Damage monitoring in rock specimens with pre-existing flaws by non-linear ultrasonic waves and digital image correlation

Abstract

Pre-existing discontinuities act as the primary source of damage initiation processes in rock masses, and consequently govern their overall deformational behavior. As laboratory tests are critical for establishing constitutive behavior of rocks, in this study, the impact of a pre-existing flaw on rock damage behavior has been analyzed through laboratory-scale experiments. For this purpose, prismatic Barre granite specimens with an existing flaw were progressively damaged under uniaxial loading condition. A state-of-the-art non-linear ultrasonic testing (NLUT) method – the Scaling Subtraction Method (SSM) – was implemented in tandem with the two-dimensional (2D) digital image correlation (2D-DIC) technique for monitoring the impact of the flaw on the rock damage processes. NLUT-SSM allowed for experimental tracking of damage progression by analyzing the elasto-dynamic non-linear response of the rock volume originating at dynamic levels of strain. The non-linear response was characterized through the frequency-domain based evaluation of the non-linear indicator in the ultrasonic imaging areas (UIA). The 2D-DIC full-field strain profiles enabled a strain-based quantitative assessment of the magnitude of tensile and shear damage visible on the rock surface through the estimation of non-elastic apparent tensile and shear strains. The stress-induced changes in the magnitude of the non-linear indicator in the UIA regions showed that the NLUT-SSM technique is sensitive to capture the signatures of rock damage initiation and progression, with the pre-flawed region showing a higher degree of variation in the magnitude of the non-linear indicator. The results also quantitatively demonstrate that damage initially nucleates at the flaw tips, with the flaw tips having a relatively higher magnitude of tensile and shear damage in comparison to other areas of the specimen. The correlation between the non-linear indicator and damage showed that non-linear indicator has a near-linear direct correlation with tensile damage. In contrast, the magnitude of the non-linear indicator increased in an inconsistent non-linear manner with continued accumulation of shear damage. These findings are significant, in the sense that very few prior studies quantitatively analyzed the influence of a pre-existing flaw on rock damage behavior and examined the primary rock damage mechanism (tensile or shear) influencing changes in non-linear ultrasonic characteristics.