Amandine Miksic

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.

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.

Deformation and fracture of echinoderm collagen networks

Collagen networks provide the main structural component of most tissues and represent an important ingredient for bio-mimetic materials for bio-medical applications. Here we study the mechanical properties of stiff collagen networks derived from three different echinoderms and show that they exhibit non-linear stiffening followed by brittle fracture. The disordered nature of the network leads to strong sample-to-sample fluctuations in elasticity and fracture strength. We perform numerical simulations of a three dimensional model for the deformation of a cross-linked elastic fibril network which is able to reproduce the macroscopic features of the experimental results and provide insights into the internal mechanics of stiff collagen networks. Our numerical model provides an avenue for the design of collagen membranes with tunable mechanical properties.

Influence of strain rate, temperature and fatigue on the radial compression behaviour of Norway spruce

Abstract A dynamic elastoplastic compression model of Norway spruce for virtual computer optimization of mechanical pulping processes was developed. The empirical wood behaviour was fitted to a Voigt-Kelvin material model, which is based on quasi static compression and high strain rate compression tests (QSCT and HSRT, respectively) of wood at room temperature and at high temperature (80–100°C). The effect of wood fatigue was also included in the model. Wood compression stress-strain curves have an initial linear elastic region, a plateau region and a densification region. The latter was not reached in the HSRT. Earlywood (EW) and latewood (LW) contributions were considered separately. In the radial direction, the wood structure is layered and can well be modelled by serially loaded layers. The EW model was a two part linear model and the LW was modelled by a linear model, both with a strain rate dependent term. The model corresponds well to the measured values and this is the first compression model for EW and LW that is based on experiments under conditions close to those used in mechanical pulping.