Abstract
Several granular materials are hydro-sensitive, i.e., contact with water severely affects their morphology and mechanical behaviour. To this broad class belong a series of materials of great relevance for the pharmaceutical and food industry. Former studies have been conducted on the effect that moisture or humidity have on granular flow or on the individual grain mechanical response, but the processes occurring at the microlevel and their influence on the overall granular packing behaviour is yet to be fully understood. This study presents an experimental investigation of the response of a water-sensitive material (couscous) exposed to high relative humidity (97%). 4D (3D plus time) x-ray tomographies were acquired in operando. A data treatment approach based on Dicrete Digital Image Correlation (dDIC) is developed and detailed here. This allows for following each individual grain throughout the 4+ days of the humidification test. This, in turn, allows the study of the interand intra-granular strain, which is particularly pronounced in water sensitive materials. The expansion of the individual grains also severely affects the contact network, whose evolving properties are here analysed and correlated to the macroscopic (sample-scale) deformation.
Highlights
Numerical simulations could help contain and preventSeveral engineering problems originate from the complex chemo-hydro-mechanical interactions of granular media
While the role of moisture on the mechanical response of impermeable granular media is relatively well understood, or at least thoroughly studied, organic materials have the additional complexity of the chemical interaction with water, and they are referred as water-sensitive, enclosing the subset of effects that has to do with water influencing the mechanics of the individual grains in terms of chemistry and swelling as opposed to the capillarity effect on the granular skeleton induced by partial saturation
This paper presents the study conducted on a watersensitive granular material: couscous
Summary
Several engineering problems originate from the complex chemo-hydro-mechanical interactions of granular media. 3D greyscale images are acquired continuously throughout the entire testing duration by means of x-ray tomography These images are analysed and measurements were made both macro (i.e., of the entire sample) and micro (of the single grain) scale. The evolution of the attenuation coefficient of the material is analysed, and the porosityvolumetric strain relation is examined Together with these macro measurements, Discrete Digital Image Correlation (dDIC) allows following individually all ≈2200 grains and measuring their deformation and kinematics. 2.1 Sample preparation and experimental set up haviour of the individual grains or on the flow behaviour, leaving a dark area around what happens at the micro level or at macro-response in quasi-static conditions, which are the ones at which the product is stored (for instance silos).
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