Influence of Stress on Propagation of Shear Wave in Piezoelectric ‐ Piezoelectric

Abstract Shear waves SH propagation in piezoelectric composite under the influence of initial stress is investigated analytically and numerically. The dispersion equation of shear waves propagation in direction normal to the layering is obtained in presence of initial stress. Numer‐ ical solutions were obtained for evaluating the effect of stress on dimensionless frequency and phase velocity. The effect of stress on stop band is discussed in this study. It can be concluded from the results that initial stress has significant effect on propagation charac‐ teristics of shear waves. The variation of initial stress has small effect on the phase velocity of shear waves. This study provides insight for development of piezoelectric composite structure under the influence of initial stress.


INTRODUCTION
The propagation behavior of shear horizontal wave is most important phenomena to be understood due to its invariably application in surface acoustic wave SAW sensors.It is well known fact that when mechanical stress is applied to piezoelectric material, an electric voltage is generated and application of electric field across material mechanical deformation is observed.It is important to know the propagation properties of shear wave in composite material for designing the SAW sensor Burkov and Piliposian 2011 .The propagation characteristics depend upon the properties of piezoelectric materials Danoyan et al. 2008 .Due to these properties, piezoelectric materials found potential applications in sensors, valves, actuators and smart structures Yuan et al. 2014, Zhanga et al. 2014, Li et al. 2012 .SAW sensor essentially comprises of alternative layers of piezoelectric materials bonded together to form a composite structure.These composite structures of two materials have better properties as compare to single material structure Levin et al. 2011 .Numerous investigations have been carried out in past which mainly focuses on propagation characteristics of shear horizontal waves in composite layered structure.Liu et al. 2003 discussed the propagation of B-G waves in pre-stressed piezoelectric structure.It is revealed in literature that, thickness ratio is affected by presence of initial stress in composite structure Son and Kang 2011 .Mahmoud 2013 found that velocity of shear waves is influenced by presence of stress in multi layered structure.The effect of initial stress on propagation behavior of shear waves in functionally graded half space was discussed by Qin et al. 2010 extensively and reveals the influence of initial stress on dispersion relation and group velocity.Kayestha et al. 2011 investigated the wave propagation in pre-stressed elastic layers.Qian et al. 2004 investigated the effect of initial stress on characteristics of shear waves in polymer-piezoelectric structure.Du et al. 2008 discussed the Love waves propagation in pre-stressed piezoelectric structure and concluded that intimal stress has considerable effect on phase velocity.
The presence of initial stress in piezoelectric composite structure introduces number of problems such as frequency shift, variation in phase velocity of SH waves that degrades the response of SAW sensors Jin et al. 2005, Su et al. 2005, Du et al. 2007. Zhang et al. 2013 investigated the effect of stress on lateral modes frequencies and the stop band in piezo composites structures.Recently some of researchers studied the propagation of a Love wave in an initially stressed fluid-saturated anisotropic porous layer and concluded that presence of stress significantly affect the porosity Chattraj et al. 2012 .But till now, no work has focused to consider the effect of initial stress in pie-zoelectric-piezoelectric composite structure and consideration of stress is important issue in designing and manufacturing of SAW sensors.
In this study, we have made an attempt to consider the influence of initial stress on propagation behavior of shear wave in piezoelectric layered structure.In section 2, we have formulated the constitutive equations for PE-PE interface and these equations are utilized to derive the dispersion relation under the presence of initial stress.In section 3, the solutions of dispersion equation were obtained by considering the bulk shear velocities for alternative layer of piezoelectric material respectively.In section 4, the numerical computation was carried out by considering two different piezoelectric materials i.e.PVDF and PZT-8.Finally the effect of initial stress on phase velocity with variation in wave number is observed for perfectly bonded piezoelectric layers.

