358 research outputs found
Size scaling of strength in thin film delamination
We investigate by numerical simulation the system size dependence of the
shear delamination strength of thin elastic films. The films are connected to a
rigid substrate by a disordered interface containing a pre-existing crack. The
size dependence of the strength of this system is found to depend crucially on
the crack shape. For circular cracks, we observe a crossover between a
size-independent regime at large crack radii which is controlled by propagation
of the pre-existing crack, and a size-dependent regime at small radii which is
dominated by nucleation of new cracks in other locations. For cracks of finite
width that span the system transversally, we observe for all values of the
crack length a logarithmic system size dependence of the failure stress. The
results are interpreted in terms of extreme value statistics.Comment: 10 pages, 4 figure
Fluctuation phenomena in crystal plasticity - a continuum model
On microscopic and mesoscopic scales, plastic flow of crystals is
characterized by large intrinsic fluctuations. Deformation by crystallographic
slip occurs in a sequence of intermittent bursts ('slip avalanches') with
power-law size distribution. In the spatial domain, these avalanches produce
characteristic deformation patterns in the form of slip lines and slip bands
which exhibit long-range spatial correlations. We propose a generic continuum
model which accounts for randomness in the local stress-strain relationships as
well as for long-range internal stresses that arise from the ensuing plastic
strain heterogeneities. The model parameters are related to the local dynamics
and interactions of lattice dislocations. The model explains experimental
observations on slip avalanches as well as the associated slip and surface
pattern morphologies
Slip avalanches in crystal plasticity: scaling of the avalanche cutoff
Plastic deformation of crystals proceeds through a sequence of intermittent
slip avalanches with scale-free (power-law) size distribution. On macroscopic
scales, however, plastic flow is known to be smooth and homogeneous. In the
present letter we use a recently proposed continuum model of slip avalanches to
systematically investigate the nature of the cut-off which truncates scale-free
behavior at large avalanche sizes. The dependence of the cut-off on system
size, geometry, and driving mode, but also on intrinsic parameters such as the
strain hardening rate is established. Implications for the observability of
avalanche behavior in microscopic and macroscopic samples are discussed.Comment: 12 pages, 4 figure
Damage growth in fibre bundle models with localized load sharing and environmentally-assisted ageing
Dynamical correlations near dislocation jamming
Dislocation assemblies exhibit a jamming or yielding transition at a critical
external shear stress value . Nevertheless the nature of this
transition has not been ascertained. Here we study the heterogeneous and
collective nature of dislocation dynamics within a crystal plasticity model
close to , by considering the first-passage properties of the
dislocation dynamics. As the transition is approached in the moving phase, the
first passage time distribution exhibits scaling, and a related peak {\it
dynamical} susceptibility diverges as , with . We relate this scaling
to an avalanche description of the dynamics. While the static structural
correlations are found to be independent of the external stress, we identify a
diverging dynamical correlation length in the direction perpendicular
to the dislocation glide motion.Comment: 4 pages, 5 figure
Dislocation jamming and Andrade creep
We simulate the glide motion of an assembly of interacting dislocations under
the action of an external shear stress and show that the associated plastic
creep relaxation follows Andrade's law. Our results indicate that Andrade creep
in plastically deforming crystals involves the correlated motion of dislocation
structures near a dynamic transition separating a flowing from a jammed phase.
Simulations in presence of dislocation multiplication and noise confirm the
robustness of this finding and highlight the importance of metastable structure
formation for the relaxation process.Comment: 4 pages, 4 EPS figure
Role of density fluctuations in the relaxation of random dislocation systems
We study the relaxation dynamics of systems of straight, parallel crystal
dislocations, starting from initially random and uncorrelated positions of the
individual dislocations. A scaling model of the relaxation process is
constructed by considering the gradual extinction of the initial density
fluctuations present in the system. The model is validated by ensemble
simulations of the discrete dynamics of dislocations. Convincing agreement is
found for systems of edge dislocations in single slip irrespective of the net
Burgers vector of the dislocation system. It is also demonstrated that the
model does not work in multiple slip geometries.Comment: 25 pages, 11 figures; submitted to Journal of Statistical Mechanics:
theory and experiment after 2nd round of referenc
A compact dual atom interferometer gyroscope based on laser-cooled rubidium
We present a compact and transportable inertial sensor for precision sensing
of rotations and accelerations. The sensor consists of a dual Mach-Zehnder-type
atom interferometer operated with laser-cooled Rb. Raman processes are
employed to coherently manipulate the matter waves. We describe and
characterize the experimental apparatus. A method for passing from a compact
geometry to an extended interferometer with three independent atom-light
interaction zones is proposed and investigated. The extended geometry will
enhance the sensitivity by more than two orders of magnitude which is necessary
to achieve sensitivities better than rad/s/.Comment: 9 pages, 8 figure
Dislocation patterning in a two-dimensional continuum theory of dislocations
Understanding the spontaneous emergence of dislocation patterns during plastic deformation is a long standing challenge in dislocation theory. During the past decades several phenomenological continuum models of dislocation patterning were proposed, but few of them (if any) are derived from microscopic considerations through systematic and controlled averaging procedures. In this paper we present a two-dimensional continuum theory that is obtained by systematic averaging of the equations of motion of discrete dislocations. It is shown that in the evolution equations of the dislocation densities diffusionlike terms neglected in earlier considerations play a crucial role in the length scale selection of the dislocation density fluctuations. It is also shown that the formulated continuum theory can be derived from an averaged energy functional using the framework of phase field theories. However, in order to account for the flow stress one has in that case to introduce a nontrivial dislocation mobility function, which proves to be crucial for the instability leading to patterning
noise and avalanche scaling in plastic deformation
We study the intermittency and noise of dislocation systems undergoing shear
deformation. Simulations of a simple two-dimensional discrete dislocation
dynamics model indicate that the deformation rate exhibits a power spectrum
scaling of the type . The noise exponent is far away from a
Lorentzian, with . This result is directly related to the
way the durations of avalanches of plastic deformation activity scale with
their size.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
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