Correlation of Damage in Rocks Dynamically Loaded by Percussive Tools Using Piston Coefficient of Restitution (COR)

Abstract Efficient drilling operations require optimal drilling parameters to achieve higher penetration rates and minimize tool wear. This study focuses on characterizing the piston coefficient of restitution (COR) as a damage indicator for rocks dynamically loaded by percussive tools. The COR offers a nuanced understanding of damage, particularly at low impact energies where plastic deformation beneath the insert and sub-surface fractures predominates over chip mass removal. Various design parameters, including piston-to-bit mass ratio, piston length and impact velocity, were varied to evaluate their impact on the piston COR in pristine rock samples. Tests were conducted on granite, sandstone and limestone, all common rocks in down-the-hole hammer drilling. Despite variations in the impact energy level, the influence of mass or length ratios on the COR and the resultant destroyed rock volume was minimal. X-ray computed tomography revealed the significance of the crushed zone beneath the crater, affecting the rock ability to exert mechanical work on the bit during unloading. This characteristic led to the identification of a distinctive non-linear shape in the COR curves, with clearly delineated regions for each rock type, corresponding to different damage rates. These findings underscore the potential of the piston COR as a valuable tool for aiding in the identification of optimal drilling parameters and understanding rock formation characteristics during drilling.