From mild to wild fluctuations in crystal plasticity, and associated size effects


Jérôme Weiss

ISTerre, CNRS/Univ. Grenoble Alpes, France

Wednesday, 17. June 2015, 17:15
WW8, Raum 2.018, Dr.-Mack-Str. 77, Fürth

Crystal plasticity was historically viewed as a continuous process where uncorrelated dislocation motions produce only near Gaussian (mild) fluctuations. An apparently conflicting picture emerged in the last decades emphasizing an intermittent plastic flow characterized by power law distributed (wild) fluctuations. I will discuss recent results [1] of acoustic emission recorded during the deformation of different materials which argue that intermittent and continuous plastic flows are not incompatible. Mild fluctuations prevail in materials where highly constrained dislocation patterns screen long-range interactions and prevent cooperative behavior. Instead, scale free fluctuations are observed in materials where long-range elastic interactions dominate and dislocations self-organize into a critical state.  Between these two limiting types of behavior are crossover regimes where strongly intermittent events coexist with a Gaussian quasi-equilibrium background. In cyclic loading, the wild behavior progressively vanishes with increasing number of cycles, as a dislocation cells pattern forms and the material hardens. We provided the first experimental evidence of such a crossover, showed that the power law exponents are not universal, and proposed a simple theoretical framework compatible with our observations. This model also suggests a size effect, “smaller is wilder”, in agreement with data on micropillars.

[1] J. Weiss, W. Ben Rhouma, T. Richeton, S. Deschanel, F. Louchet, and L. Truskinovsky, Phys. Rev. Lett. 114, 105504 (2015).