The phenomenon behind the use of the AE technique is that a lack of micro cracking, and accompanying AE activity, occurs when a rock material is loaded at levels below its previous stress state (s m ). At the previously ‘memorised’ maximum stress (s m ), there is an abrupt increase in the level of micro cracking and collapsing of pores within the material. This closure and propagation of fractures is associated with a significant increase in AE activity is known as Kaiser Effect, first discovered by Joseph Kaiser in the mid-20 th Century.
Published in Chapter:
A New Acoustic Energy-Based Method to Estimate Pre-Loads on Cored Rocks
Murat Karakus (University of Adelaide, Australia), Ashton Ingerson (University of Adelaide, Australia), William Thurlow (University of Adelaide, Australia), Michael Genockey (University of Adelaide, Australia), and Jesse Jones (University of Adelaide, Australia)
Copyright: © 2018
|Pages: 45
DOI: 10.4018/978-1-5225-2709-1.ch008
Abstract
The Acoustic Emission (AE) due to the sudden release of energy from the micro-fracturing within the rock under loading has been used to estimate pre-load. Once the pre-load is exceeded an irreversible damage occurs at which AE signals significantly increase. This phenomenon known as Kaiser Effect (KE) can be recognised as an inflexion point in the cumulative AE hits versus stress curve. In order to determine the value of pre-load (sm) accurately, the curve may be approximated by two straight lines. The intersection point projected onto the stress axis indicates the pre-load. However, in some cases locating the point of inflexion is not easy. To overcome this problem we have developed a new method, The University of Adelaide Method (UoA), which use cumulative acoustic energy. Unlike existing methods, the UoA method emphasises the energy of each AE, the square term of the amplitude of each AE. As the axial pre-load is exceeded, the micro cracks become larger than the existing fractures and therefore energy released with new and larger cracks retain higher acoustic energy.