Chen Yun-min

Effects of symmetrisation of net-stress tensor in anisotropic damage models

In anisotropic damage mechanics, matrix symmetrisation scheme or treatment in net-stress space is used for mathematical convenience without a strong physical basis. Two different symmetrisation schemes for anisotropic damage model have been compared with the non-symmetrised models proposed for this study. The effects of these symmetrisation schemes are assessed in this paper by deriving the corresponding explicit form of various key parameters for anisotropic damage modelling. It is found that in most cases the symmetrisation process markedly influences the resultant parameters that may lead to spurious results. Hence such a symmetrisation treatment of an anisotropic damage model should be carefully examined. A numerical analysis of a direct shear experiment is also presented to support the assessment of the effects of the symmetrisation schemes considered.

Analytical modelling and free vibration analysis of piezoelectric bimorphs

An efficient and accurate analytical model for piezoelectric bimorph based on the improved first-order shear deformation theory (FSDT) is developed in this work. The model combines the equivalent single-layer approach for mechanical displacements and a layerwise-type modelling of the electric potential. Particular attention is devoted to the boundary conditions on the outside faces and to the interface continuity conditions of the bimorphs for the electromechanical variables. Shear correction factor (k) is introduced to modify both the shear stress and the electric displacement of each layer. And the detailed mathematical derivations are presented. Free vibration problem of simply supported piezoelectric bimorphs with series or parallel arrangement is investigated for the closed circuit condition, and the results for different length-to-thickness ratios are compared with those obtained from the exact 2D solution. Excellent agreements between the present model prediction withk=8/9 and the exact solutions are observed for the resonant frequencies.

Experimental study of seismic cyclic loading effects on small strain shear modulus of saturated sands

The seismic loading on saturated soil deposits induces a decrease in effective stress and a rearrangement of the soil-particle structure, which may both lead to a degradation in undrained stiffness and strength of soils. Only the effective stress influence on small strain shear modulusGmax is considered in seismic response analysis nowadays, and the cyclic shearing induced fabric changes of the soil-particle structure are neglected. In this paper, undrained cyclic triaxial tests were conducted on saturated sands with the shear wave velocity measured by bender element, to study the influences of seismic loading onGmax. AndGmax of samples without cyclic loading effects was also investigated for comparison. The test results indicated thatGmax under cyclic loading effects is lower than that without such effects at the same effective stress, and also well correlated with the effective stress variation. Hence it is necessary to reinvestigate the determination ofGmax in seismic response analysis carefully to predict the ground responses during earthquake more reasonably.

Dynamic responses of a multilayer piezoelectric hollow cylinder under electric potential excitation

The dynamic responses of a multilayer piezoelectric infinite hollow cylinder under electric potential excitation were obtained. The method of superposition was used to divide the solution into two parts, the part satisfying the mechanical boundary conditions and continuity conditions was first obtained by solving a system of linear equations; the other part was obtained by the separation of variables method. The present method is suitable for a multilayer piezoelectric infinite hollow cylinder consisting of arbitrary layers and subjected to arbitrary axisymmetric electric excitation. Dynamic responses of stress and electric potential are finally presented and analyzed.

Propagation of Electromagnetic Wave in the Three Phases Soil Media

The fundamental parameters such as dielectric permittivity and magnetic permeability are required to solve the propagation of electromagnetic wave (EM Wave) in the soil. Based on Maxwell equations, the equivalent model is proposed to calculate the dielectric permittivity of mixed soil. The results of calculation fit the test data well and will provide solid foundation for the application of EM wave in the soil moisture testing, CT analyzing of soil and the inspecting of geoenvironment.

Engineering aspects of landfilling municipal solid waste

Sanitary landfilling is the most important method of municipal solid waste disposal in China. Landfill sites are always set up in mountain valley, on plain or beside seashore. A complete landfill consists of base system, cover system, and leachate collection and gas extraction system. This paper reviews the state-of-the-art landfilling technology in China and collection discusses research projects for engineers.

Study on pile drivability with one dimensional wave propagation theory

Pile drivability is a key problem during the stage of design and construction installation of pile foundations. The solution to the one dimensional wave equation was used to determine the impact force at the top of a concrete pile for a given ram mass, cushion stiffness, and pile impedance. The kinematic equation of pile toe was established and solved based on wave equation theory. The movements of the pile top and pile toe were presented, which clearly showed the dynamic displacement, including rebound and penetration of pile top and toe. A parametric study was made with a full range of practical values of ram weight, cushion stiffness, dropheight, and pile impedance. Suggestions for optimizing the parameters were also presented. Comparisons between the results obtained by the present solution and in-situ measurements indicated the reliability and validity of the method.

Experimental investigation of the influence on static and cyclic deformation of structural soft clay of stress level

This paper presents the experimental results performed to study the static and cyclic deformation behavior of undisturbed and remolded soft clays sampling from Xiaoshan. The consolidation tests indicated that the vertical strains of undisturbed soft clay could be divided into three stages with load increasing, however, the remolded clays were almost independent of stress level. The two cut-off points of these three stages are the preconsolidation stress and the structural yielding stress of the original clay, which could be determined by shear wave velocity measurement. The strains developed during cyclic tests of undisturbed and remolded soft clay, both having one turning point under different amplitude of cyclic stress. The strain developed slowly and stayed at a low level at the early stage, but developed quickly in a different way when the turning points were achieved and finally became great. The turning strains changed with different cyclic stress amplitudes, but they almost fell on a linear line whether undisturbed or remolded. Furthermore, the turning points of the remolded clay all fell on the same line of different confining stresses, including which of the undisturbed soft clay under confining pressure was larger than structural yielding stress. It was also found that the deformation characteristic of undisturbed and remolded Xiaoshan clay tend to be consistent when the structure of undisturbed soft clay is damaged.

Analysis of moving load induced ground vibrations based on thin-layer method

A time-domain solution of layered ground vibration due to moving load has been developed based on the thin layer method. Fourier-Laplace transforms are applied to derive the transformed domain solution that satisfies the boundary conditions of horizontal infinities. The eigen-decomposition approach is used with respect to the Laplace parameter, and the final ground response solution is constructed with the mode superposition method. The reliability and computation accuracy of the solution are proved by comparison with a closed-form solution. A single soil stratum on rigid bedrock is used to reveal the vibration features induced by a rectangular load moving at speeds below or above ground Rayleigh wave velocity.

Transient response of piles-bridge under horizontal excitation

Moving ships and other objects drifting on water often impact a bridges pile foundation. The mechanical model of the piles-bridge structure under horizontal forcing was established, and a time-domain approach based on Finite-difference Method was developed for analyzing the dynamic response of the piles-bridge structure. For a single pile, good agreement between two computed results validated the present approach. The slenderness ratio of the pile, the pile-soil stiffness ratio and the type of the structure influence the dynamic response of the piles-bridge structure. The computed results showed that the stiffness of the structure determines the dynamic response of the piles-bridge structure under horizontal forcing.