Wood is a multiscale material exhibiting a complex viscoplastic response. We study avalanches in small wood samples in compression. "Woodquakes" measured by acoustic emission are surprisingly similar to earthquakes and crackling noise in rocks and laboratory tests on brittle materials. Both the distributions of event energies and of waiting (silent) times follow power laws. The stress-strain response exhibits clear signatures of localization of deformation to "weak spots" or softwood layers, as identified using digital image correlation. Even though material structure-dependent localization takes place, the avalanche behavior remains scale-free.

Materials flow-under creep or constant loads-and, finally, fail. The prediction of sample lifetimes is an important and highly challenging problem because of the inherently heterogeneous nature of most materials that results in large sample-to-sample lifetime fluctuations, even under the same conditions. We study creep deformation of paper sheets as one heterogeneous material and thus show how to predict lifetimes of individual samples by exploiting the "universal" features in the sample-inherent creep curves, particularly the passage to an accelerating creep rate. Using simulations of a viscoelastic fiber bundle model, we illustrate how deformation localization controls the shape of the creep curve and thus the degree of lifetime predictability.

We consider fatigue fracture in the low-cycle limit. We use paper as the tested material and we study the failure and the deformation of individual samples, with a main emphasis on the possible predictability. The experimental procedure is to study fatigue in tension and follow the evolution of mechanical properties of paper samples. The primary quantity is the strain developing during load cycles and its evolution. Two concurrent methods are used to this end: vertical displacement measured by a laser interferometer sensor, and the digital image correlation technique (DIC). By the DIC, we obtain accurately at the same time displacement data and the spatial fields of strain and strain rate. The final rupture is signalled by a sharp final increase of different variables, like deformation, strain rate and their fluctuations. We find interestingly enough that looking at the evolution of these quantities during the first fatigue cycle only is already a good indicator about the lifetime of the sample. Crackling noise is also recorded during loading via acoustic emission (AE). AE is mostly accumulated during a short time interval before breakage. The results are compared with a fibre bundle model of fatigue in heterogeneous materials.

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