Showing papers by "YuanTong Gu published in 2008"

Investigation on mechanical behaviourof AM60 magnesium alloys

Abstract: Purpose: In this work, tension, impact, bend and fatigue tests were conducted in an AM60 magnesium alloy. The effects of environmental temperature and loading rates on impact and tension behavior of the alloy were also investigated. Design/methodology/approach: The tests were conducted using an Instron universal testing machine. The loading speed was changed from 1 mm/min to 300 mm/min to gain a better understanding of the effect of strain rate. To understand the failure behavior of this alloy at different environmental temperatures, Charpy impact test was conducted in a range of temperatures (-40~35°C). Plane strain fracture toughness (KIC) was evaluated using compact tension (CT) specimen. To gain a better understanding of the failure mechanisms, all fracture surfaces were observed using scanning electron microscopy (SEM). In addition, fatigue behavior of this alloy was estimated using tension test under tension-tension condition at 30 Hz. The stress amplitude was selected in the range of 20~50 MPa to obtain the S-N curve. Findings: The tensile test indicated that the mechanical properties were not sensitive to the strain rates applied (3.3x10-4~0.1) and the plastic deformation was dominated by twining mediated slip. The impact energy is not sensitive to the environmental temperature. The plane strain fracture toughness and fatigue limit were evaluated and the average values were 7.6 MPa.m1/2 and 25 MPa, respectively. Practical implications: Tested materials AM60 Mg alloy can be applied among others in automotive industry aerospace, communication and computer industry. Originality/value: Many investigations have been conducted to develop new Mg alloys with improved stiffness and ductility. On the other hand, relatively less attention has been paid to the failure mechanisms of Mg alloys, such as brittle fracture and fatigue, subjected to different environmental or loading conditions. In this work, tension, impact, bend and fatigue tests were conducted in an AM60 magnesium alloy.

Investigation on mechanical behaviour of AM60 magnesium alloys

Abstract: Purpose: In this work, tension, impact, bend and fatigue tests were conducted in an AM60 magnesium alloy. The effects of environmental temperature and loading rates on impact and tension behavior of the alloy were also investigated. Design/methodology/approach: The tests were conducted using an Instron universal testing machine. The loading speed was changed from 1 mm/min to 300 mm/min to gain a better understanding of the effect of strain rate. To understand the failure behavior of this alloy at different environmental temperatures, Charpy impact test was conducted in a range of temperatures (-40~35°C). Plane strain fracture toughness (KIC) was evaluated using compact tension (CT) specimen. To gain a better understanding of the failure mechanisms, all fracture surfaces were observed using scanning electron microscopy (SEM). In addition, fatigue behavior of this alloy was estimated using tension test under tension-tension condition at 30 Hz. The stress amplitude was selected in the range of 20~50 MPa to obtain the S-N curve. Findings: The tensile test indicated that the mechanical properties were not sensitive to the strain rates applied (3.3x10-4~0.1) and the plastic deformation was dominated by twining mediated slip. The impact energy is not sensitive to the environmental temperature. The plane strain fracture toughness and fatigue limit were evaluated and the average values were 7.6 MPa.m1/2 and 25 MPa, respectively. Practical implications: Tested materials AM60 Mg alloy can be applied among others in automotive industry aerospace, communication and computer industry. Originality/value: Many investigations have been conducted to develop new Mg alloys with improved stiffness and ductility. On the other hand, relatively less attention has been paid to the failure mechanisms of Mg alloys, such as brittle fracture and fatigue, subjected to different environmental or loading conditions. In this work, tension, impact, bend and fatigue tests were conducted in an AM60 magnesium alloy.

Geometrically nonlinear analysis of microswitches using the local meshfree method

Abstract: In the modeling and simulation of microelectromechanical system (MEMS) devices, such as the microswitch, the large deformation or the geometrical nonlinearity should be considered. Due to the issue of mesh distortion, the finite element method (FEM) is not effective for this large deformation analysis. In this paper, a local meshfree formulation is developed for geometrically nonlinear analysis of MEMS devices. The moving least squares approximation (MLSA) is employed to construct the meshfree shape functions based on the arbitrarily distributed field nodes and the spline weight function. The discrete system of equations for two-dimensional MEMS analysis is obtained using the weighted local weak form, and based on the total Lagrangian (TL) approach, which refers all variables to the initial configuration. The Newton–Raphson iteration technique is used to get the final results. Several typical microswitches are simulated by the developed nonlinear local meshfree method. Some important parameters of these microswitches, e.g. the pull-in voltage, are studied. Compared with the experimental results and results obtained by linear analysis, nonlinear meshfree analysis of microswitches is accurate and efficient. It has demonstrated that the present nonlinear local meshfree formulation is very effective for geometrically nonlinear analysis of MEMS devices, because it totally avoids the issue of mesh distortion in the FEM.

