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Mechanism of dwell fatigue crack initiation in Ti-7Al under biaxial tension-tension loads
Hommer, Garrison Michael
Hommer, Garrison Michael
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2018
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Advanced structural alloys often possess complex microstructures and low symmetry crystal structures that exhibit twinning, phase transformation and variations in strength between families of slip systems. These attributes give rise to anisotropic and asymmetric mechanical behaviors. Because of this, their three-dimensional mechanical properties and mechanisms of deformation cannot be fully understood through uniaxial characterizations. To investigate these behaviors with multiaxial macroscopic loading, a custom planar biaxial load frame capable of in situ X-ray diffraction experimentation has been built. The instrument was designed to study any arbitrary plane-stress loading condition, in addition to load path change events. Thus, the micromechanics of full plane stress yield and transformation loci may be quantified in addition to path-dependent behaviors. Many previous planar biaxial experiments have primarily focused on tension-tension loading of sheet metals. Thus, specimen geometries capable of planar biaxial compression-compression, tension-compression, and tension-tension were designed using finite element analysis, mechanical testing, and digital image correlation. The design of the experimental setup and its capabilities are discussed. The hexagonal close-packed (hcp) phase of alpha titanium alloys has limited deformation mechanisms. Additionally, twinning deformation, observed in many HCP alloys, is suppressed by adequate aluminum content. Critical resolved shear stress anisotropy between remaining slip systems gives rise to soft grains preferentially oriented for slip, and hard grains that are not. Dwell fatigue, where prolonged peak load is applied each cycle, is known to adversely affect life compared to regular cyclic fatigue for uniaxial loading. It is generally accepted that this dwell debit is due to load shedding between soft and hard grains that occurs because of low hardening and propensity to creep at low temperatures. However, our understanding of biaxial dwell fatigue life and mechanisms, relevant to loading of aircraft turbine compressor blades, is lacking. These topics were studied using the aforementioned experimental setup and far-field high-energy diffraction microscopy, which enabled grain-averaged and spatially-resolved 3D information, including center of mass, volume, phase, orientation, and lattice strain tensors.
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