Test methods for characterizing concrete properties at elevated temperature

Fire resistance of structural members is dependent on the thermal and mechanical properties of constituent materials and these properties vary as a function of temperature. Currently, there are limited standardized test procedures for evaluating thermal and mechanical properties of construction materials at elevated temperatures. This paper provides a review and assessment of test methods and procedures for evaluating high temperature thermal and mechanical properties of concrete. The drawbacks and variations in currently available test procedures and methods in standards are discussed. Recommendations on the most suitable methods and procedures for measuring thermal and mechanical properties at elevated temperature is presented. In addition, applicability of the proposed high temperature test methods and procedures is illustrated through a case study on conventional concrete specimens. Further, the need for developing standards by organizations such as American Society for Testing and Materials (ASTM), with standardized specifications and test procedures for measuring high temperature properties of construction materials, is laid out.     

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when [6,6]‐phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.     

Assembly-level reliability of flex-substrate BGA, elastomer-on-flex packages and 0.5 mm pitch partial array packages

The assembly-level reliability of the 0.8 mm pitch flex-substrate BGA, 0.65 mm pitch elastomer-on-flex package, and 0.5 mm pitch partial array BGA has been characterized in thermal fatigue, out-of-plane deformation, low frequency repeated bending, and thermal aging. Non-linear finite element models have been used to identify and predict the dominant failure mechanisms and identify the assembly parameters which have a dominant impact on reliability. The model predictions have been verified with accelerated test data. The results have been bench marked against other technologies including 1.5 mm pitch OMPAC^TM and 1 mm pitch, 196 I/O Glob-top BGA.     

An optimal distributed solution to the dining philosophers problem

An optimal distributed solution to the dining philosophers problem is presented. The solution is optimal in the sense that it incurs the least communication and computational overhead, and allows the maximum achievable concurrency. The worst case upper bound for concurrency is shown to ben div 3,n being the number of philosophers. There is no previous algorithm known to achieve this bound.     

Development of Al-Ti-C grain refiners containing TiC

Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A1₄C₃ and Ti₃AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB₂ commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals. On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate.     

Dimensionality of an early childhood scale using rasch analysis and confirmatory factor analysis

This paper explores the use of Rasch analysis and linear confirmatory factor analysis as methods for investigating the dimensionality of an early childhood test (Gesell School Readiness Screening Test), taking into account the theoretical basis of test construction. The paper presents the results of empirical analyses using both approaches and discusses the theoretical and psychometric considerations that guide the selection and application of each technique.     

Chromophoric chain β-aryl-substituted styryl cyanines: effect of substituents on visible absorption spectra and photosensitisation properties

A number of chromophoric chain β-aryl-substituted (CCBAS) styryl cyanines (hemicyanines) have been synthesised by condensing substituted quaternised quinaldine bases with substituted benzophenones, with a view to studying the effect of the aryl substituents on their visible absorption and silver halide photosensitisation properties.The dyes absorb at longer wavelengths than the unsubstituted analogues, showing uniform bathochromic shifts and extra-photosensitisation properties, which corroborate the authors' previous findings.Irrespective of the nature of the substituent attached to the chromophoric β-phenyl group, electron withdrawing and donating groups result in a bathochromic shift of the visible absorption band and extension of the extra-photosensitisation in most cases.     

Abrasive wear of cast aluminium alloy-zircon particle composites

The abrasive wear rates of particulate composites of an Al-11.8Si-4Mg alloy containing up to 0.35 volume fraction of zircon particles (average size, 100 μm) were measured on an 80 grit aloxide cloth sheet as a function of the volume fraction of zircon, the applied load and the number of passes over the abrasive paper. When the volume fraction of zircon is above a critical value of 0.09, the abrasive wear resistance (reciprocal of the wear rate) increases with the volume fraction of zircon according to the rule of mixtures. When the volume fraction is fixed, the abrasive wear resistance increases with the number of passes possibly because of blunting of the alumina particles of the abrasive cloth. No improvement in the abrasive wear resistance of composites over the matrix alloy was observed when the volume fraction of zircon was below 0.09. Scanning electron microscopy studies of the abraded surfaces of composites revealed fractured zircon particles but no evidence of filler particle pull-outs or debonding at the interface was obtained.     

Influence of grain refinement on the high temperature fretting behaviour of IN 738 LC

The wear and friction behaviour of Inconel 738 LC in contact with SiSiC was studied for the case of oscillating sliding motion. The test temperature was varied in the range from room temperature up to 700 °C. A large-grain IN 738 LC and a grain-refined modification were compared. In both cases, the wear dropped drastically at temperatures above 400–500 °C and remained low for the grain-refined modification, but increased again for the large-grain modification at temperatures above 600 °C. The high wear/low wear transition was accompanied by a transfer of metal on the ceramic surface.