Shiuh-Kuang Yang

Reflection and transmission of elastic waves by a structurally chiral arrangement of identical uniaxial layers

Layered composites are constructed by stacking identical uniaxial plates, their consecutive symmetric axes describing either a right- or a left-handed spiral. A matrix representation method is used for solving the reflection and transmission characteristics of such layered composites. The field equations governing the harmonic motions of the layered composite are written as a first-order matrix ordinary differential equation, where the field variables form a 6-vector consisting of the displacement and traction components. Numerical results of the plane wave reflection and transmission characteristics for chiral arrangements are compared with those for a non-chiral arrangement. It is observed that the co-polarized characteristics are unaffected by the structural chirality, while the cross-polarized reflected and transmitted fields are greatly influenced by it.

Rotation of elastic shear waves in laminated, structurally chiral composites

Abstract We present experimental and theoretical data to support the hypothesis that elastic shear waves can sense the microstructural handedness, if any, in a material. Structurally chiral composites are constructed by stacking identical uniaxial plates, their symmetry axes describing either a left- or a right-handed spiral. Four glass-reinforced laminated composites-left- and right-handed, pure uniaxial and random-were made to verify the effect of form or structural chirality on elastic shear wave propagation. The composites were characterized by using shear wave transducers in an ultrasonic through-transmission experiment. Test results indicate that shear waves transmitted through left- and right-handed chiral composites have polarization states that are rotated clockwise (CW) or counter-clockwise (CCW) with respect to the incident linearly polarized wave. The uniaxial sample and the random sample did not show this effect. This effect is similar to the rotation of the plane of polarization of electromagnetic waves in “optically” (electromagnetically) active media. Wave propagation in the samples was also analyzed numerically using a matrix representation method. The experimental data compares with the numerical results qualitatively.

Reflection and Transmission of the Longitudinal Wave at an Achiral-Chiral Boundary

The effective chiral (isotropic, noncentrosymmetric) composite can be fabricated by embedding the helical arrangement of microstructures in a host medium. In such a composite, six independent wavenumbers can be found from the dispersion equations. Two of the wavenumbers represent the nondispersive longitudinal waves while the remaining four represent the dispersive circularly polarized shear waves. According to the dispersion equations derived in our previous research that two cut-off frequencies divide the frequency spectrum of the transverse wavenumbers into three different groups, and hence the four transverse modes can only be distinguished in a specified frequency range. In this paper, we study the longitudinal plane wave propagation at an isotropic-effective chiral boundary. Numerical results show that an additional horizontal shear wave (SH wave), due to the mode conversion in the chiral composite, is enhanced between the first and second cut-off frequencies. The embedded handed microstructures may hence be properly chosen that the resulting composite is effectively chiral in a specified frequency range.

Mechanical Activity of a Chiral Slab Sandwiched between Achiral Half-Spaces

A plane wave propagating in effectively chiral composites (ECC) has been asymptotically investigated. In general, the ECC can be fabricated by embedding a helical arrangement of microstructures in the soft matrix. Such a material is mirror asymmetric or chiral; therefore, six independent wave numbers can be found. They are associated with two sets of nondispersive longitudinal fields and four sets of dispersive circularity polarized transverse fields. In this work, we study the reflected and transmitted fields of a normally incident plane wave propagating at achiral-chiral-achiral interfaces. The numerical study indicates that the longitudinal wave is unaffected by the sandwiched chiral layer, while the polarization of the transverse transmitted wave is significantly influenced by it. Furthermore, when a normally incident SV-wave (vertical shear wave) propagates in the upper half-space, the coupled SV- and SH-waves (horizontal shear wave) can be found in the lower half-space. The polarization of the transmitted field is affected by the handedness of the intermediate chiral layer.

