ABSTRACT:

The behavior of sandstone Brazilian disks is investigated under loading in two perpendicular directions. The vertical force was applied at a rate of 200 N/sec while the horizontal load was adjusted to a predetermined ratio of the vertical load until the sample failure. This testing process was repeated for various ratios (including 0%, 10%, 20%, 25%, 33%, 40%, 45%, and 50%) and the failure of samples were captured using a high-speed camera at 25 kHz. While the superposition theory suggests that there will be no tension at the center of the disks in a biaxial loading mode for ratios of 33% and above, results suggest tensile crack occurrence for ratios up to 45%. The numerical results further show that there is always a tensile stress close to the Indirect Tensile Strength happening within the disks central line. It was also noted that if the material transitions into a plastic mode due to extremely high stress ratios, the induced tensile stress would be always considerably higher.

INTRODUCTION

Brittle materials, such as rocks and concrete, have high compressive and shear strength but are weak in tensile and flexural behaviour. Therefore, accurate assessment of the tensile strength of these materials is crucial. The Brazilian indirect tensile strength method is preferred over direct tension test methods due to its practical convenience and ease of sample preparation. However, the accuracy of the Brazilian test has always been challenged due to several reasons. The repeatability of the test has a high range, and the tensile stress induced at the center of the disks is not pure and has a compressive principal stress three times that of the tensile stress. The contact and shape of loading platens can also lead to different results (Fairhurst, 1964; Mellor and Hawkes, 1971; Barla and Innaurato, 1973; Serati et al., 2014 and 2016). The biaxial loading of Brazilian disks with rock-like samples has also been largely under-investigated. In this study, sandstone Brazilian disk specimens under biaxial loading is investigated. Using the prescribed formula outlined in the ASTM (2008) and ISRM (2015) standard recommendations for calculating the induced tensile and compressive stresses at the centre of the disk and according to the principle of superposition, the equation (1) is obtained for the biaxial loading (Serati et al., 2012, 2018 and 2023).

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