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.     

Investigation of rheological and mechanical properties of wood flour reinforced polypropylene

This study aims to investigate the rheological and mechanical properties of polypropylene when reinforced by wood flour. In the study, wood flour (WF) was added as filling material to polypropylene (PP), and its effects were investigated. The grain size of the wood flour was 300 μm. Wood flour was mixed into polypropylene material at different rates of weight. Viscosity changes of the polypropylene at various temperatures and pressure values of the concentration rates were documented. As a result, viscosity increases as the amount of wood flour filling is increased; yet as temperature, pressure and shear rate are increased, viscosity decreases. The study also focuses on the changes in its mechanical properties of polypropylene reinforced with wood flour. The tensile strength of reinforced polypropylene at varying percentages (wt. 5 %, wt. 10 % and wt. 15 %) was observed to decrease by 11 %, 16 % and 21 %, respectively, compared to that of non‐reinforced polypropylene. Additionally, impact tests showed that the highest energy absorption was in 10 % wood flour reinforced polypropylene.     

A novel classification-based shilling attack detection approach for multi-criteria recommender systems

Recommender systems are emerging techniques guiding individuals with provided referrals by considering their past rating behaviors. By collecting multi-criteria preferences concentrating on distinguishing perspectives of the items, a new extension of traditional recommenders, multi-criteria recommender systems reveal how much a user likes an item and why user likes it; thus, they can improve predictive accuracy. However, these systems might be more vulnerable to malicious attacks than traditional ones, as they expose multiple dimensions of user opinions on items. Attackers might try to inject fake profiles into these systems to skew the recommendation results in favor of some particular items or to bring the system into discredit. Although several methods exist to defend systems against such attacks for traditional recommenders, achieving robust systems by capturing shill profiles remains elusive for multi-criteria rating-based ones. Therefore, in this study, we first consider a prominent and novel attack type, that is, the power-item attack model, and introduce its four distinct variants adapted for multi-criteria data collections. Then, we propose a classification method detecting shill profiles based on various generic and model-based user attributes, most of which are new features usually related to item popularity and distribution of rating values. The experiments conducted on three benchmark datasets conclude that the proposed method successfully detects attack profiles from genuine users even with a small selected size and attack size. The empirical outcomes also demonstrate that item popularity and user characteristics based on their rating profiles are highly beneficial features in capturing shilling attack profiles.     

Two-phase multi-expert knowledge approach by using fuzzy clustering and rule-based system for technology evaluation of unmanned aerial vehicles

Neural Computing and Applications - Unmanned aerial vehicles (UAVs) are utilized in many different areas for different aims such as the benefit of humanity, safety control, traffic control, crop...     

The Oxidative Polycondensation of Benzylidene-4′-hydroxyanilene using Air O<Subscript>2</Subscript>, NaOCl and H<Subscript>2</Subscript>O<Subscript>2</Subscript>: Synthesis and Characterization

In this work, the oxidative polycondensation reaction conditions of benzylidene-4′-hydroxyanilene (B-4′-HA) were studied using oxidants such as air O₂, H₂O₂ and NaOCl in an aqueous alkaline medium between 40 and 95 ^○C. Oligo-benzylidene-4′-hydroxyanilene was characterized by ¹H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. The solubility of oligomer using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl₃, CCl₄, toluene, acetonitrile, ethyl acetate was investigated. According to air O₂ oxidant (flow rate 8.5 L/h), the conversion of B-4′-HA was 82.0% in optimum conditions such as [B-4′-HA]₀=[KOH]₀=0.1015 mol/L at 50 ^○C for 25 h. According to the SEC analysis, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of O-B-4′-HA were found to be 1852 g mol⁻¹, 3101 g mol⁻¹ and 1.675; 2123 g mol⁻¹, 4073 g mol⁻¹ and 1.919; 2155 g mol⁻¹, 4164 g mol⁻¹ and 1.932, using air oxygen, NaOCl and H₂O₂ oxidants, respectively. Also, Thermo gravimetric analysis (TGA) showed oligo-benzylidene-4′-hydroxyanilene to be unstable against thermo-oxidative decomposition. The weight loss of O-B-4′-HA was found to be 95.87% at 1000 ^○C.     

Effect of stress levels applied to pre‐stressing strands on post‐tensioned beam‐to‐column connections

This article presents an experiment study on precast beam‐to‐column connections with 15.24 mm diameter pre‐stressing strands. Stress levels of 40%, 50% and 60% tensile stress were applied to the pre‐stressing strands. Following the experiments, precast specimens were compared with those of the reference specimen. The load capacity of the precast specimen stressed at the level of 60% tensile stress reached 98% of the load capacity of the reference specimen, and the amount of energy dissipated by the same specimen reached 48% of that dissipated by the reference specimen. The stiffness of the same precast specimen at 1.5% drift was observed to be 71% of the reference specimen at the same drift. In the experimental studies, it was seen that the maximum stress determined in this specimen, which pre‐stressing strands provide connection, reached approximately 75% of the tensile strength of the pre‐stressing strands. It could not be said that topping concrete in experiment specimens increased strength. Copyright © 2010 John Wiley & Sons, Ltd.     

