The Influence of Platen Stiffness on a Specimen’s Failure Initiation Point and the Failure Pattern of Brittle Materials in the Standardized Uniaxial Compression Test

Abstract

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 Ep/Es=3, Ep/Es=1 and Ep/Es=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.

Influence of Platen Stiffness on the Contact Stress Distribution in the Standardized Uniaxial Compression Test

“…In this sense, if all induced stress fields are independent of the tested material properties, provided that specimens are sufficiently long, no differences should be registered in the obtained failure patterns. In contrast to this assumption, many authors have suggested different failure patterns when testing conditions are altered [17,[24][25][26].…”

“…Additionally, it is worth mentioning here that the purpose of this article is to address, from a scientific perspective, the influence of the relative stiffness between the press plates and the specimen (the ratio between their Young's modulus) on the different contact stress distributions. While according to best knowledge of the authors, no previous proposal regarding the use of press plates with different materials, depending on the tested specimen, can be found in the current literature, previous research proves that contact conditions influence the obtained failure pattern, the location of the failure initiation point, and hence, the registered material's compressive strength [25,26]. The results expressed in this article confirm that the influence of shearing stresses across the contact can be minimized, reaching neglectable levels for practical engineering purposes, by using platens, or intermediate thick cushions, with similar elastic properties to those of the tested specimen.…”