Teng Yong Ng

ACTIVE CONTROL OF FGM PLATES WITH INTEGRATED PIEZOELECTRIC SENSORS AND ACTUATORS

Abstract In this paper, a finite element formulation based on the classical laminated plate theory is presented for the shape and vibration control of the functionally graded material (FGM) plates with integrated piezoelectric sensors and actuators. The properties of the FGM plates are functionally graded in the thickness direction according to a volume fraction power law distribution. A constant velocity feedback control algorithm is used for the active control of the dynamic response of the FGM plate through closed loop control. The static and dynamic responses are presented in both tabular and graphical forms for an FGM plate of aluminum oxide/Ti–6A1–4V material composition. The effects of the constituent volume fractions and the influence of feedback control gain on the static and dynamic responses of FGM plates are examined.

Meshfree method for large deformation analysis–a reproducing kernel particle approach

This work concerns the analysis of large displacement problems using the meshfree approach. Meshless algorithms based on the reproducing kernel particle estimate are proposed and applied to the analysis of typical two-dimensional large displacement problems. Due to the lack of Kronecker delta properties in meshless shape functions, the penalty method is explored and employed to enforce the essential boundary conditions. Results of numerical examples involving both geometrical and material nonlinearities show that the meshless model has at least similar effectiveness and accuracy as compared to the finite element method.

Advances in Mechanics of Soft Materials: A Review of Large Deformation Behavior of Hydrogels

Hydrogels possess magnificent properties which may be harnessed for novel applications. However, this is not achievable if the mechanical behaviors of hydrogels are not well understood. This paper aims to provide the reader with a birds eye view of the mechanics of hydrogels, in particular the theories associated with deformation of hydrogels, the phenomena that are commonly observed, and recent developments in applications of hydrogels. Besides theoretical analyses and experimental observations, another feature of this paper is to provide an overview of how mechanics can be applied.

Vibrations of rotating cross-ply laminated circular cylindrical shells with stringer and ring stiffeners

In this paper, the vibration analysis of simply supported rotating cross-ply laminated cylindrical shells with axial and circumferential stiffeners, i.e., stringers and rings, is presented using an energy approach. The effects of these stiffeners are evaluated via two methods, namely: by a variational formulation with individual stiffeners treated as discrete elements; and by an averaging method whereby the properties of the stiffeners are averaged over the shell surface. The effects of initial hoop tension, centrifugal and Coriolis forces due to the rotation are considered in the present formulation. Also, stiffener eccentricity is accounted for. The present formulation is verified by comparison with experimental and numerical results available in open literature. Excellent agreement is observed and a new range of results is presented for rotating shells which can be used as a benchmark to approximate solutions. Detailed studies on the depth-to-width ratios of the stiffeners on the forward and backward frequencies were conducted for the transverse, circumferential and longitudinal modes.

INHOMOGENEOUS LARGE DEFORMATION KINETICS OF POLYMERIC GELS

This work examines the dynamics of nonlinear large deformation of polymeric gels, and the kinetics of gel deformation is carried out through the coupling of existing hyperelastic theory for gels with kinetic laws for diffusion of small molecules. As finite element (FE) models for the transient swelling process is not available in commercial FE software, we develop a customized FE model/methodology which can be used to simulate the transient swelling process of hydrogels. The method is based on the similarity between diffusion and heat transfer laws by determining the equivalent thermal properties for gel kinetics. Several numerical examples are investigated to explore the capabilities of the present FE model, namely: a cube to study free swelling; one-dimensional constrained swelling; a rectangular block fixed to a rigid substrate to study swelling under external constraints; and a thin annulus fixed at the inner core to study buckling phenomena. The simulation results for the constrained block and one-dimensional constrained swelling are compared with available experimental data, and these comparisons show a good degree of similarity. In addition to this work providing a valuable tool to researchers for the study of gel kinetic deformation in the various applications of soft matter, we also hope to inspire works to adopt this simplified approach, in particular to kinetic studies of diffusion-driven mechanisms.

