PSI - Issue 35

Martin Ferreira Fernandes et al. / Procedia Structural Integrity 35 (2022) 141–149 Martin Ferreira Fernandes et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Gas turbines and other engineering components operate at a steady state after reaching peak loading during each duty cycle. The stress value increases during the climbing stage of the aircraft, achieves a steady maximum stress level during the flight and returns to a minimum stress level after landing. The combination of cyclic and constant loadings is called dwell-fatigue (Goswami and Hänninen, 2001). The introduction of dwell periods reduces the fatigue life of titanium alloys. The study of the dwell-fatigue behavior is relevant to the aeronautical industry to prevent premature structural components failures. Titanium alloys are applied in the aeronautical industry due to high specific mechanical strength, fracture toughness, fatigue strength and stability at high temperatures. Depending on the alloying elements, titanium alloys are classified into α, near - α, α+β, and β alloy classes. Ti -6Al-4V is an α+β alloy since aluminum and vanadium act as α -phase and β -phase stabilizers, respectively (Ghonem, 2010). The α+β alloys usually offer a combination of mechanical strength, toughness, fatigue resistance, and high-temperature properties for aerospace industry applications . The α phase hexagonal structure has anisotropic characteristics, which influence, for example, the elastic properties of titanium. The modulus of elasticity (E) and shear modulus (G) reach maximum values when the stress is applied parallel to the c-axis and minimum values at a perpendicular direction to the c-axis. Titanium alloys are the material mostly applied by volume in aeronautical turbine engines. However, titanium alloys present dwell sensitivity (Everaerts et al., 2017). The application of dwell periods during each fatigue cycle reduces the fatigue life of dwell-sensitive titanium alloys even at room temperature. The importance of the dwell-fatigue at room temperature or cold dwell-fatigue was first recognized after aircraft turbine disk failures in the decade of 1970 (Lefranc et al . , 2008). The most widely accepted mechanism to explain the dwell sensitivity in dwell loadings is based on the variations in crystallographic orientation between α grains that result in load shedding. The stress redistribution occurs from favorably orientated grains, with a basal plane inclined to the applied stress, to the grains with the c-axis close aligned with the loading direction (Everaerts et al., 2017). The mechanism is responsible for an early plastic deformation process and the reduction of fatigue life. Sinha et al. (2004) observed that introducing the dwell period in the Ti-6242 titanium alloy significantly reduced the fatigue life compared to pure fatigue loading and reduced the total lifetime compared to creep conditions. Cuddihy et al. (2017) showed possible evidence that the facet nucleation in aero-engine disks can occur through the mechanism of load shedding in hard-soft grain combinations. Joseph et al. (2020) showed that the stress redistribution activated pyramidal and basal dislocation slip in hard grain under dwell loadings in a near alpha titanium alloy. Lavogiez et al. (2020) showed that the dwell-fatigue loadings result in the activation of more slip systems than fatigue loadings, which contribute to the dwell-fatigue damage. The present paper aimed to investigate the fatigue performance, damage and fracture surfaces of the Ti-6Al-4V alloy with equiaxed microstructure under dwell-fatigue loadings.