Research On Quasi-static Contact Stress Peak Of Die Surface In High Strength Steel Stamping

Automobile covering panel parts are the important automobile components. A largeamount of high strength steel (HSS) was used instead of traditional low strength steel(LSS), because it meets the requirements of energy saving, environmental protectionand crash safety. However, the wear of the stamping die has become the prominentproblem in HSS sheet forming because of the high yield strength and large deformationresistance of the HSS. The problem can cause the dimensional deviation, the localdamage of the die surface and the die premature failure. Moreover, the above problemcan lead to the consistency error among the parts in batch manufacturing.In this dissertation, the U-shaped stamping part of HSS was investigated. In order toanalyze the wear of die surface, the contact stress was researched through FEMmodeling using ABAQUS. Firstly, the analytic expression of the distribution of thecontact pressure on the die surface was deduced from the analysis of the loading duringthe forming process. The FEA model was modeled based on the structure of thestamping tools. The simulation results showed the evolution trend of the peak of thecontact stress on the die corner surface. Secondly, on the basis of the above researchabout the influence factors of die surface contact stress, the relative bending radius andultimate tensile strength were chosen as two main variants to analyze the distributionand the peaks of the contact stress. Thirdly, the maximum contact stress peak value ofthe die surface was set as the optimization goal. The following variants: the sheetthickness, the yield strength of the sheet metal, the punch radius, the die radius, thehardening index of the sheet metal, the coefficient of friction, the die clearance and theblank holder force, were taken as the optimization variables. The BP neural networkmodel was established based on the above setting. The mapping relationship betweeneach pair of the above variants was obtained and the forecasting model about maximumcontact stress peak value of die surface was built. Finally, wear loss estimation formulawhich based on die surface contact stress was derived through improving the traditionalformula. According to the contact stress of the die surface node and the relative slidingdistance, the quantitative relation between the contact stress and the wear loss on the diesurface was obtained.The research shows, the evolution of the peak of the contact stress on the die surfacehas two regions: transient region and steady region. In the transient region, the distribution of the peak was centralized, changeable and of high level. The contact stressin transient area is one of the main cause of die wear. Through adjusting the processparameters in the permitted range, the magnitude of the contact stress can be reduced.The above research results provided useful information on decreasing the magnitudeof the contact stress and reducing the wear of the die surface for the HSS stamping.