Experimental investigation on brittleness characteristics of rock based on the ubiquitiformal complexity: strain rate effect and size effect

Brittleness is a crucial parameter for assessing the mechanical behavior of rocks, including fracturability, drillability, and rockburst tendency. The ubiquitiform geometry has been widely used to describe complex fine structures due to its advantages including the distribution of fragment size, energy dissipation patterns, and crack branching. In this study, a new method is proposed to evaluate the brittleness characteristics of rock from the perspective of fragmentation pattern using ubiquitiformal complexity. The uniaxial compression tests were conducted on tuff and slate samples with different L/D ratios (ranging from 0.3 to 1.5) at four strain rates (0.0005, 0.001, 0.005, and 0.008 s−1) to investigate the fragmentation pattern and brittleness behavior of rocks. The relationship between ubiquitiformal complexity and brittleness index is analyzed to evaluate the failure behavior of rock using the ubiquitiform theory. The results indicate that the criterion that considers the post-peak phase of the stress-strain curve is more accurate compared to others. The brittleness indexes B1 and B3 increase with the strain rate, whereas B2 decreases with the strain rate, leading to a decrease or increase in the ubiquitiformal complexity. The ubiquitiformal complexity of slate and tuff ranges from 1.77 to 1.90 and 1.76 to 1.86, respectively. A more comprehensive assessment of the rock can be obtained by analyzing the relationship between the size effect, strain rate, and brittleness index of the rock after damage.