavelength shift.
And 4, step 4: and comparing the wavelength shift tendency acquired by the fiber Bragg grating 7 with a grating wavelength shift identification mechanical model of the axial section crack to acquire the position and the severity of the crack.
The grating wavelength shift recognition mechanical model comprises 7 stages:
when cracks on the surface of the core rod initiate, fibers break, the stressed cross section area is reduced, the external axial force of the cross section is basically unchanged, the stress at the cracks is increased, the wavelength offset of the grating is increased, and the phase I is defined; when the cracks are tiny, the stress change at the symmetrical part of the central axis of the microcrack is small, the wavelength offset is basically unchanged (in one embodiment of the invention, the basically unchanged wavelength offset is within the fluctuation range of the grating, and does not exceed 1.1 times of the maximum value and 0.9 times of the minimum value), and the stage II is defined; when the crack on the surface of the core rod develops to a certain depth (such as 1 mm), a large number of fibers are broken, the stressed area is reduced, and the displacement is generated to reduce the external axial force of the tensile machine on the cross section, but the area reduction rate is smaller than the external axial force reduction rate, the stress at the crack begins to be reduced, the wavelength offset is reduced, and the stage III is defined; meanwhile, the axial stress of the core rod begins to incline, and the symmetrical position of the central axis of the crack has extrusion stress; the larger the crack is, the deeper the core rod is stressed, the more the extrusion stress is, the smaller the comprehensive tensile stress under the external axial force of the cross section is, and the wavelength offset is reduced, which is defined as a stage IV. Most of the fibers near the fiber Bragg grating are broken in the process that the crack is rapidly developed to be broken, so that the external axial force at the crack is small, namely the wavelength offset is small; meanwhile, when the crack develops to a certain depth (such as 3 mm), the stretching displacement of the core rod is increased, the axial force of the whole axial direction where the crack is located begins to be reduced, the wavelength offset of the grating in the axial direction begins to be reduced, and the stage V is defined; the comprehensive tensile stress under the axial force of the cross section is correspondingly reduced because the extrusion force is increased at the symmetrical position of the central axis of the crack, and the wavelength offset is continuously reduced, which is defined as a stage VI; and in the final stage of brittle fracture, the grating which is positioned in the symmetrical axial unaffected region of the central axis of the crack and is not influenced by the axial section of the crack is positioned, because the axial stressed area of the cross section is reduced, the axial force of the cross section is also reduced, but the area reduction rate is greater than the force reduction rate, so that the stress is increased, and the wavelength offset is increased, and the stage VII is defined.
When the fiber Bragg grating completely detects the development process of a crack, the wavelength offset of the fiber Bragg grating is inverted V-shaped, namely I-III; however, when the fiber bragg grating is damaged on the way of increasing the wavelength shift, the wavelength shift amount shows a monotonous increase, i.e., I only. When a plurality of cracks at different relative positions on the surface of the core rod are detected by a certain fiber Bragg grating, the wavelength offset of the fiber Bragg grating can show any process of I-III-I, I-III-I-III, IV-VI-I and IV-VI-I-III, namely the typical compound types of N, M, V, reverse N and the like appear, and the wavelength offset of the grating can be obviously more than or less than the number of the cracks. In one embodiment of the present invention, significantly greater or lesser refers to a 1/3 difference in wavelength shift.
According to the invention, based on stress distribution analysis in the core rod crack development process and the fiber grating strain wavelength shift principle, a grating wavelength shift identification mechanical model of the axial section crack is provided, and the connection between the crack development and the grating wavelength shift is established, so that the method can be used for identifying the crack on the surface of the core rod in the composite insulator brittle fracture process.
