105 research outputs found
The Breakdown of Linear Elastic Fracture Mechanics near the Tip of a Rapid Crack
We present high resolution measurements of the displacement and strain fields
near the tip of a dynamic (Mode I) crack. The experiments are performed on
polyacrylamide gels, brittle elastomers whose fracture dynamics mirror those of
typical brittle amorphous materials. Over a wide range of propagation
velocities (), we compare linear elastic fracture mechanics (LEFM)
to the measured near-tip fields. We find that, sufficiently near the tip, the
measured stress intensity factor appears to be non-unique, the crack tip
significantly deviates from its predicted parabolic form, and the strains ahead
of the tip are more singular than the divergence predicted by LEFM.
These results show how LEFM breaks down as the crack tip is approached.Comment: 4 pages, 4 figures, first of a two-paper series (experiments); no
change in content, minor textual revision
Direct Identification of the Glass Transition: Growing Length Scale and the Onset of Plasticity
Understanding the mechanical properties of glasses remains elusive since the
glass transition itself is not fully understood, even in well studied examples
of glass formers in two dimensions. In this context we demonstrate here: (i) a
direct evidence for a diverging length scale at the glass transition (ii) an
identification of the glass transition with the disappearance of fluid-like
regions and (iii) the appearance in the glass state of fluid-like regions when
mechanical strain is applied.
These fluid-like regions are associated with the onset of plasticity in the
amorphous solid. The relaxation times which diverge upon the approach to the
glass transition are related quantitatively.Comment: 5 pages, 5 figs.; 2 figs. omitted, new fig., quasi-crystal discussion
omitted, new material on relaxation time
Non-universality in Micro-branching Instabilities in Rapid Fracture: the Role of Material Properties
In spite of the apparent similarity of micro-branching instabilities in
different brittle materials, we propose that the physics determining the
typical length- and time-scales characterizing the post-instability patterns
differ greatly from material to material. We offer a scaling theory connecting
the pattern characteristics to material properties (like molecular weight) in
brittle plastics like PMMA, and stress the fundamental differences with
patterns in glass which are crucially influenced by 3-dimensional dynamics. In
both cases the present ab-initio theoretical models are still too far from
reality, disregarding some fundamental physics of the phenomena.Comment: 4 pages, 6 figures, PRL submitte
Nonequilibrium Thermodynamics of Amorphous Materials III: Shear-Transformation-Zone Plasticity
We use the internal-variable, effective-temperature thermodynamics developed
in two preceding papers to reformulate the shear-transformation-zone (STZ)
theory of amorphous plasticity. As required by the preceding analysis, we make
explicit approximations for the energy and entropy of the STZ internal degrees
of freedom. We then show that the second law of thermodynamics constrains the
STZ transition rates to have an Eyring form as a function of the effective
temperature. Finally, we derive an equation of motion for the effective
temperature for the case of STZ dynamics.Comment: 8 pages. Third of a three-part serie
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