HR: 08:30h
AN: MR31B-03    [Abstracts]
TI: Observations of Fracture in Westerly granite under AE feedback and Constant Strain Rate loading: Nucleation, Quasi-static Propagation, and the transition to Unstable Fracture Propagation
AU: * Thompson, B D
EM: b.thompson@liv.ac.uk
AF: Dept. Earth Science, University of Liverpool, 4 Brownlow St, Liverpool, L69 3GP United Kingdom
AU: * Thompson, B D
EM: b.thompson@liv.ac.uk
AF: Lassonde Inst, University of Toronto, 170 College St, Toronto, ON M5S 3E3 Canada
AU: Young, R P
EM: paul.young@utoronto.ca
AF: Lassonde Inst, University of Toronto, 170 College St, Toronto, ON M5S 3E3 Canada
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS/977 , Menlo Park, CA 94025 United States
AB: New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite (length 190.5 mm and diameter 76.2 mm), using continuous Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture nucleation process is demonstrated for quasi-static fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a `slow' constant strain rate, 2.5 x10-6 /s) and an unstable fracture that develops near instantaneously (loaded at `fast' constant strain rate, 5 x 10-5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. AE source locations, b-values and velocity data are presented. For the sample loaded under a low constant strain rate, three phases of fracture nucleation were measured in the 21 seconds prior to failure. Firstly an initial nucleation or slow growth stage was observed as strain weakening occurred in the sample up to a fracture length of 24 mm, with a velocity of approximately 1.3 mm/s. Secondly a sudden acceleration in propagation speed to 17 mm/s occurred, with the fracture length extending to 54 mm. The high level of AE activity after this time (as seen on the continuous AE record) prevented the location of discrete AE events. We infer that a third stage of growth must occur in the final 17 ms period before rupture, to a total length of 84 mm, which requires an average propagation velocity of 5 m/s. Under AE feedback loading the nucleation process is similar to that observed in the slow constant load test, to the point of initial fault propagation. From here, the fracture stably propagates to bisect the sample, at a velocity of between 12mm/hour and 50 mm/hour. Under fast loading conditions, the nucleation process is accelerated into a shorter period (approximately three seconds). In addition to showing the growth rates of fracture during nucleation or quasi-static propagation (of order æm/s to mm/s), unstable growth (of order mm/s to cm/s), and final propagation (of order m/s) we suggest the ability to constrain the size of the evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005