Influence of geometry upon crack healing rate in calcite

Abstract

Crack healing experiments were conducted at 780°, 815°, and 850° C in dry carbon dioxide over periods of 0.5 to 1320 h on small penny-shaped cracks of known geometry in Stahl calcite. As observed by previous investigators, healing initiates with the formation of tubes around the leading edge of the crack, followed by the pinching off of spherical bubbles from these tubes. Pillars and peninsular structures formed and grew at cleavage steps and other surface irregularities, greatly accelerating the healing process in some cracks. The crack tip regression distance at constant temperature follows a simple power law in time; although, with the exception of one crack at 850° C, the time exponent systematically decreases from 0.59 at 780° C to 0.15 and 0.07 at 815° and 850° C, respectively. Only the largest exponent is consistent with existing theory: the two smaller time exponents are, as yet, unexplained. Healing rates are strongly inversely dependent upon crack aperture, in qualitative agreement with theory. For example, cracks with maximum apertures of less than 60 nm (6×10⁻⁸ m) healed completely in 0.5 to 25 h at 850° C; while fatter cracks, with maximum apertures of about 700 nm, showed very little healing after up to 1320 h at the same temperature. At 780° C, the functional dependence of healing rate upon crack aperture was consistent with several aspects of the present model.