Anisotropic Layered Structures Research Articles

Acoustoelastic effect in anisotropic layered structures

Mechanical stress alters the velocity of acoustic waves, a phenomenon known as AE (acoustoelastic) effect, which is of particular importance for the wave propagation in layered heterostructures. In order to calculate the AE effect of layered systems in the presence of stress we extended the transfer-matrix method for acoustic wave propagation by considering the change of the density, the influence of residual stress, and the modification of the elastic stiffness tensor by residual strain and by third-order constants. The generalized method is applied to the calculation of the angular dispersion of the AE effect for transverse bulk modes and surface acoustic waves on the Ge(001) crystal cut. The AE effect is found to depend significantly on the propagation direction and can even change sign. The maximum velocity change occurs for transversally polarized waves propagating parallel to the [110] direction. For the layered Ge/Si(001) system the AE effect is investigated for Love modes propagating in the [100] and [110] directions. The AE effect increases rapidly with increasing layer thickness and reaches almost its maximum value when the wave is still penetrating into the unstressed substrate. For higher-order Love modes the increase of the AE effect is even steeper and, furthermore, can reach higher values.

The Long Term Mechanical Behaviour of Filled and Unfilled Injection Moulded Thermotropic Liquid Crystalline Polymers

Injection mouldings of thermotropic liquid crystalline polymers (TLCPs) have complex, highly anisotropic layered structures, and correspondingly large spatial variations in orientation and mechanical properties. The room temperature axial mechanical properties of standard tensile test specimens moulded from TLCPs are remarkable (ten-fold and two-fold increases in modulus and strength respectively, compared with conventional resins). Nevertheless, in practice glassfibre or mineral fillers are recommended to overcome problems associated with the anisotropy of unfilled TLCP mouldings, such as warping. The resulting reduction of anisotropy and more uniform local properties are not found to compromise seriously the axial mechanical properties, and indeed may improve both the short and the long-term behaviour, depending on the moulding conditions. Short glass-fibre addition appears particularly beneficial for the creep and fatigue resistance.

The Long Term Mechanical Behaviour of Filled and Unfilled Injection Moulded Thermotropic Liquid Crystalline Polymers

Injection mouldings of thermotropic liquid crystalline polymers (TLCPs) have complex, highly anisotropic layered structures, and correspondingly large spatial variations in orientation and mechanical properties. The room temperature axial mechanical properties of standard tensile test specimens moulded from TLCPs are remarkable (ten-fold and two-fold increases in modulus and strength respectively, compared with conventional resins). Nevertheless, in practice glassfibre or mineral fillers are recommended to overcome problems associated with the anisotropy of unfilled TLCP mouldings, such as warping. The resulting reduction of anisotropy and more uniform local properties are not found to compromise seriously the axial mechanical properties, and indeed may improve both the short and the long-term behaviour, depending on the moulding conditions. Short glass-fibre addition appears particularly beneficial for the creep and fatigue resistance.

Acoustic Surface Wave Excitation in Layered Structures

An analytical and computational procedure for the study of acoustic surface wave excitation in anisotropic layered structures has been developed. The analysis is based upon solution of the coupled wave equations in both layer and substrate and is directed toward materials which can be oriented such that the particle displacements are in the sagittal plane. The admittance characteristics (complex) of interdigital transducers, as used to excite surface waves in such structures, are calculated and found to be in excellent agreement with experimental results. ducers in such structures (g) , ( 181 . However, a basic theoret- ical study of surface wave excitation in layered structures has not yet been published.' This paper attempts to fill this ap- parent gap and to show that the admittance characteristics of the ID transducers used to launch surface waves in layered structures can be accurately predicted from the material con- stants which are determined from bulk wave measurements. The intent of this study is to concentrate on low coupling materials, such as CdS and ZnO, in conjunction with nonpie- zoelectric materials for the layer and substrate, respectively. Thus the analysis is based on a low-coupling approach, and certain crystalline symmetry restrictions are imposed to reduce the mathematical complexity. Nevertheless, the analysis pre- sents a method which could be used for a more general situa- tion if so desired. 11. THEORY

Surface motion from pulse transmission through anisotropic layered structures

abstract Theoretical surface motion for pulse transmission through transversely isotropic elastic structures is computed by Fourier inversion of the product of the Fourier transform of an input pulse, complex transfer coefficients for anisotropic structures, and a seismograph system response. Introduction of anisotropy causes subtle changes in the angles of refraction and shifts in arrival times of different phases as intuitively expected from Snell's law. Computations suggest no obvious criteria for recognizing anisotropy from surface motion.