Constraining Internal States in Progressive Rock Failure of Carrara Marble by Measuring Residual Strains With Neutron Diffraction

Abstract

Knowledge of the internal state of rock is key to anticipate its rheological response and susceptibility to external factors. Time-dependent failure in rock is controlled by internal state changes, like damage accumulation or strength degradation. But assessing internal states and changes thereof, nondestructively and independent of external forcing is not straightforward. Residual strains, measured with neutron diffraction techniques are used as a proxy for the internal state in material sciences. We investigated its potential for progressive rock failure by measuring residual strain states of an untested and three mechanically and chemomechanically pretested Carrara marble samples. We collected neutron diffraction data for three crystal lattice planes {10̅14}, {0006}, and {11̅20}. Measurements showed an initial overall contractional spatially homogeneous residual unit cell volume strain state of about −400 μstrain, though magnitudes were strongly partitioned among measured crystal lattice planes. However, they are equal within the spatial orientations of the intact sample. For the pretested samples, the induction and relaxation of strains varied spatially with the pretesting stress field and environmental conditions. The vertical extent of superposition of the initial residual strain state was greatest in wet samples, the magnitude of induced extensional strain highest in the dry sample. This indicates chemomechanically enhanced subcritical crack growth with concomitant residual strain relaxation as well as the mitigation of extensional strain built up by the presence of water during pretesting. Our experiments show that residual strain has a significant potential to provide insights into past and actual internal states to anticipate progressive rock failure.