Conditions for Strong Ellipticity of Anisotropic Elastic Materials

We consider the problem of finding the transversely isotropic elasticity tensor closest to a given elasticity tensor with respect to the Frobenius norm. A similar problem was considered by other authors and solved analytically assuming a fixed orientation of the natural coordinate system of the transversely isotropic tensor. In this paper we formulate a method for finding the optimal orientation of the coordinate system—the one that produces the shortest distance. The optimization problem reduces to finding the absolute maximum of a homogeneous eighth-degree polynomial on a two-dimensional sphere. This formulation allows us a convenient visualization of local extrema, and enables us to find the closest transversely isotropic tensor numerically.      

Theory of indentation on multiferroic composite materials

This article presents a general theory on indentation over a multiferroic composite half-space. The material is transversely isotropic and magneto-electro-elastic with its axis of symmetry normal to the surface of the half-space. Based on the corresponding half-space Green’s functions to point sources applied on the surface, explicit expressions for the generalized pressure vs. indentation depth are derived for the first time for the three common indenters (flat-ended, conical, and spherical punches). The important multiphase coupling issue is discussed in detail, with the weak and strong coupling being correctly revisited. The derived analytical solutions of indentation will not only serve as benchmarks for future numerical studies of multiphase composites, but also have important applications to experimental test and characterization of multiphase materials, in particular, of multiferroic properties.     

On well-posedness of the dynamic problem for an anisotropic fluid-saturated solid

It is known that a high degree of anisotropy in the constitutive behaviour of a solid may result in the loss of hyperbolicity of the dynamic equations in the form of either complex-conjugate or purely imaginary characteristic wave speeds (flutter ill-posedness and shear band formation, respectively). In the present paper we investigate the characteristic wave speeds in the dynamic problem for a transversely isotropic fluid-saturated porous solid. Three cases are considered: a dry solid and a saturated solid under locally undrained and drained conditions. It is shown that, for given constitutive parameters of the solid skeleton, the dynamic problem for a drained solid may become ill-posed due to the flutter-type loss of hyperbolicity, while the dynamic equations for a dry and an undrained solids remain hyperbolic. For a given solid skeleton, the characteristic wave speeds are strongly influenced by the pore fluid compressibility which, in turn, is extremely sensitive to the presence of a small amount of free gas.     

安装偏差对光纤布喇格光栅应变测量的影响

通过理论推导和试验分析研究了安装偏差对光纤应变测量的影响,得到安装误差所带来测量误差的计算公式.发现光纤的轴向与预定安装方向的角偏差越大,测量误差也越大.用光纤光栅进行应变测量时,必须考虑光纤的安装偏差,它直接关系到测量结果的准确性.实际测量中要尽量确保传感器安装方位的准确.     

Piezoelectric multilayer transducers for ultrasonic pulse compression

This paper describes piezoelectric multilayer transducers for application in ultrasonic pulse compression systems. The transducers are constructed in such a way as to produce binary pulse sequences, in particular Barker-coded sequences. This is achieved by the polarization pattern of the active layers. Pulse compression is effected without any electronic circuitry by using a receiver transducer with a pattern corresponding to that of the transmitter.     

圆锥磁绝缘传输线的横向空间电荷流

 根据电磁场基本理论及电子运动守恒方程，导出圆锥传输线横向空间电荷流的数值模型和磁绝缘临界条件。通过数值计算，讨论了电压及圆锥几何结构参数等对横向空间电荷流和磁绝缘性能的影响。电压较高时,无磁场的空间电荷流较大，而磁绝缘性能更好。在传输线的三个几何参数中，几何因子对传输性能影响最大。     

Study on the multifrequency Langevin ultrasonic transducer

Langevin ultrasonic transducers are widely used in high-power ultrasonics and underwater sound. In ultrasonic cleaning, a matching metal horn rather than a metal cylinder is used as the radiator in order to enhance the radiating surface and improve the acoustic matching between the transducer and the processed medium. To raise the effect of ultrasonic cleaning, the standing wave in the cleaning tank should be eliminated. One method to eliminate the standing wave in the tank is to use the multifrequency ultrasonic transducer. In this paper, the Langevin ultrasonic horn transducer, with two resonance frequencies, is studied. The transducer consists of two groups of piezoelectric ceramic elements: the back metal cylinder, the middle metal cylinder and the front matching metal horn. The vibrational modes of the transducer are analysed, and resonance frequency equations of the transducer in the half-wave and the all-wave vibrational modes are derived. According to the resonance frequency equations, transducers with two resonance frequencies are designed and made. The resonance frequencies, the effective electromechanical coupling coefficients and the equivalent electric impedances of the transducers are measured. It is shown that the measured resonance frequencies are in good agreement with the computed results, and the transducer can be excited to vibrate at two resonance frequencies, which correspond to the half-wave and the all-wave vibrational modes of the transducer.     

Geometry of minimal flows

Our main result is that for a minimal flow φ on a compact manifold M, either M is a torus and the flow is topologically conjugate to an irrational flow, or M is not a torus and every parametrization of φ has sensitive dependence on initial conditions. Our results depend heavily on the work of Lopez and Candel.     

A Polyimide Nanolayer as a Metal‐Free and Durable Organic Electrode Toward Highly Efficient Oxygen Evolution

The exploitation of metal‐free organic polymers as electrodes for water splitting reactions is limited by their presumably low activity and poor stability, especially for the oxygen evolution reaction (OER) under more critical conditions. Now, the thickness of a cheap and robust polymer, poly(p‐phenylene pyromellitimide) (PPPI) was rationally engineered by an in situ polymerization method to make the metal‐free polymer available for the first time as flexible, tailorable, efficient, and ultra‐stable electrodes for water oxidation over a wide pH range. The PPPI electrode with an optimized thickness of about 200 nm provided a current density of 32.8 mA cm^?2 at an overpotential of 510 mV in 0.1 mol L^?1 KOH, which is even higher than that (31.5 mA cm^?2) of commercial IrO₂ OER catalyst. The PPPI electrodes are scalable and stable, maintaining 92 % of its activity after a 48‐h chronoamperometric stability test.     

Modelling of nonlinear dynamics for reciprocal multi-layer piezoceramic transducer systems

The dynamics of multi-layer transducer systems are modelled and simulated by use of a multi-domain spectral method. The model accounts for a nonlinear constitutive relation between the electric displacement and the electric field. Simulation of resonance and wave propagation in the transducer is implemented using a domain-decomposition method and governing equations in each layer are discretized using a Chebyshev collocation method. The model is applied to a reciprocal transducer system, as used in many engineering applications including ultrasonic flowmeter applications and blood-velocity measurement setups, where the transducer consists of a PZT5 piezoceramic layer and a matching layer.