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

采用计算力学方法研究了新近发展起来的上级疲劳强度双相船用结构钢的疲劳裂纹萌生和扩展行为与其微观性质之间的关系，以期获得有助于进一步提高这类材料疲劳强度的知识。本文采用晶体塑性理论，因为细观力学性能和几何方面（形状、尺寸、晶粒排列等）可以将硬相和软相结合到计算模型中。研究结果如下：（1）开发了一套自动化的有限元网格前处理软件系统，该系统可以生成具有所需晶粒尺寸和形状的多晶裂纹介质有限元网格。（2）在低应力幅条件下，硬质相的硬度增加导致硬质相的微观疲劳裂纹萌生参数（FP）降低， 关于我们 这种效应在中等和高应力幅条件下消失。这意味着，当硬质相的硬度增加时，在小应力幅条件下可以预期疲劳裂纹萌生和扩展特性的改善。（3）当裂纹尖端靠近硬相边界时，软相晶粒中裂纹尖端附近的变形行为由Ⅰ型转变为Ⅱ型。这意味着在软相和硬相的边界处，可以预期扩展速率的降低和疲劳裂纹路径的迂回。（4）当循环软化速率较大时，裂纹张开应力增大，裂纹张开位移减小。这意味着，对于表现出循环软化的材料，疲劳裂纹扩展的驱动形式应该被削弱。（5）当疲劳裂纹在一个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，这意味着当其中存在一个非常狭窄的通道时，疲劳裂纹会滑过一个硬相。

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