Zhi Qian Chen

Numerical Modeling of Tensile Behavior of Fiber- reinforced Polymer Composites

Based upon micromechanical analysis of multiple damage events, a 2D theoretical model has been proposed to study tensile mechanical response of fiber-reinforced polymer composites, by considering the effect of statistic variations in fiber strength and shear failure of matrix. Stress profiles for any configuration of breaks in composites are derived by a superposition method under the framework of shear-lag arguments. A simulation scheme coupled with Monte-Carlo method has been executed to simulate progressive damage and to investigate failure behavior. Results show that the ultimate strength of composites is influenced by fiber strength statistics and stress distribution due to progressive microdamage.

First Principle Study of the Optical Properties of Transition Metal Nitrides XN (X=Ti, Zr, Hf)

The optical properties of face-centered cubic IVB group transition metal nitrides such as TiN, ZrN, and HfN were calculated using the plane wave pseudopotential method based on first-principle density function theory. The results of band structures show that conduction bands are mainly formed by the metal atom d-state, whereas valence bands are mainly formed by the N 2p-state. In optical properties research, the computed results of complex dielectric functions, absorptions, reflectivities, conductivities and loss functions of the three materials are analysed in terms of band structures. The results agree with experiment data. Analysis results show that the optical properties of these materials in low-energy regions are metallic because of the free electrons intraband-transition, and the transit to semiconducting properties in high-energy area is caused by valence electrons interband-transition. The sharp peaks of the transmissivity spectra indicate excellent optical selectivity in the visible light area. Moreover, lowering the starting energies of interband-transitions as a possible method to improve optical selectivities is discussed

Micromechanical Modeling of the Effect of Progressive Damage on the Tensile Behavior in Fiber-Reinforced Polymer Composites

In this article a methodology based upon micromechanical analysis of multiple damage events is developed to predict stress-strain response and failure behavior of fiber-reinforced polymers composites under tensile loading, by considering the effect of variations in fiber strength and local shear failure of the matrix. A simulation scheme coupled with Monte-Carlo method including these failure mechanisms is proposed to investigate failure process and determine ultimate strength of the composites. It is shown that the size dependence of composite ultimate strength is dominated by fiber strength statistics and stress distribution due to progressive microdamage.

Effect of Stepped Aging Treatment on Microstructure and Properties of AA7085 Aluminum Alloy

The effect of stepped aging treatment including two-stepped retrogression aging and retrogression reaging treatment on the mechanical properties, electrical conductivity and the microstructure of AA7085 has been investigated. Electron microscopy observations were used to analyze the microstructures and tensile fracture surfaces of AA7085 processed via various treatment schedules. Besides, X-ray diffractometer and differential scanning calorimeter were used to explore the thermodynamic factors of heat treatment. Through the investigation of the effect of the retrogression time on the properties and microstructure of AA7085, the optimized retrogression time was confirmed. The results of comparing retrogression aging and retrogression reaging treatment showed that through RRA treatment, higher conductivity and fracture toughness were gained. Through the optimized RRA treatment based on appropriate retrogression time, the tensile strength, elongation, fracture toughness and conductivity of AA7085 were raised to 660MPa, 12%,36.6MPa•m1/2and 38.1%IACS.

Thermodynamic Properties of Nb4AlC3

We use ab initio plane-wave pseudopotential density functional theory method within local density approximation (LDA) to investigate the thermodynamic properties of Nb4AlC3 under high pressure (0-100 GPa) and high temperature (0-3000 K). Through the quasi-harmonic Debye model, we investigate the thermodynamic properties of Nb4AlC3.The variation of the relative volume, bulk modulus, entropy parameter S, the thermal expansion coefficient α , the heat capacity and the Gruneisen parameter γ with pressure P and temperature T, and many other thermodynamic parameters of Nb4AlC3 are obtained systematically. The bulk modulus of Nb4AlC3 is 250 GPa under zero temperature and zero pressure.

Hydrogen Storage Mechanism of Fe3O4: A First-Principles Study

The adsorption and dissociative adsorption of a hydrogen molecule on Fetet1- terminated Fe3O4 (111) surfaces are investigated within the framework of density function theory. The surface Fetet1 atom site with the parallel mode of hydrogen molecule is found to be the most stable equilibrium adsorption state. The most possible dissociative adsorption pathway of a hydrogen molecule needs an energy barrier of 2.85 or 2.87 eV at the surface Oa or Oc atom site, and generates a water molecule, remarkably in agreement with Otsuka.K’s conclusion on the reaction condition and hydrogen storage mechanism of Fe3O4 from microscopic point of view. The calculation of the electronic properties of the termination states with the adsorption and dissociative adsorption of a hydrogen molecule shows strong interactions between atomic hydrogen and Fe3O4 (111) surface.

Research on the Elastic Property of Three New Superhard Materials

In this thesis, elastic properties of three BCN superhard materials with different structures are computed by using CASTEP software developed according to the first principles which are based on density functional theory (DFT) and plane wave method. CA-PZ of local density approximation (LDA) and PBE of generalized gradient approximation (GGA) are adopted to describe the exchange-correlation effect between electrons. The results are compared with other findings and c-BN data. Through analysis, it is found that the spatial anisotropy do exist in the Youngs modulus of single crystals all three BCN compounds.

Study of Thermodynamic Properties of Superhard w-BC2N

We investigate the thermodynamic properties of superhard w-BC2N by using ab initio plane-wave pseudopotential density functional theory method within local density approximation (LDA). Through the quasi-harmonic Debye model, we investigate the thermodynamic properties of w-BC2N. The variation of the thermal expansion, the heat capacity and the Gruneisen parameter γ with pressure P and temperature T, and many other thermodynamic parameters of w-BC2N are obtained systematically.

Effect of Carbon Doped in BN on Thermodynamic Properties

The thermodynamic properties of (B1-xCx)(N1-xCx) (x=0, 0.25, 0.5) are calculated by the ultra-soft pseud-potentials within local density approximation in frame of density functional theory with virtual crystal approximation and using the quasi-harmonic Debye model. The normalized relative volume V/V0, bulk modulus, thermal expansion coefficient α and heat capacity were computed with different temperature and pressure. Meanwhile the lower limit of the thermal conductivity κmin is also examined in details.

Effect of Carbon Doped in BN on Electronic Structures and Mechanical Properties

The structural properties and elastic constants of (B1-xCx)(N1-xCx) (x=0.0, 0.2, 0.4 and 0.6) are calculated by the ultra-soft pseudo-potentials within the generalized gradient approximation and local density approximation in frame of density functional theory with virtual crystal approximation. The elastic constants, the aggregate elastic modulus, poisson’s ratio, energy gap and hardness are computed too. The energy band structure, DOS, and Hellmann-Feynman stresses are also examined in details.