High Temperature Strain Measurement Of HAZ Based On 3D Digital Image Correlation

As an important processing technology in manufacturing industry,welding is widely used in mechanical manufacturing,nuclear power,aerospace and other fields.The effective evaluation of the quality stability and structural integrity of some key components is always a technical problem that can not be ignored by welding workers.The high temperature strain distribution and the final residual stress level in the HAZ of the welded joint seriously affect the quality of the welded joint,the rigidity of the welded component and its service life.In view of the non-linear thermal cycle of the welded joint in the welding process,the residual stress and deformation will inevitably occur in the heat affected zone and inside the weld after welding.Preheating before welding can effectively adjust the stress distribution of the welded joint,while the heat affected zone and internal stress state of the welded joint are closely related to the temperature history and strain level in the whole welding process.Accurate measurement and characterization of the strain distribution in the HAZ during welding is an effective way to control the stress level and quality of the joint.Different from the traditional twodimensional digital image correlation(DIC),three-dimensional DIC is not affected by out of plane displacement.In this paper,the traditional 3D DIC algorithm is improved to accurately obtain the high-temperature strain distribution in the HAZ of the welded joint,and the influence of preheating process on the strain field distribution and stress level in the HAZ of the welded joint is further studied.Firstly,the three-dimensional matching strategy and the whole field calculation strategy of the traditional three-dimensional DIC method are improved,and the three-dimensional matching strategy based on the combination of similarity and the global path calculation method based on the similarity threshold which can improve the efficiency of the whole field strain calculation are proposed.According to the principle of the 3D DIC algorithm,a 3D DIC strain calculation method with strong adaptability and easy expansion is developed Software.Secondly,the influence of shape function type selection on digital image matching error is discussed and analyzed through the simulation speckle test.The selection principle of shape function type in the process of self matching and mutual matching of 3D DIC method is given.After that,the rigid body translation experiment and one-way tensile experiment are carried out to explore the measurement error sources of 3D DIC,analyze and compare the calculation accuracy of three kinds of stereo matching strategies,and verify the accuracy of the stereo matching strategy of similarity combination and the calculation efficiency of the global path algorithm of similarity threshold.Finally,aiming at the whole field high temperature strain measurement of HAZ in the actual welding process,a strain measurement system of HAZ based on active vision is built.The auxiliary light source and light reducing filter system are used to effectively restrain the arc light interference.Based on the system,the butt test of 304 stainless steel robot argon arc welding plate is carried out,and the image of HAZ and temperature field information are collected Based on the increment theory and material constitutive equation,the residual stress distribution in the HAZ was quantitatively solved.The influence of preheating temperature on the strain distribution and stress level in the HAZ near the seam was studied.