Guanshui Xu
University of California, Riverside
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by Guanshui Xu.
Philosophical Magazine | 1995
Guanshui Xu; A. S. Argon; M. Ortiz
The variational boundary integral method of Xu and Ortiz is taken as a basis for studying dislocation nucleation from atomically sharp cracks under combined mode I-mode II loading. The tension-shear potential of Rice et al. is extended to allow for skewness in the shear resistance curve and to account for the surface production resistance which accompanies ledge formation. The calculated unstable equilibrium configurations of the incipient dislocations and the dependence of the associated activation energies on crack tip energy release rate are found to differ from the Rice-Beltz perturbation solution and the Schock-Puschl more approximate solution. Simulations of dislocation nucleation on inclined slip planes reveal that, while tension softening facilitates nucleation, surface production resistance impedes it. The extent to which these two effects influence critical conditions for dislocation nucleation is quantified. The calculations suggest that homogeneous dislocation nucleation on inclined planes is not favoured for materials with all but the lowest of unstable stacking-energy-to-surface-energy ratios. This emphasizes the importance of heterogeneous dislocation nucleation and nucleation on oblique slip planes on which free surface production should play a much weaker role. The implications of these findings on the nucleation-controlled brittle-ductile transition in cleavage fracture are discussed.
Philosophical Magazine | 1997
Guanshui Xu; A. S. Argon; M. Ortiz
In the present paper, we analyse several activation configurations of embryonic dislocations nucleated from the tip of a cleavage crack. The activation configurations include nucleation on inclined planes, on oblique planes and on cleavage ledges and are treated within the classical framework of Peierls. A variational boundary integral method with an interplanar tension-shear potential developed earlier is used to solve for the saddle-point configurations of embryonic dislocation loops and their associated energies. Based on the assumption that the brittle-to-ductile transition in cleavage fracture is a nucleation-controlled process (as is expected to be the case in bcc transition metals such as α-Fe) the results of the calculations are used to estimate the brittle-to-ductile transition temperatures. It is concluded that only dislocation nucleation on cleavage ledges furnishes realistic values of the transition temperature. The homogeneous nucleation of dislocations on either inclined or oblique planes requires transition temperatures well above the melting point. This implies that nucleation of dislocations from a crack tip in intrinsically brittle crystals is only possible at local crack front heterogeneities such as cleavage ledges, and that the homogeneous nucleation of dislocations from a straight crack front is not possible. This conclusion is supported by the experimental observation that dislocation nucleation from a crack tip is a rare event which occurs preferentially at heterogeneities.
Journal of The Mechanics and Physics of Solids | 2003
Guanshui Xu; Chuanli Zhang
Abstract Dislocation nucleation from a stressed crystal surface is analyzed based on the Peierls–Nabarro dislocation model. The variational boundary integral approach is used to obtain the profiles of the embryonic dislocations in various three-dimensional nucleation configurations. The stress-dependent activation energies required to activate dislocations from their stable to unstable saddle point configurations are determined. Compared to previous analyses of this type of problem based on continuum elastic dislocation theory, the present analysis eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Moreover, atomic information can be incorporated to reveal the dependence of the nucleation process on the profile of the atomic interlayer potential as compared to continuum elastic dislocation theory in which only elastic constants and Burgers vector are relevant. Finally, the presented methodology can also be readily used to study dislocation nucleation from the surface heterogeneities such as cracks, steps, and quantum structures of electronic devices.
Philosophical Magazine Letters | 2000
Guanshui Xu; A. S. Argon
We present an analysis and results on the homogeneous nucleation of a dislocation loop under stress in a perfect crystal. By using a variational boundary integral method in the Peierls-Nabarro framework, we have determined the saddle-point configurations of embryonic dislocation loops and their associated activation energies under stress levels up to the ideal shear strength. The high-energy barriers under the usual levels of applied shear stresses, differing markedly from the ideal shear strength, confirm the widely held view that thermal motion should play no role in such nucleation. The result provides means for more definitive solutions of fundamental problems involving homogeneous nucleation of dislocation loops and has significant implications for models based on the mechanism of nucleation of dislocations from a perfect crystal.
Journal of Micromechanics and Microengineering | 2004
C Zhang; Guanshui Xu; Q Jiang
The squeeze film damping effect on a beam resonator is analyzed on the basis of the coupled elastic beam theory and the Reynolds equation for isothermal incompressible gas films. Under the condition of small amplitude oscillation, linearization of the governing equations leads to the solution that characterizes the squeeze film damping effect on the beam resonator by two dimensionless parameters. These two parameters, in analogy with the damping and mass parameters of the simple mass-spring-damping system, are completely determined by the physical properties of the beam and gas. It is shown that the calculation of these two parameters can be considerably simplified according to the value of the squeeze number.
