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Effect of Stress-Strain Behavior on Low-Cycle Fatigue of Alpha-Beta Titanium Alloys.

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Memorandum rept.,

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In earlier NRL work, Ti-6Al-4V in four levels of interstitial oxygen content and a single Ti-8Al-1Mo-1V alloy plate were heat treated to alter grain size andor microstructural character the effect on fatigue crack propagation rate was measured. In this work, small tensile specimens of these materials are subjected to slow strain-controlled cyclic deformation leading to rupture in the 5-500 cycle range. Indication of crack initiation as well as rupture life are compared relative to the plastic excursion strain. On this basis, effects of grain size and oxygen content are not clearly discriminated. Yet, some of the materials exhibit markedly performance. This improvement seems to be related to a characteristic evolution in the shape of the cyclic stress-strain curve. Here, relative to a full convex hysteresis loop of early cycles, the later cycles exhibit a reduced stress level, or cyclic softening, in the first half of the excursion, followed by a resurgence of strength to initial stress levels in the latter portion. The enhanced strain hardening rate enabling this terminal strength restoration is thought to stabilize the deformation, reducing the amount of stress-relaxation-induced tensile strain. Taking such strain as an increment of damage in a cumulative cyclic creep strain criterion provides a correlation between the evolving shape of the cyclic stress-strain curve and the low cycle fatigue endurance. Results indicate the absolute increase in the terminal plastic strain hardening rate to be a constant of a material, independent of the cyclic strain excursion. Author

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  • Properties of Metals and Alloys
  • Mechanics

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