Abstract - Extension fractures under long-term loading conditions are likely to grow by slow, stable fracture growth characteristic of subcritical crack propagation. The subcritical crack-tip velocity of a mode I fracture is:
where, v is the velocity, A is a constant of proportionality, KI is the mode I stress intensity and n is the subcritical crack-tip velocity exponent; n corresponds to the sensitivity of the propagation velocity to stress intensity. Published experimental determinations of the n value range between 15 and 130 for limestone. Superbly-exposed mode I calcite veins of the Burren have straight, regular fracture traces and show no sign of crack-branching, thus they probably grew subcritically. Fracture patterns, mapped on the tiered Burren summits, have been quantitatively characterised. Numerical simulation of these fracture patterns has been attempted to ascertain the values of n appropriate to the Burren veins. Our simulations suggest that ‘natural’ n values range between 0 and 2. These values are one to two orders of magnitude less than those experimental values, implying that in nature crack propagation velocity is much less sensitive to stress intensity than experiments predict. We provide a simple mathematical rationale for low n values arising from the fluid-dependence of fracture growth.
Abstract of talk given to:
Irish Geological Research Meeting, Dublin, February 2002