Study On Hardening Behavior And Constitutive Model Of TRIP Steels Under Different Strain Paths

Because the retained austenite in TRIP?Transformation Induced Plasticity?steels will progressively transform to martensite during plastic forming processes,the mechanical behavior of TRIP steels is affected by both of the dislocation movement and the martensite transformation.Furthermore,sheet metals generally undergo complex strain path changes in real stamping engineering practices.The combination of the effect of martensite transformation and strain path changes brings great difficulties in the development of constitutive model for TRIP steels.To improve the numerical simulation accuracy,it is necessary to investigate the effect of strain path on deformation behavior of TRIP steels and develop a corresponding constitutive model.The related work have an engineering significance for widening the application of TRIP steels.In this paper,the expriments of TRIP steels under three different proportional loading paths,reverse loading and strain path change were carried out to investigate the effect of strain paths on the hardening behavior of TRIP steels.Based on the experimental results,some hardening models to reflect these strain paths on the hardening behavior of TRIP steels were proposed.Corresponding parameter calibration processes for the proposed hardening models were established based on optimization methods and experimental data.Abaqus/UMAT subroutines were also developed to validate the proposed hardening models.The main contents and conclusions of this paper are as follows:?1?The stress and strain curves of TRIP steels under three proportional loading paths were obtained through uniaxial tension,pure shear and plane strain experiments.The experimental results showed that the hardening behaviors of TRIP steels under these three proportional loading paths have significant diffenences.A new hardening model named HVTL model,was proposed to reflect the effect of different loading paths on the hardening behaviors.In this model,the loading paths were characterized by stress triaxiality and Lode angle.HVTL model was constructed by combining Voce and Hollomon hardening models.The influence of different loading paths was reflected in HVTL model by changing the proportion of Hollomon and Voce hardening component.The HVTL model was applied to simulate the above mentioned experimentations.The results showed that the stress triaxiality and Lode angle together have a good distinction on the change of the loading paths,and the HVTL model can reproduce the hardening behavior of TRIP steels in different proportional loading modes well.?2?Based on the cyclic shear test of TRIP steels,the stress-strain curves of cyclic shear of TRIP steels were obtained.The Bauschinger effect,transient effect and permanent softening effect were clearly observed when the material were reversely loaded.The effect of martensitic transformation on the hardening behavior of TRIP steels under reverse loading was considered by adding the martensitic transformation rate into the definition of k_1,g₃,g₄ item,which were respectively related to the transient effect and permanent softening effect.The corresponding UMAT subroutine of HAH model was developed.Based on experimental data and numerical simulation optimization,a set of parameter calibration method of HAH model was established.The results showed that the improved HAH model can better describe the transient behavior and permanent softening effect of TRIP steels under reverse loading.?3?An experimental method of two-step loading composed of biaxial loading test and uniaxial tensile test was established.The Marciniak test method was used to load the TRIP steels specimens of different sizes to a certain strain,and then unloading.The tensile specimens were cut out in the uniform deformation area of bulging specimens.In the second step,uniaxial tensile tests were carried out using these small specimens.The experimental results showed that the subsequent hardening ability of TRIP steels is enhanced when the second stress in the Marciniak tests changes from positive stress to negative stress.A new hardening model named VH model,considering strain path change,were proposed by using the normalized microstructure deviator of HAH model and Voce model.The simulation results of VH model were compared with the original Voce model.It is concluded that the VH model can better reproduce the hardening behavior of TRIP steels in the second step of uniaxial tension and have a good agreement with the experimental data.