Plastic deformation is a paradigmatic problem of multiscale materials modeling. Relevant processes range from the atomistic scale where the atomic arrangement of a material are of crucial importance for its deformation properties, up to macroscopic scales where deformation instabilities manifest themselves in the form plastic instabilities. The crucial question how defect and microstructural properties link to the macroscopic constitutive equations of continuum mechanics is still not completely answered. Very often it has been assumed that the transition from discrete defects and microstructural features to continuum mechanics can be accomplished by studying single defect models or by performing simple homogenization procedures. I contributed to challenge this conventional viewpoint, showing that large fluctuations occur under conditions where plastic deformation was expected to occur in a smooth and stable manner. I have collaborated with experimentalists in Grenoble to show that plastic deformation is associated with intermittent acoustic emission activity, with amplitudes distributed as a power law. Inspired by the experimental results, we showed that similar fluctuations occur in simple dislocation dynamics models. The results of these investigations, demonstrate that deformation occurs in a spatially heterogeneous and temporally intermittent manner not only on atomic scales, where spatial heterogeneity is expected because of the discreteness of the defects, but also on scales where deformation fluctuations literally involve millions of these defects. These findings pose intriguing questions about the possibility of homogenization, and hence about the applicability of continuum descriptions. The limits of continuum mechanics define the limits of our current way of dealing with engineering mechanics problems. Where continuum approaches to deformation become spurious, also the finite element tools which are at the core of engineering design may be bound to fail. Increasing miniaturization of mechanical devices is rapidly leading us to scales where the collective dynamics of defects is bound to interfere with the finite element mesh. The small scales at which plasticity becomes intermittent are now becoming the subject of experimental investigations. In their pioneering work, the group of Dimiduk performed compression experiments on single crystalline micron-scale pillar, obtained by Focused Electron Beam machining. The results show a dramatic increase of the yield stress as the pillar diameter decreases and intermittent strain bursts, reminiscent of the acoustic emission data. We have contributed to elucidate the relation between sample size and plastic fluctuations, using 3D dislocation dynamics and more recently with continuum and atomistic models. Our results had a great impact in the field and were featured on the cover of Nature. Our current research is focusing on understanding scaling features in amorphous plasticity and their relation with crystal plasticity.

Key publications:
P. D. Ispánovity, L. Laurson, M. Zaiser, I. Groma, S. Zapperi, and M. J. Alava, “Avalanches in 2D Dislocation Systems: Plastic Yielding is not Depinning” Phys. Rev. Lett. 112, 235501 (2014)
S. Papanikolaou, D. M. Dimiduk, W. Choi, J. P. Sethna, M. D. Uchic, C. F. Woodward, and S. Zapperi, “ Quasi-periodic events in crystal plasticity and the self-organized avalanche oscillator” Nature 490, 517-521 (2012). Featured on the cover of the journal.
F. Csikor, C. Motz, D. Weygand, M. Zaiser and S. Zapperi, Dislocation Avalanches, Strain Bursts, and the Problem of Plastic Forming at the Micrometer Scale, Science  318, 251 (2007).