Critically Stressed Fractures and Their Relation to Elastic Moduli

The analysis of critically stressed fractures’ relationship to elastic moduli of the rock is carried out. Series of triaxial loading tests were performed for rock samples. The behavior of differential stress vs. volumetric strain curve in the vicinity of elasticity limit of the rock is particularly analyzed. It is revealed that the slope of this curve changes considerably when the principal stresses exceed the specific values determined by the criterion of critically stressed fractures appearance. Thus the relationship between fracture initiation Griffith criteria and critical stress state of fractures in the medium is established. It is shown that it is possible to define a specific elastic modulus which is influenced by the fraction of critically stressed fractures in the rock. This modulus may be used to solve the inverse problem of stress state estimation from independent observations related to fractures. Moreover, a formalized approach for estimation of the parameters of rock failure criterion from a cyclic loading test is proposed.