Juha Koivisto

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.

Effect of Interstitial Fluid on the Fraction of Flow Microstates That Precede Clogging in Granular Hoppers

We report on the nature of flow events for the gravity-driven discharge of glass beads through a hole that is small enough that the hopper is susceptible to clogging. In particular, we measure the average and standard deviation of the distribution of discharged masses as a function of both hole and grain sizes. We do so in air, which is usual, but also with the system entirely submerged under water. This damps the grain dynamics and could be expected to dramatically affect the distribution of the flow events, which are described in prior work as avalanche-like. Though the flow is slower and the events last longer, we find that the average discharge mass is only slightly reduced for submerged grains. Furthermore, we find that the shape of the distribution remains exponential, implying that clogging is still a Poisson process even for immersed grains. Per Thomas and Durian [Phys. Rev. Lett. 114, 178001 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.178001], this allows for an interpretation of the average discharge mass in terms of the fraction of flow microstates that precede, i.e., that effectively cause, a stable clog to form. Since this fraction is barely altered by water, we conclude that the crucial microscopic variables are the grain positions; grain momenta play only a secondary role in destabilizing weak incipient arches. These insights should aid ongoing efforts to understand the susceptibility of granular hoppers to clogging.

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.

Effect of fatigue and annual rings’ orientation on mechanical properties of wood under cross-grain uniaxial compression

The mechanics of fresh wood with and without a fatigue pre-treatment that mimics a mechanical pulping process was experimentally studied. The mechanical properties of Norway spruce samples under compression are considered with the macroscopic stress–strain data and from local strain properties via digital image correlation technique. The results highlight the effects of the orientation of the wood annual rings compared to the loading direction and of the pre-fatigue. The wood presents a low yield point when the annual rings are tilted compared to the load axis, but the Young’s modulus and yield stress are higher when the annual rings are either parallel or perpendicular to the load direction. In the last case, buckling of softest layers occurs. The fatigue treatment makes the wood less stiff as deduced from the decreases of Young’s modulus and yield stress, whatever the orientation of annual rings. Secondly, it creates a thin and localized softened layer.

The sands of time run faster near the end

Grains exiting an underwater silo exhibit an unexpected surge in discharge rate as they empty. This contrasts with the constant flow rate of dry granular hoppers and the decreasing flow rate of pure liquids. Here we find that this surge depends on hopper diameter and happens also in air. The surge can be turned off by fixing the rate of fluid flow through the granular packing. With no flow control, dye injected on top of the packing gets drawn into the grains. We conclude that the surge is caused by a self-generated pumping of fluid through the packing. The effect is modelled via a driving pressure set by the exit speed of the grains. This highlights a surprising and unrecognized role that interstitial fluid plays in setting the discharge rate, and perhaps in controlling clog formation, for granular hoppers whether in air or under water.

Statistical properties of low cycle fatigue in paper

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.

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.

Path (un)predictability of two interacting cracks in polycarbonate sheets using Digital Image Correlation

Crack propagation is tracked here with Digital Image Correlation analysis in the test case of two cracks propagating in opposite directions in polycarbonate, a material with high ductility and a large Fracture Process Zone (FPZ). Depending on the initial distances between the two crack tips, one may observe different complex crack paths with in particular a regime where the two cracks repel each other prior to being attracted. We show by strain field analysis how this can be understood according to the principle of local symmetry: the propagation is to the direction where the local shear - mode KII in fracture mechanics language - is zero. Thus the interactions exhibited by the cracks arise from symmetry, from the initial geometry, and from the material properties which induce the FPZ. This complexity makes any long-range prediction of the path(s) impossible.

Thermal conductivity of wood: effect of fatigue treatment

The effect of fatigue treatment on the thermal conductivity of wood was studied. Fresh Norway spruce samples both with and without fatigue were analyzed in a temperature rise experiment by means of an infrared camera. The experimental temperature profiles were compared to finite-element simulations of heat conduction. The temporal features of temperature profiles indicate an increase in the conductivity of fatigued wood, which points to changes in the cellular structure of wood. The importance of fatigue for thermal conductivity and consequently for mechanical pulp-making is discussed.