Critical Crack Opening of Charcoal Granite

ABSTRACT: A simple testing apparatus has been designed for measurement of tensile strength of rock and characterization of fracture parameters such as critical crack opening displacement. The apparatus is a hardened-steel beam with a notch and a cut-out, designed so that the rock specimen is subjected to uniform stress through its thickness with negligible bending. The dimensions of the apparatus are length = 280.0 mm, height = 100.0 mm, and thickness = 26.5 mm; the notch length = 60.0 mm and width = 2.0 mm. Two strain gages, SG 1 and 2, are attached near the notch tip and the apparatus is calibrated to provide the tensile stress in the specimen. A Charcoal granite specimen with side notches at its center was used for testing within a closed-loop, servo-hydraulic load with crack opening displacement (COD) at a rate of 0.02 micron/s used as the feedback signal. The rock specimen was attached to the bottom cut-out using an epoxy adhesive. Two strain gages were installed on the rock surface close to the notch tips. Digital image correlation (DIC) was used to investigate the displacement field and the symmetry of displacement along the ligament length. The crack initiation and propagation in the post-peak regime is discussed using the data from the strain gages and DIC. The results show that the apparatus can be used to investigate post-peak response and critical crack opening displacement of a brittle rock. 1. INTRODUCTION The cohesive crack model (Dugdale 1960; Barenblatt 1962;) and its extension (Hillerborg et al. 1976) have been widely used to describe the process of fracture in geomaterials. Hillerborg et al. (1976) assumed that in a direct tensile test, the strains are uniformly distributed along the specimen width until the peak stress is achieved. At the peak, a cohesive crack develops that is assumed to transfer tensile stresses. As the crack opening increases, the specimen softens.