Mechanics of Advanced Composite Structures 5 (2018) 187 - 198
Mechanics of Advanced Composite Structures
journal homepage: http://MACS.journals.semnan.ac.ir
Thermoelastic Interaction in a Three-Dimensional Layered Sandwich Structure
A. Sur*, M. Kanoria
Department of Applied Mathematics, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India
Received in revised form
The present article investigates the thermoelastic interaction in a three-dimensional homogeneous and isotropic sandwich structure using the dual-phase-lag (DPL) model of generalized thermoelasticity. The incorporated resulting non-dimensional coupled equations are applied to a specific problem in which a sandwich layer of unidentical homogeneous and isotropic substances is subjected to time-dependent thermal loadings; the two outer sides are traction-free. The analytical expressions for the displacement components, stress, temperature, and strain are obtained in the physical domain using the normal mode analysis. The mathematical difficulties in dealing with the hyperbolic heat conduction equation are overcome and the thermophysical quantities of the sandwich structure are depicted graphically. The effect that the two phase lags have on the studied field are highlighted. The results demonstrate the phenomenon of a finite speed of wave propagation in a sandwich structure for each field.
Hyperbolic heat conduction
Finite wave speed
Normal mode analysis
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The conventional dynamic theory of thermoelasticity is based on the hypothesis of Fourier’s law of heat conduction in which the temperature distribution is governed by a parabolic-type partial differential equation. Thus, the theory predicts that a thermal signal is felt instantaneously everywhere in a body. This implies that there is an infinite speed of propagation of the thermal signal, which is unrealistic from a physical point of view, especially for short-time responses. It is also well known that heat transmission at low temperatures propagates by means of waves. These aspects have aroused much interest and activity in the field of heat propagation and given rise to the subject of generalized thermoelasticity. Generalized thermoelasticity theories involve hyperbolic-type governing equations and the finite speed of thermal signals. Lord and Shulman  proposed an extended thermoelasticity theory that involves one thermal relaxation time. Green and Lindsay  proposed a temperature-rate-dependent theory of thermoelasticity that includes two relaxation times. These generalized models are familiar to many researchers and numerous studies have been done using these theories.
Green and Naghdi [3-5] introduced three models, which are subsequently referred to as models I, II, and III, to provide sufficient basic modifications in the constitutive equations in order to permit the treatment of a wide class of heat flow problems. The Green and Naghdi models include a term called the thermal displacement gradient among the independent constitutive variables. Although the thermal wave model can capture the microscale response in time, it does not capture the microscale response in space . Therefore, the validity of the thermal wave model becomes debatable for the fast transient responses with respect to microstructural interactions . To remove the precedence assumption implied by the thermal wave model, the dual-phase-lag (DPL) model of heat conduction was developed  and verified . The model accounts for the spatial and temporal effects of both macroscale and microscale heat transfer for one temperature formulation with the form: where is the phase lag of the temperature gradient. In the DPL model with , the temperature gradient within the medium induces heat flux; hence, the temperature gradient is the cause of the energy transport and the heat flux is the effect. However, for , the heat flux is the cause of the energy transport and the temperature gradient is the effect. For with a homogeneous initial temperature, the DPL model reduces to Fourier’s law .
Prasad, Kumar, and Mukhopadhyay  studied the propagation of finite thermal waves in the context of the DPL model. El-Karamany and Ezzat  recently solved some remarkable problems using the DPL model. Chiriţă  studied the time-differential DPL model. Sur, Pal and Kanoria [13–16] produced several remarkable works on finite thermal wave propagations in generalized thermoelasticity.
Modern structural elements are often subjected to temperature changes of such magnitude that their material properties are considered to no longer have constant values . Because the thermal and mechanical properties of materials vary with temperature, it must be taken into consideration during the thermal stress analysis of these materials [18-22].
One of the major subjects of the mechanics of multilayered composites is the elaboration of the mathematical modeling, methods, and algorithms used for the numerical solutions of engineering problems. One class of interesting elastodynamic problems has a wide range of applications not only in the mechanics of composite materials, but also in other branches of modern engineering. Therefore, a large number of theoretical and computational investigations have been reported in this field, and a systematic review of the obtained results is available in a series of monographs [23–25].
It is widely accepted that an inappropriate selection of interface elements can lead to regions of unrealistically high stress gradients and erroneous results. Considering the fact that many media are naturally layered, some researchers have shifted their focus to the study of thermoelastic problems in a multilayered medium. Vishwakarma  reported on the torsional wave propagation in a reinforced sandwich medium. Tokovyy and Ma  presented an analytical solution for the axisymmetric and transient-thermoelastic problem of body forces and a heat source in vertically inhomogeneous media. Wu  proposed an analytical theory of the nonlinear unstable pavement temperature fields for a 2D layered composite structure; the theory was based on the fundamental principles of meteorology thermodynamics. The exact solution of the transient analysis of a multilayered magneto-electro-thermoelastic strip subjected to a nonuniform heat supply has also been reported . Zhong and Geng  demonstrated the solutions of thermal stress problems of a multilayered elastic half-space using the transfer matrix method. Sur and Kanoria  studied the problem of generalized thermoelasticity for different composite structures.
