Fracture of rock: Effect of loading rate

Fracture parameters of limestone at loading rates ranging over four orders of magnitude in the static regime are determined using the size effect method. Three sizes of three-point bend notched specimens were tested under crack-mouth opening displacement control. The fracture toughness and nominal strength decrease slightly with a decrease in rate, but the fracture process zone length and the brittleness of failure are practically unaffected. The effect of material creep on the fracture of limestone is negligible in the time range studied here. However, the methodology developed for characterizing rate effects in static fracture can be easily applied to other brittle-heterogeneous materials. The decrease of fracture toughness as a function of the crack propagation velocity is described with a power law. A formula for the size- and rate-dependence of the nominal strength is also presented.