| With the development of automotive lightweight,aluminum alloy has been widely used in automotive industry.Fracture is one of the important behaviors of aluminum alloy.The fracture of aluminum alloy under complex loading condition is affected by many factors.Characterizing the fracture of aluminum alloy is of great significance to the lightweight and evaluation on the passive safety of automobiles.First of all,in order to comprehensivly characterize the fracture of aluminum alloy,experiments were carried out on Al6061-T5 in different stress states,including shear,uniaxial and notched tensile tests,high-speed tensile tests with strain rates ranging from1/s to 1000/s,and anisotropy tests in different directions.The experimental results show that the fracture of aluminum alloy exhibits obvious strain rate and anisotropic effect.In addition,the fracture characteristics of aluminum alloy under different loading conditions were analyzed by fractography.In order to comprehensively reflect the mechanical behavior of the material,the stage of yielding and hardening were characterized using the Hill-48 anisotropic yield criterion,Swift hardening equation,and M-JC strain rate equation.As the mesh convergence was investigated,the combined experimental–numerical approach was used to obtain plastic strain,stress triaxiality,Lode angle parameter at onset of fracture under all above loading conditions,in which the average of stress triaxiality and Lode angle parameter was used as the calibration data to avoid the parameter deviating obviously from the theoretical value.In succession,the calibration of the MMC fracture criterion was completed.The fracture data at each strain rate was use to determine the location of MMC fracture strain surface under each strain rate.The normalization of the MMC surfaces at different strain rates based on the quasi-static MMC surface was conducted,and the strain rate equation was established with the normalization coefficient and the corresponding real-time strain rate.The strain rate equation was combined with the MMC criterion to achieve the correction of the strain rate effect.Referring to the method of transforming the isotropic yield criterion into the anisotropic yield criterion,the fracture anisotropy coefficient was added to the equivalent plastic strain formula to characterize the anisotropic fracture,so as to achieve the anisotropic correction of the MMC fracture criterion.The GTN damage model was calibrated by optimizing,and the calibrated model could characterize the fracture characteristics of 6061-T5 aluminum alloy in different stress states.The yield stress term in the plastic potential equation of GTN was replaced by the Swift strengthening equation and the customized strain rate equation,therefore the GTN model can reflect the strain rate effect at the hardening stage.The1)?8? and11)was determined by the design of experiment?DOE?,in which the determined values can accurately reflected the fracture of the material at the medium and high strain rates.Accordingly,1)?8? and11)were further modified as functions of strain rate.Finally,the improved GTN model can be used to represent the effect of the strain rate on the fracture.UMAT of the modified MMC fracture criterion and the modified GTN damage model were implemented into LS-DYNA and used in finite element simulation.The simulation results were compared with the experiments,in which the results of the simulation were highly consistent with experiments,demonstrating the reliability of the subroutine. |
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