Multi-scale experimental deformation and damage initiation of clay-rich rocks
Résumé
Understanding the damage processes in clay-bearing rocks is a decisive factor in geological engineering, and for instance considering nuclear waste deep geological repositories. But, more generally they may also contribute to localized deformation, and thus the rupture of fault gauges in seismic zones. However, owing to their complex mineralogy, multiscale microstructures and anisotropy, the mechanisms of clay-rich rock damage and their chronology are not yet well understood.
Here we focus on the impact of micro-damage on ultrasonic wave propagation velocity, which is confronted with the corresponding full deformation fields calculated by digital image correlation (DIC).
The aim is to associate the acoustic signature with the active deformation mechanisms identified by DIC. To this end, an integrated experimental approach is proposed to characterize localization and to identify the related deformation micro-mechanisms during uniaxial compression of natural clayey rock samples (Tournemire shales) with two simultaneous measurements: (i) the evolution of P-wave velocity within the sample by active acoustics, (ii) the development of the 2D mechanical full field by digital image correlation.
Both experimental techniques are well known, but the innovation of our approach is to combine simultaneously both measurements. Deformation localization is a multiscale problem, which obviously occurs at the sample scale, but also at the fines scales of the microstructure. Therefore, we developed two different experimental setups. On the one hand, during uniaxial compression with a standard MTS loading frame the macro-scale localization patterns are characterized by optical observations, which image resolution is well suited to the cm sample scale (sample diameter: 3.6 cm and double in length). On the other hand, in order to characterize the initiation of micro-damage at the microstructure scale of the composite type of rock, the same loading protocol is reproduced (while keeping the acoustic diagnosis) on smaller scale mm-sized specimens (sample diameter : 8 mm, double in length), using a home-designed miniature loading frame fit for an environmental scanning electron microscope (ESEM). The latter analysis is carried out under controlled relative humidity of RH = 80%, hence preventing the samples to dry out due to the high vacuum
Damage processes measured by digital image correlation (DIC) are usually represented over time as a series of deformation maps. This method is well suited to identify major damage events (macro-cracks). However, these highly localized events have little impacts on acoustic wave propagation velocity, unlike densely distributed micro-cracks. Therefore, we propose a method for analyzing the same data, to our knowledge original, revealing the temporal evolution of all deformation scales. A single map summarizes the entire temporal process with deformation classes as x-axis and time as y-axis. We call this representation the “Chronogram of Strain Distribution" (CSD).
This projection has a number of advantages: (i) it removes micro-deformations from the ground noise of DIC; (ii) it explicitly links volumetric integration, inherent to Vp wave velocity measurement, to the full deformation field; (iii) it keeps the chronology of the different deformation process.
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