Theoretical And Experimental Research On The Prediction Of The Ductile Fracture Of QP980 Advanced High-Strength Steel Under Complex Stress States

Advanced high strength steel sheets have the advantages of high strength,light weight,low cost,have been widely used in the field of automobile manufacturing.However,due to the poor formability of advanced high strength steel,defects such as edge cracking occur during the stamping process.And the traditional fracture prediction methods cannot accurately predict the possible fracture phenomena in the sheet forming process,which seriously limit the application of advanced high strength steel sheets in industrial production.Therefore,an indepth study of the uncoupled ductile fracture criteria applicable to advanced high strength steel sheets and the completion of forming fracture prediction under complex stress states is of great industrial application value for improving the forming fracture theory research of advanced high strength steel sheets and improving the accuracy of fracture prediction.From microscopic viewpoints,the ductile fracture of metals can be regarded as the integral manifestation of nucleation,growth and coalescence of voids.The stress triaxiality and Lode parameter as can describe the deformation of metal material microscopic stress state have been widely used in the study of fracture criteria.This paper firstly studies the three common stress characterization methods in three-dimensional space and introduces the methods for the conversion of these three characterization methods in detail.Through the analysis of the stress states,the thirteen ductile fracture criteria which are commonly used at present are uniformly converted into the three-dimensional space representation of the stress triaxiality,Lode parameter and equivalent fracture strain.For preliminary choose appropriate fracture criterion to predict fracture behaviors in the process of numerical simulation,the experimental results of Al2024-T351 under different stress states are selected to calibrate the fracture criteria parameters to verify the accuracy of the prediction of the fracture by the different fracture criteria.It’s found that the DF2012 ductile fracture criterion can accurately describe the ductile fracture of metallic materials.This article mainly takes the QP980 advanced high-strength steel sheet with a thickness of1.0mm as the research object,and mainly processes four different shapes of specimens in nine different directions to test the mechanical properties and fracture behaviors of QP980.The experimental results show that the anisotropy of strength and fracture behavior of QP980 sheet is not obvious,so QP980 can be considered as isotropic material for analysis.Accurately describing the yield deformation behaviors of QP980 sheet is the premise of describing the fracture behaviors.In this paper,a constitutive model composed of P＿Drucker yield function and Swift-Voce hardening model are selected to describe the plastic deformation behaviors of QP980.It’s found that the constitutive model could accurately describe the plastic deformation behaviors of QP980 through correcting the material parameters by inverse engineering.On this basis,the material parameters of fracture criteria are modified by inverse engineering,and it’s proved that the fracture behaviors of QP980 sheet under complex stress states could be accurately predicted based on the DF2012 ductile fracture criterion.In order to verify the accuracy and applicability of the selected constitutive model and fracture criterion,the tests under different stress states are carried out by using QSTE550 and QSTE420 advanced high strength steels,and the fracture prediction of QSTE series steels can be completed by combining the above parameter calibration method.