Effects of loading rate on rock fracture

By means of a wedge loading applied to a short-rod rock fracture specimen tested with the MTS 810 or SHPB (split Hopkinson pressure bar), the fracture toughness of Fangshan gabbro and Fangshan marble was measured over a wide range of loading rates, (k)over dot = 10(-2)-10(6) MPa m(1/2) s(-1). In order to determine the dynamic fracture toughness of the rock as exactly as possible, the dynamic Moire method and strain-gauge method were used in determining the critical time of dynamic fracture. The testing results indicated that the critical time was generally shorter than the transmitted wave peak time, and the differences between the two times had a weak increasing tendency with loading rates. The experimental results for rock fracture showed that the static fracture toughness K-Ic of the rock was nearly a constant, but the dynamic fracture toughness K-Id of the rock ((k)over dot greater than or equal to MPa m(1/2) s(-1)) increased with the loading rate, i.e. log(K-Id) = a log (k)over dot + b. Macroobservations for fractured rock specimens indicated that, in the section (which was perpendicular to the fracture surface) of a specimen loaded by a dynamic load, there was clear crack branching or bifurcation, and the higher the loading rate was, the more branching cracks occurred. Furthermore, at very high loading rates ((k)over dot greater than or equal to 10(6) MPa m(1/2) s(-1)) the rock specimen was broken into several fragments rather than only two halves. However, for a statically fractured specimen there was hardly any crack branching. Finally, some applications of this investigation in engineering practice are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.