Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 18-31; DOI: 10.3221/IGF-ESIS.40.02 20 T HEORETICAL CONSIDERATIONS Uniaxial compression test n uniaxial compression test a cylindrical specimen of diameter D and height h is subjected to a uniformly applied stress σ y , acting on the ends of the specimen (Fig. 1a), following the ISRM suggested method [19]. In addition to the peak stress value, the complete stress-strain curve is recorded in order to calculate the tangent intact rock modulus E 50 (Fig. 1b). (a) (b) Figure 1: (a) Uniaxial compression test and calculation of axial and lateral strains and Poisson’s ratio. (b) Stress-strain curve and the calculation of the tangent intact rock modulus. Indirect tensile test (Brazilian test) In the Brazilian test a cylindrical specimen of diameter D and thickness t is subjected to a uniform radial pressure - p , acting along an arc of length b at each end of the diameter (Fig. 2a). The angle subtended at the center of the disc by the loaded section of the rim is equal to 2a . If the material behavior is assumed to be linear elastic, this geometry and loading procedure ensures a nearly uniform tensile stress state in the center plane of the specimen (Fig. 2b). According to this distribution, the expected failure mode is the splitting of the specimen in two halves across the plane of loading. For brittle elastic materials, the maximum tensile stress ( f st ) is a material property called splitting tensile strength and is linearly related to the failure load ( P f ):   2 f st P f Dt (1) Using measurements from electrical strain gages ( ε xx , ε yy ) that are attached to the center of a specimen, the elastic parameters can be calculated for an isotropic material using the following relationships [20]:              16 1 3 xx yy yy P E Dt (2) I