HR: 1340h
AN: T23B-0549    [Abstracts]
TI: Elastic Property Development in Variably Fractured Crystalline Rock
AU: Heap, M J
EM: michaeljheap@hotmail.com
AF: University of Liverpool, Rock Deformation Laboratory Department of Earth and Ocean Sciences 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AU: * Faulkner, D R
EM: faulkner@liv.ac.uk
AF: University of Liverpool, Rock Deformation Laboratory Department of Earth and Ocean Sciences 4 Brownlow Street, Liverpool, L69 3GP United Kingdom
AB: The pervasive damage of rock by dilational microcracks strongly affects macroscopic elastic properties. Fractured rocks around fault zones have vastly altered elastic properties. A change in elasticity for fault rock has implications for the mechanics, stress distribution and fault displacement models within and surrounding fault zones. Our study focuses on the most important mechanisms of microcrack development: stress-induced microcracking, cyclic fatigue microcracking and static fatigue microcracking, and their effect on the evolution of static elastic properties and rock micromechanics. Experiments were performed on oven-dry, strain gauged, cylindrical samples of initially intact Westerly granite and conducted using an unconfined uniaxial compression apparatus. Two types of experiments were employed to understand the contribution of the different micromechanisms to the observed elastic changes: constant- and increasing- amplitude cyclic experiments. Elasticity was assumed if the gradients of the loading and unloading curves are equal. Results from increasing-amplitude cyclic experiments outline the strong stress-dependency of elastic properties. Young's modulus, E, is seen to increase rapidly (total increase of ~46%), whereas Poisson's ratio, v, demonstrates only very modest changes with respect to increasing stress. E increases since stress acts to close oblique and transversely orientated cracks. Constant-amplitude cyclic experiments were performed with hold periods between cycles during which stress was kept at a constant 0.8 of the UCS (unconfined compressive strength). Therefore, static fatigue microcracking could also be investigated. A modest decrease in E (total decrease of ~1%) and a modest increase in v (total increase of ~6%) are observed with respect to number of cycles. The gradual degradation in sample stiffness with respect to increasing number of cycles suggests that the predominant mechanism during experimentation is the steady growth, opening and coalescence of pre-existing microcracks. Results indicate that stress-induced microcracking is the dominant crack-forming mechanism and hence promotes the greatest elastic property changes. The effect of cyclic fatigue produces a minor contribution, and results seen thus far indicate that static fatigue has an almost negligible contribution owing to the relative short timescales of the experiments and the rock type used.
DE: 3902 Creep and deformation
DE: 3909 Elasticity and anelasticity
DE: 5104 Fracture and flow
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005