Ab initio and finite-temperature molecular dynamics studies of lattice resistance in tantalum

This manuscript explores the apparent discrepancy between experimental data and theoretical calculations of the lattice resistance of bcc tantalum. We present the first results for the temperature dependence of the Peierls stress in this system and the first ab initio calculation of the zero-temperature Peierls stress to employ periodic boundary conditions, which are those best suited to the study of metallic systems at the electron-structure level. Our ab initio value for the Peierls stress is over five times larger than current extrapolations of experimental lattice resistance to zero-temperature. Although we do find that the common techniques for such extrapolation indeed tend to underestimate the zero-temperature limit, the amount of the underestimation which we observe is only 10-20%, leaving open the possibility that mechanisms other than the simple Peierls stress are important in controlling the process of low temperature slip.Comment: 12 pages and 9 figure

Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed

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Not AvailableStationary bagnets are one of the most extensively operated traditional fishing gears along Maharashtra Coast. Despite being the most important traditional gear of the State, no systematic studies on the temporal variations in catch composition and the influence of environmental factors on the bagnet fishery of Maharashtra Coast have been conducted till date. Thus, the present study aimed at evaluating temporal variations in catch composition and the influence of environmental variables on the same. To this end, fishing experiments and environmental assessments were conducted every month at four stations for two years, and analysed to achieve the objectives.A total of 156 species belonging to 63 families were recorded in the catch. The most dominant species in the catch were Harpadon nehereus, Acetes spp., Nematopalaemon tenuipes, Coilia dussumieri, Chrysaora sp. and Lepturacanthus savala. No significant spatial variations were observed for the different resources, whereas temporal variations were significant. Major environmental variables were analysed and compared among the seasons.Significant temporal variations were observed for all the environmental variables. Catch composition and environmental data were subjected to different multivariate analyses. The results of cluster analysis, ANOSIM and SIMPER established a significant difference in catch composition among the seasons. The diagnostic species for each season were identified through CCA and SIMPER. The results of CCA, Pearson’s correlation analysis revealed that current speed, temperature, salinity, pH, DO, turbidity, chlorophyll-a and plankton density play significant roles in structuring the catch composition acrossthe different seasons and current speed exerts maximum influence on catchrate. The information from the study could be used as baseline data for framing management measures for a sustainable bagnet fishery along Maharashtra Coast.Not Availabl

Buckling behavior of nanowires predicted by a new surface energy density model

The axial buckling behavior of nanowires is investigated with a new continuum theory, in which the surface effect of nanomaterials is characterized by the surface energy density. Only the surface energy density of bulk materials and the surface relaxation parameter are involved, instead of the surface elastic constants in the classical surface elasticity theory. Two kinds of nanowires with different boundary conditions are discussed. It is demonstrated that the new continuum theory can predict the buckling behavior of nanowires very well. Similar to the prediction of the classical elasticity theory, the critical compressive load of axial buckling of nanowires predicted by the new continuum theory increases with an increasing characteristic length, such as the diameter or height of nanowires. With the same aspect ratio, a nanowire with a rectangular cross section possesses a larger critical buckling load than that with a circular one. However, the surface effect could enhance the critical buckling load not only for a fixed-fixed nanowire but also for a cantilevered one in contrast to the classical elastic model. All the results predicted by the new continuum theory agree well with predictions by the surface elasticity models. The present research not only verifies the validation of the new continuum theory, but also gives a much more convenient characterization of buckling behaviors of nanowires. This should be helpful for the design of nanodevices based on nanomaterials, for example, nanobeams in NEMS or high-precision instruments