W.J. Dan

Influences of cyclic loading on martensite transformation of TRIP steels

While austenite transformation into martensite induces increasing of the crack initiation life and restraining of the growth of fatigue cracks in cyclic-loading processes, TRIP-assisted steels have a better fatigue life than the AHSS (Advance High Strength Steels). As two key parameters in the cyclic loading process, strain amplitude and cyclic frequency are used in a kinetic transformation model to reasonably evaluate the phase transformation from austenite into martensite with the shear-band intersections theory, in which strain amplitude and cyclic frequency are related to the rate of shear-band intersection formation and the driving force of phase transformation. The results revealed that the martensite volume fraction increased and the rate of phase transformation decrease while the number of cycles increased, and the martensite volume fraction was almost constant after the number of cycles was more than 2000 times. Higher strain amplitude promotes martensite transformation and higher cyclic frequency impedes phase transformation, which are interpreted by temperature increment, the driving force of phase transformation and the rate of shearband intersection formation.

Simulation Investigation the Effect of Heating-Lines on Tensile Mechanical Properties of Sheet Metal after Laser Scanning

In this study, tensional mechanical properties of sheet metal with heating-lines after laser scanning are investigated based on the thermal-microstructure-mechanical model. The phase transformations, during laser scanning of sheet metal, are calculated by coupling the thermal history from finite element analysis with a phase transformation kinetic model. The flow stresses of material are obtained from the constitutive relationship of the phases based on the mixture strain hardening laws. The results show that the influence of the heating-lines number on tensional mechanical properties of material is obvious. The mechanical properties are related to the distribution of microstructure in heat affected zone after laser scanning.

A Tensile Characterization Study of Metal Sheet in Large-Strain

A method for determining the strain-stress curve of larger-strain is proposed when plastic instability occurs in standard tension tests. Thin tested steel sheet is subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera. Displacement and strain field of material deformation can be calculated by a mesh-free PIM method. A tensile experiment is simulated to verify that local measuring stress-strain curve by PIM method near the center of the specimen can describe a full stress-strain curve clearly. Numerical simulation results, at different location along the specimen axial, present that different parts of specimen have different deformation distribution in tensile and the center fracture part of tensile specimen is the only region which can experience full strain. The true stress- true strain curves, based on the estimated parameters, are validated in all strain regions by comparison with curves from standard tension tests. The measured curves by PIM method are very stabilization. Compared with several material constitutive equations, The Swift’s equation is very close to experiment curve at plastic deformation.Copyright

A Strain-Induced Martensite Transformation Model for TRIP Steels in Cyclic Loading Condition

TRIP-assisted steels are ideal for lightweight automotive applications due to not only its high strength and ductility but also the fatigue resistance, which result from the special strengthening mechanism of phase transformation in deformation processes. In this paper, a kinematic transformation model is developed, based on shear-band intersections, to reasonably evaluate the transformation from austenite into martensite with cyclic loading. The transformation process is controlled by parameters, such as, increase temperature, transformation driving force, shear-band formation rate and shear-band intersection volume fraction, by which the influences of cyclic loading variables (such as, number of cycles, strain amplitude and frequency) on transformation from austenite into martensite are predicted. The microstructure volume fraction is tested by X-Ray to verify the model.

Hardening Behaviours Prediction of TRIP Steels with Pre-Strain

The hardening behaviours of transformation induced plasticity (TRIP) steels depended on the martensite transformation from the retained austenite. As a key factor, pre-strain can influence martensite transformation during sheet metal multi-step processes. In order to learn about the material’s properties after pre-deformation, the effect of pre-strain on mechanical behaviours of TRIP steels has been investigated with the strain-induced martensite transformation in pre-strain condition in this study. The analyzed results show that: Hardening rate, strain hardening exponent and necking strain of TRIP steels are related to martensite transformation in deformation

Investigation on the TRIP Steel Sheet Forming Processes with Respect to Punch Speed Loading-Paths

Loading path is one of importance factors that influence the formability of sheet forming process. In this study, the effect of punching speed loading paths (PSLP) on forming and spring-back processes of TRIP (transformation induced plasticity) steel has been investigated. Four kinds of loading paths with three punch speeds are introduced to verify the cup drawing and U-channel spring-back processes based on a constitutive model accompanying the strain-induced martensite transformation. The results show that higher punch speed results in the thickness uniformity of drawing cup and the spring-back angle of stamping U-channel increased with the same loading path. Furthermore, a given loading path (C4) not only increases the minimum thickness of cup but also decreases the spring-back angle of U-channel.

Spring-Back Prediction of U-Channel Stamping with Martensite Transformation

As an advanced high strength steels, transformation-induced plasticity (TRIP) steel has obtained more and more attentions in automotive body components forming. The TRIP effect of this steel can improve the strength of the material, which results in the spring-back angle is different with that without TRIP effect. In this paper, the spring-back process of U-channel stamping with TRIP effect is studied by deformation energy method, which is based on a constitutive model accompanying strain-induced transformation in strain rate condition. The results have shown that martensite volume fraction distribution in key parts is related to the deformation energy of material, which induces the spring-back angle higher than those without TRIP.

Study on Loading Path Influences on the Thickness Distribution of TRIP Steels During Sheet Metal Forming

Loading path is one of key factors that influence the formability of sheet metal forming processes. In this study, the effect of several kinds of loading paths on the thickness distribution of TRIP steel is investigated in a deep drawing process based on a constitutive model accompanying the strain-induced martensite transformation. A kinetic model of transformation, that describes the relationship between the thickness distribution of a deep drawing process and the martensite transformation, is used to calculate the martensite volume fraction. The influences of loading path on the martensite transformation are also evaluated through the change in the stress-strain state, the forming temperature, the transformation driving force, the nucleation site probability and the shear-band intersection controlled by the stress-strain state and forming temperature at the minimum thickness location in the formed part.Copyright