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Theoretical and Applied Mechanics Letters is jointly published by the Chinese Society of Theoretical and Applied Mechanics and American Institute of Physics. The Chinese Society of Theoretical and Applied Mechanics is the sole distributor of the print and electronic versions in Mainland China, and American Institute of Physics is the sole distributor of the print and electronic versions outside Mainland China.

Select this Article		Stress and fabric in granular material Ching S. Chang and Yang Liu Theor. Appl. Mech. Lett. 3, 021002 (2013); http://dx.doi.org/10.1063/2.1302102 (6 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract It has been well recognized that, due to anisotropic packing structure of granular material, the true stress in a specimen is different from the applied stress. However, very few research efforts have been focused on quantifying the relationship between the true stress and applied stress. In this paper, we derive an explicit relationship among applied stress tensor, material-fabric tensor, and force-fabric tensor; and we propose a relationship between the true stress tensor and the applied stress tensor. The validity of this derived relationship is examined by using the discrete element simulation results for granular material under biaxial and triaxial loading conditions.

Select this Article		Modeling of softsphere normal collisions with characteristic of coefficient of restitution dependent on impact velocity Youhe Zhou Theor. Appl. Mech. Lett. 3, 021003 (2013); http://dx.doi.org/10.1063/2.1302103 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract This letter presents a theoretical model of the normal (head-on) collisions between two soft spheres for predicting the experimental characteristic of the coefficient of restitution dependent on impact velocity. After the contact force law between the contacted spheres during a collision is phenomenologically formulated in terms of the compression or overlap displacement under consideration of an elastic—plastic loading and a plastic unloading subprocesses, the coefficient of restitution is gained by the dynamic equation of the contact process once an initial impact velocity is input. It is found that the theoretical predictions of the coefficient of restitution varying with the impact velocity are well in agreement with the existing experimental characteristics which are fitted by the explicit formula.

Select this Article		Granular pressure—temperature relation during regime change in a simple shear state Hayley Shen Theor. Appl. Mech. Lett. 3, 021004 (2013); http://dx.doi.org/10.1063/2.1302104 (4 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract In a recent study, a regime chart was established for sheared granular systems. For a soft particle system under simple shear, the internal stress showed a range of rate dependency. As the particle concentration increased, the system transitioned from a kinetic gas to a plastic solid. This transition was gradual for low stiffness systems but became more abrupt as the stiffness increased. In this study, the relationship between the granular temperature and pressure is investigated for the same system of particles. The granular temperature is defined as the average kinetic energy per unit volume and the pressure is defined as the trace of the stress tensor. It is found that this pressure-temperature (P−T) curve depicts a sharp turn when the system moves away from the kinetic gas regime. However, no signature is found in the P-T relationship in other regimes.

Select this Article		Distinct element method investigation on mechanical behavior within shear bands in granulates under the Earth and the Moon conditions Mingjing Jiang and Wangcheng Zhang Theor. Appl. Mech. Lett. 3, 021005 (2013); http://dx.doi.org/10.1063/2.1302105 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract This letter mainly aims to investigate the mechanical behavior within shear bands in regolith both under the Earth and the Moon conditions via the distinct element method, in which a novel contact model considering interparticle van der Waals forces and rolling resistance is employed. The results show that for regolith under both conditions the stress paths are almost identical inside and outside the shear bands but void ratio, average pure rotation rate, and strain paths are rather distinct with dilation, particle rotation and the second invariant of strain tensor mainly occurring within the bands. However, the regolith under the Moon condition has higher peak strength and more significant strain localization than those under the Earth condition.

Select this Article		Micromechanics-based model for cement-treated clays Zhenyu Yin and Pierre Yves Hicher Theor. Appl. Mech. Lett. 3, 021006 (2013); http://dx.doi.org/10.1063/2.1302106 (7 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract Cementation is produced by mixing a certain amount of cement with the saturated clay. The purpose of this paper is to model the cementation effect on the mechanical behavior of cement-treated clay. A micromechanical stress-strain model is developed considering explicitly the cementation at inter-cluster contacts. The inter-cluster bonding and debonding during mechanical loading are introduced in two ways: an additional cohesion in the shear sliding and a higher yield stress in normal compression. The model is used to simulate isotropic compression and undrained triaxial tests under various confining stresses on cement-treated Singapore clay with various cement contents. The applicability of the present model is evaluated through comparisons between numerical and experimental results. The evolution of local stresses and local strains in inter-cluster planes is discussed in order to explain the induced anisotropy due to debonding at contact level under the applied loads.

