It is well known that a tensile overload applied during constant amplitude cycling at a positive stress ratio R can lead to a retardation in the rate of subsequent crack propagation. On the other hand relatively little is known about crack growth behavior following a tensile overload when constant amplitude cycling is carried out at negative stress ratio R. To provide such information, in the present study the effect of a tensile overload was investigated at three R values; R=0, -1 and -1. 5. Also, the behavior of crack closure and propagation were compared, particularly at negative R values. In the experimental program, center cracked plates of a 0. 35% carbon steel were used. At a baseline of R=0, after an overload retardation in the crack propagation was observed.
However, in the case of R=-1. 0, little or no retardation was observed, and at R=-1. 5 the fatigue crack growth rate actually accelerated after a tensile overload. The crack propagation behavior is related to the blunting behavior of crack tips and the residual stress distributions.
The fatigue life became shorter by applying a single overload while the crack propagated under negative mean stress conditions. This behavior is very dangerous, if the overloading condition and the succeeding cyclic loading conditions are unknown.

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Nondestructive remaining life estimation system for Cr-Mo steel furnace tubes has been developed. The system is based on the relationship among microstructure, the types of carbides and creep rupture strength that obtained from the numerous creep rupture testing results of aged furnace tubes. Type of carbides and microstructure of aged Cr-Mo steel change by using the furnace tubes for a long period. Remaining life will be calculated by the creep rupture testing database of aged furnace tubes. The important feature of this technology is to identify the carbides which are extracted from the filed fine particles by using X-ray diffraction method. This system is applicable to2. 25Cr-1Mo low alloy steel tubes at present.

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This paper describes a prediction method for liquid zinc induced cracking at the toe of fillet weld between thin tube of 590MPa steel and thick ring-plate during hot-dip galvanizing. Design conditions to prevent cracking in such structures are also suggested. To investigate the cracking mechanism in the above structures, elastic-plastic-thermal stress analyses (FEM) were performed at three processes of galvanizing : dipping into molten zinc, air-cooling and water-cooling. The results are summarized as follows. (1)The strain under tensile stressing is accumulated at the toe of fillet weld, by the deformation of the tube due to the temperature difference between the tube and ring-plate during galvanizing. (2)The final accumulated strain (ε_fa) and the critical strain for crack initiation (ε_ca) could be estimated by the three parameters of tubular structures : thickness ratio of the ring-plate to the steel tube (t_r⁄t_p), stress concentration factor of fillet weld (kt) and carbon equivalent for zinc induced cracking (CEZ) . (3)The hypothesis that a crack might be initiated when ε_fa exceeds ε_ca is validated by reviewing the results of the experiments conducted before under variuos conditions. (4)To prevent a crack initiation, t_r⁄t_p should be kept less than 2. 5 under the appropriate quality control of materials and welding procedures.

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Considering corrosion problem on bottom plate of oil storage tanks, fundamental research especially on environmental effects to corrosion phenomena is carried out. From laboratory corrosion tests in sand, it is found that water content is the most important factor influencing localized corrosion, and, localized corrosion rate shows the maximum at 12wt% of water content. Lift of temperature and the chloride concentration also accelerate localized corrosion rate, but compared to water content, those effects are small. Statistical extreme value distributions of localized corrosion depth of SS400 steel and pure zinc show equal variation. By observations, localized corrosion occurs at metal surface where grain of sand adhered to, hence existence of oxygen concentration cell between inside of pit and surrounding airspace is estimated.
From spray corrosion test, evolution of localized corrosion is also observed in spite of uniform wet surface condition. Chloride concentration is considered as acceleration factor of localized corrosion, but its effect is small at concentration less than 1000ppm. Statistical distribution of localized corrosion depth with SS400 steel and pure zinc also shows equal distribution according to double exponential distribution as same as corrosion tests in sand.
From the the results described above on laboratory experiments and analysis, it is estimated that localized corrosion of bottom plate of oil storage tank takes place and develops by the formation of oxygen concentration cell. Based on the results considering field survey data, simulated corrosion test, which provides corrosion prediction of real tanks, came into view.

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