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  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.