Corrosion fatigue crack growth rate (da/dN) of 80kgf/mm2 grade high strength steel HT80 has been investigated in synthetic sea water under the open circuit, anodic and the various cathodic potentials at a frequency of 0.17Hz.The effect of cathodic potential on da/dN as a function of 4K or 4Keff was examined and the crack closure effect caused by corrosion reaction products within crack was discussed.The arrangement by the effective stress intensity factor range 4Keff, which was taken from the crack closure effect of white cathodic reaction deposits (a mixture of CaCO3, MgCO3 and Mg(OH)2), was useful to evaluate the influence of cathodic potential on the crack propagation.The daldN under the cathodic protection potential of -0.7-0.9V (Ag/AgCI) became smaller than that in open circuit, while cathodically polarized potential more negative than -1.0V gave rise to the large acceleration of crack growth rate due to hydrogen embrittlement.
The three suction pump dredgers built at Hitachi Sakurajima Shipyard last year have adopted the new launching method called "Wire loosing and winding method", by launching her slowly with wires which is rather different from the common sliding method.The new method was adopted because :(1) The ship type is an Ark.(2) All the outfittings were installed on the building way and her launching weight will be about 1, 000 ton.(3) Having special apparatus as the suction ladder and spuds on her fore and aft.Under these conditions, the authors have considered of using this new methods not to cause any damage to the ways and to the side walls, also not to change the sliding direction by water stopping resistance, tidal current and wind pressure that work unequally to each part of her.The launching way for the wire loosing and winding method was exactly the same as ordinary ships in using greases, but the chocking, loosing, winding and adjusting were all done from the ground by wires.
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