Abstract
This paper discusses the effect of basketweave microstructure on the very high cycle fatigue behavior of TC21 titanium alloy. Ultrasonic fatigue tests at 20 kHz are done on a very high cycle fatigue (VHCF) property of the alloys with 60 μm and 40 μm basketweave size, respectively. Results show that the alloys illustrate step-wise S-N characteristics over the 105–109 cycle regimes and that fatigue fracture in both of the alloys occur beyond the conventional fatigue limit of 107 cycles. Subsurface crack initiation occurs at low stress amplitude. A fine granular area (FGA) is observed along the α lamella at the subsurface crack initiation site. The mechanism for the subsurface crack initiation is revealed using layer-by-layer-polishing, due to the micro-voids that are introduced at the granular α phase. The colony of α lamella is due to the local stress concentrated between them under the cyclic load. The stress intensity factor range at the FGA front is regarded as the threshold value controlling the internal crack propagation. Furthermore, the effect of the baseketweave size on the very high cycle fatigue limits of the TC21 titanium alloy is evaluated based on the Murakami model, which is consistent with the experimental results. The fatigue life of TC21 titanium alloy is well predicted using the energy-based crack nucleation life model.
Highlights
High strength titanium alloys are usually used in aeronautical force-bearing components as a result of their high specific strength and excellent corrosion properties [1]
Fatigue failure in high strength titanium alloys occurs beyond the conventional fatigue limit of 107 cycles, and the stress-number of cycles to failure (S-N) curves exhibit a step-wise shape [2] or a decreasing shape [4]
Another important characteristic of these titanium alloys is that the crack initiation site shifts from the surface-induced fracture at low cycle regimes to a subsurface-induced fracture at very high cycle regimes [5], which may be responsible for the variability of the fatigue properties and the step-wise shape of the S-N curve
Summary
High strength titanium alloys are usually used in aeronautical force-bearing components as a result of their high specific strength and excellent corrosion properties [1]. Fatigue failure in high strength titanium alloys occurs beyond the conventional fatigue limit of 107 cycles, and the stress-number of cycles to failure (S-N) curves exhibit a step-wise shape [2] or a decreasing shape [4] Another important characteristic of these titanium alloys is that the crack initiation site shifts from the surface-induced fracture at low cycle regimes to a subsurface-induced fracture at very high cycle regimes [5], which may be responsible for the variability of the fatigue properties and the step-wise shape of the S-N curve. The effect of basketweave on fatigue strength and initiation life is quantitatively estimated based on the fatigue fracture theory
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