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10 April 2006, Volume 28 Issue 2
Around the mechanics
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2004-394
Abstract ( 1109 )   PDF (918KB) ( 541 )
Vehicle longitudinal-vibration road spectrum and its application in a road-vehicle system are discussed. The vehicle longitudinal shearing force and the stress intensity factor of the pavement under the force are computed. Compared with the vertical road spectrum, the road level of the longitudinal road spectrum is better. The vehicle longitudinal shearing force is represented by a steady stochastic process whose mean value is zero. The pavement crack growth speed from upper to lower is evidently faster than from lower to upper. The speed is proportional to the fourth power of the vehicle longitudinal shearing force, by which, the pavement damage is by far greater than the vertical tyre force.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-044
Abstract ( 1458 )   PDF (700KB) ( 768 )
By means of the fractionated multi resolution analysis of the signals with wavelet transform, many problems can be solved, which can not be solved by Fourier transform. From the differences between singular points of noise and signal, and the important features of wavelet transform in highlighting the local profiles of signals both in time-frequency domains, i.e., the fact that the maximum of the noise wavelet transform reduces dramatically with the increase of the scale, the pressure fluctuation is analyzed. The results show that the wavelet transform is a good way to eliminate noise; and by the power profiles of de-noised signals, the pressure fluctuation shows a fractal construction, as revealed in the previous study.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-091
Abstract ( 1492 )   PDF (767KB) ( 533 )
On the basis of modal synthesis analysis, a parallel algorithm of solving large-scale structural eigenproblems is presented in this paper. The eigenmodes of the structure are obtained using a subspace iterative parallel method. The substructure subspace iterative method is implemented using the stiff matrix and mass matrix of the substructures without assembling the stiff and mass matrix of the whole structure. The numerical results show that this parallel algorithm is effective for large scale structure eigenproblems
Interesting mechanics story
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-098
Abstract ( 2057 )   PDF (1425KB) ( 802 )
Nonlocal constitutive models are widely used for effectively simulating strain localization. The major models may be classified into the following two kinds: the weighted-area-averaged nonlocal model and the gradient-enhanced-nonlocal model. Comparison between the two models is made in this paper. It is found that the Aifantis model is a reasonable choice for numerical simulation of a localized non-homogeneous material, but it is subject to some improvements.
Around the mechanics
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-101
Abstract ( 1649 )   PDF (607KB) ( 669 )
According to two-phase hydrodynamic theories, the motion of the aerosol particles in a Negretti sample head is analyzed in this paper. The motion trajectory of the aerosol particles during the sampling of PM$_{10}$ and PM$_{2.5}$ is computed. The influence of various factors such as airflow flux, diameter of aerosol particles on the movement trajectory is discussed. The results show that the two-phase hydrodynamics is effective in simulating the motion of aerosol particles within an impactor. It can offer a powerful help for the design and manufacture of impactor.
Application study
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-102
Abstract ( 1522 )   PDF (958KB) ( 632 )
The slab corner surface transversal cracks and longitudinal cracks during a multi-pass V-H rolling process with plain-barreled vertical roll and shape vertical roll were simulated by the explicit dynamic finite element method and updating geometric method, and the effects of vertical roll shape during rolling process on crack closure and growth were analyzed. Results show that, when applying plain-barreled vertical roll during multi-pass V-H rolling process, the deformation of slab corner crack is large, and it tends to spread, for slab corner transversal cracks, the nodes on crack tip lines may diffuse outwards, for triangle transversal cracks and longitudinal cracks, one side of crack may be folded; when applying shape vertical roll during multi-pass V-H rolling process, after vertical rolling, the cracks are closed well, after horizontal rolling, the longitudinal cracks and transversal cracks on slab side surface may open again, but the transversal cracks on slab top surface can be closed well, at the same time, only triangle longitudinal cracks show the phenomenon that one side of crack is folded.
