A series of fracture tests have been carried out to determine the mode-I fracture toughness of marble rock samples from Ma On Shan area in Hong Kong. The rock specimens can be subdivided into white, light grey and dark grey specimens, and present notable difference in the mean grain size. The ISRM suggested chevronnotched three-point bend test method is chosen in this study. The fracture toughness parameter and some relevant mechanical properties are summarized in this paper. Noticeable difference is shown in the test results between the three groups of marble specimens. Moreover, nonlinear fracture mechanics approach, capable of incorporating the effect of FPZ, is applied in this study. The cohesive crack model is chosen for describing the mode-I FPZ in rock, and a three-dimensional finite element model has been established. A series of numerical study has been carried out to simulate the fracture test. Typical simulation results of load versus load point displacement are shown and compared with test measurements. Some preliminary discussions are given in the final part, mainly focused on the influence of strain softening property on the nonlinearity correction factor and the predetermined crack length at failure load in the suggested method.
To date, a wide variety of testing methods have been employed to determine the plane strain fracture toughness of rock material, including the ASTM Fracture toughness Test Method developed for metals [1]. In the Blue Book "The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974–2006"[2], three suggested methods have been included for measuring the fracture toughness of rocks. In 1988, the ISRM Testing Commission recommended two suggested methods using two types of specimens that can be machined directly from pieces of rock core, the Chevron Bend specimen and the Short Rod specimen. In 1995, the cracked chevron notched Brazilian disc (CCNBD) specimen was introduced into the suggested methods, which would form a complete set of specimens for a full rock anisotropic fracture toughness investigation since the crack orientations of these three suggested specimen geometries can be easily arranged to be orthogonal to each other if they are machined from the same rock core. Similar as other quasi-brittle materials (such as ceramic and concrete), the crack development and growth in rocks would be influenced by the grain size due to the presence of a sizable microcrack zone formed ahead of the crack tip, called the fracture process zone. Hence the conventional test methods based on linear elastic fracture mechanics cannot be used directly, and correction arisen from the influence of nonlinear material behaviour was usually considered in the above suggested methods. Also it should be noted that some approximations or assumptions made in the above methods may require further consideration. This paper describes the results of mode-I fracture toughness tests conducted on chevron-notched three-point bend specimens. The rock specimens are marble rock samples from Ma On Shan area in Hong Kong, which can be subdivided into three groups by the difference in grain size.
