Research On Structural Damage Identification Methods Based On Strain Mode Under Ambient Excitation

The damage identification method based on vibration theory has been a hot research topic in the field of structural health monitoring(SHM)in recent years due to its advantages of convenient detection and high precision.Since the structural strain contains more structural local information than the displacement,the strain mode is more sensitive to structural damage.For this reason,this paper focuses on the damage identification method based on strain mode under ambient excitation,and proposes a new damage identification index.The validity verification is completed by numerical simulation and experiment.Finally,the damage identification performance of the polyvinylidene fluoride(PVDF)sensor and traditional resistance strain(RS)sensor is compared.The main research contents include:(1)Through the in-depth learning of displacement modal theory,the theoretical derivation of strain mode is completed by using three methods such as finite element method,displacement and strain relationship method and continuum vibration differential equation method.At the same time,some important properties of strain mode,such as mode orthogonality,transient strain response function and frequency response function(FRF)in strain mode,are introduced and derived;then,in order to obtain the structural strain modal parameters under ambient excitation,this paper derives the strain modal parameter identification method based on the Natural Excitation Technique(NExT)and the eigensystem realization algorithm(ERA)from the displacement mode theory,and uses the MATLAB software to complete the programming according to the calculation flow chart of NExT-ERA algorithm.(2)This paper focuses on researching the damage identification index based on strain mode,including strain modal shape,modal strain energy and modal flexibility,and proposes a new damage index based on flexibility matrix LU decomposition.The simulation model of one-dimensional simply supported beam and two-dimensional elastic thin plate is established to complete the numerical simulation of each damage index,and the effectiveness of these damage indices is verified,and the advantages and disadvantages of each index in damage identification is compared.The results show that each damage index can identify the damage of the numerical model,determine the damage location,and assess the damage degree to some extent.The damage index based on modal flexibility has better damage identification effect than the damage index based on modal shape.The damage index based on modal strain energy and curvature mode has better damage identification ability and has certain superiority in structural damage identification.(3)This paper designed an elastic thin plate experimental platform,which carried out modal identification experiment,damage identification experiment and sensor contrast experiment.The modal identification experiment validates the effectiveness of the NExT-ERA modal identification method based on strain mode,and can effectively identify the structural strain modal parameters under ambient excitation.The damage identification experiment verifies the validity of the damage indices proposed in this paper,and shows that the damage indices proposed have good damage identification performance.The sensor contrast experiment focuses on compare the damage identification performance of polyvinylidene fluoride(PVDF)sensor and traditional resistance strain(RS)sensor from the perspectives of structural mode,damage location,damage distance and damage degree.And the results show that compared with the RS sensor,the PVDF sensor has higher damage sensitivity,better damage recognition and the ability to recognize farther from the sensor,and is suitable for the field of damage identification in structural health monitoring.