Understanding contact acoustic nonlinearity through coupled synchrotron x-ray imaging and dynamic acoustoelastic measurements under stress

Abstract

Dynamic acoustoelastic testing (DAET) is a vibro-acoustic technique, which has been used to measure acoustic nonlinearity in disparate solid media. DAET is a pump-probe approach, where the strain pump-induced changes in the elastic properties of the medium are monitored by a pair of ultrasonic probes at frequencies much higher than that of the strain pump. In a fractured/cracked medium, the measured changes in the elastic properties are attributed to fracture breathing (opening/closing) or shearing caused by the low-frequency pumping. In a set of unprecedent experiments, we use time-lapse synchrotron x-ray imaging in tandem with quasi-static loading/unloading and DAET to visualize the fast dynamics mechanisms in four different materials systems under various stress levels. These include two samples with localized CAN: fractured Westerly granite and Berea sandstone and two samples of granular media with distributed CAN: glass beads and sand. The synchrotron x-ray images are analyzed to obtain true contact areas and changing strain fields under stress. The ultrasonic signals are analyzed in relation to the changes in contact areas during both the quasi-static and dynamic loading to examine the state-of-the-art understanding of ultrasonic transmission in fractured media as well as acoustoelasticity and dynamic acoustoelasticity.