Adiabatic shear instability based mechanism for particles/substrate bonding in the cold-gas dynamic-spray process

Particles/substrate interactions during the cold-gas dynamic-spray deposition process are studied using a dynamic axisymmetric thermo-mechanical finite element analysis. In addition, the particles/substrate bonding mechanism has been investigated using a one-dimensional thermo-mechanical model for adiabatic strain softening and the accompanying adiabatic shear localization. The results obtained show that the minimal impact particles velocity needed to produce shear localization at the particles/substrate interface correlates quite well with the critical velocity for particles deposition by the cold-gas dynamic-spray process in a number of metallic materials. This finding suggests that the onset of adiabatic shear instability in the particles/substrate interfacial region plays an important role in promoting particle/substrate adhesion and, thus, particles/substrate bonding during the cold-gas dynamic-spray process.     

Synthesis and Hydrodesulfurization Properties of Noble Metal Phosphides: Ruthenium and Palladium

Silica-supported ruthenium and palladium phosphide catalysts (Ru₂P, RuP, Pd₃P, Pd₅P₂) were investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The Ru and Pd phosphide catalysts were prepared by temperature-programmed reduction of hypophosphite-based precursors consisting of uncalcined or calcined Ru/SiO₂ or Pd/SiO₂ impregnated with ammonium hyposphosphite (NH₄H₂PO₂). The Ru₂P/SiO₂ and RuP/SiO₂ catalysts prepared from uncalcined precursors had smaller average crystallite sizes, higher CO chemisorption capacities, and higher HDS activities than the catalysts prepared from the calcined precursors, while the effect of preparation method on catalytic properties was less clear for the Pd₃P/SiO₂ and Pd₅P₂/SiO₂ catalysts. Following HDS testing at 673?K, X-ray diffraction analysis revealed that the Pd₅P₂/SiO₂ catalysts decomposed to give Pd₃P on the silica support, while the other phosphides exhibited good stability during the testing period. At temperatures at which high DBT conversion was observed (>598?K), the Ru and Pd phosphide catalysts were less active than sulfided Ru/SiO₂ and Pd/SiO₂ catalysts prepared from the uncalcined metal precursors.     

A Combined Experimental/Computational Analysis of the Butt-Friction-Stir-Welded AA2139-T8 Joints

Combined experimental and computational investigations are carried out of the mechanical properties of materials residing in different weld zones of friction stir-welded (FSW) joints of thick plates of AA2139-T8. The experimental portion of the work comprised (a) identification of the weld zones within the FSW joints, through the use of optical-microscopy characterization of a transverse section; (b) validation of the weld zones identified in (a) via the generation of a micro-hardness field over the same transverse section; (c) extracting and subsequently testing miniature tensile specimens from different weld zones; and (d) extracting and testing a larger-size tensile specimen spanning transversely the FSW weld. The computational portion of the work comprised (i) validation of the mechanical properties, as determined experimentally using the miniature tensile specimens, of the material residing within different zones of the FSW joint; and (ii) clarification of the benefits yielded by the knowledge of the local material properties within the FSW joint. These benefits arise from the fact that (a) joint mechanical properties are generally inferior to those of the base metal; (b) the width of the weld in thick metallic-armor is often comparable to the armor thickness, and therefore may represent a significant portion of the armor exposed-surface area; and (c) modeling of the weld-material structural response under loading requires the availability of high-fidelity/validated material constitutive models, and the development of such models requires knowledge of the local weld-material mechanical properties.     

Fiber-Level Modeling of Dynamic Strength of Kevlar? KM2 Ballistic Fabric

In recent years, modeling of the high-performance ballistic fabric has gradually shifted from the continuum and yarn length scales to the sub-yarn length scale which enabled establishment of the relationships between the fabric penetration resistance and various fiber-level phenomena such as fiber-fiber friction, fiber twist, transverse properties of the fibers, and the stochastic nature of fiber strength. In general, these sub-yarn modeling schemes involve special numerical techniques (e.g., digital-element method) and customized computational codes. This status of the sub-yarn fabric-modeling methods and tools makes them not readily available to wider academic and industrial research communities. In the present work, an attempt is made to use conventional finite-element methods and tools in order to carry out sub-yarn numerical analysis of the penetration resistance of Kevlar^? KM2 ballistic fabric. The goal was to demonstrate that results could be obtained which are comparable to their digital-element method?=?based counterparts. Specifically, a series of transient nonlinear dynamics finite-element analyses was carried out in order to investigate the role of the following two important sub-yarn phenomena on the penetration resistance of Kevlar^? KM2 fabric: (a) fiber transverse properties including nonlinear elastic and plastic response and (b) fiber-fiber friction within the context of stochastically distributed fiber axial strength. It is generally found that the results obtained are consistent with their digital-element method-based counterparts.     