Acoustic Wave Propagation at a Fluid-Micropolar Boundary

In contrast to a homogeneous isotropic elastic medium with only two elastic constants, a linear elastic micropolar solid is characterized by six independent constants. The solid may consist of particles uniformly distributed in a host matrix. Thus, six different types of wave propagation with four distinct speeds can be found in the dispersion equations. The four distinct speeds are associated with one nondispersively longitudinal displacement wave; one dispersively longitudinal microrotation wave; and two dispersively transverse waves in the presence of couple-stress. In this work, we illustrate the reflected and transmitted fields of an incident acoustic wave propagating at the fluid-micropolar interface. The result is compared to the fluid-isotropic case. It is observed that, due to the mode conversion in the micropolar medium, the micropolar medium should instigate a reducible reflected acoustic wave.

Wave motions in an effective chiral composite : Dispersion equations and elastic moduli

In this study, we investigate elastic wave propagation in an effective chiral (isotropic, noncentrosymmetric) composite. This composite is constructed by embedding the structural chiral microstructures or springs in a host medium. To ensure that the chiral effect can be found in the samples, a hypothesis about the material constants to characterize the effective chiral composites is proposed herein. Such a material is mirror asymmetric or chiral; therefore, six independent wave numbers can be found. Two of the wave numbers represent nondispersive longitudinal fields, the remaining four are dispersive circularity polarized transverse fields. According to the dispersion equations, two transition frequencies (90 Hz and 12 kHz) divide the frequency response of the transverse wave numbers into three different groups and the four transverse modes can only be distinguished in a specified frequency range. The above observation may account for why the microstructural size of the chiral composite should be sufficiently larger than the transverse wavelength so that an incident elastic shear wave can sense its chirality while the microstructural size should be adequately small that the composite is effectively chiral.

ERYTHROCYTES DAMAGE BY ULTRASOUND AT DIFFERENT HYDROSTATIC PRESSURES

The study of the biological effect of ultrasound on tissue has become an important branch of biophysical ultrasonics. Among the important biological effects, which have been discovered, are chromosomal anomalies, cell death, destruction of cellular structures. Although heating may be important in some results, nonthermal mechanics, such as cavitation, are often suggested as being responsible for the observed effects. In this paper, the biological subject of this study is the red blood cells (RBC); the relationship between the threshold lesion of RBC and frequency of ultrasound was studied. It has been concerned that a suitable frequency value of ultrasound was selected, capable of driving the bubbles to an interesting reaction. The effective mass of the bubble.for the oscillating system is due to the motion of the surrounding medium. For the purposes, by using the approximate expressions for the effective stiffness and mass of a gas body, one can determine the resonance frequency of the gas body. In the closed system, three types of frequencies were employed with different pressure. The cell death threshold varies with the ultrasonic frequency of exposure, the minimum at about 10 MHz. This report is intended to provide a useful way for the study of the biological effect of ultrasound at medically relevant frequencies.

USE OF THE SECOND HARMONIC IN STRAIN IMAGING: A PHANTOM STUDY

The clinical images of an ultrasound system are normally obtained by analyzing the ultrasonic echoes from the discontinuities in a patient. Information such as wave speed, acoustic impedance, and the attenuation coefficient computed from the ultrasonic echoes are the primary parameters used in constructing an ultrasonic image. However, the traditional medical imaging is insensitive to identify the abnormal regions of small stiff change. It often fails to reveal the extent or existence of tumors, which are found to be more stiff than surrounding normal tissues. To overcome these problems, this research proposes an ultrasonic imaging method based on tissue elasticity through the harmonic filter to construct a strain-harmonic image. This technique is formed by a process involving two steps. First, the phantom is compressed externally to generate an internal strain field. Second, ultrasonic transmitting signals before and after compression are acquired and processed by harmonic filter. The strain profiles of a soft sample are converted into two-dimensional gray images using MATLAB software. Experiments were conducted for the comparison between the strain and strain-harmonic image. It is observed that the strain-harmonic images can illustrate layer location of the phantoms better than those plotted in the strain images.