Multi-Length Scale Modeling of High-Pressure-Induced Phase Transformations in Soda-Lime Glass

Molecular-level modeling and simulations are employed to study room-temperature micro-structural and mechanical response of soda-lime glass when subjected to high (i.e., several giga-Pascal) uniaxial-strain stresses/pressure. The results obtained revealed the occurrence of an irreversible phase-transformation at ca. 4 GPa which was associated with a (permanent) 3-7% volume reduction. Close examination of molecular-level topology revealed that the pressure-induced phase transformation in question is associated with an increase in the average coordination number of the silicon atoms, and the creation of two- to fourfold (smaller, high packing-density) Si-O rings. The associated loading and unloading axial-stress versus specific-volume isotherms were next converted into the corresponding loading Hugoniot and unloading isentrope axial-stress versus specific-volume relations. These were subsequently used to analyze the role of the pressure-induced phase-transformation/irreversible-densification in mitigating the effects of blast and ballistic impact loading onto a prototypical glass plate used in monolithic and laminated transparent armor applications. The results of this part of the study revealed that pressure-induced phase-transformation can provide several beneficial effects such as lowering of the loading/unloading stress-rates and stresses, shock/release-wave dispersion, and energy absorption associated with the study of phase-transformation.     

A Computational Investigation of the Multi-Hit Ballistic-Protection Performance of Laminated Transparent-armor Systems

Multi-hit ballistic-protection performance of a prototypical laminated glass/polycarbonate transparent armor is investigated using a series of transient nonlinear dynamics analyses of armor impact with a sequence of four M2AP full metal jacket (FMJ) armor-piercing bullets. All calculations were carried out using ABAQUS/Explicit commercial finite element program (ABAQUS Version 6.7, User Documentation, Dessault Systems, 2007), and the computational results obtained were compared to their experimental counterparts obtained by Dolan (Ballistic Transparent-armor Testing Using a Multi-hit Rifle Pattern, Bachelors, Thesis, Kettering University, December 2007). The comparison revealed that (a) The proposed computational procedure can reasonably well account for the observed multi-hit ballistic-protection performance of the laminated transparent armor; (b) The role of prior bullet hits in reducing armor??s ballistic-protection performance is clearly revealed; (c) The role of polycarbonate lamina in preventing glass fragments from entering the vehicle interior is clearly demonstrated; and (d) Experimentally observed inability of the transparent armor to defeat 0.50-caliber Fragment Simulating Projectiles (FSPs) is confirmed.     

Design-Optimization and Material Selection for a Proximal Radius Fracture-Fixation Implant

The problem of optimal size, shape, and placement of a proximal radius-fracture fixation-plate is addressed computationally using a combined finite-element/design-optimization procedure. To expand the set of physiological loading conditions experienced by the implant during normal everyday activities of the patient, beyond those typically covered by the pre-clinical implant-evaluation testing procedures, the case of a wheel-chair push exertion is considered. Toward that end, a musculoskeletal multi-body inverse-dynamics analysis is carried out of a human propelling a wheelchair. The results obtained are used as input to a finite-element structural analysis for evaluation of the maximum stress and fatigue life of the parametrically defined implant design. While optimizing the design of the radius-fracture fixation-plate, realistic functional requirements pertaining to the attainment of the required level of the devise safety factor and longevity/lifecycle were considered. It is argued that the type of analyses employed in the present work should be: (a) used to complement the standard experimental pre-clinical implant-evaluation tests (the tests which normally include a limited number of daily-living physiological loading conditions and which rely on single pass/fail outcomes/decisions with respect to a set of lower-bound implant-performance criteria) and (b) integrated early in the implant design and material/manufacturing-route selection process.     

Crystal plasticity analysis of stress–assisted martensitic transformation in Ti–10V–2Fe–3Al (wt.%)

A new model based on crystal–plasticity, crystallography, thermodynamics, kinetics and statistics is developed for stress–assisted martensitic transformation. The model includes the essential features of the stress–assisted martensitic transformation, such as: nuclei of progressively lower potency are activated in the course of transformation, the martensite phase appears in the form of thin plates, the parent phase exerts a higher resistance toward the growth of a plate in the thickness than in the radial direction, the average plate size decreases while the average plate aspect ratio increases with the extent of transformation, etc. The model is implemented in the commercial finite element code ABAQUS/Standard to analyze the evolution of martensite, materials texture and the resulting equivalent stress–equivalent strain curve during the stress–assisted martensitic transformation under different stress and strain states in a polycrystalline Ti–10V–2Fe–3Al (wt.%) alloy. The equivalent stress–equivalent strain curves and the volume fraction of martensite–equivalent strain curves are found to be mainly controlled by the applied stress state. Conversely, the texture observed in the transformed Ti–10V–2Fe–3Al is found to be primarily controlled by the imposed macroscopic strain state. The validity of the proposed materials constitutive model has been established by demonstrating a reasonable agreement between the model predictions and the available experimental data.     

