Structural and mechanical aspects of fracture processes under gigacyclic and dynamic loading

Abstract

The study of the kinetics of propagation of fatigue cracks in an aluminum-magnesium alloy AlMg6 preloaded dynamically in the gigacyclic fatigue regime has been held. The research holds a considerable practical interest for assessing the resource of materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. The preloading of samples was realized using a split Hopkinson pressure bar set-up at strain rates of~103 s-1, after which the samples were subjected to gigacyclic loading. Fatigue tests were carried outusing a Shimadzu USF-2000 ultrasonic testing machine. The quantitative analysis of fracture fractography was performed on the basis of profilometry and scanning electron microscopy data.For alloys used in the aircraft engine industry, an important point is to retain their bearing capacity under dynamic loading conditions. To this end, the mechanisms of plastic shear localization during the dynamic deformation were studied in the present work both experimentally and theoretically. It has been found that they are caused by the collective effects observed in the ensembles of microdefects in spatially localized areas. In-situ infrared scanning of the instability region and analysis of a dislocation substructure provide evidence for a crucial role of non- equilibrium transitions in the ensembles of defects during the development of a localized plastic flow. Based on the equations describing a relationship between non-equilibrium transitions and mechanisms of structural relaxation and plastic flow, numerical modeling of plastic shear localization was performed.