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1.Introduction A.M.Abd-Alla, S.M.Abo-Dahab, andF.S.Bayones RayleighWavesinGeneralizedMagneto-Thermo-ViscoelasticGranularMediumundertheInfluenceofRotation,GravityField,andInitialStress ResearchArticle

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Volume 2011, Article ID 763429,47pages doi:10.1155/2011/763429

Research Article

Rayleigh Waves in Generalized

Magneto-Thermo-Viscoelastic Granular Medium under the Influence of Rotation, Gravity Field, and Initial Stress

A. M. Abd-Alla,

1

S. M. Abo-Dahab,

1, 2

and F. S. Bayones

3

1Mathematics Department, Faculty of Science, Taif University, Taif 21974, Saudi Arabia

2Mathematics Department, Faculty of Science, South Valley University, Qena 83523, Egypt

3Mathematics Department, Faculty of Science, Umm Al-Qura University, P.O. Box 10109, Makkah 13401, Saudi Arabia

Correspondence should be addressed to S. M. Abo-Dahab,sdahb@yahoo.com Received 4 December 2010; Revised 14 January 2011; Accepted 25 February 2011 Academic Editor: Ezzat G. Bakhoum

Copyrightq2011 A. M. Abd-Alla et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The surface waves propagation in generalized magneto-thermo-viscoelastic granular medium subjected to continuous boundary conditions has been investigated. In addition, it is also subjected to thermal boundary conditions. The solution of the more general equations are obtained for thermoelastic coupling. The frequency equation of Rayleigh waves is obtained in the form of a determinant containing a term involving the coefficient of friction of a granular media which determines Rayleigh waves velocity as a real part and the attenuation coefficient as an imaginary part, and the effects of rotation, magnetic field, initial stress, viscosity, and gravity field on Rayleigh waves velocity and attenuation coefficient of surface waves have been studied in detail. Dispersion curves are computed numerically for a specific model and presented graphically. Some special cases have also been deduced. The results indicate that the effect of rotation, magnetic field, initial stress, and gravity field is very pronounced.

1. Introduction

The dynamical problem in granular media of generalized magneto-thermoelastic waves has been studied in recent times, necessitated by its possible applications in soil mechanics, earthquake science, geophysics, mining engineering, and plasma physics, and so forth. The granular medium under consideration is a discontinuous one and is composed of numerous large or small grains. Unlike a continuous body each element or grain cannot only translate

(2)

but also rotate about its center of gravity. This motion is the characteristic of the medium and has an important effect upon the equations of motion to produce internal friction. It was assumed that the medium contains so many grains that they will never be separated from each other during the deformation and that each grain has perfect thermoelasticity. The effect of the granular media on dynamics was pointed out by Oshima1. The dynamical problem of a generalized thermoelastic granular infinite cylinder under initial stress has been illustrated by El-Naggar2. Rayleigh wave propagation of thermoelasticity or generalized thermoelasticity was pointed out by Dawan and Chakraporty 3. Rayleigh waves in a magnetoelastic material under the influence of initial stress and a gravity field were discussed by Abd-Alla et al.4and El-Naggar et al.5.

Rayleigh waves in a thermoelastic granular medium under initial stress on the prop- agation of waves in granular medium are discussed by Ahmed6. Abd-Alla and Ahmed 7 discussed the problem of Rayleigh wave propagation in an orthotropic medium under gravity and initial stress. Magneto-thermoelastic problem in rotating nonhomogeneous orthotropic hollow cylinder under the hyperbolic heat conduction model is discussed by Abd-Alla and Mahmoud8. Wave propagation in a generalized thermoelastic solid cylinder of arbitrary cross-section is discussed by Venkatesan and Ponnusamy 9. Some problems discussed the effect of rotation of different materials. Thermoelastic wave propagation in a rotating elastic medium without energy dissipation was studied by Roychoudhuri and Bandyopadhyay 10. Sharma and Grover 11 studied the body wave propagation in rotating thermoelastic media. Thermal stresses in a rotating nonhomogeneous orthotropic hollow cylinder were discussed by El-Naggar et al.12. Abd-El-Salam et al.13investigated the numerical solution of magneto-thermoelastic problem nonhomogeneous isotropic material.

