Corrosion fatigue crack growth-thresholds for cast nickel-aluminum bronze and welds

Fatigue crack-growth rate tests of cast NAB and NAB weld metal specimens we re conducted to determine the threshold for fatigue crack,growth (Delta K-t h); The tests were conducted in accordance with ASTM E 647, Standard Test M ethod for Measurement of Fatigue Crack Growth Rates, in artificial seawater : at a cyclic frequency of 2 Hz at R-ratios of +0.05, +0.30, and +0.60. The ; specimens were machined from both poor and good quality, as-cast NAB. Pro duction 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 determi ne Delta K-th. With increasing stress intensity ratio (R),Delta K-th, decre ased. Compared to the values,for cast NAB higher Delta K-th, and higher cra ck closure levels in NAB weld metal tests were noted, due to the residual s tress in the weldment. The same fatigue crack growth-rate curve sets were c orrected for crack closure, and the near- threshold stress intensity decrea sed significantly when the closure loads were deducted. The closure-correct ed, near-threshold stress intensity, based on Delta K-eff, ranged from 4.4 to 6.1 MPa-root m for cast NAB versus 4.4 to 7.4 MPa-root m for NAB weld me tal.