Stress of rocks generally can be known, but at the point of fracture initiation is often not known with certainty. To study the stress around fracture initiation, it is difficult to do using mathematical calculations so that the necessary methods of alternative solution is photoelastic method. Major principal stress can be obtained from photoelastic method. In this research, photoelastic method is conducted to study the stress on fracture toughness tests mode I (tension).

Sample testing conducted at the Laboratory of Geomechanics and Mine Equipment Institute of Technology Bandung involved the physical properties and mechanical properties test to obtain the sample characteristics as input data for numerical modeling using Rocscience 3 version 1.0, such as the natural density, uniaxial compressive strength, Young's modulus, Poisson's ratio, and tensile strength. Moreover, fracture toughness tests mode I are conducted on Modified Ring Test (MRT) and Central Straight-Through Crack (CSCBD) using photoelastic method. The stress can be calculated using colors observed in photoelastic method by following the equation of optical stress. Major principal stress obtained from photoelastic method would be compared to numerical method. From this research, it could be concluded that the results of the stress distribution obtained from numerical modeling have similar results to the stress distribution obtained from photoelastic method, although there are slightly different results, but it still tolerable.

1. Introduction

Fracture toughness has been recognized as an intrinsic property of the rock material that indicates its ability to resist fracture initiation and propagation. Fracture toughness has an important contribution in designing the structure of the rock, rock excavation, and process of rock fragmentation.

In studying the stress distribution, the mathematical theory of elasticity has proven very useful to the behavior of material. This theory is not only the easiest, but has also provided very accurate prediction of the behavior on structure of rock mass. But in case of studying the stress concentration at the initiation point of fracture, mathematical difficulty increases so that the alternative solution method is necessary. The solution method that can be used is photoelastic method.

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