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March 2013

Volume 3, Issue 2, Articles (02xxxx)

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Select this Article		Editorial: Computational mechanics of granular materials Xikui Li and Xiaojing Zheng Theor. Appl. Mech. Lett. 3, 021001 (2013); http://dx.doi.org/10.1063/2.1202101 (1 page) Full Text: Read Online (HTML) \| Download PDF
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Select this Article		Numerical modeling of fluid-particle interaction in granular media Jidong Zhao and Tong Shan Theor. Appl. Mech. Lett. 3, 021007 (2013); http://dx.doi.org/10.1063/2.1302107 (3 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract Fluid-particle interaction underpins important behavior of granular media. Particle-scale simulation may help to provide key microscopic information governing the interaction and offer better understanding of granular media as a whole. This paper presents a coupled computational fluid dynamics and discrete element method (CFD-DEM) approach for this purpose. The granular particle system is modeled by DEM, while the fluid flow is simulated by solving the locally averaged Navier–Stokes equation with CFD. The coupling is considered by exchanging such interaction forces as drag force and buoyancy force between the DEM and CFD. The approach is benchmarked by two classic geomechanics problems for which analytical solutions are available, and is further applied to the prediction of sand heap formation in water through hopper flow. It is demonstrated that the key characteristic of granular materials interacting with pore water can be successfully captured by the proposed method.

Select this Article		Granular materials: Bridging damaged solids and turbulent fluids Q. Sun, S. Song, J. Liu, M. Fei, and F. Jin Theor. Appl. Mech. Lett. 3, 021008 (2013); http://dx.doi.org/10.1063/2.1302108 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract Granular materials exhibit abundant dissipations due to fluctuations in both granular motions and configurations (i.e., granular skeleton) evolutions. Twin granular temperatures T_k and T_e are introduced accounting for two types of fluctuations, and the so-called twin granular temperatures theory is established as an extension of granular solid hydrodynamics. By using simulations, the nonaffine deformations in a 2D assembly are simulated by using discrete element methods. By analogizing with microdamages in deformed solids, double scalar damage variables, D_P and D_q, are proposed to describe the deformed granular solid under triaxial compressions. Granular flows are found intrinsically turbulent due to the presence of T_k and the Naiver–Stokes equation is obtained for granular flows.

Select this Article		Discrete element modeling of acoustic emission in rock fracture Shunying Ji and Shaocheng Di Theor. Appl. Mech. Lett. 3, 021009 (2013); http://dx.doi.org/10.1063/2.1302109 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract The acoustic emission (AE) features in rock fracture are simulated numerically with discrete element model (DEM). The specimen is constructed by using spherical particles bonded via the parallel bond model. As a result of the heterogeneity in rock specimen, the failure criterion of bonded particle is coupled by the shear and tensile strengths, which follow a normal probability distribution. The Kaiser effect is simulated in the fracture process, for a cubic rock specimen under uniaxial compression with a constant rate. The AE number is estimated with breakages of bonded particles using a pair of parameters, in the temporal and spatial scale, respectively. It is found that the AE numbers and the elastic energy release curves coincide. The range for the Kaiser effect from the AE number and the elastic energy release are the same. Furthermore, the frequency-magnitude relation of the AE number shows that the value of B determined with DEM is consistent with the experimental data.

Select this Article		Estimation of dynamic stress spectrum distribution in structural fatigue test Guangjin Xue, Kai Li, Wu Pan, Qiang Li, Bingjie Wang, and Shonquang Su Theor. Appl. Mech. Lett. 3, 021010 (2013); http://dx.doi.org/10.1063/2.1302110 (6 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract The determination of structural dynamic stress spectrum distribution is of great significance in the structural fatigue strength evaluation as well as reliability design. In previous empirical data processing methods, the data grouping and distribution fitting were excessively coarse and contained distinctive defects. This paper proposed an effective approach to statistically group actual measured dynamic stress data and validly extrapolate the combined distribution to fit the dynamic stress spectrum distribution. This approach has been verified its effectiveness through chi-square test, stress spectrum extrapolation and damage calculation in dynamic stress study.