The objective of this paper is to consider three-dimensional thermoelastic layers of unidentical substances, each of which is homogeneous and isotropic. The outer surfaces of the medium are free of tractions and are subjected to time-dependent thermal loadings. The heat conduction equation has been formulated to incorporate the DPL model of heat conduction. Using the normal mode analysis, the governing equations have been expressed in Cartesian coordinates and are applied to a thermal shock problem in a composite structure that fills the half-space. The numerical estimates of the thermophysical quantities have been computed for copper and stainless steel, and are depicted graphically. To the best of the authors’ knowledge, this is the first time that a problem based on DPL heat conduction has been modeled. Excellent predictive capability is demonstrated in the layered composite structure due to the DPL heat conduction model.
The stress-strain-temperature relations are as follows:
where is the stress tensor, is the temperature field, ∆ the cubical dilation, and are Lamé's constants, , is the coefficient of linear thermal expansion, and is the Kronecker delta. In addition, and is the strain tensor given by:
The stress equation of motion in the absence of body force is
The heat equation for the dynamic coupled generalized thermoelasticity based on the DPL thermoelasticity model is given by:
where is the displacement components, is the density, and is the specific heat at constant strain.
We now consider an isotropic, homogeneous, and thermoelastic layered medium of a sandwich structure with a three-dimensional space that fills the region , which is defined as where layers I and III are made from same material and layer II is a different material. Layer II is in the middle of the space and its thickness is half of the entire thickness. We consider that the outer sides of the medium are thermally shocked and traction-free. The rectangular Cartesian system is used in which the origin is on the surface . The components of the displacement vector are given as .
The constitutive relations are as follows:
The equations of motion in the absence of body forces are
The heat conduction equation for the DPL model is given by:
where . Neglecting the term , we have the Lord–Shulman (LS) heat conduction model.
Equations (11)–(13) can be rewritten in the following form:
The nondimensional variables are as follows:
After removing the primes, the equations can be written in a nondimensional form as follows:
In a similar manner, we can transform the constitutive relations into nondimensional forms. The dimensionless expressions for the constitutive are obtained by summation of Equations (18) – (20) as follows:
We shall consider the invariant stress to be the mean value of the normal stresses as follows:
Substituting the expressions for , , and into the above expressions, we obtain
In this method, the solutions of the physical variables can be decomposed in terms of the normal modes in the following form:
where , , , , and are the amplitudes of the functions , is the angular frequency, and and are the wave numbers in the and directions, respectively.
By rewriting Equations (21) - (23) based on the normal modes and eliminating from the resulting expressions, we obtain a system of ordinary differential equations:
Eliminating from Equations (25) and (26) yields the following fourth-order differential equation:
We consider the layered plates in a sandwich structure in which layers I and III are made from the same material and layer II is a different material. Layer II is placed in the middle of the plane; its thickness is equal to half thickness of the plate. We consider that the two outer sides of the sandwich structure are subjected to thermal loading and are traction-free.
(i) Region I : The solutions of Equations (25) and (26) take the following form:
where and and are the roots of the equation
(ii) Region II : The solutions of Equations (25) and (26) take the following form:
where and are the roots of the equation
(iii) Region III : The solutions of Equations (25) and (26) take the following form:
5.1. Thermal Boundary Condition
We suppose that a thermal load is applied to the medium in the two outer sides:
5.2 Mechanical Boundary Condition
We consider that the normal stresses disappear on the two sides of the medium. For example,
5.3 Continuity Conditions of Heat Flux
The continuity conditions of heat flux are given as
The conditions in Equation (38) can take the following form:
where the heat flux is subjected to the DPL model of heat conduction. For example,
Employing the normal mode analysis, heat flux takes the following form:
5.4 Continuity Conditions of Stresses
The continuity conditions of stresses are given as
Applying the above conditions, the stress and temperatures in regions I, II, and III are obtained as follows:
where Using Equation (23), we can determine the strain component as follows:
Furthermore, using Equation (18), we can determine the displacement component as follows:
The aim of this section is to illustrate the results obtained in the preceding sections. First, we present the analytical numerical results. For the numerical computations, we have considered a copper-like material for layers I and III, and stainless steel for layer II. Since is the complex time constant, we have then . The values of the material constants for the copper (layers I and III) are
The values of the material constants for stainless steel (layer II) are
Furthermore, the values of the other non-dimensional parameters are Figures 1–4 have been plotted to study the variation of the temperature, stress, strain, and displacement against the distance for the DPL model at when the depths of the layer are at and , respectively. In the figures, the continuous lines correspond to , while the dotted lines correspond to .
