405 research outputs found
An automated procedure for material parameter evaluation for viscoplastic constitutive models
An automated procedure is presented for evaluating the material parameters in Walker's exponential viscoplastic constitutive model for metals at elevated temperature. Both physical and numerical approximations are utilized to compute the constants for Inconel 718 at 1100 F. When intermediate results are carefully scrutinized and engineering judgement applied, parameters may be computed which yield stress output histories that are in agreement with experimental results. A qualitative assessment of the theta-plot method for predicting the limiting value of stress is also presented. The procedure may also be used as a basis to develop evaluation schemes for other viscoplastic constitutive theories of this type
An experimental comparison of several current viscoplastic constitutive models at elevated temperature
Four current viscoplastic models are compared experimentally for Inconel 718 at 593 C. This material system responds with apparent negative strain rate sensitivity, undergoes cyclic work softening, and is susceptible to low cycle fatigue. A series of tests were performed to create a data base from which to evaluate material constants. A method to evaluate the constants is developed which draws on common assumptions for this type of material, recent advances by other researchers, and iterative techniques. A complex history test, not used in calculating the constants, is then used to compare the predictive capabilities of the models. The combination of exponentially based inelastic strain rate equations and dynamic recovery is shown to model this material system with the greatest success. The method of constant calculation developed was successfully applied to the complex material response encountered. Backstress measuring tests were found to be invaluable and to warrant further development
Dimensional Reduction for Directed Branched Polymers
Dimensional reduction occurs when the critical behavior of one system can be
related to that of another system in a lower dimension. We show that this
occurs for directed branched polymers (DBP) by giving an exact relationship
between DBP models in D+1 dimensions and repulsive gases at negative activity
in D dimensions. This implies relations between exponents of the two models:
(the exponent describing the singularity of the
pressure), and (the correlation length exponent of
the repulsive gas). It also leads to the relation ,
where is the Yang-Lee edge exponent. We derive exact expressions
for the number of DBP of size N in two dimensions.Comment: 7 pages, 1 eps figure, ref 24 correcte
Ice Age Epochs and the Sun's Path Through the Galaxy
We present a calculation of the Sun's motion through the Milky Way Galaxy
over the last 500 million years. The integration is based upon estimates of the
Sun's current position and speed from measurements with Hipparcos and upon a
realistic model for the Galactic gravitational potential. We estimate the times
of the Sun's past spiral arm crossings for a range in assumed values of the
spiral pattern angular speed. We find that for a difference between the mean
solar and pattern speed of Omega_Sun - Omega_p = 11.9 +/- 0.7 km/s/kpc the Sun
has traversed four spiral arms at times that appear to correspond well with
long duration cold periods on Earth. This supports the idea that extended
exposure to the higher cosmic ray flux associated with spiral arms can lead to
increased cloud cover and long ice age epochs on Earth.Comment: 14 pages, 3 figures, accepted for publication in Ap
Numerical study of the transition of the four dimensional Random Field Ising Model
We study numerically the region above the critical temperature of the four
dimensional Random Field Ising Model. Using a cluster dynamic we measure the
connected and disconnected magnetic susceptibility and the connected and
disconnected overlap susceptibility. We use a bimodal distribution of the field
with for all temperatures and a lattice size L=16. Through a
least-square fit we determine the critical exponents and . We find the magnetic susceptibility and the overlap
susceptibility diverge at two different temperatures. This is coherent with the
existence of a glassy phase above . Accordingly with other simulations
we find . In this case we have a scaling theory with
two indipendet critical exponentsComment: 10 pages, 2 figures, Late
Weighted Mean Field Theory for the Random Field Ising Model
We consider the mean field theory of the Random Field Ising Model obtained by
weighing the many solutions of the mean field equations with Boltzmann-like
factors. These solutions are found numerically in three dimensions and we
observe critical behavior arising from the weighted sum. The resulting
exponents are calculated.Comment: 15 pages of tex using harvmac. 8 postscript figures (fig3.ps is
large) in a separate .uu fil
On the thermodynamics of first-order phase transition smeared by frozen disorder
The simplified model of first-order transition in a media with frozen
long-range transition-temperature disorder is considered. It exhibits the
smearing of the transition due to appearance of the intermediate inhomogeneous
phase with thermodynamics described by the ground state of the short-range
random-field Ising model. Thus the model correctly reproduce the persistence of
first-order transition only in dimensions d > 2, which is found in more
realistic models. It also allows to estimate the behavior of thermodynamic
parameters near the boundaries of the inhomogeneous phase.Comment: 4 page
How can a glacial inception be predicted?
The Early Anthropogenic Hypothesis considers that greenhouse gas
concentrations should have declined during the Holocene in absence of humankind
activity, leading to glacial inception around the present. It partly relies on
the fact that present levels of northern summer incoming solar radiation are
close to those that, in the past, preceded a glacial inception phenomenon,
associated to declines in greenhouse gas concentrations. However, experiments
with various numerical models of glacial cycles show that next glacial
inception may still be delayed by several ten thousands of years, even with the
assumption of greenhouse gas concentration declines during the Holocene.
Furthermore, as we show here, conceptual models designed to capture the gross
dynamics of the climate system as a whole suggest also that small disturbances
may sometimes cause substantial delays in glacial events, causing a fair level
of unpredictability on ice age dynamics. This suggests the need of a validated
mathematical description of the climate system dynamics that allows us to
quantify uncertainties on predictions. Here, it is proposed to organise our
knowledge about the physics and dynamics of glacial cycles through a Bayesian
inference network. Constraints on the physics and dynamics of climate can be
encapsulated into a stochastic dynamical system. These constraints include, in
particular, estimates of the sensitivity of the components of climate to
external forcings, inferred from plans of experiments with large simulators of
the atmosphere, oceans and ice sheets. On the other hand, palaeoclimate
observations are accounted for through a process of parameter calibration. We
discuss promises and challenges raised by this programme.Comment: Contribution to the special issue of 'The Holocene' on the Early
Anthropogenic Hypotheses. W.R. Ruddiman, M. Crucifix, F. Oldfiel
Positive temperature versions of two theorems on first-passage percolation
The estimates on the fluctuations of first-passsage percolation due to
Talagrand (a tail bound) and Benjamini--Kalai--Schramm (a sublinear variance
bound) are transcribed into the positive-temperature setting of random
Schroedinger operators.Comment: 15 pp; to appear in GAFA Seminar Note
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