J. Rosti

Fluctuations and scaling in creep deformation

The spatial fluctuations of deformation are studied in the creep in Andrades power law and the logarithmic phases, using paper samples. Measurements by the digital image correlation technique show that the relative strength of the strain rate fluctuations increases with time, in both creep regimes. In the Andrade creep phase characterized by a power-law decay of the strain rate ϵt∼t(-θ), with θ≈0.7, the fluctuations obey Δϵt∼t(-γ), with γ≈0.5. The local deformation follows a data collapse appropriate for a phase transition. Similar behavior is found in a crystal plasticity model, with a jamming or yielding transition.

Relaxation of creep strain in paper

In disordered, viscoelastic or viscoplastic materials a sample response exhibits a recovery phenomenon after the removal of a constant load or after creep. We study experimentally the recovery in paper, a quasi-two-dimensional system with intrinsic structural disorder. The deformation is measured by using the digital image correlation (DIC) method. By the DIC we obtain accurate displacement data and the spatial fields of deformation and recovered strains. The averaged results are first compared to several heuristic models for viscoelastic polymer materials in particular. The most important experimental quantity is the permanent creep strain, and we analyze whether it is non-zero by fitting the empirical models of viscoelasticity. We then present in more detail the spatial recovery behavior results from DIC, and show that they indicate a power-law-type relaxation. We outline results on variation from sample to sample and collective, spatial fluctuations in the recovery behavior. An interpretation is provided for the relaxation in the general context of glassy, interacting systems with barriers.

Crackling noise and its dynamics in fracture of disordered media

Laboratory experiments on fracture yield ample evidence on the stick-slip or intermittent character of the dynamics of failure. We discuss the experimental evidence about such intermittency based on in particular acoustic emission (AE), the detection and analysis of ultrasound which reflects the elastic energy released in the fracture process. The main points are the statistical characterization of AE, the correlations that possibly exist in AE signatures or time series, and the universality of such features upon changing material and fracture setting (loading mode, loading rate and so forth). We also discuss implications on theory and on materials science via failure time or ultimate strength prediction.

Crackling noise in paper peeling

Acoustic emission or crackling noise is measured from an experiment on splitting or peeling of paper. The energy of the events follows a power law, with an exponent β ~ 1.8 ± 0.2. The event intervals have a wide range, but superposed on scale-free statistics there is a time scale, related to the typical spatial scale of the microstructure (a bond between two fibers). Since the peeling takes place via steady-state crack propagation, correlations can be studied with ease and shown to exist in the series of acoustic events.

Spatial fluctuations in transient creep deformation

We study the spatial fluctuations of transient creep deformation of materials as a function of time, both by digital image correlation (DIC) measurements of paper samples and by numerical simulations of a crystal plasticity or discrete dislocation dynamics model. This model has a jamming or yielding phase transition, around which power law or Andrade creep is found. During primary creep, the relative strength of the strain rate fluctuations increases with time in both cases?the spatially averaged creep rate obeys the Andrade law t ~ t ? 0.7, while the time dependence of the spatial fluctuations of the local creep rates is given by ?t ~ t ? 0.5. A similar scaling for the fluctuations is found in the logarithmic creep regime that is typically observed for lower applied stresses. We review briefly some classical theories of Andrade creep from the point of view of such spatial fluctuations. We consider these phenomenological, time-dependent creep laws in terms of a description based on a non-equilibrium phase transition separating evolving and frozen states of the system when the externally applied load is varied. Such an interpretation is discussed further by the data collapse of the local deformations in the spirit of absorbing state/depinning phase transitions, as well as deformation?deformation correlations and the width of the cumulative strain distributions. The results are also compared with the order parameter fluctuations observed close to the depinning transition of the 2d linear interface model or the quenched Edwards?Wilkinson equation.

Acoustic Emission or Crackling Noise in Paper Fracture

Acoustic emission experiments in paper, a disordered material, give evidence for power‐law probability distributions of statistical quantities as event energies or intervals. Two setups, usual mode I tensile tests and a nip‐in‐peel one have been studied in our work. In both cases, even using the same material, we observe “criticality”. This has wide implications for theories of statistical fracture.