Thermomechanics of Swelling Biopolymeric Systems

A two-scale theory for the swelling biopolymeric media is developed. At the microscale, the solid polymeric matrix interacts with the solvent through surface contact. The relaxation processes within the polymeric matrix are incorporated by modeling the solid phase as viscoelastic and the solvent phase as viscous at the mesoscale. We obtain novel equations for the total stress tensor, chemical potential of the solid phase, heat flux and the generalized Darcy's law all at the mesoscale. The constitutive relations are more general than those previously developed for the swelling colloids. The generalized Darcy's law could be used for modeling non-Fickian fluid transport over a wide range of liquid contents. The form of the generalized Fick's law is similar to that obtained in earlier works involving colloids. Using two-variable expansions, thermal gradients are coupled with the strain rate tensor for the solid phase and the deformation rate tensor for the liquid phase. This makes the experimental determination of the material coefficients easier and less ambiguous.     

Thermomechanics of solids: Nonequilibrium and irreversibility

Reconciliation of thermal and mechanical interdependence is made possible by casting away the concept of heat. Defined are the dissipation and available energy density as two mutually exclusive quantities; thermal and mechanical changes become two aspects of the same process that coexist and are in continual operation. This is accomplished by interlacing the rotation, deformation and change in element size into one single operation; irreversibility is embedded inherently into the theory. No a priori assumption needs to be made on the constitutive relations which are, in fact, derived for each individual element and time increment. Only the initial slope of the reference material and load history need to be specified. Instead, the surface and volume energy density are assumed to be exchangeable without letting the change of volume with surface area to vanish in the limit, a simplification of classical physics and continuum mechanics that results in the decoupling of thermal and mechanical effects. Complete nonlinearity and finiteness of deformation are retained such that boundary problems can be solved directly by specifying the tractions and/or displacements. Nonequilibrium/irreversible solutions are shown to possess definite limits and to be bounded by the equilibrium/irreversible states whose solutions are proved to be unique. The existence of the isoenergy density function provides an elegant means of resolving the multidimensionality of the problem; the translation of unidimensional data to multidimensional states.     

Electrodynamics and Thermomechanics of Material Bodies

We consider a material body emersed in space and, relative to an aether frame, the balance laws which govern its interaction with an electromagnetic, thermal and mechanical environment. A detailed formulation of a non-relativistic theory for studying thermomechanical–electromagnetic processes in deformable media is presented and certain invariance issues are discussed. The idea of an isolated process in a given aether frame is introduced and we identify a related non-increasing Lyapunov function for such processes. This function suggests the structure of a class of minimization problems within the statical theory and we discuss a typical problem within the area of elastic dielectrics.      

Thermomechanics of shells undergoing phase transition

The resultant, two-dimensional thermomechanics of shells undergoing diffusionless, displacive phase transitions of martensitic type of the shell material is developed. In particular, we extend the resultant surface entropy inequality by introducing two temperature fields on the shell base surface: the referential mean temperature and its deviation, with corresponding dual fields: the referential entropy and its deviation. Additionally, several extra surface fields related to the deviation fields are introduced to assure that the resultant surface entropy inequality be direct implication of the entropy inequality of continuum thermomechanics. The corresponding constitutive equations for thermoelastic and thermoviscoelastic shells of differential type are worked out. Within this formulation of shell thermomechanics, we also derive the thermodynamic continuity condition along the curvilinear phase interface and propose the kinetic equation allowing one to determine position and quasistatic motion of the interface relative to the base surface. The theoretical model is illustrated by two axisymmetric numerical examples of stretching and bending of the circular plate undergoing phase transition within the range of small deformations.     

Thermomechanics of a heterogeneous fluctuating chain

In this paper we present a theory to efficiently calculate the thermo-mechanical properties of fluctuating heterogeneous rods and chains. The central problem is to evaluate the partition function and free energy of a general heterogeneous chain under the assumption that its energy can be expressed as a quadratic function in the kinematic variables that characterize the configurations of the chain. We analyze the effects of various types of boundary conditions on the fluctuations of the rods and chains and show that our results are in agreement with recent work on homogeneous rods. The results for the heterogeneous chains are verified through Monte Carlo simulations. Finally, we consider a special heterogeneous chain with only two bending moduli and use it as a model to interpret experiments on partially unfolded protein oligomers.     