Comparing the effects of dispersed Stone–Thrower–Wales defects and double vacancies on the thermal conductivity of graphene nanoribbons

Classical molecular dynamics with the AIREBO potential is used to investigate and compare the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing various densities of Stone-Thrower-Wales (STW) and double vacancy defects, within a temperature range of 100-600 K. Our results indicate that the presence of both kinds of defects can decrease the thermal conductivity by more than 80% as defect densities are increased to 10% coverage, with the decrease at high defect densities being significantly higher in zigzag compared with armchair nanoribbons. Variations of thermal conductivity in armchair nanoribbons were similar for both kinds of defects, whereas double vacancies in the zigzag nanoribbons led to more significant decreases in thermal conductivity than STW defects. The same trends are observed across the entire temperature range tested.

The modelling and design of smart structures using functionally graded materials and piezoelectrical sensor/actuator patches

Finite element formulations are derived for static and dynamic analysis and the control of functionally graded material (FGM) plates under environments subjected to a temperature gradient, using linear piezoelectricity theory and first-order shear deformation theory. The multi-input–multi-output (MIMO) system with four collocated sensors and actuators is applied to provide active feedback control of the integrated FGM plate in a closed loop system. The distributed piezoelectrical sensors monitor the structural deformation due to the direct piezoelectrical effect and the distributed actuators control the deformation via the converse piezoelectrical effect. Numerical results for the static and dynamic control have been presented for the FGM plate, which consists of zirconia and aluminum. The purpose of the examples, which consist of a FGM plate with four collocated sensors and actuators used as MIMO system, is to determine the optimum configurations of the sensor/actuator pairs under various thermal and mechanical load fields.

Three-dimensional modelling of elastic bonding in composite laminates using layerwise differential quadrature

In an effort to overcome the limitations of existing rigid bonding analysis of composite laminates, the current three-dimensional elastostatic model is proposed. In this model, the three-dimensional interlaminar elastic stress field is determined using the technique of layerwise differential quadrature. The new formulations allowed us to determine the influence of a natural bonding layer upon the field variables in the laminated structure. The interfacial characteristics of continuity and discontinuity satisfy the kinematic continuity conditions through the elastic-bonding layer. A number of case studies are examined, comparisons with rigid bonding and finite element analyses are provided, and the influence of the pertinent parameters on the interlaminar stress field is evaluated and discussed.

Enhanced thermal characterization of silica aerogels through molecular dynamics simulation

Porous structures of silica aerogels are generated using classical molecular dynamics, with the Tersoff potential, which has been re-parametrized for modeling silicon dioxides. This work demonstrates that this potential is superior to the widely used BKS potential in terms of characterizing the thermal conductivities of amorphous silica, by comparing the vibrational density of states with previous experimental studies. Aerogel samples of increasing densities are obtained through an expanding, heating and quenching process. Reverse non-equilibrium molecular dynamics is applied at each density to determine the thermal conductivity. A power-law fit of the results is found to accurately reflect the power-law variation found in experimental bulk aerogels. The results are also of the same order of magnitude as experimental bulk aerogels, but they are consistently higher. By analyzing the pore size distribution on different simulation length scales, we show that such a disparity is due to finite sizes of pores that can be represented, where increasing simulation length scales lead to an increase in the largest pore size that can be modeled.

A new REBO potential based atomistic structural model for graphene sheets

A new atomistic structural model is developed here for graphene sheets based on the stiffnesses from the REBO potential. Using this model, the flexural vibration natural frequencies and buckling loads of rectangular single-layer graphene sheets of different sizes, chiralities and boundary conditions are calculated. The newly developed atomistic structural model is verified by comparing the calculated fundamental natural frequencies for small-sized graphene sheets with those obtained from ab initio density functional theory (DFT) frequency analysis. The vibration and buckling analysis results are also compared with those of an earlier atomistic structural model based on the AMBER potential as well as the equivalent continuum model for graphene sheets. Through this study, it is observed that graphene sheets display very slight anisotropic characteristics in flexural vibration and buckling. Also, it is shown that the atomistic structural model cannot be replaced by a classical equivalent continuum model such as a plate model. Most significantly, we verify that the new atomistic structural model based on the REBO potential predicts more accurate natural frequencies and buckling loads for graphene sheets, which are considerably lower than those predicted by the earlier atomistic structural model based on the AMBER potential.