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EFFECT OF STRESS‐STRAIN BEHAVIOR ON LOW‐CYCLE FATIGUE OF α‐β TITANIUM ALLOYS
Author(s) -
KRAFFT J. M.
Publication year - 1981
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1981.tb01114.x
Subject(s) - materials science , composite material , hardening (computing) , ultimate tensile strength , grain size , cyclic stress , strain rate , metallurgy , stress (linguistics) , plasticity , stress relaxation , fatigue limit , strain hardening exponent , creep , linguistics , philosophy , layer (electronics)
— Tensile specimens of Ti‐6A1‐4V with four levels of interstitial oxygen content and a Ti‐8Al‐1Mo‐1V alloy with different heat treatments to alter grain size and/or microstructural character 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, the effects of grain size and oxygen content are not clearly discriminated. Yet, some of the materials exhibit markedly superior 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.