Zhenhuan Li

Effective electromechanical properties of transversely isotropic piezoelectric ceramics with microvoids

In most piezoelectric materials and smart structures, the presence of microvoids can hardly be avoided. In the present paper, detailed three-dimensional (3-D) finite element analyses based on the unit cell method are carried out to investigate the inherent relations between the effective properties and the non-uniform distributions of microscopic electromechanical coupling fields resulting from microvoids. The emphases are placed on the influences of void volume fraction, void distribution, void shape and configuration on the effective properties of voided piezoelectric ceramics. The influences rooting in the permeability of voids and piezoelectricity of matrix materials on the effective properties of voided piezoelectric ceramics are also studied. The results obtained by this study have prospective guiding significances to the structural integrity analyses and fabrication of piezoelectric ceramics.

The influence of plasticity mismatch on the growth and coalescence of spheroidal voids on the bimaterial interface

Abstract To investigate the macro-mechanical response and micro-mechanism of damage by void growth and coalescence on the interface in a bimaterial system, detailed finite element computations of a representative cylindrical cell containing a spherical void are performed. By comparison with the response of a homogeneous material cell model, significant effects of the matrix plasticity mismatches due to the yield stress and the strain hardening exponent on the void growth and coalescence are revealed: (1) The growth rate of the void on the bimaterial interface is much faster than that in the homogeneous material, and the critical coalescence strain of the void on the interface is only about half of that in homogeneous material. (2) Due to the difference in the deformation resistance of the matrix materials in the bimaterial system, all computations indicate that deformed voids are seriously distorted and the linking of adjacent voids takes place in the softer matrix material. Comparison of the computational results with the classical Rice–Tracey (R-T) model shows that the R-T model cannot make good prediction for the growth of the void on the bimaterial interface. On the basis of large numbers of numerical simulations, a correction coefficient is introduced to improve the R-T model.

The evolution of voids in the plasticity strain hardening gradient materials

Abstract The main aim of this paper is to opens out the meso-mechanism of void growth and coalescence in the matrix materials with graded strain-hardening exponent distribution. For this end, detailed finite element computations of a representative cylindrical cell containing a spherical void have been carried out. According to the FE analyses, significant effects of the strain-hardening exponent gradient (SEG) in the matrix on the void growth and coalescence are revealed: (1) In the homogeneous materials, the void growth and coalescence are slightly dependent on the strain-hardening exponent, however, the SEG distribution in the matrix can increase remarkably the void growth rate and decrease seriously the void coalescence strain. (2) The critical void shapes in the homogeneous materials are mainly governed by the macroscopic stress triaxiality, but due to earlier plastic flow localization in the softer matrix layer, the SEG distribution in the matrix has very significant effects on the deformed void shapes, especially when the stress triaxiality is lower. (3) When the triaxial stress levels are lower, in the homogeneous materials, the shape change mode of the void evolution is dominate so the void growth rate is very low; however, the SEG distribution in the matrix can bring the volume change mode out, as a result of increasing the void growth rate. (4) Comparisons of the numerical results with the existing damage model indicate that the classic damage model cannot give satisfying prediction to the void growth in both the homogeneous strain-hardening matrix and the SEG materials. On the basis of large numbers of numerical computations, a new damage model, which can uniformly describe the void growing in the homogeneous and plasticity gradient materials, is suggested. A mass of element computations have validated that the new damage model can give satisfying agreement with the FE results of cell model.

The size effect and plastic deformation mechanism transition in the nanolayered polycrystalline metallic multilayers

The strength and deformation mechanisms of the nanolayered polycrystalline metallic multilayers (NPMMs) are investigated via molecular dynamics simulation, with special attentions to the coupling effect of grain size and layer thickness. The results indicate that the strength of multilayers does not always increases sensitively with the decrease of layer thickness or grain size, and the smaller one of them governs substantially the size effect on the strength. Due to the constraint of GBs and phase interface to gilding dislocations, there are several possible deformation mechanisms, which can govern the strength of NPMMs, including the confined partial dislocation slip, confined extended dislocation slip, and confined grain boundary slip. With the increase or decrease of the characteristic size of multilayers (i.e., layer thickness or grain size), the dominant deformation mechanism changes from one to another, resulting in very intricate size effect on the strength of multilayers. The underlying reason of...

Modeling of abnormal mechanical properties of nickel-based single crystal superalloy by three-dimensional discrete dislocation dynamics

Unlike common single crystals, the nickel-based single crystal superalloy shows surprisingly anomalous flow strength (i.e. with the increase of temperature, the yield strength first increases to a peak value and then decreases) and tension–compression (TC) asymmetry. A comprehensive three-dimensional discrete dislocation dynamics (3D-DDD) procedure was developed to model these abnormal mechanical properties. For this purpose, a series of complicated dynamic evolution details of Kear–Wilsdorf (KW) locks, which are closely related to the flow strength anomaly and TC asymmetry, were incorporated into this 3D-DDD framework. Moreover, the activation of the cubic slip system, which is the origin of the decrease in yield strength with increasing temperature at relatively high temperatures, was especially taken into account by introducing a competition criterion between the unlocking of the KW locks and the activation of the cubic slip system. To test our framework, a series of 3D-DDD simulations were performed on a representative volume cell model with a cuboidal Ni3Al precipitate phase embedded in a nickel matrix. Results show that the present 3D-DDD procedure can successfully capture the dynamic evolution of KW locks, the flow strength anomaly and TC asymmetry. Then, the underlying dislocation mechanisms leading to these abnormal mechanical responses were investigated and discussed in detail. Finally, a cyclic deformation of the nickel-based single crystal superalloy was modeled by using the present DDD model, with a special focus on the influence of KW locks on the Bauschinger effect and cyclic softening.

