Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part II. crack path and microstructure
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this article on beta-annealed Ti-6Al-4V0.1Ru (extra-low interstitials (ELI)) applied design-ofexperiments (DOE) methods to variables that might significantly influence the fatigue-crack propagation rates for this candidate material for dynamically loaded offshore oil and gas production risers. A single growth rate (da/dN ) in the [1]
STUART R. STOCK, Professor, is with the School of Materials Science and Engineering and Mechanical Properties Research Laboratory, Georgia Institute of Technology, Atlanta, GA 30332-0245. MORTEN A. LANGØY, formerly on Assignment from Kvæmer Oil & Gas, Stavanger, Norway, at the School of Materials Science and Engineering and Mechanical Properties Research Laboratory, Georgia Institute of Technology, is Director, Strategic Business Unit-Casting Process (Foundry), Scana Stavanger AS, Stavanger, Norway. Manuscript submitted February 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
middle of the Paris-law region of the stress-intensity range (⌬K ) vs da/dN curves formed the basis for the analysis. Also, the ⌬K vs da/dN curves were compared for the DOE and supplementary tests. Temperature and aeration/deaeration had little to no effect on the crack growth rate in seawater, while the prior plastic deformation, frequency, and load ratio (R) exerted substantial influence. For the variables examined, this alloy showed robust resistance to fatiguecrack growth, that is, changes in environment, deformation state, or loading conditions produced little increase or even a decrease in the crack-propagation rate. One tenent of materials science and engineering is that microstructure determines, to a large degree, the macroscopic response of materials, even in areas such as fatiguecrack propagation, which are often envisioned as purely continuum phenomena. Thus, the fatigue resistance of betaannealed titanium alloys is often attributed to the lamellar VOLUME 32A, SEPTEMBER 2001—2315
A. Controlling Microstructural Unit The change in the slope of the ⌬K vs da/dN curve, diagrammed in Figure 1, marks the change from structuresensitive to continuum-mode fatigue-crack growth and appears to result when the reversed plastic-zone size equals the size of the controlling microstructural unit.[3,4,5] The size of the plane-stress reversed plastic zone (rcy) may be calculated from fracture-mechanics for a strip-yield model[8] as rcy ⫽
Fig. 1—Schematic of the transition in slope of the fatigue-crack growth, where ms and mc are the exponents in a Paris-law approximation for structure-sensitive and continuum modes of propagation, respectively.[8]
or Widmansta¨tten-type structure. Work comparing recrystallization-annealed, beta-annealed, beta-quenched, and solution-treated and annealed Ti-6Al-4V concluded that the fracture path was most irregular (roughest) for the betaannealed material.[2] A wide variety of characteristic features on fracture surfaces were observed, and they depended on both the underlying microstructure and loading and environmental conditions. Comparing Ti-6Al-4V samples fatigued in air and in v
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