Manifestation of localized microcrack stability by the unique pop-in events

There is a tendency to regard the crack stability equation as the balance between the driving force and the fracture resistance terms, normally expressed by the continuum theory. However, in cases of subcritical slow crack growth or in crack initiation-arrest-initiation processes, more of a localized approach is required. Here, fluctuations of the crack driving force and resistance occur on a fine scale during the semi-equilibrium period, allowing subcritical growth. Moreover, it appears that mutual interactions develop between the resistance and the driving force suggesting that a generic local crack stability equation actually governs fracture. The current study provides a viewpoint as related to the discontinuous crack growth, while considering micro/macro scale circumstances. In order to elucidate the microcrack stability aspects, Fe-3%Si single crystals beside polycrystalline Al-7075 T651 and AISI-4340 alloys were selected. In contrast to the single crystal case in which the subcritical crack growth was enhanced by environmental interactions, the polycrystalline materials indicated Pop-In (PI) events. Such initiation and arrest potential characterized by the PI enabled to develop physical insights into the dynamic nature of the subcritical crack growth phenomena.