International Journal of Solids and Structures | 1994
Guanshui Xu; Allan F. Bower; M. Ortiz
Abstract A method is presented for calculating stress intensity factors at the tip of a slightly wavy three-dimensional crack. The solution is used to calculate the direction of propagation of an initially planar semi-infinite crack, which is subjected to uniform remote loading. In particular, the combinations of remote load which cause the crack to deviate from its original plane are found. For the particular case of a two-dimensional crack subjected to combined mode I and mode II loading, we recover the results of Cotterell and Rice [1980, Int. J. Fract. 16 (2), 155]. For a crack loaded by combined mode I and mode III loading, it is shown that there is a critical ratio of K III / K I which causes the crack to deviate from its original plane. The influence of T -stresses, which act parallel to the crack plane, is also investigated. Finally, the predictions of the perturbation theory are compared with full-field numerical simulations which predict the path of a crack propagating under mixed mode remote loading.
Mathematics and Mechanics of Solids | 2003
Chuanli Zhang; Guanshui Xu; Qing Jiang
An analysis of the air-damping effect on the frequency response of a micromachined beam resonator is presented. The motion of the beam is analyzed based on the linear elastic beam theory. The air drag to the beam vibration is characterized based on the Oseen solution of the drag force acting on an infinite long cylinder that moves in incompressible viscous fluids at low Reynolds numbers. The conditions for the validation of this characterization are discussed. The analytical results show that air-damping generally shifts the resonant frequency downward and degrades the quality factor, and that this effect increases as the dimension of the beam decreases. The dependence of this effect on the dimension of the beam is illustrated. The presented analysis provides a simplified analytical approach to estimate the length-scale-dependent air-damping effect for designing high-performance micromachined beam resonators.
Journal of The Mechanics and Physics of Solids | 1998
Guanshui Xu; Allan F. Bower; M. Ortiz
Frictional crack bridging is the main mechanism of toughening in brittle fiber\brittle matrix composites. In addition, the fibers may have a second beneficial effect : they tend to trap cracks propagating through the solid, and may cause them to arrest. The effectiveness of crack trapping increases with the fracture toughness of the interface between fibers and matrix. In contrast, crack bridging tends to be more effective if the interface between fibers and matrix has a low fracture toughness. In this paper, we study the competing effects of crack trapping and bridging in a brittle fiber\brittle matrix composite. A numerical method is used to predict in three dimensions the path of a crack as it bypasses rows of fibers in an ideally brittle matrix. The results are used to deduce the influence of crack trapping on the toughness of the composite. In addition, a simple model of frictional crack bridging is used to compare the relative effects of crack trapping and bridging. It is shown that, in general, the influence of bridging greatly exceeds that of trapping. However, if the fibers have a low tensile strength and there is a large resistance to sliding between fibers and matrix, crack trapping can be significant : in this case, the best composite toughness is achieved by using a tough interface between fibers and matrix.
Journal of Intelligent Material Systems and Structures | 2001
Guanshui Xu; Qing Jiang
Electrode perturbation of the surface may cause significant second order effects on the frequency response of surface acoustic wave (SAW) devices for high frequency applications. These effects in a Y-Z lithium niobate filter are analyzed using a recently developed finite element model. In this model, the equations of wave propagation in piezoelectric materials are discretized using the Galerkin method, in which an implicit algorithm of the Newmark family with unconditional stability is implemented. The Rayleigh damping coefficients are included in the elements near the boundary to reduce the interference of reflected waves. The frequency response of the filter is obtained through the Fourier transform of the impulse response, which is solved directly from the finite element simulation. The analyses have shown that for aluminum electrodes of fixed thickness, the second order effects on the frequency response become increasingly significant as the device is scaled smaller to have higher central frequencies. Typically, for aluminum electrodes of thickness 2000 Å, the frequency response is significantly distorted for the device of the central frequency at above 2 GHz. Quantitative analyses of the second order effects in the device of various center frequencies provide insights for designing high performance SAW devices for high frequency applications.
Smart Materials and Structures | 2000
Guanshui Xu
A direct finite-element model is developed for the full-scale analysis of the electromechanical phenomena involved in surface acoustic wave (SAW) devices. The equations of wave propagation in piezoelectric materials are discretized using the Galerkin method, in which an implicit algorithm of the Newmark family with unconditional stability is implemented. The Rayleigh damping coefficients are included in the elements near the boundary to reduce the influence of the reflection of waves. The performance of the model is demonstrated by the analysis of the frequency response of a Y-Z lithium niobate filter with two uniform ports, with emphasis on the influence of the number of electrodes. The frequency response of the filter is obtained through the Fourier transform of the impulse response, which is solved directly from the finite-element simulation. It shows that the finite-element results are in good agreement with the characteristic frequency response of the filter predicted by the simple phase-matching argument. The ability of the method to evaluate the influence of the bulk waves at the high-frequency end of the filter passband and the influence of the number of electrodes on insertion loss is noteworthy. We conclude that the direct finite-element analysis of SAW devices can be used as an effective tool for the design of high-performance SAW devices. Some practical computational challenges of finite-element modeling of SAW devices are discussed.