Z.Q. Lin

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

The Effect of Macroscopic Stress State on Transformation Rate in TRIP-Assisted Steels

The dependence of the transformation rate on the multiaxial stress state corresponding to different loading paths is investigated. On the basis of the single shear, uniaxial tension, plane strain and equibiaxial stretching tests, the influence of stress state on stability of retained austenite is analyzed and an equation of transformation kinetics is developed. In sheet metal forming, the material undergoes complicated deformation, the prediction of the volume fraction of retained austenite during forming process is essential to estimate the contribution of the TRIP effect to improving formability. To this aim, the volume fraction of retained austenite in the part’s different regions has been calculated using finite element method. The calculated results were compared with the experimental data measured in deep drawing experiments.

The Ductile Damage and Fracture Mechanisms Analysis with Random Dispersion Multivoids

The cell model with twenty-five random dispersion voids was employed to analyze the damage and fracture mechanism of the nodular cast iron. The results show that the growth velocity of the voids has obvious difference with each other due to the random dispersion of voids. In the early stage of the deformation, the growth of the voids is mainly determined by the distance of the voids since the triaxiality stress parameter of the matrix around the voids is approximately equal. With the increase of the triaxiality stress parameter of the matrix materials around the void, the evolution velocity of the voids increases quickly. At the same time, this will influence the neighboring voids to grow quickly. The chain reaction of the rapidly increase of voids lead to the final material failure. The results can explain the fracture appearance of the smooth bar specimens under uniaxial tensile load really.

Transformation Behavior of Retained Austenite for a TRIP700 Steel during Sheet Forming - An Experimental Study

Transformation induced plasticity (TRIP) steel possesses favorable mechanical properties such as high strength, ductility and toughness. In this paper, the transformation behavior of retained austenite for a TRIP sheet steel in complex strain states was investigated. Uniaxial tensile, Nakazima testing and cup drawing experiments for TRIP700 sheet steels were performed and the microstructures in different deformation mode were analyzed. The volume fraction of austenite was determined with X-ray diffraction and is found to decrease with increasing equivalent strain. The amount of austenite transformed during plane strain was determined to be greater than that of transformed in biaxial tension at the same equivalent strain and the amount of transformed in uniaxial tension was the least. Due to inhomogeneous deformation of the specimen, the retained austenite is distributed inhomogeneously, which may result in the deterioration of shape precision or appearance of surface distortion under unexpected load.

The Numerical Analysis of the Formability of Aluminium Alloy Sheet in Complex Shape Box Deep Drawing Forming

Abstract. This paper analyzes the formability of aluminium alloy sheet X611-T4 in complex shape box deep drawing by means of a ductile fracture criterion, and reveals the effect of shear stress in blank plane on the formability of blank, which at different-radius corner. The obtained results are as follows: (1) the simulated thickness distribution is in agreement with the measured value; (2) the critical punch stroke and the fracture initiation position predicted by the criterion are consistent with the experiment; (3) the effect of friction coefficient, blank holder force and cut-length on the formability is obvious, among which the effect of cut-length is the most remarkable. The result proves that reasonable modification of the processing parameters can obviously increase the formability of aluminium alloy sheet X611-T4.