Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part I. Design of experiments, assessment, and crack-grown-
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ce and Engineering and Mechanical Properties Research Laboratory, Georgia Institute of Technology, Atlanta, GA 30332-0245. MORTEN A LANGØY, formerly on Assignment from Kvaerner Oil & Gas, Stavanger, Norway, at the School
METALLURGICAL AND MATERIALS TRANSACTIONS A
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.
VOLUME 32A, SEPTEMBER 2001—2297
I. INTRODUCTION
RISERS bring well fluids from the seabed to the surface and are crucial elements of systems for economical offshore production of hydrocarbons. Riser systems consist of three major components: bottom connections at the seabed, the riser pipe itself, and a top connection at the vessel on the surface. Candidate materials for risers must withstand high temperatures (well fluids from the Visund field will produce riser temperatures as high as 110 ⬚C[1]), high pressures, corrosive environments, and dynamic loads; beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials (ELI)) is attractive because of its robust resistance to fatigue-crack propagation in different marine environments, its favorable elastic moduli, and its high strength-to-weight ratio. Because defects are inevitable in these types of components,[2] fatigue-crack growth, as opposed to crack initiation, is of paramount importance. Further, the threshold regime of crack propagation is not of greatest concern in production risers because of the relatively short period of service before a given well is depleted and the riser is scrapped or inspected on the surface prior to reinstallation at another site. In seawater, five parameters expected to be important in crack growth are the temperature, frequency, load ratio (R ⫽ min/max ⫽ Kmin/Kmax, where min is the minimum and max is the maximum stress and Kmin and Kmax are the corresponding stress intensities during a fatigue cycle), aerated versus deaerated seawater, and prior cold work (due to possible deformation during installation). Traditional experimental programs assessing fatiguecrack growth rates alter one variable by small increments while keeping all other variables constant. This approach to investigating the response space for fatigue-crack growth is invariably tedious and time consuming and absorbs enormous resources. Given the large number of factors possessing the potential to affect crack growth rates, traditional approaches often raise an insurmountable barrier to the introduction of new materials. Design-of-experiments (DOE) approaches can be of great value early in investigations, reducing experimental effort by probing a large number of variables superficially rather than examining a small number thoroughly.[3] These designs later may be used as building blocks for a more complex interrogation of the response space, an interrogation whose complexity matches that of the problem at hand. The interpretation of the observations pro
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