Progressive microcrack development in tests on Lac du Bonnet granite—II. Ultrasonic tomographic imaging

Abstract

Thorough three-dimensional raypath coverage of transmitted ultrasonic waves was used to assess the development of microcrack damage in a cylinder of Lac du Bonnet grey granite subjected to uniaxial cyclic loading. The peak load for each cycle was increased gradually until volumetric strain reversal occurred during the 9th cycle. For the remaining 19 cycles, the peak load corresponded approximately to the onset of volumetric strain reversal, as determined from real-time strain measurements. Acoustic emission (AE) counts were monitored during loading and unloading. Ultrasonic wave travel time scans were done prior to the test and then following nine of the load cycles. More than 4500 travel time measurements were made for each scan. The data show an increase in slowness (the inverse of velocity) and a change in orientation of the anisotropy of wave propagation with continued load cycling. Slownesses of raypaths propagating normal to the sample axis increase the most. Slownesses for raypaths propagating parallel to the axis increased minimally. Tomographic images constructed from slowness differences (relative to the pre-test scan) show the progressive development of damage with continued cycling, primarily in a donut-shaped zone near the outer surface around the middle of the cylinder. The relatively constant axial velocities suggest the damage was primarily due to axial splitting cracks.