Abstract
The purpose of this study was to improve our understanding of the effect of macro/micro texture on dwell fatigue properties. The Ti-6Al-4V forged bar with highly oriented texture and fine equiaxed microstructure was used. Dwell fatigue, cyclic fatigue and creep tests were conducted at room temperature by using specimens taken in the longitudinal (L) direction and the transverse (T) direction of the bar. The effects of loading direction on dwell fatigue life and fracture surface morphology were examined in detail. The dwell fatigue life in T direction was shorter than that in L direction. The fracture surface morphologies were characteristically varied by loading waveform, amplitude and directions. In the range of 93 - 95% of 0.2%PS, the characteristic large facets were observed in the T direction in dwell fatigue. Detailed analyses revealed that the large facet consists of multiple initiation facets and propagation facets. The propagation facet plane and crack propagation direction appeared to correspond to alpha (0001) <10-10>. Furthermore, the relationship between strain rate and life time was compared to that for room temperature creep. The deviation from the Monkman-Grant relationship corresponded to the change of the fracture surface morphology.
Highlights
In titanium alloys used for fan disks of aircraft engines, it is known that the fatigue life significantly decreases by dwell fatigue at about room temperature to 200°C
In dwell fatigue, the relationships are obviously different according to the loading direction, and the fatigue lives for T direction significantly decreased
As the crack propagated to the inside of the test piece, the fracture surface morphology shifted to the dimple morphologies
Summary
In titanium alloys used for fan disks of aircraft engines, it is known that the fatigue life significantly decreases by dwell fatigue at about room temperature to 200°C. Many studies have been conducted on the influence of microstructure, texture or MTR (Micro Texture Region) on crack initiation sites and characteristic facet formation in dwell fatigue [1,2]. Hasija et al performed analysis accounting for plastic anisotropy and time-dependent plasticity, showed that local stress concentration generated by load shedding mechanism when grains with different orientations are adjacent, and mentioned that the stress concentration could cause the crack initiation [4]. In dwell fatigue, the interaction between fatigue and creep complicates the change in damage mechanism due to macro-/micro-texture and stress conditions. In the industrial aspect, it is required to grasp the influence of various factors on dwell fatigue damage and quantitatively predict the fatigue life
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