In this paper, the effect of magnetic field, rotation, thermal relaxation time, gravity field, viscosity, and initial stress on propagation of Rayleigh waves in a thermoelastic granular medium is discussed. General solution is obtained by using Lame’s potential. The frequency equation of Rayleigh waves is obtained in the form of a determinant. Some special cases have also been deduced. Dispersion curves are computed numerically for a specific model and presented graphically. The results indicate that the effect of rotation, magnetic field, initial stress, and gravity field are very pronounced.

2. Formulation of the Problem

Let us consider a system of orthogonal Cartesian axes, Oxyz, with the interface and the free surface of the granular layer resting on the granular half space of different materials being the planesz K and z 0, respectively. The origin O is any point on the free surface, thez-axis is positive along the direction towards the exterior of the half space, and the x- axis is positive along the direction of Rayleigh waves propagation. Both media are under initial compression stressPalong thex-direction and the primary magnetic field−−→

H0acting ony-axis, as well as the gravity field and incremental thermal stresses, as shown inFigure 1.

The state of deformation in the granular medium is described by the displacement vector

Uu, o, wof the center of gravity of a grain and the rotation vector→−

ξξ, η, ζof the grain about its center of gravity. The elastic medium is rotating uniformly with an angular velocity Ω Ωn, where n is a unit vector representing the direction of the axis of rotation. The displacement equation of motion in the rotating frame has two additional terms,Ω×Ω×u

(3)

z=K O

z Ω Granular

layer

y P P

−−→ x H0

z=0

Granular half space

g

Figure 1: Depiction of the problem.

is the centripetal acceleration due to time varying motion only, and 2→− Ω×→−

u is the Coriolis acceleration, andΩ 0,Ω,0.

The electromagnetic field is governed by Maxwell equations, under the consideration that the medium is a perfect electric conductor taking into account the absence of the displacement currentSI see the work of Mukhopadhyay14:

J curl→− h,

−μe→− h

∂t curl→− E, div→−

h0, div→−

E0,

E−μe

→−u

∂t ×−→

H

,

2.1

where

hcurl→−u×−−→

H0

, −→

H−−→

H0→− h, −−→

H0 0, H0,0, 2.2

where→−

h is the perturbed magnetic field over the primary magnetic field vector,→− E is the electric intensity,→−

J is the electric current density,μeis the magnetic permeability,−−→

H0 is the constant primary magnetic field vector, and→−uis the displacement vector.

The stress and stress couple may be taken to be nonsymmetric, that is, τijji, Mij/Mji. The stress tensorτijcan be expressed as the sum of symmetric and antisymmetric tensors

τijσijσij, 2.3

(4)

where

σij 1 2

τijτji

, σij 1 2

τijτji

. 2.4

The symmetric tensorσijσjiis related to the symmetric strain tensor

eijeji 1 2

∂ui

∂xj ∂uj

∂xi

. 2.5

The antisymmetric stressσij are given by

σ23 −F∂ξ

∂t, σ31 −F∂η

∂t, σ12 −F∂ζ

∂t, σ11 σ22σ33 0, 2.6 whereF is the coefficient of friction between the individual grains. The stress coupleMij is given by

Mijij, 2.7

where,Mis the third elastic constant,M11, M13, M33, and so forth, are the components of the resultant acting on a surface.

The non-symmetric strain tensorνijis defined as

ν11 ∂ξ

∂x, ν31 ∂ξ

∂z, ν33 ∂ζ

∂z, ν21 ν22ν230, ν12

∂x

ω2η

, ν32

∂z

ω2η

, ν13 ∂ζ

∂x,

2.8

whereω2 1/2∂u/∂z−∂w/∂x.

