Material Spreading and Compaction in Power-Based Solid Freeform Fabrication Methods: Mathematical Modeling

In this study, the spreading and compaction of powder by a counter-rotating roller, in the solid freeform fabrication (SFF) process, is modeled and characterized. The effect of layer thickness, roller diameter, amount of loose powder, and its initial density on the powder bed’s relative density and roller compaction pressure is investigated.Mechanical Engineerin

Effect of poor curing conditions and remedies on the durability of steel in concrete

An investigation was undertaken to study the effect of poor curing simulating hot climatic conditions and remedies on the durability of steel in concrete. Three different curing environments were used i.e. (1) Saturated Ca(OH)2 solution at 20°C, (2) Saturated Ca(OH)2 solution at 50°C and (3) Air at 50°C at 30% relative humidity. The third curing condition corresponding to the temperature and relative humidity typical of Middle Eastern Countries. The nature of the hardened cement paste matrix, cured under the above conditions was studied by means of Mercury Intrusion Porosimetry for measuring pore size distribution. The results were represented as total pore volume and initial pore entry diameter. The Scanning Electron Microscope was used to look at morphological changes during hydration, which were compared to the Mercury Intrusion Porosimetry results. X-ray defraction and Differential Thermal Analysis techniques were also employed for looking at any phase transformations. Polymer impregnation was used to reduce the porosity of the hardened cement pastes, especially in the case of the poorly cured samples. Carbonation rates of unimpregnated and impregnated cements were determined. Chloride diffusion studies were also undertaken to establish the effect of polymer impregnation and blending of the cements. Finally the corrosion behaviour of embedded steel bars was determined by the technique of Linear Polarisation. The steel was embedded in both untreated and polymer impregnated hardened cement pastes placed in either a solution containing NaCl or an environmental cabinet which provided carbonation at 40°C and 50% relative humidity

Electrodynamic Concentration of Non-ferrous Metallic Particles in the Moving Gas-powder Stream: Mathematical Modeling and Analysis

This paper presents theory, modeling, and analysis of a novel electrodynamic concentration approach for submillimeter-sized conductive metal particles focusing in moving gas-powder stream. Such method is of particular interest in blown-powder feeding fabrication industry (e.g., powder-fed additive manufacturing) to generate a tightly focused powder stream. Conceptual design of a concentration generator is proposed with two different configurations: The doublet Halbach permanent magnet quadrupoles (doublet-Halbach-PMQs) and the doublet electromagnet quadrupoles (doublet-EMQs). Analytical models for magnetic forces and concentration angles were built. Numerical calculations were conducted for pure aluminum particles with a radius of 50 150 μm. The particles with a radius of R_{p} = 300 μm can be concentrated with more than 15∘ angle at the frequency of 600 kHz. Therefore, the proposed doublet-EMQs configuration has a great potential to generate a narrowed and finely focused powder stream in the blown-powder feeding fabrication process

Directed energy deposition powder stream modeling using a Gaussian beam ray representation

The powder stream from a side feed nozzle, or part of the powder stream in some coaxial nozzles, in a directed energy deposition via powder feeding (DED-PF) machine, can be modeled using a particle velocity field that has a constant downward component and a linearly increasing outward component, in proportion to the powder stream's center line distance. However, when the powder stream is subject to a force field, it was found that the shape of the powder concentration function close to the center of the powder stream diverges considerably at high degrees of focusing. This discrepancy is reduced by modeling the powder stream based on the ray representation of a Gaussian beam. Experimental results from high-speed camera particle tracking and numerically extrapolating the trajectories to the nozzle exit suggests that the statistics of the powder stream correspond to this model. A novel method to compute the particle concentration along the stream using an optical system analog, with the focusing force field modeled as the transfer matrix of a graded refractive index (GRIN) lens, is also demonstrated. This method is orders of magnitude faster than the corresponding Lagrangian simulation

