Study On Dynamic Constitutive Equation And Fracture Of QP980 Steel Coupling With Phase Transformation

The dynamic constitutive equation and dynamic fracture coupling with phase transformation for QP980 steel are studied.An interruption mechanism is developed to control the elongation of specimens at high strain rate in using a tensile split Hopkinson bar(TSHB).The specimens of QP980 steel recovered from the interrupted TSHB tests are measured by using synchrotron X-ray diffraction(XRD)to analyze the volume fraction of retained austenite.It is revealed that the retained austenite volume fraction as a function of strain under static tension is almost identical with that at high strain rate tension of 500/s and 1000/s.The phase transformation kinetics equation of QP980 steel is constructed based on the O-C theory and the revealed results.The flow stress of QP980 multiphase steel is calculated from the stresses of individual constituent phase based on the continuum model whereby the internal stresses produced by the inhomogeneous distribution of plastic strain are considered.A dynamic constitutive equation coupling with transformation induced plasticity(TRIP)effect for QP980 steel was presented assuming that the effects of strain rate and temperature are separable.The stress-strain curves of quasi-static and dynamic tensile tests for QP980 steel were compared with that predicted by the presented constitutive equation.The diffuse necking of QP980 steel specimens in TSHB tests is recorded by high speed camera and analyzed using Batra and Wei's instability criterion and the presented dynamic constitutive equation.The effects of strain rate and temperature on the dynamic tensile fracture strain of QP980 steel specimens are given.The different mechanisms of microvoid evolution in QP980 steel are indicated by the SEM investigation of the ruptured zones.