The acoustic emission from a rock specimen subjected to increasing uniaxial restress, reveals a kaiser Effect recollection of the stress that existed in its native environment. If confining stress was a component of that stress environment, the Kaiser Effect reveals the maximum amount by which the major principal stress had exceeded the minor principal stress, even for a brief period.
L'emission acoustique provenant d'un echantillon de roche, soumise à un champ de contraintes uniaxial de plus en plus eleve, revèle une memoire de Kaiser Effect de la contrainte qui existait dans son environment originel. Si la contrainte laterale etait un composant de ce champ de contraintes, Kaiser Effect revelerait la deviation la plus grande entre la contrainte principale maximum et la contrainte principale minimum, même pour une brève periode.
Die akkustische Emission einer Felsprobe, die einer ansteigenden einachsigen Wiederbeanspruchung unterworfen ist, gibt eine Kaiser Effect Abbildung der Spannung wieder, die in der urgprunglichen Umgebung geherrscht hatte. Falls eine Komponente jener Spannungsumgebung eine Zwangsspannung war, so verrat der Kaiser Effect den Maximalwert, um den die grössere Hauptspannung die kleinere ueberschritten hatte, selbst fuer einen kurzen Zeitraum.
Kaiser Effect Gauging is a technique which is being developed towards providing a practical method for retrieving the Kaiser Effect recollection of the maximum previous stress to which a sample had been exposed in its native environment (Hughson and Crawford 1986). This technique is anticipated to enable the rapid and economical determination of the pre-existing in-situ stress in rock, concrete and other materials. It is based on the natural phenomenon of Acoustic Emissions (AE). Acoustic Emissions are the spontaneously generated Ultra-sonic pulses that can be detected from certain materials (examples listed in Table 1) when subjected to increasing stress. The wave form is similar to that of a seismic event with duration in the order of 2 milliseconds. In this research project, the AE detection has been filtered to limit the frequency range to between 50 and 500 kHz and is quantified in terms of the cumulative "ring-down count above a threshold" (Nishijima et al 1985). Dr. Joseph Kaiser (Kaiser 1953) noted that if the stress on a material is relaxed and then gradually increased, there is a significant increase in the AE output as the stress passes from the region of past experience into the level of new-experience. This increase in AE rate at the level of previous maximum stress has been referred to as the Kaiser Effect. There is evidence to support the possibility that AE is an acoustic manifestation of micro-sized inelastic strain and fracture occurrences within the material. These authors have also previously confirmed or established (Hughson and Crawford 1986) that:
1.1 a sample carries in it a Kaiser Effect recollection of its stress environment prior to its extraction from that environment;
1.2 their developed technique of Kaiser Effect Gauging can be an effective and economical procedure for recalling the Kaiser Effect memory held in a sample;
1.3 the Kaiser Effect does not necessarily occur at the exact previous maximum stress. There can be a variation between the two, relative to such factors as duration of the stress and the proportion of applied stress to Ultimate stress;
1.4 different materials exhibit their own characteristic pattern of AE response to stress and this has been referred to as their AE Signature Profile.
This phase of the Kaiser Effect Gauging research programme reported herein, is to determine by uniaxial testing, the effect of confining stresses on the fidelity of Kaiser Effect recollection of the sample's stress environment. If there were to be found a consistent relationship between the recalled stress and the triaxial stress state of the environment then such a testing procedure could be employed in gauging the in-situ stress field of a rock mass. This research has found that the Kaiser Effect -occurs approximately at the amount of deviatoric stress on the sample.
To achieve the experimental objective, it was necessary to determine the responses obtained from many samples, each of which had been exposed to a different combination of axial and confining stresses. Initial uniformity of material selected, including any previous stress history, should be optimised. For this reason, a single block of Berea sandstone was cored into many samples each of which was 54 mm diameter with all axes parallel and being normal to the bedding plane. Sample lengths were trimmed to about 80 mm by the removal of approximately equal amount of material sawed and ground from each end. Incremental steps of axial and confining stress increase and decrease were kept to within 2 MPa of this path. The stress combination was maintained for a period of about 2 hours in order to achieve stability indicated by substantial decay of residual AE during dwell.
