Effect of martensitic transformation in Ti–15 at %V β-phase particles on lamellar boundary decohesion in γ-TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers     

Most frequent ways of the collateral circulation in occlusive coronary artery disease

By comparison of the sectional area of the fetal head of the fronto-occipital level in the height of the biparietalic diameter and the cross sectional area of thorax in the level area of the ventrical it was tried to determine the exspected birth weight. A mean deviation of 8,9% was found. Below a birth weight of 2500 g the presented calculation is only valid to some extend as in these cases the dominance proved in fetal development grounds of the head of the fetus becomes apparent.     

Potential Improvements in Shock-Mitigation Efficacy of a Polyurea-Augmented Advanced Combat Helmet

The design of the currently used Advanced Combat Helmet (ACH) has been optimized to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface collisions. However, the ability of the ACH to protect soldiers against blast loading appears not to be as effective. Polyurea, a micro-segregated elastomeric copolymer has shown superior shock-mitigation capabilities. In the present work, a combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction analysis is used to investigate potential shock-mitigation benefits which may result from different polyurea-based design augmentations of the ACH. Specific augmentations include replacement of the currently used suspension-pad material with polyurea and the introduction of a thin polyurea internal lining/external coating to the ACH shell. Effectiveness of different ACH designs was quantified by: (a) establishing the main forms of mild traumatic brain injury (mTBI); (b) identifying the key mechanical causes for these injuries; and (c) quantifying the extents of reductions in the magnitude of these mechanical causes. The results obtained show that while the ACH with a 2-mm-thick polyurea internal lining displays the best blast mitigation performance, it does not provide sufficient protection against mTBI.     

Shock-Wave Attenuation and Energy-Dissipation Potential of Granular Materials

The propagation of uniaxial-stress planar shocks in granular materials is analyzed using a conventional shock-physics approach. Within this approach, both compression shocks and decompression waves are treated as (stress, specific volume, particle velocity, mass-based internal energy density, temperature, and mass-based entropy density) propagating discontinuities. In addition, the granular material is considered as being a continuum (i.e., no mesoscale features like grains, voids, and their agglomerates are considered). However, while the granular material is treated as a (smeared-out) continuum, it is recognized that it contains a solid constituent (parent matter), and that the structurodynamic properties (i.e., Equations of State (EOS) and Hugoniot relations) of the granular material are related to its parent matter. Three characteristic shock loading regimes of granular material are considered and, in each case, an analysis is carried out to elucidate shock attenuation and energy dissipation processes. In addition, an attempt is made to identify a metric (a combination of the material parameters) which quantifies the intrinsic ability of a granular material to attenuate a shock and dissipate the energy carried by the shock. Toward that end, the response of a typical granular material to a flat-topped compressive stress pulse is analyzed in each of the three shock loading regimes.     

A Concurrent Product-Development Approach for Friction-Stir Welded Vehicle-Underbody Structures

High-strength aluminum and titanium alloys with superior blast/ballistic resistance against armor piercing (AP) threats and with high vehicle light-weighing potential are being increasingly used as military-vehicle armor. Due to the complex structure of these vehicles, they are commonly constructed through joining (mainly welding) of the individual components. Unfortunately, these alloys are not very amenable to conventional fusion-based welding technologies [e.g., gas metal arc welding (GMAW)] and to obtain high-quality welds, solid-state joining technologies such as friction-stir welding (FSW) have to be employed. However, since FSW is a relatively new and fairly complex joining technology, its introduction into advanced military-vehicle-underbody structures is not straight forward and entails a comprehensive multi-prong approach which addresses concurrently and interactively all the aspects associated with the components/vehicle-underbody design, fabrication, and testing. One such approach is developed and applied in this study. The approach consists of a number of well-defined steps taking place concurrently and relies on two-way interactions between various steps. The approach is critically assessed using a strengths, weaknesses, opportunities, and threats (SWOT) analysis.     

Multidisciplinary Design Optimization for Glass-Fiber Epoxy-Matrix Composite 5 MW Horizontal-Axis Wind-Turbine Blades

A multi-disciplinary design-optimization procedure has been introduced and used for the development of cost-effective glass-fiber reinforced epoxy-matrix composite 5 MW horizontal-axis wind-turbine (HAWT) blades. The turbine-blade cost-effectiveness has been defined using the cost of energy (CoE), i.e., a ratio of the three-blade HAWT rotor development/fabrication cost and the associated annual energy production. To assess the annual energy production as a function of the blade design and operating conditions, an aerodynamics-based computational analysis had to be employed. As far as the turbine blade cost is concerned, it is assessed for a given aerodynamic design by separately computing the blade mass and the associated blade-mass/size-dependent production cost. For each aerodynamic design analyzed, a structural finite element-based and a post-processing life-cycle assessment analyses were employed in order to determine a minimal blade mass which ensures that the functional requirements pertaining to the quasi-static strength of the blade, fatigue-controlled blade durability and blade stiffness are satisfied. To determine the turbine-blade production cost (for the currently prevailing fabrication process, the wet lay-up) available data regarding the industry manufacturing experience were combined with the attendant blade mass, surface area, and the duration of the assumed production run. The work clearly revealed the challenges associated with simultaneously satisfying the strength, durability and stiffness requirements while maintaining a high level of wind-energy capture efficiency and a lower production cost.     

