Propagation of Frictional Fractures under Complex Loading

Purdue University, West Lafayette IN

Investigators

Abstract

One of the most important challenges in rock mechanics is the development of tools and technologies that provide accurate information about damage, i.e. new cracks produced inside the rock mass as a result of activities that change existing conditions. Recent laboratory experiments conducted at Purdue University show that it is possible to detect the formation of new cracks inside a rock by monitoring changes in amplitude of compressional and shear waves propagating through the rock. The experiments have also detected the presence of precursors to failure. These precursors have the form of a dramatic drop of amplitude of transmitted waves, which occurs at stress levels well below the stress at which damage/failure occurs. These findings have the potential of providing a methodology to predict impending failure, to detect new cracks, and to provide information about engineering properties of the new cracks such as stiffness and possibly shear strength. The focus of the project is on the experimental verification of such potential. A comprehensive series of experiments will be performed on pre-cracked anisotropic rock subjected to biaxial and cyclic compressional loads to determine: (1) fracture initiation and coalescence processes in the rock; (2) geophysical signature of these processes, in particular the presence of precursors to damage; and (3) coupling between the mechanical and geophysical response of rock under complex loading. The experiments will be conducted on granite, Austin Chalk Cordova Crème, a soft rock that exhibits seismic anisotropy, and sandstone. The work is multi-disciplinary as it combines the expertise of the two principal investigators in fracturing in brittle materials and in geophysical methods applied to fractured rock. The potential for impact of the research in science and society is very high. First, the work will be instrumental in improving our ability to scrutinize the interior of the rock for the presence of cracks and to infer engineering properties of the cracks. This is an important problem that limits advancements in fields such as rock mechanics, stiff soils, and other materials because of the limitations that exist in observing cracking processes under complex loading in the interior of geomaterials. Second, the early detection of failure, by monitoring precursors, may bring benefits to society by providing new technologies to detect damage and failure early. It will also provide a significant benefit to science by probing the link between mechanics processes and geophysical signatures.