Fracture Mechanism And Criteria For Shear Fracture Of DP780 Dual Phase Steel

Advanced high strength steels(AHSS)have been widely used in automobile industry and become key lightweight materials in auto-body.However,the phenomenon “Shear fracture” is often observed in the stretch-bending process of stamping over small radius with AHSS.It occurs parallel to and near the die radius in the stretch-bending test,with no obviously necking.Since traditional necking theory cannot explain this failure,commonly used Forming Limit Diagram(FLD)is unable to describe this type of fracture,which limits the application of AHSS.In this study,the shear fracture of dual-phase(DP)steels is investigated.Based on recent studies,a stretch-bending failure system is built and experimental works are conducted to study the failure modes characterized by the mold geometry and process conditions.Then the in-situ bending under tension test system is introduced,microstructure evolution and propagation of micro-cracks in shear fracture are observed.Digital image correlation(DIC)and GTN(Gurson-Tvergaard-Needleman)damage model are used to analyze the strain and stress distribution in order to reveal the mechanism of shear fracture.Finally,the non-uniform deformation is considered and shear fracture criterion is proposed to predict shear fracture.Engineering approach of shear fracture is explored afterwards.Main contents and innovations of this dissertation can be explained as follows:(1)Shear fracture experiments of DP steelsFailure modes are investigated with stretching-bending experiments in stamping.The difference between necking failure and shear fracture are explored,and critical relative bending radius(R/t)is obtained.A new symmetrical bending under tension tests system is designed to study the fracture in stretch-bending tests.Results show that when bending radius is large,the necking failure happens on the side walls of specimen,while shear fracture occurs at small bending radius.Transitional fracture mode is also observed.With the increase of steel strength grade and blank holder force,shear failure is more likely to occur.(2)In-situ experiments and micro fracture behaviors of shear fractureThe in-situ symmetrical bending under tension tests are introduced to explore the microscopic fracture behaviors of AHSS.Through the experiments,micro-cracks are not found in the outer surface when bending radius is large,while the micro-cracks appear very early at small bend radius.Micro-cracks appear near the interface of martensite and ferrite,and also in martensite later.The micro-cracks will cross the ferrite and martensite,and propagate along the interface,to form macroscopic cracks.In the thickness direction,martensite and ferrite are elongated in the outer and middle layer,while the inner layer ferrite appears from compressed to slightly stretched during the stretch-bending process.(3)Microstructural fracture mechanism of shear fractureIn-situ microstructure images are used for quantitative analysis by digital image correlation,to obtain displacement and strain distributions in each phase of bending surface.Representative volume element(RVE)modeling method is used to build a microstructure model of DP steels with the analyzed region.Later the GTN damage model is introduced to study the stress and strain behavior of martensite and ferrite in the stretch-bending of DP steels under small bending radius.The simulation results match the in-situ tests and DIC analysis well and thus the shear fracture mechanism of DP steels under small bending radius is revealed.(4)Shear fracture criteria and industrial applicationThe non-uniform local deformation through thickness corresponding to bending position is obtained and analyzed.The initial crack location of shear fracture is at the outer layer of bending radius.Different ductile fracture criteria are compared and Cockcroft & Latham fracture criterion is selected to determine shear fracture,and the initial crack location of shear fracture at small radius can be effectively predicted.The forming index based on ductile fracture criterion and forming limit diagram can evaluate the shear fracture and necking failure.The shear fracture of typical DP780 front rail in auto-body is analyzed and results show that ductile fracture model can be used as the engineering method to predict shear fracture in the forming process.Overall,the shear fracture in the forming of AHSS is studied,shear fracture experiments of DP steels are finished and in-situ observations of microstructure are carried out.Shear fracture mechanism is obtained by microstructure analysis,and the shear fracture criterion is established and applied to DP780 front rail parts.These works are the basis for the development and application of AHSS.