Some rock engineering design operations are mainly controlled by the tensile strength and deformability of the rock (i.e., critical span of excavations or hydraulic fracturing). The ISRM Commission on Testing Methods and the ASTM procedures recommend the use of cylindrical rock specimens, with height-to-diameter (H/D) ratio of 2.5 to 3.0 for direct tensile strength (DTS) determination, in a similar way as per UCS testing. Nevertheless, it is unclear how this ratio may affect the tensile strength of a rock when directly determined. In the present work, the authors performed some series of numerical simulations with a 3D distinct element code (3DEC), replicating DTS tests carried out on specimens presenting five H/D ratios (0.5, 1, 1.5, 2, 2.5, and 3). The study is complemented with laboratory DTS tests in granite. The results herein presented indicate that H/D ratios greater than one have almost negligible influence on DTS.
The study and understanding of rock strength and deformability represented one of the main topics in the development of rock mechanics as a discipline in the beginning of the 20th century. Although somewhat overlooked, the tensile strength of rock materials attracted the attention of some researchers at mid-century (Obert et al. 1946) leading them to even attempt to standardize some rock-mechanics laboratory tests, particularly, the direct tensile strength test (DTS). The relevance of rock tensile strength was also remarked by some researchers (Hawkes et al. 1973; Hoek, 1964), who recommended its direct determination through DTS tests under controlled conditions.
Some years later, the International Society for Rock Mechanics and Rock Engineering (ISRM, 1978) published the Suggested Method for Determining Tensile Strength of Rock Materials, where some guidelines on the determination of direct tensile strength through cylindrical rock core testing are contained. Aspects like the dimensions of the metal caps to be used, specimen geometry, linking system or loading rate are provided. Following the line of the ISRM document, the last version of the ASTM International Standard Test Method for DTS (ASTM, 2020) presents also the main features for this type of test.
