点火能对预混气体爆炸过程及管道薄壁加载响应的影响

在两端封闭的无缝不锈钢管道中,利用压力传感器、应变片以及数据采集系统实验测试了不同点火能作用下,管道内甲烷-空气预混气体爆炸波发展规律及由此造成的管道外壁的动态响应。结果表明,点火能量越大,爆炸反应程度越剧烈,管道内最大爆炸压力就越大,管道薄壁的最大动态应变也越大,爆炸波发展就越迅速,并且管壁动态应变信号和压力波信号出现较好的一致性。本文结果可为油气长输管道的爆炸破坏效应研究提供一种新的思路和方法。     

Simple Torsion of Isotropic, Hyperelastic, Incompressible Materials with Limiting Chain Extensibility

The purpose of this research is to investigate the simple torsion problem for a solid circular cylinder composed of isotropic hyperelastic incompressible materials with limiting chain extensibility. Three popular models that account for hardening at large deformations are examined. These models involve a strain-energy density which depends only on the first invariant of the Cauchy–Green tensor. In the limit as a polymeric chain extensibility tends to infinity, all of these models reduce to the classical neo-Hookean form. The main mechanical quantities of interest in the torsion problem are obtained in closed form. In this way, it is shown that the torsional response of all three materials is similar. While the predictions of the models agree qualitatively with experimental data, the quantitative agreement is poor as is the case for the neo-Hookean material. In fact, by using a global universal relation, it is shown that the experimental data cannot be predicted quantitatively by any strain-energy density which depends solely on the first invariant. It is shown that a modification of the strain energies to include a term linear in the second invariant can be used to remedy this defect. Whether the modified strain-energies, which reflect material hardening, are a feasible alternative to the classic Mooney–Rivlin model remains an open question which can be resolved only by large strain experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Large plastic strains in sheet-bending

Based on Hill's general theory of sheet bending, a closed form expression is derived for strain in any fiber of a sheet bent under the conditions of plane strain and negligible elastic strains. A numerical example indicates that the assumptions of the theory are plausible over a wide range of the angle of bending.     

Strain stiffening of non-colloidal hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region

Concentrated hard sphere suspensions often show an interesting nonlinear behavior, called strain stiffening, in which the viscosity or modulus starts to increase at critical strain amplitude. Sudden increase of rheological properties is similar to shear thickening; however, the particle dynamics in the strain stiffening under oscillatory shear flow does not necessarily coincide with the mechanism of shear thickening under step shear flow. In this study, we have systematically investigated the nonlinear rheology of non-colloidal (>1???m) hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region in order to better understand the strain stiffening behavior. The suspensions near liquid-and-crystal coexistence region are known to locally form the closed packing structure. The critical strain amplitude which is the onset of strain stiffening was different for the storage and loss modulus. But they converged to each other as the suspension forms a more crystalline structure. The critical strain amplitude was independent of medium viscosity, imposed angular frequency, and particle size, but was strongly dependent upon particle volume fraction. The onset of strain stiffening was explained in terms of shear-induced collision due to particle motion in the closed packing structure. Nonlinear stress wave-forms, which reflect the micro-structural change, were observed with the onset of strain stiffening. During the strain stiffening, enhanced elastic stress before and after flow reversal was observed which originates from changes in the suspension microstructure. Nonlinearity of the shear stress in terms of Fourier intensity was extremely increased up to 0.55. Beyond the strain stiffening, the suspension responded liquid-like and the nonlinearity decreased but the elastic shear stress was still indicating the microstructure rearrangement within a cycle.     

Strain localization in an oscillating Maxwell viscoelastic cylinder

The transient rotation responses of simple, axisymmetric, viscoelastic structures are of interest for interpretation of experiments designed to characterize materials and closed structures such as the brain using magnetic resonance techniques. Here, we studied the response of a Maxwell viscoelastic cylinder to small, sinusoidal displacement of its outer boundary. The transient strain field can be calculated in closed form using any of several conventional approaches. The solution is surprising: the strain field develops a singularity that appears when the wavefront leaves the center of the cylinder, and persists as the wavefront reflects to the outer boundary and back to the center of the cylinder. The singularity is alternately annihilated and re-initiated upon subsequent departures of the wavefront from the center of the cylinder until it disappears in the limit of steady state oscillations. We present the solution for this strain field, characterize the nature of this singularity, and discuss its potential role in the mechanical response and evolved morphology of the brain.     

