Normal Indentation of Rock Specimens with a Blunt Tool: Role of Specimen Size and Indenter Geometry

Indentation testing has been widely used in laboratory environments to investigate the processes leading to rock fragmentation in drilling, mechanized tunneling, and mining. Rock specimens for laboratory testing are limited to finite size, potentially causing size effects that have to be accounted for when transferring results to in situ applications. We present an integrated experimental and theoretical investigation of the specimen size effect in indentation testing (a) to address the limited understanding of its causes and the lack of tools to analyze tests on variable specimen sizes and (b) to identify to what extent an indenter mimicking the shape of a cutter on a tunneling machine can be approximated by a conventional indenter geometry. We performed indentation tests on cylindrical specimens of a porous sandstone with aspect ratios (diameter/height) ranging from 0.3 to 1.7, using a blunt-truncated indenter and monitoring the fracturing process by the acoustic emission technique. A damage zone, enclosing a zone of crushed grains immediately below the indenter tip, forms and grows due to tool penetration. Eventually, all specimens failed as a result of the propagation of a sub-vertical fracture, initiated close to peak indentation pressure. Peak force, its corresponding penetration depth, and peak indentation pressure increase with specimen size, more significantly with specimen diameter than with height. We developed a semi-analytical model based on cavity-expansion theory and linear elastic fracture mechanics for the formation of the damage zone and the nucleation and propagation of the macroscopic vertical fracture, respectively, whose predictions are in good agreement with our experimental data. The observed increases of peak indentation pressure with specimen size can be explained by the effect of the free surfaces on damage zone growth rather than on fracture propagation. The model permits evaluating the specimen size effect through the ratio between two geometrical parameters, specimen diameter and tip width of the truncated indenter, which has to be larger than around 10² for the size effect to be insignificant. The model permits upscaling of experimental results to in situ conditions based on geometrical indenter parameters and commonly used material parameters.