Quantifying fatigue-damage and failure-precursors using ultrasonic coda wave interferometry

Accumulation of the subtle impacts of fatigue-induced rock damage may ultimately trigger rock failure. Probing, understanding and quantifying this behavior is crucial in evaluating the mechanical response under dynamic loading. We conduct cyclic uniaxial compression tests with real-time measurement of ultrasonic velocity to probe the evolution of fatigue-induced damage in shale subjected to cycles of increasing-amplitude and constant-amplitude. The stress-strain response shows hysteretic behavior and hardening effect under cyclic compression. Time of first arrival is used to estimate apparent changes in ultrasonic velocity at incremented maximum stresses. Coda wave interferometry (CWI) is applied to detect the more subtle changes in ultrasonic velocity at minimum stress (10 MPa) under fatigue-loading. Despite a continuous increase in ultrasonic velocity measured at incremented maximum stresses the subtle signal of increasing fatigue-induced damage is apparent in the sensitive CWI signal. This is reflected in the descending trend of ultrasonic velocity, especially the S-wave velocity, observed at the minimum stress (10 MPa) – and may ultimately result in failure. The evolution of ultrasonic velocity is controlled by the competing effects of irreversible healing/closure of micro-fractures, dominating at low-stresses/early-cycles, and counteracted by the creation of fatigue-induced micro-fractures, dominating at high-stresses/late-cycles. The development of fatigue-induced micro-fractures is optimally detected by the multiply-scattered coda waves but is absent in the first arrivals. Dynamic modulus and relative changes in effective velocity inferred from CWI are sensitive and discernable indicators of fatigue-induced damage prior to failure while characteristics of the stress-strain curve and the evolution of elastic moduli are inferior indicators.