Evidence of persistent,recurring summertime hypoxia in Green Bay,Lake Michigan

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (<5?mg?L^?1) affect nearly 70% of the 38 stations sampled representing an area of ~500–600?km². Stratification typically lasts 2+?months, from late June to early September, and some stations exhibit bottom water hypoxia (<2?mg?L^?1) at a frequency of nearly 25% when sampled during this period. A monitoring program initiated in 1986 by the Green Bay Metropolitan Sewerage District has provided a 23?year, recreational season record (May–September) of continuous (15?min interval) in situ bottom water oxygen and temperature measurements at the Entrance Light station of the Green Bay navigational channel. The duration of the hypoxic season ranges from 2?weeks to over 3?months at this shallow 7?m offshore site. This variability likely results from a combination of thermal stratification, oxygen consumption in deeper waters of the bay, and physical forcing mechanisms that drive cool, oxygen depleted, bottom waters on a southerly trajectory across this sensor. These data suggest the duration of hypoxic conditions may have increased during the stratified season in recent years. Hypoxia in the bay would also appear to be sensitive to relatively small changes in these forces, particularly changes in organic carbon loading and the duration of stratification.     

A fast boundary element based solver for localized inelastic deformations

We present a numerical method for the solution of nonlinear geomechanical problems involving localized deformation along shear bands and fractures. We leverage the boundary element method to solve for the quasi-static elastic deformation of the medium while rigid-plastic constitutive relations govern the behavior of displacement discontinuity (DD) segments capturing localized deformations. A fully implicit scheme is developed using a hierarchical approximation of the boundary element matrix. Combined with an adequate block preconditioner, this allows to tackle large problems via the use of an iterative solver for the solution of the tangent system. Several two-dimensional examples of the initiation and growth of shear-bands and tensile fractures illustrate the capabilities and accuracy of this technique. The method does not exhibit any mesh dependency associated with localization provided that (i) the softening length-scale is resolved and (ii) the plane of localized deformations is discretized a priori using DD segments.     

Mechanisms for tertiary creep of single crystal superalloy

During the thermal-mechanical loading of high temperature single crystal turbine components, all three creep—stages: primary, secondary and tertiary, manifest themselves and, hence, none of them can be neglected. The development of a creep law that includes all three stages is especially important in the case of non-homogeneous thermal loading of the component where significant stress redistribution and relaxation will result. Thus, local creep analysis is crucial for proper design of damage tolerant airfoils. We have developed a crystallographic-based constitutive model and fully coupled it with damage kinetics. The model extends existing approaches for cyclic and thermal-cyclic loading of anisotropic elasto-viscoplastic deformation behavior and damage kinetics of single-crystal materials, allowing prediction of tertiary creep and failure initiation of high temperature components. Our damage model bridges the gap between dislocation dynamics and the continuum mechanics scales and can be used to represent tertiary as well as primary and secondary creep.     

Analysis of stresses during the freezing of solid spherical foods

An analytical model is presented to calculate thermal stresses and strains during the freezing of a spherical food, taking into account both the expansion during phase change and subsequent thermal contraction due to temperature decrease. The Young modulus and Poisson ratio are assumed to undergo a step change at the freezing point. The expansion due to phase change cause a uniform and virtually constant isotropic tensile stress in the unfrozen core. In the frozen shell, this expansion gives rise to tensile radial stress and compressive tangential stress. The thermal contraction subsequent to phase change causes reverse effects, i.e. uniform compressive stress in the unfrozen core and compressive radial stress in the frozen shell, while tangential stress is tensile on the outside and compressive on the inside of the frozen shell. The effect of thermal contraction is noticeable only at cryogenic temperatures.     

Air motion sensing hairs of arthropods detect high frequencies at near-maximal mechanical efficiency

Using measurements based on particle image velocimetry in combination with a novel compact theoretical framework to describe hair mechanics, we found that spider and cricket air motion sensing hairs work close to the physical limit of sensitivity and energy transmission in a broad range of relatively high frequencies. In this range, the hairs closely follow the motion of the incoming flow because a minimum of energy is dissipated by forces acting in their basal articulation. This frequency band is located beyond the frequency at which the angular displacement of the hair is maximum which is between about 40 and 600 Hz, depending on hair length (Barth et al. [1] Phil. Trans. R. Soc. Lond. B 340, 445–461 (doi:10.1098/rstb.1993.0084)). Given that the magnitude of natural airborne signals is known to decrease with frequency, our results point towards the possible existence of spectral signatures in the higher frequency range that may be weak but of biological significance.     