Guided assembly of nanostructures via elastic interactions

A solid solution can spontaneously separate into phases that self-assemble into patterns. This process can be guided via external fields to form ordered micro- and nanostructures. In this paper, we demonstrate that notions of interaction energies provide powerful insights into the coupling of these fields with the properties of the alloy. A phase-field model is developed that incorporates chemical, interfacial, and elastic energies, including heterogeneous elastic properties, and couples naturally to externally imposed mechanical fields. Aggregation in bulk and in thin films under patterned external load is investigated. The kinetics and morphology of phase separation are shown to depend significantly on elastic properties of the system, which include elastic heterogeneity and the misfit or transformation strain. Eshelby-type asymptotic estimates for interaction energies are shown to be very useful in understanding and predicting the trends observed from the simulations.     

A generalized Adadorov theory for anisotropic thin-walled beams

This paper presents a generalized Adadorov theory for anisotropic thin—walled beams. The theory takes account of the shear strain of the middle surface, which exerts a significant influence on the anisotropic thin-walled beams. A new approach is established to solve the governing equations, which have the same form for both open and closed section beams. The numerical examples show that the effects of the shear strain cannot be neglected for this class of beams.This work was part of research project supported by the National Natural Science Foundation of China     

集中荷载作用下的压电压磁双材料III型界面裂纹问题

研究了压电压磁双材料的III型界面裂纹问题,裂纹表面和界面电磁绝缘,且作用有集中荷载．通过解析延拓和复变函数技术,得到该问题的全场解,并给出了裂纹尖端的场强因子．还讨论了Wan等所定义的用以表征界面上下压电压磁材料失配比的无量纲参数G,发现G实际表征的是界面两侧应变的比值,也即界面两侧应变的不相容性．     

Extended yield condition of soils with tensile yield strength and rotational hardening

An extended yield condition of soils applicable to the range of negative pressure is proposed. A closed surface is formulated in a simple mathematical form not limited to the range of positive pressure. A hardening function of plastic volumetric strain is adopted, based on the linear relation of both logarithms of volume and pressure extended to the negative range of pressure for the isotropic consolidation. Isotropic softening, due to the deviatoric plastic strain rate, and anisotropic hardening, due to rotation of yield surface (rotational hardening) are also incorporated into the extended yield condition.     

含缺口构件高周疲劳寿命的损伤力学封闭解法

本文根据损伤力学建立含缺口板守恒积分、应力与应变集中系数公式以及疲劳裂纹形成寿命的封闭解法。对铝合金含缺口板寿命预估结果与实验结果基本一致。本方法节省机时并可代替大量实验,具有一定的工程实用性。     

An alternative approach to integrating plasticity relations

A new plasticity integration algorithm is proposed based upon observations from the closed form integration of a generalized quadratic yield function over a single time step. The key to the approach is specification of the normal to the plastic flow potential as a function of the current state and strain increment. This uniquely defines the direction of the stress tensor for a convex, non-faceted flow potential. The stress magnitude and plastic strain increment are computed to satisfy the yield function. A non-quadratic, isotropic, associative flow model is coded to demonstrate accuracy and time step convergence following a step change in loading path. The model is used in additional simulations of strain localization in an expanding ring and a perforated plate.     

Structure of adiabatic shear bands in thermo-viscoplastic materials

The shear band structure in a thermo-viscoplastic material has been analyzed under quasi-static conditions during the final stage of the localization process. A closed form expression of the bandwidth, of the size of the heat-affected zone and of the failure time has been obtained for a homogeneous material. The analysis is based on the assumption that the strain-rate distribution is steady beyond a certain strain level. Predictions have been compared with experimental measurements.     

A higher order subdomain method for finding local stress fields in composites

The problem of finding local and volume averaged stresses in a two-dimensional heterogeneous solid is formulated in terms of fundamental point load solutions (Green's function) leading to singular integral equations. The resulting equations are solved approximately using a subdomain method in which closed form solutions for a rectangular subdomain are obtained and utilized to find the full field solution. Previously, closed form solutions for a rectangular subvolume had been found, but only for the case of an assumed constant strain. In the present paper the solution is obtained for a quadratic form which includes not only the usual constant term but also linear and quadratic terms. The advantages of using the higher order solutions is illustrated by finding the local field in a periodic composite with square fibers. The numerical solution takes less than 90 CPU s on a workstation. The solution yields average properties independent of the reference modulus as would be expected for an accurate solution of the singular integral equation and the effective transverse modulus vs volume fraction is close to that from Christensen's model developed for round fibers.     