Imitation vs. innovation in the SME sector

It is frequently mentioned that during economic crisis one of the key elements of strengthening SMEs is permanent renewal and capability of innovation. However, only some businesses are able to carry out classical product/service innovation (Schumpeter). We are of the opinion that observing the best practice, adoption and/or adaption – the imitation – should be given a higher function in everyday business life. From the point of view of business development, imitation can be more important than innovation. To find ideas worth imitating it is necessary to react and start to produce rapidly, at the right time and bring to the market the essentials of im itation. The aim of the paper is to give a theoretical overview of the interpretations of innovation and imitation. Sectoral features and regional differences determine the scope of development and renewal for existing enterprises. At the same time the knowledge of entrepreneurs regarding innovation is insufficient, thus a new paradigm is needed

An Analytical Approximation to the Block Error Rate in Nakagami-m Non-Selective Block Fading Channels

With few exceptions, an analytical closed-form expression for the block error rate (BLER) is lacking in block fading channels. Thus, the BLER is often obtained by numerical methods, such as Monte-Carlo simulations, resulting in additional computational complexity. In this paper, we propose a single-parameter analytical approximation for the BLER in flat, block-fading Nakagami-m channels, which significantly reduces the computational overhead. The low computational cost of the approximation makes it feasible to include the BLER in the objective function of larger optimization problems

A Low-Complexity Semi-Analytical Approximation to the Block Error Rate in Nakagami-m Block Fading Channels

<p>There are few analytical formulas that can be used for calculating the block error rate (BLER) in block fading channels. Thus, an estimate of the BLER is often obtained using numerical methods. One such method is the threshold method which assigns 0 or 1 to the instantaneous BLER given the signal to noise ratio (SNR) level. It has been shown that utilizing such a method results in an accurate approximation of the BLER in Nakagami-m block fading channels for a wide range of m.</p> <p>In this work, we consider a recently proposed simple method of obtaining the threshold and study the effect of adopting different physical layer and channel parameters on that threshold. We show that, while the value of this threshold depends on the modulation, coding, and block size, it is almost unaffected by the m parameter of Nakagami-m channels for a wide range of practical values. In addition, for a given modulation and coding method, the threshold is shown to be a simple function of block size. As a result, the computational complexity required to obtain the threshold can be significantly reduced.</p

A Complexity Adjustable Scheduling Algorithm for Throughput Maximization in Clusterized TDMA Networks

We consider clustered wireless networks, where transceivers in a cluster use a time-slotted mechanism (TDMA) to access a wireless channel that is shared among several clusters. Earlier work has demonstrated that a significant increase in network throughput can be achieved if all the schedules are optimized jointly. However, a drawback of this approach is the prohibitive level of computational complexity is required when a network with a large number of clusters and time-slots is to be scheduled. In this paper, we propose a modification to our previously proposed algorithm which allows for the complexity to be adjusted to the available processing power, provided some minimum processing power is available. This is achieved by carefully reducing the number of interfering clusters considered when scheduling a cluster. In addition, we propose and evaluate two heuristic methods of discarding the less significant clusters. While the optimality of the obtained schedule is not proven, our results demonstrate that large gains are consistently attainable

On the impact of powder cohesion on the bulk properties of a powder bed in Additive Manufacturing using Discrete Element Method (DEM) simulations

In powder based Additive Manufacturing (AM) a uniform and compact spread of particles is required which can then be accurately fused layer by layer to form final products. As powders are spread, several parameters control the quality of the final powder bed layer; namely, spreader type, powder grain shape, powder characteristics and ambient manufacturing conditions. Utilising discrete element method (DEM) simulations this paper investigates the effect of cohesion on the quality of the powder bed post spreading. However, only cohesion due to the formation of liquid bridges as a result of moisture content of the powder is considered in this work. Simulations are run with a realistic spreader (geometry of which was created from data points from manufacturing equipment used within industry), alongside realistic particle shapes created via Multi-Sphere Approximations (MSA) of models derived from powder X-ray microtomography images, see Figure 1. A random selection of powder particles is chosen and used within simulations, with the resolution of these particles being controlled via a surface smoothing factor [1] to ensure an acceptable balance of accuracy and computational cost. Simulations are run with an appropriate subset of the total number of particles to yield a statistically accurate representation of the grain population to identify the effects of cohesion on the final quality of the powder bed layer. In this paper for the first time, the relationship between the moisture content and powder bed quality is investigated and the simulation results indicate that the cohesion has a strong effect on the powder bed quality which is quantified via a surface roughness parameter and powder's bulk density