Holographic Dynamic Displacement Testing And Damage Identification Of Simply Supported Beams Under Tilt Photography

The deformation response of a structure can reflect the safety status of the structure,therefore,it is important to obtain the deformation information of a bridge economically and efficiently.To address the problem that it is difficult to set up ortho-visual image acquisition stations to monitor the structural deformation of cross-river bridges based on digital image processing,this paper relies on the National Natural Science Foundation of China（NSFC）project"Research on the method of obtaining bridge holographic deformation and structural state deduction by image contour line superposition difference analysis"（51778094）,which was undertaken by the supervisor’s team,and the Shenzhen Natural Science Foundation of China（51778094）,the Shenzhen Municipal Natural Science Foundation（JCYJ20220818095608018）,and the National Natural Science Foundation of China（52208128）,this paper proposes a method for dynamic bridge displacement monitoring and damage identification under tilt photography conditions.The main research contents are as follows:（1）To address the deformation monitoring problem of cross-river bridges,an image monitoring method based on tilt photography is proposed,which can obtain the holographic deformation pattern of the structure without setting up target points and contact sensors;to address the problem that insufficient equipment resolution will lead to the monitoring purpose cannot be achieved,the constraint of bridge end monitoring accuracy is proposed by combining the imaging principle of optical equipment,and on this basis,the tilt photography based on the digital image characteristics of the bridge acquired under tilt conditions,a simplified model of image tilt photography is proposed,and the functional relationship between the vertical distance of the station from the bridge body h₀,the horizontal tilt angleαof the equipment and the image pixel scale factor W_iis established,which can provide a data reference for the best station selection of cross-river bridges.（2）In the process of holographic dynamic displacement acquisition of the bridge structure,it is proposed to use single-strain transformation to process the tilt image to obtain a uniform orthorectified digital image of the structure;for the problem of missing bridge edge detection due to environmental factors during video data acquisition,an improved Canny edge detection operator is proposed,which replaces Gaussian filtering with bilateral filtering,realizes high and low detection thresholds with Otsu algorithm.In order to eliminate the noise of data fluctuation on the original displacement curve,we propose an adaptive filter window calculation combined with Savitzky-Golay filter to smooth the displacement curve under the conditions of data fidelity and smoothing,which can effectively reduce the noise signal while preserving the trend of the original displacement signal.（3）To explore the damage identification indexes applicable to the dynamic deformation monitoring method of tilt images.Based on the principles of material mechanics and structural dynamics,the damage identification indexes zb₁ based on dynamic displacement time difference,zb₂ based on dynamic angular time difference and zb₃ based on dynamic curvature time difference are proposed successively.The finite element numerical calculation of an I-beam is analyzed,and the specific recognition effects of each indicator on the location,quantity and degree of damage are compared,and the application range of each indicator is discussed.The analysis shows that the damage identification based on the indicator zb₃ is the best,and it will be used as the indicator for the subsequent indoor damage test research to carry out practical verification.（4）An I-beam simply supported model bridge was designed and fabricated in the laboratory.The dynamic displacement acquisition test and damage identification test were conducted.The results showed that the dynamic displacement curves obtained by the two monitoring methods were in the same direction,and the data error was below±0.5 mm,which verified the effectiveness and accuracy of the tilt-image monitoring method to obtain the holographic dynamic displacement of the structure.After applying 15 damage conditions to the simple-supported girder bridge model and loading it with action,the effectiveness of using the indicator zb₃ to identify the location and quantity of damage was verified based on the corresponding results of the curvature time difference surface diagram;the measured data sets corresponding to these 15 groups of damage conditions were used to establish the relationship between the damage location x,the damage degree y and the peak value of the damage identification index zb_3max,and the feasibility of using the peak value of zb_3max to reverse calculate the specific value of the damage degree y was tested by applying five groups of"unknown damage".