Dispersion of Love Waves in a Composite Layer Resting on Monoclinic Half-Space
Dispersion of Love waves is studied in a fibre-reinforced layer resting on monoclinic half-space. The wave velocity equation has been obtained for a fiber-reinforced layer resting on monoclinic half space. Shear wave velocity ratio curve for Love waves has been shown graphically for fibre reinforced material layer resting on various monoclinic half-spaces. In a similar way, shear wave velocity ratio curve for Love waves has been plotted for an isotropic layer resting on various monoclinic half-spaces. From these curves, it has been observed that the curves are of similar type for a fibre reinforced layer resting on monoclinic half-spaces, and the shear wave velocity ratio ranges from 1.14 to 7.19, whereas for the case isotropic layer, this range varies from 1.0 to 2.19.
Fiber-reinforced composite materials have become very attractive in many engineering applications recently due to their superiority over the structural materials in applications requiring high strength and stiffness in light-weight material. Consequently, the characterization of their mechanical behavior is an utmost requirement. The monoclinic system is the largest symmetry system with almost a third of all minerals belonging to one of its classes. This system contains two nonequal axes ( and ) that are perpendicular to each other and a third (), that is, inclined with respect to the axes. The and axis lie in a plane. The - plane can be, but is not always, a mirror plane with left side of -axis a reflection of the right side. Fledspar which is an example of monoclinic material is the name of a group of rock-forming minerals which make up as much as 60% of earth’s crust. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, and they also can occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar is known as an orthosite. Feldspars are also found in many types of sedimentary rock. The wave propagation in reinforced medium was studied by Chattopadhyay and Choudhury  and in crystalline monoclinic plate was studied by Chattopadhyay and Bandyopadhyay . Propagation of elastic waves in laminated composite plates was studied by Datta et al. . Chattopadhyay et al.  studied the propagation, reflection, and transmission of shear waves in monoclinic media and obtained a dispersion equation for a monoclinic layer overlying monoclinic half-space. Besides these, a large number of papers on elastic wave propagation have been published in different journals. Without going into details of research works in this field, we mention papers by Kim  and Nayfeh . In this paper, we have computed the ranges of shear wave velocity ratio and the corresponding wave numbers for Love waves at a layer of fiber-reinforced material resting on monoclinic half-space and compared them with shear wave velocity ratio of Love waves at isotropic layer resting on monoclinic half-space. Using these values the dispersion curves have also been obtained.
2. Formulation of the Problem
The constitutive equations for fibre-reinforced linearly elastic medium whose referred direction is that of are (Spencer ) where , are components of stress, , are components of infinitesimal strain, and are components of , all referred to Cartesian coordinates. The vector a may be a function of position. The coefficients , and are elastic constants with dimension of stress. If is so chosen that its components are (). The stress components (2.1) become where and are the displacement components.
In this problem, we consider a fiber-reinforced of anisotropic layer resting on the monoclinic half-space () in the plane. The -axis is chosen parallel to the layer in the direction of propagation of the disturbance. The strain-displacement relations for a monoclinic crystal medium are where , and are displacement components in the directions , and , respectively, and ) are the strain components. The stress-strain relations for a rotating -cut plate of quartz which exhibits monoclinic symmetry with being the diagonal axis are where are the normal stresses are the shearing stresses, () are the elastic constants. In the study of seismic waves, when s-pulses are polarized so that all particles of the substance move horizontally during its passage, the wave motion is called SH-wave. Our problem is to investigate the propagation of such waves in the media which consist of two separate media in which upper medium is a layer of thickness , and the lower one is monoclinic half-space.
