Scaling behavior of the Portevin-Le Chatelier effect in an Al-2.5%Mg alloy

Evidence That Strain-Rate Softening Is Not Necessary for Material Instability Patterns

Strain-rate softening has been associated with a wide variety of material instabilities, from the Portevin-Le Chatelier effect in metal alloys to stick-slip motion in crust faults. Dynamic instability patterns have been recently discovered in brittle porous media: diffused, oscillatory, and erratic compaction. Using model simulations inspired by experiments with puffed rice, we question the link between these dynamic patterns and strain-rate sensitivity in such media. An important feature of our model is that it can recover strain-rate softening as an emergent phenomenon, without imposing it a priori at its microstructural scale. More importantly, the model also demonstrates that the full range of dynamic patterns can develop without presenting macroscopic strain-rate softening. Based on this counterexample model, we therefore argue that strain-rate softening should not be taken as a necessary condition for the emergence of instability patterns. Our findings in brittle porous media have implications on models that require strain-rate softening to explain earthquake and metal alloy instabilities.

Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: collective yielding

Using molecular dynamics simulation, we examine the dynamics of crystal, polycrystal, and glass in a Lennard-Jones binary mixture composed of small and large particles in two dimensions. The crossovers occur among these states as the composition c is varied at fixed size ratio. Shear is applied to a system of 9000 particles in contact with moving boundary layers composed of 1800 particles. The particle configurations are visualized with a sixfold orientation angle αj(t) and a disorder variable Dj(t) defined for particle j, where the latter represents the deviation from hexagonal order. Fundamental plastic elements are classified into dislocation gliding and grain boundary sliding. At any c, large-scale yielding events occur on the acoustic time scale. Moreover, they multiply occur in narrow fragile areas, forming shear bands. The dynamics of plastic flow is highly hierarchical with a wide range of time scales for slow shearing. We also clarify the relationship between the shear stress averaged in the bulk region and the wall stress applied at the boundaries.

Plastic flow in polycrystal states in a binary mixture

Using molecular dynamics simulation, we examine the dynamics of sheared polycrystal states in a binary mixture composed of small and large particles in two dimensions. We vary the composition c of the large particles and the shear rate ._gamma to realize changeovers among crystal, polycrystal, and glass. We find large stress fluctuations arising from sliding motions of the particles at the grain boundaries, which occur cooperatively to release the elastic energy stored. These stress fluctuations decrease as the system crosses over from polycrystal to glass. The dynamic processes are visualized with the aid of a sixfold angle alpha(j)(t) representing the local crystal orientation and a disorder variable D(j)(t) representing a deviation from the hexagonal order for particle j.

Dynamic scaling approach to study time series fluctuations

We propose an approach for properly analyzing stochastic time series by mapping the dynamics of time series fluctuations onto a suitable nonequilibrium surface-growth problem. In this framework, the fluctuation sampling time interval plays the role of time variable, whereas the physical time is treated as the analog of spatial variable. In this way we found that the fluctuations of many real-world time series satisfy the analog of the Family-Viscek dynamic scaling ansatz. This finding permits us to use the powerful tools of kinetic roughening theory to classify, model, and forecast the fluctuations of real-world time series.

Multifractal analysis of evolving noise associated with unstable plastic flow

Plastic flow instability attracts increasing interest as a self-organization phenomenon showing various dynamical regimes, including deterministic chaos and self-organized criticality. The analysis of the associated nonrandom noise--drastic jumps of the mechanical stress--however, confronts the variation of the noise average parameters due to the evolution of the dislocation microstructure. The present paper examines some limitations of the multifractal approach to the study of the evolving noise. The applicability of the multifractal analysis to practical situations is proven using the example of discontinuous deformation curves observed under conditions of the Portevin-Le Châtelier effect in an A1Mg alloy, as well as model signals generated by stretching multifractal Cantor sets. It is found that the smooth trends in the stress serration parameters may narrow the range of the scale invariant behavior associated with the multifractal structure, but do not essentially mask it.