A constitutive model for creep-plasticity interaction based on dislocation dynamics

A glide-plus-climb micromechanism of dislocation evolution with the formation of subgrains is proposed for modelling of the creep-plasticity interaction (CPI). The long-range internal stress can be divided into the resistance for dislocation climb in subgrain boundaries and that for dislocation glide within grains or subgrains. Their evolution equations are then derived based on dislocation dynamics. Furthermore, a unified constitutuve model for CPI is developed from Orowan's formula. Theoretical calculations on the basis of this model show a very good agreement between the model prediction and experimental results of benchmark tests for 2 1/4 Cr-1 Mo steel at 600°C.     

Numerical study of the detonation structure in rich H2?NO2/N2O4 and very lean H2?N2O mixtures

In some mixtures and under certain conditions, detonation soot records show substructures. In nitromethane and nitrogen tetroxide mixtures, particular cellular structures can be observed. This kind of structures has been reported as the so-called double cellular structure. One- and two-dimensional simulations of detonation have shown that the double cellular structure is related to a non-monotonous energy release. Two-step energy release is also observed in rich H₂−NO₂/N₂O₄ and in very lean H₂−N₂O mixtures. The present study aims at the investigation of the effect of the energy release profile on the detonation structure in these two mixtures through numerical simulations. The origin of the non-monotonous energy release is explained in both mixtures using one-dimensional simulations with detailed chemistry. Reduced kinetic schemes are obtained and used to perform two-dimensional simulations. It is shown that in rich H₂−NO₂/N₂O₄ mixtures, the double cellular structure appears, whereas in very lean H₂−N₂O mixtures, classical substructures are observed. Both behaviours are explained based on ZND calculations and previous stability results. Phenomenological considerations led the authors to link the formation of the double cellular structure with the appearance of a large scale instability mode (a super cellular structure).     

Investigating internal resonances and 3:1 modal interaction in an electrostatically actuated clamped-hinged microbeam

Meccanica - In this paper, we investigated the realization of all possible internal resonance conditions between the first three modes of an electrostatically actuated, straight, clamped-hinged...     

Simulation of detonation wave interaction using an ignition and growth model

A kinetics based detonation model has been integrated with an existing object oriented hydrocode. The model has been demonstrated to correctly predict the shock initiation of explosives and captures key features such as the von Neumann pressure spike and reaction zone. Comparisons to experimental flyer plate data for both primary and secondary class explosives have been performed and key features such as detonation wave form and resulting target disk velocities are reproduced. Simulations have also been performed representing the interaction of various mechanical waves. First, the interaction of detonation waves, a key process in energetic systems, is simulated. Next, the response of an established detonation wave to an explosive that has been preconditioned by a weak compression wave is modeled. These have been carried out to determine the utility of the existing kinetic schemes for such problems.     

A comprehensive dynamic model for class-1 tensegrity systems based on quaternions

In this paper we propose a new dynamic model, based on quaternions, for tensegrity systems of class-1. Quaternions are used to represent orientations of a rigid body in the 3-dimensional space eliminating the problem of singularities. Moreover, the equations based on quaternions allow to perform more precise calculations and simulations because they do not use trigonometric functions for the representation of angles. We present a thorough introduction of tensegrities and the current state of research. We also introduce the quaternions and provide in the appendix some important details and useful properties. Applying the Euler–Lagrange approach we derive a comprehensive dynamic model, first for a simple rigid bar in the space and, at last, for a class-1 tensegrity system. We present two model forms: a matrix and a vectorial form. The first more compact and easier to write, the latter more suitable to apply the tools and the theory based on vector fields.     