An elasto-plastic analysis of solids by the local meshless method based on mls

Abstract: A pseudo-elastic local meshless formulation is developed in this paper for elasto-plastic analysis of solids. The moving least square (MLS) is used to construct the meshless shape functions, and the weighted local weak-form is employed to derive the system of equations. Hencky's total deformation theory is applied to define the effective Young's modulus and Poisson's ratio in the nonlinear analysis, which are obtained in an iterative manner using the strain controlled projection method. Numerical studies are presented for the elasto-plastic analysis of solids by the newly developed meshless formulation. It has demonstrated that the present pseudo-elastic local meshless approach is very effective for the elasto-plastic analysis of solids.

A multiscale deformation analysis for mono-crystalline copper under dynamic uniaxial tension

Abstract: This paper presents a concurrent multiscale study for the deformation mechanism of mono-crystalline copper under dynamic uniaxial tension. The multiscale simulation is based on the coupled meshless and molecular dynamic (MD) method. Using it, the size of computational model can be extended to a large dimension (in micrometer) with an atomistic resolution. The pure MD simulation is difficult to reach this microscopic dimension because the number of atoms will be too large. In this study, it has been revealed that the deformation behavior and mechanism of the copper is sensitive to its size, geometry, and loading strain rate. In addition, the Young's modulus is found to be independent of the cross-sectional size and the strain rate range considered in this study. On the other hand, the yield stress decreases with specimen length and increases with the loading strain rate.

A coupled numerical approach for nonlinear dynamic fluid-structure interaction analysis of a near-bed submarine pipeline

Abstract: The near-bed submarine pipeline is a widely used structure in the marine engineering. Due to the presence of the seabed resulting in an asymmetric flow, a large negative lift (attraction) can be induced on a pipeline in a horizontal current, which has significant influence on the behaviours of the pipeline. A coupled numerical approach is proposed in this paper to assess the nonlinear dynamic responses of this pipeline by combining the meshless technique and the boundary element method (BEM). BEM is firstly used to get the nonlinear dynamic fluid loading induced by the asymmetric flow. The meshless technique is used to discretize the structure of the pipeline, and the local weighted weak form using the spline weight function is employed to get the discrete system of equations for this nonlinear dynamic analysis. A numerical example for the static and dynamic analyses of a structure is firstly presented to verify the effectivity of the present method. Then, the coupled technique is used to simulate the nonlinear dynamic fluid-structure interaction problem of a near-bed pipeline. A Newton-Raphson iteration procedure is used herein to solve the nonlinear system of equations, and the Newmark method is adopted for the time integration. Our studies reveal that there exists a critical current velocity, above which the pipeline will become instable sharply. The detailed relationship between the critical velocity and the gap is given, and it has been found that the critical velocity is significantly affected by the initial gap from the pipeline to seabed. It has demonstrated that present approach is very effective to obtain numerical solutions for the nonlinear dynamic fluid-structure interaction analysis of a near-bed submarine pipeline.

A Multiscale Deformation Analysis for Mono-Crystalline Copper under Dynamic Uniaxial Tension

Abstract: This paper presents a concurrent multiscale study for the deformation mechanism of monocrystalline copper under dynamic uniaxial tension. The multiscale simulation is based on the coupled meshless and molecular dynamic (MD) method. Using it, the size of computational model can be extended to a large dimension (in micrometer) with an atomistic resolution. The pure MD simulation is difficult to reach this microscopic dimension because the number of atoms will be too large. In this study, it has been revealed that the deformation behavior and mechanism of the copper is sensitive to its size, geometry, and loading strain rate. In addition, the Young’s modulus is found to be independent of the cross-sectional size and the strain rate range considered in this study. On the other hand, the yield stress decreases with specimen length and increases with the loading strain rate.

