Fatigue crack growth mechanics for Ti-6Al-4V (RA) in vacuum and humid air
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I.
INTRODUCTION
THE rate of growth of fatigue cracks in alloys is, in general, relatively insensitive to microstructure but quite sensitive to environment. But, as with most generalized statements, there are a number of exceptions, such as the titanium alloys, which show both a significant microstructural crack growth rate dependence I and a dependence on environment. 2 Consequently, it is useful to understand the reasons underlying these effects. Questions which arise while trying to explain and model the observed effects of microstructure and environment on fatigue crack growth include: What measurable parameters describing the microstructure are responsible for the effects seen? How does environment alter the physics of the fracture process? How can these microscopic considerations be related to the remotely applied crack-driving cyclic load? This paper reports the results of an experimental and mathematical modeling effort to describe quantitatively how microstructure is related to the physics of fatigue fracture, and what effect environment has on that process, for Ti-6A1-4V, a widely used and much studied alloy. Only one heat treatment is considered here because of the complexity of the problem, but the intent has been to provide a basis for similar work on other microstructural versions of this same alloy. II.
METHODS
A. Material Sheet Ti-6A1-4V was obtained commercially in an unspecified metallurgical condition. The microstructure consisted of equiaxed alpha phase grains 8 to 15 micrometers in diameter partially or totally surrounded by a coating of beta phase, typical of the recrystallized annealed heat treatment. Mechanical properties of similar material have been determined and are shown in Table I.
B. Experimental Technique Single edge notched specimens of gage section 21 • 1 mm thick were cut from the sheet material, and loading tabs were attached to each end by spot welding. Fatigue cracks, loaded at frequencies of 1 to 5 Hz in a hydraulic S.L. DAVIDSON, Institute Scientist, and J. LANKFORD, Staff Scientist, are with Southwest Research Institute, San Antonio, TX 78284. Manuscript submitted February 24, 1984. METALLURGICAL TRANSACTIONS A
Table I.
Ti-6AI-4V (RA) Mechanical Properties, Ref. 1
Tensile Properties yield strength ductility
862MPa 0.36
Cyclic Properties stress strain
Ao- -- 2550 MPa(-~-~)
low-cycle fatigue
N ' = 0.15 Aep A N ~
fl = 0.766
= 0.47
e~ = 0.47
closed-loop laboratory loading frame, were initiated from a 0.05 mm wide saw slit. Cracks were grown at room temperature in two environments: vacuum of 1 mPa (10 -~ torr) and ambient air of approximately 50 pct relative humidity (12,000 ppm water vapor). The stress intensity range chosen for study was 7 < AK < 15 M N / m 3/2, with Kmin/ Kmax = 0.2 to 0.3. Cracks were grown to a length of several notch diameters prior to the taking of any data. Load sheading was employed to obtain average crack growth rates. Specimens were then transferred from the laboratory machine to a special cyclic loading stage in the Scanning Electron M
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