A two-dimensional finite-element simulation of an impact fracture toughness test under high loading rates is performed. A crack subject to a time-dependent uniform pressure on its surface is considered, and caustics in reflection are generated by making assumption of the plane stress condition. The crack is assumed to begin to propagate when the theoretically exact value of KI (t) reaches a critical value Kcrit. The time history of the "measured" dynamic stress intensity factor KI (t) is determined from the peak value of the time history of the "measured" KI (t). It is found that the impact fracture toughness thus determined increases with the decrease in time-to-fracture when the loading rate is high. This may explain some of the physical meaning of the fracture incubation time .