Properties of hybrid fiber reinforced concrete under repeated impact loads

Steel, glass and polypropylene fiber reinforced concrete structures were investigated under repeated impact loads in this study. Twelve different concrete series prepared fiberless the one of all and the others reinforced polypropylene, glass, steel and hybrid fiber were produced. The polypropylene fibers in diameter of 50 mm, l/d (length/diameter) ratio of 400, glass fibers in diameter of 14 mm, l/d ratio of 857 and steel fibers in diameter of 0.75 mm, l/d ratio of 80 were used in concrete mixtures. The volumetric contents were 0.5, 0.75 and 1% in fiber reinforced concrete. The volumetric content was 0.1% in polypropylene and glass fiber reinforced concrete. Drop-weight test similar the method presented ACI 544.2R-89 was conducted using 28 days cured samples having dimensions of 100 × 100 × 100 mm and reduction of strength were determined by ultrasonic pulse velocity (UPV) test. Furthermore, initial crack formation and fracture numbers variation in samples were detected. In the present study, it was aimed to develop performance under repeated impact loads using fibers in concrete. As a result, especially impact performance was rather increased in steel fiber reinforced concrete, and hybrid fibers had also positive effect on the performance of concrete.     

Production of ferroboron powders by solid boronizing method

Ferroboron is an iron-boron alloy containing 10–20% of boron by weight. Commercial ferroboron production is made by two main processes: carbothermic reaction and aluminothermic reaction. Ferroboron also occurs in steel surfaces due to boronizing, which is applied to increase surface hardness in steel. Boronizing is a thermo-chemical surface hardening treatment. The ferroboron phases like Fe₂B, FeB form by diffusing of boron element into iron. These phases are very hard, wear strengths are high, and friction coefficients are low.In this study, ferroboron powder was obtained by boronizing ASC 100.29 iron powder that was used widely in powder metallurgy area. Solid boronizing method was preferred due to its advantages in applications and Ekabor-HM powder was used as the boronizing agent. The 80% ASC 100.29 and 20% Ekabor HM were mixed homogeneously and subjected to boronizing at 850–950 °C for 1–6 h. Formation and development of ferroboron phase on the samples was determined by metallographic studies depending on various treatment conditions. The X-ray diffraction analysis revealed that the Fe₂B phase did form but FeB phase did not. Micro hardness distributions were measured on the powder grains. Eighteen GPa hardness was measured at Fe₂B phase obtained by boronizing while hardness of non-boronized iron powders was 1.06 GPa. The thickness of ferroboron layer formed by boronizing changed with boronizing conditions. The thickness of ferroboron layer increased with boronizing temperature or boronizing time. Depending upon processing parameters, ferroboron layers was formed partially or throughout ferrous powder structure. Since boronizing can be applied to iron powders having any size or shape, ferroboron production with required shape and size is possible.Finally, a new method, namely solid boronizing method, was developed in ferroboron powder production.     

Drying behaviour of hazelnuts

A diffusion-based drying model has been used to model the single-layer drying of whole hazelnuts (hull, air-gap and kernel). The drying of whole hazelnut was simulated by means of the numerical solution of Fick's second law for a composite solid consisting of an air gap which is between two solid bodies. The Crank–Nicholson implicit numerical method has been applied to solve the diffusion equation with variable mass diffusivity for whole hazelnut. The numerical data so obtained has been presented in the form of a series of curves which represent the drying characteristics of a whole hazelnut. Experimental results from whole hazelnuts were correlated with the theoretical results. A correlation describing the diffusivity of the whole hazelnut with moisture content and temperature was established, when the expression of diffusivity for hazelnut was used. Predicting the drying behaviour of the hazelnut for a known value of initial moisture content, equilibrium moisture and equivalent radius, a good agreement was obtaind between the experimental data and prediction. Whole hazelnut diffusivity was determined at 25–45°C, air velocity 0·2–0·3 ms⁻¹, 60% relative humidity of air, using the theoretical curves that best fit the data. The diffusivity of whole hazelnut was determined as a function of drying air temperature and velocity. © 1998 SCI.     

Improvements in the analysis of a TH0mp cavity for precise complex permittivity measurements

An analysis of the relationship between complex permittivity and complex resonance frequency is proposed for a cylindrical cavity oscillating in a TM_0mp mode. Effects of wall conductivity, coupling loops, and holes for the insertion of dielectric samples are fully taken into account. With dielectric samples of small radii, insertion holes produce the most important corrections. Their effect is modelled through a finite difference solution of the field equations in the neighbourhood of the holes. A computer program is described for the analysis, the use of which is illustrated with results of measurements.     

Sensitivity minimization in signal-flow graphs

In this work, the sensitivity minimization is studied for the signal-flow graph of the state equations realizing a general biquadratic transfer function. The sensitivity measures are defined for the poles, the pole Q and the pole frequency. The formulae are derived for their evaluations. Branch transmittances minimizing each of these measures are found. Finally, it is shown that there exists a very simple relation between the measures. The results obtained in this study can be used in active filter realizations with minimum sensitivity.     

Replicated partitioning for undirected hypergraphs

Hypergraph partitioning (HP) and replication are diverse but powerful tools that are traditionally applied separately to minimize the costs of parallel and sequential systems that access related data or process related tasks. When combined together, these two techniques have the potential of achieving significant improvements in performance of many applications. In this study, we provide an approach involving a tool that simultaneously performs replication and partitioning of the vertices of an undirected hypergraph whose vertices represent data and nets represent task dependencies among these data. In this approach, we propose an iterative-improvement-based replicated bipartitioning heuristic, which is capable of move, replication, and unreplication of vertices. In order to utilize our replicated bipartitioning heuristic in a recursive bipartitioning framework, we also propose appropriate cut-net removal, cut-net splitting, and pin selection algorithms to correctly encapsulate the two most commonly used cutsize metrics. We embed our replicated bipartitioning scheme into the state-of-the-art multilevel HP tool PaToH to provide an effective and efficient replicated HP tool, rpPaToH. The performance of the techniques proposed and the tools developed is tested over the undirected hypergraphs that model the communication costs of parallel query processing in information retrieval systems. Our experimental analysis indicates that the proposed technique provides significant improvements in the quality of the partitions, especially under low replication ratios.     

Flexibility improvement of short glass fiber reinforced epoxy by using a liquid elastomer

In certain applications of fiber reinforced polymer composites flexibility is required. The aim of this study was to improve flexibility of short glass fiber reinforced epoxy composites by using a liquid elastomer. For this purpose, diglycidyl ether of bisphenol-A (DGEBA) based epoxy matrix was modified with hydroxyl terminated polybutadiene (HTPB). A silane coupling agent (SCA) was also used to improve the interfacial adhesion between glass fibers and epoxy matrix. During specimen preparation, hardener and HTPB were premixed and left at room temperature for an hour before mixing with epoxy resin to allow possible reactions to occur. In order to compare flexibility of the specimens flexural tests were conducted and the data were evaluated numerically by using a derived relation. Test data and scanning electron microscope analysis indicated that surface treatment of glass fibers with SCA, and HTPB modification of epoxy matrix improved flexural properties especially due to the strong interaction between fibers, epoxy, and rubber. It was also observed that HTPB modification resulted in formation of relatively round rubber domains in the epoxy matrix leading to increased flexibility of the specimens.