Molecular-Level Simulations of Shock Generation and Propagation in Soda-Lime Glass

A non-equilibrium molecular dynamics method is employed to study the mechanical response of soda-lime glass (a material commonly used in transparent armor applications) when subjected to the loading conditions associated with the generation and propagation of planar shock waves. Specific attention is given to the identification and characterization of various (inelastic-deformation and energy-dissipation) molecular-level phenomena and processes taking place at, or in the vicinity of, the shock front. The results obtained revealed that the shock loading causes a 2-4% (shock strength-dependent) density increase. In addition, an increase in the average coordination number of the silicon atoms is observed along with the creation of smaller Si-O rings. These processes are associated with substantial energy absorption and dissipation and are believed to greatly influence the blast/ballistic impact mitigation potential of soda-lime glass. The present work was also aimed at the determination of the shock Hugoniot (i.e., a set of axial stress vs. density/specific-volume vs. internal energy vs. particle velocity vs. temperature) material states obtained in soda-lime glass after the passage of a shock wave of a given strength (as quantified by the shock speed). The availability of a shock Hugoniot is critical for construction of a high deformation-rate, large-strain, high pressure material model which can be used within a continuum-level computational analysis to capture the response of a soda-lime glass based laminated transparent armor structure (e.g., a military vehicle windshield, door window, etc.) to blast/ballistic impact loading.     

Multiphysics Modeling and Simulations of Mil A46100 Armor-Grade Martensitic Steel Gas Metal Arc Welding Process

A multiphysics computational model has been developed for the conventional Gas Metal Arc Welding (GMAW) joining process and used to analyze butt-welding of MIL A46100, a prototypical high-hardness armor martensitic steel. The model consists of five distinct modules, each covering a specific aspect of the GMAW process, i.e., (a) dynamics of welding-gun behavior; (b) heat transfer from the electric arc and mass transfer from the electrode to the weld; (c) development of thermal and mechanical fields during the GMAW process; (d) the associated evolution and spatial distribution of the material microstructure throughout the weld region; and (e) the final spatial distribution of the as-welded material properties. To make the newly developed GMAW process model applicable to MIL A46100, the basic physical-metallurgy concepts and principles for this material have to be investigated and properly accounted for/modeled. The newly developed GMAW process model enables establishment of the relationship between the GMAW process parameters (e.g., open circuit voltage, welding current, electrode diameter, electrode-tip/weld distance, filler-metal feed speed, and gun travel speed), workpiece material chemistry, and the spatial distribution of as-welded material microstructure and properties. The predictions of the present GMAW model pertaining to the spatial distribution of the material microstructure and properties within the MIL A46100 weld region are found to be consistent with general expectations and prior observations.     

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.     

Two-Level Weld-Material Homogenization for Efficient Computational Analysis of Welded Structure Blast-Survivability

The introduction of newer joining technologies like the so-called friction-stir welding (FSW) into automotive engineering entails the knowledge of the joint-material microstructure and properties. Since, the development of vehicles (including military vehicles capable of surviving blast and ballistic impacts) nowadays involves extensive use of the computational engineering analyses (CEA), robust high-fidelity material models are needed for the FSW joints. A two-level material-homogenization procedure is proposed and utilized in this study to help manage computational cost and computer storage requirements for such CEAs. The method utilizes experimental (microstructure, microhardness, tensile testing, and x-ray diffraction) data to construct: (a) the material model for each weld zone and (b) the material model for the entire weld. The procedure is validated by comparing its predictions with the predictions of more detailed but more costly computational analyses.     

Atomistic simulation of ∑3 (111) grain boundary fracture in tungsten containing various impurities

The effect of various impurities and micro-alloying additions (B, N, C, O, Al, Si, S and P) on the intrinsic resistance of the ∑3 (111) grain boundary in tungsten has been investigated using the molecular dynamics simulation. The atomic interactions have been accounted for through the use of Finnis-Sinclair interatomic potentials. The fracture resistance of the grain boundary has been characterized by computing, in each case, the ideal work of grain boundary separation, the mode I stress intensity factor and the Eshelby's F₁ conservation integral at the onset of crack propagation. The results obtained suggest that pure tungsten is relatively resistant to grain boundary decohesion and that this resistance is further enhanced by the presence of B, C and N. Elements such as O, Al and Si however, have a relatively minor effect on the cohesion strength of the ∑3 (111) grain boundary. In sharp contrast, S and P greatly reduce this strength making tungsten quite brittle. These findings have been correlated with the effect of the impurity atoms on material evolution at the crack tip.     

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)     

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.     

Design-optimization and material selection for a femoral-fracture fixation-plate implant

The problem of size/thickness optimization of a distal femoral-fracture fixation-plate is addressed computationally using a combined finite-element/design-optimization procedure. To obtain realistic physiological loading conditions associated with normal living activities (cycling, in the present case), a musculoskeletal multi-body inverse-dynamics analysis is carried out of a human riding the bicycle. While optimizing the design of the femoral-fracture locking-plate, realistic functional requirements pertaining to attain the required level of fracture-femur fixation and longevity/lifecycle were used. It is argued that these types of analysis should be used to complement pre-clinical implant-evaluation tests, the tests which normally include a limited number of physiological loading conditions and single pass/fail outcomes/decisions with respect to a set of lower-bound implant–performance criteria.