Research On Plastic Fracture Behaviour And Ductile Fracture Model Of TRIP Steel Sheet

Experimental study under various stress states,strain rates and nonlinear strain paths were carried out using two different Transformation-Induced Plasticity（TRIP）steel in this paper.The different plastic fracture behavior and failure mechanisms of the two materials were revealed,thus provides theoretical guidance for the use of TRIP steel.This article focused on the plastic fracture mechanism of TRIP steel and fracture model,divided into four chapters from two aspects of theoretical and experimental research,and the arrangement is as follows:（1）Based on a new designed group of tensile specimens with various stress states from pure shear to plane strain,the plastic fracture mechanisms of TRIP800 and medium manganese TRIP980 and phase transformation of retained austenite（RA）under different stress states was systematically compared and studied.On one hand,moving from a stress state with a lower stress triaxiality,i.e.the pure shear state to the plane strain stress state,the shear fracture might be gradually replaced by dimple fracture,and the ductility of two steels decreased steadily.On the other hand,the primary mechanism for transformation of RA in TRIP steel during shearing deformation is the development of martensitic blocks while more martensite laths formated under uniaxial tension state and plane strain state.Finally,a stress state-dependent RA transition mechanism model was proposed.（2）Two kinds of TRIP steel materials were studied and analyzed systematically under quasi-static and medium-to-high-rate 100 s^-1 tensile tests.The phase transformation mechanism of RA at high strain rates was discussed.Firstly,TRP800 showed a pronounced positive strain rate sensitivity,whereas TRIP980 has the opposite.The fracture strain of both increased with the increase of the strain rate;the difference was that the fracture strain of TRIP800 changed monotonically with the stress triaxiality,while that of TRIP980 was not.With increasing the strain rate,the shear fracture modes of the two materials were enhanced while the dimple fracture become more uniform,and the plasticity of the materials improved as well.When the strain rate was greater than 100 s^-1,the RA phase transformation in TRIP steel was suppressed and become more stable at high strain rates.（3）The plastic hardening and fracture properties of TRIP800 under tension/compression deformation paths were investigated,and the influence of nonlinear strain path on plastic fracture and RA transformation of TRIP steel was revealed.First,a hybrid hardening model based on the Zang model of kinematic hardening and linear hardening was proposed,which can accurately describe the hardening behavior of TRIP800under nonlinear deformation paths.Through tensile and compressive nonlinear deformation,the dislocation density in the TRIP steel structure increased,the fracture strain and fracture plasticity significantly reduced.Under the non-linear deformation path,tensile stress was the main reason for the martensitic transformation of RA,while compressive stress can inhibit the martensitic transformation of RA.（4）The main factors affecting the plastic fracture behavior of sheet metal were summarized,and a new exponential fracture model was proposed,which considers the equivalent plastic strain,shear stress,stress state parameters,strain rate and other influencing factors.The new model can not only accurately describe the plastic fracture behavior caused by the void nucleation,growth and coalescence,but also the plastic fracture behavior caused by pure shearing.In addition,the new model considers non-linear damage factor,which can accurately describe the effect of tension-compression nonlinear loading path on plastic fracture.Finally,the experimental results based on TRIP steel were used to verify the prediction accuracy of the new model.Through the above research,a deeply understood on the plastic fracture failure mechanism of TRIP steel in different application scenarios have been made,which provides a basic guidance for the part design,process design and performance evaluation of high-strength-and-high-plasticity TRIP steel.