Development of a Computor Code for Crack Path Prediction in Ship Structures

在加筋板结构中，加劲肋的交叉处经常产生裂纹，局部应力集中严重，焊接残余应力较大。由于裂纹轨迹上的局部应力分布比较复杂，裂纹弯曲是问题的基本特征之一，提出了一种裂纹路径预测的计算方案，该方案通过对裂纹尖端进行逐步应力分析，并利用解析得到的裂纹尖端附近的裂纹路径来预测裂纹扩展的曲线增量。作为断裂判据，我们采用了要求II型应力强度因子沿裂纹路径消失的局部对称性条件。为了有效地处理有限元数据，并利用个人计算机系统对进出主机的数据进行交互式和图形化的前后处理，使用该计算程序对具有接近圆孔的边裂纹的板进行了考虑。数值计算表明，只有当初始裂纹线到孔边的最小距离小于孔洞半径时，孔洞才能阻止裂纹扩展。历史上首次对各种焊接接头附近的弯曲脆性断裂路径进行了数值模拟，得到了焊接残余应力强烈影响下的直线和弯曲裂纹路径，该方法同样适用于疲劳裂纹扩展，将成为焊接结构断裂控制的重要计算工具之一。

In stiffened plate structures cracks are often formed at the intersection of the stiffeners, where severe local stress concentrations and welding residual stresses exist. Since the local stress distribution is rather complicated along the crack trajectory, crack curving is one of the essential features of the problem.A computational scheme has been developed for the crack path prediction, which is performed by the step-by-step stress analysis ahead of the crack tip and the prediction of a curved increment of the crack growth by the use of analytically obtained crack path in the neighbourhood of the crack tip. As a fracture criterion we enploy the local symmetry condition which reqires the Mode II stress intensity factor vanishing along the crack path. For the efficient finite element data handling, and interactive and graphic pre-and post-processing of the data, which are transmitted to and from host computors, is introduced by the use of a personal computing system.Using this computational code, considerations are made for a plate having an edge crack approaching a circular hole. It is numerically predicted that the hole can stop the crack growth only when the minimum distance between the initial crack line and the edge of the hole is less than the hole radius. Curved brittle fracture paths near the various welded joints are numerically examined for the first time in the history, and straight and curved crack paths are obtained under the strong influence of the welding residual stresses.This method will also be applicable to fatigue crack propagation, and will become one of the essential computational tools of the fracture control of welded structures.