Select this Article		Thermal stretching in two-phase porous media: Physical basis for Maxwell model Xiaohu Yang, Tianjian Lu, and Tongbeum Kim Theor. Appl. Mech. Lett. 3, 021011 (2013); http://dx.doi.org/10.1063/2.1302111 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract An alternate yet general form of the classical effective thermal conductivity model (Maxwell model) for two-phase porous materials is presented, serving an explicit thermo-physical basis. It is demonstrated that the reduced effective thermal conductivity of the porous media due to non-conducting pore inclusions is caused by the mechanism of thermal stretching, which is a combination of reduced effective heat flow area and elongated heat transfer distance (thermal tortuosity).

Select this Article		Precise method to control elastic waves by conformal mapping Yongquan Liu, Wei Liu, and Xianyue Su Theor. Appl. Mech. Lett. 3, 021012 (2013); http://dx.doi.org/10.1063/2.1302112 (6 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract The transformation method to control waves has received widespread attention in electromagnetism and acoustics. However, this machinery is not directly applicable to the control of elastic waves, because it has been shown that the Navier's equation does not usually retain its form under coordinate transformation. In this letter, we prove the form invariance of the Navier's equation under the conformal mapping based on the Helmholtz decomposition method. The needed material parameters are provided to manipulate elastic waves. The validity of this approach is confirmed by an active stealth device which can disguise the signal source by changing its position. Experimental verifications and potential applications may be expected in nondestructive testing, structural seismic design and other fields.

Select this Article		An overview on nonlinear porous flow in low permeability porous media Yanzhang Huang, Zhengming Yang, Ying He, and Xuewu Wang Theor. Appl. Mech. Lett. 3, 022001 (2013); http://dx.doi.org/10.1063/2.1302201 (8 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract This paper gives an overview on nonlinear porous flow in low permeability porous media, reveals the microscopic mechanisms of flows, and clarifies properties of porous flow fluids. It shows that, deviating from Darcy's linear law, the porous flow characteristics obey a nonlinear law in a low-permeability porous medium, and the viscosity of the porous flow fluid and the permeability values of water and oil are not constants. Based on these characters, a new porous flow model, which can better describe low permeability reservoir, is established. This model can describe various patterns of porous flow, as Darcy's linear law does. All the parameters involved in the model, having definite physical meanings, can be obtained directly from the experiments.

Select this Article		Flexural-gravity wave resistances due to a surface-moving line source on a fluid covered by a thin elastic plate D. Q. Lu and H. Zhang Theor. Appl. Mech. Lett. 3, 022002 (2013); http://dx.doi.org/10.1063/2.1302202 (4 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract Analytical solutions for the flexural-gravity wave resistances due to a line source steadily moving on the surface of an infinitely deep fluid are investigated within the framework of the linear potential theory. The homogenous fluid, covered by a thin elastic plate, is assumed to be incompressible and inviscid, and the motion to be irrotational. The solution in integral form for the wave resistance is obtained by means of the Fourier transform and the explicitly analytical solutions are derived with the aid of the residue theorem. The dispersion relation shows that there is a minimal phase speed c_min, a threshold for the existence of the wave resistance. No wave is generated when the moving speed of the source V is less than c_min while the wave resistances firstly increase to their peak values and then decrease when V ⩾ c_min. The effects of the flexural rigidity and the inertia of the plate are studied.

Select this Article		Effect of air core on the shape and discharge of the outflow through a bottom outlet Yiyi Ma, Yu Qian, and David Z. Zhu Theor. Appl. Mech. Lett. 3, 022003 (2013); http://dx.doi.org/10.1063/2.1302203 (5 pages) Online Publication Date: March 10, 2013 Full Text: Read Online (HTML) \| Download PDF
	+ -	Show Abstract Experiments were conducted to study the generation of air core and its effect on the outflow shape and discharge in a cylindrical water tank with a bottom well-designed outlet. Depending on the stages of the air core in the tank, the outflow shape can vary from a smooth water jet to a smooth spindle shape with air-core, and to water sprays. The diameter of the nozzle size also has influence on the outflow pattern. The existence of the penetrated air core can dramatically reduce the outflow discharge, with the discharge coefficient decreasing with the nozzle diameter.