Transformation toughening in the γ-TiAl–β-Ti–V system: Part I Finite element analysis of a crack touching the interface

Atomistic simulation of transformation toughening due to martensitic transformation in Ti–V phase particles dispersed in a γ-TiAl matrix containing cracks requires knowledge of the continuum elastic stress and displacement fields for the problem of a crack touching the γ–β interface. Because of the anisotropic characters of the two phases, analytical solutions for these fields are not available and they must be determined numerically. In the present paper a finite element method-based eigenanalysis is developed and subsequently applied to the γ–β system to determine the order of the stress singularity and the angular dependences of the stress and displacement fields. These fields are subsequently used to enrich the finite elements surrounding the crack tip and, through the use of the general finite element code ABAQUS, to determine the generalized stress intensity factors and thus the total singular crack-tip stress and displacement fields. It is found that there are two coupled singular terms in the singular stress and displacement fields, and consequently pure (uniaxial) mode I loading gives rise to mixed modes I–II near-crack-tip behaviour. This revised version was published online in November 2006 with corrections to the Cover Date.     

The Potential of a Clinch-Lock Polymer Metal Hybrid Technology for Use in Load-Bearing Automotive Components

In order to help meet the needs of automotive original equipment manufacturers and their suppliers for a cost-effective, robust, reliable polymer-metal-hybrid (PMH) technology which can be used for the manufacturing of load-bearing body-in-white (BIW) components and which is compatible with the current BIW manufacturing process chain, a new approach, the so-called direct-adhesion PMH technology, was recently proposed (Grujicic et al., J. Mater. Process. Technol., 2008, 195, p 282-298). Within this approach, the necessary level of polymer-to-metal mechanical interconnectivity is attained through direct adhesion and mechanical interlocking. In the present work, a new concept for mechanical interlocking between the metal and plastics is proposed and analyzed computationally. The approach utilizes some of the ideas used in the spot-clinching joining process and is appropriately named clinch-lock PMH technology. To assess the potential of the clinch-lock approach for providing the required level of metal/polymer mechanical interlocking, a set of finite-element based sheet-metal forming, injection molding and structural mechanics analyses was carried out. The results obtained show that stiffness and buckling resistance levels can be attained which are comparable with those observed in the competing injection overmolding PMH process but with an ~3% lower weight (of the polymer subcomponent) and without the need for holes and for overmolding of the free edges of the metal stamping.     

Toward a model of social influence that explains minority student integration into the scientific community.

[Correction Notice: An erratum for this article was reported in Vol 103(1) of Journal of Educational Psychology (see record 2011-01898-001). The name of the author Mica Estrada-Hollenbeck should have read Mica Estrada. All versions of this article have been corrected.] 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)     

Statistical Analysis of High-Cycle Fatigue Behavior of Friction Stir Welded AA5083-H321

A review of the literature revealed that high-cycle fatigue data associated with friction stir-welded (FSW) joints of AA5083-H321 (a solid-solution-strengthened and strain-hardened/stabilized Al-Mg-Mn alloy) are characterized by a relatively large statistical scatter. This scatter is closely related to the intrinsic variability of the FSW process and to the stochastic nature of the workpiece material microstructure/properties as well as to the surface condition of the weld. Consequently, the use of statistical methods and tools in the analysis of FSW joints is highly critical. A three-step FSW-joint fatigue-strength/life statistical-analysis procedure is proposed in this study. Within the first step, the type of the most appropriate probability distribution function is identified. The parameters of the selected probability distribution function, along with their confidence limits, are computed in the second step. In the third step, a procedure is developed for assessment of the statistical significance of the effect of the FSW process parameters and fatigue specimen surface conditions. The procedure is then applied to a set of stress-amplitude versus number of cycles to failure experimental data in which the tool translational speed was varied over four levels, while the fatigue specimen surface condition was varied over two levels. The results obtained showed that a two-parameter weibull distribution function with its scale factor being dependent on the stress amplitude is the most appropriate choice for the probability distribution function. In addition, it is found that, while the tool translational speed has a first-order effect on the AA5083-H321 FSW-joint fatigue strength/life, the effect of the fatigue specimen surface condition is less pronounced.     

Computational Modeling of Microstructural-Evolution in AISI 1005 Steel During Gas Metal Arc Butt Welding

A fully coupled (two-way), transient, thermal-mechanical finite-element procedure is developed to model conventional gas metal arc welding (GMAW) butt-joining process. Two-way thermal-mechanical coupling is achieved by making the mechanical material model of the workpiece and the weld temperature-dependent and by allowing the potential work of plastic deformation resulting from large thermal gradients to be dissipated in the form of heat. To account for the heat losses from the weld into the surroundings, heat transfer effects associated with natural convection and radiation to the environment and thermal-heat conduction to the adjacent workpiece material are considered. The procedure is next combined with the basic physical-metallurgy concepts and principles and applied to a prototypical (plain) low-carbon steel (AISI 1005) to predict the distribution of various crystalline phases within the as-welded material microstructure in different fusion zone and heat-affected zone locations, under given GMAW-process parameters. The results obtained are compared with available open-literature experimental data to provide validation/verification for the proposed GMAW modeling effort.