Influence of loading rate on fracture toughness of rock by semi-circular bend (SCB) test

A fracture toughness test under a wide range of the loading rate from 10^-3 to 10⁶ mm/min was conducted using Kimachi sandstone in order to investigate the influence of the loading rate on the fracture toughness of the rock. Semi-Circular Bend (SCB) test was adopted to estimate the mode-I fracture toughness of the rock. The specimen was loaded in the loading rate from 10^-3 to 10³ mm/min using a servo-controlled testing machine in the static SCB test. Split Hopkinson Pressure Bar (SHPB) was used in the dynamic SCB test under the loading rate from 10⁵ to 10⁶ mm/min. As the results, the dynamic fracture toughness was higher than the static one. Each of the fracture toughness K_Ic in the static and dynamic loading condition was plotted almost linearly against the loading rate v on a logarithmic graph and increased with increasing the loading rate. This relation is represented as: K_^Ic = βvⁿ where β and n are constants. The n value was 0.016 and 0.82 in the static and dynamic loading condition, respectively. The dynamic n value was much higher than the static one. The relation between the fracture toughness and the loading rate can be drawn bi-linear in the range of the loading rate from 10^-3 to 10⁶ mm/min on the logarithmic graph. An inflection point can be identified between the range from 10³ and 10⁵ mm/min.