The dynamic equation of motion, if the magnetic field and rotation are added, can be written as15

τji,jFiρ ••

ui →− Ω×→−

Ω× −→u

i

2→− Ω×→−u

i

, i, j1,2,3. 2.9

The heat conduction equation is given by16

K∇2T ρs∂

∂t

1τ2

∂t

Tγ T0

∂t

1τ2δ∂

∂t

∇ · −→u, 2.10

whereρis density of the material,Kis thermal conductivity, s is specific heat of the material per unit mass, τ1, τ2 are thermal relaxation parameter, αt is coefficient of linear thermal expansion,λ andμare Lame’s elastic constants,θis the absolute temperature,γαt3λ2μ,

(5)

T0 is reference temperature solid, T is temperature differenceθ−T0,τ0 is the mechanical relaxation time due to the viscosity, andτm 1τ0∂/∂t.

The components of stress in generalized thermoelastic medium are given by

σ11 τm

λp∂u

∂x τmλP∂w

∂zγ

1τ1

∂t

T, σ33τmλ∂u

∂xτm

λ∂w

∂zγ

1τ1

∂t

T, σ13τmμ

∂u

∂z ∂w

∂x

.

2.11

If we neglect the thermal relaxation time, then2.11tends to Nowacki17and Biot18.

The Maxwell’s electro-magnetic stress tensorτijis given by τijμe

HihjHjhi−Hk·hkδij

, i, j1,2,3, 2.12

which takes the form

τ11−μeH022φ, τ13τ230, τ33 μeH022φ,2φ ∂u

∂x∂w

∂z. 2.13 The dynamic equations of motion are

∂τ11

∂x ∂τ31

∂z P 2

∂ω2

∂zρg∂w

∂x Fxρ 2u

∂t2∂w

∂t −Ω2u

,

∂τ12

∂x ∂τ32

∂z Fy 0,

∂τ13

∂x ∂τ33

∂z P 2

∂ω2

∂x ρg∂w

∂x Fzρ 2w

∂t2 −2Ω∂u

∂t −Ω2w

,

2.14

wheregis the Earth’s gravity and F

−μeH022φ,0, μeH022φ

, 2.15

τ23τ32∂M11

∂x ∂M31

∂z 0, τ31τ13∂M12

∂x ∂M32

∂z 0, τ12τ21∂M13

∂x ∂M33

∂z 0.

2.16

(6)

From2.3–2.8and2.11, we have

τ11 τm

λp∂u

∂x τmλP∂w

∂zγ

1τ1

∂t

T,

τ33 τmλ∂u

∂xτm

λ∂w

∂zγ

1τ1

∂t

T,

τ13 τmμ ∂u

∂z ∂w

∂x

F∂η

∂t,

τ12 −F∂ζ

∂t, τ23 −F∂ξ

∂t, M11 M∂ξ

∂x, M31M∂ξ

∂z, M33 M∂ζ

∂z, M21M22 M23 0, M12 M

∂x

ω2η

, M32 M

∂z

ω2η

, M13M∂ζ

∂x.

2.17

Substituting2.17into2.14and2.16tends to

τm λ

P∂2u

∂x2 τmλP 2w

∂x ∂zγ

1τ1

∂t ∂T

∂xτmμ 2u

∂z2 2w

∂x ∂z

P 2

2u

∂z22w

∂x ∂z

ρg∂w

∂x F 2η

∂z ∂tμeH02 2u

∂x2 2w

∂x ∂z

ρ 2u

∂t2∂w

∂t −Ω2u

, 2.18

then

τm

λ

eH022u

∂x2

τm λμ

P

2 μeH02 2w

∂x ∂z

τmμP 2

2u

∂z2

γ

1τ1

∂t ∂T

∂xρg∂w

∂x F 2η

∂z ∂t ρ 2u

∂t2∂w

∂t −Ω2u

.

