International audiencePlasticity, a key property in the mechanical behavior and processing of crystalline solids, has been traditionally viewed as a smooth and homogeneous flow. However, using two experimental methods, acoustic emission and high-resolution extensometry, to probe the collective dislocation dynamics in various single crystals, we show that its intermittent critical-like character appears as a rule rather than an exception. Such intermittent, apparently scale-free plastic activity is observed in single-slip as well as multislip conditions and is not significantly influenced by forest hardening. Strain bursts resulting from dislocation avalanches are limited in size by a nontrivial finite size effect resulting from the lamellar character of avalanches. This cutoff explains why strain curves of macroscopic samples are smooth, whereas fluctuations of plastic activity are outstanding in submillimetric structures

The purpose of this work is to explore nonuniform plastic flow at small length- and time-scales. Pure single crystal copper tensile specimens were pulled along the [6¯ 5 6] crystal axis at three nominal strain-rates: 0.01%/s, 0.04%/s, and 0.10%/s. Simultaneously, the surface deformation was monitored with in situ digital image correlation over a length-scale of ∼100 μm and a time-scale of 0.07–0.2 s. Sequential digital image correlation strain-rate fields show compelling evidence of a wavelike plastic deformation that is proportional to the nominal strain-rate and decelerates with increasing strain hardening. While a mechanism responsible for the waves is not identified, a methodology correlating observations of multiple researchers is forwarded.

Aluminium–lithium alloys provide a lower density and higher stiffness alternative to other high strength aluminium alloys. However, many Al–Li alloys exhibit a non-traditional failure mechanism called delamination, which refers to the failure of the elongated grain boundary interface. In this investigation, delaminations were observed after cyclic deformation of both uniaxial and torsion experiments. A cyclically stable rate-independent crystal plasticity framework with kinematic hardening was developed to address many experimental trends of stabilized cyclic plasticity. Utilizing this framework, meso-scale grain boundary interface stresses were estimated with uniform deformation and bi-crystal models. These models are computationally amenable to investigate both orientation dependence and the statistical nature of the grain boundary stresses for a given bulk texture and nominal loading. A coupled shear-normal Findley-based damage parameter was formulated to quantitatively characterize the nucleation of delamination consistently with experimental trends.

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Much of the damage mechanics literature has focused on void growth due to tensile hydrostatic stress. To clarify the effect of combined shear stress and hydrostatic stress on the development of damage, specimens of various geometries were employed in an experimental program to cover a wide range of triaxiality and shear stress. Digital image correlation (DIC) is utilized to measure the fracture strain of the 2D specimens. Experiments are paired with simulations utilizing J2 plasticity theory to complement the experiments and relate the fracture strain with combined hydrostatic and shear stresses. The results display accelerated damage for cases dominated by shear at low triaxiality. Crystal plasticity simulations were carried out using boundary conditions based on the DIC displacement field. These simulations indicate that tensile hydrostatic stress develops due to grain-to-grain interaction.