Measurement of Elastic Bi-modulus of Rock-type Materials: Insight from comparative and parallel tests

Rock materials exhibit different elastic moduli under tension and compression, making them typical bi-modulus materials. Few testing methods can measure both tensile and compressive moduli (E_t and E_c) of such materials simultaneously, and with their reliability not systematically validated. Taking blue and red sand rocks as the example, this study performed a series of uniaxial compression, direct tension, Brazilian disc, and three-point bending tests, in which strain gauges and DIC technology were used to observe the deformation. Through comprehensively comparative study, the following findings are obtained: (1) Direct tension and compression tests remain the most reliable methods for measuring the E_t and E_c of rock materials. (2) Only the sub-method I based on the Brazilian disc test proposed by Ye et al in (Int J Rock Mech Min Sci 46(3):568–576, 2009a) exhibits good reliability. (3) Three-point bending test is not recommended due to the adverse effect caused by the stress concentration in the region beneath the loading head. (4) Strain gauges theoretically offer a high measurement accuracy, but their results are significantly influenced by artificial factors; while DIC technology demonstrates a higher reliability when strain exceeds a threshold (e.g., > 500).