ABSTRACT:

During fracture toughness testing, two different failure mechanisms presumably act within a notched specimen. The first is a strength failure controlled by the tensile strength of the material. The second is the failure due to crack extension, governed by the fracture toughness. In this study, notch sensitivity analysis to determine which failure mechanism is critical has been employed for the CB and the SECRBB specimens. The region, given in the crack length ratio, where the crack propagation failure curves are below the ultimate strength failure curve defines the notch-sensitive range of the given specimen type. The notch sensitivity analysis can be applied to discriminate invalid fracture toughness values in screening stage. Similarly, with an estimated fracture toughness for a rock material, the valid range of the initial crack length ratio for a given type of specimen geometry can also be determined prior to the testing.

INTRODUCTION

An understanding of the mechanisms of rock fracture is a key element in the solution of many rock engineering problems. The application of linear elastic fracture mechanics (LEFM) principles has been proven to be an effective approach in rock engineering and geoscience fields. The mode I fracture toughness, K/c, represents the ability of material against the opening mode fracture propagation. Recently, fracture toughness and related fracture mechanics parameters of rock materials have been widely applied to various rock engineering problems, including the hydraulic fracturing (Takahashi & Abe 1987, Rummel 1976), stability analysis of rock slopes and underground structures (Ingraffea 1979), and interpretation of the tunnel boring machine performance. Therefore, a standardized procedure for testing and data interpretation for rock fracture toughness has been suggested by the International Society for Rock Mechanics (ISRM 1988), for which special attention was given to the difficulties in obtaining the true fracture mechanics parameter for the wide variety of rock materials. Rock specimens are usually cored from drill holes, and core-based specimens are more cost-effective for determining rock fracture toughness. Accordingly, the single-edge-cracked round-bar-in-bending (SECRBB), semi-circular bending (Chong & Kuruppu 1984), notched Brazilian discs (SzendiHorvath 1980, Guo et al. 1993), and the chevron bend (CB) specimens are most practical specimen types. Other core-based specimen types include the burst cylinder method, modified ring test, and the round compact tension (Clifton et al. 1976, Thiercelin & Roegiers 1986, Sun & Ouchterlony

1986).

However, the fracture toughness of inhomogeneous and anisotropic rock materials, which generally violate the fundamental assumptions of LEFM, often depends on the specimen geometry and test method adopted. This fact is often attributed to improper initial notch length, along with other test variables. In this paper, the notch sensitivity analysis is applied to the CB and SECRBB specimens, and the proper ranges for each specimen type are suggested.

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