Damage evolution processes in Stanstead granite during stress relaxation

Stress relaxation is a critical end-member of the continuum of time-dependent phenomena exhibited by rocks. During stress relaxation, rocks exhibit decreasing stress magnitudes under constant strain conditions, where the initial major principal stress corresponding to constant strain is referred to as the driving stress ( σ d ). To date, quantifiable insights on magnitude of tensile and shear damage that evolves in rocks undergoing stress relaxation as a function of driving stress ( σ d ) are lacking. Consequently, the deformation behavior of Stanstead granite (SG) rock specimens undergoing stress relaxation at varying σ d magnitudes was monitored using two-dimensional digital image correlation (2D-DIC) in sync with ultrasonic monitoring. Specifically, prismatic intact SG specimens were experimentally designed to undergo stress relaxation at σ d (i) just above the crack initiation stress (CI), (ii) between CI and the crack damage stress (CD), and (iii) approximately at the CD stress. The results quantitatively show the degree to which damage in the SG rock specimens induced during stress relaxation increased with σ d , with specimens tested at σ d just above CI exhibiting significantly less damage relative to the specimens tested at σ d above the CD threshold. It was observed that at higher σ d , the propensity for strain localization is more relative than that at lower σ d magnitudes. Additionally, ultrasonic signal attributes (i.e. amplitude, velocity, mean frequency) showed attenuation with increasing stress relaxation, and the magnitude of attenuation was observed to be a function of σ d The increases in the ultrasonic signal wavelength ( λ ) were also observed to be systematic with the stress relaxation.