A micromechanically based couple-stress model of an elastic orthotropic two-phase composite

We determine couple-stress moduli and characteristic lengths of a two-dimensional matrix-inclusion composite, with inclusions arranged in a periodic square array and both constituents linear elastic of Cauchy type. In the analysis we replace this composite by a homogeneous planar, orthotropic, couple-stress continuum. A generalization of the original Mindlin's (1963) derivation of field equations for such a continuum results in two (not just one!) characteristic lengths. We evaluate the couple-stress properties from the response of a unit cell under several types of boundary conditions: displacement, displacement-periodic, periodic and mixed, and traction controlled. In the parametric study we vary the stiffness ratio of both phases to cover a range of different media from nearly porous materials through composites with very stiff inclusions. We find that the aforementioned boundary conditions result in hierarchies of orthotropic couple-stress moduli, whereas both characteristic lengths are fairly insensitive to boundary conditions, and fall between 0.12% and 0.22% of the unit cell size for the inclusions' volume fraction of 18%.     

A fractional calculus based model for the simulation of an outbreak of dengue fever

We propose a new mathematical model for the simulation of the dynamics of a dengue fever outbreak. Our model differs from the classical model in that it involves nonlinear differential equations of fractional, not integer, order. Using statistics from the 2009 outbreak of the disease in the Cape Verde islands, we demonstrate that our model is capable of providing numerical results that agree very well with the real data.     

A block image encryption algorithm based on wave transmission and chaotic systems

A block-based image encryption algorithm using wave function and chaotic system is presented. A random sequence generated by the chaotic system is used to find the source point in the wave and produces a diffusion matrix for modular operation. In the encryption process, the keystream is dependent on both the plain-image and the secret key. It changes in each encryption round. Theoretical analyses and simulation results show the high security of the proposed method, including the large key space, fairly uniform histogram, zero correlation between neighbouring pixels, resistance to differential attacks, and high efficiency. Therefore, our algorithm is a practical scheme for digital image encryption.     

A model for brittle fracture based on the hybrid phase field model

We propose a phase field model for crack propagation based on the hybrid model and justify the model by constructing a family of asymptotic solutions.     

A semi-collocated ocean model based on the SOMS approach

In application to the Gulf of Mexico (GOM), a new DieCAST ocean model, which uses a modified Arakawa ‘a’ grid, and the SOMS model, which uses an Arakawa ‘c’ grid, give remarkably similar results. The new model avoids ‘null space’ problems of the standard ‘a’ grid approach by first using fourth-order interpolations to a ‘c’ grid advection velocity, then applying incompressibility to the result. Accuracy is further improved by using fourth-order pressure gradient approximations at ‘a’ grid locations. Incompressibility with general geometry is satisfied efficiently by a fast-converging iteration with a regular gteometry elliptic solver. Results are compared with satellite-measured r.m.s. sea surface elevation anomaly and detailed front structures, climatological mean thermocline and empirical orthogonal functions and other observations.     

A polycrystal plasticity model based on the mechanical threshold

A temperature and rate-dependent viscoplastic polycrystalmodel is presented.It uses a single crystal constitutive response that is based on the isotropic Mechanical Threshold Stress continuum model. This combination gives us theability to relate the constitutive model parameters between the polycrystaland continuum models. The individual crystal response is used to obtain themacroscopic response through the extended Taylor hypothesis. A Newton-Raphsonalgorithm is used to solve the set of fully implicit nonlinear equations for each crystal. The analysis also uses a novel state variable integration method which renders the analysis time step size independent for constant strain rate simulations. Material parameter estimates are obtained through an identification study, where the error between experimental and computed stress response is minimized. The BFGS method, which is used to solve theidentification problem, requires first-order gradients. These gradients arecomputed efficiently via the direct method of design sensitivity analysis.Texture augmentation is performed in a second identification study by changing crystal weights (volume fractions).     

考虑湍流模型的板片空间结构风荷载下流固耦合模拟

大跨空间结构凭借其新颖的外形和较大的内部空间,已经成为常用的建筑结构.由于大跨结构刚度、质量较小,受到风荷载作用时力学响应较一般结构大,同时大跨结构附近极易出现湍流影响.本文基于显式大涡模拟(ILES)和S-A (Spalart-Allmaras)湍流模型,使用ABAQUS软件对矢跨比为1:2的空间板片结构在不同风速下的风压系数进行流固耦合(FSI)模拟.风速以及不同湍流模型对大跨板片风压系数分布的影响得到了考察,并与相关的风洞实验比较.同时对板片结构受风载的力学响应进行了分析.结果显示选用大涡模拟湍流模型和雷诺时均方程方法湍流模型的流固耦合板片空间结构模型得到的风压系数及其分布和风洞试验实测数据吻合均较好.     

