| Metal plastic deformation is generally non-uniform flow deformation,and the metal material itself is not in an ideal uniform state.Defects such as uneven size and distribution of micro grains,voids and structural segregation will lead to unstable mechanical properties and non-uniform plastic flow.Therefore,the characterization of mechanical properties and plastic behavior of materials has always been a difficult problem for scholars.Necking is the most significant strain localization behavior of metal plastic deformation,and it is also an important failure mechanism.Although metals are widely used in engineering,it is not clear how the necking behavior evolves during loading.Most previous necking related studies were carried out around the circular section,because the necking behavior of the circular section is relatively regular and easy to observe and measure,and its mechanical properties are relatively stable,but the necking behavior of the rectangular section is complex and irregular,and there are more interference factors in the mechanical property test.In this study,using the combination of three-dimensional digital image correlation(3D-DIC)technology and uniaxial tensile test,taking 6061 aluminum alloy plate specimen as an example,a local micro cube element based on the elastic-plastic volume invariance law is constructed,and an improved thickness strain measurement method is proposed.Applying this method to the necking section position of tensile specimen,a direct measurement method of true stressstrain of rectangular section can be developed,and compared with the two most commonly used true stress-strain solution methods in engineering.Finally,the full field thickness strain history is applied to the strain distribution description of the whole axial position,so as to explore the evolution characteristics of the necking zone length of rectangular section.The main innovative research work of this study is as follows:(1)Using the traditional 3D-DIC technology combined with the uniaxial tensile test of plate specimens,the full field transverse engineering strain and longitudinal engineering strain on the specimen surface are analyzed,these data are screened and matched,and the corresponding small piece is selected from the specimen to construct the element cube.Then the corresponding thickness strain is solved by combining the elastic-plastic volume invariance law,and then the whole field thickness strain history can be obtained by applying this process to the whole field.(2)In order to accurately obtain the true stress-strain relationship of rectangular section,the fracture position of the specimen is regarded as the collection of element cubes,and the thickness strain measurement method is used to obtain the cross-sectional area history of the fracture position,so as to solve the true stress history,and then construct a complete true stress-strain relationship,which is compared and verified with two traditional true stress-strain solution methods.The application scope of this true stress-strain measurement method is extended and applied to the solution of true stress-strain of different tensile specimens and different cross-section positions of the same specimen respectively.According to the obtained result curve,the applicability,advantages and disadvantages of this measurement method are analyzed.(3)In order to study the evolution characteristics of the length of the necking zone of rectangular section metal,after comparing and discussing the three strain distributions of the axial position,the thickness strain distribution is selected to describe the whole axial position inside and outside the necking zone of the four specimens,and the edge position of the necking zone and its length evolution characteristics of each specimen are determined by using the characteristics of the strain evolution of the necking zone,The effects of material parameters(width thickness ratio and grain size)of rectangular section on the length evolution behavior of necking zone were discussed. |
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