2.19

(7)

Also,

∂t ∂ζ

∂x∂ξ

∂z

0, 2.20

τmμ 2u

∂x ∂z∂2w

∂x2

F 2η

∂x ∂tτmλ 2u

∂x ∂zτm

λ2w

∂z2γ

1τ1

∂t ∂T

∂z P

2 2u

∂x ∂z2w

∂x2

ρg∂u

∂xμeH02 2u

∂x ∂z∂2w

∂z2

ρ 2w

∂t2 −2Ω∂u

∂t −Ω2w

, 2.21

then

τm λμ

P

2 μeH02 2u

∂x ∂z

τmμP 2

2w

∂x2 τm

λ

μeH022w

∂z2

γ

1τ1

∂t ∂T

∂z ρg∂u

∂xF 2η

∂x ∂tρ 2w

∂t2 −2Ω∂u

∂t −Ω2w

,

2.22

and, from2.16, we have

2ξs2∂ξ

∂t 0, 2.23

2 ω2η

s2

∂η

∂t 0, 2.24

2ζs2∂ζ

∂t 0, 2.25

where

s2 2F

M. 2.26

3. Solution of the Problem

By Helmholtz’s theorem19, the displacement vector→−u can be written in the displacement potentialsφandψform, as

u gradφcurl→−ψ , →−ψ 0, ψ,0

, 3.1

which reduces to

u ∂φ

∂x∂ψ

∂z, w ∂φ

∂z ∂ψ

∂x. 3.2

(8)

Substituting3.2into2.19,2.22, and2.24, the wave equations tend to

α22φγ ρ

1τ1

∂t

Tg∂ψ

∂x 2φ

∂t2∂ψ

∂t −Ω2φ, 3.3 β22ψs1∂η

∂t g∂φ

∂x 2ψ

∂t2 −2Ω∂φ

∂t −Ω2ψ, 3.4

2ηs2∂η

∂t − ∇4ψ0, 3.5

where

s1 F

ρ, α2 τm

λ

eH02

ρ , β2mμP

. 3.6

Substituting3.2into2.10, we obtain

K∇2T ρs∂

∂t

1τ2

∂t

Tγ T0

∂t

1τ2δ∂

∂t

2φ. 3.7

From3.3and3.7, by eliminatingT, we obtain

2− 1 χ

∂t

1τ2

∂t

α22φg∂ψ

∂x2φ

∂t2 −2Ω∂ψ

∂t Ω2φ

ε∂

∂t

1τ1

∂t

1τ2δ∂

∂t

2φ0,

3.8

where

χ K

ρs, ε γ2T0

ρK. 3.9

From3.4and3.5by eliminatingη, we obtain

2s2

∂t

β22ψ2ψ

∂t2 g∂φ

∂x∂φ

∂t Ω2ψ

s14∂ψ

∂t 0. 3.10

For a plane harmonic wave propagation in thex-direction, we assume

φφ1eikx−ct, ψψ1eikx−ct, 3.11

ξ, η, ζ

ξ1, η1, ζ1

eikx−ct. 3.12

(9)

From3.12into2.20,2.23, and2.25, we get

1ikζ10, 3.13

D2ξ1q2ξ10, 3.14

D2ζ1q2ζ10, 3.15

where

q2ikcs2k2, Dd

dz. 3.16

The solution of3.14and3.15takes the form

ξ1 A1eiqzA2e−iqz, ζ1B1eiqzB2e−iqz, 3.17

whereA1, A2, B1, andB2are arbitrary constants.

From3.13and3.17, we obtain

q

A1eiqzA2e−iqz

k

B1eiqzB2e−iqz

0, 3.18

then

qA1kB10, qA2kB20

Aj −1j−1k

q Bj, j 1,2. 3.19

Substituting3.11into3.8and3.10, we obtain

α2D4G1D2G2

φ1

G3D2G4

ψ10,

R1D4R2D2R3

ψ1

R4D2R5

φ10,

3.20

where

Γ01−ikcτ0, Γ11−ikcτ1, Γ21−ikcτ2, Γ31−ikcτ2δ,

α2 Γ0

λ

eH02

ρ , β20μP. G1k2

c2−2α2 ikc

χ

α2Γ2χεΓ1Γ3

Ω2,

(10)

G2k4

α2c2

ikcΓ2

χ

k2 1−α2

Ω2

k2

Ω2ikεcΓ1Γ3

,

G3 ik

g−2Ωc

, G4

g−2Ωc

ik3k22

χ

,

R1β2ikcs1, R2k2

c2−2β2 ikc

s2β2−2k2s1

Ω2,

R3k2

k2ikcs2

β2c2 ikc

s2Ω2k4s1

, R4ik

g−2Ωc

, R5

2Ωc−g

ik3k2cs2

.

