Self-Similar Patterns of Damage Development and Reliability Assessment of AMg6 and D16T Aluminum Alloys under Consecutive Dynamic and Gigacycle Loading

Abstract

In the paper, we study the kinetics of fatigue crack growth in MMg6 and D16T aluminum alloys in the gigacycle fatigue mode under dynamic preloading. The relevance of the problem statement is determined by the critical applications—life estimation of materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. Specimens were preloaded by dynamic tension using the split Hopkinson (Kolsky) pressure bar at strain rates up to ~10³ s⁻¹, with consecutive gigacycle loading on the Shimadzu USF-2000 ultrasonic testing machine. Quantitative fractography of fracture surfaces was performed using profilometry and scanning electron microscopy data. We propose an original form of the kinetic equation, which relates the fatigue crack growth rate to a change in the stress intensity factor. The scale invariance of defect structures responsible for the formation of the fracture surface relief under gigacycle fatigue loading is found to be related to the power exponent of the Paris law. The fracture surface morphology of an aluminum-magnesium alloy under consecutive dynamic and gigacycle loading is studied by the multifractal detrended fluctuation analysis method. It is found that a transition from the stage of formation of a fish-eye zone of localized deformation is caused by the generation of fracture sites and accompanied by a qualitative change of the nonlinear dynamics of the system—a transition from monofractal to multifractal dynamics characterized by broadening of a multifractal spectrum at the final crack growth stage, which leads to macrofracture.