Around the mechanics
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-113
Abstract ( 1397 )   PDF (758KB) ( 640 )
It is indicated by macroscopical and microscopical experiments that most of the directions of failure particle pair are not consistent with the direction of the first principal stress. According to the microscopic mechanics with respect to brittle materials' tensile failure, the material will begin to decompose if the distance between pair particles exceeds their attraction distance. Based on the above facts, a new strength criterion of brittle materials' tensile failure is derived, which can be written as: $\sigma_1+\alpha \sigma_2+\beta \sigma_3\le [\sigma]$, in which the coefficients are dependent on the predominant angle and Poisson's ratio. It is pointed out that the reason why the classical criteria disagree with experiments is that the predominant angle is not considered in the classical criteria. The new criterion is shown to be more accurate than the classical criteria for tensile failure of brittle materials by experiments.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-141
Abstract ( 1806 )   PDF (893KB) ( 690 )
Based on the characteristics of hole-charged blasting, the mechanism of action, the stressing and the movement of the stemming material are analyzed according to the blasting theory and the stress wave theory, and the expression of the length of the stemming material is deduced. On the basis of theoretical derivation and analysis, the experiment of stemming and its effect on blasting results was carried out. The experimental results show that the stemming in hole-charge is important and necessary. It may serve as a theoretical basis in the parameter design of blasting.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-145
Abstract ( 1623 )   PDF (651KB) ( 534 )
To clarify the relation between turbulent drag reduction and self-excited vibration of flexible tubes, experiments were performed on the effects of turbulent drag reduction and the characters of self-excited vibration, by comparing the turbulent drag with that in a rigid tube and by using a double-sleeve structure and a laser displacement sensor. The results are as the follows: the thinner the flexible tubes, the larger the root mean square of the fluctuating displacement at the outer wall of the tubes, and the larger the decreasing rate of the friction coefficient of the self-excited vibration, while applying a pressure-balanced air on the outer wall, with the rates of drag reduction of the flexible tubes with thickness of 2mm, 3mm, 4mm, being about 12%, 10%, 9% at the Reynolds number of 17\,500. This would provide a reference for efficient fluid transportation.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-174
Abstract ( 1131 )   PDF (318KB) ( 1351 )
An improved method for structural finite element modal reduction is presented. The disadvantage of Guyan method is demonstrated and an improvement is proposed. A numerical simulation for a plane truss structure is carried out. Results show that the proposed method is accurate.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-175
Abstract ( 1210 )   PDF (643KB) ( 785 )
In assembling an automobile, the cable of handbrake must be pre-tightened appropriately. The tension is usually measured and controlled by a technique called the tri-point method''. However, the results obtained do not agree with the actual ones. Generally, people ascribe this discrepancy to bending rigidity of the cable. This paper presents a new calculating model without bending rigidity by means of considering the actual force condition of a cable. Calculated results agree with the actual ones properly and therefore the technical problem of quickly measuring the tension of cables is resolved theoretically.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-178
Abstract ( 1372 )   PDF (652KB) ( 697 )
Much attention has been paid to the properties of materials and the shapes of cross sections of beams in order to increase the resistance of blast-loaded structures. This paper discusses how to increase the blast resistance of beams effectively by elastic and damping supports at the ends of beams. The equations of the forced vibration of an elastically supported beam with damping are obtained by the Lagrange equations. The responses of the beam subjected to two typical blast loads with long and short durations are analyzed. The numerical results show that the displacements of the ends are decreased with the increase of the damping coefficient and the stiffness of the elastic support. In the case of blast loads with long duration, the resistance of beams can only be increased effectively by using elastic and damping supports simultaneously at the ends of beams, while the resistance of beams can be increased effectively by using elastic and damping supports either jointly or separately in the case of blast loads with short duration.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2005-184
Abstract ( 1152 )   PDF (833KB) ( 589 )
The response surface method is applied to the structural shape optimization of a two-dimensional continuum, combined with FEM, with the approximate expressions of constraints functions being fitted, and the complex analytical sensitivity analysis avoided. The program of the shape optimization is developed under the platform MSC/Patran with Patran Command Language (PCL). The examples illustrate the feasibility and efficiency of RSM.
2006, 28(2):  0-0.  doi:10.6052/1000-0879-2005-213
Abstract ( 1757 )   PDF (350KB) ( 764 )
A method to estimate the system natural frequency using the unit natural frequency is developed by deriving the relation between the first order natural frequency of the frame structure and that of a simple frame structure. The method in this paper is verified through ANSYS, by using the finite element modal analysis. The calculation for the derrick of an oil drilling rig shows that the error is not exceeding 1\%. It is shown that the method is convenient and feasible for the natural frequency estimation of frame structures in series.
2006, 28(2):  0-0.  doi:10.6052/1000-0992-2006-088
Abstract ( 1650 )   PDF (1917KB) ( 699 )
The engineering background and guiding principals of seismic safety evaluation of concrete large dams in China are presented in this paper. Particularly, the special features and the linear and non-linear analytical methods for seismic responses of high arch dam system are described. The methods of the random seismic response and reliability assessment of concrete large dams are also discussed. Finally, based on the results of dynamic tests of dam concrete with specimen of fully-graded aggregates, the study on the dynamic failure process and mechanism of dam concrete using three-dimensional meso-mechanics analysis checked by computerized X-ray topography technique is briefly reported.