Information systems outsourcing decisions using a group decision-making approach

Outsourcing refers to a company that contracts with another company to provide services that might otherwise be performed by in-house employees. Information system (IS) outsourcing policies define the criteria that organizations utilize to decide upon the scope and degree of reliance of their IS capabilities upon external sources. IS outsourcing is an innovative organizational tool for IS management in both private and public sector organizations. In this paper, an interactive group decision-making methodology is proposed to select/rank IS providers under multiple criteria. A measure for the consensus level of the group preferences is developed to satisfy an acceptable level of group agreement and reliability. The Spearman coefficients for both the aggregated rank order and each DM's rank order have also been calculated. The group and the individual evaluations are gathered through a fuzzy TOPSIS approach. The proposed methodology is applied in the largest office furniture manufacturer in Konya-Turkey. Eight alternative IS providers are evaluated based on seven criteria by five decision makers. Sensitivity analyses are also provided to see the effects of parameter changes on the final decision.     

Schiff base‐substituted polyphenol: synthesis,characterisation and non‐isothermal degradation kinetics

BACKGROUND: Polymers of phenols and aromatic amines have emerged as new materials in fields such as superconductors, coatings, laminates, photoresists and high‐temperature environments. The stability, kinetics and associated pollution of the thermal decomposition of oligophenols are of interest for the aforementioned fields. RESULTS: A new Schiff base polymer, derived from N,N′‐bis(2‐hydroxy‐3‐methoxyphenylmethylidene)‐2,6‐pyridinediamine, was prepared by oxidative polycondensation. Characterisations using Fourier transform infrared, UV‐visible, ¹H NMR and ¹³C NMR spectroscopy, thermogravimetric/differential thermal analysis, gel permeation chromatography, cyclic voltammetry and conductivity measurements were performed. The number‐average (M_n) and weight‐average molecular weight (M_w) and dispersity (D = M_w/M_n) of the polymer were found to be 61 000 and 94 200 g mol^?1 and 1.54, respectively. Apparent activation energies of the thermal decomposition of the polymer were determined using the Tang, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose and Coats–Redfern methods. The most likely decomposition process was a D_n deceleration type in terms of the Coats–Redfern and master plot results. CONCLUSION: The mechanism of the degradation process can be understood through the use of kinetic parameters obtained from various non‐isothermal methods. Copyright © 2009 Society of Chemical Industry     

Correlation between barrier heights and ideality factors of H-terminated Sn/p-Si(1 0 0) Schottky barrier diodes

Schottky barrier diodes (SBDs) were prepared by evaporation on H-terminated p-Si(1 0 0) surfaces. The Si(1 0 0)-H surfaces were obtained by wet chemical etching in diluted hydrofluoric acid. The current–voltage (I–V) characteristics of real SBDs are described by using two fitting parameters that are the effective barrier height (EBH) and ideality factor n. They were determined from I–V characteristics of SBDs (30 diodes) fabricated under experimentally identical conditions. The obtained values of EBHs varied from 0.729 to 0.749 eV, and the values of ideality factors varied from 1.083 to 1.119. The results showed that both parameters of SBDs differ from one diode to another even if they are identically prepared. The EBH distributions were fitted by two Gaussian distribution functions, and their mean values were found to be 0.739 ± 0.003 eV and 0.733 ± 0.001 eV, respectively. The homogeneous barrier height of SBDs was found to be 0.770 eV from the linear relationship between EBHs () and ideality factors (n).     

Processing and characterisation of 2-D woven metal fibre-reinforced multilayer silica matrix composites using electrophoretic deposition and pressure filtration

A novel, cost-effective and rapid processing route including electrophoretic deposition (EPD) and pressure filtration (PF) has been developed for the fabrication of 2-D woven metallic fibre mat reinforced multilayer silica matrix composites. Commercially available silica sol containing ultrafine ceramic particles (15 nm) was used as the matrix whilst 2-D woven metal stainless steel 316L fibre mat was used as the metal reinforcement to produce a composite having 2-D isotropic properties. The colloidal silica sol was modified with boria and boehmite in order to produce a silica matrix which could be sintered at 900°C (the maximum use temperature for the particular fibre architecture employed), and with densification taking place before crystallisation. An in-situ electrophoretic deposition (EPD) cell capable of measuring the weight gain in real time during deposition was designed. This technique enabled the woven fibre inter/intra tow regions to be infiltrated with the ultrafine silica particles in a very short time (2 min). Green bodies made from electrophoretically deposited fibre mats were further consolidated using pressure filtration. The EPD parameters were optimised in terms of time, voltage and deposition thickness as well as deposit formation rate. Microstructural observation indicated that the composites produced were dense and of high microstructural homogeneity.