• A novel brittleness index based on rock fracture energy is proposed and validated through multi-scale experiments.
• Increasing loading rate from 0.1 to 5 mm/min significantly enhances granite brittleness, with the brittleness index Bs rising from 0.17 to 0.28.
• Qualitative indices (projectile mass ratio and average lumpiness) also increase with loading rate, confirming the brittleness trend.
• The study provides theoretical guidance for predicting rockburst and optimizing hydraulic fracturing in deep underground engineering.