Compatibility Conditions in Continua with Microstrusture

Abstract. The present study is devoted to the statement of compatibility conditions on propagating wave surfaces of strong and weak discontinuities in thermoelastic continua with microstructure. The field formalism is used to study the problem. A natural density of thermoelastic action and the corresponding variational least action principle are stated for a varying domain. A special form of the first variation of the action is employed to obtain 4-covariant boundary conditions on the wave surfaces. These are given by jumps of the Piola–Kirchhoff stress 4-tensor and the energy–momentum tensor. Problems of propagation of weak discontinuities in type-II MPTE continua are considered. Geometrical and kinematical compatibility conditions are used to study possible wave surfaces of weak discontinuities. It is shown that the surfaces of weak discontinuities can propagate without weak discontinuities of the temperature displacement.


Introduction
Problems of micropolar continua take its origin from the classical E. and F. Cosserat's paper [1].Micropolar (MP) continuum theories include not only translational displacements but also additional degrees of freedom.These degrees of freedom are coupled with changes in reper (three directors) associated with microvolume.Such changes may be described by a rotation vector when reper associated with microvolume are rigid rotated.In contrary to conventional elasticity a continuum with microstucture is described by the asymmetric strain and stress tensors known from many previous discussions [2,3,6].Thus the asymmetric elastic theory is characterized by a comparatively large number of constitutive elastic constants need to be determined from the experimental observations.There are several phenomena (for example, additive manufacturing design [4,5], biomechanics, nematic liquid crystals [7,8] behavior, the anomalous piezoelectric effect in quartz, the dispersion of elastic waves, as well as a number of other experimentally observed elastic properties of the pure crystals) being beyond the scope of the conventional thermoelasticity (CTE) and piezoelectroelasticity, the finite propagation velocity of thermal waves known as second sound waves.
The phenomenon of the second sound is observed at very low temperatures in crystals of high purity.The observation of the second sound in the crystal NaF is discussed (see [9]).The second sound in the NaF is observed at a temperature of about 15K [10].The existence of second sound waves was also confirmed in the NaF (see [11] and [12]); the speed of the second sound waves is 1953.1m/Ssee [13].In the publication [14] the measurements of the second sound waves in bismuth (Bi) at a temperature of about 3K are described.The speed of the second sound waves in the bismuth is equal to 78 ± 5m/S .
The micropolar thermoelastic (MPTE) continuum may be described in terms of field formalism, for example, from positions of the Green-Naghdi thermoelasticity (GN-theory) [15,16].Now such mathematical frame-works of the thermoelastic behavior of solids are rapidly refined [17][18][19][20].They are based on different modifications of the classical Fourier law of heat conduction.GN-thermoelasticity theory itself can be subdivided into the three different types.Type-I theory (GNI/CTE thermoelasticity) is based on the classical Fourier law of heat conduction with an infinite velocity of propagation of an exponentially decaying heat signal.Type-II theory (GNII, hyperbolic thermoelasticity) is characterised by the energy conservation and the finite propagation velocity of thermal waves known as second sound waves.Type-III theory (GNIII) combines both type-I and type-II thermoelasticity.Thereby type-I and type-II theories are the limiting cases of type-III theory.GNIII-theory obviously encircles a wider range of phenomena, as compared with the classical Fourier heat conduction (CTE) theory.

Compatibility conditions for jumps
The mathematical models are to simultaneously fulfill the following frameworks: 1.) Finiteness of the heat signal propagation velocity, and 2.) The ability of the spatial propagation of the thermoelastic waves without attenuation, and 3.) Existence of distortionless wave forms akin to the classical d'Alembert type waves.
A field theory formalism involves mathematical description of physical fields by integral action functional.A general form of action within a variable domain of 4spacetime with the elementary volume of where k M is the physical fields array, L is the Lagrangian density.
The least action principle states that the actual field is realized in the spacetime in a way that the action of ( 1) is minimum, i.e. for any admissible variations of physical fields k M and non-variable coordinates E X are 0 = G .Then the classical Euler-Lagrange equations are held: In general, a conservation law has the following form , 0 w E E J where the vector E J is the generalized vector 4-current.

