13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental Investigation on Deformation and Failure of Rock under Cyclic Indentation Hao Zhang1, Haipeng Song1, Yilan Kang 1,*, Ganyun Huang1 1 Tianjin Key Laboratory of Modern Engineering Mechanics, School of Mechanical Engineering, Tianjin University, Tianjin 300072, People's Republic of China ∗ Correspondence. E-mail: tju_ylkang@tju.edu.cn Abstract Indentation induced cracking may have widely encountered in rock engineering, such as drilling and cutting process. It is significant to investigate the deformation and failure process for further understanding the failure mechanism of rock under cyclic indentation. In the present work, we have experimentally investigated the deformation fields on the sample surface via digital image correlation (DIC) technique. It has been found that the force-indentation displacement exhibits hysteresis during the loading-unloading cycles. The energy dissipation and the displacement field in the samples during the process has been analyzed. Furthermore, a median crack has been observed to nucleate running almost parallel to the loading axis when indentation force amounts to a critical value. Experimental analysis reveals that local tensile strain dominates the crack nucleation of rock and the shear strain has an effective influence on crack propagation. In unload-reloading process, crack behavior closure and re-opening is clearly observed by DIC technique. By the displacement distribution along the crack surface, crack opening displacement (COD) and stress intensity factor can be calculated for further fracture analysis. Keywords Rock, Cyclic indentation, Digital image correlation, Deformation and failure 1. Introduction Rock fragmentation induced by mechanical tools is an effective technique in mining and civil engineering. Rock cutting by various bits, such as drilling bit, rolling cutter, could be simplified as the cyclic indentation process [1]. Knowledge on deformation and failure process of rock is an important basis for the study of rock failure mechanisms. Therefore, it is necessary to experimentally investigate the damage and fragmentation of rock during cyclic indentation. Many experimental tests have been conducted in the past decades to investigate the fragmentation of rock. Damage pattern which consists of inelastic zone and a region of multiple cracks is observed in rock under indentation [2]. Laboratory tests, simulating the disk cutter action at the tunnel face by means of an indenter, were carried out to explore the influence of the lateral confinement acting on the rock failure [3]. Acoustic emission (AE) and electronic speckle pattern interferometry (ESPI) techniques were used to detect the events of microcrack nucleation [4]. Numerical methods, such as the finite element method (FEM) [5], rock failure process analysis of two dimensions (RFPA2D) [6], have been developed to simulate the fragmentation process in rocks under indentation. Repeated loading–unloading conditions are widely encountered in practical excavation and drilling engineering. However, affects of cyclic loading on indentation-induced failure remains unclear. In this work, cyclic indentation tests have been conducted to investigate the deformation and damage evolution of Yunnan sandstone. During the loading-unloading process, images of rock specimens have been sequently captured, from which deformation fields can be obtained by digital image correlation (DIC) processing. Displacement and apparent strain fields calculated by DIC, are
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