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In this study, fatigue crack propagation properties in sea water are investigated, when considering the usage for hull structural steels. The rate of the optimum ferrite-bainite (F/B) steels shows superior resistance even in a sea water.Corrosion fatigue properties in sea water are investigated for both conventional ferrite-pearlite (F/P) steels and the F/B steels. It is clarified that fatigue strength of the F/B steels is higher than that of the F/P steels. As to corrosion fatigue crack propagation rates, the F/B steel originally has much superior resistance in normal conditions, and acceleration factors of corrosion crack propagation rate in sea water to that of air is almost constant regardless of microstructure in steels. These two characters brings superior propagation resistance in sea water. As to corrosion fatigue strength, the F/B steel has the advantage of not only a long fatigue crack propagation life, but also long fatigue crack initiation life. This extended initiation life in the F/B steel comes from the small number of corrosion pits, which might be corrosion fatigue crack initiation sites. This difference of pit number may depend on homogeneity to general corrosion. In case of the F/P steels, the microstructure is not so homogeneous, therefore, general corrosion is not uniform. In case of the F/B steels, the general corrosion is uniform because of fine and homogeneous microstructures.

It is known that compressive plastic strain deteriorates the ductility and fracture toughness of steels. So when buckling collapse takes place in structural steel members under the cyclic bending load, cracks may easily initiate and propagate at the concave side (compressive side) of buckling deflection.In this paper, an attempt was made to investigate the crack initiation behavior and quantify the crack initiation strength for structural steel members subjected to cyclic bending load after undergoing buckling collapse. For this purpose, crack initiation tests using strip smooth specimens and fillet welded specimens under the cyclic bending load were performed with JIS SM 490 C steel plate. In these tests the specimens were given various cyclic compressive displacement amplitude after undergoing buckling and number of cycles to crack initiation and fracture were examined. Elastoplastic large deformation FEM analyses were also performed for all loading cases to investigate stress and strain distributions at crack initiation sites. It has been found that : (1) According to the crack initiation tests, for smooth specimens and fillet welded specimens, cracks initiated and propagated from the center of width on the compressive side (concave side). Finally all specimens were fractured due to propagation of these cracks from compressive side.(2) According to the FEM analyses, the largest longitudinal stress and strain were induced at the center of width on the compressive side. With the increase in number of cycles, stress was increased and strain was decreased.(3) Relationships between compressive displacement amplitudes and number of cycles to crack initiation and to fracture can be expressed by linear functions on logarithmic scale. Relationships between longitudinal strain of the crack initiation site at the first compression and number of cycles to crack initiation and to fracture can be also expressed by linear functions, and the relationships for smooth specimens and fillet welded specimens were almost the same.

Recently, to use the high tensile strength steel more effectively for a ship hull, some feasibility studies have been performed on a ship structural design which allows a local panel buckling. Such a design method needs an accurate evaluation of the post-buckling behaviour of plates. It is well known that a thin plate which deformed after the primary buckling may snap to another equilibrium state of a different deflection mode. This is called a secondary buckling.In the previous paper, the authors performed an elastic large deflection analysis for a rectangular plate under uniaxial compression and discussed the influence of initial deflections on the elastic secondary buckling behaviour of the plates. In this paper, elastoplastic large deflection analyses are performed using the finite element method, and the influence of the secondary buckling on the buckling/plastic collapse behaviour of rectangular plates is investigated. The dynamic effects of secondary buckling are also studied. It has been found that : (1) Dynamic analysis must be performed to simulate snap-through behaviour due to the secondary buckling exactly. However, when free from large plasticity effects, static analyses can give rational estimates of the behaviours except in the vicinity of snap-through path.(2) The influence of a secondary buckling on the ultimate strength of thin plates depends on the relationship between an initial yield strength and an elastic secondary buckling strength.(3) Ultimate strength of thin rectangular plates is sometimes attained by a secondary buckling. Secondary buckling reduces a load carrying capacity of plates even when it takes place in the post-ultimate strength range.(4) The change in the deflection behaviour owing to the secondary buckling is more complicated for the plates of larger aspect ratio.(5) Elastoplastic secondary buckling strength can be predicted by the elastically calculated equilibrium path after primary buckling.