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Fatigue crack-growth rate tests of cast NAB and NAB weld metal specimens were conducted to determine the threshold for fatigue crack growth (ΔKth). The tests were conducted in accordance with ASTM E 647, Standard Test Method for Measurement of Fatigue Crack Growth Rates, in artificial seawater at a cyclic frequency of 2 Hz at R-ratios of +0.05, +0.40, and +0.60. The specimens were machined from both poor and good quality, as-cast NAB. Production NAB weldments were used for NAB weld metal specimens.
The fatigue crack growth-rate curves for duplicate tests at each R-ratio of cast NAB in seawater were analyzed in the near-threshold regime to determine ΔKth. With increasing stress intensity ratio (R), ΔKth decreased. Compared to the values for cast NAB, higher ΔKth and higher crack closure levels in NAB weld metal tests were noted, due to the residual stress in the weldment. The same fatigue crack growth-rate curve sets were corrected for crack closure, and the near-threshold stress intensity decreased significantly when the closure loads were deducted. The closure-corrected, near-threshold stress intensity, based on ΔKeff, ranged from 4.4 to 6.1 MPa-√m for cast NAB versus 4.4 to 7.4 MPa-√m for NAB weld metal.
nickel-aluminum bronze, fatigue crack growth rate, fatigue crack growth rate threshold, corrosion fatigue, seawater, crack closure
Metallurgical Engineer, Naval Surface Warfare Center, West Bethesda, MD