Research On Damage Observation And Fatigue Assessment Of Steel Components Based On Digital Image Correlation Technology

Steel structure has many performance advantages and is widely used in railway bridge construction in China.However,under the long-term vehicle alternating load,the external damage caused by steel structure collision or the internal damage caused by welding defects are easy to develop into initial fatigue cracks,causing fatigue failure,and burying major safety hazards.There are more than one million existing bridges in our country.At present,we are faced with heavy tasks of bridge detection,maintenance,repair and reinforcement,and steel bridges also account for a considerable proportion.Steel bridge detection has the problems of difficult component detection and high detection cost.At present,the mature application of steel bridg fatigue assessment is limited to the arrangement of strain gauge or steel wire strain gauge measuring point at the fatigue sensitive part of the site to obtain the strain history of dynamic measuring vehicle,then investigate the operation history,classify by stages,and compare with the fatigue design curve of corresponding structural details for assessment.The method is rigorous in theory,but there are many uncertainties in the process.There is a large gap between the evaluation results and the real situation.For the standard bridge equipment,it is also difficult to grasp the past load.The purpose of this study is to avoid the traditional process of dynamic strain history sampling by optical strain images,and to directly identify the extent of fatigue damage development by optical strain measurement for steel members in any period of time,so as to provide a new approach for fatigue state assessment of steel bridge members and steel structure standard equipment.In view of the application requirements of 3D-DIC in fatigue monitoring,Q345q D specimens were designed and processed for fatigue test,and the feasibility and implementation scheme for fatigue monitoring were determined.The main research contents are as follows:Chapter 1:Systematically introduces the research of fatigue damage of steel structure and the development,research and application of fatigue crack original state image recognition technology and 3D digital image correlation method.It is determined that the image information is converted into digital detection quantity,and the fatigue abnormality can be detected earlier by identifying the change of some parameters.Chapter 2:Investigate the structural details of steel bridge members and standard bridge equipment,carry out finite element analysis,and determine the eight structural details suitable for fatigue research of steel bridge.In-depth study of ARAMIS three-dimensional digital image measurement system and fatigue testing machine linkage method,complete the optical measurement feasibility analysis,form the application of fatigue research optical strain measurement method and operational implementation rules,determine fatigue test parameters,data extraction direction and specific test scheme.Chapter 3:Through static load test,it is determined that ARAMIS system can the image data of the measurement surface without contact,which meets the accuracy requirements.Fatigue test of 47 specimens with different structural details is completed,and the recommended fatigue resistance design parameters of defect components,deformation correction components and trapezoidal lap welds are proposed.Eight kinds of S-N curve equations were obtained by regression,which created the foundation for optical fatigue damage model.Chapter 4:Analyze and study the optical image data group,and determine that the nominal strain of the cross section can be obtained through the mean value method of the cross section line ARAMIS system can dynamically collect the strain image data of the ARAMIS system,and the three-dimensional strain value and displacement value of any point can be obtained;the maximum strain characteristic value can be intercepted on the strain equivalent cloud diagram as a series of fatigue monitoring values,and the strain comes from the microscopic damage of the crack tip,and the position does not change with the change of load cycle.The measured data are objective,effective and practical,which are the optimal parameters for fatigue monitoring.Four power exponent functions were selected to fit the change trend of maximum strain with fatigue process effectively.In chapter 5,the method of automatic identification and processing of image strain eigenvalue is studied,theε_max-N fatigue life curve is obtained,and the change rule of maximum strain with fatigue process is obtained.Based on the fatigue evaluation theory based on S-N curve,a general formula of fatigue damageε-N based on digital image is proposed,and the life proportion can be obtained by taking the measured strain into the inverse function.The early identification method of fatigue damage is proposed.The verification test results show that the digital imageεmax-N evaluation method can be used to detect the fatigue period of cracks（usually 90%of the fatigue life）about 20%earlier.The results can provide a basis for the formulation of preventive fatigue maintenance measures for steel bridges.