Woodquakes and why avalanche waiting times do not follow Poisson statistics

Mikko J. Alava
Department of Applied Physics,
Aalto University, Espoo, Finland

Wood is a multiscale material exhibiting a complex viscoplastic response. We study avalanches in small wood samples in compression. “Woodquakes” exhibit distributions of event energies and of waiting (silent) times that 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. In the next part, I will analyze in general systems exhibiting such crackling noise and the waiting times which follow fairly clean power-law distributions. As the normal expectation would be instead Poissonian statistics this indicates correlations and leaves us with the question why. Here I present some results that explain this by our inability to follow properly avalanches experimentally (and in theory!): the waiting time behaviour is due to detecting sub-avalanches that all belong to the same correlated event. Experimental data from a slow (in-plane) crack propagation experiment and studies of a coarse-grained depinning model are presented to this effect.