220,221 research outputs found
On the Application of Deformation Kinetics to Nonlinear Constitutive Relations at Higher Temperatures
A single phenomenological constitutive equation is derived theoretically from first principles and applied to aluminum, tin and lead. The theory is based on deformation kinetics of steady creep in which the fundamental mechanism is atomic transport over potential barriers whose conformation is distorted by the application of a stress field. The form of the functional dependence of barrier distortion and stress over the entire temperature range is found to be a sigmoidal curve which tends to straight lines of a unit slope in the small and high stress regions. With this form of barrier distortion, the constitutive equation prediction the steady creep behavior of aluminum, tin and lead over a wide range of temperature and stress
Some Recent Developments in the Endochronic Theory with Application to Cyclic Histories
Constitutive equations with only two easily determined material constants predict the stress (strain) response of normalized mild steel to a variety of general strain (stress) histories, without a need for special unloading-reloading rules. The equations are derived from the endochronic theory of plasticity of isotropic materials with an intrinsic time scale defined in the plastic strain space. Agreement between theoretical predictions and experiments are are excellent quantitatively in cases of various uniaxial constant amplitude histories, variable uniaxial strain amplitude histories and cyclic relaxation. The cyclic ratcheting phenomenon is predicted by the present theory
Semiempirical calculation of deep levels: divacancy in Si
A study of the electronic levels associated with the divacancy in silicon is reported. The extended Huckel theory is shown to reproduce the band structure of silicon. The electronic levels of the divacancy are calculated by considering a periodic array of large unit cells each containing 62 atoms; a 64 atom perfect cell with two atoms removed to form the divacancy. The results are found to be in qualitative agreement with the results of EPR and infrared absorption measurements
Hybrid bearings for LH2 and LO2 turbopumps
Hybrid combinations of hydrostatic and ball bearings can improve bearing performance for liquid hydrogen and liquid oxygen turbopumps. Analytic studies were conducted to optimize hybrid bearing designs for the SSME-type turbopump conditions. A method to empirically determine damping coefficients was devised. Four hybrid bearing configurations were designed, and three were fabricated. Six hybrid and hydrostatic-only bearing configurations will be tested for steady-state and transient performance, and quantification of damping coefficients. The initial tests were conducted with the liquid hydrogen bearing
Fully automatic telemetry data processor
Satellite Telemetry Automatic Reduction System /STARS 2/, a fully automatic computer-controlled telemetry data processor, maximizes data recovery, reduces turnaround time, increases flexibility, and improves operational efficiency. The system incorporates a CDC 3200 computer as its central element
Coherence scale of the two-dimensional Kondo Lattice model
A doped hole in the two-dimensional half-filled Kondo lattice model with
exchange J and hopping t has momentum (pi,pi) irrespective of the coupling J/t.
The quasiparticle residue of the doped hole, Z_{(\pi, \pi)}, tracks the Kondo
scale, T_K, of the corresponding single impurity model. Those results stem from
high precision quantum Monte Carlo simulations on lattices up to 12 X 12.
Accounting for small dopings away from half-filling within a rigid band
approximation, this result implies that the effective mass of the charge
carriers at the Fermi level tracks 1/T_K or equivalently that the coherence
temperature T_{coh} \propto T_K. This results is consistent with the large-N
saddle point of the SU(N) symmetric Kondo lattice model.Comment: 4 pages, 4 figure
Effects of propagation parallel to the magnetic field on the type 1 electrojet irregularity instability
A simple analysis is presented which indicates that Type 1 irregularities which have a slight component of propagation along the magnetic field may be more unstable than those which propagate across the field. It was found that significant irregularity amplitudes may occur at the northern or southern extremities of the equatorial electrojet from those modes with large north-south group velocity, and they could significantly change our understanding of nonlinear solutions of the electrojet instability
Optical conductivity of one-dimensional narrow-gap semiconductors
The optical conductivities of two one-dimensional narrow-gap semiconductors,
anticrossing quantum Hall edge states and carbon nanotubes, are studied using
bosonization method. A lowest order renormalization group analysis indicates
that the bare band gap can be treated perturbatively at high
frequency/temperature. At very low energy scale the optical conductivity is
dominated by the excitonic contribution, while at temperature higher than a
crossover temperature the excitonic features are eliminated by thermal
fluctuations. In case of carbon nanotubes the crossover temperature scale is
estimated to be 300 K.Comment: RevTeX4 file, 6 pages, no figur
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