DIC Test Analysis Of High Gradient Stress Fields Of Defective Structures

Digital Image Correlation(DIC)is a test method to obtain the structural displacements and other information of images before and after deformation by correlation criterion function.Due to its advantages of easy operation,high accuracy,simple equipment and instruments,and full-field measurement,DIC technique has been developed and applied in many fields in recent years,especially in the measurements of object strain fields.In this thesis,the DIC method is used to experimentally study the stress concentration of metal specimens containing cracks and V-notches during the tensile process.The main research work and results are as follows:1.An overview of the relevant domestic and international research progress of the digital image correlation method for structural displacement measurement is presented.The basic principle of 2D DIC method is introduced,and the whole pixel initial value determination method and sub-pixel displacement search method are classified.2.Axial tension specimens of 6061 aluminum alloy are designed and the tension tests are performed on the specimens.The stress-strain test curves of the aluminum alloy specimens are obtained from the records of strain gauge and universal testing machine.Based on Hooke’s law of elastic theory and Ramberg-Osgood theory of plastic model,the stress-strain constitutive relationship of the elastic-plastic material during the tensile process is derived in detail.Comparing the theoretical constitutive relationship with the stress-strain test curve,the mechanical property parameters of 6061 aluminum alloy are formulated.As a result,the elastic-plastic stress-strain curve of 6061 aluminum alloy is constructed.3.Uniaxial tension tests of 6061 aluminum alloy sheet specimens with the cracks/Vnotches are completed.The displacement and strain fields of the specimens in the defective area are obtained based on the DIC method.The stress fields of the cracked and notched specimens are calculated based on the constructed elastic-plastic stress-strain curve of 6061 aluminum alloy and the obtained mechanical properties parameters.According to the stress results by DIC test,the plastic zone in the tip regions of the specimens is also obtained4.Using Irwin plastic zone through linear elastic theory,combining principal stress and von Mises yield criterion in the crack tip region,the boundary and size of plastic zone for the specimens is obtained.Then the outline of plastic zone boundary is drawn.The linear elastic theoretical results of Irwin’s plastic zone are compared with the plastic zone obtained by the DIC test.5.Finally,the finite element method in ABAQUS software is used to simulate and analyze the stress concentration problems of the specimens containing V-notches and cracks under the tensile process.The stress results from the DIC test for the specimens are compared and analyzed with the simulation results of the finite element method.Hence,the relative errors are calculated to verify the reliability of the experimental results by DIC method.