Tribological Performance of Laser-Textured Polyoxymethylene under Boundary Lubricated Sliding Condition

Micro texturing was fabricated on polyoxymethylene surfaces using laser texturing, and the tribological characteristics were studied. Micro texturing was proven to be very effective in lowering the coefficient of friction, regardless of the texturing depth used in this study. Laser texturing was practical and efficient owing to its fast process time with reasonable machined shapes. An optimum aspect ratio and area density of texturing would exist under the conditions used in this study. Wear of polyoxymethylene was negligible.     

Polymer-based hydrogel scaffolds for skin tissue engineering applications: a mini-review

Polymer hydrogels consist of a three-dimensional (3D) structure with cross-linked networks rich in a huge amount of water through hydrogen-bonding interactions, making them highly hydrophilic. Due to their impressive hydrophilic characteristics and cell non-cytotoxicity, polymer hydrogels are useful tissue engineering tools for the organization of cells and tissues and organ regeneration. Many biomedical engineers and researchers have recently begun to utilize polymer hydrogels as tissue or cell culture environments and as scaffolds for the stable growth of organs in tissue engineering and regeneration medicine. This paper focuses on skin regeneration in polymer hydrogels where skin is a means of protecting the body from infection or physical or chemical damage. Generally, skin tissue that has incurred minor damage or wounds can regenerate and heal in a relatively short time, while severe injuries may require transplantation or artificial skin. For those purposes, skin culturing in an in vitro environment is essential, and the environment produced using polymer hydrogel scaffolds needs to be both similar to the real environment and safe for skin cell growth. This paper reviews post-2000 skin regeneration research in the field of tissue engineering, focusing specifically on polymer hydrogels; it also discusses some of the central perspectives and key issues.     

Minimizing the Consequences of Intentional Attack on Water Infrastructure

Abstract:  Since September 11, 2001, protecting the nation's water infrastructure and improving water network resiliency have become priorities in the water industry. In this work, we develop methods to mitigate the consequences of water shortage resulting from destruction of facilities in water networks. These methods integrate search techniques, such as branch-and-bound and genetic algorithms, with a hydraulic solver to check demand feasibilities across a residual water network. The objective is to identify a feasible customer demand pattern that minimizes the consequences of water shortage in the downgraded network. We present a mathematical model of the problem addressed along with an exact solution methodology and several heuristics. We apply these methodologies to three water networks ranging in size from approximately 10–700 nodes and compare the solution quality and computational efficiency.     

Combined effect of copper, cadmium, and lead upon Cucumis sativus growth and bioaccumulation

Cucumis sativus (cucumber) was tested to assess an ecotoxicity in soils contaminated by the heavy metals copper (Cu), cadmium (Cd), and lead (Pb) separately and in combinations. The toxicity endpoint was plant growth, which was measured as shoot and root lengths after 5 day exposure. Sum of toxic unit (TU) at 50% inhibition for the mixture (EC50mix) was calculated from the dose (TU-based)-response relationships by the Trimmed Spearman-Karber method. Binary metal combinations of Cu+Cd, Cu+Pb, and Cd+Pb produced all three types of interactions; concentration additive (EC50mix=1TU), synergistic (EC50mix<1TU), and antagonistic (EC50mix>1TU) responses. Ternary combination of Cu+Cd+Pb produced an antagonistic response for the growth of Cucumis sativus. Bioaccumulations of Cu, Cd, and Pb were observed in Cucumis sativus and the bioaccumulation of one metal was influenced by the presence of other metals in metal mixtures. In general, antagonistic and/or synergistic responses reflected bioaccumulation patterns in some binary combinations, but the patterns in mixtures were not always consistent with toxicity data. This study indicated that TU approach appears to be a good model to estimate the combined effect of metals in plant systems, and mixture toxicity may be closely-related to the bioaccumulation pattern within plants. Combined effects of mixtures have to be taken into account to ecological risk assessment.     

Interaction of gaseous pollutants with aerosols in Asia during March 2002

The Asian Dust Aerosol Model (ADAM) and the aerosol dynamic model with the output of the fifth generation of mesoscale model (MM5) in a grid of 60x60 km2 over the Asian domain have been performed with and without the heterogeneous reaction (gas-aerosol interaction) to estimate the effect of the gas-aerosol interaction on the formation of aerosol for the period of 1-31 March 2002 when a severe Asian dust event has been observed during this period. The simulated gas-phase pollutants concentrations and aerosols are compared with those observed in South Korea and the East Asia Network (EANET). The results indicate that the present modeling system including ADAM, aerosol dynamic model and MM5 model simulates quite well and the gas-phase pollutants concentrations observed in South Korea and the simulated aerosol concentrations with the gas-aerosol interaction yield much better results in concentrations than those without the gas-aerosol interaction. It is found that the favorable regions for the gas-aerosol interaction in Asia are eastern China (high pollutants emissions), Korea, Japan and the East China Sea that are downstream regions of the Asian dust sources and relatively high relative humidity. In these regions the concentrations of SO2 and O3 decrease whereas the concentrations of sulfate and nitrate increase significantly due to the gas-aerosol interaction. In particular, the increase of sulfate concentration due to the interaction is more than 30% of the corresponding concentration without the gas-aerosol interaction. It is also found that the time-area mean column concentrations of PM10, sulfate, nitrate in the model domain are respectively to be 154.9, 3.2, 3.6 mg m(-2) without the gas-aerosol interaction. However, with the gas-aerosol interaction these values have been increased to 0.6% (155.8 mg m(-2)), 16% (3.7 mg m(-2)), and 14% (4.1 mg m(-2)) of the corresponding concentration without the gas-aerosol interaction. On the other hand, the time-area mean concentration of ammonium is found to decrease about 13% (1.8 mg m(-2) to 1.6 mg m(-2)) due to the gas-aerosol interaction. The result clearly indicates the importance of the gas-aerosol interaction on the tropospheric chemistry during the long-range transport period.     

Zoning‐Based Vertical Transportation Optimization for Workers at Peak Time in a Skyscraper Construction

In a skyscraper construction, a great number of workers and materials must be vertically transported to the proper positions depending on their roles. Particularly, the optimal vertical transportation logistics of lift cars for workers at peak time should be established to enhance the entire project performance in a skyscraper construction. For achieving this objective, the zoning‐based concept can be introduced to improve the effectiveness of the vertical transportation logistics of lift cars for workers at peak time in a skyscraper construction. In developing the zoning‐based vertical transportation logistics, it is necessary to consider the minimization of the electricity consumption as an environmental index as well as the minimization of the operating time and the maximization of the cost effectiveness. Therefore, this study aims to develop a multi‐objective optimization model for solving the time–cost–environment trade‐off problem in establishing the zoning‐based vertical transportation logistics of lift cars for workers at peak time in a skyscraper construction.     

Buckling behavior of steel girder in integral abutment bridges under thermal loadings in summer season during deck replacement

Integral abutment bridges (IABs) have been built and successfully served for several decades, especially in the U.S. and Europe, because of their many advantages, such as their structural efficiency, stability and low construction and maintenance costs. Recently, the deck slabs of IABs have reached the end of their service lives and bridge administrators have decided to replace the deck slabs to extend the bridge service life. Due to the restraints at both abutments, the steel girder IAB is subjected to high axial forces and the steel girders have laterally buckled during deck slab replacement. This study performed numerical simulations to identify the buckling modes that may occur during deck replacement and the results were compared to the recent accident which happened in Missouri, U.S. Key parameters such as the length of the girder, width and thickness of the flange and the imperfection level were selected and a parametric study was performed. Using the obtained critical buckling stress, an equation for predicting the critical buckling stress of the girder in IABs during deck removal was developed. The results provide a better and safer, long-lasting IAB design and maintenance regime.     

Smart damper using sliding friction of aramid brake lining and self-centering of rubber springs

The purpose of this study is to develop a smart damper with flag-shaped behavior by using the sliding friction of aramid brake lining and the restoring capacity of pre-compressed rubber springs. For this purpose, the friction force of aramid brake lining pressed by bolts was used along with polyurethane rubbers, each with a length of 80 mm, a diameter of 95 mm, and a circular hole of 37 mm. In the experiments, loading frequency and torque force were considered. The loading frequency varied from 0.1 to 2.0 Hz, and frictional force was controlled by variable bolt torque force. The tests were conducted to demonstrate that the clamping force by the bolts could provide normal force to frictional material. The friction force by the aramid brake lining sliding was tested, followed by the pre-compressed rubber springs' behavior. Afterward, a damper combining the two components was tested to verify flag-shaped behavior by using a dynamic actuator, and the damping ratios were evaluated from the hysteretic curves. The behavior of the damper closely matched flag-shaped behavior, resulting in self-centering and energy dissipation capacity.