Fabrication of sintered porous polymeric materials: effect of chain interdiffusion time on mechanical properties

In this study, sintered porous polymeric materials made of high density polyethylene (HDPE) were fabricated through controlling the chain interdiffusion time at the transition temperature of semicrystalline and melt states. At this intermediate state, where both crystalline and amorphous phases coexist, the interfacial welding of HDPE particles is facilitated thanks to interdiffusion caused by chain relaxation phenomena. Then, by assuming a spherical shape and a cubic packing configuration of particles, a geometrical model was developed to predict porosity variations as sintering progresses. Moreover, the HDPE used, as a broad molecular weight distributed polymer, has different family chains with different specific molecular weight ranges. Accordingly, the melt coalescence rate of the particles was tracked using an optical microscope equipped with a hot stage, in order to determine the diffusion characteristic times for each family. During the characterization stage, SEM images proved the presence of porous structures in the sintered samples. In addition, mechanical properties were assessed through the shear punch test. It was shown that the mechanical properties are governed by the interdiffusion of long chains which occurs at relatively long sintering times. The results also demonstrated the role of reptation motion of long chains in the interfacial welding of polymeric particles. They revealed the compatibility of macroscopic properties of the samples and chain motions at microscopic levels. © 2017 Society of Chemical Industry     

Bootstrap Technique for Risk Analysis with Interval Numbers in Bridge Construction Projects

Bridges are principal and vital transportation structures. If risk management is not considered in bridge construction projects, objectives cannot be delivered on time, on budget, or with suitable quality results. Risk data set sizes and experts’ judgments are not usually sufficient for analyzing significant risks in bridge construction projects; moreover, the statistical distributions for risk parameter estimates are usually unknown. Standard parametric statistical techniques cannot provide appropriate solutions for cases with small data sets or unknown distributions. This paper proposes a new hybrid approach by using a nonparametric resampling technique and interval computations for risk analysis, in particular, for bridge construction projects. Bootstrap techniques produce more accurate inferences for comparing parametric techniques and are an alternative when the underlying parametric assumptions are not considered. Increasingly, because of the complexity and uncertainty in decision making at bridge projects, it is easier or more natural to provide interval values for parts or all of decision-making judgments. Furthermore, the goal of reducing standard deviations for both risk probability and risk impact compared with the conventional approach is another conclusion of this paper. The proposed approach is applied to a case in Iran to show the validity of the approach.     

Mechanical properties of transparent poly(methyl methacrylate) nanocomposites reinforced with core–shell polyacrylonitrile/poly(methyl methacrylate) nanofibers

Having considered the mechanical and optical properties related to microstructure, the authors of the present work did a study of the in situ interface formation between polyacrylonitrile/poly(methyl methacrylate) (PAN/PMMA) core–shell nanofibers and PMMA resin so as to prepare reinforced PMMA nanocomposites (NCs). The NCs were produced using the dip-coating method. The core–shell nanofibers were generated via phase separation of PAN/PMMA solution during the conventional electrospinning. The results of attenuated total reflection-Fourier transform infrared spectroscopy, transmission electron microscope, and energy dispersive X-ray spectrometer confirmed the formation of core–shell structure of the PAN/PMMA nanofibers. According to the findings of the study, the NCs reinforced with 1.7% volume fractions (v _f) of the core–shell nanofibers, having the composition of 50/50 (PAN/PMMA), had the highest tensile and bending properties. The obtained results showed that by increasing the v _f of nanofibers from 1.7 to 2.9%, the tensile and bending moduli increased by 29.9 and 44.2%, respectively. Increasing v _f to 5.7% decreased the just-mentioned properties. Moreover, the transparency of NCs decreased by less than 1, 10, and 18%, respectively, when the aforementioned volume fractions were applied. The theoretical values for the tensile modulus were calculated using the models proposed by Manera, Pan, and Halpin–Tsai–Nielsen. The best prediction was made when the model proposed by Halpin–Tsai–Nielsen was applied.     

Possibility of NiO,ZrO 2 , ZnO and GrO 2 Attached to Silicon and Carbon Nanostructures as Anode of Battery: Computational Examination

In current study, possibility of Si-nanocage (Si48) and C-nanocage (C60) as anodes in LIB, NIB and KIB are examined via computational methods. Adsorption o     

A possibilistic programming approach for the location problem of multiple cross-docks and vehicle routing scheduling under uncertainty

This article considers the design of cross-docking systems under uncertainty in a model that consists of two phases: (1) a strategic-based decision-making process for selecting the location of cross-docks to operate, and (2) an operational-based decision-making process for vehicle routing scheduling with multiple cross-docks. This logistic system contains three echelons, namely suppliers, cross-docks and retailers, in an uncertain environment. In the first phase, a new multi-period cross-dock location model is introduced to determine the minimum number of cross-docks among a set of location sites so that each retailer demand should be met. Then, in the second phase, a new vehicle routing scheduling model with multiple cross-docks is formulated in which each vehicle is able to pickup from or deliver to more than one supplier or retailer, and the pickup and delivery routes start and end at the corresponding cross-dock. This article is the first attempt to introduce an integrated model for cross-docking systems design under a fuzzy environment. To solve the presented two-phase mixed-integer programming (MIP) model, a new fuzzy mathematical programming-based possibilistic approach is used. Furthermore, experimental tests are carried out to demonstrate the effectiveness of the presented model. The computational results reveal the applicability and suitability of the developed fuzzy possibilistic two-phase model in a variety of problems in the domain of cross-docking systems.     