Thermomechanics of volumetric growth in uniform bodies

A theory of material growth (mass creation and resorption) is presented in which growth is viewed as a local rearrangement of material inhomogeneities described by means of first- and second-order uniformity “transplants”. An essential role is played by the balance of canonical (material) momentum where the flux is none other than the so-called Eshelby material stress tensor. The corresponding irreversible thermodynamics is expanded. If the constitutive theory of basically elastic materials is only first-order in gradients, diffusion of mass growth cannot be accommodated, and volumetric growth then is essentially governed by the inhomogeneity velocity “gradient” (first-order transplant theory) while the driving force of irreversible growth is the Eshelby stress or, more precisely, the “Mandel” stress, although the possible influence of “elastic” strain and its time rate is not ruled out. The application of various invariance requirements leads to a rather simple and reasonable evolution law for the transplant. In the second-order theory which allows for growth diffusion, a second-order inhomogeneity tensor needs to be introduced but a special theory can be devised where the time evolution of the second-order transplant can be entirely dictated by that of the first-order one, thus avoiding insuperable complications. Differential geometric aspects are developed where needed.     

Thermomechanics of elastoplastic and superplastic deformation of metals

Using the process theory of A. A. Il’yushin, we consider the problem of determining the thermomechanical parameters of a material element for specified deformation and temperature-variation processes with allowance for the elastic, plastic, and viscous properties of superplastic deformation. The relations obtained are applicable for the case of arbitrary stresses and finite strains. The strain and stress measures are decomposed into elastic, plastic, and viscous components by classifying the processes into reversible, irreversible equilibrium, and nonequilibrium processes. Tula State University, Tula 300600. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 164–172, September–October, 1999.     

Thermomechanics of solids with general imperfect coherent interfaces

The objective of this contribution is to develop a thermodynamically consistent theory for general imperfect coherent interfaces in view of their thermomechanical behavior and to establish a unified computational framework to model all classes of such interfaces using the finite element method. Conventionally, imperfect interfaces with respect to their thermal behavior are often restricted to being either highly conducting (HC) or lowly conducting (LC) also known as Kapitza. The interface model here is general imperfect in the sense that it allows for a jump of the temperature as well as for a jump of the normal heat flux across the interface. Clearly, in extreme cases, the current model simplifies to HC and LC interfaces. A new characteristic of the general imperfect interface is that the interface temperature is an independent degree of freedom and, in general, is not a function of only temperatures across the interface. The interface temperature, however, must be computed using a new interface material parameter, i.e., the sensitivity. It is shown that according to the second law, the interface temperature may not necessarily be the average of (or even between) the temperatures across the interface. In particular, even if the temperature jump at the interface vanishes, the interface temperature may be different from the temperatures across the interface. This finding allows for a better, and somewhat novel, understanding of HC interfaces. That is, a HC interface implies, but is not implied by, the vanishing temperature jump across the interface. The problem is formulated such that all types of interfaces are derived from a general imperfect interface model, and therefore, we establish a unified finite element framework to model all classes of interfaces for general transient problems. Full details of the novel numerical scheme are provided. Key features of the problem are then elucidated via a series of three-dimensional numerical examples. Finally, we recall since the influence of interfaces on the overall response of a body increases as the scale of the problem decreases, this contribution has certain applications to nano-composites and also thermal interface materials.     

