Research On Structural Health Monitoring Method Using Long-gauge Optical Fiber Sensing Technology

Nowadays,our country is in a rapid development period of new-type urbanization and industrialization;a large amount of infrastructure has been completed or under construction.Civil engineering structures are susceptible to environmental influence,material degradation and external operational loads during daily service,which result in gradual degradation of performance.Besides,impact of vehicles and ships and even natural disasters such as earthquakes and typhoons may also lead to damage of civil engineering structures.Huge economic losses and casualties may be brought if an accident occurs.Therefore,to guarantee the safety of civil engineering structure and its long service life is an important national demand.In fact,structural health monitoring technology is considered to be one of the effective ways to improve structural health and safety and to achieve long-life service and sustainable management of the structure.At present,one of the bottleneck problems in the field of structural health monitoring is sensing technology is either too macroscopic or too local,which results in that even if advanced theoretical algorithms are developed,the "health monitoring" of the structure cannot be effectively realized.A kind of novel long-gauge optical fiber sensing technology has been developed for both local and global structural monitoring.Furthermore,it has the merit of realizing dynamic and static measurement with high-precision and can be connected in series to make a distributed sensor network.These features make the developed sensor suitable for monitoring of large civil engineering structures.Based on this kind of novel long-gauge optical fiber sensor,this paper mainly studies the structural health monitoring methods under different loads to provide a theoretical basis for structural damage identification and multi-level analysis,which can help to realize structural long-term performance evaluation.Specific research content of this paper are as follows:(1)A structural damage identification method based on long-gauge optical fiber sensing technology under ambient vibration is developed.Based on the basic equations of dynamics,this paper deduces the estimated expression of the long-gauge frequency response function(FRF)under ambient vibration and identifies the scaling long-gauge strain mode shapes,which enriches the theory of long-gauge strain modal identification and theoretically reveals the essential relationship between the estimated frequency response function and the real frequency response function,that is the shape of the two function is the same while the amplitude is different.The ratio of two FRFs' peak magnitude in the r-th mode is a constant.It is varying with the mode number r but it has no relation with the output node number.Thus it is proved that picking the peaks of the strain FRFs estimated from ambient vibration data in fact is exactly the strain mode shape elements.Then,based on the identified long-gauge strain mode shapes,a convenient method for performing structural damage identification and indirect identification of bearing damage under ambient vibration is proposed.Furthermore,American IBS benchmark bridge test and its numerical example are used to verify the validity of this method.(2)A two-level detection strategy to locate and quantify structural damage via the long-gauge strain flexibility under impact test is proposed.This paper reveals the essential of impact test and establishes the identification theory of long-gauge strain flexibility.A two-level damage detection strategy is proposed based on long-gauge strain flexibility,which divides damage identification problem into two aspects: damage location and damage quantification.According to the long-gauge strain response and impact force time-history,the structural strain flexibility can be identified and the damage can be located by using the difference of strain flexibility before and after damage occurs.When structural damage location is determined,the number of unknown parameters can be simplified from the stiffness of all elements to that of the damaged elements.As a result,the sensitivity equations of the long-gauge strain flexibility matrix concerning the structure sectional stiffness are established on the damaged area and the sectional stiffness reduction can be further quantitatively identified.The thought is to solve location and quantification problem separately and identifies damage step by step.The number of unknown parameters to be identified can be significantly reduced at the first stage and then the establishment of the parametric identification equation can be focused on the damaged area,which is simplified and guarantees the validity of the equation solution.Finally,a corrosion experiment of a reinforced concrete beam is carried out in the lab to validate the effectiveness of this proposed method.(3)A three-stage method for predicting the bearing capacity of corroded structures is studied by using the long-gauge strain response under moving loads.By using the long-gauge strain influence line theory,a method is provided for identify structural corrosion damage.Moreover,based on the result of corrosion quantification,a further study is performed to predict the bearing capacity of corroded structures.When the bar is corroded,changes occur in geometrical dimensions,yield strength and modulus of elasticity,which will result in the reduction of bearing capacity.Therefore,according to the identified amount of corrosion,correction is made to modify the geometrical dimensions of steel bars and the constitutive relationship of reinforced concrete structure in the finite element model.Then the prediction of residual bearing capacity of the corroded structure is conducted base on the modified model.In order to verify the effectiveness of the method,corrosion and failure experiments of reinforced concrete beams are carried out in the lab.It shows that the result of finite element numerical analysis is consistent with that of damage test,which proves that bearing capacity predicted using this method is correct.(4)A seismic fragility analysis method considering health monitoring data is studied in this paper.Dynamic characteristic parameters of the structure can be identified based on health monitoring data,which can be used in the correction of the initial finite element model combining with static test result.As a result,a model which is closer to the actual structural state can be acquired.Furthermore,seismic structural vulnerability analysis can be carried out based on this modified model and effective evaluation of the true anti-seismic performance of structures under current conditions can be realized.Finally,seismic structural vulnerability analysis of a long-span suspension bridge is conducted using the method proposed in this paper.(5)Based on the information geometry and long-gauge optical fiber sensing technology,a damage detection method which can identify structural apparent damage detection and internal damage is suggested.For apparent damage detection,an algorithm is proposed for extracting structural crack edge using Ricci curvature operator on image manifold and a filter based on geodesic distance on image manifold is designed.Secondly,for internal damage identification,a non-reference state damage identification method is illustrated,it use fisher information distance to measure the correlation between different element responses.Finally,a maintenance decision design concept is explored based on the stochastic distributed control theory.