SUMMARYWe investigate the stress-strain behaviour and failure of a cohesive granular material both by experiments and numerical simulations. The material is an assembly of aluminium rods glued together by means of an epoxy resin. The behaviour of cohesive bonds (force-displacement relationship, failure conditions) is characterized by performing simple loading tests (tension/compression, shear. . .) on a couple of rods. Then, this local behaviour is introduced in a numerical code based on a discrete element method in order to perform numerical compression tests on large samples. The validation of this approach was the main goal of the present investigation that is essentially achieved by a direct comparison between the numerical results and similar experimental tests. As a basic application, we derive the macroscopic cohesion and friction characteristics of random cohesive materials by systematic numerical simulations in a biaxial geometry.

When dealing with porous media, the liquid-gas phase-change is generally considered instantaneous, while a retardation time is observed in the case of hygroscopic soils. So far, little research has been done to characterize the non-equilibrium behaviour of water phase change. Therefore, we propose a macroscopic model of the liquid-gas phase-change rate in porous media, based on the difference of chemical potentials between the liquid and its vapour, which is taken as the driving force. It introduces a phenomenological coefficient that must be determined experimentally. An original experiment able to create a macroscopic non-equilibrium between the liquid and its vapour is described. Analysing the return to equilibrium leads to the determination of the phenomenological phase-change coefficient. Depending on the range of partial vapour pressure, two different behaviours are observed: a linear domain close to equilibrium and a nonlinear one far from equilibrium. The results emphasize the relation between water retention properties in hygroscopic porous media and these phase-change characteristics.

Cereal Chem. 75(5):673-676 A rapid, simple method was developed to prepare small, parallelepipedal test samples of endosperm of wheat, corn, and rice grains. Compression tests performed on endosperm samples revealed the following mechanical properties: modulus of elasticity (Ε, GPa), breaking stress (σ rup , MPa), and maximum breaking strain (ε rup , %). All tests were performed on several endosperm test samples of each cereal species. The results displayed good repeatability and several significant differences in the mechanical behavior of different endosperm structures, especially among soft, hard, and durum wheats. Rice and corn endosperm displayed mechanical behavior similar to that of durum wheat endosperm. The method proposed appears to be sufficiently sensitive and repeatable for studying the incidence of hardness and vitreousness of cereal grain endosperm in relation to its suitability for milling.Publication no. C-1998-0805-06R.

The phenomenological relation of non-equilibrium liquid-gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hygroscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilibrium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.