A coupled numerical approach for nonlinear dynamic fluid‐structure interaction analysis of a near‐bed submarine pipeline

Abstract: Purpose – The purpose of this paper is to develop an effective numerical approach to assess the nonlinear dynamic responses of a near‐bed submarine pipeline.Design/methodology/approach – A coupled numerical approach is proposed in this paper to assess the nonlinear dynamic responses of this pipeline. The boundary‐element method is first used to get the nonlinear dynamic fluid loading induced by the asymmetric flow. The meshless technique is used to discretize the structure of the pipeline. A numerical example is first presented to verify the effectivity of the present method. Then, the coupled technique is used to simulate the nonlinear dynamic fluid‐structure interaction problem of a near‐bed pipeline. A Newton‐Raphson iteration procedure is used herein to solve the nonlinear system of equations, and the Newmark method is adopted for the time integration.Findings – The presence of seabed results in a large negative lift on a pipeline in a horizontal current. Studies reveal that there exists a critical cu...

An effective multiscale approach for deformation analyses of carbon nanotube-based nanoswitches

Abstract: This paper aims to develop an effective multiscale simulation technique for the deformation analysis of nanotube-based nanoswitches. In the multiscale simulation, the key material parameters, (e.g., Young's modulus and moment of inertia) are extracted from the MD simulation which can explore the atomic properties. Then, the switches are simplified to continuum structure which is discretized and simulated by the advanced RBF meshfree formulation. The system of equations is nonlinear because the nonlinear loading is calculated from coupled the electrostatic, the elastostatic, and the van der Waals energy domains. Besides the normal deformation analysis, the pull-in voltage characteristics of different nanoswitches based on the double-walled nanotubes are analyzed. Comparing with the results in the literature and from experiments, it has proven that the developed multiscale approach is accurate and efficient.

An advanced meshless technique for large deformation analysis of metal forming

Abstract: The large deformation analysis is one of major challenges in numerical modelling and simulation of metal forming. Although the finite element method (FEM) is a well-established method for modeling nonlinear problems, it often encounters difficulties for large deformation analyses due to the mesh distortion issues. Because no mesh is used, the meshless methods show very good potential for the large deformation analysis. In this paper, a local meshless formulation is developed for the large deformation analysis. The Radial Basis Function (RBF) is employed to construct the meshless shape functions, and the spline function with high continuity is used as the weight function in the construction of the local weak form. The discrete equations for large deformation of solids are obtained using the local weak-forms, RBF shape functions, and the total Lagrangian (TL) approach, which refers all variables to the initial (undeformed) configuration. This formulation requires no explicit mesh in computation and therefore fully avoids mesh distortion difficulties in the large deformation analysis of metal forming. Several example problems are presented to demonstrate the effectiveness of the developed meshless technique. It has been found that the developed meshless technique provides a superior performance to the conventional FEM in dealing with large deformation problems in metal forming.

Confidence interval of lifetime distribution using bootstrap method

Abstract: Lifetime estimation is significant in engineering asset management. However, the estimate of lifetime or failure time could be misleading as it may not reveal the real value. Therefore, confidence intervals need to be built to quantify the prediction uncertainty. For the Gamma process, which is a commonly used method for lifetime estimation, the conventional confidence interval construction methods do not perform well. This paper adopts bootstrap methods to build confidence intervals of lifetime distribution when the Gamma process is used. Three bootstrap methods, i.e. bootstrap percentile, bootstrap-t and bootstrap bias corrected & accelerated (BCa), are utilized and investigated. These methods are first applied to the basic Gamma process and further to the Gamma process which considers random effects. Moreover, bootstrap calibration is conducted to assess the coverage probability of the confidence intervals built by these bootstrap methods. Applications to the fatigue crack growth data are carried out as a case study. The results show that the BCa method is recommended for generating confidence intervals for Gamma processes in this application.

Crack analysis using an improved meshless technique with irregular nodes

Abstract: Meshless method has advantages in analyzing the deformation around a crack. However, the effectiveness of such method is influenced by the treatment of local support domain which is a base for an appropriate selection of field nodes in the construction of shape functions. In the current practice, the methods to determine support domain have some drawbacks. It is therefore beneficial to develop a more flexible technique for determining the support domain in crack simulation. This paper presents such a technique which could be used in both regularly and irregularly distributed nodes. Numerical examples show that the technique produces accurate results in these two situations. Moreover, the new technique can be integrated with different types of meshless methods to provide an effective way in handling arbitrary nodal distribution to meet the needs of solutions to problems with complex geometric boundaries.