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 KIc 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:
KIc = ß vn
where ß 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.
