Understanding creep in nanocrystalline materials

Only a limited number of creep investigations have been carried out on nanocrystalline materials to-date. These studies have remained largely inconclusive in establishing the mechanisms of creep in nanocrystalline materials. The stress exponent and activation energy values obtained from nanocrystalline materials do not correlate well with conventional well established creep models. Furthermore discrepancies between experimentally determined deformation rates and theoretical predictions suggest that an entirely new mechanism of creep could be operational in these exotic materials. In this work we aim to develop an understanding of the creep behavior of nanocrystalline materials by considering a stress assisted grain growth mechanism that has been recently identified in these materials. In turn a model has been developed that provides a quantitative understanding of some of the observations made in creep literature.     

A multi‐start threshold accepting algorithm for multiple objective continuous optimization problems

A multi‐start threshold accepting algorithm with an adaptive memory (MS‐TA) is proposed to solve multiple objective continuous optimization problems. The aim of this paper is to find efficiently multiple Pareto‐optimal solutions. Comparisons are carried out with multiple objective taboo search algorithm and genetic algorithm. Experiments on literature problems show that the proposed algorithm is more effective. The presented multi‐start adaptive algorithm improves the best‐known results by a significant margin. Copyright © 2010 John Wiley & Sons, Ltd.     

Three-dimensional generalized thermoelastic diffusion and application for a thermoelastic half-space subjected to rectangular thermal pulse

In this article, a model of three-dimensional generalized thermo-diffusion in a half-space thermoelastic medium subjected to permeating gas and the rectangular thermal pulse has been constructed. The half-space is considered to be made of an isotropic homogeneous thermoelastic material. The chemical potential is also assumed to be known on the bounding plane. Laplace transform techniques have been applied, and the solution is obtained in the Laplace transform domain using a direct approach. The solution of the problem in the physical domain is obtained numerically using a numerical method based on a Riemann-sum approximation for the inversion of Laplace transform. The temperature increment, stress, strain, diffusion concentration, and chemical potential distributions are represented graphically. The nonzero value of the relaxation time parameter predicts the finite speed of thermal, mechanical, diffusion waves.     

Effect of Gradual Heating and Fat/Oil Type on Fat Stability,Texture, Color,and Microstructure of Meat Batters

The effects of endpoint cooking temperature (40, 50, 60, 70, 80, and 90 °C) on emulsion stability, texture, color, and microstructure of meat batters prepared with different fats/oils were studied. Canola oil treatments showed the highest cooking loss whereas hydrogenated palm oil provided the most stable meat batters. Rendered beef fat was less stable than regular beef fat. Increasing endpoint cooking temperatures resulted in a progressive reduction of water holding capacity in all treatments. As temperature was raised, meat batters showed higher hardness and cohesiveness values, but no appreciable changes in cohesiveness above 60 °C. Canola and hydrogenated palm oil treatments showed the highest hardness and chewiness values. Lightness (L^*) values of all meat batters increased significantly with increasing temperature from 40 to 60 or 70 °C; no major changes observed above 70 °C. Light microscopy revealed no substantial changes in the microstructure of all the stable meat batters cooked to between 50 and 70 °C. Heating to 90 °C changed the microstructure in all meat batters except the hydrogenated palm oil treatments, which still showed nonround fat particles and a less aggregated protein matrix.     

Tribological behaviour of proposed polymeric bearing materials

The aim of the present work is to introduce new polymeric bearing materials. The proposed polymeric composites are consisting of polyethylene (PE), polypropylene (PP) and polystyrene (PS) and filled by fibres of polytetrafluoroethylene (PTFE) in concentration up to 25 wt.% as well as different types of natural oils such as (corn oil, olive oil, paraffin oil, glycerin oil, castor oil and sunflower oil) in concentration up to 10 wt.%. The frictional behavior of the proposed composites and wear resistance are investigated at different values of applied load. Based on the experimental observations, it was found that for composites free of oil, friction of PS and PE specimens decreased, while friction of PP specimens slightly increased with increasing PTFE content. PP composites filled by corn oil showed slight friction increase. Besides, friction coefficient displayed by PS and PE specimens filled by glycerin oil decreased with increasing oil content, while friction coefficient displayed by PP specimens showed consistent trend. It was noted that, PE filled with 7.5% glycerin oil and 20 wt.% PTFE displayed the minimum value of friction coefficient (0.07). This friction coefficient values recommend those composites to be used as bearing materials. PE filled by glycerin oil displayed relatively lower friction values due its common known good lubricating property. Friction of PE composites filled by paraffin oil drastically decreased with the increasing oil content. PP composites showed the lowest wear values. In addition to that, it was shown that wear displayed by composites filled by glycerin oil was higher than that containing corn oil, while wear of the tested composites filled with olive oil showed lower values than that displayed by corn oil filled composites. Composites containing 5.0–7.5% paraffin oil content showed good wear resistance which recommends them to be used as bearing material. Wear of PP, PS and PE composites filled with sunflower oil and 15 wt.% PTFE drastically decreased with increasing oil content. The minimum wear was displayed by PP and PE composites filled with 10% oil content.     

