INTRODUCTION
Although discovered more than 40 years ago, [ 1], there has been little or no development of a theory of the Kaiser effect, particularly for rocks. The purpose of this paper is to supply a theoretical framework for thinking about the Kaiser effect under general stress states. A model of microcrack growth developed by Costin [2] is the basis for the theory. First, a review of the crack growth model will be given. Then the model will be used to compute the effect of various stress states when applied to an ensemble of cracks. The natural way to show the computational results is as a damage surface, defined as the locus of points in stress space within which stress causes no "damage." The Kaiser effect will be seen to be due to the load path intersecting the damage surface. Examples will show the form of the damage surface for common load paths for comparison with available experimental results. The implications of these results for the use of {he Kaiser effect to determine in situ stress will be discussed. In particular, it will be shown that the conventional approach using uniaxial stressing of oriented sub-cores cannot be correct, if the phenomenon responsible for the Kaiser effect induced by laboratory loading is the same as for that observed in situ, i.e., in cores stressed by the earth. Finally, an hypothesis for a different mechanism for the in situ Kaiser effect will be presented. In the following discussion the Kaiser effect observed as a result of a known, laboratory applied stress history will be referred to as the "laboratory Kaiser effect" and for the Kaiser effect observed from cores whose stress history was applied in situ by the earth the term "in situ Kaiser effect" is appropriate.
THEORETICAL CONCEPTS
Acoustic emissions produced by the growth of cracks in response to shear stresses are the mechanism responsible for the laboratory Kaiser effect. A theory developed by Costin [2], based on observations of the anisotropic growth of stress-induced, cracks [3,4] was used. The connection to the Kaiser effect is the assumption that crack growth produces acoustic emissions. The goal is to apply the theory to an ensemble of cracks and to decide which stress states will produce crack growth and acoustic emissions. By mapping all such states, a locus of points in stress space is determined where at least one crack in the ensemble will grow and produce AE. For stress states in the space enclosed by the locus, no crack growth is predicted. The locus is called a damage surface because any stress state that lies on the surface will be on the verge of causing damage in the form of crack growth, with associated AE. Thus a Kaiser step will be observed whenever the stress path reaches the damage surface. For stress states that surpass the damage surface, the theory is used to calculate the new length of cracks and a new damage surface.
