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TECHNICAL PAPERS

Mechanistic Features of Short Fatigue Crack Growth Kinetics for High Strength Steels in Sea Water

[+] Author and Article Information
Kijoon Kim

Division of Marine System Engineering, Korea Maritime University, No. 1 Dongsam-Dong, Youngdo-Ku, Pusan, Korea

William H. Hartt

Center for Marine Materials, Department of Ocean Engineering, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431

J. Offshore Mech. Arct. Eng 128(2), 169-176 (Apr 15, 2004) (8 pages) doi:10.1115/1.2185127 History: Received November 19, 2003; Revised April 15, 2004

The importance of fatigue to the integrity of offshore structures is well documented. Also, it has been demonstrated that much of the service life of members and components such as tendons and risers is comprised of an extension of cracks from initial surface defects to a size of several millimeters. At the same time, the growth kinetics of such short cracks has been shown to be more rapid than those of long cracks; however, it is upon the latter that most historical studies have focused. In the present paper, the results of scanning electron microscope fractographic analyses performed upon five high strength steels fatigued in air and seawater are presented. These revealed fracture surface morphology distinctions that were a unique function of material, environment (air versus seawater), potential, and crack length, and that the enhanced fatigue crack growth rate in the short crack regime was relatable to these morphological features. Of particular importance were (1) the development of secondary cracks as a precursor for the short crack to long crack growth rate kinetics transition, and (2) a change in fracture mode, either from quasicleavage (QC) to microvoid coalescence (MVC) or from intergranular to QC or MVC with increasing crack length. The results are discussed within the context of (1) alloy development for applications where a significant portion of the fatigue life transpires while cracks are relatively short such that the enhanced growth rate kinetics apply; and (2) materials selection and fatigue design of riser and tendon systems for deep water offshore structures.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 10

Fracture surface morphology for QT80 steel tested in seawater at −800mV (SCE): (a) short crack zone and (b) long crack zone

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Figure 9

Fracture surface morphology for A537-DQ steel tested in seawater at −950mV (SCE): (a) short crack zone for a=0.45−0.60mm and (b) long crack zone a∼1.9mm

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Figure 8

Fracture surface morphology for AC70 steel tested in seawater at −950mV (SCE): (a) short crack zone for a=0.15−0.30mm and (b) long crack zone a=0.75−0.90mm

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Figure 7

Fracture surface morphology for AC70 steel tested in seawater at −800mV (SCE): (a) short crack zone for a=0.15−0.30mm, (b) short crack zone for a=1.20−1.35mm

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Figure 6

Fracture surface morphology for QT80 steel tested in seawater (FC): (a) short crack zone for a=0.2−0.3mm, (b) short crack zone for a=0.3−0.4mm, (c) transition zone (a=0.80−0.95mm), and (d) long crack zone a∼1.2mm

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Figure 5

Fracture surface morphology for A537-DQ steel tested in air: (a) short crack zone for a=0.15−0.30mm, (b) short crack zone for a=0.45−0.6mm, (c) transition zone (a=0.75−0.90mm), and (d) long crack zone (a=0.9−1.1mm)

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Figure 4

Schematic illustrations of test specimen geometry

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Figure 3

Fatigue crack growth rate data in air and in seawater for AC 70 steel at three cathodic potentials

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Figure 2

Schematic of analysis upon which Table 2da∕dN increase was based

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Figure 1

Fatigue crack growth rate data in air and in seawater for AC 70 steel

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