| As the service environment of complex welded structures is getting worse in the fields,such as engineering machinery,rail transit,automobile manufacturing and so on,structural components are always deformed and damaged due to wear,corrosion,fatigue,etc.,which seriously affects the overall performance of mechanical equipment and vehicles,even leading to the serious construction and life safety accidents.As the weakest part of the structural component that is prone to failure,the damage behavior of the weld directly determines the performance and service life of the overall component.Therefore,reliable monitoring of the change in the damage state of the weld during service is the key to assessing and predicting the safety of the welded structure.Besides,welding deformation is not only the most intuitive response of structural components to external loads or residual stresses,but an important means to grasp the damage status of various parts of the structure.In this case,in view of the complexity and uncertainty of the current service environment of welded structural parts,the difficulty in establishing an effective failure physical model,and the evaluation and prediction of the life cycle and reliability test of structural parts,the long and costly problems,this article takes the welding structure deformation as the research object(Deformation during welding and external load after welding),measures the amount of deformation with the digital image correlation methods,and carries out further research on the early warning model of stress state assessment on structural parts,based on the measured deformation data of the welded structure.Further,it also puts forward the criteria for reasonably evaluating the safety status of welds based on the stress distribution status data,and meanwhile provides theoretical and methodological support for avoiding the mis-warning of safety situation and non-warning of dangerous state.The main research contents of this article are as follows:(1)Establishing a TIG flat plate and welding test system,high-temperature speckle production system,tensile deformation measurement test system and welding deformation measurement test system for docking,at the same time,designing to achieve the reliable acquisition of traditional / laser prefabricated speckle images,and providing original data support for welding deformation measurement based on digital image correlation method.(2)An improved Harris corner detection algorithm is proposed to study the reliable calibration and error accuracy analysis of the camera.Based on the theoretical analysis and solution of binocular vision parameters,an improved Harris corner detection algorithm is used to obtain accurate corner extraction results;by correcting the entire pixel coordinates of the corner points,more accurate sub-pixel coordinates,internal and external calibration parameters of the left and right cameras are obtained.Compared with the traditional calibration method,the results reflect that the camera's focal length error calculated with the improved calibration method is less than 0.2%,and the average reprojection error is distributed between [-1,1],which holding the small error as well as the high calibration accuracy.(3)Aiming at the problem that the traditional speckle is easy to fall off,not resistant to high temperature,giving rise to the failed deformation measurement,the article has designed a laser prefabricated speckle,researched the deformation measurement algorithms on traditional speckle images and laser-preformed speckle images,improved the traditional DIC algorithm process,and the algorithm calculation speed;in addition,the speckle image feature extraction algorithm design and speckles of laser-prepared speckle images is performed,the matrix is modeled and the speckle image deformation matrix is solved,along with the accuracy of the traditional speckle image measurement algorithm and the laser pre-speckle speckle image measurement algorithm are compared and analyzed.The results show that the average accuracy of the traditional speckle image measurement algorithm is 99.43%;the average accuracy of the laser precast speckle image measurement algorithm is 99.27%;the average processing time of the traditional speckle algorithm is 9.283 S,and the average processing time of the laser preform speckle array algorithm is 6.975 S.Ultimately,by taking the processing time and average accuracy of different speckle algorithms as the comprehensive evaluation coefficients,conducting a comprehensive evaluation and comparison analysis of the two speckle measurement algorithms,it is concluded that the comprehensive evaluation coefficient of the laser pre-speckle algorithm is 0.9697,which is less than that of the traditional speckle algorithm,with the evaluation coefficient of 1.0303,so that the laser pre-speckle algorithm is more efficient and faster.Therefore,the laser pre-speckle speckle measurement algorithm is selected for subsequent deformation measurement experiments of welded structures.(4)In this paper,a TIG butt welding experiment with a welding current of 60 A and an arc voltage of 12 V was carried out using a stainless steel test plate with a size of 150 mm × 70 mm × 2mm,and the welding deformation was measured.It showed that the deformation trend of the welded specimen in the X and Y directions is consistent with the theoretical analysis results.In the whole welding process,the maximum deformation in the X and Y directions were 1.66 mm and 1.62 mm.At the same time,after the experimental research on the measurement of tensile deformation of multiple welded structures,the stress-strain curves of multiple groups of welds were obtained,and the average stress-strain curves of the welds were obtained by median fitting.Based on this,an early warning model for the stress state of the weld is established,the safe stress variation range of the weld as well as the early warning range of the first,second,and third stress state of the weld are given.Moreover,based on the established early warning model,before the strain reaches plastic deformation,the probability of weld fracture is about 12%.At this time,the maximum load that the welded structure can withstand is 240Mpa;When the strain of the welded structure is greater than 0.04 and less than or equal to 0.18,the probability of fracture of the weld is about 36%,the maximum load that the welded structure can bear at this time is 480 Mpa,and a three-level early warning forecast of the weld danger zone is issued;When the strain of the welded structure is greater than 0.18 and less than or equal to 0.35,the probability of fracture of the welded structure is 32%,and a second-level warning forecast of the weld danger zone is issued;When the strain of the welded structure is greater than 0.35,the probability of fracture of the weld is about 80%.At this time,the maximum load that the welded structure can bear is 700 Mpa,and a firstlevel warning of the dangerous zone of the weld is issued. |
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