Papers by Author: Xiang Guo Zeng

Abstract: Experiments have shown that initial voids may exist in the manufacturing processes of pure aluminum, which adversely affect its mechanical properties. In this study, the process of plastic deformation around voids in pure aluminum was examined at atomic scale through molecular dynamics (MD) simulation. The Modified Embedded Atom Method (MEAM) was employed to characterize the atomic interactions in the pure aluminum with two voids. The calculation results revealed that the interaction of two voids endures three phases when the interval of the voids is increased: void coalescence, void coactions followed by the formation of a stress shield zone, and interaction vanishing. The critical parameters of the interval for the three phases were defined as well in this work. It was observed that crack initiated and further propagated near the voids along the slip systems of FCC crystal, which eventually caused structural failure. Meanwhile, the evolution of micro structure in the crack propagation process was investigated by means of Common Neighbor Analysis (CNA). The results showed that the phase transformation occurred near the voids during loading process.

Abstract: The voids in pure Aluminum always exit in the manufacturing process. The Modified Embedded Atom Method (MEAM) potential is employed in the molecular dynamics (MD) simulation at atomic scale to investigate the interaction between voids under the impact loading for pure Aluminum. The distance between the voids distributed along the loading orientation affects the failure mechanism seriously. The results show that there are 3 kinds of mechanisms with the change of the distance between voids: 1) coalescence takes place within a critical distance between voids under extra loading, 2) when the distance between voids reaches a certain value, each void cracks at 4 locations along with the slide direction <110> of face-centered cubic (fcc), respectively, 3) a stress shield zone appears when the ligament between the voids is at the size between the cases mentioned above, which brings out the phenomena that each of the voids cracks only at 2 locations, and no crack appeared at the stress shield zone.

Abstract: Micro-cracks which seriously affect the strength of metal materials always exist in metal or alloys during the manufacturing process. In order to investigate the pre-crack effects on deformation and failure mechanisms for pure aluminum at atomic scale, the plastic deformation processes of pure aluminum with face-centered cubic (fcc) crystal structure around the pre-crack tips at atomic scale were examined by means of molecular dynamics (MD) method. The Modified Embedded Atom Method (MEAM) potential was used to describe the interaction among atoms of pure aluminum. The crack propagation and failure processes for fcc structure were observed near the pre-crack tip zone. The calculation results reveal that the pre-crack blunting occurred at first, then the dislocation emitted at the pre-crack boundary and moved along with the specific direction obviously, eventually, cracks propagated along the crystallographic direction family of <110>. By means of VMD software, the graphic pictures of dislocation movement and crack propagation were obtained under different load conditions. The results and methodology given in this study are very significant for understanding more about plastic deformation and destruction at atomic scale for pure Aluminum with fcc structure.

Abstract: Multiple interacting crack problems for 3-point bending specimen were studied in this article. Two symmetrical minor cracks were placed in the structure, besides a main crack at the middle, and using the finite element method program ABAQUS, the energy release rate (G) and the stress intensity factor (SIF) were evaluated based on the virtual crack closure technology（VCCT）in conjunction with finite element analysis(FEA). Then, effects of variation in relative lengths and locations of the minor cracks on the stress intensity factors of the main crack were obtained and analyzed. Finally, the approach was applied to dynamic analysis, and influences of interacting effects among the cracks on dynamic fracture parameters were also studied.

Abstract: The crack propagation for pure Magnesium at an atomic scale level under external loading was carried out by using a molecular dynamics method. In this study, the Modified Embedded Atom Method (MEAM) was used to characterize the interactions of atoms and the Newtonian equations were solved by Velocity-Verlet algorithm. The crack propagation and failure processes were observed around the crack tip. The calculation results reveal that vacancies were formed near the crack tip during the failure processes for pure Magnesium, and the coalescence between crack tip and vacancies induced the crack growth with the increase in loading.

Abstract: Magnesium alloys are among the best light-weight structural material with a relatively high strength-to-weight ratio end excellent technological properties. Therefore, magnesium attracts special attention of researchers working in automotive and aircraft industry. This work paid the efforts to the structural components made out of magnesium alloy AM60 such as chassis, transmission case in automotive, where the components are subject to cyclic loading after being pre-loaded. In this study, the cyclic stress-strain behaviors were investigated by strain-controlled fatigue testing. In order to investigate the effects of R-ratio on mean stress relaxation, the R-ratio ranged from 0.1 to 0.7 at the strain amplitude of 0.3%. The experimental results indicate that the mean stress relaxation increases with the increasing R-ratio. A constitutive model was proposed to simulate the mean stress relaxation. The calculation results show that the constitutive model developed in this work is capable of reproducing the stress relaxation behaviors of magnesium alloy AM60 under strain control.

Abstract: The mechanical testing for magnesium alloy AM60 was conducted on an MTS servohydrolic material testing system. In order to examine the effects of impact velocity on fracture toughness, finite element method was applied to compute the impact fracture toughness of AM60 specimens with single notched edge at room temperature. The specimens were impacted by steel balls (Diameter 20 mm) at the velocities of 80m/s, 120m/s and 160m/s. The simulation results well described the effects of impact velocities on fracture toughness. The finite element analysis performed in this work offered an effective method to compute dynamic fracture toughness for engineering applications.

Abstract: Founded on the energy storing characteristics of microstructure during irreversible deformation, a viscoplastic constitutive model with no yielding surface introduced was developed for single crystals by adopting a spring-dashpot mechanical system. Both plastic dashpots reflecting the material time-independent responses and Newtonian dashpots mirroring the material time-dependent viscous responses were introduced to describe the viscoplasticity of slip systems. The single crystal constitutive model was established based on the thermodynamics of internal variables and the theory of absolute reaction rate. By implementing the KBW self-consistent theory, a polycrystal viscoplastic constitutive model was formed. The numerical analysis in corresponding algorithm was significantly simplified as no searching process for the activation of the slip systems and slip directions was required. The numerical simulation of creep-plasticity behaviors demonstrated excellent agreement with the corresponding experimental data.

Abstract: The stress intensity factor (SIF) for tubular specimens were calculated based on virtual crack closure technique （VCCT）. The effects of geomet rical factors (inner radius, wall thickness and relative crack length) and external loads on the SIF were analyzed, respectively, by use of the single-variable approach. Finally, an approximate formula for SIF of compact-tensile tubular specimens was obtained as all factors mentioned were considered, which was presented as a reference for the tubular engineering design.

Abstract: The cast magnesium alloys as AM50 offer a good strength, ductility and surface finish for automotive industry. But the poor creep resistance limited its application to power components such as engine and transmission cases at temperatures in excess of 100°C. In order to investigate the cyclic creep behavior of Magnesium Alloy at high temperature, creep tests of plate specimens AM50 were conducted in this work. Based on the analysis about the microstructure and defects of AM50 under the condition of cyclic creep, a cyclic creep constitutive model with isotropic and scalar damage parameter was developed. Furthermore, the proposed model was experimentally verified by analyzing the cyclic creep and recovery response of Cast Magnesium alloy under cyclic loading with dwell time. Comparisons between calculated results and experimental data showed good agreement.