Study on Fatigue Crack Initiation and Propagation Process of Ship Structural Materials by Computational Mechanics Approach

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The relationship between the fatigue crack initiation and propagation behaviors and the microscopic nature of dual-phases ship structural steels with superior fatigue strength, which were developed recently, are investigated by computational mechanics approach in order to acquire knowledge conducive to the additional improvement of the fatigue strength of these materials. The crystalline plasticity theory is employed in this study because the mezoscopic mechanical properties and geometrical aspects (shape, size, arrangement of grains, etc.) of hard and soft phases can be incorporated in the calculation models. As results, followings are found ;(1)An automated preprocessing software system which can generate FE-meshes of cracked polycrystalline media with desired grain size and shape is developed.(2)The increase in the hardness of the hard phase leads to the decrease in the microscopic fatigue crack initiation parameter (FP) of the hard phase under low stress amplitude conditions while … More such effect vanishes under medium and high stress amplitude conditions. This means that an improvement in the fatigue crack initiation and propagation properties can be expected under small stress amplitude conditions, when the hardness of the hard phase increases.(3)The deformation behavior in the vicinity of the crack tip in a soft phase grain changes from mode I to mode II when the crack tip comes near the boundary of the hard phase. This means that the decrease in the propagation rate and the detour of the fatigue crack path can be expected at the boundary of the soft and hard phases.(4)The crack opening stress increases and the crack opening displacement decreases when the swiftness of cyclic softening is large. This means that the driving form of fatigue crack propagation is supposed to be weakened for the materials which show cyclic softening.(5)When a fatigue crack propagates in a narrow soft phase channel, both the crack opening stress and the crack opening displacement are not affected by the channel width This means that a fatigue crack slips through a hard phase when there exists a very narrow interstice in it. Less