Investigation of the influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of titanium alloy VT-8

The paper investigates the cyclic durability of VT-8 titanium alloy in the gigacycle fatigue mode with preliminary dynamic deformation. This has important applications – the assessment of the resource of materials and structural elements of aircraft gas turbine engines in flight cycle conditions under random dynamic effects. The dynamic preloading test program for specially designed specimens of VT-8 alloy was implemented experimentally using a split Hopkinson (Kolsky) pressure bar. Very-high-cycle fatigue tests were performed on a Shimadzu USF-2000 ultrasonic testing machine, in which specimens are subjected to 108–1010 cycle loading with an amplitude of several tens of micrometers and a frequency of 20 kHz. The experimental program tests high-cycle and very-high-cycle loading realized using the ‘in situ’ method for determining the accumulation of the irreversible fatigue damage by analyzing nonlinear forms of feedback in a closed-system ultrasonic fatigue setup. This makes it possible to establish the connection of the microscopic fatigue mechanisms with the model representations as well as to consider the stages of the damage development based on the nonlinear kinetics of the defect accumulation under cyclic loading in high-cycle and gigacycle fatigue modes. Cyclic loading in the specified modes showed a significant decrease in fatigue life: at a stress level of 585 MPa – from the 2.23·109 critical number of cycles in the initial (undeformed) state to 6.38·103 for preloaded alloy VT-8.