Thermodynamic nature and control of the elastic limit in solids

The elastic limit of a solid is implicit in its thermo-elastic properties and can be determined from the constitutive equations of internal energy and entropy in the elastic range. The second law of thermodynamics is responsible for this, as it sets an upper bound to the internal energy that a material can store during isothermal elastic deformation processes. A link between irreversibility and elasticity can thus be established, which allows for a better control of the properties of strength, ductility and elastic limit of the material. For elastic-plastic materials of practical interest it implies that the yield limit cannot be assigned independently of the elastic constitutive equations, although the current approaches do so. An application to elastic-plastic materials with linear thermo-elastic properties reveals that, in the one-dimensional case, all information on the entropy of the material can be drawn from standard uniaxial tests. An easy procedure can then be devised to design the preparation process of the material so that the desired combination of strength, ductility and elastic limit can be achieved within the admissible values for these quantities.     

Thermodynamic activation functions of viscous flow of non-polar liquids

The thermodynamic activation function of viscous flow may be determined from the expression for the pre-exponential factor in the Eyring relationship (the viscosity coefficient), which is a function of density and relative permittivity, together with the thermal dependence of the viscosity coefficient. This method of determination is demonstrated for a series of n-alkanes C₆–C₂₀.List of symbols A, B, C parameters in empirical viscosity-temperature dependence - E molecular bond energies of liquid calculated from heat of evaporation in vacuum - E _v viscous flow activation energy - G activation Gibbs function of viscous flow - H _v heat of evaporation - H _vb heat of evaporation at boiling point - H viscous flow activation enthalpy - k Boltzmann constant - m weight of molecule - M relative molecular weight - N _A Avogadro constant - q total number of bonds in one mole of liquid - R gas constant - S _id entropy of one mole of compound in ideal gas state - S _sat entropy of one mole of compound in saturated vapour state - S activation entropy of viscous flow - T absolute temperature - T _c critical temperature - T _r reduced temperature - T _c critical volume - T _f free volume of liquid per molecule - V ₀ molar volume of molecules - V _m molar volume of liquid - relative permittivity - viscosity - density - _r reduced density     

Torque suppression in mechanically stirred liquids and multiphase liquid systems

Both polymeric and fibrous drag-reducing additives can be used to suppress the torque of agitators operating in a turbulent regime. Unlike the tubular flow, the influence of the fibrous additive is stronger than that of the polymer but their combined action seems to be additive. The action of the drag-reducing additives is further enhanced by the presence of a second phase (gaseous or liquid) in the stirred tank. In aerated tanks, the torque on the shaft of the agitator is not only reduced by more than 70% but also the flooding point is shifted to higher gas rates. An application of this phenomenon to fermentation tanks and similar technologies has been suggested.     

Non-linear heat transfer of solids in orthogonal coordinate systems

Solutions to the non-linear partial differential equation of heat conduction, (Poisson type), are obtained in which the conductivity is temperature dependent, by solving a linear partial differential equation and transforming it to the non-linear form using the Kirchhoff transformation. The method applies to any orthogonal coordinate system.

Transformations for handling boundary conditions of the Dirichlet, Neumann, convection and non-zero type are developed. The method is extended to solve a special class of non-linear unsteady-state conduction problems.

Two non-linear examples are solved to illustrate the method.     

Axial dispersion of solids in rotary solid flow systems

Summary A simple unidirectional diffusion model is employed to analyze the axial dispersion of solid particles flowing through a rotary solid flow system, namely a rotary dryer. It is shown that the reciprocal of the Peclet number D/uL is uniquely correlated as a function of the dimensionless number F/dSN which characterizes the operating conditions of the rotary dryer.List of Symbols C concentration of tracer, mass/(length)³ - d diameter of rotary dryer, length - d _p diameter of solid particles, length - D longitudinal dispersion coefficient or axial mixing coefficient, (length)²/time - F volumetric flow rate of solid, (length)³/(length)² time - L length of rotary dryer, length - N rate of rotations of dryer, time^–1 - Q volume of tracer injected, based on bulk density of particles, (length)³ - S slope of the rotary dryer - u average flow velocity, based on effective flow volume of dryer, length/time - v volumetric flow rate, based on bulk density of particles, (length)³/time - V effective volume of rotary dryer, (length)³ - x distance from entrance of experimental section of dryer, length Greek letters time, measured from instant of introducing tracer into flowing material - 1– volumetric solid hold-up fraction - standard deviation - ² variance - _r relative standard deviation     