沥青路面反射裂缝问题的损伤力学守恒积分

基于损伤力学理论，建立了沥青路面反射裂缝问题的损伤力学守恒积分，证明了在反射裂缝形成过程中应变能密度的守恒性。在此基础上，导出了预估反射裂缝形成寿命的简便方法与公式。本文研究成果对于沥青路面设计与旧路补强设计具有一定理论指导意义。     

Plane problems in piezoelectric media with elliptic inclusion

I.IntroductionPiezoelectricmedia,asa"ex\'typeoffullctionalmaterial.arex'idel}'appliedtomanytechnologicalfieldsduetoitselectronlechallicalcouplillgeffect.Defects.likethatofothermaterials.arenotlimitedtocracks.x'oidsandinclusionsillpiezoelectricmaterialsorelements.Yet,stressconcentrationsornoll-ullitbrllldistl-ibutionsofelectricfieldillducedbythosedefectsareoneofthehe}l'filctorswllicllwouldleadpiezoelectricstructurestonon-normalfailure.Therel'ore.itisofgrealimportancetostudythepropertiesofthos…     

Torsion in nonlinearly elastic incompressible circular cylinders

We consider a variable torsion deformation for incompressible right-circular cylinders and investigate the possibility of equilibrium states other than simple torsion in isotropic hyperelasticity. Our problem formulation also provides generalized versions of Rivlin's universal relations in torsion. In this more general setting, it is shown that simple torsion persists as the only solution for large classes of strain energies. When one allows for more general boundary conditions, variable torsion solutions are possible for special forms of the stored energy function. We derive such a form and develop general results.     

The significance of pure measures of distortion in nonlinear elasticity with reference to the Poynting problem

The objective of this paper is to show the significance of expressing the strain energy function in terms of a scalar pure measure of dilatation and a tensor pure measure of distortion, which were essentially introduced by Flory [1]. It is shown that convenient representations for the strain energies of dilatation and distortion, and the pressure and deviatoric Cauchy stress may be recorded in terms of these deformation measures. After specializing to the case of an isotropic material, specific constitutive equations are proposed and the Poynting problem is considered. It is shown that the Poynting effect (extension of a bar in torsion) is significantly influenced by coupling between dilatational and distortional strain energies, which is caused by the dependence of the shear modulus on dilatation.     

On Saint Venant - Kirchhoff imperfect interfaces

Using matched asymptotic expansions with fractional exponents, we obtain original transmission conditions describing the limit behavior for soft, hard and rigid thin interphases obeying the Saint Venant-Kirchhoff material model. The novel transmission conditions, generalizing the classical linear imperfect interface model, are discussed and compared with existing models proposed in the literature for thin films undergoing finite strain. As an example of implementation of the proposed interface laws, the uniaxial tension and compression responses of butt joints with soft and hard interphases are given in closed form.     

Longitudinal elastic stress impulse induced by impact through a spring-dashpot system: Optimization and inverse problems for the spring stiffness

In the present paper, an exact analytical solution is obtained for the problem of a semi-infinite elastic rod struck by a rigid mass through a linear Kelvin–Voigt element. The optimization problem resulting in generating compressed pulses of minimum duration is addressed. The case of undamped impact has been studied in detail, and the optimal value for the spring stiffness is presented in a closed form. Inverse problems of recovery of the impact system parameters from the parameters of the strain wave impulse are also considered.     

A mechanical model on elastic cavitation in solid under hydrostatic tension

In this paper, a mechanical model is proposed to study cavitation in solids. The material is elastic and obeys Hook's law. Logarithm strain and Cauchy stress in nonlinear elasticity is used. A sphere with a central hole is loaded under hydrostatic tension on surface. In spherical symmetry, closed form solution is got. As radius of the hole tends to zero, a pitchfork bifurcation appears on load-displacement curve. The pitchfork bifurcation indicates that there is a cavitation at center of solid sphere as load reaches a critical value.