CN108007935B - On-line monitoring system for buried cracks based on distributed optical fiber sensing technology structure - Google Patents

Abstract

The invention provides an on-line monitoring system for a structure buried crack based on a distributed optical fiber sensing technology, which comprises an on-line monitoring module, a loss mechanism research module, a multi-scale dynamic analysis module, a damage characteristic extraction module, a damage intelligent identification module and a damage evaluation module, wherein the on-line monitoring module carries out external excitation on a hoisting equipment structure, adopts the distributed optical fiber sensing technology to carry out real-time monitoring on the hoisting equipment structure, detects surface and internal cracks, captures the crack state in real time, and carries out tracking monitoring on crack formation, crack propagation and critical damage to obtain a monitoring signal.

Description

On-line monitoring system for structural buried cracks based on distributed optical fiber sensing technology

technical field

The invention relates to an on-line monitoring system for structural buried cracks based on distributed optical fiber sensing technology.

Background technique

Looking at the development status of distributed optical fiber sensing technology at home and abroad, the current distributed optical fiber sensing technology can be applied to long-term online monitoring of structural surface cracks. At the same time, the common conventional static non-destructive testing methods for structural cracks include visual inspection, penetration, eddy current, magnetic particle, ultrasonic, and ray. Comprehensive inspection. The detection position and interval must be based on the statistical characteristics of the defects in the previous use of the structure. There are often cracks that were not found in the first inspection, and the cracks are already very long in the second inspection at the next interval, so the structure needs to be split for fracture analysis. To inversely determine the crack formation life.

Optical fiber sensing technology has the advantages of light weight, anti-electromagnetic interference, high humidity resistance, electrical insulation, and low signal attenuation, and is widely used in engineering monitoring, industrial and agricultural production, life science and other fields. In particular, the optical fiber can be used as both a sensing element and a transmission element, and a distributed optical fiber sensor can be formed through the series connection of the transmission optical fiber sensing units. Compared with the point sensor, it has a significant advantage in that it can continuously monitor the components under test, make up for the blind spot problem of the point sensor monitoring, avoid the danger of major missed detection, and improve the reliability of the monitoring of the component under test.

SUMMARY OF THE INVENTION

The invention provides an online monitoring system for structural buried cracks based on distributed optical fiber sensing technology. The system includes an online monitoring module, a loss mechanism research module, a multi-scale dynamic analysis module, a damage feature extraction module, a damage intelligent identification module and a damage assessment module. , wherein the online monitoring module includes external excitation of the lifting equipment structure, real-time monitoring of the lifting equipment structure using distributed optical fiber sensing technology, detection of surface and internal cracks, real-time capture of crack status, and detection of crack formation and crack propagation. Tracking and monitoring of critical damage were carried out, and monitoring signals were obtained.

The invention adopts the advanced distributed optical fiber sensing network technology, which belongs to the invention of dynamic crack monitoring at the same time. The dynamic crack monitoring technology can make up for the insufficiency of conventional non-destructive testing, and capture the crack state in real time. - Crack propagation - critical failure tracking monitoring.

The present invention performs high-speed on-line monitoring and evaluation of mechanical equipment for multiple types of defects (surface/internal cracks, stress-strain, deformation), mainly structural surface and near-surface cracks. It mainly focuses on two scientific and technical issues: on-line monitoring technology for fatigue crack damage and defect evolution of structural materials; intelligent identification and risk assessment technology for structural damage.

The invention can solve the problems of inspection and monitoring of structural cracks, evolution mechanism of structural crack defects, residual life evaluation of structures, etc., adapt to the difficult application requirements of system maintenance and life evaluation based on service states, and realize mobile pressure vessels, large-scale amusement facilities or start-up In-service non-destructive monitoring and evaluation of heavy equipment and other major machinery and equipment.

Description of drawings

Fig. 1 is the schematic diagram of distributed optical fiber arrangement; Fig. 2 is the technical route and design drawing;

Figure 3 is a distributed optical fiber sensor network remote monitoring system;

Figure 4 is a linear interpolation of the overall strain field obtained from the monitored local strain;

Figure 5 is a quadratic interpolation of the overall strain field obtained from the monitored local strains.

Detailed ways

When a macroscopic crack occurs in the structure and the crack expands macroscopically, the stress-strain field around the crack will change (outer surface). For complex composites, the inner and outer surfaces are connected with external components through rivets and bolts, and the overall structure is under the action of external loads. , the force will be transmitted through multiple transmission methods. When a large macroscopic crack appears on the internal component, the surface stress-strain field closest to the external component and the crack will also change. Therefore, distributed optical fiber sensing technology can be used to detect surface and internal cracks. The distributed optical fiber sensing wiring is shown in Figure 1.

Among them, the structural fatigue damage accumulation criterion is judged by the following formula:

The i-th optical fiber monitoring value

Initial monitoring value without cracks

则认为该部位或附近至少在第i次测量时已出现裂纹，其中，D为系统误差。if satisfied

Then it is considered that a crack has occurred at or near this part at least at the i-th measurement, where D is the systematic error.

