Materials often do not deform continuously, but in an intermittent manner with sudden bursts of activity separated by periods of low activity. The sizes of these bursts, or avalanches, typically follow power law distributions, and spatial and temporal correlations are observed between avalanches. Experimental evidence of such behaviour in material deformation ranges from compressed single crystals on the micrometer scale to the movement of tectonic plates. In the first part of this work (Publication I-III) we study avalanches and other collective phenomena in the deformation of crystalline materials with simple numerical models. We extend a standard 2d discrete dislocation dynamics model to include point defects, which interact with dislocations and act as obstacles for their movement. With immobile defects, we find two new phases different from the pure 2d system. For a moderate pinning strength, the critical properties match those seen in depinning transitions, with critical exponents different from mean field theory. At a very high pinning strength, critical avalanche dynamics ceases. With mobile solute atoms, dislocations and solute clouds form growing structures, which we characterize through spatial correlations. The structure formation leads to Andrade creep in an extended range of external stress for creep simulations. The effect of mobile solute atoms in avalanches is seen as a linear size-duration relationship, and a stationary cutoff regime for avalanche distributions. In a pure 2d system, we show that the avalanche statistics depend on the avalanche triggering mechanism, and that critical avalanche dynamics is seen even at zero external stress. In the second part (Publication IV) we study avalanches in wood compression by analyzing the acoustic emission recorded during compression experiments. The acoustic event time series displays several similarities to earthquakes and experiments on brittle porous materials. These include power law distributions of event energies and waiting times between avalanches, as well as the Omori law for aftershocks. By using digital image correlation, we show that the peaks of highest acoustic activity are related to the sudden collapse of softwood layers.
|Translated title of the contribution||Vyöryt materiaalien plastisessa muodonmuutoksessa|
|Publication status||Published - 2016|
|MoE publication type||G5 Doctoral dissertation (article)|
- plastic deformation
- acoustic emission