Collective phenomena in deformation

In this work the collective phenomena in material deformation, here meaning the collective dynamics of crystal defects (dislocations) or cellulose fibers, are studied experimentally using various techniques, which include imaging techniques, such as laser speckle technique and Digital Image Correlation, and also Acoustic Emission monitoring. Due to the nature of these collective bursts, or avalanches, they can be modelled with mesoscopic models and here each of the experimental systems is compared to simulations based on one of these simple models. In Publication I the dynamics of Portevin–Le Chatelier (PLC) deformation bands are studied using statistical methods. The results are mapped to a model based on the well-known model of mean-field depinning, the Alessandro–Beatrice–Bertotti–Montorsi (ABBM) model. This shows that a PLC deformation band, consisting of a large number of dislocations, behaves essentially as a single excitation of a quasiparticle. In Publication II the creep deformation of a quasi-2D fibrous material, paper, was studied close to failure. The localization of strain in tertiary creep was observed, concomitantly with scalefree behavior, divergence, of associated quantities. A serial fiber bundle model qualitatively reproduces this behavior and this implies that the behavior is not due to correlated avalanche phenomena. In Publication III the compression of foam-formed fibrous materials is studied. It is compared with the predictions of a mean-field model of fiber buckling, which reproduces the observed phenomena up to a certain strain, at which a crossover to collective phenomena is observed. The exponents of the acoustic emission energy and waiting time distributions seem to belong to a novel universality class.