ABSTRACT

ABSTRACT: Experimental work is conducted to detect acoustic emissions produced during propagation of discrete Mode I and Mode II fractures in large rock specimens. Equipment developed is described in detail. Preliminary results of testing granite and dolostone rock specimens are presented. Results indicate that several clearly different types of AE signals are produce during testing of one rock type to failure. In addition, very similar signals may be produced by very different rocks, suggesting a similarity of internal mechanism during crack propagation.

1 INTRODUCTION

Acoustic emissions (AE) are elastic waves generated in conjunction with crack propagation and internal deformations in materials. This release of energy is manifested as transient stress waves which propagate from the locus of a structural change brought on by changes in the local stress field. These micro-structural changes or displacements occur very rapidly and can be produced by a wide variety of material responses to stress changes, from small scale changes within a crystal lattice structure to growth of macro-cracks. Each AE is a signature of an actual mechanism, a discrete event of material response. The waveform of each AE is "free" information produced by the material. It is the job of the scientist to identify characteristics and correlate with mechanisms, translating this AE "noise" to allow meaningful and insightful conclusions.

The study of AE has developed into an increasingly popular form of nondestructive testing. The technique has long been employed to monitor pressure vessels, and an ASTM method is widely used in the nuclear industry for this purpose. Metallurgists have successfully used AE to identify discrete molecular level events. AE has been used, especially in Japan, to model earthquake activity. In the field of rock mechanics, AE methods have been used to address problems such as rock burst predictions, hydraulic fracturing, mine pillar stress and deformation, the influence of blasting on rock mass stability, and the velocity of groundwater movement.

Detection and analysis of AE are made difficult for several reasons. When an event occurs, emitted stress waves propagate through the surrounding material, filtering the original signal. Stress waves are detected by observing and measuring material response only at an accessible surface. The displacements related to AE events are extremely small, necessitating the use of elaboratelectronic transducers and analytical devices. Simple AE events are not instantaneous, and the appearance of the stress wave will be complex as energy is released over the duration of the event. The duration of these events can be from microseconds to many milliseconds depending on the material, the loading, and the nature of the source. The waveform is very complex and difficult to analyze. In near-field, wave modes are not easily defined, while in far field different wave modes can be discriminated.

An experimental study of AE is made difficult because actual exact source mechanisms are not fully characterized beforehand, and the propagating medium is not an ideal, homo- geneous, isotropic, elastic solid. Additional problems in AE analysis are complicating influences and effects of boundary conditions and transducers. Elastic waves are distorted and converted at specimen boundaries.

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