3.21 The solution of3.20takes the form

φ14

j1

CjeikNjzDje−ikNjz ,

ψ14

j1

EjeikNjzFje−ikNjz ,

3.22

where the constantsEjandFjare related to the constantsCjandDjin the form Ej mjCj, FjmjDj, j1,2,3,4,

mj 1

g−2Ωc

ikNj2ik

εΓ2

×

α2k2Nj4

k2

c2−2α2 ikc

χ

α2Γ2χεΓ1Γ3

Ω2

Nj2

ikcΓ2

χ

1−α2Ω2 k2

Ω2ikεcΓ1Γ3

.

3.23

Substituting3.22into3.11, we obtain

φ4

j1

CjeikNjzDje−ikNjz

eikx−ct,

ψ 4

j1

EjeikNjzFje−ikNjz

eikx−ct,

3.24

(11)

and values of displacement componentsuandware

uik 4 j1

1−Njmj

CjeikNjz

1Njmj

Dje−ikNjz

eikx−ct,

wik 4 j1

Njmj

CjeikNjz

mjNj

Dje−ikNjz

eikx−ct,

3.25

whereN1, N2, N3, andN4are taken to be the complex roots of the following equation

N8t1N6t2N4t3N2t40, 3.26

where

t1 k2 α2

c2−2α2 ikc

α2χ

α2Γ2χεΓ1Γ3

Ω2 1 β2ikcs1

× k2

c2−2β2 ikc

s2β2−2k2s1

Ω2 ,

3.27

t2 1 α2

k4

α2c2

ikcΓ2

χ

k2 1−α2

Ω2

k2

Ω2ikεcΓ1Γ3

1 α2

β2ikcs1

k2

c2−2β2 ikc

s2β2−2k2s1

Ω2

×

k2

c2−2α2 ikc

χ

α2Γ2χεΓ1Γ3

Ω2

1 β2ikcs1

k2

k2ikcs2

β2c2

ikc

s2Ω2k4s1

− 1

α2

β2ikcs1

k2

g−2Ωc2 ,

3.28

t3 1 α2

β2ikcs1

× k2

c2−2β2 ikc

s2β2−2k2s1

Ω2

×

k4

α2c2

ikcΓ2

χ

k2 1−α2

Ω2

k2

Ω2ikεcΓ1Γ3

(12)

k2

k2ikcs2

β2c2 ikc

s2Ω2k4s1

×

k2

c2−2α2 ikc

χ

α2Γ2χεΓ1Γ3

Ω2

ik

g−2Ωc2

ik3k22

χ

ik3

g−2Ωc2

ik−cs2 ,

3.29 t4 1

α2

β2ikcs

× k2

k2ikcs2

β2c2 ikc

s2Ω2k4s1

× ik

g−2Ωc

2Ωc−g2

ik3k2cs2

ik3 k22

χ

.

3.30

From3.4,3.11,3.12,3.22, and3.23, we obtain

η14

j1

1

ikcs1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

×

CjeikNjzDje−ikNjz . 3.31

Using3.22and3.11into3.3, we obtain

T ρ γΓ1

4 j1

−α2k2

1Nj2

k2c2ikgmj

CjeikNjzDje−ikNjz

eikx−ct. 3.32

With the lower medium, we use the symbols with primes, forξ1, ζ1, η1, T, φ, ψ, andq, forz > K,

ξ1k

qB2e−iqz, ζ1 B2e−iqz, η1 4

j1

1

ikcs1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

cg

Dje−ikNjz,

T ρ γΓ1

4 j1

−α2k2

1Nj2

k2c2ikgmj

Dje−ikNjzeikx−ct,

(13)

φ4

j1

Dje−ikNjzeikx−ct,

ψ4

j1

Fje−ikNjzeikx−ct.