Evolution of Texture from a Single Crystal Ti-6Al-4V Substrate During Electron Beam Directed Energy Deposition

Additive manufacturing of Ti-6Al-4V commonly produces 〈001〉 _β -fiber textures aligned with the build direction. We have performed wire-feed electron beam directed energy deposition on the {112} _β plane of a single prior β-grain. The build initially grew epitaxially from the substrate with the preferred 〈001〉 growth direction significantly angled away from the build direction. However, continued layer deposition drove the formation of a 〈001〉 _β -fiber texture aligned with the build direction and the direction of the strongest thermal gradient.     

Microstructures and Grain Refinement of Additive-Manufactured Ti-xW Alloys

Metallurgical and Materials Transactions A - It is necessary to better understand the composition–processing–microstructure relationships that exist for materials produced by additive...     

Determination of Bulk Residual Stresses in Electron Beam Additive-Manufactured Aluminum

Additive-manufactured aluminum alloy deposits were analyzed using neutron diffraction to characterize the effect of intermediate stress relief anneal heat treatment on bulk residual stresses in the final part. Based on measured interplanar spacing, stresses were calculated at various locations along a single bead, stacked wall deposit. A comparison between an uninterrupted deposited wall and an interrupted, stress-relieved, and annealed deposited wall showed a measureable reduction in residual stress magnitude at the interface with a corresponding shift in stress character into the deposit. This shift changes the interface stresses from purely compressive to partially tensile. The residual stress profile varied along the length of the deposit, and the heat-treatment procedure reduced the overall magnitude of the stress at the interface by 10 through 25 MPa. These results are interpreted in terms of thermal gradients inherent to the process and compared with prior residual stress-characterization studies in additive-manufactured metallic structures.     

Dethermalization Theory: Thermal/Dielectric Analogy for Heat Flow Through Porosity in Composites

The flow of heat in solids has long been known to possess an electric current analogy applicable to both steady state and transient flows. In the present work we assume a vector analogy between Fourier’s law and the classical electric displacement to develop a method of handling distributed porosity in composite materials subject to heat flow in a way analogous to dealing with distributed dielectric regions in solids subject to an external electric field. The effect of the geometry of “depolarization” regions in an electric displacement field and “demagnetization” regions in a magnetizing field can be carried over to the effect of “dethermalization” regions in a heat-flux field. The analogy provides a simple analytic way of determining the effects of porosity shape on thermal conductivity which can be significant and can violate the usual law of mixtures approach. For uniformly distributed porosity of known aspect ratio in a given region, the volume-fraction porosity of the region can then be evaluated from a simple measurement of the thermal diffusivity. This approach was originally successfully tested over a limited range of variables when the model was developed and has recently been validated to good accuracy over a large range of porosity aspect ratios.     

Productivity of yam-based systems with herbaceous legumes and short fallows in the Guinea-Sudan transition zone of Benin

The principal driving force in agricultural research is to increase the yield of food crops. For farming to remain productive, it will be necessary to replenish the nutrients removed or lost from the soil. The objective of this study was to determine the impact of yam-based systems on soil productivity (dry matter production, nutrients recycled or removed, profitability and soil fertility changes). We compared smallholders’ traditional systems (1-year fallow of Andropogonon gayanus -yam rotation; maize-yam rotation) with yam-based systems with legumes (intercropped Aeschynomene histrix with maize-yam rotation; intercropped Mucuna pruriens with maize-yam rotation). The production of dry matter (tubers, shoots), nutrients removed or recycled, and soil properties were significantly improved on yam-based systems with legumes in comparison with traditional systems. Year × Treatment interactions influenced significantly the tuber dry matter production. Site × Treatment and Treatment × Farmer interactions affected significantly nutrients removed or recycled. The amount of nutrients recycled or removed was dependent on the dry matter production that, in turn, depended on soil fertility, rainfall and farmers’ effect.Yam-based systems with legumes brought a higher present value than traditional systems in the first 4 years and appeared attractive for land, labour and cash productivities.