In this paper incubation time fracture criterion is applied to perform numerical investigation of relation between stress intensity factor (SIF) and crack velocity observed in experiments by J.F. Kalthoff [1] for Araldite B. Two sample geometries were studied – double cantilever beam (DCB) and single edge notched sample (SEN). Quasistatic loading of these samples revealed existence of two branches of stress intensity factor – crack speed dependence corresponding to each sample geometry. Finite element method was used to perform numerical simulations of experiments by J.F Kalthoff and to study non-unique SIF – crack speed dependence.