Data-driven Structural Damage Identification Method Based On Strain Responses

Damage identification in structural health monitoring has become a very important problem.In order to improve the reliability of civil engineering and reduce the risk of accidents,the research of damage identification has very important economic value and engineering significance.It is of great practical significance to identify the damage of structures quickly and accurately under earthquake.Structural response is affected by structural characteristics,and also related to complex factors such as seismic load and working environment,which makes it a challenging task to identify damage based on structural response under unknown seismic excitation.One idea is that the system input is only used as the power source,so that it does not participate in the identification process,thus avoiding the dependence on the system input measurement.This kind of method can identify the structural damage only based on the response data,and does not need to impose specific artificial excitation on the structure,so as to avoid the interruption of the normal operation of the structure.Not only the test cost is low,but also the measurement time is greatly shortened,which is convenient for engineering application.At present,damage identification methods can be divided according to the dependence on health data,it can be divided into methods that need data of undamaged and damaged structures and methods that only need data of damaged structures,that is,baseline method and baseline free method.Most of the existing structures,especially the older ones,are usually lack of health data,and the undamaged data as a reference may be affected by the complex environment,and the inaccurate health data will make it difficult to identify the damage,or even cause wrong identification results.Therefore,it is very important to develop baseline free damage identification for structures.Under fatigue load or strong excitation(such as earthquake),engineering structures usually have obvious nonlinear behaviors,such as breathing cracks in concrete and yielding of steel bars.After fatigue load or strong excitation,the structure often needs health assessment to ensure its safety and reliability.At present,the research of damage element location and quantitative identification generally assumes that structural damage is a linear damage form such as element stiffness reduction,which is different from the actual engineering.Because the structure is in the nonlinear state,the theory of transmissibility function,frequency response function and other linear structures will not be applicable.At the same time,most of the existing methods are based on nonlinear model for damage identification,but the establishment of nonlinear model is more difficult.Therefore,model free localization and restoring force identification of nonlinear damage in multi degree of freedom system based on vibration signal is a very important problem in civil engineering.In chapter 2,the feasibility of wavelet based transmissibility function in damage identification under unknown earthquake excitation is discussed and proved.Combining the advantages of strain transmissibility function and wavelet transform,strain is more sensitive to the small changes in structural response than displacement,and the strain transmissibility function shows better performance than the traditional one.And this chapter only uses strain response data to identify structural damage without artificial excitation,which avoids the interruption of normal operation of structure and low testing cost.Through the damage identification of the model of the overhanging beam and the cable-stayed bridge under the unknown earthquake excitation,it is proved that the method can accurately locate single or multiple damage units in the structure.The experiment of cable-stayed bridge tower proves that the method has high accuracy and good noise resistance.In chapter 3,a baseline free damage identification method combining power spectrum density transmissibility(PSDT)and mode wavelet transform is proposed.Firstly,the strain mode shape is identified based on the PSDT.Based on the identified strain mode shapes,the modal anomalies at the damage location are amplified by wavelet transform to locate the damage.The proposed method does not need baseline data as comparison,and realizes baseline free damage identification based on strain response.Finally,the accuracy and robustness of the method are verified by simply supported beam,actual bridge and the bridge tower test.In chapter 4,an integrated method of nonlinear damage element location and nonlinear restoring force identification is proposed.Considering that the relative wavelet entropy can accurately describe the difference between elements,the relative wavelet entropy of strain response of each element is calculated as an index to locate the structural damage element without baseline.Based on the location results,the nonlinear restoring force of the damaged element is regarded as the "unknown fictitious force" acting on the linear structure,and the generalized Kalman filter under unknown input(GKF-UI)proposed by the research group is further used to identify the modelfree nonlinear restoring force.This method does not need baseline data or the assumption of nonlinear force model.At the same time,since GKF-UI can identify unknown force without feedback,this method does not need to observe the response at the position of nonlinear force,only needs the strain signal and a small amount of acceleration signal of the structure.Finally,numerical examples show that the proposed method can accurately locate and identify the nonlinear forces for different numbers and types of nonlinear damage elements,which has proved the engineering significance of this method.