H G G
/ the 4-current can be obtained as if the variational symmetries of the action are known from previous considerations.The finite variations of action is (see [21]) Assuming the temperature field is continuous and temperature gradient of the first order can be discontinuous by passing through some bilateral surface Hereafter square brackets denote the jumps.
One can see, that the compatibility conditions (5) on strong discontinuities surfaces contain the jumps of the energy-momentum 4-tensor and the Piola-Kirchhoff 4tensor The compatibility conditions for jumps of the energymomentum 4-tensor in view of ( 4) is rewritten in form Equations ( 7) can be transformed by the compatibility conditions of the strong discontinuities of the Piola-Kirchhoff 4-tensor into The only significant equation from the compatibility conditions for jumps of the energy-momentum 4-tensor by using the Hadamard-Thomas geometric first order compatibility conditions [22] The three-dimensional form of the compatibility conditions on the surface of strong discontinuity of the field is derived from the resulting 4-covariant form (4):

, P O
The compatibility conditions for strong discontinuities are complemented by well known three-dimensional geometrical and kinematic Hadamard-Thomas compatibility conditions of the first and the second order [22] valid for an arbitrary field k ; the prime in superscripts are denoted by the Greek indices related to surface Gaussian coordinates the delta-derivative operator.Those are also due to Rankine and Hugoniot [23,24].

Action for micropolar continua
The theory GNII is the only thermodynamically correct theory which satisfies the principles listed in Sec. 1.The action taking into account of the polar microstructure can be adopted in the following form [21], with following field variables: X α (α = 1, 2, 3) are the Lagrangian coordinates; x j ( j = 1, 2, 3) are the Eulerian coordinates;  -T ).Action density (1) we define in the form ). , , , The field equations in this case read: ), Compatibility conditions on strong discontinuity surface propagating in MPTE-II continuum, according to eqs. ( 9), (10), take the forms:

4
Governing equations of GNII continuum.Linear case A mathematical model of MPTE-II continuum (see [2] for details) is used throughout the paper.Such continua permit the existence of rotational degrees of freedom (a rotation vector φ) along with translational degrees of freedom (the translational displacement vector u).Thus the natural action density may be accepted in form where ρ is the mass density tensor, J is a tensor of inertia, \ is the free energy per unit of volume, T is the actual temperature, is the three-dimensional Hamiltonian differential operator (the nabla symbol), dot over a symbol denotes partial differentiation with respect to time at fixed spatial coordinates.The asymmetric strain tensor e and bending-torsion tensor Γ are associated with translational displacements u and microrotations φ by formulae For MPTE-II continuum, equations of motion of micropolar medium are written (according to eq. ( 13)) in direct tensor representation for the case of the absence of mass forces and mass moments where ε is the three-dimensional Levi-Civita symbol (permutation symbol, antisymmetric symbol, or alternating symbol).Equations ( 19) in rectangular co-ordinate system can be deduced in form The entropy balance equation are taken in form where s denotes the entropy referred to the unit of volume, j R is the entropy flux vector.The Clausius-Duhem inequality is satisfied by using the law of heat conduction, which states the proportionality of the heat velocity flux vector h and negative spatial temperature gradient T , =

T Λ h
where Λ is the material thermal conductivity tensor (thermal diffusion tensor).Λ is a positive definite matrix.
Since the free energy ) , , , ( is the function of the independent variables e , Γ , T ,then the constitutive equations ( 14) can be rewritten Therefore the constitutive equations of MPTE-II continuum can be derived according to equations ( 22) and ( 23) re e e (24) wherein λ, μ, η, γ, β, ε are isothermal constitutive constants of type-II micropolar thermoelastic (GNII) continuum; α, ς are constitutive constants providing coupling of equations of motion and heat conduction; Λ is the material thermal conductivity (thermal diffusion); κ is the heat capacity (per unit volume) at constant (zero) strains.Constants α, ς depend not only on the mechanical properties of the continuum, but also depend on the thermal properties.
Consequently the hyperbolic heat conduction equation can be derived by transforming equations ( 21) and ( 24) as After substituting the stress tensors σ and m from the formulae (24) in the equations of motion (19) and taking account of (17) the system of coupled partial differential equations of motion and heat conduction for a linear isotropic GNII continuum in the absence of mass forces, moments, and heat sources can be written as [2,6]: Hereafter θ is the temperature increment over the referential temperature.
As pointed out the nonzero constitutive constants α, ς provide coupling of micropolar thermoelasticity equations.It is usually assumed that ς = 0 (see [2]).This constitutive constant is kept in all further considerations for the completeness analysis.
The scalar equation in the system (26) is called a gen eralized heat conduction equation conjugate to the equations of motion (the first and the second equations in (26)).