The technical route and design diagram are shown in Figure 2. In order to establish a distributed optical fiber sensor network remote monitoring system for structural crack damage, from the damage mechanism, damage model, damage feature identification, online intelligent monitoring, risk assessment, safety early warning and other progressive levels, combined with international leading theories and technologies, to achieve structural It can intelligently identify the occurrence, location and degree of structural damage in the running state, and realize online emergency warning. The monitoring system includes an online monitoring module, a loss mechanism research module, a multi-scale dynamic analysis module, a damage feature extraction module, an intelligent damage identification module and a damage assessment module. The optical fiber sensing technology monitors the lifting equipment structure in real time, detects surface and internal cracks, captures the crack state in real time, tracks and monitors crack formation, crack expansion and critical failure, and obtains monitoring signals.

Key core technology A: Establishing a cross-scale crack damage evolution model for major equipment structures

Solution: According to the characteristics of structural deterioration on two different spatial scales, namely the overall structural scale and the local micro- and meso-scale, combined with the analysis and processing of existing monitoring data, an efficient and accurate structural damage refinement across scales is established. Analytical model. Based on the established refined model and nonlinear damage accumulation criterion, the law of occurrence and evolution of structural damage across scales is studied, the structural deterioration process coupled with multiple factors is analyzed, and the mechanism of structural damage initiation and expansion is further revealed.

Key Core Technology B: Establishment of multi-scale dynamic model of structure (including damage)

Solution: Use the sub-structure method in the finite element method to divide the complex structure into smaller sub-structures that are easier to handle. Inside the sub-structure, use meso-scale units to finely simulate the local details of interest; the sub-structure It is only used as a unit to connect with the overall structure finite element model at the overall macroscopic scale to form a "three-scale" multi-scale finite element model of full-scale-construction-scale-local refinement. The overall properties of the structure can be obtained by solving the multi-scale finite element model at the macro scale, and then the internal expansion solution of the substructure can obtain the mechanical properties of the local details at the small scale, which constitutes a multi-scale numerical calculation method of the structure (including damage).

Key core technology C: Propose wavelet-fractal multi-scale singular spectrum damage feature extraction technology in noisy environment

Solution: Based on multi-scale mathematical analysis of the similarity between fractal and wavelet transform. On this basis, a new structural damage detection model of "wavelet suppression of measurement noise and fractal enhancement of damage features" is established, and a structural damage feature quantity suitable for low signal-to-noise ratio environment is established in the implementation of the model: wavelet-fractal multi-scale singular spectrum. The feature quantity includes two basic elements of wavelet suppression of measurement noise and fractal enhancement of damage characteristics: (1) Wavelet suppression of measurement noise - a new wavelet packet transform algorithm with frequency-time order and no "down-sampling", which transforms structural dynamics into The response is decomposed into a set of multi-scale wavelets, and the multi-scale characteristics of the wavelets will decompose the damage characteristics and measurement noise of the structure into different scales, effectively realizing the separation of signal and noise; (2) Fractal enhancement of damage characteristics - using correlation Dimension and Katz waveform dimension are used as singularity detection operators to perform singularity analysis at the multi-scale wavelet level. The mutation of fractal dimension characterizes the damage effect of the structure.

Among them, the structural cross-scale damage evolution model adopts multiple experimental studies, and multiple judgments are made as follows. The expression of the structural cross-scale damage variable D is:

where η is the residual strain coefficient; ε ₀ is the unit principal strain; ε _r is the unit residual strain; ε _u is the unit ultimate strain, and ε _max is the maximum tensile strain value corresponding to a certain load value during loading.

When ε _max <ε ₀ , there is no damage to the unit; when ε ₀ <ε _max <ε _r , some units have the first-stage damage, and when ε _r <ε _max <ε _u , some units have the second-stage damage, and ε _max ＞ε The unit is completely destroyed when _u .

When D=0, the material has no initial damage, and when D=1, fatigue damage occurs and cracks form.

The cross-scale damage evolution model of the structure can be used to quickly analyze the damage mechanism and obtain a more reasonable and accurate crack situation.

As shown in Figure 3, build a distributed optical fiber sensor network remote monitoring system, mobile pressure vessels, large amusement facilities or lifting equipment and other major equipment, these equipment and embedded wireless transceiver and remote monitoring integrated unit for data transmission and command reception, send all data in real time to the safety monitoring and emergency management support center, the safety monitoring and emergency management support center and the embedded wireless method and the remote monitoring integrated unit regularly send typical data or temporarily send abnormal data, realize and mobile communication links for major equipment such as pressure vessels, large amusement facilities or hoisting equipment.