According to the invention, through the fiber Bragg gratings adhered to different positions on the surface of the core rod, the wavelength offset which is monitored when the relative positions of the fiber Bragg gratings and the generated cracks are different is different, so that the severity and the position of the crack of the core rod in the brittle fracture process can be judged according to the trend of the wavelength offset, the timely monitoring and early warning of the crack in the brittle fracture process of the core rod can be realized, the cost of manpower inspection is saved, and the artificial error is reduced.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
Claims (9)
1. The method is characterized in that the method is used for identifying the composite insulator brittle fracture process based on a fiber grating monitoring composite insulator brittle fracture system, the system comprises an acid container (1) for containing a corrosive medium, a horizontal tensile machine (2), a fiber grating demodulator (3), a computer (4) and a fiber composite insulator (5), the fiber composite insulator (5) comprises a core rod (51), hardware fittings arranged at two ends of the core rod (51) and at least two fibers (52) arranged on the surface of the core rod (51), each fiber (52) is connected with the fiber grating demodulator (3), and the fiber grating demodulator (3) is connected with the computer (4) through a network cable; the optical fiber composite insulator (5) is fixed on the horizontal tensile machine (2) through hardware fittings at two ends, the acid container (1) is used for providing a stress corrosion condition for the core rod (51), and an optical fiber Bragg grating (7) used for acquiring wavelength offset to judge brittle fracture is arranged on each optical fiber (52) relative to the position of the acid container (1);
the method comprises the following steps:
soaking a core rod (51) of the optical fiber composite insulator (5) in an acid container (1), and performing a stress corrosion experiment under the action of a horizontal tensile machine (2);
monitoring the brittle failure process of the optical fiber composite insulator (5) by an optical fiber Bragg grating (7) arranged on the optical fiber (52) to obtain the wavelength offset;
comparing the obtained wavelength shift trend with a grating wavelength shift recognition mechanical model of the axial section crack to obtain the position and the severity of the crack;
the grating wavelength migration recognition mechanical model comprises 7 stages, wherein when cracks on the surface of the core rod crack, fibers break, the stressed cross-sectional area is reduced, the external axial force of the cross section is basically unchanged, the stress at the cracks is increased, and the grating wavelength migration amount is increased, and is defined as stage I; when the cracks are tiny, the stress change at the symmetrical part of the central axis of the microcrack is small, the wavelength offset is basically unchanged, and the stage II is defined; when the crack on the surface of the core rod develops to a certain depth, a large number of fibers are broken, the stressed area is reduced, the displacement is generated, so that the external axial force on the cross section is reduced, but the area reduction rate is smaller than the external axial force reduction rate, the stress at the crack begins to be reduced, the wavelength offset is reduced, and the stage III is defined; meanwhile, the axial stress of the core rod begins to incline, the central axis of the crack generates extrusion stress at a symmetrical position, the larger the crack is, the deeper the core rod is stressed, the more the extrusion stress is, the smaller the comprehensive tensile stress under the external axial force of the cross section is, the smaller the wavelength offset is, and the phase IV is defined; when the crack develops to a certain depth, the stretching displacement of the core rod is increased, the axial force of the whole axial direction where the crack is located begins to be reduced, the wavelength offset of the grating in the axial direction begins to be reduced, and the stage V is defined; the comprehensive tensile stress under the axial force of the cross section is correspondingly reduced at the symmetrical position of the central axis of the crack because of the increase of the extrusion force, and the wavelength offset is continuously reduced, which is defined as a stage VI; and in the final stage of brittle fracture, the optical fiber Bragg grating is positioned in the region which is symmetrical to the central axis of the crack and is not influenced by the section of the axial section of the crack, because the axial stressed area of the cross section is reduced, the axial force of the cross section is also reduced, but the area reduction rate is greater than the force reduction rate, so that the stress is increased, and the wavelength offset is increased, which is defined as stage VII.
2. The method for identifying the cracks in the composite insulator brittle fracture process according to claim 1, wherein in the brittle fracture system, an acid container (1) is used for containing a nitric acid solution.
3. The method for identifying the cracks in the brittle fracture process of the composite insulator as claimed in claim 1, wherein each optical fiber (52) is fixedly adhered to the surface of the core rod (51).
4. The method for identifying the cracks in the brittle fracture process of the composite insulator according to claim 1, wherein 3-5 fiber Bragg gratings (7) are arranged on each optical fiber (52) at positions corresponding to the acid container (1).
5. The method for identifying the cracks in the brittle fracture process of the composite insulator according to any one of claims 1-4, wherein three optical fibers (52) are uniformly distributed on the surface of the core rod (51) along the central axis of the composite insulator in a circumferential direction of 120 degrees.
6. The method for identifying the cracks in the brittle fracture process of the composite insulator according to claim 1, wherein the grating wavelength shift identification mechanical model is obtained based on stress distribution analysis in the core rod crack development process and a strain wavelength shift principle of a fiber Bragg grating.
7. The method for identifying the crack in the brittle fracture process of the composite insulator according to claim 1, wherein the relative positions of the fiber Bragg grating (7) and the surface crack (6) of the core rod comprise four conditions: the fiber Bragg grating (7) is respectively positioned on the crack or near the crack tip (71), a central axis symmetry point (72) of the axial section of the crack, a near axial position (73) of the core rod of the crack and an axial position which is axially symmetrical to the central axis of the crack and is not influenced by the axial section of the crack (74).
8. The method for identifying the crack in the brittle fracture process of the composite insulator according to claim 1, wherein when the fiber Bragg grating (7) completely detects the development process of a crack, the wavelength offset is inverted V-shaped, namely stage I-stage III; however, when the fiber Bragg grating (7) is damaged in the middle of increasing the wavelength shift, the wavelength shift amount is monotonously increased, namely, only in the stage I.
9. The method for identifying the cracks in the brittle fracture process of the composite insulator according to any one of claims 7 to 8, wherein when a plurality of cracks at different relative positions on the surface of the core rod are detected by a certain fiber Bragg grating (7), the wavelength offset of the grating can occur in any one of stage I-stage III-stage I, stage I-stage III-stage I-stage III, stage IV-stage VI-stage I and stage IV-stage VI-stage I-stage III.
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