Chipping Mechanisms in Rotary-Percussive Drilling: Experimental Insights into the Role of the Median Crack as Precursor of the Dynamic Fragmentation Process
Authors: Aising J. , Gerbaud L. , Sellami H. , Senechal P. , Moonen P. , Ugarte I. 
Published: August 2024

ABSTRACT:
Understanding the mechanisms governing the production of large fragments during rotary-percussive drilling is critical to optimizing drilling efficiency. According to the prevailing theory, these fragments can only be produced during the unloading phase of indentations by the closure of a specific crack, named the median crack. The experimental evidence for this argument mostly comes from quasi-static indentations in amorphous materials, such as soda-lime glass. However, this material drastically differs from typically drilled rocks, where the degree of heterogeneity, grain size, and initial defects may modify the chipping mechanism. To investigate the fragmentation mechanism in typically drilled rocks, dynamic indentation tests were conducted on limestone and granite. Results of high-speed camera observations revealed that chipping occurred during the loading phase in the granite, and through segmentation analyses of tomographic images, that the median crack was often absent. Moreover, in the limestone, a correlation between the onset of a Hertzian cone above the median crack and the change in the growth rate of the side cracks experimentally supports that the chipping mechanism in these two rocks must be related to the expansion of the crushed zone in the loading phase. Therefore, the prevailing theory cannot be generalized to any rock type.

1. INTRODUCTION
Roto-percussive drilling is the preferred method for drilling medium to high-strength rock formations, such as those encountered in quarries, mining, and geothermal operations. In this method, a piston repeatedly impacts a drill bit equipped with multiple tungsten carbide inserts that come into direct contact with the rock (L. E. Chiang & Elías, 2008). The impact generates a stress wave that travels down the drill bit towards the rock. When the stress wave reaches the end of the drill bit, the inserts indent the rock, inducing highly localized stresses that create dust and fragments of various sizes (Reyes et al., 2015), which are then removed by drag through a high-velocity air flow (C. H. Song et al., 2014), ensuring that the rock surface is clean for the next impact. Using the minimal amount of energy in the impact to generate the maximum number of fragments is key to optimizing the Rate of Penetration (ROP) and reducing carbon emissions in the industry (Gilbert et al., 2010). In this regard, various authors have noticed that the large fragments observed in both laboratory experiments and field tests are generated by the coalescence of two specific types of cracks, known as radial cracks and side cracks (also called lateral cracks). Fig. 1 shows the typical idealized crack types developed in brittle materials by indentation (Adapted from Cook & Pharr, 1990; and Liu et al., 2008). Although the formation of radial cracks due to tangential stresses is well understood and accepted for crystalline and amorphous materials (Cook & Pharr, 1990), both with and without confinement (Wu et al., 2021), the mechanism that generates the side cracks, and particularly the moment at which they occur, is still debated (Cook & Pharr, 1990; Saksala, 2011). The prevailing theory follows the work of Lawn et al. (Lawn & Swain, 1975), who, through quasi-static indentations on soda-lime glass and other brittle, amorphous materials, characterized and described the evolution of median cracks and side cracks, suggesting that the side cracks occur as a consequence of the closure of the median crack during the unloading phase. In the same approach, Chiang et al. (S. S. Chiang et al., 1982) through a plasticity cavity model, further substantiated the notion that side cracks occur during unloading.