Abstract
Uniaxial fatigue tests were conducted for a β titanium alloy Ti-22V-4Al up to a very high cycle fatigue (VHCF) regime. The initiation and propagation processes of the internal fatigue cracks were investigated using 3D fractographic analysis. Multiple facets were observed at the crack initiation site. Three facet initiation models were proposed based on the surface appearances and the 3D facet bonding patterns of the multiple facets, and the major facet was determined to be the true crack initiation site. Using the size of the major facet, a Tanaka–Akiniwa model, which can determine the material constants for the Paris law using only conventional fatigue tests, was applied to reveal the propagation process of the internal cracks. A reverse fatigue life prediction was also conducted to evaluate the accuracy of the material constants obtained using the Tanaka–Akiniwa model. When the facet initiation models were applied, the predictions showed less deviation and better agreement than when the facet initiation process was not considered. The findings of this study indicate that the formation of multiple facets in β titanium alloys is sequential rather than simultaneous.
Highlights
Titanium and its alloys are widely used as aerospace materials owing to their excellent mechanical properties, their fatigue resistance. β titanium alloy has emerged as a promising titanium alloy and has been used in recent years in several landing gear systems in aircrafts such as Boeing 787 and Airbus A380 [1,2,3,4].Generally, it is considered that when the applied stress is less than the fatigue limit, the alloy material exhibits an infinite fatigue life
Tremendous efforts have been devoted by several researchers to investigate the fatigue behavior of β titanium alloys and these studies have confirmed the occurrence of internal fractures in the very high cycle fatigue (VHCF) regime [7,8,9,10,11]
This indicates that the deviation of the crack propagation rate due to various crack initiation life has no significant effect on the internal fracture
Summary
Titanium and its alloys are widely used as aerospace materials owing to their excellent mechanical properties, their fatigue resistance. β titanium alloy has emerged as a promising titanium alloy and has been used in recent years in several landing gear systems in aircrafts such as Boeing 787 and Airbus A380 [1,2,3,4]. Atitthe early stage of with inclusions which are the initiation of steels, is more difficult the internal crack propagation process, the facet initiation and crack propagation behavior to determine and identify multiple facets, which are commonly observed at the internal are strongly affected severalalloys factors such In as addition, microstructures andstage grainoforientation, crack initiation sites ofby titanium [15,16,17]. This affected by several factors such as microstructures and grain orientation, cause indicates that it is important to develop a general model that can illustrate which the facet initi-a significant deviation inbehavior the fatigue estimation of internal cracks This indicates that it is ation and propagation of life titanium alloys. A reverse fatigue life estimation was conducted to evaluate the accuracy of the obtained constants.and Results
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