The Investigation On Forming Limit Of Sheet Metal Involving Thickness Stress

The forming limit diagram is the most direct form to evaluate the formability of sheet metal,and it plays an absolutely important role in the metal plastic forming field.The traditional theoretical forming limit diagram(FLD)is based on the condition of plane stress.However,for some plastic forming processes,the stress in the thickness direction plays a very important role.If the traditional FLD is used to describe the ultimate strain of the material,the accuracy will be greatly reduced.For the rapidly developing sheet metal forming industry,new materials and new processing technologies are emerging one after another,and the precision requirements for sheet metal processing will be higher and higher.It's very urgent to study a more complete FLD,that is,the FLD with the consideration of the thickness stress.This article based on the currently widely accepted C-H and MMFC instability theory,the theoretical forming limit curve is solved after considering the thickness stress.In the calculation,the thickness stress is considered as the constant thickness stress and the thickness stress induced in the process of sheet metal deformation.The effect of two different forms of thickness stress on the theoretical forming limit diagram and the forming limit stress diagram are discussed,as well as the effect of the material property coefficient on the forming limit after the introduction of thickness stress.Results show,The introduction of thickness stress changes its sensitivity to the effect of FLD.The finite element simulation of the bulging experiment proves that the thickness stress will be induced in the process of deformation,which provides a strong support for the project;TRIP780,304 stainless steel,AA5182 aluminum alloy and 57540 aluminum alloy were used to verify the theoretical forming limit curve after the introduction of thickness stress.The forming limit diagram after the introduction of the thickness stress is more consistent with the experimental data,which confirms the necessity of considering the thick stress to solve the forming limit.The M-K instability criterion is currently the most widely used theory to solve the theoretical forming limit curve.The core idea is that there is an initial thickness imbalance in the initial state of the sheet metal,and the instability starts from this groove.From another point of view,in some materials with sufficiently smooth surface,the initial thickness unevenness is not the main cause of damage.At the same time,due to the slight irregularity of the shape of the material or the problems in the processing technic,the internal stress is uneven,and it is this uneven stress that causes the material to eventually break.In view of this idea,the M-K theory is modified as the initial stress unevenness,and the effect of initial stress unevenness on FLD is discussed.And based on the modified M-K theory,a constant thickness stress is introduced to study its effect on FLD.Finally,TRIP780 and 304 stainless steel are used to verify its accuracy.