How are Nanoindentation-based Hardness and Bulk Indirect Tensile Strength of Ultramafic Rocks Related?

ABSTRACT: The study investigates the relationship between indentation hardness index and the nanoindentation hardness index, using the latter to attempt a validation of existing correlations between indentation hardness and bulk strength tests. Additionally, it evaluates the impact of serpentinization on these relations. Four cylindrical specimens and four square prismatic specimens were extracted from the same olivine-rich rock sample. The four small specimens undergo nanoindentation testing, with one specimen serving as a control. The four core plugs were subjected to bulk tensile strength and UCS testing before and after hydrogen generation. Key findings indicate that the actual tested values of the samples are generally twice the estimated values from existing correlations, leading to a modified correlation for UCS and BTS estimation from nanoindentation measurements. The measurements of the UCS and BTS post-serpentinization resulted in values that did not fit the modified correlation. We suspect that either the mineral alteration due to serpentinization or the increased pressure in the batch reactor for the process could have impacted the mechanical properties of the rock specimen. The novelty of this study lies in its determination of the relationship between bulk tensile strength and nanoindentation hardness, along with examining how serpentinization could affect these mechanical properties.