Thermodynamic optimization of finned crossflow heat exchangers for aircraft environmental control systems

This paper shows that the main geometric features of a flow component can be deduced from the thermodynamic optimization of the global performance of the largest flow system that incorporates the component. This approach represents a departure from the usual approach, where a flow component is optimized in isolation. The example chosen is the counterflow heat exchanger of the environmental control system (ECS) used on modern aircraft. The heat exchanger is fitted with a diffuser and a nozzle for the ram air, and the ECS runs on the boot strap air cycle, employing an additional compressor and turbine. Two heat transfer surface types are considered, finned and smooth parallel plates. Numerical results are reported for the external geometric aspect ratios of the heat exchanger, and for the plate-to-plate spacing of the smooth-plates model. It is shown that the optimized geometry for the core with finned surfaces is nearly the same as the optimized geometry for the core with smooth plates. Several of the optimized geometric features are robust with respect to changes in external parameters that vary from one application to the next. The method illustrated in this paper – the thermodynamic (constructal) optimization of flow geometry – is applicable to any system that runs on the basis of a limited amount of fuel (exergy) installed onboard, e.g., automobiles, ships, portable tools.     

Couple stress theory for solids

By relying on the definition of admissible boundary conditions, the principle of virtual work and some kinematical considerations, we establish the skew-symmetric character of the couple-stress tensor in size-dependent continuum representations of matter. This fundamental result, which is independent of the material behavior, resolves all difficulties in developing a consistent couple stress theory. We then develop the corresponding size-dependent theory of small deformations in elastic bodies, including the energy and constitutive relations, displacement formulations, the uniqueness theorem for the corresponding boundary value problem and the reciprocal theorem for linear elasticity theory. Next, we consider the more restrictive case of isotropic materials and present general solutions for two-dimensional problems based on stress functions and for problems of anti-plane deformation. Finally, we examine several boundary value problems within this consistent size-dependent theory of elasticity.     

Monotonicity theorems for two-phase solids

Certain alloys, such as gold-copper, have two solid phases. We establish a general mathematical framework in which we show that the fraction in one phase and the compositions within each phase are in some sense decreasing in the overall composition. The tools used include useful new lemmas on minima of functions of several variables and parameters.     

Constraint manifolds for isotropic solids

Communicated by M. E. Gurtin     

Lattice-Boltzmann method for yield-stress liquids

In the present study we propose a new version of the lattice-Boltzmann (LB) method for the simulation of flow of yield-stress liquids. Unlike traditional LB methods, collisions are treated implicitly, i.e., the collision term is chosen in such a way that the stress and strain rate tensors satisfy the constitutive equation after the collision. This approach requires the solution of a (one-dimensional) non-linear algebraic equation at each point and at each time step. In the practically important cases of a Bingham liquid this equation can be solved analytically. We calculated the flow of Bingham fluid through a channel and periodic mesh of cylinders.     

Noncanonical Poisson brackets for elastic and micromorphic solids

This paper investigates the Lagrangian-to-Eulerian transformation approach to the construction of noncanonical Poisson brackets for the conservative part of elastic solids and micromorphic elastic solids. The Dirac delta function links Lagrangian canonical variables and Eulerian state variables, producing noncanonical Poisson brackets from the corresponding canonical brackets. Specifying the Hamiltonian functionals generates the evolution equations for these state variables from the Poisson brackets. Different elastic strain tensors, such as the Green deformation tensor, the Cauchy deformation tensor, and the higher-order deformation tensor, are appropriate state variables in Poisson bracket formalism since they are quantities composed of the deformation gradient. This paper also considers deformable directors to comprise the three elastic strain density measures for micromorphic solids. Furthermore, the technique of variable transformation is also discussed when a state variable is not conserved along with the motion of the body.     

