鋼材の脆性破壊発生特性におよぼす疲労の影響

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Previously the authors investigated the effect of low cycle fatigue on the brittle fracture propagationarrest characteristics of ship steels. This paper concerns with some fundamental aspects of brittle fracture initiation from fatigue crack. It is of practical interest to know quantitatively to what extent the fatigue cracks, once they happened to grow in ship hull members, should affect the susceptibility to catastrophic failure.Toughness of ship steel against brittle fracture initiation was investigated using wide plate specimen (300 & 500mm wide) containing fatigue crack. Some observations on fatigue crack growth rate were also made. The steels tested are 15 & 20mm thick ordinary ship steel plates and fatigue loading considered here is repeated tension.The followings have been obtained conclusively : The material deterioration due to repeated plastic straining seems to be relatively slight.The specimens with low cycle fatigue crack showed higher toughness than that of the base plate specimen with machined notch, for which the tilting of fatigue crack and the residual compression stress developed in crack tip zone seemed to be responsible.On the other hand reduction in fracture toughness is observed in the specimen with high cycle fatigue crack where the above two factors are almost absent.A simple expression is formulated with respect to fatigue crack growth rate under general yielding condition.Further investigation is required to ensure the validity of the conclusions obtained here.Additional research program on high strength steel for ship structure is now in progress.

鋼材の脆性破壊発生特性におよぼす疲労の影響

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