Figure 2 depicts the variation of the temperature versus distance for different depths at and . As the temperature attains its maximum magnitude on the two outer sides of the sandwich structure to satisfy the thermal boundary conditions of the problem, the magnitude of decreases sharply toward layer II of the sandwich structure at . In addition, the magnitude of for regions I and III (copper) is greater than the magnitude of in region II (stainless steel). Furthermore, at depth , the magnitude of decreases slowly compared with the rate at . As time increases, the magnitude of the temperature also increases; however, as the depth of the composite medium increases, the magnitude of decreases.
Figure 1. Geometry of the Problem
Figure 2. Variation of temperature against for the DPL model at
Figure 3. Variation of stress against for the DPL model at
Fig. 3 shows the variation of the stress against the distance at and for different depths of the sandwich structure ( and , respectively). The stress has a value of zero on both outer sides of the sandwich structure, which satisfies the mechanical boundary condition of the problem given in Equation (37). Furthermore, the stress is compressive in nature near the two planes of application of thermal loading for different depths of the medium. For different and , the magnitude of for stainless steel is less than the of the copper. As the depth of the layer increases, the magnitude of decreases throughout the body. In addition, an increase in time also increases the magnitude of .
Fig. 4 shows the variation of the strain against distance for different values of depth at . The magnitude of is greater for copper than for stainless steel. In addition, the strain is maximum near the two outer boundaries of the sandwich structure. The magnitude decreases sharply as we move toward layer II. With an increase in depth, the magnitude of elongation decreases.
Fig. 5 shows the variation of displacement against the distance for . The magnitude of the displacement increases at intervals and to attain the maximum magnitude near . The magnitude then decreases sharply as we move toward layer II when . Furthermore, the displacement magnitude almost negligible near the two outer surfaces of the sandwich structure at the interval
The figures show that at the interfaces of the composite structure because the medium satisfies the continuity conditions of heat flux and stress as given in Equations (38) and (39), respectively. Thus, the graphical representations of the thermophysical quantities are compatible with the continuity conditions at the interfaces.
Figs. 6–9 have been plotted to study the profile of variation of the thermophysical quantities for and at depth . The figures show that as time increases, the profile of the thermophysical quantities also increase, which is quite plausible.
Figs. 10–13 show the variation of the thermophysical quantities for the same set of parameters as mentioned earlier at . The figures show that as the depth increases, the magnitudes of the thermophysical quantities decrease. However, as time increases, the magnitudes increase.
Figs. 14 and 15 show the variation of the stress and displacement , respectively, against the thickness of the sandwich structure at depth and time for both the LS and the DPL models.
Fig. 14 shows that for both the LS and the DPL models, the value become zero on the two outer sides of the sandwich structure, which satisfies the mechanical boundary condition of the problem. Furthermore, for and , the magnitude of for the LS model is greater than in the DPL model.
Fig. 4. Variation of strain against for the DPL model at
Figure 5. Variation of displacement against for the DPL model when
Figure 6. Profile of temperature for the DPL model for different at
Figure 7. Profile of stress for the DPL model for different x and t at
Figure 8. Profile of strain for the DPL model for different x and t when
Figure 9. Profile of displacement for the DPL model for different x and t at
Figure 10. Profile of temperature for the DPL model for different x and t at
Figure 11. Profile of stress for the DPL model for different x and t at
Figure 12. Profile of strain for the DPL model for different x and t at
Fig. 15 shows the variation of the displacement for the same set of parameters as mentioned earlier. The magnitude of for the LS model is greater than in the DPL model for both earlier situations and later on.
Figure 13. Profile of displacement for the DPL model for different x and t at .
Figure 14. Variation of stress against for the DPL and LS models at
Figure 15. Variation of displacement against for the DPL and LS models at
Fig. 16 shows the variation of the stress component against the distance for the DPL model for . The temperature on both sides of the structure is considered to be constant. The stresses have a significant effect on the structure as observed in Fig. 11. The magnitude of the stress component is the same on both boundaries of the structure. The magnitude of the profile of the stress component increases as we move within the medium.
Figure 16. Variation of stress against for the DPL and LS models at
In this paper, a mathematical treatment has been presented to explore the wave propagation in a three-dimensional isotropic thermoelastic medium based on the DLP model. The LS model can be obtained as a particular case. The problem has been solved theoretically and exemplified through the use of the LS and DPL models. All the figures exhibit the different peculiarities that occur during the propagation of waves. The conclusions may be summarized as follows.
We are grateful to Prof. S. C. Bose of the Department of Applied Mathematics, University of Calcutta for his valuable suggestions and guidance in preparation of this paper. We also express our sincere thanks to the reviewers for their valuable suggestions on ways to improve the paper.
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