13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Investigation on the Failure Mechanisms in Sandstones under Uniaxial Compression by Digital Image Correlation Ganyun Huang*, Hao Zhang, Haipeng Song, Yilan Kang Department of Mechanics, Tianjin University, Tianjin 300072, China * Corresponding author: g.y.huang@hotmail.com Abstract Sandstone samples have been compressed uniaxially till failure. The displacement field during the deformation process has been obtained by digital image correlation (DIC) technique. It has been observed that the rock samples ruptured through splitting parallel to the compression direction, as conventional experiments indicated. But upon analyzing the apparent strain field, no sliding cracks or shear cracks have been found that are suggested to drive the splitting failure. Instead, deformation band like structure has been detected. The splitting failure has been observed to be the direct result of the localization of the some scattered small cracks. The micromechanism underlying may be the buckling of force chains that induces delamination between them. Keywords uniaxial compression, failure mechanism, sandstone, digital image correlation 1. Introduction The failure of rocks under compression may hold important implications in many aspects such as geophysics, civil engineering and mining sciences. In fact, uniaxial compressive strength has even been a key parameter in estimating the performance of cutters in rock drilling. Despite the wide variety of rock types, a large body of experiments have demonstrated the rock fails under uniaxial compression via axial splitting or shear faulting [1-4]. Such macroscopic failure mode can be well explained by the nucleation and propagation of the wing cracks that is assumed to depend on the existence of sliding cracks. Experiments on samples with pre-existing cracks, or more exactly cuts inclined to the compression direction have also proved the wing crack formation and propagation that leads to axial splitting [5-7]. Model of wing crack formation and propagation has been widely applied to the damage and micromechanics of rock like brittle materials [8-11]. However, the model depends critically on the existence of sliding or shear cracks. No experiments have been reported on the nucleation of the shear cracks. Thus what the physical micromechanisms drive the axial splitting under uniaxial compression remains an open question. In rock like brittle materials, it has been conventionally believed that microcracks with a random distribution of various orientations exist, which may make the nucleation of sliding cracks dispensable. If it is true, then analyzing the full-field deformation behavior of as-received rock samples under uniaxial compression may help to reveal splitting induced by the sliding crack or the physical micromechanisms for axial splitting. Combination of the uniaxial compression test and DIC technique may suit the purpose very well. Indeed, through DIC technique, full field displacement on the sample surface can be obtained efficiently without contact and hence damage. Applicability of the technique to the rock deformation has already been demonstrated in our previous work [12]. Thus, we will use DIC technique to study the deformation behavior of rocks under uniaxial compression so as to cast some new light on the failure mechanisms at microscopic level. 2. Experiments Samples of dimension 25×25×50 mm were cut from a large sandstone block from Yunnan, China. All the sample surfaces were randomly painted with artificial speckles. Then the sandstone samples were uniaxially compressed along the longest sides on the electric universal machine
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