Effect of moist curing and use of lightweight sand on characteristics of high-performance concrete

Factorial design approach was undertaken to determine the effect of initial moist-curing duration (0 and 6 days) on properties of high-performance concrete (HPC) made with different water-to-cementitious materials ratios (w/cm) of 0.30 and 0.40 and sand substitution rates with lightweight sand (LWS) of 0 and 30 %, by volume. Mechanical properties of HPC are shown to be mainly affected by w/cm and total shrinkage of concrete by the LWS replacement rate and initial moist-curing period. It should be noted that concrete made with 30 % LWS replacement that did not receive any moist-curing exhibited lower shrinkage than moist-cured concrete prepared without any LWS, regardless of the w/cm. Combined use of 30 % LWS and 7 days of moist curing can lead to greater increase in compressive strength and larger decrease in total shrinkage compared to the use of 30 % LWS without moist curing or 7 days of moist curing without LWS. The use of LWS, however, is more practical and efficient to reduce shrinkage of HPC for deep concrete elements.     

Centrifugal casting of an Al-12Si-2Mg/Al2O3-particulate MMC. Part II: Evaluation of soundness and structure

A technique for the preparation of an MMC using centrifugal casting has been developed and tested for its feasibility in preparing Al-12Si-2Mg/Al₂O₃- particulate composites. The process is evaluated by observing the structure, measuring the homogeneity in the distribution of the ceramic particles, the porosity type and distribution, and by analysing the metal/ceramic interface for possible reactions.

The different processing conditions applied are: rotational frequency 16, 22.7 and 33.3 Hz (960, 1360 and 2000 rpm), Al₂O₃ particle size 30, 47, 60 and 89 μn, melt superheat 20, 100 and 150°C, specimen radius of rotation from 145 to 180 mm.

Because the ceramic particles are close packed, a uniform particle distribution with no agglomeration is obtained, and the interparticle distance depends only on the alumina particle size. The metal/ceramic interface was sharp with no reaction. Microporosity is observed in some locations due to incomplete infiltration between the alumina particles. Increasing rotational speed, particle size, superheat, and radius of rotation help to decrease the microporosity. The macrostructure along the composite length showed columnar grains followed by equiaxed grains. The type and size of the structure depend mainly on the composition of the matrix and not on the presence of the alumina particles.     

Benchmarking of the 3-D CAD-based Discrete Ordinates code “ATTILA” for dose rate calculations against experiments and Monte Carlo calculations

Shutdown dose rate (SDDR) inside and around the diagnostics ports of ITER is performed at PPPL/UCLA using the 3-D, FEM, Discrete Ordinates code, ATTILA, along with its updated FORNAX transmutation/decay gamma library. Other ITER partners assess SDDR using codes based on the Monte Carlo (MC) approach (e.g. MCNP code) for transport calculation and the radioactivity inventory code FISPACT or other equivalent decay data libraries for dose rate assessment. To reveal the range of discrepancies in the results obtained by various analysts, an extensive experimental and calculation benchmarking effort has been undertaken to validate the capability of ATTILA for dose rate assessment. On the experimental validation front, the comparison was performed using the measured data from two SDDR experiments performed at the FNG facility, Italy. Comparison was made to the experimental data and to MC results obtained by other analysts. On the calculation validation front, the ATTILA's predictions were compared to other results at key locations inside a calculation benchmark whose configuration duplicates an upper diagnostics port plug (UPP) in ITER. Both serial and parallel version of ATTILA-7.1.0 are used in the PPPL/UCLA analysis performed with FENDL-2.1/FORNAX databases. In the FNG 1st experimental, it was shown that ATTILA's dose rates are largely over estimated (by ～30–60%) with the ANSI/ANS-6.1.1 flux-to-dose factors whereas the ICRP-74 factors give better agreement (10–20%) with the experimental data and with the MC results at all cooling times. In the 2nd experiment, there is an under estimation in SDDR calculated by both MCNP and ATTILA based on ANSI/ANS-6.1.1 for cooling times up to ～4 days after irradiation. Thereafter, an over estimation is observed (～5–10% with MCNP and ～10–15% with ATTILA). As for the calculation benchmark, the agreement is much better based on ICRP-74 1996 data. The divergence among all dose rate results at ～11 days cooling time is no more than 15% among all participants.     

Polyurethane methacrylate/silicone interpenetrating polymer networks synthesis,thermal and mechanical properties

The synthesis of an interpenetrating polymer network (IPN) combining a polyurethane methacrylate network (PUMA) and a silicone network is reported. The PUMA network is synthesized by UV-light cure. The silicone network is formed through a condensation between α, ω dihydroxy polydimethyl siloxane and γ-methacryloxypropyl trimethoxy silane (γ-MPS) as a cross-linking agent. The IPN is prepared by different mechanism: radical and condensation types. According to thermogravimetric analysis of the hybrid material, the thermal stability stayed unchanged but the kinetic of degradation changed. Tg decreased with increasing silica content. The thermal cure process under humid atmosphere influence properties just for PUMA/4.2%Si^UV+T. Condensation between γ-MPS decreases the penetration depth from 158 to 82 μm and increases the mechanical glass transition temperature from 106 to 141 °C.