Anisotropic Fracture Model For The Failure Prediction Of Advanced High Strength Steel Sheets

Advanced high strength steel?AHSS?sheets have been widely used in automobile industry and become key lightweight materials in auto-body.Different from the conventional high strength steel sheets,martensite is the main strengthening phase of advanced high strength steel?AHSS?sheets.Due to the large difference in strengths between martensite and ferrite,the distributions of stress and strain between these two phases are extremely heterogeneous during the deformation.Such uncoordinated deformation will promote void nucleation and crack formation in the material,which makes ductile fracture a critical issue for AHSS sheets.Besides,the anisotropy/directionality in crack initiation and propagation has always been observed.AHSS sheets usually exhibit varying degrees of in-plane anisotropy due to formation of texture during rolling process.In addition,the martensite always distributes in bands throughout the thickness due to segregation of alloying elements inherited from the casing process and subsequent continuous rolling process.Such martensite morphology and inhomogeneous distribution will have a significant influence on the deformation and fracture behavior of AHSS sheets under out-of-plane loading conditions,which cannot been characterized through the traditional in-plane experimental methods.In this study,the anisotropic fracture of AHSS sheets is investigated.Firstly,a new stress invariant I₁-J₂-J₃ based ductile fracture model is developed for isotropic materials.Secondly,the proposed isotropic model is extended to account for the direction dependency of ductile fracture.Meanwhile,relevant stress state characterization and parameter calibration methods are developed for anisotropic materials.Then the proposed anisotropic fracture model is used to predict the anisotropic fracture behaviors during two different types of hole expansion tests.Effective guidance will be provided to predict the anisotropic fracture of actual parts.Finally,a brand new out-of-plane shear test method is proposed for sheet metals.Out-of-plane shear tests on AHSS sheets of different strength grades are carried out to investigate the out-of-plane properties of these metals.Main contents and innovations of this dissertation can be explained as follows:?1?A stress invariant based isotropic fracture modelFor isotropic materials,the stress states can be characterized by three stress invariants or Mises equivalent stress,stress triaxiality and Lode angle.The ductile fracture of sheet metal is not only related to hydrostatic stress,but also related to deviatoric stress.A new stress invariant I₁-J₂-J₃ based isotropic fracture model is proposed by introducing a polynomial function of stress triaxiality and normalized third invariant.The study on DP590 and DP780 indicates that the proposed isotropic fracture model has a good applicability for different materials over a variety of stress states.?2?Anisotropic fracture model and its parameter calibration methodThe stress tensor as normalized by the anisotropic equivalent stress is used to characterize the stress state of anisotropic materials.The isotropic fracture model is further extended to an anisotropic one through the linear transformation of stress tensor.Corresponding parameter calibration method for anisotropic materials is developed.The study on a DP590 sheet validates the accuracy of the proposed anisotropic fracture model.A new in-plane fracture experimental program,including central hole specimen tension,V-bending test,Nakajima test and in-plane shear,is developed.The anisotropic fracture model and experimental method proposed above are then used to investigate the in-plane anisotropic fracture behavior of a DP980sheet.?3?A case study on in-plane anisotropic fracture behavior of AHSS sheetsA new hole expansion device with conical punch is developed.The results of DP590 and DP980 show that cracks usually initiate at the edge of the hole and propagate along the rolling direction.Compared to the isotropic fracture model,the proposed anisotropic fracture model can predict the location and direction of fracture initiation more accurately.For the hole expansion test with cylindrical punch,the material will deform greatly at the edge of the hole and within a certain range,where the stress state is between uniaxial tension and plane strain tension.By using the Yld2004 yield criterion calibrated from uniaxial tension and plane strain data in conjunction with the proposed anisotropic fracture model,the location and direction of fracture initiation is accurately predicted for the Numisheet 2018 Benchmark 1.Through the comparative study,the parameter identification strategy,which considers the stress state of deformation zone,is put forward.?4?Experimental study on the out-of-plane anisotropic fracture behavior of AHSS sheetsA brand new out-of-plane double-notched shear specimen and relevant experimental approach are proposed for AHSS sheets with thickness of 1-3mm.Test results of a DP980 sheet show that the out-of-plane shear fracture strength is lower than that measured under the in-plane shear condition.And the out-of-plane anisotropic fracture behavior of the DP980 sheet is accurately predicted by the proposed fracture model.Out-of-plane shear tests on another four kinds of AHSS sheets are further carried out.The testing data revealed a significant reduction in out-of-plane shear fracture strength in banded structure.Therefore,the martensite banding structure should be suppressed throughout processing.Overall,this dissertation has systematically studied the anisotropic fracture of AHSS sheets.A new stress invariant I₁-J₂-J₃ based anisotropic fracture model is proposed,which can describe the anisotropic fracture loci in the stress space accurately.A brand new out-of-plane shear test method is developed and the influences of martensite banding on the out-of-plane behavior of AHSS sheets are studied.Corresponding prediction method is proposed and accurate prediction of anisotropic fracture of AHSS sheets subjected to hole expansion is realized.The achievements of this dissertation will provide guidance for the failure prediction and engineering application of AHSS sheets.