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Uniaxial compressive strength is an essential mechanical parameter to adequately characterize any given material. Numerous standards have been developed to guarantee reliable testing execution, as well as the repeatability of results. In this sense, not only the geometric dimensions and tolerances of both the platen and the specimen have been prescribed, but also the testing parameters, such as the load application speed. However, all these recommendations are based on the assumption that the stresses are uniformly distributed across the contact interface between the platen and the specimen. Nevertheless, this is major elastic simplification that allows for obtaining a handy and useful formula to determine the compressive strength, but this strongly deviates the theoretical foundations from the actual experimental reality. Experimental and numerical research to determine the influence of relative stiffness between the specimen and the platen on the stress distribution generated during the execution of the uniaxial compressive test is performed. The results prove that the stresses are not uniformly distributed across the contact when the platen material is significantly stiffer or softer (less stiff) than that of the tested specimen, and additionally, an undesired triaxial stress field is induced inside the specimen. For these reasons, the use of platens with a similar stiffness to that of the specimen is strongly recommended, as it allows for the uniform distribution of the compressive contact stresses and minimizes the influence of the triaxial stress field. [ABSTRACT FROM AUTHOR]

Uniaxial compressive strength is a variable necessary for adequately characterizing a material's mechanical properties. However, a specimen's geometric deviations and elastic properties may lead to undesirable stress states, which cause strong discrepancies between the results of the uniaxial compression test and its theoretical foundations. While geometric deviations may cause non-uniform contact between the platen and the specimen, elastic properties can provoke severe end effects that disturb the local stress field near the points of contact. To address how the relative stiffness between the platen and the specimen influences the induced stress field, numerical simulations considering the stiffness ratios E p / E s = 3 , E p / E s = 1 and E p / E s = 0.05 were performed. Subsequently, these results were employed to establish the relation between relative stiffness and specimen failure patterns in brittle materials, particularly in three different rocks. The results prove that the platen stiffness must be accurately selected to match that of the tested material, in order to avoid undesirable local stress fields near the point of contact and to induce homogeneous uniaxial compression that guarantees reliable uniaxial compressive strength characterization. Furthermore, the brittle failure patterns reported in previous studies were correlated with the induced stress fields inside the specimen depending on its platen stiffness, allowing the validity of the test results to be verified based on a simple visual inspection. [ABSTRACT FROM AUTHOR]