Quantifying progressive pre-peak brittle fracture damage in rock during uniaxial compression

This paper presents the findings of an extensive laboratory investigation i nto the identification and quantification of stress-induced brittle fractur e damage in rock. By integrating the use of strain gauge measurements and a coustic emission monitoring, a rigorous methodology has been developed to a id in the identification and characterization of brittle fracture processes induced through uniaxial compressive loading. Results derived from monocyc lic loading tests demonstrate that damage and the subsequent deformation ch aracteristics of the damaged rock can be easily quantified by normalizing t he stresses and strains observed in progression from one stage of crack dev elopment to another. Results of this analysis show that the crack initiatio n, sigma(ci), and crack damage, sigma(cd), thresholds for pink Lac du Bonne t granite occur at 0.39 sigma(UCS) and 0.75 sigma(UCS) respectively. Acoust ic emissions from these tests were found to provide a direct measure of the rapid release of energy associated with damage-related mechanisms. Simplif ied models describing the loss of cohesion and the subsequent development o f microfractures leading up to unstable crack propagation were derived usin g normalized acoustic emission rates. Damage-controlled cyclic loading test s were subsequently used to examine the effects of accumulating fracture da mage and its influence on altering the deformation characteristics of the r ock. These tests revealed that two distinct failure processes involving the progressive development of the microfracture network, may occur depending on whether the applied cyclic loads exceed or are restrained by the crack d amage stress threshold. (C) 1999 Elsevier Science Ltd. All rights reserved.