244 research outputs found
Strain bursts in plastically deforming Molybdenum micro- and nanopillars
Plastic deformation of micron and sub-micron scale specimens is characterized
by intermittent sequences of large strain bursts (dislocation avalanches) which
are separated by regions of near-elastic loading. In the present investigation
we perform a statistical characterization of strain bursts observed in
stress-controlled compressive deformation of monocrystalline Molybdenum
micropillars. We characterize the bursts in terms of the associated elongation
increments and peak deformation rates, and demonstrate that these quantities
follow power-law distributions that do not depend on specimen orientation or
stress rate. We also investigate the statistics of stress increments in between
the bursts, which are found to be Weibull distributed and exhibit a
characteristic size effect. We discuss our findings in view of observations of
deformation bursts in other materials, such as face-centered cubic and
hexagonal metals.Comment: 14 pages, 8 figures, submitted to Phil Ma
Acceleration and localization of subcritical crack growth in a natural composite material
Catastrophic failure of natural and engineered materials is often preceded by an acceleration and localization of damage that can be observed indirectly from acoustic emissions (AE) generated by the nucleation and growth of microcracks. In this paper we present a detailed investigation of the statistical properties and spatiotemporal characteristics of AE signals generated during triaxial compression of a sandstone sample. We demonstrate that the AE event amplitudes and interevent times are characterized by scaling distributions with shapes that remain invariant during most of the loading sequence. Localization of the AE activity on an incipient fault plane is associated with growth in AE rate in the form of a time-reversed Omori law with an exponent near 1. The experimental findings are interpreted using a model that assumes scale-invariant growth of the dominating crack or fault zone, consistent with the Dugdale-Barenblatt âprocess zoneâ model. We determine formal relationships between fault size, fault growth rate, and AE event rate, which are found to be consistent with the experimental observations. From these relations, we conclude that relatively slow growth of a subcritical fault may be associated with a significantly more rapid increase of the AE rate and that monitoring AE rate may therefore provide more reliable predictors of incipient failure than direct monitoring of the growing fault
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.
Self-affine surface morphology of plastically deformed metals
We analyze the surface morphology of metals after plastic deformation over a
range of scales from 10 nm to 2 mm, using a combination of atomic force
microscopy and scanning white-light interferometry. We demonstrate that an
initially smooth surface during deformation develops self-affine roughness over
almost four orders of magnitude in scale. The Hurst exponent of
one-dimensional surface profiles is initially found to decrease with increasing
strain and then stabilizes at . By analyzing their statistical
properties we show that the one-dimensional surface profiles can be
mathematically modelled as graphs of a fractional Brownian motion. Our findings
can be understood in terms of a fractal distribution of plastic strain within
the deformed samples
Depinning transition of dislocation assemblies: pileup and low-angle grain boundary
We investigate the depinning transition occurring in dislocation assemblies.
In particular, we consider the cases of regularly spaced pileups and low angle
grain boundaries interacting with a disordered stress landscape provided by
solute atoms, or by other immobile dislocations present in non-active slip
systems. Using linear elasticity, we compute the stress originated by small
deformations of these assemblies and the corresponding energy cost in two and
three dimensions. Contrary to the case of isolated dislocation lines, which are
usually approximated as elastic strings with an effective line tension, the
deformations of a dislocation assembly cannot be described by local elastic
interactions with a constant tension or stiffness. A nonlocal elastic kernel
results as a consequence of long range interactions between dislocations. In
light of this result, we revise statistical depinning theories and find novel
results for Zener pinning in grain growth. Finally, we discuss the scaling
properties of the dynamics of dislocation assemblies and compare theoretical
results with numerical simulations.Comment: 13 pages, 8 figure
Risk-taking Behavior in Police Officers and Martial Artists: Investigating potential Differences using the Balloon Analogue Risk Task
Threat-Related Attentional Biases in Police Officers and Martial Artists: Investigating Potential Differences Using the E-Stroop and Dot Probe Task
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