905 research outputs found
The Peierls--Nabarro FE model in two-phase microstructures -- a comparison with atomistics
This paper evaluates qualitatively as well as quantitatively the accuracy of
a recently proposed Peierls--Nabarro Finite Element (PN-FE) model for
dislocations by a direct comparison with an equivalent molecular statics
simulation. To this end, a two-dimensional microstructural specimen subjected
to simple shear is considered, consisting of a central soft phase flanked by
two hard-phase regions. A hexagonal atomic structure with equal lattice spacing
is adopted, the interactions of which are described by the Lennard--Jones
potential with phase specific depths of its energy well. During loading, edge
dislocation dipoles centred in the soft phase are introduced, which progress
towards the phase boundaries, where they pile up. Under a sufficiently high
external shear load, the leading dislocation is eventually transmitted into the
harder phase. The homogenized PN-FE model is calibrated to an atomistic model
in terms of effective elasticity constants and glide plane properties as
obtained from simple uniform deformations. To study the influence of different
formulations of the glide plane potential, multiple approaches are employed,
ranging from a simple sinusoidal function of the tangential disregistry to a
complex model that couples the influence of the tangential and the normal
disregistries. The obtained results show that, qualitatively, the dislocation
structure, displacement, strain fields, and the dislocation evolution are
captured adequately. The simplifications of the PN-FE model lead, however, to
some discrepancies within the dislocation core. Such discrepancies play a
dominant role in the dislocation transmission process, which thus cannot
quantitatively be captured properly. Despite its simplicity, the PN-FE model
proves to be an elegant tool for a qualitative study of edge dislocation
behaviour in two-phase microstructures, although it may not be quantitatively
predictive.Comment: 29 pages, 11 figures, 5 tables, abstract shortened to fulfill 1920
character limit, small changes after revie
Characterization and Simulation of High-Speed-Deformation-Processes
The combination of the processes deep drawing and electromagnetic pulse forming is a promising way to cope with the ever higher complexity of new sheet metal designs. A cooperation between the Institute of Materials Science (IW) of the Leibniz Universität Hannover and the Institute of Applied Mechanics (IFAM) of the RWTH Aachen is investigating these processes both experimental and in simulation. Aim is the characterization of the combined process. Therefore the material properties of the investigated aluminum alloy EN AW 6082 T6 have to be determined quasi-static as well as at high speed. These properties are then used as a basic for the simulations. Anisotropic behaviors as well as dynamic hardening effects are investigated in the quasi-static state. Several experiments for analyzing "Bauschinger" respectively "Ratcheting effects" have been conducted resulting in a new measuring set-up for thin sheets. For the determination of high speed forming limit diagrams a novel testing device on the basis of the Nakajima-test has been developed allowing for strain rates of approximately 10^3 s^-1. Both testing methods are described in this paper; the results are then used to adapt the simulation models for the combined processes. The high speed deformation process is simulated by means of finite elements using a material model developed at the IFAM. The finite strain constitutive model combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamic setting. It is based on the multiplicative split of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong-Frederick kinematic hardening which is widely adopted as capable of representing the above metal hardening effects. To prevent locking of the simulated thin sheets a new eight-node solid-shell finite element based on reduced integration with hourglass stabilization developed at IFAM has been used. With these features it was possible to simulate the Bauschinger effect obtained by the previous experiments
Comparative predictions of discharge from an artificial catchment (Chicken Creek) using sparse data
Ten conceptually different models in predicting discharge from the artificial Chicken Creek catchment in North-East Germany were used for this study. Soil texture and topography data were given to the modellers, but discharge data was withheld. We compare the predictions with the measurements from the 6 ha catchment and discuss the conceptualization and parameterization of the models. The predictions vary in a wide range, e.g. with the predicted actual evapotranspiration ranging from 88 to 579 mm/y and the discharge from 19 to 346 mm/y. The predicted components of the hydrological cycle deviated systematically from the observations, which were not known to the modellers. Discharge was mainly predicted as subsurface discharge with little direct runoff. In reality, surface runoff was a major flow component despite the fairly coarse soil texture. The actual evapotranspiration (AET) and the ratio between actual and potential ET was systematically overestimated by nine of the ten models. None of the model simulations came even close to the observed water balance for the entire 3-year study period. The comparison indicates that the personal judgement of the modellers was a major source of the differences between the model results. The most important parameters to be presumed were the soil parameters and the initial soil-water content while plant parameterization had, in this particular case of sparse vegetation, only a minor influence on the results
Thermal analysis of hadron multiplicities from relativistic quantum molecular dynamics
Some questions arising in the application of the thermal model to hadron
production in heavy ion collisions are studied. We do so by applying the
thermal model of hadron production to particle yields calculated by the
microscopic transport model RQMD(v2.3). We study the bias of incomplete
information about the final hadronic state on the extraction of thermal
parameters.It is found that the subset of particles measured typically in the
experiments looks more thermal than the complete set of stable particles. The
hadrons which show the largest deviations from thermal behaviour in RQMD(v2.3)
are the multistrange baryons and antibaryons. We also looked at the influence
of rapidity cuts on the extraction of thermal parameters and found that they
lead to different thermal parameters and larger disagreement between the RQMD
yields and the thermal model.Comment: 12 pages, 2 figures, uses REVTEX, only misprint and stylistic
corrections, to appear in Physical Review
Remnants of Initial Anisotropic High Energy Density Domains in Nucleus-Nucleus Collisions
Anisotropic high energy density domains may be formed at early stages of
ultrarelativistic heavy ion collisions, e.g. due to phase transition dynamics
or non-equilibrium phenomena like (mini-)jets. Here we investigate hadronic
observables resulting from an initially created anisotropic high energy density
domain. Based on our studies using a transport model we find that the initial
anisotropies are reflected in the freeze-out multiplicity distribution of both
pions and kaons due to secondary hadronic rescattering. The anisotropy appears
to be stronger for particles at high transverse momenta. The overall kaon
multiplicity increases with large fluctuations of local energy densities, while
no change has been found in the pion multiplicity.Comment: Submitted to PR
The climate sensitivity of carbon, timber, and species richness covaries with forest age in boreal–temperate North America
Climate change threatens the provisioning of forest ecosystem services and biodiversity (ESB). The climate sensitivity of ESB may vary with forest development from young to old-growth conditions as structure and composition shift over time and space. This study addresses knowledge gaps hindering implementation of adaptive forest management strategies to sustain ESB. We focused on a number of ESB indicators to (a) analyze associations among carbon storage, timber growth rate, and species richness along a forest development gradient; (b) test the sensitivity of these associations to climatic changes; and (c) identify hotspots of climate sensitivity across the boreal–temperate forests of eastern North America. From pre-existing databases and literature, we compiled a unique dataset of 18,507 forest plots. We used a full Bayesian framework to quantify responses of nine ESB indicators. The Bayesian models were used to assess the sensitivity of these indicators and their associations to projected increases in temperature and precipitation. We found the strongest association among the investigated ESB indicators in old forests (\u3e170 years). These forests simultaneously support high levels of carbon storage, timber growth, and species richness. Older forests also exhibit low climate sensitivity of associations among ESB indicators as compared to younger forests. While regions with a currently low combined ESB performance benefitted from climate change, regions with a high ESB performance were particularly vulnerable to climate change. In particular, climate sensitivity was highest east and southeast of the Great Lakes, signaling potential priority areas for adaptive management. Our findings suggest that strategies aimed at enhancing the representation of older forest conditions at landscape scales will help sustain ESB in a changing world
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