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The indirect tensile strengths (ITSs) of more than 200 disc specimens for 19 different rock and rock-like materials were determined by considering the concept of fracture toughness under different loading conditions. Steel loading arcs with various contact angles and flat platens were used in experiments to investigate whether they can be used as alternatives for the standard Brazilian jaw. Many of the disc specimens tested under the standard jaw did not fail with ideal tensile cracking at the centre, whereas simple splitting cracks were obtained along the vertical diameter of most of the discs tested under the flat platens. However, flat platens caused cracks to initiate in the compressive zone just under the loading points. This is an important outcome for criticizing the validity of the test because tensile failure is expected to occur along the vertical diameter of the disc. Friction conditions between the disc and jaw have important roles in the failing load. Moreover, fracture toughness is an additional parameter for investigating the ITSs of rock materials. According to the experimental results, fracture toughness values were found to increase with an increase in contact angle. In addition to the experimental study, finite element analyses were performed on several types of rock discs under various loading conditions. As a result, a comparison between all loading conditions was made to determine the best ITSs of the rocks. Both experimental and numerical analyses concluded that deciding the best loading type depends on the rock type. Therefore, the standard jaw is not recommended to use for determination of the ITS values of all rock types.

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In this study, a new direct tensile strength determination method of drilled core specimen testing was investigated with numerical and experimental studies. Effect of different drill diameters of holes of the rock specimens was analysed to assess the ideal drill-hole design. The aim of drilling process is decreasing the failure load and stress concentration at the adhesive part to prevent invalid failures. Drilled specimens were determined to fail due to the start of cracking under the condition of uniaxial tensile stress distribution at sidewalls of holes. It was found that drill bits with the diameter of 11 mm is usable for a practical specimen drilling process to make a significant decrease in the failure load of the core specimens with the NX size (@54 mm) diameter. According to the results obtained from numerical and experimental analyses, an equation was suggested to determine uniaxial tensile strength values of drilled core specimens with the hole diameter of 11 mm.