EAGER: Developing an Experimental Technique for Measuring Very Slow Crack Velocities in Rock Using the Atomic Force Microscope

University Of Arizona, Tucson AZ

Investigators

Abstract

Subcritical crack growth is one of the dominant mechanisms for time-dependent rock degradation and failure. In spite of the substantial amount of work that has been conducted on subcritical crack growth in rocks, some very important issues still remain. The double torsion test and other conventional techniques for subcritical crack growth testing measure crack velocities between 10-8 and 10-3 m/s. Within this range, it is not possible to determine the shape of the crack velocity vs. KI curve for very low crack velocities, which is needed in order to accurately predict the long term behavior of geologic structures subjected to low stresses. Also, there is controversy about the origin of shear crack growth in rocks that cannot be adequately resolved with traditional microscopy. To address these issues, experimental techniques based the Atomic Force Microscope (AFM) will be developed. The resolution of the AFM for crack growth measurements is less than 4 nanometers, which allow crack velocities as small as 10-13 m/s to be measured. This will provide fundamental information on the shape of the crack velocity vs. KI curve and the subcritical cutoff for rocks. The experimental procedure developed will involve periodic mechanical loading of small rock samples to create very small amounts of crack growth, followed by AFM investigations to measure the amount and pattern of crack growth. Laboratory and AFM techniques developed for mode I crack growth in glass will initially be used for the rock specimens, and modifications will be made to account for the complex rock microstructure and also to investigate both tensile and shear crack growth. The research will increase our ability to predict the long-term stability of critical geologic structures such as dam foundations, tunnels, underground nuclear waste storage facilities, underground CO2 sequestration sties, highway slopes, and many other structures. Also, the results could impact other science and engineering fields that are interested in environmentally assisted crack growth and failure, such as material science, fracture mechanics, mining, and civil and mechanical engineering. By disseminating the results of this research through international publications and distance courses, this research will be part of the training for undergraduate and graduate students worldwide.