PROBLEM FORMULATION
Consider a two layer composite structure with alternative layer of piezoelectric materials of thickness h 1 and h 2 bonded perfectly together as shown in figure 1.The shear wave propagates along the positive direction of x axis with poling of piezoelectric material taken along the z axis.
), 0 Where ρ is mass density, σ ij , j is stress tensor, σ 0 kj is initial stress tensor, u i and D i represents the mechanical and electrical displacements in the i th direction.D j is the initial electrical displacement.For the piezoelectric layer I media, the constitutive equations of 1 can be represented as Gaur and Rana 2015 where c' 11 , c' 12 , c' 13 and c' 14 are elastic constants, e' 15 , e' 31 and e' 33 are piezoelectric constants and ε' 11 and ε' 33 are dielectric constants.
The strain tensor s ij and electrical intensity E k can be represented as follows , , We consider the Shear wave propagates along the direction normal to layering in presence of initial stress σ 0 x .The mechanical displacement component W and electrical function φ can be described by following equations, and z is the polarization direction Gaur and Rana 2014 0, ( , ), ( , ) Substituting equations 10 into 1 -2 and 7 -9 , we have Let us assume w 1 represents mechanical displacement and φ 1 electrical potential function in P.E-I layer.From equation 11 , 12 and 16 -18 the following equations can be obtained Similarly w 2 represents mechanical displacement and φ 2 electrical potential function in P.E-II layer.
The continuity conditions at x 0 in direction normal to the PE-PE interface i.e. ) The following conditions must be also satisfied i.e.
For shear waves propagating in positive direction of x axis, solutions of the mechanical displacement and electrical potential function can be expressed as follows, Where k 2π/λ is the wave number, λ is the wavelength, i 2 -1, and c is the phase velocity of wave propagation.Let W 1 x , W 2 x , Ф 1 x , and Ф 2 x are the undetermined functions, respectively.Substituting equation 27 -28 into equation 19 -20 leads to, Inserting the value of equation 29 -30 into 21 -22 yields, Latin American Journal of Solids and Structures, 2018, 15 1 , e04 5/9 Where bulk shear velocity can be represented By inserting the value of Ф 1 x from equation 32 , the solution of equation 31 can be found The solution of equation 32 is sum of particular and homogeneous solution.So the particular solution of equation is obtained as The homogeneous solution of equation 32 can be represented The complete solution of equation 32 leads to Similarly the solution of equation 33 is obtained The solution of equation 34 is sum of particular and homogeneous solution.So the particular solution is found as ( ) So complete solution of equation 34 is represented The components of stress and electrical displacement can be obtained as Pikb A e B e exp ik x ct t , , , , , , , .In order to obtain the nontrivial solution of equations 53 -60 , the determinant of the coefficient matrix must be equate to zero Jin et al. 2005 i.e. Where The equation 61 and 62 represents the dispersion equation for shear wave propagating in direction normal to x axis.The equation provides relation between phase velocity c and wave number k under the influence of initial stress σ 0 x .

NUMERICAL EXAMPLES AND RESULTS
In above section, we have obtained analytically computation solution of the dispersion relation for propagation of shear wave in PE-PE layered structure.The thickness of PVDF is h 1 and thickness of PZT-8H is taken as h 2 respectively.In this section numerical computation is utilized to illustrate graphically the variation of initial stress on dispersion relation.For numerical calculation, the   In this study, we have considered the effect of initial stress on propagation characteristics of shear wave propagation in piezoelectric-piezoelectric multi-layer structure.Analytical and numerical solutions were found to investigate the effect of stress on the interface of two different piezoelectric materials i.e PVDF and PZT-8.The numerical results indicates that the initial stress up to 10 2 MPa has very small effect on dimensionless frequency of shear waves propagating in the direction normal to layering.But effect of stress becomes more pronounced when stress increased upto 10 4 MPa.Further increase in the value of initial stress may cause obstruction in propagation of shear waves.So variation in value of stress can be utilized for designing the filter for wireless applications.It is also observed that low value of initial stress has very little impact on phase velocity.This investigation is limited to two layer laminates which restricts the applicability to only some practical areas.Other numerical tools such as finite element method can be utilize further to investigate the shear wave propagation in multilayered laminates.The present analysis provides insight for design and manufacturing of new class of surface acoustic wave sensors.Further the imperfection at the layer interface can be also considered to study the effect of stress on the wave number and phase velocity.

Figure 1 :
Figure 1: Schematic of piezoelectric layered structure.The wave motion equation for piezoelectric material with initial stress can be described by following equation Mahmoud 2013, Qian et al. 2012 0 , , ( ), , c 12 , c 13 and c 14 are elastic constants, e 15 , e 31 and e 33 are piezoelectric constants and ε 11 and ε 33 are dielectric constants.For the piezoelectric layer II media, the constitutive equations of 1 can be represented as follows Latin American Journal ofSolids and Structures, 2018, 15 SOLUTIONS AND DISPERSION RELATIONSSubstituting the continuity conditions of Equations 23 -26 in equations 45 -52 yields the following set of linear algebraic equations with unknown coefficients 1 MPa with volume fraction at constant value of η 0.4.

FigureFigure 2 :Figure 3 :
Figure 2 a -2 d shows the variation of stop band for initial stress 0 0 x s = and 0 10 x s = MPa.It can be observed from plots with subsequent increase in value of initial stress, the number of stop bands increases but width of each stop band decreases considerably.Further increase in value of stress beyond 0 4 10

Table 1 :
Table 1 lists material properties of PVDF and PZT-8.Material constants of PVDF and PZT-8.