The optical fiber distributed sensing technology is adopted, and the optical fiber itself is used as the signal pickup. It adopts the characteristics of passive, anti-electromagnetic interference, no power supply at the front end, light and easy to install, etc., to realize the monitoring of the disturbance of the small strain field. The following aspects can be considered:

A. Monitoring of micro-strain field disturbance by high-sensitivity grating sensor network A. The structure of white light interference system is to be used.

B. The two-way interference signal output structure is adopted to effectively reduce the noise brought by the optical system, and at the same time provide the system with an ideal working point with high sensitivity.

C. The single-core feedback structure is adopted. As shown in the previous figure, the light input to the fiber grating sensor is reflected by the reflection unit and then returns to the fiber interference module in the same way. This structure makes the structural sensitivity of the system independent of the monitoring distance. Moreover, due to reflection, the optical signal passes back and forth through the same sensing fiber twice, so that the signal is multiplied and the pickup sensitivity is increased.

D. Analyze and design the structural parameters of the all-fiber interference module and the fiber grating sensor.

E. Since the fiber light is easily affected by the polarization of the fiber itself, according to the specific structure of the optical path and the characteristics of white light interference, polarization control technology is used to reduce the influence of polarization on the system.

Based on the arrangement of the fiber grating sensor network, the overall strain field is obtained from the monitored local strain.

(a) The overall strain field is obtained from the monitored local strains—linear interpolation, as shown in Fig. 4,

in

_Ni is a shape function, which is a function of local coordinates. The shape function satisfies the following conditions

The element local strain field is written as

By assembling all monitoring points, the strain field of the entire surface field can be obtained.

(b) The overall strain field obtained from the monitored local strains—quadratic interpolation, as shown in Fig. 5,

in the formula

The displacement interpolation form is

By assembling all monitoring points, the strain field of the entire surface field can be obtained.

The invention can be specifically applied to major equipment such as mobile pressure vessels, large amusement facilities or lifting equipment. The present invention performs high-speed online monitoring and evaluation for major equipment with complex shapes and multiple types of defects (surface/internal cracks, stress-strain, deformation) coexisting, and is a new measurement method with breakthroughs; to the macro-evolution mechanism. It can solve the problems of fatigue and crack damage inspection and monitoring, defect evolution mechanism, and equipment remaining life evaluation, adapt to the difficult application requirements of system maintenance and life evaluation based on service status, and realize in-service non-destructive testing and evaluation and structural health monitoring of major equipment. integrated detection and monitoring.

Claims (5)

1. An on-line monitoring system for a structural buried crack based on a distributed optical fiber sensing technology comprises an on-line monitoring module, a loss mechanism research module, a multi-scale dynamic analysis module, a damage characteristic extraction module, a damage intelligent identification module and a damage evaluation module, the on-line monitoring module is characterized in that the on-line monitoring module carries out external excitation on the hoisting equipment structure, adopts the distributed optical fiber sensing technology to carry out real-time monitoring on the hoisting equipment structure, detects surface and internal cracks, captures the crack state in real time, tracking and monitoring crack formation, crack propagation and critical damage to obtain a monitoring signal, wherein the loss mechanism research module comprises a structure trans-scale damage evolution model and a structure fatigue damage accumulation criterion, the structure trans-scale damage evolution model adopts multiple times of experimental research to perform multiple times of judgment, wherein the expression of the structure trans-scale damage variable D is as follows:

in the formula: eta is residual strainA coefficient;₀is unit principal strain;_ris unit residual strain_{； u}In the form of a unit of ultimate strain,_maxis the maximum tensile strain value corresponding to a certain load value during loading,

when in use_max<₀Time cell damage does not occur;₀<_max<_rwhen the 1 st stage damage of partial unit occurs,_r<_max<_uwhen part of the cells are damaged in stage 2,_max〉_uwhen the unit is completely destroyed,

when D is 0, the material has no initial damage, and when D is 1, fatigue damage occurs and cracks form;

the structural fatigue damage accumulation criterion is judged by adopting the following formula:

ith optical fiber monitoring value

Monitoring value without crack initially

If it is satisfied with

It is assumed that a crack has occurred at or near the site at least at the ith measurement, where D is the systematic error.

2. The online monitoring system of claim 1, wherein the multi-scale kinetic analysis model comprises a structure multi-scale kinetic model and a cross-scale sensitivity of structural damage.

3. The on-line monitoring system of claim 2, wherein the structural multi-scale dynamic model employs a substructure method in a finite element method, the substructure being connected as only one unit with the finite element model of the overall structure at the overall macro-scale to form a full-scale-build-scale-locally refined "three-scale" finite element model.

4. The on-line monitoring system of claim 1, wherein the damage feature extraction module comprises wavelet-fractal multi-scale singular spectrum extraction structure damage features, structural local damage factor characterization structure nonlinear damage features, and wavelet neural network damage identification and intelligent identification.

5. The on-line monitoring system of claim 1, wherein the distributed fiber sensing technology employs a network arrangement to obtain an overall strain field from the monitored local strain.

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