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

用计算力学方法研究了近年来研制的高疲劳强度双相船用结构钢的疲劳裂纹萌生和扩展行为与微观性质的关系，以期获得有利于进一步提高这些材料疲劳强度的知识。在本研究中采用了晶体塑性理论，因为介观力学性能和几何方面(形状、大小、颗粒排列等)。硬相和软相的组合可以包含在计算模型中。研究结果表明：(1)开发了能自动生成具有所需晶粒度和形状的裂纹多晶介质有限元网格的自动前处理软件系统。(2)硬质相硬度的增加导致了硬质相在低应力幅值条件下的微观疲劳裂纹萌生参数(FP)的降低，而…在中等和高应力幅值条件下，这种影响更多地消失。这意味着在小应力幅值条件下，随着硬质相硬度的增加，疲劳裂纹的萌生和扩展性能有望得到改善。(3)当裂纹尖端接近硬质相的边界时，软相颗粒中裂纹尖端附近的变形行为由I型转变为II型。这意味着在软硬相的交界处，疲劳裂纹扩展速率和路径的绕行是可以预期的。(4)循环软化速度越大，裂纹张开应力越大，裂纹张开位移越小。这意味着对于循环软化的材料，疲劳裂纹扩展的驱动形式应该被削弱。(5)当疲劳裂纹在狭窄的软相通道中扩展时，裂纹张开应力和裂纹张开位移都不受通道宽度的影响，这意味着当裂纹中存在很窄的间隙时，疲劳裂纹滑过硬相。较少

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