損傷解析への破壊力学的手法の適用

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Structural failure of a bulk carrier was caused in gale sea under service condition of carrying vehicles enroute from Japan to Canada, where cracked damage was found at upper deck plate and slant plate of topside tank near the after corner of No.3 cargo hatchway port side. The cracks propagated from hatch side almost up to the stringer angle and total length of the cracks was about 14 m. Investigation on the fractured surface disclosed a fact that fast fracture started near the intersection of the sidecoaming and endcoaming of the hatch. It was observed that low cycle Fatigue crack had been initiated there prior to the fast fracture, of which the crack length was estimated about 75 mm as a result of the observation by means of microfractographs. In this paper, the failure analysis is made on the phenomenon such as the fast fracture of upper deck plating and the fatigue crack initiation in upper deck as well as fatigue crack propagation and initiation in hatch endcoaming bracket. Fracture toughness of the failured material as obtained by deep notch test is directly compared with the maximum working stress intensity factor which has been evaluated from hull girder analysis by means of three dimensional FEM analysis on the failured deck structures. Fatigue crack initiation and propagation are analysed by using the cumulative fatigue damage and fatigue crack growth law respectively. It is assumed that the maximum expectation value of working stress range in her service life corresponds to the value when the fast fracture occurs and that the long term stress range distribution is logarithmic one. From the above estimations, it is considered that the maximum wave height which the ship encountered varies from 10 m to 12 m. The life of fatigue crack propagation on the endcoaming bracket without taking into account of corrosion is long enough as compared with the inspection period after her delivery, but the effect of corrosion on the propagation life becomes of practical significance. It is found from the study that the stress intensity at the end of hatch sidecoaming is increased when the height of hatch coaming is higher than usual and that the fracture mechanics analysis is applicable to predict reliability on the failure life of ships at the stage of design on their new type of structures.
社団法人日本船舶海洋工学会の論文

損傷解析への破壊力学的手法の適用

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