Study On A Ductile Fracture Prediction Model For Advanced High Strength Steel And Its Application

Advanced high strength steel(AHSS)is a promising material for producing light weight car bodies coupled with preferable crashworthiness.However,AHSS.shows a strong tendence to fracture during forming processes,such as fracture without noticeable necking,edge fracture and shear-induced fracture.The widely used forming limit curves have no solutions in the abovementioned ductile fracture,consequently restricting the application of AHSS sheets in automobile industry.In order to accurately predict the complicated ductile fracture of AHSS,the current paper focuses on a mathematical model of ductile fracture and its application in AHSS.The main research aspects and conclusions are as follows:A micro-mechanically-motivated phenomenological ductile fracture model was devised,which focused on the void deformation modes.To this end,it was assumed by the model that the ductile fracture onset is a limit state of void deformation followed by void coalescence.Thus,a strain based void nucleation model was proposed according to the observation from X-ray microtomography for a dual phase steel.Also,two major void deformation modes were carefully considered,namely the void dilation and the void elongation/rotation.In order to capture the continuous ductile damage caused by plastic deformation,the formulated models,which were responsible for the void nucleation and void deformation,were combined in an integral framework,where five model parameters with clear physical meanings were introduced into the finalized prediction model.A consistent transformation was performed for the proposed model from the space of {?1,?2,?3} to the space of {?,L,?p}.With the assumption of proportional loading,the model proposed was reformulated as a function of 3D fracture surface.A parametric study on the newly developed model was performed to better understand the performance of the model.A hybrid experimental-numerical method was adopted to obtain the necessary information regarding the deformation history in ductile fracture tests for DP780 and DP590 sheets.The method made use of four distinct shapes of specimens,by which ductile fractures were achieved under distinctly different stress states.The experimental reports on strain field,punch force versus displacement curve,and the thickness distribution along a selected section for No.4 specimen validated the robustness of the current simulations.The proposed model was calibrated by minimizing the MSE.The constructed 3D fracture surfaces were in a good agreement with experiment.Besides,the SEM images of fracture surfaces at identified critical locations were took for the DP780.The SEMimages exhibited a stress state induced change in appearance.This served as a contribution to verify the physical backbone of the model proposed.In the current research,the MMC model and Lou-Huh model could also give accurate 3D fracture surfaces with acceptable errors for the considered meaterials,although they were developed from different points of views.The newly developed model was also successfully used to construct the 3D fracture surface for an aluminum alloy and a docol 600DL dual phase steel with high accuracy.It was noteworthy that the constructed 3D fracture surfaces for the considered materials presented pronounced differences in "surface height" and surface curvature.However,the new ductile fracture model still provided a good agreement with test data.Therefore,the developed model was thought to hold a good capability to predict ductile fracture in a broad range of stress state for various materials.In order to predict fracture for the sheared edges of AHSS sheets,a fully integrated simulation framework was proposed with the shear-induced pre-damage considered.With the help of the simulation framework,the effects of pre-damage were analyzed.It was concluded that considering the shear-induced pre-damage played an important role in accurate edge fracture prediction.A series of edge fracture simulations were performed to study the effects of different ductile fracture models,among which the newly proposed model and Lou-Huh model achieved very good prediction when compared to the results of hole-expansion test,whereas the Oh model and Brozzo model did not produce acceptable predictions.In sense of the nonlinear load path involved in the issue of edge fracture,effects of hardening model were also studied.To this end,a semi-implicit integration scheme was proposed to incorporate Hill'48 quadratic yield function with mixed kinematic-isotropic hardening model.With the help of the integration scheme,user-defined material subrountes were coded to simulate edge fracture behavior.It was found that although nonlinear loading path occurred from shearing process to hole expansion process,isotropic hardening model was efficient.Forming limit curve at fracture(FFLC)was obtained for the DP780 sheets by using Zwick/Roell BUP 600 test machine.The predicted FFLC by the newly proposed moded showed an acceptable deviation compared to the experimental one.This confirmed a good capability of the proposed moded to predict FFLC for AHSS.The FFLC of a TRIP690 sheet was also constructed by the model proposed.With the help of the FFLC,the shear-induced ductile fracture in square cup drawing of the TRIP690 sheet was studied by finite element simulation.The fracture location and fracture stroke were accurately predicted by the constructed FFLC.Since the material subjected to the shear-induced ductile fracture experienced a pure shear loading path,the traditional FLC had no solutions for this case.Therefore,the proposed model in current work served as a contribution to accurate prediction of shear-induced duction fracture for AHSS sheets.