Papers by Author: Wen Jun Hu

Abstract: Cubic structures having spherical pores ranged as BCC and FCC lattices are constructed to simulate the microstructures of cellular polymers with various relative densities. The Mooney-Rivlin strain energy potential model is adopted to characterize the hyperelasticity of the constituent solid from which the foams are made. Finite element analysis on the influences of the polymer hyperelasticity upon the macroscopic mechanical properties of the foams is carried out. Numerical results show that there is no obvious buckling plateau segment in the uniaxial compressive stress-strain curves of the regular spherical cell models as most low density foams have. Moreover, it is found that the initial tangent modulus is a power function of the foam’s relative density, and the index is smaller than 2 for lower relative density models, bigger than 2 for moderate relative density models, and closed to 2 for higher relative density models.

Abstract: Two-dimensional (2D) random cell models composed of circular cells with different sizes are developed to simulate the microstructure of silicone rubber foams. The two-parameter Mooney-Rivlin strain energy potential model is employed to characterize the hyperelasticity of the solid of which the foams are made. Finite element method is used to simulate the large deformation of the foams. Predicted uniaxial compressive stress-strain curves exhibit universally three deformation regions: an initial linear-elastic response, an extended plateau region, and a final densification stage as cell collapsing. It is also found that with the foam relative density increasing, the initial linear-elastic region is found to extended, the buckling plateau section becomes indistinctive, and final densification stage is hastened.

Abstract: In this work the static and dynamic properties of vanadium alloy V-5Cr-5Ti over a wide range of temperature from 20 to 1000 degree at strain rates ranged from 10-4/s~103/s were studied experimentally under uniaxial quasi-static tension with MTS universal testing machine, uniaxial dynamic compression and tension with split Hopkinson bar system with temperature control. The stress-strain curves of V-5Cr-5Ti at various temperatures and various strain rates were obtained. Experimental data show that V-5Cr-5Ti behaves strain-rate sensitive and temperature dependent, for instance the material parameters yield stress, tensile strength and failure strain. And fracture mode of the material is also dependent on strain-rate and temperature. Based on experimental data the temperature-rate-dependent constitutive relations were established in the form of Johnson-Cook and Cowper-Symonds models which are widely used in numerical simulation of dynamic processes of structures under impact loading. The material microstructures and failure modes were analyzed using optical microscope, TEM etc, and results shows that the yield stress and strength are increased with strain rate. The brittle-ductile transition strain-rate is from 101/s to102/s.

Abstract: In this work the static and dynamic tensile properties of vanadium alloy V-5Cr-5Ti were investigated at strain rates ranged from 3.3x10-5/s to 1.2x102/s. The material microstructures were analyzed using optical microscope, SEM, TEM, XRD and EDS. Results show that the yield strength a increases with strain rate. The brittle-ductile transition strain-rate is about 101/s to102/s. At room temperature the tensile fracture at a low strain rate occurs via mixed modes of microvoid aggregating and transgranular cracking; at high strain rate the fracture occurs via a brittle mode. The analysis by TEM, XRD and EDX shows the existence of lath martensite, and on there exits precipitate phase of Ti(O,C, N) in grain boundaries.

Abstract: In this work, LS-DYNA program was adopted to simulate the loading process of Vanadium Alloy specimen conducted on the Split Hopkinson Pressure Bar (SHPB) in two dimensions. Based on the Johnson-Cook material constitutive relation and criterion of Johnson-Cook failure, the initiation, propagation process of an adiabatic shear band (ASB) and the corresponding distribution of temperature field in the vanadium alloy V-5Cr-5Ti hat-shaped specimen are analyzed. The field of stress, strain and temperature in the tip of an ASB, and the spread speed, the width as well as the type of the ASB are all studied. It is shown that the formation of the ASB is related to the loading velocity and the size of the hat-shaped specimen. And formation of mircocracks and their interlinkage are primary shearing failure mechanism of hat-shaped specimen.