Thermomechanics of the interface between a body and its environment

We formulate integral statements of force balance, energy balance, and entropy imbalance for an interface between a body and its environment. These statements account for interfacial energy, entropy, and stress but neglect the inertia of the interface. Our final results consist of boundary conditions describing thermomechanical interactions between the body and its environment. In their most general forms, these results are partial differential equations that account for dissipation and encompass as special cases Navier’s slip law, Newton’s law of cooling, and Kirchhoff’s law of radiation. When dissipation is neglected, our results reduce to the well-known zero-slip, free-surface, zero-shear, prescribed temperature, and flux-free conditions. Dedicated to James K. Knowles: teacher, colleague, friend     

Thermomechanics of transformation pseudoelasticity and shape memory effect in alloys

Transformation pseudoelasticity and shape memory effect of alloy materials are investigated from the thermomechanical point of view. The thermomechanical constitutive equations and the kinetics of transformation established by the theory are applied to explain the stress-strain-temperature behavior of the material. Numerical illustrations for the uniaxial stress state are given.     

Energy Approach to the Formulation of Boundary Problems of Nonlinear Thermomechanics for Elastic Systems

An energy approach is proposed to derive the physical constitutive equations of nonlinear thermomechanics for inertial elastic systems. A potential of local inertial thermodynamic state and a potential of thermoelastic energy dissipation are introduced. The variational formulation of nonlinear boundary problems of thermoelasticity is implemented on the basis of the Hamiltonian energy functional. Sufficient conditions for the convexity of the functional are formulated __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 9, pp. 52–59, September 2005.     

Thermomechanics of shape memory polymers: Uniaxial experiments and constitutive modeling

Shape memory polymers (SMPs) can retain a temporary shape after pre-deformation at an elevated temperature and subsequent cooling to a lower temperature. When reheated, the original shape can be recovered. Relatively little work in the literature has addressed the constitutive modeling of the unique thermomechanical coupling in SMPs. Constitutive models are critical for predicting the deformation and recovery of SMPs under a range of different constraints. In this study, the thermomechanics of shape storage and recovery of an epoxy resin is systematically investigated for small strains (within ±10%) in uniaxial tension and uniaxial compression. After initial pre-deformation at a high temperature, the strain is held constant for shape storage while the stress evolution is monitored. Three cases of heated recovery are selected: unconstrained free strain recovery, stress recovery under full constraint at the pre-deformation strain level (no low temperature unloading), and stress recovery under full constraint at a strain level fixed at a low temperature (low temperature unloading). The free strain recovery results indicate that the polymer can fully recover the original shape when reheated above its glass transition temperature (T_g). Due to the high stiffness in the glassy state (T < T_g), the evolution of the stress under strain constraint is strongly influenced by thermal expansion of the polymer. The relationship between the final recoverable stress and strain is governed by the stress–strain response of the polymer above T_g. Based on the experimental results and the molecular mechanism of shape memory, a three-dimensional small-strain internal state variable constitutive model is developed. The model quantifies the storage and release of the entropic deformation during thermomechanical processes. The fraction of the material freezing a temporary entropy state is a function of temperature, which can be determined by fitting the free strain recovery response. A free energy function for the model is formulated and thermodynamic consistency is ensured. The model can predict the stress evolution of the uniaxial experimental results. The model captures differences in the tensile and compressive recovery responses caused by thermal expansion. The model is used to explore strain and stress recovery responses under various flexible external constraints that would be encountered in applications of SMPs.     

Thermomechanics of Porous Media - I: Thermohydraulic Model for Compacted Bentonite

A general thermomechanical model is derived for a mixture. The model describes the behavior of the mixture via proper choices of free energy and dissipation function. A model for any combination of the mixture constituents can be reduced from the general model. The theory is applied to a thermohydraulic model for a mixture of compacted bentonite, liquid water, vapor, and air with the assumption of rigid skeleton and constant uniform porosity. The free energy of the system is chosen to take into account the individual nondissipative behaviors of the constituents and their mutual interactions, namely, adsorption and mixing of the gaseous constituents. The choices for the interaction terms are based on the equilibrium conditions for the water species in different combinations of the constituents. The resulting thermodynamically consistent macroscopic model is fitted to a suction experiment and applied to a simple one-dimensional thermohydraulic simulation of the bentonite buffer of the Febex in situ test. The results calculated with finite element method are successfully compared to measurements.