3.33

4. Boundary Conditions and Frequency Equation

In this section, we obtain the frequency equation for the boundary conditions which are specific to the interfacezK, that is,

iuu, iiww, iiiξξ, ivηη,

vζζ, viM33M33, viiM31M31, viiiM32M32,

ixτ33τ33 τ33 τ33 xτ31τ31 τ31 τ31, xiτ32τ32 τ32 τ32, xiiT T,

xiii ∂T/∂z θT ∂T/∂z θT.

The boundary conditions on the free surfacez0 are xivM330,

xvM310, xviM320, xviiτ33τ33 0, xviiiτ31τ31 0, xixτ32τ32 0,

xx ∂T/∂z θT 0.

(14)

From conditionsiii,v,vi, andvii, we obtain

B1eiqKB2e−iqK−B2e−iqK, B1eiqKB2e−iqKB2e−iqK, M

B1eiqKB2e−iqK

−MB2e−iqK,

M

B1eiqKB2e−iqK

−MB2e−iqK.

4.1

Hence,

B1B2B20, ξζξζ0. 4.2

The other significant boundary conditions are responsible for the following relations:

i

4 j1

1−Njmj

CjeikNjK

1Njmj

Dje−ikNjK

1Njmj

Dje−ikNjK0, 4.3

ii

4 j1

Njmj

CjeikNjK

mjNj

Dje−ikNjK

mjNj

Dje−ikNjK0, 4.4

iv

4 j1

1 cs1

k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

×

CjeikNjKDje−ikNjK

4

j1

1

cs1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

cg

Dje−ikNjK0,

4.5

(15)

viii MNj

4 j1

k2mj

Nj21 1

ikcs1

k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

×

CjeikNjKDje−ikNjK MNj

4 j1

k2mj

Nj21 1

ikcs1

×

k2β2mj

1Nj2

mj

k2c2 Ω2 ik

cg

Dje−ikNjK0,

4.6

ix

4 j1

Γ0λμeH02

1−Njmj

Γ0

λ

μeH02

Nj2mjNj

×CjeikNjK

Γ0λμeH02

1Njmj

Γ0

λ

μeH02

Nj2mjNj

×Dje−ikNjKρ

−α2

1Nj2

c2ig kmj

CjeikNjKDje−ikNjK

Γ0λμeH02

1Njmj

Γ0

λ μeH02

Nj2mjNj

Dje−ikNjK

−ρ

−α2

1Nj2

c2ig kmj

Dje−ikNjK

0,

4.7

x

4 j1

−2k2Γ0μNj

CjeikNjKDje−ikNjK

−k2Γ0μmj

1−Nj2 F

s1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

×

CjeikNjKDje−ikNjK

−2k2Γ0μNjDje−ikNjK

−k2Γ0μmj

1−Nj2 F

s1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

cg

×Dje−ikNjK 0,

4.8

(16)

xii

4 j1

ρ γ

−α2k2

Nj21

k2c2igkmj

CjeikNjKDje−ikNjK

ρ γ

−α2k2

Nj21

k2c2igkmj

Dje−ikNjK0,

4.9

xiii

4 j1

ρ γ

−α2k2

Nj21

k2c2igkmj

θikNj

CjeikNjK

θikNj

Dje−ikNjK

ρ γ

−α2k2

Nj21

k2c2igkmj

×

θikNj

Dje−ikNjK0,

4.10

xvi MNj

4 j1

k2mj

Nj21 1

ikcs1

k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

× CjDj

0,

4.11

xvii

4 j1

Γ0λμeH02

1−Njmj

Γ0

λ

μeH02

Nj2mjNj Cj

Γ0λμeH02

1Njmj

Γ0

λ

μeH02

Nj2mjNj

Dj

ρ

−α2 1Nj2

c2ig kmj

CjDj

0,

4.12

xviii

4 j1

−2k2Γ0μNj

CjDj

−k2Γ0μmj

1−Nj2 F

s1 k2β2mj

1Nj2

mj

k2c2 Ω2 ik

2Ωc−g

× CjDj

0,

4.13

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