Combined heat transfer of radiation and forced convection flow of participating gases in a three-dimensional recess

This research work presents a numerical investigation of three-dimensional combined convection-radiation heat transfer over a recess including two inclined steps in a horizontal duct. To simulate the inclined surface boundaries, the blocked off method is employed for both fluid mechanic and radiation problems. The fluid is treated as a gray, absorbing, emitting and scattering medium. In numerical solution of the governing equations including conservation of mass, momentum and energy, the three-dimensional Cartesian coordinate system is used. These equations are solved numerically using the CFD techniques to obtain the temperature and velocity fields. Discretized forms of the governing equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation terms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of radiation-conduction parameter, optical thickness and albedo coefficient on heat transfer behavior of the system are presented. Comparison of numerical results with the available data published in open literature shows a good agreement.     

An interaction stress analysis of nanoscale elastic asperity contacts

A new contact mechanics model is presented and experimentally examined at the nanoscale. The current work addresses the well-established field of contact mechanics, but at the nanoscale where interaction stresses seem to be effective. The new model combines the classic Hertz theory with the new interaction stress concept to provide the stress field in contact bodies with adhesion. Hence, it benefits from the simplicity of non-adhesive models, while offering the same applicability as more complicated models. In order to examine the model, a set of atomic force microscopy experiments were performed on substrates made from single-walled carbon nanotube buckypaper. The stress field in the substrate was obtained by superposition of the Hertzian stress field and the interaction stress field, and then compared to other contact models. Finally, the effect of indentation depth on the stress field was studied for the interaction model as well as for the Hertz, Derjaguin-Muller-Toporov, and Johnson-Kendall-Roberts models. Thus, the amount of error introduced by using the Hertz theory to model contacts with adhesion was found for different indentation depths. It was observed that in the absence of interaction stress data, the Hertz theory predictions led to smaller errors compared to other contact-with-adhesion models.     

Risk constrained self-scheduling of hydro/wind units for short term electricity markets considering intermittency and uncertainty

In view of the intermittency and uncertainty associated with both the electricity production sector of restructured power system and their competitive markets, it is necessary to develop an appropriate risk managing scheme. So that it is desirable to trade-off between optimum utilization of intermittent generation resources (i.e. renewable energy resources), uncertain market prices and related risks in order to maximize participants' benefits and minimize the corresponding risks in the multi-product market environment. The main goal of this paper is to investigate risk management by introducing a novel multi-risk index to quantify expected downside risk (EDR) which is caused by both the wind power and market price uncertainties. Value-at-Risk (VaR) method is used to assess the mentioned risk issue by the proposed weighted EDR, so that an optimal trade-off between the profit and risk is made for the system operations. Also, the roulette wheel mechanism is employed for random market price scenario generation wherein the stochastic procedure is converted into its respective deterministic equivalents. Moreover, the autoregressive integrated moving average (ARIMA) model is employed to characterize the stochastic wind farm (WF) generation by predetermined mean level and standard deviation of wind behavior as well as temporal correlation. The problem is formulated as a mixed-integer stochastic framework for a hydro-wind power system scheduling and tested on a generation company (GENCO).     

Continuous-time state-space unsteady aerodynamic modeling based on boundary element method

In this paper a continuous-time state-space aerodynamic model is developed based on the boundary element method. Boundary integral equations governing the unsteady potential flow around lifting bodies are presented and modified for thin wing configurations. Next, the BEM discretized problem of unsteady flow around flat wing equivalent to the original geometry is recast into the standard form of a continuous-time state-space model considering some auxiliary assumptions. The system inputs are time derivative of the instantaneous effective angle of attack and thickness/camber correction terms while the outputs are unsteady aerodynamic coefficients. To validate the model, its predictions for aerodynamic coefficients variations due to the various unsteady motions about different wing geometries are compared to the results of the direct BEM computations and verified numerical and theoretical solutions. This comparison indicates a good agreement. Since the resulting aerodynamic model is in the continuous-time domain, it is particularly useful for optimization and nonlinear analysis purposes. Moreover, its state-space representation is the appropriate form for an aerodynamic model in design or control applications.