Experimental investigation of deformation and failure mechanisms in rock under indentation by digital image correlation Hao Zhang, Ganyun Huang, Haipeng Song, Yilan Kang ⇑ Tianjin Key Laboratory of Modern Engineering Mechanics, School of Mechanical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China article info Article history: Received 13 April 2012 Received in revised form 14 September 2012 Accepted 15 September 2012 Keywords: Indentation Digital image correlation (DIC) Crack identification Splitting fracture model abstract Sandstones have been indented by a cylinder till failure. During indentation, the force-pen- etration curve has been recorded and the full-field displacements on the sandstones’ sur- faces have been measured by the nondestructive optical technique of digital image correlation (DIC). By analyzing the results with DIC, the onset of fracture and the crack length evolution have been identified. Just before cracking, DIC analysis has further revealed slip in some regions, which may act as the shearing crack in splitting fracture model. In that sense, adaptation of the splitting fracture model for the present indentation and comparison with experiments suggest that splitting fracture mechanism may be the dominant one in the failure of sandstones under indentation. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Mechanical excavation, such as drilling and TBM tunneling, plays an important role in mining industry, transportation, underground space exploitation [1,2]. Rock cutting by various excavation machines could be simplified as indentation by a wedge or other general shaped indenters [3,4]. Rock indentation represents the fundamental process for rock excavation and fragmentation by using mechanical methods. Therefore, it is necessary to investigate the basic deformation and failure mechanisms during the process of rock indentation. The indentation induced rock deformation and failure involves complicated phenomena. Various forms of cracks such as Hertzian cracks, radial cracks, median cracks and lateral cracks have been observed [5]. Based on the Boussinesq solution, the criteria such as the maximum shear stress and tensile stress criteria have been believed to be the failure mechanism in rocks under indentation [6]. Cavity expansion model with account of inelastic deformation zone has been introduced since the Boussinesq stress is singular at the loading point [7]. Other mechanisms such as splitting fracture mechanism have also been proposed for rock indentation failure [8]. Experiments, on the other hand, have been performed to observe the rock fragmentation phenomena for further under- standing of the failure mechanism under indentation [3,9]. In combination with acoustic emission and electronic speckle pattern interferometry techniques, it has been demonstrated that under indentation an inelastic deformation zone develops that may drive the subsequent formation of tensile crack [10]. It is worth mentioning, full-field displacement information during the indentation process is still lacking. Acquirement of such information nevertheless would be of great significance since the possible nucleation of cracks can be detected. Consequently obtaining the full-field displacement during indenta- tion till eventual failure will improve our understanding of the mechanisms involved. Technique like digital image correla- tion (DIC) [11–13] can achieve the purpose very well. In fact, DIC offers the opportunity to measure displacement and strain 0013-7944/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfracmech.2012.09.012 ⇑ Corresponding author. E-mail address: tju_ylkang@tju.edu.cn (Y. Kang). Engineering Fracture Mechanics 96 (2012) 667–675 Contents lists available at SciVerse ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech