Int J Fract (2009) 158:73–80 DOI 10.1007/s10704-009-9365-0 © Springer Science+Business Media B.V. 2009 LETTERS IN FRACTURE AND MICROMECHANICS 13 OBSERVATION AND MODELING OF BRITTLE FRACTURE INITIATION IN A MICRO-HETEROGENEOUS MATERIAL H. Jasarevic*, A. Chudnovsky*, Dudley, J.W.**, Wong, G.K.*** * University of Illinois at Chicago, Illinois 60607, U.S.A., ** Shell Canada Limited, Calgary, Alberta, Canada *** Shell Exploration and Production Company, Houston, Texas, USA Abstract. Observations and characterization of brittle fracture initiation in a micro-heterogeneous material (sandstone) are conducted using the standard indirect tensile strength test. Acoustic emissions, optical microscopy and scanning electron microscopy (SEM) are employed for monitoring and characterizing the discrete micro-mechanical events preceding macroscopical fracture. The observations suggest that brittle fracture initiation is the end result of a microscopic damage accumulation process. A simple statistical model of micro damage accumulation leading to brittle fracture in a micro-heterogeneous material is also proposed. The model is calibrated by matching the coefficient of variation of measured ultimate stress with that resulting from the proposed model. Keywords: fracture initiation, sandstone, Brazilian test, optical microscopy, acoustic emission. 1. Introduction. Fracture initiation (FI) is the least studied and the most uncertain stage of fracture phenomena. It is the end result of a microscopic-scale damage accumulation process, rather than an instantaneous event (see, e.g., Kanaun and Chudnovsky (1999)). The objective of the present work is to quantitatively characterize and model FI in a brittle material. For this purpose, we select the standard “Indirect tensile strength test,” also known as the Brazilian test (ASTM (1989) and ISRM (1978)). In this test, a disc is compressed by point forces along the diameter of the disc. The analytical solution for elastic stress distribution in such a setting is available (Chen at al. (1998)). It includes the expression for the maximum tensile stress in terms of the magnitude of the applied force and dimensions of the disc. The effect of rock anisotropy, i.e., layering orientation with respect to the loading direction, is also addressed in Chen at al. (1998). In the Brazilian test the fracture initiation is commonly observed in the central region of the disc, where the tensile stresses are maximal. To detect the processes that accompany fracture initiation in micro-heterogeneous materials such as rocks, acoustic emission (AE) monitoring techniques has been applied for many decades (see, e.g., Hoaglend at al. (1975) Suzuki at al. (1980)). Hashida and Takahashi (1993) have examined a relation between the AE characteristics and fracture behavior in granite and demonstrated the importance of an AE energy parameter for quantitative characterization of the fracture process. In the present work we use Torry Buff sandstone for an illustrative purpose; it displays a typical brittle behavior. Our observations support the findings of Hashida and Takahashi (1993), and we also employed AE cumulative energy parameter to detect and quantify the damage accumulation prior to FI. In addition to AE monitoring, we have collected the loose material fragments from the fracture surfaces and performed statistical analysis of the fragment sizes based on transmission optical microscopy observations. The fragments vary in size from a fraction of an individual grain to a large cluster of grains. The structure of fragments has been observed using scanning electron microscopy (SEM). The presence of a large number of fragments is direct evidence of multiple micro-fracture events leading to macroscopic FI, in agreement with the AE data. AE energy and the average fragment size can be linked by the energy analysis of fragmentation (Glenn and Chudnovsky, 1986).