Where curling stones collide with rock mechanics: cyclical damage accumulation and fatigue in granitoids

Abstract. Fatigue and damage accumulation in granitoids are classical, but poorly characterised, rock mechanics problems. We explore these phenomena by examining curling stone impacts. Curling stones are slid on ice and made to collide along a circumferential striking band. This well constrained scenario involves uniaxial compression of convex surfaces (i.e., Hertzian contacts). Conservatively, each stone experiences about 2900 impacts per season, over a lifespan of 10–15 years before refurbishment, providing a unique opportunity to study fatigue and damage accumulation under dynamic cyclic loading. Here, we first determine the stress magnitudes of head-on curling stone impacts using on-ice experiments involving a high-speed camera and pressure-sensitive films. We then characterise the damage observed in aged stones using photogrammetry, microtomography, and microscopy. For high-velocity impacts (2.93±0.15ms-1), a curling stone is locally and momentarily stressed to 300–680 MPa, exceeding its quasi-static unconfined compressive strength and exceeding the threshold for fatigue damage for repeated dynamic loadings. Curling stone impacts are dynamic in nature, as evidenced by (1) high strain rates (24±4s-1) that lie below those of co-seismic rock pulverization; (2) ejection of rock powder during collisions and the presence of potential spalling microcracks; and (3) presence of striations on crescent-shaped fractures, which resemble mirror-mist-hackle patterns indicative of dynamic microcrack propagation. In the striking band, damage is confined to macroscopic Hertzian cone fractures and their immediate collet zones, and does not appear to extend beyond about 3–5 cm into the stones (radially). The circumferential density of cone fractures is limited to about 2–2.5 cm−1. We propose that (1) early, high-velocity impacts initiate cone fractures up to a specific spatial density, and (2) with subsequent collisions in the same regions of the striking band, cone fractures progressively propagate and coarsen. This concentrates and channels the accumulated damage, shielding the rest of the stone from reaching critical stress levels for damage. Our findings are significant for applications where rocks are exposed to repetitive, high-stress impacts and suggest that narrow damage zones can dissipate high-impact stresses.