ABSTRACT

Standard methods to determine fracture toughness which is one of the important factors in rock mechanics were suggested in 1988 and 1995 by the commission on testing methods of ISRM. It is difficult to verify lab test because of the wide variation of test results and difficulty in reproduce. Therefore verification by numerical analysis is preferred to experimental one. It is also possible to observe fracture propagation into the model and to repeat the same simulation in numerical analysis. In this study 4 kinds of numerical analysis, FLAC 2D and FLAC 3D as continuum method and PFC 2D as discontinuum method, were performed in order to estimate fracture toughness and results were compared with actual Chevron Bend test results. It was found that fracture toughness values of 3D simulations were closer to the lab test results than 2D simulations and values of discontinuum method were better than those of continuum methods.

1 INTRODUCTION

Rock failure can be evaluated with theoretical explanation of facture mechanics like as microscopic behavior that crack is created inside the rock and grew enough to fragment rock with propagation. Fracture mechanics is primarily used to prevent and predict catastrophic failure of structures by evaluation of crack creation, growth and propagation. Especially, the fracture toughness is used in order to determine the possibility of failure.The fracture toughness expresses resistance to crack extension or fracture energy consumption rate required to create new surface. After the suggestion of standard fracture toughness test methods by ISRM commission on testing methods (1988, 1995), several studies were carried out to estimate fracture toughness by numerical analysis(Synn et al, 2000, Yoon & Jeon, 2003). In this study, 3 numerical analysis codes, FLAC 2D and FLAC 3D as continuum method and PFC 2D as discontinuum method, were carried out to estimate the influence of each code to the calculation of fracture toughness and to compare results of each code.

2 CHEVRON BEND TEST(CB)

Suggested standard test method for determining fracture toughness with Chevron Bend test by ISRM has two levels, one is called level I test, that only maximum load is considered to calculate fracture toughness, and the other is called level II and continuous measurement of load and displacement beyond the maximum load are considered to calculate fracture toughness. The description of level I test is run under load control and load has to record until the specimen fails.The minimum average stress intensity rate during the test is 0.25MPa √ (m)/sec and the rate has to be controlled that failure occurs within 10 sec of initial load application.

3 NUMERICAL ANALYSIS

In order to observe effect to fracture toughness by numerical analysis method, 4 codes were used to estimate and compare result with that of lab test in previous study. FLAC 2D and FLAC 3D were carried as continuum method and PFC 2D as discontinuum. Specimen size and properties by lab test of reference were used as numerical model size and assigned properties to numerical model.

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