Plastic potentials for anisotropic porous solids

The aim of this paper is to incorporate plastic anisotropy into constitutive equations of porous ductile metals. It is shown that plastic anisotropy of the matrix surrounding the voids in a ductile material could have an influence on both effective stress–strain relation and damage evolution. Two theoretical frameworks are envisageable to study the influence of plastic flow anisotropy: continuum thermodynamics and micromechanics. By going through the Rousselier thermodynamical formulation, one can account for the overall plastic anisotropy, in a very simple manner. However, since this model is based on a weak coupling between plasticity and damage dissipative processes, it does not predict any influence of plastic anisotropy on cavity growth, unless a more suitable choice of the thermodynamical force associated with the damage parameter is made. Micromechanically-based models are then proposed. They consist of extending the famous Gurson model for spherical and cylindrical voids to the case of an orthotropic material. We derive an upper bound of the yield surface of a hollow sphere, or a hollow cylinder, made of a perfectly plastic matrix obeying the Hill criterion. The main findings are related to the so-called ‘scalar effect’ and ‘directional effect’. First, the effect of plastic flow anisotropy on the spherical term of the plastic potential is quantified. This allows a classification of sheet materials with regard to the anisotropy factor h; this is the scalar effect. A second feature of the model is the plasticity-induced damage anisotropy. This results in directionality of fracture properties (‘directional effect’). The latter is mainly due to the principal Hill coefficients whilst the scalar effect is enhanced by ‘shear’ Hill coefficients. Results are compared to some micromechanical calculations using the finite element method.     

Universal relations for non-linear magnetoelastic solids

In the light of recent and growing interest in the applications of magneto-sensitive elastomers and the corresponding theoretical analysis of their properties, this paper is devoted to the derivation of universal relations for these materials, that is connections between the components of a stress tensor and the components of the magnetic (induction) field vector that hold independently of the choice of constitutive law within a considered class of such laws. Here, attention is focussed on isotropic magnetoelastic materials. In particular, within this framework, it is shown that in general there is only one possible universal relation for these materials, but for particular classes of constitutive laws or for special deformations there can be more than one. The theory is exemplified by application to the problem of homogeneous triaxial deformation combined with a simple shear.     

Stability conditions for elastic-plastic prandtl-reuss solids

Summary The algebraic condition ensuring the validity of Hadamard's criterion of local stability in a Prandtl-Reuss solid is examined. Four fundamental situations depending on the possible transitions of state are considered.

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Sommario Si esaminano le condizioni algebriche assicuranti la validità del criterio di Hadamard di stabilità locale in un solido di tipo Prandtl-Reuss. Quattro situazioni fondamentali dipendenti dalle possibili permanenze o conversioni di stato sono considerate.

     

Physical invariants for nonlinear orthotropic solids

Seven invariants, with immediate physical interpretation, are proposed for the strain energy function of nonlinear orthotropic elastic solids. Three of the seven invariants are the principal stretch ratios and the other four are squares of the dot product between the two preferred directions and two principal directions of the right stretch tensor. A strain energy function, expressed in terms of these invariants, has a symmetrical property almost similar to that of an isotropic elastic solid written in terms of principal stretches. Ground state and stress–strain relations are given. Using principal axes techniques, the formulation is applied, with mathematical simplicity, to several types of deformations. In simple shear, a necessary and sufficient condition is given for Poynting relation and two novel deformation-dependent universal relations are formulated. Using series expansions and the symmetrical property, the proposed general strain energy function is refined to a particular general form. A type of strain energy function, where the ground state constants are written explicitly, is proposed. Some advantages of this type of function are indicated. An experimental advantage is demonstrated by showing a simple triaxial test can vary a single invariant while keeping the remaining invariants fixed.     

Minimum principles for incompressible viscoelastic solids

Summary Four minimum principles are established, governing the quasi-static deformations of linear, isotropic, incompressible viscoelastic solids.

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Sommario Si stabiliscono quattro principi di minimo che governano le deformazioni quasi statiche di solidi viscoelastici, lineari, isotropici e incompressibili.