Cyclic Hardening Behavior of Polycrystals with Penetrable Grain Boundaries: Two-Dimensional Discrete Dislocation Dynamics Simulation

A two-dimensional discrete dislocation dynamics (DDD) technology by Giessen and Needleman (1995), which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocation-penetrable grain boundaries (GBs) on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

Effect of Crystal Orientation on Femtosecond Laser-Induced Thermomechanical Responses and Spallation Behaviors of Copper Films

Ultrafast thermomechanical responses and spallation behaviours of monocrystal copper films irradiated by femtosecond laser pulse are investigated using molecular dynamics simulation (MDS). Films with 〈100〉, 〈110〉 and 〈111〉 crystal orientations along the thickness direction were studied. The results show that the crystal orientation has a significant effect on femtosecond laser-induced thermomechanical responses and spallation behaviors of monocrystal copper films. The discrepancy between normal stresses in copper films with different crystal orientation leads to distinct differences in lattice temperature. Moreover, the copper films with different crystal orientations present distinct spallation behaviors, including structural melting (atomic splashing) and fracture. The melting depth of 〈100〉 copper film is lower than that of 〈110〉 and 〈111〉 copper films for the same laser intensity. The dislocations and slip bands are formed and propagate from the solid-liquid interface of 〈110〉 and 〈111〉 copper films, while these phenomena do not appear in 〈100〉 copper film. Additionally, numerous slip bands are generated in the non-irradiated surface region of copper films due to reflection of mechanical stress. These slip bands can finally evolve into cracks (nanovoids) with time, which further result in the fracture of the entire films.

Interaction between three-dimensional crack and near hard piezoelectric fiber

Abstract Due to intrinsic electromechanical coupling behaviors, various fiber-shaped piezoelectric inhomogeneities (sensor) have been widely applied to detect or arrest the crack and damage in various engineering structures. Knowledge about interaction between three-dimensional (3D) through-thickness crack and near piezoelectric fiber is frequently required for designing various fiber-shaped piezoelectric sensors or actuators. In the present paper, detailed 3D finite element analyses are carried out to probe into the influences of the cylindrical piezoelectric fiber on the 3D crack-tip stress fields. Four main influencing factors, i.e. the ligament length ( δ / ρ ) between the crack-tip and the piezoelectric fiber, the plate thickness ( B / ρ ), the applied mechanical loading, K I , and the electromechanical coupling loading, E 3 e 13 r 0 /K I , are considered. For comparison, deeper investigation on the interaction between the 3D crack and an elastic fiber-shaped inhomogeneity has been also carried out in the current study. The results show clearly that the applied electromechanical loading, E 3 e 13 r 0 /K I , the ligament length, δ / ρ , and plate thickness, B / ρ , have great influence on the interaction between the 3D crack and near piezoelectric fiber. The results obtained by this study have important guiding significances for understanding the interaction mechanism between the 3D crack and the sensor or actuator in a non-piezoelectric elastic matrix.

Crystal orientation-dependent mechanical property and structural phase transition of monolayer molybdenum disulfide

By performing molecular dynamics simulations, we investigate the mechanical property and structural phase transition in monolayer molybdenum disulfide (MoS2) with different crystal orientations under uniaxial tensions systematically. The results show that both the mechanical property and structural phase transition are strongly dependent on the crystal orientation, specifically, for some crystal orientation angles lower than about 20°, the structural phase transition takes place with the plastic deformation; for other crystal orientation angles, plastic deformation cannot occur. Further studies have found that plastic deformation results from irreversible changes of the angles between the bonds, rather than the variation of bond length. Youngs modulus increases while ultimate strength and fracture strain decrease with the increase of the crystal orientation angle. The critical strain at which the first structural phase transition occurs increases with the increase of the crystal orientation angle. The pl...

Thermal damage and ablation behavior of graphene induced by ultrafast laser irradiation

Abstract Ultrafast laser-induced damage and ablation of graphene is the one of the most critical parts of precise nanopatterning of graphene by using laser ablation. In this article, we have studied the local damage and ablation behavior of monolayer graphene irradiated by femtosecond single pulse laser using molecular dynamics simulation. A theoretical model of phonon-dominated absorption of laser energy is proposed to describe the interaction between graphene and femtosecond single pulse laser. The simulation results based on this model are quantitatively consistent with experimental and theoretical ones. Furthermore, the effects of laser fluences on the atomic ablation behavior and nanogroove generation are investigated. The results show that the relationship between depth of the induced ablation and laser fluence follows a logarithmic function instead of a simple linear relationship. These results will be useful in providing guidance in femtosecond laser processing of graphene.