The traditional manual inspection method is subjective, and it takes too long to quickly find the health status of the structure after an emergency. Therefore, the health monitoring technology of engineering structures has attracted more and more attention from all walks of life. Early warning of disasters or post-disaster assessment of the damage degree and residual life of structures has become a hot issue for scholars all over the world.
1 concept of structural health monitoring
Structural health monitoring refers to the use of on-site nondestructive sensing technology to detect structural damage or degradation through structural system characteristic analysis including structural response [1]. The process of structural health monitoring includes: obtaining the measured values of the dynamic response sampled by the system at regular intervals through a series of sensors, and then extracting the characteristic factors sensitive to damage from these measured values, and statistically analyzing these characteristic factors, so as to obtain the current health state of the structure.
Structural health monitoring includes (1) load monitoring: including wind, earthquake, temperature and traffic load; (2) Geometric monitoring: monitoring the static displacement of each part of the structure (such as the settlement and inclination of the bridge tower and anchor ingot, etc.). ); (3) Static and dynamic responses of structures, such as strain, acceleration, frequency and modal information [2].
Real-time health monitoring of major engineering structures, timely identification of structural damage accumulation, evaluation of its service performance and service life, establishment of corresponding safety early warning mechanism and early warning of possible disasters are not only of great scientific significance for improving the safety and reliability of structures, but also can reduce the operation and maintenance costs of structures, so it has become an important issue that human beings urgently need to solve in the 2 1 century.
2 Composition of structural health monitoring system
Structural health monitoring system is mainly composed of sensor subsystem, information acquisition and processing subsystem, data management subsystem, damage identification subsystem, model updating subsystem, safety assessment subsystem and early warning subsystem [3].
(1) The sensor subsystem mainly includes intelligent sensing elements, which sense the load and response information of the structure and output it in the form of light, electricity, sound, heat and other physical quantities. This system is the most basic subsystem of health monitoring system.
(2) Information acquisition and processing subsystem, which completes sensor data acquisition and signal preliminary processing.
(3) Data management subsystem, which mainly stores and manages data collected by sensors, data processed by digital signals and subsequent analysis data.
(4) Damage identification, model modification, safety assessment and early warning subsystem, which consists of damage identification software, model modification software, structural safety assessment software and early warning equipment.
3 sensor
3. 1 sensor introduction
The sensor can feel the measured information, and can convert the information into electrical signals or other required information output forms according to certain rules. The traditional sensors used in vibration measurement include piezoelectric force sensor, acceleration sensor, impedance sensor and strain gauge, which have been widely used in the measurement of various engineering structures [4].
3.2 Optimal arrangement of sensors
The arrangement of sensors is of great significance for parameter identification and damage monitoring. Generally speaking, the optimal layout of sensors should follow the following principles [4]:
(1) Minimizes the cost of equipment, data processing, transmission and data channels of the sensor system.
(2) Better estimation of structural model parameters is obtained from the measurement data containing noise.
(3) The structural control is improved through the experimental study of large-scale structural model.
(4) Effectively determine the structural characteristics and their changes, and improve the overall performance evaluation system of the structure.
(5) For large structures, the ability of early damage identification is improved.
4 structural damage identification
Damage identification is the core of structural health monitoring. The basic idea is that damage will significantly change the stiffness, mass or energy dissipation capacity of the structure, and then cause changes in the dynamic characteristics or response of the measured structure. By extracting the information of different parts from the monitoring data and comparing the corresponding information in the undamaged state of the structure, the health detection and evaluation of the structure can be realized, so as to judge the occurrence, location and degree of damage.
Damage identification technology can be divided into whole method and local method. The whole method tries to evaluate the state of the whole structure, while the local method relies on mature nondestructive testing technology to detect a specific structural member. Usually, the whole method is used to determine some possible damage parts, and then the components in these parts are detected in detail by the local method, so as to accurately evaluate the damage of the structure.
4. 1 integral detection method
Structural integrity detection methods mainly include: dynamic fingerprint analysis, model modification and system identification, neural network method, genetic algorithm and so on.
If the structure is damaged, the structural parameters such as mass and stiffness will change, which will lead to the corresponding change of dynamic fingerprint, which is the principle on which dynamic fingerprint analysis is based. The basic idea of model updating and system identification method is to use dynamic test data to continuously modify the stiffness distribution in the model through conditional optimization constraints, and then compare it with the measured data of the modified model response. When two parameters are basically the same, this set of parameters is considered as the current parameters of the structure.
Neural network is based on biological nervous system and simulates the function of human brain. It is a highly nonlinear very large-scale network system connected by simple neurons. Genetic algorithm is to search for the best individual of the next generation according to the evolutionary principle of survival of the fittest and survival of the fittest in Darwin's evolution theory, so as to get the optimal solution that meets the requirements.
4.2 Local detection method
Traditional local detection methods include dyeing method, visual inspection method, indentation method, rebound method, ultrasonic pulse method, ray method and so on. In recent years, some new local damage detection methods have appeared, such as acoustic emission method, Lamb method, frequency domain ARX method, ultrasonic spectrum method, geometric time domain method and so on.
5 engineering application
Civil engineering structure is an important object of structural health monitoring system. At present, the application and function verification of health monitoring system in civil engineering structures such as large bridges, dams and high-rise buildings is a hot topic. In Britain, sensors are laid on the 522-meter-long foyle Bridge and the three-span continuous box girder with variable height to monitor the response of the main girder to vibration, disturbance and strain under the action of vehicles and wind load during the bridge operation, and to monitor the ambient wind and structural temperature field [5]. The structural health monitoring system has also been adopted in Jiangyin Yangtze River Highway Bridge in China. In order to monitor and study the working environment of the bridge, the structural state of the bridge and the response of the bridge under various external load factors such as vehicles, fully understand the operation status and quality degradation of the bridge, and provide basis for the operation management, maintenance, reliability evaluation and scientific research of the bridge, a structural health monitoring system of Jiangyin Bridge was established on the basis of the original monitoring system. In the new health monitoring system, an optical fiber strain testing system is added to monitor the internal force of the main girder, and a GPS displacement testing system is added to monitor the linear shape of the main girder and the displacement of the bridge tower, so as to realize long-term online collection and management of the bridge structural response, including internal force, displacement, vibration, temperature and other signals, carry out effective data accumulation and analysis, obtain characteristic information reflecting the health status of the bridge, and evaluate the safety and reliability of the bridge.
6 existing problems and prospects
The research of structural health monitoring system is still in the basic exploration stage, which is far from the practical system goal. At present, there are still many problems to be solved, which can be summarized as follows: lack of unified and effective damage quantification index, inaccurate signal processing, high cost, difficulty in optimal arrangement of sensors, uncertainty of civil engineering structure and sensitivity of complex working environment to structural modal parameters will have adverse effects. Therefore, the future development direction appears in the following aspects: First, the study of structural damage mechanism. Second, the study of identifying small damage. Thirdly, the research of nonlinear damage diagnosis technology. Fourthly, the research of intelligent sensor. Fifth, optimize the layout of sensors. Sixth, research on parameter identification. Seventh, the research of expert system. Eighth, the study of structural system reliability. If the above problems can be solved well, it will undoubtedly promote the development of structural health monitoring technology to a great extent.
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