Experimental Characterization and Material-Model Development for Microphase-Segregated Polyurea: An Overview

Numerous experimental investigations reported in the open literature over the past decade have clearly demonstrated that the use of polyurea external coatings and/or inner layers can substantially enhance both the blast resistance (the ability to withstand shock loading) and the ballistic performance (the ability to defeat various high-velocity projectiles such as bullets, fragments, shrapnel, etc. without penetration, excessive deflection or spalling) of buildings, vehicles, combat-helmets, etc. It is also well established that the observed high-performance of polyurea is closely related to its highly complex submicron scale phase-segregated microstructure and the associated microscale phenomena and processes (e.g., viscous energy dissipation at the internal phase boundaries). As higher and higher demands are placed on blast/ballistic survivability of the foregoing structures, a need for the use of the appropriate transient nonlinear dynamics computational analyses and the corresponding design-optimization methods has become ever apparent. A critical aspect of the tools used in these analyses and methods is the availability of an appropriate physically based, high-fidelity material model for polyurea. There are presently several public domain and highly diverse material models for polyurea. In the present work, an attempt is made to critically assess these models as well as the experimental methods and results used in the process of their formulation. Since these models are developed for use in the high-rate loading regime, they are employed in the present work, to generate the appropriate shock-Hugoniot relations. These relations are subsequently compared with their experimental counterparts in order to assess the fidelity of these models.     

Electrochemical and AFM study of cobalt nucleation mechanisms on glassy carbon from ammonium sulfate solutions

Nucleation mechanisms of cobalt on a glassy carbon electrode (gce) from aqueous ammonium sulfate solutions were investigated through the electrochemical techniques of cyclic voltammetry (cv) and chronoamperometry (ca), coupled with atomic force microscopy (AFM) studies. The studied parameters were pH, cobalt concentration, temperature, scanning rate, and deposition potential. It was found that scanning in the cathodic direction produced two peaks, corresponding to cobalt and hydrogen reduction, respectively. Scanning in the anodic direction was characterized by cobalt dissolution, which was interrupted by formation of cobalt hydroxide, causing a second anodic peak. The amperometric study found progressive nucleation mechanisms, in contrast to the instantaneous nucleation mechanisms determined by the AFM study. An explanation for the contradictory nucleation mechanisms shown in the two studies is provided.     

Optimization of the VARTM process for enhancement of the degree of devolatilization of polymerization by-products and solvents

Devolatilization of the polymerization by-products and the impregnation solvent during Vacuum Assisted Resin Transfer Molding (VARTM) of the polyimide polymers is analyzed using a combined continuum hydrodynamics/chemical reaction one-dimensional model. The model which consists of seven coupled partial differential equations is solved using a finite element collocation method based on the method of lines. The results obtained reveal that the main process parameters which give rise to lower gas-phase contents in the VARTM-processed polymer matrix composites are the vacuum pressure and the tool-plate heating rate. Lower tool-plate heating rates are found to be beneficial since hey promote devolatilization of the impregnation solvent at lower temperatures at which the degree of polymerization and, hence, resin viscosity are low.     

Correction to Estrada-Hollenbeck et al. (2010).

Reports an error in "Toward a model of social influence that explains minority student integration into the scientific community" by Mica Estrada-Hollenbeck, Anna Woodcock, Paul R. Hernandez and P. Wesley Schultz (Journal of Educational Psychology, np). The name of the author Mica Estrada-Hollenbeck should have read Mica Estrada. All versions of this article have been corrected. (The following abstract of the original article appeared in record 2010-22529-001.) Students from several ethnic minority groups are underrepresented in the sciences, indicating that minority students more frequently drop out of the scientific career path than nonminority students. Viewed from a perspective of social influence, this pattern suggests that minority students do not integrate into the scientific community at the same rate as nonminority students. Kelman (1958, 2006) described a tripartite integration model of social influence by which a person orients to a social system. To test whether this model predicts integration into the scientific community, we conducted analyses of data from a national panel of minority science students. A structural equation model framework showed that self-efficacy (operationalized to be consistent with Kelman's rule orientation) predicted student intentions to pursue a scientific career. However, when identification as a scientist and internalization of values were added to the model, self-efficacy became a poorer predictor of intention. Additional mediation analyses supported the conclusion that while having scientific self-efficacy is important, identifying with and endorsing the values of the social system reflect a deeper integration and more durable motivation to persist as a scientist. (PsycINFO Database Record (c) 2011 APA, all rights reserved)