Oscillations induced by crack growth in compression and morphology of fracture surface

Abstract

<p>Growth of internal cracks in compression is a primary mechanism of catastrophic rock failures.  Since the cracks are internal, only indirect methods are currently available for investigation and monitoring of failures of this kind.  In order to understand the fundamentals of 3D (internal) crack growth in compression, special physical models using transparent polymer prisms with embedded penny-shaped flaws were performed.  A biaxial stress field was applied, to simulate conditions near the face or walls of deep tunnels.  Previously, such experiments were conducted within a fully-enclosed polyaxial testing machine, preventing observation of the growing crack itself.  Recent experiments at UWA utilized a plane-strain restraint device, which develops the secondary principal stress (σ<sub>2</sub>) by limiting the lateral expansion that would otherwise result from the imposition of the primary principal stress (σ<sub>1</sub>).  This device leaves clear line-of-sight to the sample free face, allowing visual observation and the installation of other instrumentation. </p><p>As expected, the biaxial stress field resulted in growth of an extensive, nearly planar crack, parallel to the σ<sub>1</sub>-σ<sub>2</sub> plane.  Growth of this crack was observed using high-speed video.  The crack surface morphology shows similarity to many natural and excavation-induced fractures in geomaterials.  This similarity justifies the usage of the transparent materials for investigating rock failure. Furthermore, the observed morphological features can be linked to specific events in the crack growth process.</p><p>In addition, experiments incorporated acoustic emission sensors, both on the sample, and in the air near the sample.  Two categories of vibration were observed at the sample’s rear free face: a short-duration wavelet, representing the crack initiation, and a long-duration high-amplitude “ringing” waveform.  The “ringing” was also observed in air (as audible sound) and in video (as movement of the whole sample). The observed vibrations could be utilized for monitoring dangerous rock failures.</p>