Fracture behavior in cracked or jointed rock masses is an important task for rock structure designers for investigating the onset of crack growth and the overall failure. In the real applications like tunneling, mining, rock cutting and excavation process, the fracture of rock mass may occur under a combination of tension and shear (or mixed mode I/II) loading. Therefore, it is important to investigate mixed mode fracture in rock samples using suitable test specimens. In this research, a test configuration called the centrally cracked ring (CCR) specimen subjected to diametral compression is introduced for mixed mode fracture tests. The mode I and mode II stress intensity factors (KI, KII) of the CCR specimen are determined by means of the finite element analysis in the full range from pure mode I to pure mode II. Then a series of mixed mode fracture experiments are conducted on a type of granite using the CCR specimen. It is shown that the CCR specimen can be used as an appropriate test sample for fracture toughness studies in rock materials.
1. INTRODUCTION
Cracks, bedding planes, joints and inherent discontinuities are frequently found in rock structures and materials. Under mechanical or environmental loads, the probability of crack propagation from these discontinuities is increased. Meanwhile, in many practical applications, such as tunneling, mining, excavation process, blasting, rock cutting, oil and gas wells drilling and analyses of rock slope stability, the evaluation of crack growth and overall failure are important tasks for rock structure designers. For assessing the fracture resistance of a cracked rock, the fracture toughness of rock should be known. In real applications, the cracked rock structures are usually subjected to complex loading conditions and the fracture of rock masses may occur under a combination of tension and shear (mixed mode I/II) loading. Therefore, it is important to investigate mixed mode fracture in rocks. There are some standard procedures and testing methods for rock fracture studies. However, all of the test methods suggested by the international society for rock mechanics (ISRM) are confined only to the crack opening case or pure mode I fracture of rocks. Three standard test methods accepted by the ISRM for evaluating the mode I fracture toughness of rocks are: the chevron-notched round bar in bending [1], chevronnotched short rod in splitting [1] and chevron-notched Brazilian disc in diametral compression [2]. Linear elastic fracture mechanics (LEFM) can provide an appropriate discipline for analyzing the fracture of jointed rock masses subjected to complex stress conditions. In LEFM, it is assumed that the overall failure and fracture of a cracked body initiates from the tip of pre-existing cracks. For analyzing mixed mode fracture through LEFM, the stress intensity factors of crack under opening and sliding modes and fracture toughness of rock should be known. Fracture toughness of rock materials is found experimentally by means of suitable test samples. Several test configurations have been used in the past for investigating mixed mode I/II fracture toughness of rocks.
