Time Evolution Of Deformation Research Articles

MULTI-FLUID HYDRODYNAMICS IN CHARGE DENSITY WAVES WITH COLLECTIVE, ELECTRONIC, AND SOLITONIC DENSITIES AND CURRENTS

We present a general scheme to approach the space - time evolution of deformations, currents, and the electric field in charge density waves related to appearance of intrinsic topological defects: dislocations, their loops or pairs, and solitons. We derive general equations for the multi-fluid hydrodynamics taking into account the collective mode, electric field, normal electrons, and the intrinsic defects. These equations may allow to study the transformation of injected carriers from normal electrons to new periods of the charge density wave, the collective motion in constrained geometry, and the plastic states and flows. As an application, we present analytical and numerical solutions for distributions of fields around an isolated dislocation line in the regime of nonlinear screening by the gas of phase solitons.

Time Evolution of Deformation Using Time Series of Differential Interferograms: Application to La Palma Island (Canary Islands)

Differential interferometry is a very powerful tool for detecting changes in the Earth’s crust where coherence conditions are good, but is difficult to employ in some volcanic areas due to dense vegetation. We apply two differential InSAR methods using the time series associated with the interferograms to perform a phase analysis on a data set for La Palma island (Canary Islands) from the ERS-1 and ERS-2 European Space Agency (ESA) satellites for the time period 1992 to 2000. Both methods involve choosing a master image from the database and creating a series of interferograms with respect to this image. The “Coherent Pixel Time Series” (CPTS) technique chooses pixels with good average coherence, aligns the unwrapped interferograms with a stable area and then performs an inversion to calculate the linear velocity to quantify the deformation. The Coherent Target Modeling (CTM) method calculates the temporal coherence of each pixel to identify stable targets and then determines the best velocity for each pixel by using a linear fit that maximizes the temporal coherence. Using these two methods we have been able to detect deformation on La Palma Island that has been previously undetectable by conventional InSAR methods. There is a roughly circular region on the Southern part of the island that is actively deforming at ∽ -4 to -8 mm/yr. This region is located near the Teneguia valcano, the host of the last known eruption on La Palma in 1971. A thorough investigation of the possible sources for this deformation revealed that it was most likely created by a subsurface thermal source.

Optical measurement of the specimen deformation at high strain rate

During recent years, the investigation of the strain-rate-dependent properties of materials has become more and more important. The experimental techniques used to establish the specific dynamic behavior of materials all have in common that the acquisition of information concerning the deformation of the specimen is cumbersone and often questionable. Mostly, only limited information on the spatial distribution and time evolution of the deformation in specimen can be obtained. In this paper, a non-contact, optical technique is presented, providing the time evolution and spatial distribution of the axial deformation in specimens during a high strain rate test. The deformation of a line grid attached to the specimen is recorded during an experiment by means of a rotating drum camera. The time history of the axial displacements is subsequently derived by an advanced technique based on digital geometric moire combined with a phase-shift method, specially developed to this purpose. The technique can be applied to a wide range of materials and high strain rate tests, and is illustrated by means of a split Hopkinson tensile bar experiment on a steel sheet specimen.

A Geometrical Approach to Dynamic Strain Localization in Planar Motion

A geometrical theory for the time-evolution of strain localization (flow-localization) is discussed. The purpose is to obtain a reduction of the continuum mechanical problem of time-evolution of deformation to one involving few scalar invariants rather than the full field variables and thereby obtain a 'particle-like' description of the evolving localization zones. The present paper deals with these issues on cases restricted to those of planar deformation. Several concepts are introduced in the description, such as path-continuity, stable and unstable localization, width of localization zones, uncertainties in the description of time evolution of localization, coherent and non-coherent flow localization. Simulation results for the high-rate planar extension with rectangular elastic-viscoplastic blocks are used to demonstrate several of the theoretical concepts used. A fully non-linear formulation of the continuum mechanical problem is used accounting for finite deformations as well as irreversible elastic-viscoplastic material behavior. Several characteristic localization phenomena are observed in the simulations such as branching of the critical path of the impact wave, multiple neck formation and elastic-plastic wave interaction. (Less)

Hydrodynamic theory of plastic flows with conversion

Experimental studies show that the sliding state is essentially inhomogeneous with different effects near contacts, in the bulk or at obstacles. We present a general scheme to approach the space - time evolution of deformations, electric fields and currents with particular emphasize t o the current conversion process in Charge Density Waves. We derive general equations for the multifluid hydrodynamics of plastic flows which allow, in principle, to study the transformation of injected carriers from normal electrons to new periods, collective motion in constrained geometry as well as the plastic contribution to non-linear current-voltage characteristics.

Measurement of the temporal evolution of periodic induced displacement derivatives using stroboscopic electronic speckle-shearing interferometry

Single pulsed subtraction electronic speckle-shearing pattern interferometry is used to measure the time evolution of deformations induced by periodic impulse loading on a metal plate. This is accomplished with a stroboscopic system based on a pulsed laser and an interline-transfer CCD camera running at 60 frames/s. In a few seconds, a sequence of hundreds of interferograms is recorded with an effective sampling time interval of some tens of microseconds. We describe the experimental setup used to generate the transient deformation on the metal plate and also the synchronization of laser pulses and image acquisition. Several correlation fringe patterns are presented, showing the time evolution of the deformation. Finally, these data are used to calculate optical phase maps and displacement derivative fields for different times after the application of the dynamic load on the object.