Dynamic instability of spinning launch vehicles modeled as thin-walled composite beams

This paper presents the results of the dynamic stability analysis of a flexible spinning launch vehicle subjected to thrust modeled as a thin-walled composite beam with circular cross section. Due to the presence of gyroscopic forces, we mainly aimed to find divergence and/or flutter instabilities and establish the stability boundaries of the spinning beam. For this purpose, we implemented a circumferentially uniform stiffness (CUS) layup configuration to exhibit the coupled motion of bending–bending–shear. The solution of the eigenvalue problem is handled by the extended Galerkin method (EGM), and we computed the results addressing the effects of various parameters such as spin speed, axial load, ply angle, aspect ratio and transverse shear on the dynamic stability of the beam. Insights revealed by this study contribute to the design of advanced aerospace structures modeled as thin-walled composite beams reflecting the potential influence of transverse shear and aspect ratio on dynamic stability characteristics. A notable contribution is that we show that divergence/flutter instabilities can be delayed or even avoided using the directionality property of composite materials.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

Vibration analyses of FGM plates with in-plane material inhomogeneity by Ritz method

In this study, functionally graded plates which the properties of material varying through the in-plane direction is considered. The analysis is based on a five-degree-of-freedom shear deformable plate theory with different boundary conditions. The vibration solutions are obtained using the Ritz method and assumed displacement functions are in the form of the Chebyshev polynomials. The material properties are assumed to vary as a power form of the in-plane direction. The convergence and comparison studies demonstrate the accuracy and correctness of the present method. Effects of the different material composition, the Poisson ratio and the plate geometry (side-side, side-thickness) on the free vibration frequencies and mode shapes are investigated.     

Optical and structural properties of zinc oxide films with different thicknesses prepared by successive ionic layer adsorption and reaction method

In this work, zinc oxide semiconducting films belonging to the II-VI group have been produced by successive ionic layer adsorption and reaction (SILAR) method on glass substrates with 10, 15, 20 and 25 cycles at room temperature. Following the deposition, the samples were dried in air at 400 °C for 1 h. The films were characterized by X-ray diffraction, field emission scanning electron microscopy and optical absorption measurement techniques. The X-ray diffractions of the films showed that they are hexagonal in structure. The crystallite size of ZnO films varied between 34 and 38 nm accordingly with the number of SILAR cycles. The material has exhibited direct band gap transition with the band gap values lying in the range between 3.13 and 3.18 eV. The red shift is observed in the absorption edge as the cycles increased. Transmission of the films decreased from 65 to 40% with increasing the number of cycles.     

Selection of porous materials in marine system design: The case of heat exchanger aboard ships

Porous materials are processed in various areas of material science and manufacturing. Furthermore, the varieties of porous materials ensure great advantages to designers. This paper proposes a decision aid mechanism based on fuzzy axiomatic design (FAD) to select adequate form of porous materials in marine systems design. It enables elimination and choice of suitable material alternatives in respect to two sets of attributes: generic material selection attributes (GMSA) and specific material selection attributes (SMSA). Furthermore, the paper specifically addressed use of porous materials in plate type heat exchanger design to demonstrate the proposed model. It is expected that the paper ensures a novel procedure for marine engineers and naval architectures in conceptual design process of marine systems.     

Broadband impedance matching via lossless unsymmetrical lattice networks

In this paper, a broadband impedance matching network (equalizer) design algorithm has been proposed. In the equalizer, a lossless unsymmetrical lattice network has been utilized. The branch impedances of the lattice network are considered as singly terminated lossless LC networks, since it is not desired to dissipate power in the equalizer. After giving the algorithm, its usage has been illustrated via an example.     

Comment on "Electronically tunable current-mode second-order universal filter using minimum elements"

This comment, relates to a recently published paper by M. Sagbas and K. Fidanboylu (see ibid., vol.40, p.2-4, 2004) which presents a current-mode second-order universal filter for realising lowpass, bandpass and highpass responses. This filter employs two second-generation current controlled conveyors (CCCIIs) and two capacitors. It is shown here that this circuit is not new, simply derived from another filter proposed by S. Ozoguz and C. Acar (see ibid., vol.33, p.948-9, 1997), employing CCCIIs instead of second-generation current conveyors (CCIIs).     

Global robust stability analysis of neural networks with multiple time delays

Global robust convergence properties of continuous-time neural networks with discrete delays are studied. By employing suitable Lyapunov functionals, we derive a set of delay-independent sufficient conditions for the existence, uniqueness, and global robust asymptotic stability of the equilibrium point. The conditions can be easily verified as they can be expressed in terms of the network parameters only. Some numerical examples are given to compare our results with previous robust stability results derived in the literature. One of our main results is shown to improve and generalize a previously published result. Other results proved to establish a new set of robust stability criteria for delayed neural networks.     

Direct Assembly of Vertically Oriented,Gold Nanorod Arrays

Although many nanoscale materials such as quantum dots and metallic nanocrystals exhibit size dependent optical properties, it has been difficult to incorporate them into optical or electronic devices because there are currently no methods for precise, large-scale deposition of single nanocrystals. Of particular interest is the need to control the orientation of single nanocrystals since the optical properties are usually strongly anisotropic. Here a method based on electrophoretic deposition (EPD) is reported to precisely assemble vertically oriented, single gold nanorods. It is demonstrated that the orientation of gold nanorods during deposition is controlled by the electric dipole moment induced along the rod by the electric field. Dissipative particle dynamics simulations indicate that the magnitude of this dipole moment is dominated by the polarizability of the solution phase electric double layer around the nanorod. The resulting vertical gold nanorod arrays exhibit reflected colors due to selective excitation of the transverse surface plasmon mode. The EPD method allows assembly of arrays with a density of over one million, visually resolvable, vertical nanorods per square millimeter.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.