The tensile strength of a rock is determined indirectly by means of a test better known as the Brazilian test. The Brazilian test for rocks is regulated by ASTM D3967: this test allows to indirectly evaluate the tensile strength of a rock starting from the crushing of a specimen. The indirect tensile test is a quick test that allows a quick assessment of the toughness of a rock.

The Brazilian test for rocks consists in squeezing a rock specimen between two concave jaws: starting from the value of the force applied to reach the failure of the specimen, the tensile strength of the rock is ascertained.

According to ASTM D3967, the rock specimen to be subjected to the Brazilian test must have specific characteristics: it must be a circular disc in which the ratio between thickness and diameter must fall within the range 0.2-0.75. In general, the diameter of the specimen must always be at least ten times greater than the largest grain of the rock. To have a significant value of tensile strength it is advisable to perform the test on several specimens, in order to mediate the results as a function of the intrinsic anisotropy of the rock: the more homogeneous a rock is, the less tests will be required. The tensile strength by the Brazilian test is determined by inserting the rock specimen between two curved jaws, which distribute a load to the specimen which is applied to them via two plates. LRRT owns a Brazilian GCTS test rig like the one in the figure below.