Damage-failure transition in VT6 titanium alloy under consecutive dynamic and VHCF loads

Abstract

The lifetime of VT6 (Ti-6Al-4V) titanium alloy under consecutive dynamic and very-high cycle fatigue loads is studied. The relevance of the problem is determined by the applications to the prediction of the endurance fatigue limit for materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. The titanium alloy specimens were used to perform High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) tests on the ultrasonic testing machine, which allows fatigue loading for 108-1010 cycles with amplitude up to several tens of micrometers and frequency of 20 kHz. It is shown that VT6 alloy specimens in the 108 cycle regime subject to dynamic preloading reduces the fatigue strength by 70%. Structural study of the fracture surface for the specimens after consecutive loading was conducted using the profilometry data to identify the roughness scale invariants induced by defects for corresponding areas responsible for the staging of fatigue damage-failure transition. The scale invariants and corresponding lengths were used to formulate the generalized Paris law governing the crack growth in damaged materials.