Principal effects of dynamic instability of the fracture process are investigated. Particularly an issue of a dependence of stress intensity factor (K) on crack velocity (v) is discussed. Uniqueness, related to the given material, and even existence of this dependence has been a matter of discussion among researchers, since contradicting experimental results have been reported in the literature over last decades. In this paper results of numerical simulations of the crack propagation process are presented. A numerical scheme based on finite element method and incubation time fracture criterion was developed. Scattering of the K values during the propagation process is considered to be a feature principally related to a spatial-temporal nature of the fracture process. It is found that quasistatic loading is characterized by a small scatter of the K values and fitting of the data can be performed in order to obtain some well-known K - v curve. In the case of a pulse loading the scatter is much higher and it can be concluded that a wide range of K values correspond to a particular crack velocity and no unique continuous K - v curve can be associated with the process. These results are supported by well-known experimental observations. Thus, it is proved that the incubation time fracture criterion makes it possible to investigate dynamic crack propagation for a wide variety of loading conditions (quasistatic, high rate and short pulse loading) and only one extra material parameter - the incubation time is needed to predict a big variety of effects of the dynamic instability of the fracture process. This is a huge advantage comparing to a widespread approach, which involves complicated experimental determination of material strain rate dependencies and an a priori given K - v relationship.