Propagating weak discontinuities of thermomechanical fields in MPTE-II continuum
This section deals with the processes of propagating weak discontinuities of translational displacements u, microrotations φ and temperature θ in the MPTE-II continuum.System of partial differential equations (26) includes partial derivative order not higher than the second.Let a wavefront (wave surface Σ) of weak discontinuities displacements u, microrotations φ and temperature θ be propagated in three-dimensional space  Therefore, kinematical and geometrical compatibility conditions of the second and the first order according to eqs. ( 11) and ( 27) are read are fields defined on this surface, and the equalities 0 = S 0, = A 0, = B cannot be satisfied simultaneously at any point of the surface, if the surface is indeed the surface of weak discontinuities.Equations ( 26) and (28) give the following relations between the jumps of partial derivatives of the second order across the wave surface: The polarization vectors of the wave A, S can be splited into a sum of projections onto the tangent plane and on the normal direction to the wave surface: where τ is the tangential unit vector and n is the normal unit one of the wave surface respectively.Taking account of equations (30) the system (29) is transformed into After rearrangements in equations (31) in view of (30) the following system is derived: We proceed to consider the discriminated cases separately.Evidently in the case A = 0, S = 0 and returning to system (29) the scalar equation in ( 29) is satisfied identically, so the surface Σ is actually not a surface of weak discontinuities.
The propagation of transverse waves in the MPTE-II continuum is possible in the following cases: In the first case a weak discontinuity of translational displacements exists only on the surface propagating with the velocity In third case the propagation of wave surfaces of weak discontinuities displacements, microrotations and temperature is impossible if the constitutive characteristic of the type-II micropolar thermoelastic continuum does not satisfied certain limitations as determined by (32), i.e.

Conclusions
In the present study problems of propagating surfaces of discontinuities of displacements, microrotation and thermal displacements of the micropolar type-II thermoelastic media (GNII) has been considered.The dynamical and constitutive equations for hyperbolic thermoelastic type-II micropolar continuum have been derived by field theory technique and formalism of the variational calculus.The special form of the first variation of the action integral has been used us in order to obtained 4-covariant jump conditions on wave surfaces.
The three-dimensional form of the jump conditions on the surface of a strong discontinuity of field can be derived as the result of its four-dimensional co-variant form.The possible normal velocities of the prop-agating surfaces of weak discontinuities are found.The possible normal velocities of the propagating surfaces of weak discontinuities have been found.The distinguished cases of propagating wave surfaces in MPTE-II continuum have been separately considered.Propagating surfaces of weak discontinuities have been discriminated depending on the spatial orientations of polarization vectors of discontinuities.

r 6
propagating with the normal velocity G and normal unit 4-vector E N in 4-spacetime.We replace the integral over 4-volume in equation (3) on sum of surface integrals.Then only two surface integrals over surface 6 are remained when the variations k

.
The equation 0 = G is valid for the actual field and a variations of k M G and D G X are continuous in passing through the surface 6 .Therefore following 4-covariant compatibility conditions for strong discontinuities is obtained from equation (4): j a d (a = 1, 2, 3) are micropolar directors associated with microvolume;is the thermal displacement field coupled with temperature T by equation (

I
is the microinertion tensor; ij U is the mass density tensor; \ is the volume density of the Helmholtz's free energy.For tensors ij ab

P
this notation the energy-momentum 4-tensor is rewritten in following forms is the Eshelby stress tensor, P * is the Umov-Poynting vector, O P is pseudo-momentum vector, H is quasidensity of the Hamiltonian.
the free energy \ into the Taylor series in the vicinity of the natural state 0 of higher order than the second one.The following form of the expansion is obtained for isotropic, homogeneous and centrosymmetric bodies

g and second DE b fundamental forms of surface 6
in (31) allows us to couple a weak discontinuity of temperature displacement with a normal projection of weak discontinuities of translational displacements and microrotations by .

cc
It can be elucidated that the velocity G (34) exactly coincides with the one of transverse elastic wave if the constitutive micropolar constant η equals to zero case, the third equation of the system (32) allows to compute the propagation velocity of a weak discontinuities Focus attention that a weak discontinuity of temperature is not associated with the tangential projections of polarization vectors of weak discontinuities A⊥ and S⊥.
In fourth case the wave surface propagation is impossible if the normal projections of discontinuities polarization vectors are equal to zero.Thus equations (32) can be transformed into are only two cases of transverse wave propagation in MPTE-II continuum:In second case the propagation of weak discontinuities surfaces is impossible if the constitutive characteristic of the type-II micropolar thermoelastic continuum does not satisfied certain limitations as determined by (32), i.e.