Theory And Experiment Analysis On The Compressive And Tensile Instability Properties Of Sheet Metal Forming

Wrinkling and fracture are two common defects caused by plastic instability in sheet metal forming.The essential factor for determining wrinkling and its fracture is the material property,only on the basis of reasonably evaluating the ability of resistance to plastic instability,can accurately analyze and effectively control the two kinds of defects.At present,the most commonly used test methods for sheet wrinkling performance such as the Yoshida Buckling Test(YBT)can not actually reflect the complex loading conditions during sheet stamping due to the limited stress state.And this metod is not proper to test the wrinkling features of some low plastic materials.In addition,how to characterize and analyze all kinds of plastic instability deformation scientifically with numerical methods still needs to be further explored.Therefore,this paper discusses the wrinkling characteristics of perforated and asymmetric plate which can provide complex in-plane stress state,analyzes the mechanical behavior during deformation,and accordingly puts forward new wrinkling test methods.The stress-strain evolution law and fracture morphologies in the process of tensile instability are investigated considering the nonuniform material properties,and the influencing factors on the results of the numerical simulation are addressed.The problems in engineering production are solved.The main work and conclusions are as follows:(1)A wrinkling test method based on a rectangular plate with a center circular hole is proposed.The stress concentration effect at the hole edge of a finite plate under uniaxial tension is analyzed using elastic mechanics,complex variable function and equilibrium differential equation.According to the engineering plasticity theory,the energy method and finite element simulation,the wrinkling analysis model of perforated plate under nonuniform compression is established based on the stress type and distribution state,and the formula for predicting the critical wrinkling stress is deduced,which is verified by the analysis with SUS304 samples.(2)The wrinkling test method with a perforated plate is extended to YBT.The stress at the onset wrinkling point changes from transverse compression and axial tension to mainly transverse compression.Wrinkling is more sensitive and will occur under subtle deformation on the whole,which provides a feasible method for wrinkling test of low ductile materials such as aluminum alloy Al6061.Compared with the methods of modifying the shape,length and width,fillet or curvature of the specimen,the stress type significantly changes in perforated YBT,which is more diverse with uneven distribution state.(3)The “transverse/axial stress ratio” at wrinkling onset increases with the hole diameter,showing that a more dominant role of transverse compressive stress in the deformation process of perforated strips and YBT,which results in a higher wrinkling tendency and wrinkle height.However,the bending stiffness decreases sharply when the hole is excessively large,the inhibitory effect of transverse tensile stress nearby in perforated strip increases,and the second wrinkling occurs in perforated YBT,leading to the reduction of wrinkling height.(4)A shear stress wrinkling test method for sheet metal using the asymmetric specimen under offset loading is proposed.Because the external force can not be coaxially balanced,it can provide the complex stress state consisting of shear stress and nonuniform tension,with the advantages of simplicity and quickness.The principal stress determines the occurrence and evolution of wrinkle hight.For the SUS304,the errors of the inclined angle θ of the wrinkle among numerical simulation,theoretical calculation and experiment are 7.4% and 2.9% respectively.(5)A method for calibrating the fluctuation range of material properties based on microhardness is proposed to analyze the nonuniform material characteristics.The finite element model of standard tensile test with nonuniform material is automatically generated using a self-programmed Python script,which can reproduce the dynamic strain transfer phenomenon in a large scale during diffuse instability,the critical instability strain can be accurately predicted,and more practical fracture morphologies are obtained.(6)The feasibility of wrinkling test methods with perforated plate under uniaxial tension is verified through the forming example of a door panel and tube bending.Based on the theoretical analysis,the wrinkling tendency and the risk of fracture at the welding line is greatly reduced by optimizing the drawbead of the stamping die,the wrinkling tendency of the thin-walled square tube is inhibited and the depressions on the internal/external faces is improved by modifying the bending core die and adjusting the fillet,which can provide a reference for engineering application.