Compared to uniaxial or triaxial compression tests on rocks, direct tension tests on rocks are not common in practice. As is well known, the reasons are because underground rock masses are usually under compressive stress state and/or because the direct tension test on rock is rather difficult and time-consuming. However, the direct tension test is important to understanding mechanical characteristics of rocks under tensile stress state. Moreover, the number of case examples requiring accurate tensile strength of rock is recently increasing in rock engineering. This paper summarized the methods and points of attention for the direct tension test on rock, based on the previous studies and the authors' experiences. The direct tension tests on rock have adopted a variety of specimen shape, loading apparatus and testing conditions. This paper presented some important factors affecting the deformation and failure of rocks under tensile stress state. The relations of the direct tensile strength to the other mechanical properties, such as the Young's modulus, the elastic wave velocity and the compressive strength, was discussed. The rock tests related to the direct tension test were explained: tension test using centrifugal action, tension test under confining pressure, dynamic tension test and in situ tension test on rock masses. This paper introduced the authors' ongoing standardization activity for the direct tension test on rock.
