Investigation of optimum sample shape for the Luong core tension test

Theoretically, the direct tension (DT) test is the simplest and most effective method to determine the tensile strength of rocks, which is meaningful in the practice of rock engineering and geophysics applications. However, clamping the specimens is a problem that causes difficulty in specimen preparation and complicates the use of test equipment. The Luong core tension (LCT) test is an innovative approach to estimate the tensile strength of rocks in a laboratory and overcome the disadvantages of the conventional DT test; thus, this approach has strong practical appeal. This study first discusses the mechanics of the LCT test, and then experimentally and numerically investigates the sample shape effect and optimal sample shape range to obtain a reliable tensile strength (TS). Results show that the maximum stress near the inner and outer bottom are close to each other, and a reliable tensile strength can be obtained when the ratio of the outer ring diameter to specimen diameter, r₁/R, is 0.62 ± 0.08; the ratio of the inner ring diameter to specimen diameter, r₂/R, is 0.45 ± 0.12; and the ratio the of inner ring diameter to outer ring diameter, r₂/r₁, is 0.64 ± 0.06. Tensile strength results obtained from these tests are well consistent with that obtained from the present DT test. Furthermore, compared with results of Brazilian and hydraulic fracturing hollow-cylinder tests, this method yields the lowest standard error and minimum coefficient of variation.