Simulation of dynamic crack growth under quasistatic loading was performed using finite element method with embedded incubation time fracture criterion. The uniqueness of the stress intensity factor - crack velocity relationship (K-v) is discussed. It is shown that the use of the structural-time approach and the fracture incubation time criterion enables us to predict successfully the results of experiments both on quasistatic and on impact loading of samples with cracks. Comparison of calculated K-v relationships with experimental data for various loading conditions leads to the conclusion that the dependence of the crack velocity on the stress intensity factor cannot be considered as a unique material law because the properties of this dependence are strongly determined by the sample configuration, the history, and the loading method.