3. Solution of the Problem
For wave propagating in the -direction and causing displacements in the -direction only, we assume that and . For the shear wave propagating in the - plane, . So the equation of motion for shear wave takes the form Putting the values of and , the above equation takes the form given below
For wave changing harmonically , where wave number, is the angular frequency, and is the speed of simple harmonic waves of wavelength . Substituting this value of in the equation of motion, it transforms into The solution of this equation is Therefore, Equation of motion for the lower half-space has been obtained by using the values of and . Finally, the equation has been obtained as where is the displacement in the direction in the lower half-space. We assume that is the solution of the above differential equation. So, by substituting this to the above equation, we obtain the solution as
3.1. Boundary Conditions
The boundary conditions in the plane at the top of the system At the interface, that is, at , From (3.10), From (3.11), we obtain From (3.12), we obtain Therefore, So, where Finally, this equation (3.19) gives the velocity of love wave in an elastic fiber-reinforced layer of finite thickness resting on the monoclinic half-space which may be naturally a stable rock layer of large thickness. So, it is a possible case that a road of fiber-reinforced composite material has been constructed on a rock surface. To make it seismically stable, the knowledge of probable velocity of Love wave will be a necessary precondition. In this context, this study will be helpful. The real root of this equation can be found for the values of given below If we transform the upper and lower layer as isotropic medium with different rigidity and density, the wave equation (3.19) takes the form shown below, by taking and , where rigidity and density for upper and lower media are and , respectively. This equation is well-known Love wave equation for classical case. By taking, , the wave equation (3.19) transforms to the case of wave equation for a fiber-reinforced layer resting on fiber-reinforced half-space derived by Sengupta and Nath .
4. Numerical Results and Discussion
The range of shear wave velocity ratio of Love wave has been obtained from (3.20), and within this range, the corresponding values of are obtained from (3.19) by considering different material constants of fiber-reinforced layer and monoclinic half-space. These values have been plotted to obtain the shear wave velocity ratio curve. We have obtained a set of shear wave velocity ratio curve for fiber-reinforced layer resting on half-spaces of different materials (monoclinic and isotropic). Similarly, a set of shear wave velocity curves for isotropic layer resting on monoclinic and isotropic half-spaces have also been obtained. These are specified in details in Cases 2 and 1, respectively, below.
Case 1. Here we have considered three sets of arrangements; in all these, top layer is isotropic-I, and lower half-spaces are (i) isotropic-II with different material constants from isotropic-I, (ii) lower half-space is monoclinic, and (iii) lower half-space is monoclinic II, respectively. The material constants are , for isotropic-I and , for isotropic-II. The material constants for monoclinic are as , , , and and for monoclinic-II are , , , given by Tiersten . Using these values, shear wave velocity ratio curves have been plotted for corresponding values of obtained from (3.19). The range of values for shear wave velocity ratio and are given in Table 1, and curves are given in Figure 1.
Case 2. In this case, we have plotted three dispersion curves as in Case 1. Here the upper layer is taken as fibre-reinforced layer, and the lower half-spaces are taken as in Case 1 of three separate arrangements, and the material constants are the same as earlier. The material constants for fibre-reinforced layer are, , and. Shear wave velocity ratio curve for Love waves at fibre-reinforced layer has been plotted for all these three cases and shown in Figure 2, and the ranges of shear wave velocity ratio and wave number are shown in Table 2.
From the results it has been observed that shear wave velocity ratio for Love waves at fibre-reinforced layer is very much higher than at isotropic layer.
From the curves plotted and results tabulated, it has been clearly observed that the shear wave velocity ratio for a fiber-reinforced layer resting on any layer whether it is isotropic or monoclinic is always much higher than on the isotropic layer resting on similar half-spaces. For the case of fiber-reinforced layer, shear wave velocity ratio ranges from 1.14 to 6.15, 1.14 to 7.16, and 1.14 to 7.19, for isotropic half-spaces, monoclinic and monoclinic-II half-spaces, respectively. In the contrary for the case of an isotropic layer, the shear wave velocity ratio ranges from 1.0 to 1.85, 1.0 to 2.19, and 1.0 to 2.19 for the case isotropic, monoclinic, and monoclinic-II half-spaces, respectively.
S. Saha is very much thankful to Director CRRI, for his permission to publish this paper. S. Saha is also very much thankful to the reviewer for valuable comments to modify this paper.
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