Multiscale Full-Field Strain Measurements for Micromechanical Investigations of the Hydromechanical Behaviour of Clayey Rocks

Abstract

International audienceDigital image correlation techniques (DIC) are applied to sequences of optical images of argillaceous rock samples submitted to uniaxial compression at various saturation states at both the global centimetric scale of the samples and the local scale of their composite microstructure, made of a water-sensitive clay matrix and other mineral inclusions with a typical size of 50 μm. Various scales of heterogeneities are revealed by the optical technique. Not only is it confirmed that the clay matrix deforms much more than the other mineral inclusions, but it also appears that the deformation is very inhomogeneous in the matrix, with some areas almost not deformed, while others exhibit deformation twice the average overall strain (for a gauge length of 45 μm), depending on the local distribution of the inclusions. In almost-saturated rocks, overall heterogeneities are also linked to the presence of a network of cracks, induced by the preliminary hydric load. On such wet samples, DIC analysis shows that the overall strain results both from the bulk deformation of the sound rock, with deformation levels similar to those in dry samples, and the closing or opening of these mesoscopic cracks

Inputs from in situ experiments to the understanding of the unsaturated behaviour of Callovo-Oxfordian claystone

“…As the material progressively dehydrates, water menisci form in the porous space. These menisci strengthen the cohesion between grains and improve the apparent stiffness of the rock [15,11]. This issue is highlighted by triaxial tests that were performed at the end of the drying tests or in other study.…”

“…At a smaller scale, Wang et al [17] also studied the evolution of the COx claystone under hydric cycles using techniques based on the combination of environmental scanning electron microscope (ESEM) and digital image correlation (DIC) [11], in order to quantify local strain field. The observation is carried out on zones of several hundred micrometers, so the evolution of such material under hydric loadings can be studied at the scale of inclusion-matrix composite.…”

“…Sample are subjected to air with a control Relative Humidity and mass or/and strain changes are measured. Some devices rely on a static atmosphere with saline solution [10][11][12], other used complex system with a steam generator and a heating channel, in which air is blown through the chamber [13].…”

Damage of Wood-Concrete Composite subjected to variable hygrometric conditions

“…The image recorded at the beginning of the test is considered as the reference image. The analysis of displacement and strain fields by the CMV software [7] provided information about the swelling of plywood and the initiation of damage, as a function of time. Images have been taken every half an hour through an automatic system.…”

“…Digital image correlation (DIC) is used to characterize the local strains and the mechanical incompatibilities, at the scale of the interface and of the microstructure heterogeneities [7,8]. A rather standard optical setup is used to record images of the whole sample (plane X Y, field: 60 x 60 mm 2 , pixel size: 30 Pm).…”