Time-Dependent Compressive Strength of Unidirectional Viscoelastic Composite Materials

The strength of unidirectional composites is often lower in compressionthan in tension, making compressive strength an especially importantdesign criterion. Compressive strength in the fiber direction depends onthe stiffness of the matrix material, which softens over time due toviscoelastic effects, and thus lowers the strength of the composite.This reduction must be accounted for in assessing the long-termdurability of composite structures.The dependence of compressive strength on time and temperature hasbeen investigated for a unidirectional carbon/epoxy composite material.Experimental results for strength versus time are compared withanalytical predictions based on a one-dimensional fiber microbucklingmodel and numerical results from finite element analysis.     

Optimization of CH fluorescence diagnostics in flames: range of applicability and improvements with hydrogen addition

This study quantifies the range of premixed flame conditions for which CH fluorescece diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios (phi) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at phi = 1.25 and phi = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as afactor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-MECH 3.0 chemistry. Sources experimental uncertainties are discussed.     

Density-controlled, solution-based growth of ZnO nanorod arrays via layer-by-layer polymer thin films for enhanced field emission

A simple, scalable, and cost-effective technique for controlling the growth density of ZnO nanorod arrays based on a layer-by-layer polyelectrolyte polymer film is demonstrated. The ZnO nanorods were synthesized using a low temperature (T = 90?°C), solution-based method. The density-control technique utilizes a polymer thin film pre-coated on the substrate to control the mass transport of the reactant to the substrate. The density-controlled arrays were investigated as potential field emission candidates. The field emission results revealed that an emitter density of 7?nanorods?μm(-2) and a tapered nanorod morphology generated a high field enhancement factor of 5884. This novel technique shows promise for applications in flat panel display technology.     

Isomorphism and external support in conflicting institutional environments: a study of drug abuse treatment units.

Using institutional theory, we developed predictions about organizational units that moved from an environment making consistent demands to one making conflicting demands. Many community mental health centers have diversified into drug abuse treatment. The units providing those services face conflicting demands from the traditional mental health sector and the new drug abuse treatment sector about which clients to serve, how to assess their problems, and who should provide treatment. We propose that in response to such demands these units will adopt apparently conflicting practices. Also, isomorphism with the traditional sector will be positively associated with external support from parent mental health centers and other actors in the mental health sector. Results generally support those predictions.     

Mixed-mode fracture of ductile thin-sheet materials under combined in-plane and out-of-plane loading

Cracks in thin structures often are subjected to combined in-plane and out-of-plane loading conditions leading to complex mixed mode conditions in the crack tip region. When applied to ductile materials, large out-of-plane displacements make both experimentation and modeling difficult. In this work, the mixed-mode behavior of thin, ductile materials containing cracks undergoing combined in-plane tension (mode I) and out-of-plane shear (mode III) deformation is investigated experimentally. Mixed-mode fracture experiments are performed and full, three-dimensional (3D) surface deformations of thin-sheet specimens from aluminum alloy and steel are acquired using 3D digital image correlation. General characteristics of the fracture process are described and quantitative results are presented, including (a) the fracture surface, (b) crack path, (c) load-displacement response, (d) 3D full-field surface displacement and strain fields prior to crack growth, (e) radial and angular distributions of the crack-tip strain fields prior to crack growth and (f) singularity analysis of the crack-tip strains prior to crack growth. Results indicate that the introduction of a mode III component to the loading process (a) alters the crack tip fields relative to those measured during nominally mode I loading and (b) significantly increases the initial and stable critical crack-opening-displacement. The data on strain fields in both AL6061-T6 aluminum and GM6208 steel consistently show that for a given strain component, the normalized angular and radial strains at all load levels can be reasonably represented by a single functional form over the range of loading considered, confirming that the strain fields in highly ductile, thin-sheet material undergoing combined in-plane tension and out-of-plane shear loading can be expressed in terms of separable angular and radial functions. For both materials, the displacement and strain fields are (a) similar for both mixed-mode loading angles Φ = 30° and Φ = 60° and (b) different from the fields measured for Mode I loading angle Φ = 0°. Relative to the radial distribution, results indicate that the in-plane strain components do not uniformly exhibit the singularity trends implicit in the HRR theory.     

Chemical interactions between an active pharmaceutical ingredient and its counterion in a tromethamine salt under forced degradation conditions

In this study, the tromethamine salt of an active pharmaceutical ingredient containing both a carboxylic acid and ethyl ester functionality was subjected to forced degradation conditions. Based on HPLC-MS analysis, it was found that tromethamine formed both amide and ester type condensation products with the API, with amide formation predominating over ester formation. Addition of tromethamine at the carboxylic acid group of the API was favored over addition at the ethyl ester group. Tromethamine condensation products were observed only under the harshest stress conditions (80 degrees and 75% relative humidity), in which the salt physically changed from a crystalline form to a deliquesced state. Under stress conditions in which the crystalline structure of the salt remained intact, good stability was observed. Thus, the interaction between tromethamine and API occurred only in cases where the crystallinity of the salt was compromised.     

What do the hands externalize in simple arithmetic?

In 4 experiments, the authors examined the use of the hands in simple arithmetic tasks. Experiments 1 and 2 demonstrated that pointing increases both accuracy and speed in counting arrays of items, whether those items are identical or distinctive. Experiment 3 demonstrated that individuals tend to nod their heads when not allowed to point and that nodding is associated with greater accuracy, suggesting that pointing is functional for reasons other than simply providing additional visual information. Experiment 4 examined changes in speech when adding arrays of digits, depending on whether participants were allowed to use their hands to manipulate the tokens on which the digits were presented. Taken together, the results of these experiments are consistent with recent research suggesting that gesture can serve cognitive functions and that the hands can support the binding of representational elements to their functional roles by providing phase markers for cyclic cognitive processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Computational steering in Monte Carlo simulations of thin film polystyrene

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.     

Characterization of triacetone triperoxide by ion mobility spectrometry and mass spectrometry following atmospheric pressure chemical ionization

The atmospheric pressure chemical ionization of triacetone triperoxide (TATP) with subsequent separation and detection by ion mobility spectrometry has been studied. Positive ionization with hydronium reactant ions produced only fragments of the TATP molecule, with m/z 91 ion being the most predominant species. Ionization with ammonium reactant ions produced a molecular adduct at m/z 240. The reduced mobility value of this ion was constant at 1.36 cm(2)V(-1)s(-1) across the temperature range from 60 to 140 °C. The stability of this ion was temperature dependent and did not exist at temperatures above 140 °C, where only fragment ions were observed. The introduction of ammonia vapors with TATP resulted in the formation of m/z 58 ion. As the concentration of ammonia increased, this smaller ion appeared to dominate the spectra and the TATP-ammonium adduct decreased in intensity. The ion at m/z 58 has been noted by several research groups upon using ammonia reagents in chemical ionization, but the identity was unknown. Evidence presented here supports the formation of protonated 2-propanimine. A proposed mechanism involves the addition of ammonia to the TATP-ammonium adduct followed by an elimination reaction. A similar mechanism involving the chemical ionization of acetone with excess ammonia also showed the formation of m/z 58 ion. TATP vapors from a solid sample were detected with a hand-held ion mobility spectrometer operated at room temperature. The TATP-ammonium molecular adduct was observed in the presence of ammonia and TATP vapors with this spectrometer.