Gas Metal Arc Welding Process Modeling and Prediction of Weld Microstructure in MIL A46100 Armor-Grade Martensitic Steel

A conventional gas metal arc welding (GMAW) butt-joining process has been modeled using a two-way fully coupled, transient, thermal-mechanical finite-element procedure. To achieve two-way thermal-mechanical coupling, the work of plastic deformation resulting from potentially high thermal stresses is allowed to be dissipated in the form of heat, and the mechanical material model of the workpiece and the weld is made temperature dependent. Heat losses from the deposited filler-metal are accounted for by considering conduction to the adjoining workpieces as well as natural convection and radiation to the surroundings. The newly constructed GMAW process model is then applied, in conjunction with the basic material physical-metallurgy, to a prototypical high-hardness armor martensitic steel (MIL A46100). The main outcome of this procedure is the prediction of the spatial distribution of various crystalline phases within the weld and the heat-affected zone regions, as a function of the GMAW process parameters. The newly developed GMAW process model is validated by comparing its predictions with available open-literature experimental and computational data.     

Analysis of Fe---Ni---Cr---N austenite using the Embedded-Atom Method

An overview is given of the Embedded Atom Method (EAM) and its application to the analysis of atomic interactions in substitutional and interstitial metallic solid solutions. The method is subsequently applied to the Fe-Ni-Cr-N austenite. Experimental bulk properties such as the elastic constants and the heats of solution etc. for pure metals, and binary alloys are used to determine EAM functions for the constituent elements, Fe, Ni, Cr and N. The results are used to determine the corresponding f.c.c.-b.c.c. energy differencies, which are in turn compared with their CALPHAD counterparts.     

Atomistic simulations of the solubilization of single-walled carbon nanotubes in toluene

Solubilization of the armchair, metallic (10,10) single-walled carbon nanotubes (SWCNTs) in toluene is modeled using molecular dynamics simulations. Inter- and intra-molecular atomic interactions in the SWCNT + toluene system are represented using COMPASS (Condensed-phased Optimized Molecular Potential for Atomistic Simulation Studies), the first ab initio forcefield that enables an accurate and simultaneous prediction of various gas-phase and condensed-phase properties of organic and inorganic materials.The results obtained show that due to a significant drop in the configurational entropy of toluene, the solvation Gibbs free energy for these nanotubes in toluene is small but positive suggesting that a suspension of these nanotubes in toluene is not stable and that the nanotubes would fall out of the solution. This prediction is consistent with experimental observations.     

Factors affecting farmers’ willingness and ability to adopt and retain vitamin A-rich varieties of orange-fleshed sweet potato in Mozambique

The addition of orange-fleshed sweet potato (OFSP) to the food environment is an effective nutrition-sensitive agricultural approach to improve vitamin A intakes. However, the adoption of this biofortified crop merits further study. The objective of our research was to understand factors that affect Mozambican farmers’ adoption and retention of OFSP varieties, with a specific interest in the retention of planting material. Field research was conducted in three provinces of Mozambique during 2015. Provinces with different OFSP intervention histories were selected to allow for the identification of site-specific factors and the impact of variable approaches over time. Qualitative inquiry was used to assess participants’ progress through the five stages of the Innovation-Decision process in the Diffusion of Innovations Theory. Ninety-five producers, consumers, and market stakeholders of OFSP participated in semi-structured in-depth interviews and focus groups. Results indicate that diverse factors influenced the adoption and retention of OFSP, including organoleptic qualities, taste preferences, access to planting material, agronomic traits, environmental conditions, lack of capital for inputs and labor, unstable markets, and limited sharing of information and planting material across farmer networks. Current OFSP varieties were acceptable to Mozambican farmers and consumers, but there are several remaining challenges to reaching a critical mass such as lack of access to planting material, perceptions of superior drought tolerance of white-fleshed sweet potato (WFSP), and the belief that OFSP requires additional effort to cultivate (e.g. weed removal, measuring space between plants). Key recommendations which may be considered in future planning for OFSP interventions in Mozambique and other countries include enabling decentralized vine multipliers to provide vines to community members at no cost, continued focus on breeding and distribution of more drought tolerant varieties of OFSP, and training on the similarities in agronomic practices required for producing and preserving OFSP and WFSP.