Study On Instability And Deformation Behavior Of Uniaxial Tensile Dispersion Of Sheet Metal

The true Stress-Strain data obtained by uniaxial tensile test is an important parameter to characterize the material property.And it is also the basis for judging the material strength and the ability of deformation resistance.The true stress-strain curve usually obtained from the engineering stress-strain under the assumption that the tensile specimen's volume is constant.This method can only be applied to the macroscopic uniform deformation stage.When the deformation reaches the diffuse instability,the method is no longer applicable.Therefore,in this paper,the deformation behavior of the diffuse instability stage of sheet metal is studied.In order to obtain a large range true stress and strain of rectangular section specimen,and to apply it to simulation and guide Engineering practice.The main research contents and conclusions are as follows:304 stainless steel,ST12 cold rolled steel and 5052 aluminum alloy with thickness of1 mm sheets are selected as the research object.According to the national standard,tensile specimens were designed.The circular mesh was printed on the specimens by silk printing method.The grid size change before and after deformation of the specimen is measured and analysed by the non-contact measurement method,then the deformation behavior of metal sheet specimens in the stage of diffuse instability deformation is studied.At the same time,the thickness anisotropy coefficient of the specimen is obtained by the size change before and after the mesh deformation in the uniform deformation stage.Thus,the minimum cross-section area of the specimen at any time at the stage of dispersion instability is obtained.And then the true stress and strain of the specimen in the whole deformation stage are obtained.Tensile tests were carried out with extensometer,and the true stress and strain of the specimen during uniform deformation were obtained by data processing.The results obtained by grid method and extensometer method are compared and analyzed.The maximum relative error is 4.2%.And the feasibility of grid method was verified.The results show that the true stress-strain curve of metal sheet under large plastic deformation can be obtained by grid non-contact measurement method.According to the basic assumption of constant volume,the critical width and the originallength of the necking are derived,and a theoretical formula for calculating the true stress true strain of the metal sheet in the phase of the dispersion instability is derived.The true stress true strain of the test piece in the dispersion stability stage can be obtained based on the force elongation curve of the grid-based tensile test.The accuracy of the theoretical formula was verified by comparing the results obtained by the grid method and the theoretical method,and the maximum relative error is 1.8%.The stress and strain data obtained by non-contact mesh measurement method were applied to the Abaqus finite element software to establish the simulation model of unidirectional tension.The simulation results of force-stroke curve was compared with the Grid method experimental results.The relative error is less than 5%.the accuracy of the grid method and the theoretical formula result were verified.