Instability effects of the dynamic crack propagation process

•Stress intensity factor – crack velocity dependence (K“ − ȧ) is numerically studied.•Space-time discreetness of the dynamic fracture is the fracture process feature.•Incubation time fracture criterion is applied.•Scatter of the K“ data defines possibility of the K” − ȧ curve construction.•Various loading schemes and specimen types result in different K“ − ȧ dependencies. Dependencies and parameters describing the crack propagation process can often exhibit unstable behavior which is not predicted by classic fracture mechanics. Here such phenomena as the crack velocity oscillations and floating stress intensity factor (SIF) values for the initiation of a pulse loaded crack can be mentioned. This paper considers SIF – crack velocity dependence (KI-ȧ dependence, mode-I fracture is considered in the paper) for dynamically propagating cracks as another example of instabilities observed in the crack propagation. Uniqueness, related to a given material, and even existence of this dependence has been a matter of discussion among researchers, since contradicting experimental results were reported in the literature over the last decades. Experiments with different loading regimes and specimen types demonstrated various shapes of the KI-ȧ curve for a particular material. However, common theoretical tools for the crack propagation prediction and simulation imply the KI-ȧ dependence to be a material property and thus suppose it to be unique for a particular material. In this work crack propagation is simulated using incubation time fracture criterion, which provides possibility to avoid the a priori given KI-ȧ dependence. A computational scheme based on finite element method and the incubation time fracture criterion was developed and various types of the KI-ȧdependence were numerically obtained for various loading conditions and materials.