Free axial vibration of nanorods with elastic medium interaction based on nonlocal elasticity and Rayleigh model

In this paper, the free axial vibration of single walled carbon nanorod embedded in an elastic medium is investigated by the use of Rayleigh model. The stress gradient model introduced by Eringen is used to formulate the governing equations. Explicit expressions are derived for eigenfrequencies of fixed-fixed and fixed-free boundary conditions.     

含液颗粒材料液固耦合分析的离散颗粒模型及特征线SPH法

对含液颗粒材料流固耦合分析建议了一个基于离散颗粒模型与特征线SPH法的显式拉格朗日-欧拉无网格方案。在已有的用以模拟固体颗粒集合体的离散颗粒模型[1]基础上,将颗粒间间隙内的流体模型化为连续介质,对其提出并推导了基于特征线的SPH法。数值例题显示了所建议方案在模拟颗粒材料与间隙流相互作用的能力和性能以及间隙流体对颗粒结构承载能力及变形的影响。     

A nonlinear creep model based on the concept of slip

Relations between the creep strain and the stress are obtained for a complex stress state on basis of the hypothesis that the tangent strain components depend, in a certain slip system, only on the history of variation of the corresponding shear stress component. It is assumed that the slip system orientation is chaotically distributed over all directions and the total strain of the polycrystal is considered as the sum of contributions of all slips. This permits describing the creep strains in an arbitrary stress state by using a common operator defining the dislocation slip in a certain slip system. The model can be used for arbitrary creep operators. Computational results are analyzed and compared with experimental data obtained by other authors.     

基于细观力学的混凝土数值模型研究

详细分析了界面单元模型、接触面单元模型、CT图像三维重建模型三种细观混凝土数值模型的优缺点,并将数值计算结果与物理试验结果进行了对比.结果表明:混凝土接触面模型克服了界面单元模型中的界面层太厚、计算单元多、计算速度慢的弊端,且解决了原模型裂纹空洞区大的问题;而CT图像三维重建模型与实际物理模型能够一一对应,可以弥补传统随机混凝土骨料模型的不足.不仅可以使混凝土数值模型更接近于实际物理模型,而且可以较好地反映出了混凝土材料内部的力学特性.CT三维重建模型可为最终能够深入研究混凝土材料的细观破损机理开辟新的途径.     

A shock tube study on the thermal decomposition of CS2 based on S(3P) and S(1D) concentration measurements

The thermal decomposition of CS₂ highly diluted in Ar was studied behind reflected shock waves by monitoring time-dependent absorption profiles of S(³P) and S(¹D) using atomic resonance absorption spectroscopy (ARAS). The rate coefficient of the reaction:

Mechanical interaction between spherical inhomogeneities: an assessment of a method based on the equivalent inclusion

This paper assesses the ability of the Equivalent Inclusion Method (EIM) with third order truncated Taylor series (Moschovidis and Mura, 1975) to describe the stress distributions of interacting inhomogeneities. The cases considered are two identical spherical voids and glass or rubber inhomogeneities in an infinite elastic matrix. Results are compared with those obtained using spherical dipolar coordinates, which are assumed to be exact, and by a Finite Element Analysis. The EIM gives better results for voids than for inhomogeneities stiffer than the matrix. In the case of rubber inhomogeneities, while the EIM gives accurate values of the hydrostatic pressure inside the rubber, the stress concentrations are inaccurate at very small neighbouring distances for all stiffnesses. A parameter based on the residual stress discontinuity at the interface is proposed to evaluate the quality of the solution given by the EIM. Finally, for inhomogeneities stiffer than the matrix, the method is found to diverge for expansions in Taylor series truncated at the third order.